After our one-day run of seasonably cool temperatures yesterday, we’re headed back to mild weather today as southwesterly winds increase ahead of an approaching cold front. That front will spread clouds into the region from west to east this morning. A quick glance at satellite imagery reveals that most of the area is already beneath thick overcast as of 6:45 AM. The front will push some precipitation our way too, but everything moving in from the west will have a hard time making much headway today as a summer-like Bermuda High makes its presence known. With that in mind, the best chance for spotty light snow or mixed precip today will be found in the north and mountains. No accumulations of significance are expected.
High temps today will range from 30 in the northern mountains to the upper 40’s along the NH Seacoast.
Today will feature a brief shot of colder air behind the storm that narrowly missed to our east yesterday. The chilly airmass has been moving into the region on gusty NW flow overnight. Gusty NW winds will continue this morning before tapering off midday as high pressure begins to crest over the region. Skies this morning are mainly clear except for in the mountains where upsloping remains in progress. Mainly sunny conditions will continue through the mid afternoon hours before high clouds ahead of our next storm begin approaching from the west.
High temps today will be on the cooler side, but believe it or not that’s actually to be expected in January. We’ve just been so warm for so long that “normal” temps now feel cold. The mountains should top out around 15 this afternoon while the coastline gets up into the mid 20’s.
Unsettled weather is in the forecast for today as a coastal storm intensifies to our southeast, and a strong cold front approaches from the west. Unfortunately, these two features won’t join together until they’re well east of our area, so we’ll have to wait a bit longer for a blockbuster snow event.
That said, an inverted trough has developed between the offshore system and the cold front back to our west. While not intense enough to qualify as a Norlun trough, the dynamics powering this feature are similar and a narrow band of snow showers has already developed along this feature from Casco Bay to Bethel, as seen on radar imagery below.
This light snow will linger for another couple hours this morning with a coating-1″ of accumulation expected. This afternoon, steady snow will be replaced by more isolated snow showers and squalls as the cold front moves in from the west. These showers and squalls will be most intense in the mountains, where an inch or two of accumulation is expected in the higher terrain. Dustings are more likely in the valleys.
Cold air will begin pouring into the region on west-northwesterly winds behind the cold front this evening. Winds will gust up to 30 mph overnight, which will be enough to push wind chill values into uncomfortable territory but shouldn’t cause any major disruptions.
High temps will range from 25 in the mountains to 40 along the NH Seacoast.
Today will feature another round of light snow as a weak clipper system cruises through the region. Clouds from this system are already here across most of the region but northeastern parts of the area could see some sunny breaks this morning. The snow will arrive in SW NH around 10-11 AM before moving into Maine after noon. Snow is expected to be light, especially outside the mountains. Only a dusting-2” is expected for most places, with 1-3” in the mountains. High temps will range from 20 in the mountains to 35 in SW NH.
I’m headed out today for a little backcountry excursion into the White Mountains and will be staying through tomorrow morning. Therefore, I don’t anticipate being able to send out an update tomorrow morning. Tomorrow’s weather looks a bit sunnier and warmer, with bright skies returning to all outside the mountains and temps rising into the mid 20’s for the north and upper 30’s for southern NH.
Our next light snow event arrives tomorrow night and continues into Wednesday.
The system that brought light snow to the region yesterday and earlier this morning will be rapidly intensifying as it approaches Newfoundland today. This means that we’ll be left with clearing skies and cooler temps as NW winds bring a refreshing airmass into the region from Canada. Skies will clear most quickly along the coastal plain where downslopimg will have the strongest influence. The mountains will remain mostly cloudy until this evening. High temperatures will be recorded this morning for most areas as temps fall this afternoon in response to the strong advection of cold air from Canada. Currently, temps range from the upper 20’s north to the mid 30’s south. By this evening, temps will be in the low 10’s north and mid 20’s south.
A weak storm system will approach the region from the southwest today and will bring widespread cloud cover and precipitation with it. The system is relatively weak, so precipitation is expected to remain light/moderate today with the exception of some brief and localized heavier bursts possible later this evening.
Radar imagery shows a band of precipitation extending from Portland into central NH this morning. While radar imagery suggests that this band is fairly heavy, a look at more advanced radar products shows that this is actually a case of “bright banding” where melting snowflakes clump together and trick the radar into thinking it’s raining/snowing harder than it actually is. Portland reported light rain as of 9 AM.
Dew point observations give us a good clue as to where rain/snow/mix lines will set up today. Over the mountains and foothills where temps are in the low/mid 30’s and dew points are around 30, snow is the most likely form of precipitation. Along much of the coastal plain and central/SW NH, dew points are just a hair above freezing with temps in the mid 30’s which means that light precipitation will fall as rain, but if any heavier bursts are able to develop, they will probably hit the ground as snow. Along the peninsulas of the Midcoast and the NH Seacoast, dew points are well into the mid/upper 30’s so plain rain is expected.
