Quantitative Precipitation Forecast
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FXUS04 KWBC 250756

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
356 AM EDT Fri May 25 2018

Prelim Day 1 QPF Discussion
Valid May 25/1200 UTC thru May 26/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

The flow pattern is not too exciting except for a closed low
entering the west coast and a broad area of anomalously low
heights in the Gulf of Mexico / northern Caribbean where the
National Hurricane Center places high probabilities of subtropical
or tropical depression development in the near future. A shortwave
digging into the back side of the Gulf trough early this morning
will help induce inverted troughing and enhanced southerly flow of
very moist air downstream over south Florida today. Elsewhere
northern stream waves will promote a few mesoscale thunderstorm
clusters over the upper Midwest / Great Lakes and New England.
Diurnal convection should again become fairly widespread in
coverage over the southern tier, and the low along the west coast
will yield organized rainfall, especially northern California and
southern to eastern Oregon. WPC QPF was derived from a near 50/50
blend of our in-house ensemble and the HREF blended mean. Six-hour
QPF probabilities from the GEFS were also helpful.

...Western U.S...
Models are very generous with rainfall in the difluent region
north to northeast of the closed low center. Although the low is
forecast to open up late in the Day 1 period there should be a
lengthy duration of favorable ascent profiles and southeasterly
low level inflow from northern California into Oregon, and
catching part of northwest Nevada. The event may begin with some
convective enhancement and then transition to longer duration
stratiform rainfall behind the surface boundary. An environment
more supportive of deep convection and/or more repetitive
convection should exist farther east, from southeast Oregon into
parts of northern Nevada and much of Idaho. Instability drops off
with eastward extent through NV/UT given drier air in that region,
but thunderstorms should be able to make some eastward progress
before drying out. WPC favored the NAM CONUS NEST for its more
expansive coverage, but perhaps the HREF mean for more toned down
amounts. Still, areal average rainfall greater than an inch is
forecast in parts of CA/OR. The odds of training cells appears
lower today, but hourly rain rates will likely exceed a half an
inch locally during the stronger individual thunderstorms.

With a shortwave trough dropping in to strengthen the broad upper
trough over this region, the models develop an inverted trough
axis over south Florida along with upwind convergence in the
strengthening and very moist southeasterly flow coming off the
Caribbean. There is an increased model signal for widespread 1 to
2 inch areal average rainfall, and the potential for locally more
intense amounts as the pattern remains locked in, allowing
repeated tropical convection into the Keys and parts of south

...Central / Eastern U.S...
The small vort max that has aided in rainfall production over the
southeast will lift a bit and begin to shear toward the Carolinas.
On the broader scale, the response to progression in the northern
stream will start to draw higher octane air back into the eastern
Tennessee and upper Ohio Valleys. The coverage of measurable
precipitation, therefore, is expected to increase or at least
expand toward the north compared to recent days. The pattern
overall still lacks a lot of focus, however, with very weak winds
in much of the column over the eastern and southern states. Some
stronger shear profiles and forcing could lead to more
organization over the upper Midwest, and early day warm advection
followed by strong heating will again allow for a few clusters of
pseudo-organized convection in the lower Missouri Valley and
southern Plains. WPC favored many of the details in the NAM CONUS
Nest, although the lack of synoptic control on the forecast does
give us less confidence, and for QPF we mainly relied on a blend
of low-res and high-res ensemble output, using our in-house
ensemble and the HREF blended mean. The result was areal average
rainfall of a quarter inch or more, but not exceeding one inch,
over broad areas from the upper peninsula of Michigan to Oklahoma
and over across the southern states.



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