Mesoscale Precipitation Discussion 0545
NWS Weather Prediction Center College Park MD
329 PM EDT Fri Jul 27 2018

Areas affected...Eastern PA...NJ...DE...MD...DC...Northern
VA...Ext Eastern WV...Ext Southeast NY/NYC...

Concerning...Heavy rainfall...Flash flooding possible

Valid 271930Z - 280030Z

SUMMARY...Increasing moisture, convective converge into saturated
ground conditions leads to likely flash flooding this evening.

DISCUSSION...19z Surface analysis depicts a coastal stationary
front delineating mid-60s Tds to mid-70s Tds from FVX to DCA
through the Delaware Valley...this q-axis is demarcated by 1.75"
total PWATs currently.  A cold front is analyzed further west
crossing Western PA into W WV at this time, which matches up with
mid-level height falls with a shortwave/ entrance to the 80kt 3H
jet extending into the Northeast.  CIRA Layered PW shows a narrow
ribbon of increased mid-level moisture pooling in the vicinity
axis with total values near 1.5". Both the forcing/convergence and
the moisture will march eastward and continue to provide strong
ascent for a increasing coverage line of thunderstorms from
Central PA to S central WV.  As the mid-level moisture layer
advances...it will combine with the deeper low level tropical
moisture layer across the I-81 to I-95 corridors and further
enhance rainfall efficiency through a greater depth.  Mean cloud
bearing flow will support cells moving north-northeast in the line
but eastward propagation should limit, but not completely
eliminate repeat track of cores capable of 1.5"/hr increasing
toward 2"/hr as the moisture layers overlap, and total PWAT exceed
2". 

To exacerbate flooding concerns, the area delineated remains very
wet with the vast majority of areas where FFG values remain below
1"/hr, which even in the weakest cells could be exceeded. 
Additionally, there are some warm sector cells that have formed in
advance of the main forcing line across North Central VA and along
rivers/arms of the bay where sfc convergence is maximized.  Given
the deep moisture and slower mean cell motions prior to the main
forcing, some localized spots may exceed 2" prior to the main line
of 1-2" totals. 

Gallina

ATTN...WFO...AKQ...BGM...CTP...LWX...OKX...PBZ...PHI...RLX...RNK...

ATTN...RFC...MARFC...NERFC...OHRFC...SERFC...

LAT...LON   41857703 41477452 40747356 39757525 37537628
            37997810 37367981 37888060 39537865