I was hoping to get an update on this evening’s system out last night but other commitments prevented that so here’s a quick rundown of my thinking and why we’re not going to see a significant event.
One inhibitor of large snowfall accumulations will be ample dry air ahead of the storm that will evaporate some of the snow before it can reach the ground. A good inch or two worth of snow could be lost to mid level dry air. Do notice though that the entire atmosphere is plenty cold enough for snow. There is zero threat of any mixing with this storm but there are plenty of other forecast challenges to make things interesting!
Looking at the mid levels, it is fairly easy to see why we’re not in for a big storm and why the highest snowfall amounts will be along the coast. Both disturbances associated with the storm are positively tilted and separate while winds ahead of the system are out of the WSW which pushes the storm ENE. There is some respectable divergence (winds blowing away from each other) over the system but the setup lacks the explosive dynamics needed for a stronger storm that tracks farther west. Development of these dynamics will occur eventually but too late for us. Drive to New Brunswick or Nova Scotia for those.
Upper level dynamics are similar to the mid level dynamics: modestly favorable but not explosive. The storm is in the right entrance region of a very strong but low amplitude jet streak. There is only one jet and no jet coupling meaning that we lack the explosive dynamics needed for a strong storm. This is the same story across all levels of the atmosphere and explains why we’re not in for any big snow. However, that doesn’t mean some areas won’t get a moderate storm.
A look towards the mid levels shows pretty much the same story with one exception. Notice all the WSW flow and the positively tilted troughs but also notice the kink in the flow right over the storm itself. This kink will gradually sharpen and develop along with SE winds ahead of it. The timely development of these SE winds will be crucial to pulling the deeper moisture NW towards our area. If the kink intensifies more quickly, the SE winds will as well, and the moisture will be able to move NW therefore giving us more snow. If the kink lags in development, the opposite will be true.
Now that we’ve dug into all the dynamics behind this storm and what some of them mean, we can focus on impacts. This map from the Kachelmann Swiss model shows how the coast will see the moderate snow, the foothills the lighter snow, and the mountains only flurries. Also notice the bands of ocean enhanced snowfall across eastern MA as Arctic air currently over ME is drawn across the warm Gulf of Maine waters on NE winds ahead of the storm. In those heavy bands, over a foot of snow is possible along with blizzard conditions. This will be limited to eastern/coastal MA and no impacts that significant are forecast for the ME or NH coastlines.
Snow will arrive in the next hour across southern NH and will be falling across the ME coast by sundown this evening. Notice how the mountains see hardly anything while the coast enjoys moderate snowfall. Also check out all the ocean enhancement as bitterly cold air gets drawn into the storm and passes over the warm Gulf of Maine. The snow will move out in the predawn hours as the storm moves farther offshore.
Here’s my snowfall forecast for tomorrow. This will be a light/fluffy snow that will be easy to move, both by human and by wind. With steady northerly winds, some blowing snow is certainly possible which could create lower visibilities and some drifting in exposed areas though nothing major is expected.
Another cold few days are in store before a brief warmup mid week as low pressure passes to the west.
A very interesting and exciting ~18 hours is in store from tomorrow evening through early Friday morning. During this time, look for torrential downpours along the coast, whiteout snows inland, and strong gusty winds for all. This will be brought to you by a rapidly intensifying coastal storm that will deepen roughly 20mb in 12 hours. This is double the rate of deepening needed for official bombogenesis!
Just a quick note before the fun… If you’ve been reading my storm updates for a while, you know I get into some fairly high level (and interesting!) meteorology while outlining what I expect to happen. If you’re new to my blog, this is how I like to operate as I think it gives you the reader a unique look “under the hood” of weather forecasting. I try to explain technical concepts in an accessible way so you too can become part weather geek. If you’re not into the “why behind the what” or just simply don’t have time to enjoy lots of weather geekery, scroll to the bottom where I’ll put my snowfall map and a concise forecast. Maybe you’ll even see a map interesting enough to read about on the way down!