Here’s a look at forecast model guidance thoughts on precipitation/precip type today and tonight. The initial batch of light rain/snow we’re watching this morning will likely fizzle out midday before steadier precip redevelops over northern/central areas this afternoon. While I’m not sure the coast stays quite as dry as this model seems to think, precipitation will certainly be lighter/spottier over southern regions. Steadier and slightly heavier snow will develop later tonight as a coastal storm begins to intensify to our south.
Unfortunately for those of us wanting a good old fashioned coastal snowstorm, this system won’t reach its peak intensity until it nears Newfoundland in a few days.
Our next chance for a substantial winter storm arrives Wednesday though rain may fall along the coast.
Speaking of snowstorms, in case you missed yesterday’s retrospective look at the pre-New Year’s snow event, you can find it by clicking this link.
In any field that requires making predictions such as meteorology, post-prediction verification and reflection is a key part of the job. I won’t ever be able to claim that I have a perfect forecast, but I owe it to all those who trust my forecasts to learn from the mistakes I inevitably make so that I don’t get tripped up again. That process of learning from busted predictions is why the weather forecasts you get from me or from elsewhere are so accurate despite the immense challenges associated with predicting the future state of the atmosphere.
With that in mind, as you might recall a few days ago we had a storm that I did an exceptionally bad job predicting. I expected 2-4″ to fall over the course of the day on Monday (December 30th) and some places reported over 8″. More snow fell on Tuesday which bumped storm totals well into the double digits not only in parts of the mountains where that was expected, but also in parts of York and Cumberland counties. Given that the nuances of mesoscale banding such as that which occurred on Tuesday are always hard if not impossible to pin down in advance, I feel pretty good about what happened during the second part of the storm’s evolution. Therefore this retrospective look will focus mostly on Monday.
Observed snowfall totals estimated from observations reported to the NWS office in Gray
I prepared Monday’s forecast fairly early in the morning, as per usual. I analyzed radar imagery, current observations, and forecast model guidance. Having spent a considerable amount of time over the weekend analyzing the storm for weather.us, I had a fairly solid mental model of what I anticipated would happen before I even opened my computer. All I needed to do was tweak my previous expectations to account for newly available information and summarize it for you all to read.
My expectation was that a batch of precipitation associated with the warm front of a storm over the Great Lakes would approach the region from the southwest during the morning hours. As it moved northeast into Maine, it would lose momentum due to a combination of low level dry air surging southwest from strong high pressure in Canada, and a loss of mid-level upward motion as the Great Lakes storm weakened and moved westward away from our area.
This forecast model image, posted in Monday morning’s update, highlights some of the reasoning behind my forecast for Monday.
As a result, I was expecting light snowfall accumulations around or below 2″ over SW ME and moderate snowfall accumulations between 2-4″ over NH (closer to the storm, and thus the upward motion needed for precipitation). I expected little to no snow across northeastern parts of the area due to the presence of very dry air in the lower atmosphere which would evaporate snowflakes before they could make it to the ground. Strong upward motion could have offset the effect of the dry air, but the fact that the storm was weakening and moving away from ME/NH supported forecast model guidance which suggested that the best upward motion would remain to our south.
ECMWF forecast guidance available on Monday morning showed light to moderate snow struggling to push into northeastern parts of our region during the day on Monday.
Helpfully, forecast model precipitation guidance such as the ECMWF shown above was in agreement with my conceptual model based on a general understanding of the large-scale pattern. I actually thought the ECMWF was a bit too bullish on its forecast for precipitation given that it was overpredicting dew points in the airmass to our northeast (the air moving in from Canada was drier than the model thought) but I didn’t adjust my forecast too much in that direction.
Radar imagery available Monday morning showed precipitation receding as dry air worked into the area on a northeasterly breeze.
When I analyzed radar imagery, my conceptual model and the forecasts offered by model guidance seemed to be on track. A wide area of light to moderate snow was gradually receding as dry air surged southwest. The snow was perhaps a bit more widespread and intense than I would’ve thought initially, but the apparent error wasn’t all that noticeable except for folks in/near Lewiston who would wake up with a dusting of snow when I thought they would only see flurries at best.
Looking outside in Yarmouth, I saw about 2″ of snow which was towards the high end of what I thought might happen, but given that the dry air seemed to be approaching quickly, I didn’t give it too much thought. One tricky aspect of preparing a forecast early in the morning is that few people have stepped outside with their rulers by that point in the day. While radar imagery is helpful, it doesn’t tell me how much snow has actually fallen. The automated weather stations located at airports across the area are very helpful too, but most of them don’t report snowfall totals until 12z (7 AM) which is after I sent my forecast. Perhaps if I had known there was already 4″+ on the ground in parts of central NH and interior SW ME, I could’ve adjusted my forecast to be a bit closer to what actually happened.
With forecast model guidance and my mental model in good agreement both with each other and with the observations I was analyzing, I went ahead and typed up my forecast with fairly high confidence. In hindsight, that confidence was misplaced.
