Quantitative Precipitation Forecast
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FOUS30 KWBC 170828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sat Feb 17 2018

VALID 12Z Sat Feb 17 2018 - 12Z Sun Feb 18 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 E GLW JKL 35 WNW I16 I16 10 ENE 6V3 10 ESE TRI 10 SE TYS
30 WNW CHA 30 NW MDQ 55 SW BNA 20 WNW BNA 25 E GLW.


...Lower-Mid MS Valley / TN-OH Valleys / Central-Southern
Appalachians / Mid Atlantic / Northeast...

a rather dynamic setup is in store for the day 1 period, which is
not reflected very well in the mass fields per the relatively flat
upper flow and weak surface low. Southern stream shortwave energy
emanating form the subtropical jet will quickly traverse the
southern plains early today, then move across the TN valley by
late afternoon and the mid Atlantic region this evening.
Meanwhile, at the same time a strengthening/amplifying northern
stream jet and associated robust vort lobe will move from the
central CONUS and across the Northeast. The northern stream wave
and associated 150+ kt 250 MB upper jet streak in the lee of the
trough will provide a broad (SW-NE) area of robust right-entrance
region forcing (upper divergence and low-mid level frontogenesis).
Meanwhile, the nearby presence of the elongated southern stream
and infusion of subtropical moisture along with the fast-moving
shortwave impulses will result in impressive (anomalous) moisture
parameters for mid February -- especially south of 40N --
including 850-700 MB moisture flux anomalies around 3-3.5 standard
deviations above normal per the SREF/GEFS. The swift progression
of both northern/southern waves along with the absence of elevated
instability (at most <200 j/kg) will mitigate the risk of
excessive rainfall across the TN valley-lower OH into the western
slopes of the southern and central Appalachians. However, given
the antecedent conditions/wet soils and thus still low FFGs, WPC
will maintain the marginal risk across this area from yesterday`s
day 2 ERO.

In terms of the QPF, WPC discarded the GFS north of the mid
Atlantic due to what looks like convective feedback (i.e.
noticeably lighter totals north of an axis of 0.50-1.0" amounts).
Otherwise, a non-GFS multi-model blend was utilized, which
compared favorably to the continuity (previous forecast).

Hurley
$$





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