Showers are moving in this evening and will continue to do so through the overnight hours. Rain will be heavy at times and steadiest in the northern part of the mountains. The rest of the area will see more scattered heavy rain with thunder and gusty winds possible in the stronger cells. Showers and storms sweep east tomorrow morning with clearing in the afternoon. A few showers are possible Thursday with the passage of a cold front but otherwise the end of the week and the weekend look warm and dry. Another threat for heavy rain arrives early next week with significant heat relief likely following for the rest of next week.
Today’s forecast was fairly good. Morning sun slowly faded behind increasing clouds and showers began encroaching from the west this afternoon. Temps stayed mainly in the 70’s as forecast with only a few low 80’s in far southern NH. The showers arrived a hair earlier than forecast and continue to do so this evening but the margin of error there was fairly small. Again, there is no such thing as a perfect forecast but today’s was fairly solid.
A round of rain ahead of a warm front is moving NE through the area currently. The warm front will lift north before stalling right around Portland tonight. Meanwhile, low pressure will track through the mountains during the same overnight timeframe before pushing a cold front south early tomorrow morning. This is the surface setup and is shown with the 4PM surface analysis map from the SPC mesoanalysis page. The biggest forecast challenge for tonight will be pinpointing exactly where that warm front sets up. North of it, steady heavy rains will put a noticeable dent in the drought. South of it, heavy rains will be hit or miss in a more convective format with some towns getting a much needed drink and others staying more or less completely dry.
Aloft, a disturbance over the Eastern Great Lakes will move rapidly NE tonight. SW winds ahead of this disturbance will pump a warm moist airmass into the region. This airmass will feature more Precipitable Water values in excess of 2″ which will support very heavy rain. Pictured is the 850mb moisture field (RH) and vectors showing where that moisture is going (arrows). Notice all the arrows pointing in our direction. This shows that the moisture is headed our way overnight tonight.
So what does this all mean for the forecast tonight into tomorrow morning? The HRRR model does a fairly good job summing it up. Rain will continue to overspread the region from SW to NE this evening and will become heavy at times. The heaviest rain will be concentrated over northern areas with the cutoff somewhere between Portland and Lewiston. North of that cutoff point, steady rain, heavy at times, can be expected with storm totals of 1-2″. Some areas have a shot at seeing up to 3″ if they can manage to sit under a downpour for a while. South of that cutoff, heavy rain will be more scattered with those same 1-2″ amounts falling only where the heaviest downpours track. Those that don’t see individual storms track overhead are likely to miss out on any substantial rain. Most areas in this southern region will see between a tenth and a third of an inch of rain with some spots under the heaviest downpours seeing amounts closer to an inch or two.
Some of the downpours over southern areas could feature thunder and gusty winds as some marginal instability tries to work in aloft. This will not be a widespread severe event but some gusty winds are possible especially across SW NH. The bigger severe threat is to our SW over SNE and NY but we will get some of those storms in a much weakened state. Be aware and have a plan if a warning is issued for your area but otherwise, this won’t be a significant event. Showers and storms sweep NE tomorrow morning with just a few showers left along the coast by sunrise.
The rest of Wednesday will be mostly dry with just an isolated pop up shower here or there. Temps will be warm but not hot and dew points will be sticky but not oppressive. Another cold front will approach the area Thursday which will bring the chance for some more showers and possibly a storm or two. Notice the two low pressure systems back to the west on this map valid Thursday afternoon. Those will slowly work east during the weekend and will end up near us by Monday with a slow moving cold front in tow.
Next week’s rain threat will be centered around Monday. The upper level pattern will shift by then and the seeds of that shift are being sown today. The thick black line is tonight’s disturbance. Thursday’s disturbance is the red line. The lines for the next heavy rainfall threat are the blue one in the top left over NW Canada, the green one south of Bermuda, and the pink one over the Rockies. The blue disturbance will drop south in the coming days and carve out a deep trough. The pink ridge will move offshore by Monday and will end up evolving into a large block off of Nova Scotia. The southerly flow around this block and ahead of the Canadian disturbance will drag tropical moisture provided by the green disturbance northward. There is no reason to change from last night’s forecast anything except the timing. The amplified pattern means that the forward (eastward) progress of each feature will be slowed. Last night, it looked like the heavy rain threat was primarily Sunday into Monday, now guidance indicates it’s more of a Monday/Tuesday affair. Based on the pattern and guidance trends, don’t be surprised if this ends up being more of a middle of next week event. It is still far too early for specifics but if you have plans around this time, keep updated with latest forecasts both from me as well as the NWS and local media.
The deep trough over the Central US that will be providing the focus for this rain threat looks to move east by late next week with more substantial heat relief. How long will that last? It’s anyone’s guess at this point but that should become clearer as we get closer. There is plenty of weather to figure out between now and then.