The upper level pattern is relatively zonal today meaning that winds are mostly blowing west to east and there are no significant disruptions in the flow (storms, blocking highs, etc.). This will change to a certain extent in the next 24 hours but it’s important to remember that the overall zonal flow will prevent this storm from sitting and dumping. Also notice the lack of any substantial blocking high pressure over NE Canada. The zonal flow and lack of blocking means that this storm will be a fast mover and totals will be limited by the short duration of heavy snow. That doesn’t mean some hefty numbers will be recorded, it just means that this storm probably isn’t one for the record books.
At the surface, the setup isn’t quite ideal for a major storm but it does show that heavy snow is likely across the interior. The limiting factor for coastal Maine and New Hampshire will be the lack of deep cold air. This is a function of a) the zonal pattern discussed above, and b) the relatively weak high to the north and the relatively strong high to the south. This means the high to the south will be the primary driver of a) the flow ahead of the storm and b) the antecedent airmass. For big snows along the coast, we need a strong high to the N/NW to lock in the cold air and keep the cold air flooding south through the storm. This will not be the case tomorrow evening. As for the low pressure systems, the primary low was over MN this morning (it is now over SW Ontario) and the secondary low (our storm) was over northern OK (it is now over AR/SW OK).
Snow will move into the area beginning in SW NH late tomorrow morning and ending up in the Augusta area by sundown. There are some indications precip will begin a little earlier in the Portland/Midcoast area as snow showers move in off the ocean but we’ll have to wait until mid afternoon for any more meaningful snowfall. Precip will begin as a period of snow for most if not all areas but will quickly change to rain along the immediate coast (east/south of rt 1). By sundown tomorrow, a couple of inches will be on the ground over SW NH with dustings elsewhere.
As the evening wears on, heavier bands of snow will begin to pivot into the area as coastal low pressure kicks into gear offshore. Guidance is hinting at two circulations being present initially. Which one becomes dominant will dictate which track the low takes and thus how much warm air can wrap into the coast. If the western circulation develops, the storm will track farther to the west and the coast will be warmer, warm enough perhaps for all rain. If the eastern circulation develops, the opposite would happen with the coast seeing slightly more snow. The difference here is not between 3 and 12″, it is rather between 0 and 3-6″. This is not a storm for the coast to see big snows but if the eastern track pans out, the coast would see moderate accumulations as opposed to light/nonexistent accumulations.
Precip will begin to fall very heavily as powerful mid/upper level dynamics move into place Thursday night. At 500mb, it’s hard to imagine a better setup for rapid cyclogenesis. The shortwave trough is negatively tilted and a very strong vortmax is racing NNE out ahead of it. Winds ahead of both the vort and the trough are strongly divergent which favors intense upward motion across the entire area.
Another factor aiding in explosive cyclogenesis will be favorable jet dynamics in the upper levels of the atmosphere. The polar outflow jet of the storm is forecast to be situated in such a way that its right entrance region will overlap with the left exit region of the inflow jet to the south/south-west. This overlapping of zones favorable for divergence will allow the storm at the surface to rapidly strengthen, deepening around 20mb in 12 hours!
What does all this divergence mean? Upward motion Thursday night will be truly incredible. When the scale ends at 20 and values are forecast to exceed 70, you know the event is highly anomalous. The rapid deepening of the surface low, the intense vort at 500mb, and the jet dynamics at 300mb will combine to lift the air at a very high rate. What does this mean for us? Precip will be falling and it will be falling hard.
A band of precip will set up Thursday evening across eastern NH and western ME that will feature whiteout snow falling at rates of 1-3″/hr+, torrential downpours along the coast, very strong winds, and possibly thunder. The fact that precip will be falling so heavily is important because of a phenomenon known as dynamic cooling. When there’s so much upward motion and so many snowflakes melting into raindrops, the atmosphere (especially the above freezing parts) will cool rapidly. This is the key to snowfall east of I-95 and N/W of the peninsulas. The next paragraph explains dynamic cooling and is borrowed from last evening’s update.
