Quantitative Precipitation Forecast
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810
FOUS30 KWBC 261500
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1059 AM EDT Thu Apr 26 2018

VALID 15Z Thu Apr 26 2018 - 12Z Fri Apr 27 2018

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 S LYH 25 SW CHO KW66 10 NNE CGS FME ADW 15 SE NYG 20 NW PTB
15 N AVC 15 SE TDF 20 W 45J 10 SSW EQY 10 SW EXX 15 S LYH.


For the 1500 utc update...the northern portion of the marginal
risk area was extended approximately 75 nm to the northeast across
north central Virginia and into the urban corridor from DC to
Baltimore.  This was to cover the northward extent of the heavy
rains expected to push northeastward tonight into early Friday
across these areas.  The latest arw and nam conest do bring heavy
rains into the urban corridor from DC to Baltimore after 0900 utc
with hourly precip  rates of .50-.75"+ possible.  This may result
in isolated runoff issues, especially across the urbanized regions.

Oravec

...Central NC-VA...

Another compact mid-upper low will pivot across the lower MS and
TN Valleys today, then northeast into the mid Atlantic region
tonight. This feature, along with a more open shortwave in its
wake approaching the ARKLATEX Thu night, will reinforce the
longwave trough/height falls across the Gulf coast region Thu
night into Fri. Despite the largely curved upper flow and thus
favorable Qs forcing ahead of the initial deep-layer circulation
(upper difluence, dpva), the dynamical support will remain
transient given the continued steady progression of the shortwave.
This along with the modestly anomalous moisture parameters
(including PW and 850-700 mb) and minimal deep-layer instability
will restrict the QPF potential to some degree. However, 850 mb
moist southerly inflow increases to 30-35 kts Thu night/early Fri
morning as the compact circulation lifts across the Piedmont and
areas east across central NC-VA. The uptick in the low level
inflow, along with aligning parallel to the mean 850-700 mb flow,
will allow for an enhanced potential for repetitive linear
convective segments, as depicted per the high-res simulated
reflectivity. This scenario is also manifested in the multi-CAM
consensus in terms of the deterministic QPF -- with an axis of
1.5-2.0" noted over a 3-6 hour period. Instability is rather
meager (500 j/kg tops), however given the antecedent moist
soils/current FFGs, could be enough to allow for isolated areas of
excessive short-term runoff. As a result, a MARGINAL risk has been
noted over this area in the day 1 Excessive Rainfall Outlook
(ERO). The MARGINAL risk area also encompasses the highest (50+
percent) probabilities of 1 hour QPF exceeding 1 inch per the
latest (00Z) HREF overnight Thu-early Fri morning.

Hurley
$$





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