The key error I made on Monday was putting too much trust in the ability of forecast models to accurately predict the upward motion needed to generate precipitation. Instead of analyzing the wind and temperature fields and using my knowledge about what physical processes can produce upward motion in the atmosphere, I trusted the models to do that work for me.
Because the volume of data available to forecasters is so enormous, there will always be a need to place some amount of trust in the models. The human brain just can’t process that much information in a reasonable amount of time. One of the skills that’s becoming more important as our world becomes oversaturated with data is sorting the signal from the noise. When can we accept what’s given to us (by models) verbatim and when do we need to dig into the weeds and figure things out for ourselves? This problem isn’t unique to meteorology.
The map below is a bit noisy but to the eye of someone who has spent far too much time digging into forecast model guidance, it should scream heavy snow across central NH/SW Maine. Winds and temperatures at 700mb (about 10,000 feet) are plotted along with relative humidity at 700mb (thin contours) and 850mb or ~5,000 feet (shading). The model’s forecast for upward motion is also plotted in orange contours.
Forecast model guidance valid 1 PM Monday shows (in retrospect) a setup ripe for narrow bands of heavy snow across central New Hampshire/SW Maine.
If we focus on the 700mb winds and temperatures for a minute, we can see that Maine is at the center of something called a “deformation zone”. This happens when winds blowing from opposite directions run into each other. In our case, northeasterly winds moving in from Quebec are crashing into southwesterly winds advancing from the Mid Atlantic. Deformation zones are often focal points for heavy banded precipitation because when two airmasses crash into each other, upward motion develops via a process known as frontogenesis. For a more in-depth look at frontogenesis and why it leads to upward motion, check out my article for weather.us which digs into that process.
SPC RUC objective analysis showing strong deformation and related frontogenesis across central NH and SW Maine on Monday afternoon.
Sure enough, objective analysis products valid 1 PM on Monday show a narrow zone of frontogenesis producing heavy snow located just south of the deformation axis. This is consistent with the forecast model guidance analyzed above, and would also be consistent with the large scale setup visible on satellite imagery. A close look at the fundamentals (wind/temperature/moisture) in the mid levels would have made it pretty clear that heavy banded snow would continue through most of the day on Monday across central NH and SW Maine. Unfortunately, I figured that the model’s prediction of strong upward motion over MA/SW NH was reasonable and kept heavy snow out of the forecast.
In short, the forecast bust on Monday resulted from a model error that I failed to catch because I didn’t think to double check the relevant part of the model’s analysis. In retrospect, this seems pretty obvious but without the answer key, it’s very difficult to figure out where model guidance is doing a good job and where it might miss a key feature or two.
Given that I don’t have the capacity to double check every aspect of the model’s work, choices must be made about what information to accept verbatim and what to analyze more closely. I think the biggest lesson I’ll carry away from this storm is that paying closer attention to the fundamentals (wind/temperature/moisture) instead of derived parameters (such as upward motion) is valuable, especially in tricky setups like this. In situations where the fundamentals are telling a different story than the derived parameters, I should be better about trusting my knowledge of the fundamentals rather than leaning on the model-derived parameters.
I would like to apologize again for any inconveniences caused by the forecast error on Monday. I would also like to say thanks for the understanding so many of you have expressed in the past few days. Most meteorologists have inboxes and comment sections full of hate mail after a busted forecast. I have yet to receive a single negative message about the recent storm. Given the state of the internet as we enter 2020, I think that’s pretty cool.
Today will feature mild and cloudy weather as a weak storm system moves by to our north. Light SW breezes on the southern side of this system will be responsible for the warmer air. High temps will range from 35 in the mountains to near 50 in SW NH. The system to our north will be too weak and moisture-starved to produce any precipitation outside a brief flurry over the mountains. Our next round of precipitation will arrive tomorrow once cooler air has returned, and is likely to fall as light to moderate snow.
Today will feature more quiet weather as high pressure meanders offshore over the Mid Atlantic. Skies across the region this morning feature a mix of sun and clouds and that trend should generally continue through this afternoon, though the balance will shift a bit towards cloudier conditions later this evening. Milder temps will continue as warm air slowly filters northeast behind the departing high pressure. Highs will range from around freezing in the mountains to 45 along the NH Seacoast. No precipitation is expected today as our next storm system remains well to the west.
Today will feature the return of quiet weather as the storm that has been impacting the region over the past few days moves away to the northeast. Behind the system, expect WNW breezes to set up our classic upslope/downslope pattern in terms of temps, precip, and cloud cover. The mountains and northwestern foothills can expect mostly cloudy skies, intermittent snow showers, and temps in the upper 20’s to low 30’s. The coastal plain and southeastern foothills will see partly to mostly sunny skies, dry conditions, and temps in the mid to upper 30’s.
Enjoy the nice weather, and I hope you have a great start to 2020!
Forecasterjack 