Quantitative Precipitation Forecast
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FXUS04 KWBC 200955

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
554 AM EDT Fri Apr 20 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 20/1200 UTC thru Apr 23/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Central U.S...

A closed low over the Southwest will progress slowly, in mid April
fashion, producing mountain snows and increasing coverage of
rainfall / some convective / on the high Plains and Plains.
Certainly the precipitation up through Thursday evening had been
sparse, with the system having no connection to sub-tropical
moisture. It will entrain Gulf moisture and should produce an
expanding coverage of rain and snow on Friday. Still, moisture
return is confined to a narrow corridor and hampered somewhat by
extensive surface ridging extending from the Great Lakes to the
northern Gulf of Mexico. The greatest intensity / longevity of
precipitation should occur along the inverted trough axis north of
the low center (generally northern Colorado and adjacent WY/NE),
also in the zone of max height falls downstream of the low center
over western Kansas, and finally in the marginally unstable
environment developing near / north of an advancing warm front
into the Texas panhandle, Texas South Plains, and western/southern

Given this is a well defined, closed, medium wavelength system,
model differences are minimal, and a consensus approach to the QPF
should perform well. The inherited WPC forecast was already in
good shape, and we made minor adjustments, favoring the WRF-ARW2
and some of our in-house ensemble / consensus tools.

...Pacific Northwest...

A progressive, open trough will move onshore Friday evening and
overnight. Forcing will be strong, with 12-hour height falls of
150 meters, but also short-lived for this quick moving system.
Precipitable water values in the GFS just briefly spike above a
half inch. These factors will minimize areal average rainfall, but
given the time of year and intensity of the cold air aloft, one
could imagine some weak convective enhancement for a time this
evening over western Washington. Even with this occurring, showers
will be moving quickly, and any given location is unlikely to
exceed an inch of liquid for the event. WPC QPF is in line with
consensus, and changed very little since the previous model cycle.


Height rises in the Northeast should tend to suppress showery
activity relative to recent days, although there may be enough
residual low level moisture to shake out additional showers early
in the day over northern New England. Easterly onshore flow will
begin to promote similarly shallow rain showers along the east
coast of Florida, but based on the dry and stable look of the NAM
forecast soundings WPC took a conservative approach to coverage
and intensity.

Days 2/3...

...Central-Southern Plains to the Southeastern U.S....

While there are plenty of differences in the details, there
remains a good model signal for widespread moderate to heavy and
potentially excessive rains to develop and move east across the
region as a compact mid-upper level low tracks east-southeast from
the central Great Plains on Saturday to the lower Mississippi and
Tennessee valleys by Monday morning.

Moderate to locally heavy rains are expected to continue
developing Saturday morning across the Red River into Oklahoma as
strong southerly low level flow draws moisture from the western
Gulf of Mexico into a region of strong lift supported in part my
left-exit region upper jet dynamics.  Overall, models are in good
agreement showing precipitation translating southeast with the
better moisture advection and dynamics into the lower Mississippi
valley Saturday night into Sunday morning.  Models suggest a
period of training convection which will help bolster the
potential for heavy to excessive amounts from Northeast Texas
through southern Arkansas and northern Louisiana into the
ArkLaMiss region.  A slight risk for excessive was maintained
across this region for the Day 2 period (ending 12 UTC Sun).

From Sunday morning to early Monday, the heavy rain threat will
continue to shift further east.  Moisture anomalies are forecast
to further increase as southerly flow begins to tap deeper
moisture from the Caribbean and western Atlantic.  By late Sunday,
the GFS shows anomalies increasing to over 1.5 standard deviations
above normal across the Southeast ahead of the low.  This deep
moisture along with the strong lift supported by the low is
expected to produce widespread moderate to heavy rainfall
accumulations across portions of the Southeast and Tennessee
valley on Day 3.  Despite the antecedent dry conditions across
portions of the region, these amounts may prove problematic.
Therefore, a slight risk was introduced in the Day 3 excessive
(ending 12 UTC Monday) - focused mainly across portions of
northern Alabama and northern/central Georgia - where some of the
heaviest totals are forecast to occur and flash flood guidance
values are relatively lower.

Overall, WPC QPF followed a blend of the GFS, with recent runs of
the ECMWF and ensemble guidance.  Less weighting was given to the
NAM, particularly late in the period when it begins to show a
broader upper low and a significantly slower northern stream
shortwave over the northeastern U.S. - which helps to draw heavier
amounts further north along the southern Appalachians.


Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml


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