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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0058Z Apr 24, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Mon Apr 23 2018

VALID 01Z Tue Apr 24 2018 - 12Z Tue Apr 24 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 SE CHS 35 E HXD 15 ESE NBC 25 SSW OGB 15 SE TBR VDI
15 SW MLJ CCO 25 E GAD 35 ENE MDQ 20 NW MDQ 60 N MSL 25 SE PAH
35 ENE M30 25 N HOP 25 SW BWG 30 SSW GLW 30 ESE GLW EKQ
30 SE EKQ 15 N OQT 45 SW TYS 20 ENE DNN 25 S RHP 10 NE 1A5
25 NW AVL 10 N AVL 30 S TRI 20 NW GEV 10 SSW PSK 15 N BCB
15 NE ROA 15 SSW LYH 10 WNW TDF POB 15 WSW EYF 10 W ILM
25 SE ILM 40 ESE SUT 40 SSE SUT.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NNE AVL 25 WSW TNB 10 NNW GEV HLX 20 SE PSK 25 SE BCB MTV
10 WSW MWK 15 SE TNB FQD 20 SE GSP 15 W GMU 25 ENE GVL
30 NNE GVL 20 W AVL AVL 25 NNE AVL.


0100z Update:

Significant changes were made to the remaining day 1 ERO period --
most notably the removal of the SLIGHT risk area along the SC
coast. Continued robust upper level difluence ahead of the
slow-moving mid-upper low continues to support widespread rainfall
within the warm conveyor belt (wcb), as the southerly low-level
ageostrophic maintains the vigorous moisture transport off the
Gulf Stream. The favorable thermodynamic response is manifested
via the axis of ~1.75" PWs nudging toward the SC coast; however,
deep-layer instability in this rain-cooled column north of the
surface warm front has been lacking all day, and continues to do
so this evening. The latest high-res CAM guidance indicate
continued tempered short-term rainfall rates near the coast
overnight, which given the current state of the soil moisture
(FFGs around 2-2.5" in 1 hour/2.5-3" in 3 hours), would pose at
most a marginal or isolated risk for excessive rainfall as
supported by the latest HREF exceedance probabilities.

Elsewhere, maintained the MARGINAL risk over central TN given the
continued convection (albeit weakening this evening) toward the
occlusion/comma head. Much of this area had already received
between 1-3" today over the past 24 hours, which prompted a fairly
sizable drop in the FFG (in some areas an inch or less within 3
hours). Thus despite the decreased convective contribution as the
activity becomes more stratiform, isolated short term runoff
issues will remain plausible though at least midnight.

Hurley


1530z Update:

Not much change to the risk areas with this update. Can see a well
defined spin on radar in southwest SC associated with a shortwave
feature. This is the feature responsible for the heavy QPF signal
in some of the high res models
over coastal SC. Thus far rain rates in reality have not been as
impressive as forecast by some of the high res models, with weak
instability likely a limiting factor. So think amounts shown in
some HRRR runs and a few HREF members may be too high, but still
could see some locally heavy rates along ahead of this shortwave
into the afternoon hours. Should see some development from eastern
GA into SC this afternoon as instability increases, although a
displacement between this developing instability and the better
low level moisture transport should help limit convective
organization later today. Will maintain the Slight risk over
coastal SC, although trends would appear to suggest that this
threat could be coming more marginal with time.

The favored upslope regions in western SC/NC will experience an
extended period of strong upslope southeasterly low level flow
beneath difluent upper flow. Rainfall rates will be lower here
given little to no instability, however the persistent strong
southeasterly flow into the terrain supports some 0.5" in an hour
amounts in the more favored terrain, which given the duration,
could result in localized 5"+ totals. Totals of this magnitude
should result in some flooding concerns developing with time. The
Slight was trimmed some, as the main threat should primarily be
focused into and just east of the terrain.

Went ahead and added a Marginal risk for portions of the OH/TN
Valley near the closed mid/upper level low. Should see low topped
convection near/underneath the low given steep lapse rates in
place. Not the best moisture in place, however cells will likely
be slow moving underneath the low, and storm motions are conducive
for some repeat south to north cell activity east of the low.
Localized 1-3" amounts are possible in this Marginal risk area,
and with FFGs lowered over portions of the area due to recent
rainfall, some localized flooding concerns could arise.

Chenard