Mid Day Severe Storm Analysis

Hello everyone!

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.

1:15 PM Radar And Satellite
1:15 PM Radar And Satellite

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.

Surface Analysis. All Maps Valid At 12 Noon Unless Otherwise Stated. All Image Credit Goes To The SPC Mesoanalysis Page Unless Otherwise Stated.

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?

WV Imagery Showing Both Today's Disturbance And Tomorrow's. Image Credit: COD Nexlab
WV Imagery Showing Both Today’s Disturbance And Tomorrow’s. Image Credit: COD Nexlab

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?

Surface Based CAPE Showing Modest Instability
Surface Based CAPE Showing Modest Instability

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.

0-6km Wind Shear
0-6km Wind Shear

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.

Downdraft CAPE Indicating The Potential For Strong Wind Gusts.
Downdraft CAPE Indicating The Potential For Strong Wind Gusts.

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.

12Z GFS Showing The Setup For Afternoon Mountain Showers Tomorrow.
12Z GFS Showing The Setup For Afternoon Mountain Showers Tomorrow.

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.

NHC 5-Day Tropical Outlook. Image Credit: NHC

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.

12Z GFS With The Pattern Overview For Next Weekend
12Z GFS With The Pattern Overview For Next Weekend

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.






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