As snowflakes melt, the water they contain goes from solid (frozen) form to liquid form. This phase change requires outside energy to energize the water molecules into moving around more, thus changing the phase from solid to liquid. Where does this energy come from? The air around the melting hydrometeors (a fancy word for water (hydro) that falls from the sky (meteor) ). What happens when you remove energy from the air? It cools because, by definition, air with less energy is colder. If you have tons and tons of snowflakes melting at the same time, the temperature will continue to cool and eventually, the entire column will cool below freezing and snow can reach the ground. This process is aided by the intense lifting as air rises, cools, and is replaced at the surface by colder air moving in from the NW.
The problem along the coast is that the dynamic cooling has a lot of warm air to overcome. The warm layer in Portland is modeled to be around 5,000 feet deep by most guidance. Some models are colder but not by a lot. While melting and lifting will cool the atmosphere a bunch, it likely won’t be enough to get significant accumulations. The accumulating snow along the coast will come at the very end as cold air rushes in from the west while precip moves out. IF the easterly track pans out, the warm layer would be shallower and the dynamic cooling would have a chance at cooling the column enough for more substantial snows near the coast. As I mentioned above, there is a cap on snowfall potential east of I-95. I’d say this is about 6″ which would only fall in this area if a) the storm tracked a little east, b) the dynamic cooling worked out as strong or stronger than forecast and c) moisture aloft continued to keep snow falling longer as cold air rushed in behind the storm Friday morning. If none of that happens, most coastal areas would only see an inch or two at best and parts of the midcoast could see no snow at all.
Besides the heavy precip, the other big story with this storm will be the wind. There will be two rounds of very strong winds, one on the front side of the storm and one on the back side. On the front side, winds will be out of the ESE along the coast with gusts to 60mph possible along the midcoast. Gusts to 50-55 mph are likely in the Portland area and points along the coast SW of that. On the back side, winds will flip to the west and begin blasting at similar speeds. While the front side winds will be mostly a coastal issue, the westerlies on the back side will impact everyone.
Here’s another visualization of the winds at a single point (Rockland) through time. Much like a hurricane, there will be front side winds, a calm period as the center of the storm passes overhead, and then back side winds. Winds will not be of hurricane strength though a gust to hurricane force can’t be ruled out offshore and possibly at an exposed Midcoast point. These winds will definitely be strong enough to knock down trees and power lines especially those anchored in soggy ground (midcoast) or those weighed down by heavy snow (inland). With colder air moving in behind this storm, it will be important to be prepared for a night or two in the cold should your power go out.
Here’s the latest snowfall forecast. The main adjustment was to trim back totals a bit near the coast as guidance has shifted towards a warmer solution. The bullseye of around 2 feet in the Whites/Mahoosics still looks good. The area with the sharpest gradient (near the coast) still has a bit of uncertainty attached as some guidance still wants to hang onto a colder solution. This will be watched and any adjustments needed made tomorrow.
To summarize: snow will arrive from SW to NE midday tomorrow and will change to rain along the coast tomorrow evening. Heavy snow and rain will arrive tomorrow evening and last through tomorrow night with whiteouts possible in areas of heavy snow. Thunder will also be possible as dry air moves in aloft and instability is created with warmer and moister air near the surface. Winds will be strongest along the coast and will blast the area in two parts. The first will be along the coast with ESE winds gusting up to 60mph. The second will be enjoyed by everyone with WNW winds gusting up to 50mph. These winds will be strong enough for power outage concerns.
No coastal flooding concerns are expected as tides are astronomically low though a few instances of minor splashover are possible along the midcoast where onshore flow will be strongest.
The pattern doesn’t stop here, more snow is in the forecast for New Year’s Eve night and mixed precipitation may threaten to begin the first week of 2017. I will have an update on those storm threats later this evening or tomorrow.
After a weekend of on and off storms and heavy rain, we finally got a break from the heat, humidity, and storms today. Enjoy the drier air while it lasts because it won’t last long. Warm, sticky air will again flood the region tomorrow as a warm front tries to drive north. After heavy rain for many tomorrow evening, rain will become lighter and more scattered for the rest of this week with more dry hours than wet. That changes by this upcoming weekend when a deep trough to our west will bring more tropical moisture and accordingly more heavy rain.
