Tuesday, September 29, 2015

Sheets and Sheets of Rain

At times the weather is boring and I have to put it on the back burner for a little while.  The pattern over the past several months has been so dull I've turned off the stove.  Given how the year started this isn't a terrible thing.  Mother nature also has a funny way of snapping back to life and that is what we will see over the next week.  The biggest threat is flooding rains although we will have to watch Tropical Storm Joaquin carefully.  He may act up by the end of the weekend.

General Overview
August and September have been extremely dry.  April and May were also dry.  Some parts of New England are experiencing drought conditions while most others are abnormally dry.
US drought monitor as of 9/22/2015
The soil is holding less moisture than normal.  From the US Drought Portal webpage
US VIC Soil Moisture % wrt 1916-2004 average
There is an old saying that a drought ends in a flood.  Heavy rain is expected to move in overnight and the National Weather Service in Taunton has issued a flood watch for much of Southern New England.
NWS Boston watches and warnings
So what happens meteorologically speaking to create such a set up?

The Set Up
A slow moving but strong early fall cold front is pushing across the Great Lakes.  High pressure behind the system is ushering in below average cold into the Northern Plains.
WPC surface analysis 5 am
2 m temps 8 am (image weatherbell)
So our first ingredient is in place, a good temperature contrast. At the same time a strong blocking ridge is present in the western Atlantic
06z NAM 500 mb vort valid 8 PM Tues (image NCEP)
A ridge also exists in the Southwest with a developing trough in the Southeast.  So in addition to the temperature contrast we have effective moisture transport to the north.  I'm sure most of you can already feel the humidity.   As the front approaches rain will break out tonight.  The morning commute will be a mess.  By 10 AM the available moisture is over 2" for most of the East Coast
06z NAM Hires precipitable water 11 am Wednesday (image weatherbell)
In english that translates to a radar picture like this
06z NAM hires simulated radar 11 am Wednesday (image Weatherbell)
Other threats tomorrow include some coastal flooding impacts at the coast.  Minor to moderate coastal flooding is possible.  Some wind damage is possible but this is not a big hazard.  However some wind gusts will approach 40-50 mph in the coastal plain.  All told 2-4" of rain are likely in Southern New England with higher amounts in Northern New England. 
WPC 1-2 day precipitation forecast
Left over rain is certainly possible Thursday into Friday as waves of low pressure move along the front.  The front wants to stall offshore.
WPC surface forecast Thursday 8 am
This is when Tropical Storm Joaquin will enter the picture.  Guidance is all over the place but we do expect him off the SE coast late this week before beginning a trek north.  Until we are sure what happens with Joaquin its premature to speculate on potential impacts.  At the very least he will enhance the potential for rainfall along the Mid Atlantic/New England coasts.
NHC official Joaquin track
WPC 7 day precipitation forecast
The 7 day precipitation forecast from the WPC calls for 6-9" of rain through next Tuesday. Locally higher amounts are possible.  I am hoping for some dry weather on Saturday.  I'll continue to monitor this over the next several days.

-Zack Green

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