Quantitative Precipitation Forecast
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000
FOUS30 KWBC 120132
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
831 PM EST Sun Feb 11 2018

VALID 01Z Mon Feb 12 2018 - 12Z Mon Feb 12 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ENE CYD 15 SE KMDJ 15 SE 9F2 10 SSE NBG 10 NNW MOB
15 ENE LSF 20 WNW EZM 15 NNW DQH 10 SE VLD 15 S 40J 65 SSE AAF
90 SSW AAF 90 ENE KIPN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SE OXB 25 W MFV 25 N FYJ 15 WSW 2W6 15 S NAK PHL LGA
15 SE BDR 10 SE HTO 40 SSE MTP 70 SSE HTO.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 SE TOI 30 N ABY 30 E ABY 20 SW MGR 30 WSW TLH 35 SW PAM
45 ENE KVOA 30 N KVOA JKA 25 SE TOI.


0100 UTC Update

Trimmed the back edge of the MARGINAL risk area over the eastern
Gulf region, based on the latest mosaic radar trends, while
refining the SLIGHT risk to match the lower FFGs and (thus) higher
probabilities of exceedance per the 18Z HREF. MUCAPE trends are
down this evening (and are expected to fall further to below 1000
j/kg overnight); however, continued favorable wind profiles
(deep-layer SW flow with the low-level magnitude nearing the mean
850-300 mb mag) would support training of convection given the
upwind propagation and slow W-E progression of the heavier linear
segments. Therefore while the hourly rainfall rates will become
somewhat mitigated overnight, 3-6 hourly totals will nevertheless
challenge FFGs in spots -- especially if the past few HRRR and
experimental HRRR models come close to verifying (23 and 00Z runs).

Hurley


1900 UTC Update

The previous excessive rainfall potential outlook was updated for
current radar trends.  The marginal and slight risk areas over the
mid Atlantic were pushed eastward approximately 30-60 nm to
account for the current area of training that has set up over the
mid Atlantic---east of the DC-Baltimore area---northeastward into
the coastal northern mid Atlantic.  In areas of training rainfall
rates of up to an inch possible.  See WPC mesoscale precipitation
discussion #0016 valid until approximately 2300 utc for additional
information across this area.

Over the central gulf coast---no major changes made to the
previous outlook areas.  See WPC`S mesoscale precipitation
discussion #0015 valid until approximately 2000 utc for additional
information across this region.

Oravec

1500 UTC Update

Changes to the previous excessive rainfall potential outlook were
to expand the slight risk slightly eastward over the mid Atlantic
to cover areas experiencing southwest to northeast training this
morning.  The slight risk area was also extended slightly westward
into far eastern WV and NW VA to cover the lowered ffg values
across this area.

Along the central gulf coast---training of cells in a southwest to
northeast direction continues off the gulf and into portions of
far southern MS--the FL panhandle---southeast AL and southwest GA.
 Hi res guidance is not depicting this activity well.  See WPC`S
mesoscale precipitation discussion #0015 valid until approximately
2000 utc for additional information across this region.

Oravec

Previous discussion


...Gulf Coast...

In the Florida panhandle and adjacent parts of Al/GA, even over
into a sliver of southeast MS, Florida there is some concern for
flooding and flash flooding. This started out as mainly a longer
duration concern given the very high flash flood guidance values
in this region. But antecedent rainfall of 1 to 2 inches and
expectations of additional showery weather with convectively
enhanced rain rates, leads to the possibility of flash flooding.
Additionally, a squall line had firmed up as of 08z along the
leading edge of a shortwave / ripple in the flow. This slow moving
squall line may produce truly intense short duration rates for
several hours this morning until deep layer ascent reorganizes
farther inland and away from the Gulf-based source of instability.
See MPD number 0014 for details.

The intensity of the event may peak near 12-15z or even prior,
during the best overlap of low level confluence and mid level
height falls. Beyond 12Z there are actually height rises as the
shortwave over Colorado rounds the base and the upper jet / indeed
the whole large scale positively tilted trough starts to finally
lift toward the east-northeast, which would take deep layer
forcing inland and away from the warm / unstable gulf air over the
gulf waters. Until things quiet down, there is a Slight Risk of
excessive rainfall for parts of this region.


...Mid-Atlantic States...

Extrapolating model performance in the early hours and tying this
to the global model consensus mass fields going forward, we
preferred QPF from the Canadian Gem Regional as far as placement,
with more emphasis on the WRF-ARW and WRF-ARW2 for the intensity
details as well as placement. We blended these sources with WPC
continuity, resulting in not too great an overall change. The
amounts beyond 12Z today actually did increase, however, in the
I-95 corridor from D.C. to New York. Earlier a low amplitude
shortwave evident in 00Z upper air data had suppressed heavier
rain farther south into Virginia and southern Maryland, but expect
this to redevelop back northward as the large scale pattern
continues to respond to the lowering heights in the base of the
trough back west, and continued enhancement of the strong upper
jet over the Great Lakes / Canada. Model QPF consensus certainly
moved back north a bit over the mid Atlantic and southern New
England this cycle. With good model agreement and a deterministic
WPC forecast of near 2 inches areal average rainfall, we upgraded
to a Slight Risk of excessive rainfall from central VA to
northeast NJ for much the same reason as the outlook issued
yesterday, including long duration saturation, very anomalous PW
values, and frontogenetical forcing capable of yielding some
enhanced short term rain rates within the urban corridor.

Burke
$$





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