Today’s forecast worked out fairly well with the exception of cloud cover. In typical upslope/downslope situations, the clouds would reside over the mountains while the sun would be over the southern part of the area. Today, however, warm air advection aloft brought clouds to the south while the atmosphere dried out so much (and was still so warm aloft) that no widespread clouds were observed over northern areas. The cloud cover aside, everyone stayed dry as forecast and forecast temps panned out well with mid/upper 70’s north and low to mid 80’s south. Overall, not a disaster, but as always, it could’ve been better.
Tomorrow will feature the return of tropical moisture to the area as a disturbance passes through Northern VT and into Northern ME. A low pressure system will move right through the middle of the area tomorrow as well. This means that SW winds aloft and at the surface will again bring tropical moisture to the area. Tomorrow’s forecast will be basically split up into three parts, the delineation between them being location. The first part will be fairly heavy, fairly steady rain across the north.
Tropical moisture will once again be present across the region by tomorrow afternoon and evening and Precipitable Water values will be running near or over 2″ which is about as loaded as it gets here in Maine in terms of tropical moisture. Low pressure will pass right through the middle of the area leaving the northern part of the area to the north and eventually north west of the low. This is where steady precip is most likely, The tropical moisture will encounter some residual low level cool air leftover from today’s airmass and rise, cool, and condense, causing rain. Areas mainly north of route 2 could see 1-3″ of rain from this event.
The middle part of the area, north of a Portland/Hanover NH line, but south of route 2, will see more scattered showers and possibly a storm or two. This is the area through which the low itself will track which means that it will be missing both the instability from the warm sector south of the low, and the forcing for steady rain north of the low. This area will see the least amount of precip overall because it is neither here nor there so to speak. The precip for this middle stripe will range from a quarter to a half inch and will come from showers and storms that form farther south tracking NE.
Southern areas, south of that Portland/Hanover NH line, will see showers and storms with torrential rains and also with some limited severe potential. The nearby low, strong upper level disturbance, and warm front/cold front combo will provide the trigger. Strong shear associated with the developing low (pictured above) will provide the organizer, but the big question will, as per the norm here in Maine, be instability which, right now, looks limited at best. Due to the lack of instability and the presence of an inversion aloft which will help keep strong winds high in the sky, the severe threat looks limited. That being said, the SPC has this southern area in a marginal risk for severe weather and a stray strong wind gust can’t be ruled out. This severe threat will be least small overnight Tuesday into early Wednesday morning. Some scattered showers and possibly a stray storm is possible Wednesday morning before afternoon drying and clearing. Southern areas will see rainfall amounts vary widely depending on exactly where storms set up going into the evening hours. Some places could see over 1-2″ if storms train over a specific location while it is possible others struggle to see even a quarter of an inch of rain. There is no way of knowing exactly where storms will set up this far in advance.
Highs Tuesday look to remain cooler as clouds and showers overspread the area from west to east. Look for temps largely in the 70’s with a few low 80’s possible in SW NH.
Thursday and Friday at least look warm but not hot with some scattered showers in the afternoon but no organized rain. By the beginning of next weekend, however, a deep trough will be present over the Central US with deep S/SE flow from the tropics over our area out ahead of it.
There are several important factors for next weekend’s rainfall potential. The pattern looks a lot like last winter’s with a big trough just to our west flanked by large blocking ridges. Warm S/SE flow ahead of the trough looks to bring another threat for heavy rain, just like it did so many times last winter. At this point, all that seems clear is that another heavy rain threat exists and must be watched between 6 and 8 days from now. The pattern supports it as does all available guidance that I’ve seen. Specifics are still uncertain at this point but they will become less so as the week goes on. I’ll have more updates as those details become clearer.
Following that heavy rain threat, more significant heat relief seems likely as we look to move into the third and fourth weeks of August.
One last note, this Wednesday (8/17) at 6:30 PM, I will be giving a presentation at the Freeport Community Library where I will discuss me and my background, some of the quirks of Maine weather, how to be a better informed consumer of weather information, and finally what you can do to help meteorologists make better forecasts. For more details, click on over to the FB event page that the folks at the Freeport Library created. I hope to see you there!
