Quantitative Precipitation Forecast
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FOUS30 KWBC 171439
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
938 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.


...15z update...

WPC kept the marginal threat across parts of TN into eastern KY,
as guidance shows the arrival of subtropical moisture reaching the
OH/TN valleys this afternoon and lasting till early evening.
Global and hi-res models show possible shear-axis or line, which
may produce locally heavy steady rainfall from extreme southern
WV/southwestern VA/southeast KY and a large portion of TN. This is
on the southern extent of lower flash flood guidance values from
the eastern OH valley. Areal average amounts should generally be
an inch or two but may be locally heavier and the eastern extent
of the marginal threat begins to encounter hills/mountains of the
central and southern Appalachians. By later this evening into the
overnight hours, this threat should diminish very quickly.

Musher


...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, though
the model reflection is rather subtle 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 track 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  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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