Heavy Rain Possible This Weekend Following Oppressive Heat Tomorrow

Hello everyone

Current Conditions This Afternoon (4:30 PM) Verifying Today's Forecast
Current Conditions This Afternoon (4:30 PM) Verifying Today’s Forecast

This morning’s showers are now well offshore as an upper level disturbance rotates eastward. Looking at the forecast verification, temps worked out fairly well with widespread 70’s under cloudy skies. Some 80’s were reported in SW NH as expected. I didn’t think clouds would clear out enough in the far north for 80’s but by the looks of obs across SE Quebec, some warmer temps likely found their way into those areas. This morning’s showers evolved as forecast but so far no storms have been reported in NE areas though some leftover convection from Quebec could still find its way in later this evening. Overall, today was a pretty good forecast.

4km NAM Showing Oppressive Dew Points Tomorrow. Image Credit: Weatherbell
4km NAM Showing Oppressive Dew Points Tomorrow. Image Credit: Weatherbell

Tomorrow will begin a two-day sufferfest as we sit to the south of a frontal boundary. Look for highs to climb into the low to mid 90’s for most SW areas with upper 80’s elsewhere. SW winds will keep any sea breeze attempts at bay. Humidity will be the big story with dew points rising into the low 70’s for most with some upper 60’s north and east. The heat and humidity will combine to offer up heat index values near or over 100. Heat advisories are up for SW NH and York County ME due to this dangerous heat.

12Z 4km NAM Showing Widely Scattered Showers Possible Tomorrow. Image Credit: Weatherbell
12Z 4km NAM Showing Widely Scattered Showers Possible Tomorrow. Image Credit: Weatherbell

While there is no organized trigger for showers or storms tomorrow, there will be plenty of moisture around and as a result, even the differences in heating of the mountains could be enough to trigger a spot shower or two tomorrow afternoon. Again, this does not look widespread by any means but don’t be surprised to see a few drops if you’re out and about in the mountains. Most of the area looks to bake under the hot sun with no relief from clouds or precip.

12Z 4km NAM Showing More Dangerous Heat Friday. Image Credit: Weatherbell
12Z 4km NAM Showing More Dangerous Heat Friday. Image Credit: Weatherbell

More dangerous heat is on tap for Friday as we continue to sit south of the front. Temps are likely to be only slightly cooler than Thursday but dew points are likely to soar even higher. Temps in the low to mid 90’s combined with dew points in the 70’s will result in heat indexes exceeding 100 for much of the SW portion of the area. This is astronomical for Northern New England and nothing to take lightly. Be sure to take proper precautions such as hydration and limiting outside activity during the hottest part of the day. By evening, storms will be rumbling into the area from the NW and will begin to bring relief along with heavy rain, gusty winds, and frequent lightning.

12Z GFS Showing Sufficient Shear For Severe Storms Friday. Image Credit: Accuweather
12Z GFS Showing Sufficient Shear For Severe Storms Friday. Image Credit: Accuweather

The hot temps and oppressive humidity will result in a very unstable atmosphere Friday. Remember the three severe weather ingredients? We will have plenty of fuel. The approaching cold front will be our trigger and indications are that there will be more than enough shear to organize storms. This means that there is a threat for severe storms tomorrow with gusty winds being the main threat. This threat is greatest in the mountains where the front will be closer during peak daytime heating.

Current Water Vapor Satellite Showing All The Players For The Weekend Rain Event. Image Credit: COD Nexlab
Current Water Vapor Satellite Showing All The Players For The Weekend Rain Event. Image Credit: COD Nexlab

The pattern change I’ve been talking about for almost a week is finally here. A strong area of high pressure is setting up over the Western Atlantic as a trough is digging into the Western US. The SW flow on the periphery of the high and on the eastern side of the trough is directing a pool of tropical moisture associated with the tropical disturbance over Florida our way. Meanwhile, a powerful storm over Hudson Bay is pushing a cold front SE. Energy will eject from the Western trough and move east combining with the front and the moisture to bring rounds of showers and storms beginning Friday evening and wrapping up Monday.

12Z GFS Showing The Pattern For The Weekend
12Z GFS Showing The Pattern For The Weekend

I show the GFS 500mb vorticity (energy) and SLP map as it gives a general overview of what the pattern will be like from Friday evening through Monday morning. A front will be stalled overhead with several disturbances aloft and waves of low pressure at the surface promoting heavy rainfall. A final upper level disturbance and associated surface low will move east Monday bringing a final round of heavy rain. The exact location of this rain is still a bit up in the air. Some guidance suggests it will be right over us while some suggest we only see the far northern fringes of it. Those details should come into more clarity as that part of the event draws closer.

12Z GFS Showing The Moisture Plume Being Forced Offshore Monday
12Z GFS Showing The Moisture Plume Being Forced Offshore Monday

The front will continue to provide the focus for showers Sunday before the whole Western trough swings through with one final round of heavy rain Sunday night into Monday morning. This round is the most uncertain as there are some indications that it will primarily impact Southern New England leaving us dry. After this round passes out to sea, either via us or SNE, drier air looks to move in for the beginning of next week.

12Z GFS With This Week's Overview
12Z GFS With This Week’s Overview

The trend for this week will be a stifling start tomorrow and Friday before clouds and showers bring gradual cooling through the beginning of next week. Rounds of showers and storms could be heavy at times with flash flooding possible. Each round of rain will remove some moisture from the atmosphere so that by the beginning of next week we are left with a fairly dry and comfortable airmass. The pattern supports more rain by the middle or latter part of next week but there are no significant signals for a specific event yet.

I will have another update tomorrow morning. As a reminder, I will be gone from Friday afternoon through Sunday afternoon. I will post Friday morning but not again until Monday morning (there is the low potential I will return in time for a post Sunday evening but alas that depends on the weather). During this time, please revert to your primary source for weather information, such as the NWS or local media.

-Jack

 

 

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