I have a quick break between Beach to Beacon this morning (awesome!) and heading north to Katahdin tonight so I figured I’d sneak in an update to bring you up to speed on how things look for severe storms today as well as a more detailed look at tomorrow’s forecast as I will be up and hiking early, thus unable to post.
I’ll start out with the short forecast for those who just want to know what the weather will be. The why behind the what will be explained in full detail below. Temps are on track to rise into the mid to upper 80’s for most with low to mid 80’s along the island and peninsulas. Some low 80’s are also likely across far northern areas where some thunderstorms have already gone through. Showers and storms move through over the next several hours from NW to SE. The main threats from these storms will be strong winds, heavy rain, and lightning. Remember, when thunder roars, go indoors!
A dry and mild night is in the cards tonight with temps sinking into the upper 50’s north and low to mid 60’s south. For tomorrow, mainly sunny skies are expected with dry and cool NW winds keeping temps right around 80 region wide with noticeably less humidity. An isolated shower or storm is possible in far NE areas but nothing significant is forecast and most folks will be staying dry.
Now for the why behind the what. What are the factors driving thunderstorms today? What will the rest of the afternoon look like and why? How about tomorrow? To answer those questions, we’re going to get to take a little bit of a deeper dive into the world of weather.
A prefrontal trough moved through the area this morning and is now producing showers over NE MA and the Gulf of Maine. The actual front is draped across NY and the St Lawrence valley which is fairly significantly west of where most guidance had it for this time. This was an idea I forecasted in last night’s discussion. Storms are beginning to fire across the mountains as forecast while a lone cell travels across Central Maine towards Augusta. The front will continue to move slowly to the east this evening and storms will continue to fire out ahead of it. When discussing thunderstorms, I always talk about three things: the trigger, the fuel, and the organizer. How are we doing in each category this afternoon?
The trigger at the surface is the cold front pictured in the surface analysis above and the trigger aloft is a shortwave over Quebec (orange line) which will be moving ESE through the area this evening. Tomorrow’s disturbance is farther west, just south of Hudson Bay. A 90kt+ jet streak (not pictured) is currently just north of the area and will also be sagging SE as the afternoon goes on. In short: we have plenty of trigger. How about fuel?
Most of the area is currently seeing about 1,000 j/kg of CAPE with the exceptions being the immediate coast (due to cloud cover and some weak marine influence) and far NW areas (due to earlier convection using up some of the fuel). One important thing to notice is the unstable air even farther to the NW over SE Quebec. That will translate ESE with the whole system this afternoon and will keep NW areas in the mix as far as storms go. The days long debate as to unstable or not has been more or less resolved and there appears to be enough instability to sustain some storms. However, that being said, instability is still modest/marginal. There does not appear to be quite enough for widespread severe storms. The result will be scattered/isolated severe storms with mainly sub-severe storms as well as showers. The lack of deep instability will also impact storm coverage which will be scattered as opposed to widespread.
The final piece of the puzzle is the organizer. Will storms have enough shear to get organized enough so that they are capable of damaging winds? 0-6km shear is currently in excess of 40kts across the entire area with some parts seeing shear in excess of 50kts. This amount of shear is more than enough to organize some strong storms. We have our trigger, our fuel, and our organizer which means we are a go for strong storms this afternoon.
Most storms will remain below severe limits but a few could grow strong enough for some marginally severe wind gusts. Shown at left is downdraft CAPE, which measures how fast air can sink in downdrafts (as opposed to normal CAPE which measures how fast air can rise in updrafts), is plenty high enough for some strong wind gusts. Also to note is the drying out of the mid levels of the atmosphere on WV satellite (look at the trigger graphic and see the dry air (black) moving in). This will aid in the development of strong downdrafts by evaporationally cooling the air which will result in even more intense negative buoyancy due to the cooled parcels being even cooler than their environment. However, this development comes with a caveat: the drying aloft will be robbing the storms of any deep moisture. This is another factor that looks to limit any serious severe weather today.
