Overcast and Unsettled Through the Weekend

A upper level 500 mb cut off low will contribute to gray skies, occasional drizzle and brisk northeast winds for Wednesday and Thursday.  A steadier rain is likely overnight Thursday into Friday.  Steady showers look likely all weekend.  Thanks to low level moisture max temperatures will be stuck in the low 60's.

Short Term Forecast
Currently it is overcast with temperatures in the 50's.  Expect some periods of drizzle or light showers today with temperatures struggling to hit 60 north of the MA Pike.
NWS Max temperature forecast Wednesday (image Weatherbell)
A front is stalled offshore which is allowing for onshore flow out of the Northeast.  This means the greatest chance of showers is closer to the coast but a few could work inland.
WPC Surface Forecast Wednesday 2 PM
Overnight lows will be around 50 thanks to cloud cover and onshore flow keeping moisture around.  Lows will be in the mid 50's to near 60 along the coast.  Now for Thursday the weather is nearly identical to today although a hair warmer.  When I talk about a closed 500 mb low I am talking about ~18k ft in the atmosphere.  It looks like this on an upper air chart.
06z NAM 500 mb vort Thursday 2 PM (image NOAA/NCEP)
The reason the front is stalled is thanks to a stagnant atmosphere in the Atlantic.  Remember the atmosphere is a fluid so it something is preventing the "waves" from moving through the fluid anomalies will occur.  The cut off low is one of them.  The upper low drops out of the upper level steering pattern.  For example here is the jet stream at the same time at 250 mb (around flight level or ~34k ft).  The steering winds are up in Canada in blue.
06z NAM 500 mb 250 mb heights, winds Thursday 2 PM (image NOAA/NCEP)
Rain begins to develop Thursday night.

Weekend Outlook
By Friday AM low pressure off the Carolina coast begins to move north.  At the same time the front will still be stalled offshore.  High pressure will also build north of New England in Quebec.
WPC Surface Forecast Friday 8 AM
The 500 mb chart at the same time shows the big ridge in the Atlantic and the upper low in the Ohio Valley.  The UL will transfer some energy to the surface low at the coast.  This is similar to the evolution of a winter storm.  If this high was in place during winter snow lovers would be dancing in the streets.  That high will allow for moisture to lift over the frontal thanks to a clash of air masses.  That will add beneficial rain to Southern New England even when the low is hundreds of miles to our south.
06z NAM 500 mb vort Friday 8 AM (image NOAA/NCEP)
Look where this system is able to pull moisture- the Gulf of Mexico and Caribbean.  Bursts of heavy showers embedded with lighter showers is likely beginning Thursday night and continuing into Friday.  Light to moderate showers continue through Sunday.  The system looks like it finally kicks out next Sunday night into Monday.  Here is the total rainfall through Monday 8 AM.
WPC 120 hour precipitation forecast through Monday 8 AM
After that its time to watch this tropical disturbance in the Atlantic.  It is moving into a spot favorable for a north turn in October.
NHC 5 day tropical cyclone formation area's
Most models turn it north and most make it a Hurricane but its a good 8-12 days away so just be aware something is possible at the end of next week.

-Zack Green


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