WHY THE HEAVY RAINS OVER SE VA ?
STAR DATE 201609.21 1500 EDT CLICK ON IMAGES TO SEE FULL SIZE
We begin by pointing out that the of the September 15 edition of the NEXT 3 WEEKENDS newsletter it correctly forecasted that for the beginning of THIS week --specifically September 20 - 21 -- was going to see widespread significant and heavy rain across much of southeastern Virginia and into northeast and North Carolina. For those of you that don't get the NEXT 3 WEEKENDS newsletter ...spending the $5/ a month is really one of the best bargains you can get out there with regard to whether information for the Middle Atlantic region..
This surface map shows the overall conditions at the surface and help show WHY the rain has been so persistent and so heavy over Hampton Roads and into portions of north central and northeast North Carolina over the past two days. Note the LOW pressure area along the Carolina Coast -- which is actually the remains of tropical storm Julia. To the north the area of HIGH pressure is located over New England. That HIGH is providing a northeast / east wind but it is blocking the remains of tropical storm Julia--- now just that LOW pressure area along the Carolina Coast--from moving either "up the coast" or out to sea. The result is a persistent EAST wind that is enhanced by the interaction of the LOW on the Carolina Coast and the big HIGH over New England.
One of the things meteorologist look for when it comes to significant precipitation events on the East Coast is a strong easterly wind at the mid levels of the atmosphere. In this analysis we look at the winds at the 850mb level which is 1 mile above the ground.
This first image shows what the mid level winds at 850mb as of Tuesday afternoon. Notice the dark green and yellow areas over Southeast Virginia. These represent winds of 30 to 40 knots (35 -45mph). All these winds are blowing from the East to West -- that is to say transporting Atlantic moisture into Hampton roads which is feeding and enhancing the rainfall rates.
As long as the East winds are strong and located over Southeast Virginia ...the rain will continue. It is really just that simple. And of course the strong East winds at 850mb will stay in place because of the HIGH to the north and the LOW pressure area over the Carolinas. This next image shows the 850mb winds as of Tuesday night into early Wednesday morning. Again notice that the core of the strongest winds over Hampton roads so the rain continues.
This next image shows that the 850 mb winds are diminishing which means a break in the rain on Wednesday afternoon and evening as the strong as winds are now all located over Northwest North Carolina. This is a good sign telling us that the heavy rains are breaking down.
However as we can see the low level East winds rebuild early on Thursday morning for several hours which bring back more moderate rains into Hampton roads. Now the model data does not showed nearly as much rain as what we saw on Tuesday and / or today (Wednesday) but it clearly indicates that another round of rain is going to redevelop Thursday morning over Hampton roads.
Eventually the 850mb Low level winds will break down be cause the LOW to the south will begin to fall apart and the HIGH to the north will slide off the New England Coast. This will finally allow the rains to come to an end over southeastern Virginia and northeast North Carolina later on Thursday.
REMEMBER .. the East wind is always bad in he cold season monthsn