By tonight, the front will be offshore and cooler air will be filtering in on NW winds. Lows will settle into the upper 50’s north and low to mid 60’s south. Skies will be mainly clear.
Tomorrow will be a mainly quiet day but there could be an afternoon shower or storm in the NE mountains due to a cold pool aloft associated with a lively upper level disturbance (shown in the left panel). It will trigger some daytime heating driven showers and storms. They will be clustered in the mountains and especially the NE mountains (shown in the right panel). Because freezing levels are so low (~10,000 feet), some small hail is possible in any of the stronger cells but no severe weather is expected. That activity will die down tomorrow evening with the loss of daytime heating.
What’s after tomorrow? A period of cool, quiet weather is likely through the middle of next week with highs each day in the low to mid 80’s with full sun and low humidity. As we approach next weekend, however, things change. Some beneficial rainfall could arrive next weekend if everything comes together right, which, as of now, is possible. This rainfall chance will be connected to that yellow ‘X’ over the NE Gulf of Mexico which the NHC is monitoring for possible tropical development. Right now that appears unlikely.
A pattern change driven by the recurvature of Typhoon Omais off Japan will result in the development of strong high pressure near Bermuda by next weekend. SW flow on the west side of that high will direct moisture from a tropical disturbance over the NE Gulf of Mexico (shown on the tropical disturbance map above) towards us. At the same time, a front will be draped near the region which could help focus rainfall. Model guidance is quite optimistic we see drought easing rains from this setup and the pattern supports it so while it is certainly not a lock, it is something to keep a serious eye on as we enjoy another week of lovely weather.
I will not have a post tomorrow morning as I will be climbing Katahdin. Please refer to this post as well as to the NWS and local media for your forecast tomorrow. I will be back with a post Monday morning.
A cold front is slowly approaching from the west this evening and will arrive in far NW areas tomorrow morning before slowly sinking SE through the day tomorrow. This will bring showers and storms to the area, some of which could be severe. The risk for severe storms is greatest across far southern areas where the most instability will reside. Cooler and drier air filters in Sunday though a pop up shower is possible in the mountains. Cooler and drier air sticks around for Monday before heat and humidity slowly build back up again for mid-late next week. For more details as to the why behind the what, read on for a full analysis of the upcoming week of weather.
Conditions this evening verify this morning’s forecast well with mainly sunny skies and warm temps observed across the area. As forecast, some more clouds are located over NW areas closer to the front. Temps are generally in the mid to upper 80’s with a few overachieving spots hitting 90. Temps up north and along the coast are cooler. Overall, this morning’s forecast worked out pretty well. The only exception might be that forecasted showers didn’t materialize in the mountains.
The front will continue to approach this evening and a batch of showers and storms currently just west of the St Lawrence valley will be drifting into the area by early tomorrow morning. Expect this batch of showers to be weakening as it approaches and it may just be some clouds by the time it reaches the coast. The moral of the story is that by tomorrow morning, the umbrella will need to be on standby as showers mainly of the light variety drift across the area. Temps tonight will remain in the 60’s for most with low 70’s south and west. The more noticeable factor will be the humidity which will be not too far below the temps. Soupy readings near 70 are likely by tomorrow morning. That moisture will help fuel storms tomorrow afternoon.
As I’ve discussed many times over the course of the summer, there are three things you need for widespread severe storms: a trigger, some fuel, and something to fan the flames. The trigger will be the cold front at the surface and an upper level disturbance aloft. The main storm both at the surface and aloft will remain well north of the area in Northern Canada but a filament of it will be swinging through our area tomorrow afternoon. There is little doubt that we will have enough of a trigger for storms tomorrow.
There is plenty of shear around tomorrow to organize storms into segments capable of strong winds. Not only that, winds aloft will be pulling apart (black arrows) which will leave a ‘gap’ in the atmosphere aloft. To fill the gap, air from below will be rising. Rising air aids in storm development and the divergent flow aloft will be aiding in synoptic scale rising air. 30-40kts of 0-6km shear will be the organizer. There is high confidence that we will have a trigger and an organizer for tomorrow. The big question will be how much fuel do we have.
