Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1000 AM EDT Tue Apr 24 2018
VALID 15Z Tue Apr 24 2018 - 12Z Wed Apr 25 2018
MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 S RUQ MRN 25 NNW AVL 25 SW RHP 40 ENE CHA 35 ESE EKQ
15 N JKL 25 NNW I16 EKN 25 S CBE 15 SE MDT 10 ENE TTN VAY W29
20 ENE OFP RZZ 15 N HRJ 10 S RUQ.
...15 UTC Update...
No change to previous (below).
...Eastern TN / Northern NC / Mid-Atlantic...
The slow moving, stacked cyclone affecting the eastern United
States will bring a wet, showery day to areas from Ohio to eastern
Tennessee and toward North Carolina and the Mid-Atlantic states.
Rain will spread northward into New York and southern New England
Tuesday night. The greatest odds for intense rainfall will be
limited to offshore areas near a surface triple point exiting the
Carolina coast. Inland, a Marginal Risk of excessive rainfall is
justifiable over some part of the Carolinas and Mid-Atlantic
states, although any truly heavy rainfall will probably occur on
small scales, with no real stand-out signal on the large scale.
Our primary concern is with parts of North Carolina where
antecedent rain this morning will lower flash flood guidance
values before late afternoon convection ramps up within an axis of
MUCAPE greater than 1000 J/KG extending west to east across the
state. Upslope-enhanced rainfall over Virginia, also weakly
convective, may lead to a sufficient lowering of FFG values and an
eventual risk of flash flooding. Diurnal heating beneath the upper
low will likely contribute to small scale swaths of convective
rainfall to the other side of the mountains in eastern KY/TN. The
big cities / I-95 corridor is more of a question mark, being
located north of the surface low and occlusion. The literal FFG
numbers requiring 1.50 to 2.0 inches in 3 hours will have a low
chance of being exceeded, but there may be enough local convective
enhancement, especially Tuesday night as the upper low approaches,
that the combined effects of daytime and nighttime rainfall could
eventually lead to some isolated issues with high water -
particularly in prone areas.
Adding these mesoscale concerns together, we maintained a
relatively broad Marginal Risk area, but did remove it from the
Carolina coast given better hi-res model agreement in keeping the
deepest warm sector convection offshore.