Quantitative Precipitation Forecast
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678
FOUS30 KWBC 171004
QPFERD

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

VALID 15Z 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 as depicted from the
models by 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 to 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).

The coupled northern and southern stream forcing mechanisms, along
with the the infusion of subtropical moisture (especially south of
40N) will make for a favorable setup for areas of moderate-heavy
precipitation within a relatively short period of time (<12 hours)
despite the predominant stratiform structure. Moisture anomalies
from the lower-mid MS Valley, TN Valley, lower OH Valley, and into
the mid Atlantic are rather impressive (anomalous) for mid
February, especially the 850-700 MB layer moisture flux (3-3.5
standard deviations above normal) per the latest 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
Valley into the western slopes of the southern and central
Appalachians. However, given the antecedent conditions/wet soils
and remnant low FFGs, WPC will maintain the MARGINAL risk across
this area today from yesterday`s day 2 excessive rainfall outlook.

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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