There are two possibilities for how much fuel is available tomorrow. Either there’s a bunch or there’s not a bunch. If the front is slower and there is more time for the sun to heat the ground ahead of it, we’ll have more fuel for bigger storms. The opposite is also true if the front is faster. Notice how over Northern New England (NE black box) there is instability on both sides of the front? Notice how this contrasts with the conditions over the OH valley (SW black box). The atmosphere is set up so that not all the moisture is chased off the instant the front arrives. As a result, even with a slightly faster front, there could be some storms. Now how fast do I think the front will move? For that, let’s examine the pattern as a whole.
The current pattern is quite amplified for the summer months. A large ridge extends from New England to Baffin Bay (red line) with an Omega block over Baffin Island (red box). Low pressure flanks this high both over the North Atlantic just south of Greenland and over Hudson Bay (blue boxes). The Hudson Bay low is the one we really care about because our disturbance is moving along the southern flank of it (pink line and arrow). Because of the wall of high pressure to its east (red line/box), both the low and the disturbance have nowhere to go. The high isn’t moving particularly fast because the whole pattern is blocked up (low S of Greenland, high to its east, low to the east of that, high to the east of that, and so on). Because of the blocked pattern, I tend to think that everything will be moving a little slower which would lead me to lean a little bit towards the NAM solution of higher energy.
Of course, even with a slower front, strong storms are far from a guarantee. Remember that there will be showers ongoing across the area tomorrow morning. Showers come with clouds and clouds don’t let as much sun through which means that the ground can’t heat up as fast or as much. A cooler surface temperature means that the difference between the surface and the upper atmosphere is slightly less which results in less energy for storms. For more on how this works, check out my UpPortland column from July (now online as a direct link!).
The greatest chance for strong to severe storms will be across southern areas where strong instability is most likely. The greatest threat will be for strong winds with lightning and heavy rain always possible with any storm, even those that are non severe. When thunder roars, go indoors!
What happens after the front drifts offshore Saturday night? Cooler and drier weather is in the forecast for Sunday and into early next week (orange box, look at those dew points (green line)!). A spot shower is possible Sunday but otherwise the rest of the outlook looks fairly dry as we continue to grow our rain deficit. More heat and humidity threatens by mid to late next week.
Looking even farther into the future, a typhoon will be recurving off the coast of Japan this week which will set off a chain event across the entire Northern Hemisphere by the end of next week. Tropical moisture will be pooling off the Florida coast over the Gulf of Mexico during the same time. The pattern will be changing as a result of the typhoon so that a high pressure system will organize near Bermuda. The flow on the west side of the Bermuda high will be pointing that tropical moisture in our general direction by next weekend. At this point, that looks like our next chance for a steady, heavy rainfall. I’ll continue to watch it as we get closer.
Today will feature slightly cooler temps, less humidity, and another chance for strong/severe thunderstorms. The thunderstorm threat will develop later this morning and continue through the early evening hours. Storms are more likely in SW areas where more energy will be available aloft. While most of the storms are not likely to become severe, some of the stronger cells do have the potential to mix down some strong wind gusts and perhaps some small hail. Frequent lightning and heavy rain are the main threats. Highs will range through the 80’s with dew points in the 60’s which is still warm and certainly not dry but far better than yesterday.
A vigorous disturbance is currently located north of the Great Lakes and is forecast to drop SE and intensify, arriving in our area bu sunset. Storms will form out ahead of it late this morning/early this afternoon. When I talk about forecasting thunderstorms, I talk about the trigger, the organizer, and the fuel. This disturbance will be the trigger for today’s storms.
A band of strong winds and powerful shear will rotate around the disturbance today. The strongest winds will be pointing at Southern New England while Northern Maine sits in the calm aloft associated with the disturbance being to their south. In southern Maine/New Hampshire, we’re a bit on the edge. We likely see enough shear to get some small clusters but large squall lines are unlikely. If that disturbance can intensify enough fast enough, it could pull some of those winds north and we could see a bit more action. If it remains a little weaker, those winds will continue screaming towards SNE. These winds will be our organizer.
