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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0104Z Apr 27, 2018)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
904 PM EDT Thu Apr 26 2018

VALID 01Z Fri Apr 27 2018 - 12Z Fri Apr 27 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 N EXX 30 NE DAN 25 SSW CHO 15 NE CHO 10 SSE CJR 20 NNE OFP
15 SW PTB 15 E HNZ 15 SW RDU 20 WNW SOP 10 E JQF 10 N EXX.


Still appears to be a Marginal flash flood risk into the overnight
hours from portions of central NC into central VA. As of 01z
Convection is beginning to increase in intensity over central NC
as low level convergence is increasing to go along with an uptick
in 850 mb moisture transport and the development of marginal
instability values. The expectation is that this activity will
move northeastward into the overnight hours...supported by this
enhanced convergence/moisture transport and a few hundred J/KG of
CAPE that should develop into south central VA over the next
several hours. Most of the 12z HREF members appear to be too far
west with this activity, with recent runs of the HRRR and
Experimental HRRR doing a better job. The Experimental HRRR is a
bit further east, and may be closer to reality...with areas
slightly further east having a better opportunity to see the
development of the marginal CAPE. Did nudge the Marginal risk a
bit east to account for this. Cut back a bit on the northward
extent of the risk area, as think rates will be lower here as the
showers/storms out run instability.

Both HRRRs show localized rainfall amounts upwards of 1.5" within
the Marginal risk area, and think this is reasonable. Convection
will be fast moving, and thus think heavier rates will generally
be short lived, preventing totals from getting much higher than
1.5". While amounts of this magnitude may approach and locally
exceed FFG, think the quick progression and limited instability
will prevent a more widespread/organized flash flood threat. Thus
will maintain the Marginal for now.

Chenard