Hot, Humid Weather for Next Several Days

The calender says mid August but it is still summer time.  The next week looks to be hot and humid with a few chances for thunderstorms.  Despite a soaking rain on Tuesday dry conditions have expanded again and these thunderstorm chances will not put a big dent into the moisture deficit.  The Atlantic basin remains quiet thanks it large part to El Nino, but also relatively cool sea surface temperatures in the tropical Atlantic and a drought in the Caribbean.  First up the upcoming heat and humidity.

General Overview/Short Term Forecast
A weak cold front will push through New England today but in general high pressure will be in charge.
8 am surface analysis (WPC)
Temperatures today should climb into the upper 80s/near 90.  Dew points will climb into the 60s in Southern New England.  The biggest threat for thunderstorms will start in Southern VT, NH, ME around 1-3 PM.  The threat shifts to Western MA around 4 PM and further east around 5-7 PM.  The steering in the upper atmosphere is weak so the biggest threat will heavy rain in a short period of time.  This may sound good because we are dry but this type of rain doesn't absorb into the soil as well as a steady soaking day long rain.
14z HRRR simulated radar 7 PM (image weatherbell)
Notice the dip in Northern Maine and the X in PA at 8 PM tonight .  This is the upper level support driving this system
12z NAM 500 mb vort 8 PM (image NCEP)
The shortwave will push offshore tonight and a ridge builds in for Sunday.  The tail of the front may trail near the coast tomorrow therefore we may have an isolated shower or storm tomorrow.  In general everyone stays dry however.  Here is the upper level chart
12z NAM 500 mb vort 2 PM Sunday (image NCEP)
With the trough not moving completely offshore I am expecting temperatures in the upper 80s tomorrow. It will be cooler at the Cape.  This is setting up a very warm Monday as the trough should push completely offshore by tomorrow evening.  Southwest flow will set enable a warmer, more humid air mass to invade next week.
WPC Surface forecast Sunday 8 PM
Monday highs should stay around 90, to perhaps as high as 94-95 in some spots like the Merrimack Valley, or Connecticut River Valley.  There is a low probability some could see isolated showers/thunderstorms in the PM.
Mid Range Outlook
A cold front will be approaching from the Midwest by Tuesday AM.  This will help to increase chances for showers and thunderstorms Tuesday into Wednesday though not a washout.  The front looks to slow down as it hits the Great Lakes.
WPC Tuesday 8 am surface forecast
A developing ridge in Southern Canada and the Northwest Atlantic will prevent the atmosphere from moving anything too quickly through.  It is not a very progressive pattern.  Picture it like the Mass Pike from Boston out to Framingham.  After the Weston Tolls 128/95 merges onto the Pike.  Then there is a lane drop another 1-2 miles up.  All the volume has to merge from 4 to 3 lanes and it just slows everything down.  That's the type of set up we are looking at.  There is good confidence in the block according to the relative measures of predictability (RMOP).
NCEP Ensemble RMOP Wednesday PM
The oranges in the middle of the US and in Atlantic Canada are showing high confidence in both the ridge being present in the Western Atlantic and the trough driving a cold front through the center of the country.  I would expect this ridge offshore to strengthen as we approach next week and into week 2.  That means any cold fronts will likely weaken and stall as they head towards the east coast.  That doesn't mean we see no rain but organized rain/storms will have a hard time getting towards the coast.  Northern and Western New England will have higher chances of rain.  Here is the drought monitor
CPC US Drought Monitor 8-11-15
The finer details can be sorted in the coming days.  If needed I will post a thunderstorm update for today at the top of the page later on.  See Part 2 for more on El Nino and the longer range tropical outlook.

-Zack Green


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