The biggest question today is how unstable the air will be. A cold front moved through last night (remember those storms?). That front has moved offshore and has taken the extremely soupy/unstable air with it. We are left with marginal moisture and several areas of clouds to limit surface heating. Model guidance suggests that more unstable air will try to filter in from the west later today but I have to wonder if that will make it in time. Also of note with regard to instability is wind direction. Westerly winds don’t exactly bring in the warm moist air you need for big storms. Regardless, we do still have leftover moisture and dew points remain in the 60’s which is sufficient for at least some storms. Also, wide swaths of clear skies this morning support some solid surface heating which will bring temps up into the 80’s, also plenty sufficient for storms.
All that to say, ingredients are at least sufficient for some storms, a few of which could be severe with strong winds and small hail. However, I do have doubts as to how unstable the atmosphere is. Also, we don’t have optimal winds aloft for organizing storms into lines capable of widespread wind damage. While organized severe storms are not forecast, still do keep an eye to the sky today and be prepared to duck inside for a few minutes if you plan on heading outside.
More storms are possible Monday and Thursday with two more cold fronts.
As one light snow event races off towards Newfoundland, another waits on our doorstep for tomorrow. Following light snow tomorrow, temps stay cool into Wednesday before warming up ahead of a more significant storm Thursday into Friday. This one should be mainly rain but quite a lot of it is expected. That storm departs Friday night leaving afternoon showers and thunderstorms around for the weekend as we sit under an upper low.
Overview: This Week’s Weather At A Glance
Here’s this week’s weather at a glance for those who just want the short version. I hate to say it, but there are no long term warm ups in the forecast and even the mid/late week warm spell will struggle to top out above 50F. April in Maine is cruel. Details on our two incoming storms are below.
The Setup: Tomorrow’s Snow
Low pressure is currently diving SE across the Great Lakes and will be making its way to SNE by tomorrow morning. It will be too far south for significant impacts here in ME but points south could see several inches. Snow will move in around daybreak and move out in the evening. It will be light and accumulations will be similarly unobtrusive. Because this system is much weaker than the one today, no crazy winds are expected.
Tomorrow’s Snow: Weak Upper Support Means Weak Storm
The reason this storm will remain weak and to the south is that above 15,000 or so feet, the storm doesn’t exist. In the image to the right you can see the upper level footprint of our storm today with very strong winds and a big dip. Tomorrow’s storm? It’s circled in red. If you can see any significant storm there, let me know because I’m having a hard time. The lack of significant upper level support means this one won’t have a chance to intensify or turn NE when it reaches the coast. The net result? Very little snow for ME and NH.
The Setup: Late Week Heavy Rain
Our next storm arrives late week with heavy rain likely. Notice the NW/SE orientation of the longwave features. This means that the trough to our west will be feeding tropical air northward into our area on strong S/SE winds. Those S/SE winds also mean that storms will be more likely to move up the coast when they develop as the individual shortwaves rounding the base of the trough. The end result? Heavy rain is likely at some point between Thursday and Friday. Exactly when and exactly how much rain falls has yet to be determined.
Late Week Heavy Rain: The Tropical Connection
When the rain does come, it will bring with it air straight from the tropics as the 12Z GFS PWAT map shows. This helps to establish fairly high confidence we will see heavy rain at some point late this week. The greatest threat for heavy rain will be Thursday night but it could fall any time Thursday or Friday. Guidance is indicating fairly significant rain could fall with amounts likely in the 1-3″ range. This could cause some rapid stream rises so be aware of that potential. Despite that, no widespread flooding issues are expected.
Next Weekend: The Return Of The Cold
Looking ahead to next weekend, the upper level low that brought the heavy rain Thursday/Friday will park over our area bringing cool and unsettled conditions. Expect temps to again drop back towards freezing with rain/snow showers possible. No accumulations or organized storm systems are on the horizon but showers driven by daytime heating will likely become a fact of life heading into the second week of April.