Quantitative Precipitation Forecast
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000
FOUS30 KWBC 151434
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
934 AM EST Thu Feb 15 2018

VALID 15Z Thu Feb 15 2018 - 12Z Fri Feb 16 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 ESE LEX FTK 10 SE CUL 15 ENE SLO 10 SE PRG 20 NW AOH CGF
20 ENE GKJ 30 ESE BFD 25 ESE IPT 15 NNW RDG CXY 10 WNW HGR
25 SSW W99 15 WNW LWB HTS 35 ESE LEX.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SSW KHLB 25 E GEZ MNN 25 E CAK 10 SSW FIG 15 SSE AOO
25 NW W99 48I 10 NW 3I2 40 SE LUK 30 SSW KHLB.


1500 UTC update

No changes made to the previous excessive rainfall potential
outlook.

Oravec

Previous discussion

...Ohio Valley eastward into the central and northern
Appalachians, the Laurel Highlands, and central Pennsylvania...

The broad mid-upper level WSW flow, with subtropical origins ahead
of the upper trough southwest of Baja, will allow for an
elongated, robust warm conveyor belt/moisture plume ("atmospheric
river" per the ARDT integrated water vapor transport anomalies)
into the central and eastern CONUS during the day 1 period prior
to the cold frontal passage. The 00Z guidance continues to denote
a heavy rainfall signal across the OH valley into the
central-northern Appalachians, Laurel Highlands, and into central
PA in light of the abnormally rich deep-layer moisture/theta-e
profile along with a vigorous upper jet streak (170 kts at 250 mb)
that will provide a relatively prolonged period of robust,
deep-layer ascent via the approaching left-exit region initially
and (by overnight Thursday into early Friday) associated more from
right-entrance region upper jet forcing and strengthening
low-level frontogenesis ahead of the front. PW anomalies during
the event will peak around 3.5 standard deviations above normal
per the SREF and GEFS, which along with the 850 MB west-southwest
inflow of 50-55+ kts and moisture flux anomalies between 4-5
standard deviations above normal, will make for nearly optimal
rainfall efficiency/short-term rates for mid February considering
the lack of instability (mucapes < 400 j/kg).

In terms of the QPF, the majority of the high-res CAM guidance
with recent events characterized by strong dynamical forcing,
impressive moisture profiles, and minimal instability (i.e.
Largely stratiform) were generally underdone in terms of the
breadth of moderate-heavy rainfall area when compared to the
global guidance. With this in mind, the WPCQPF was essentially a
blend of the 00Z ECMWF, GFS, and in-house bias corrected ensemble
mean, which matches up favorably with the RGEM as well as a blend
of the WRF-ARW and ARW2. The 00Z NAM meanwhile was a northern
outlier with respect to the max axis of 1.5+ inch totals.

Given the stratiform component, the high-res guidance does not
show much deviation above the mean QPF -- i.e. max totals are
expected to be in the 2.5-3.0" range. However, given the
antecedent soil conditions and relatively low 3 and 6 hourly FFGs,
the probabilities of exceeding 1.0-1.5" within 3 hours and
especially 1.5-2.0" within 6 hours across the SLIGHT risk area are
decidedly elevated per the latest HREF.

Hurley
$$




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