Quantitative Precipitation Forecast
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FXUS04 KWBC 240709

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
308 AM EDT Thu May 24 2018

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

Day 1

The pattern looks like May, but overall, considering the full
CONUS, it is quite a bit warmer than climatology and the mid level
flow is generally weaker than usual, at least where it is
precipitating. A shear axis and embedded circulation centered
around 700 mb will provide some level of organization to diurnal
thunderstorms over Georgia and South Carolina. A better defined
northern stream trough will bite into the ridge over the northern
Plains and upper Midwest today, producing thunderstorm clusters
from the Dakotas to Kansas and eastward...with more widely
scattered activity over the southern Plains and the rest of the
Southeast. In much of the central and southeast United States the
convection will be driven as much or more by the available
instability / strong May sun angle and plentiful surface moisture
as it will be driven by the mid level flow regime. Meanwhile an
upper low poking into California will provide lift enough to yield
organized thunderstorms from the Sierras to southern Oregon.

...Southern / Southeast U.S...
With slow cell movement but also a lack of deep layer shear this
afternoon, the greatest areal average rainfall should occur where
CAPE and low level inflow are maximized. This especially describes
central to eastern Georgia and western South Carolina, where low
level inflow will be slightly enhanced by the presence of the 700
mb circulation over central GA. Models also signal heavy rain
around the southern periphery of the low in somewhat a repeat of
what occurred Wednesday. Precipitable water values of 1.50 inches
and greater provided a pretty good trace of where organized
diurnal convection took place on Wednesday. For Thursday this
threshold led us to favor the WRF-ARW2 and the HREF blended mean.
We combined these sources with 30 to 40 percent of continuity to
derive the updated forecast. The ECMWF ensemble QPF probabilities
were also very useful at separating some of the mesoscale areas of
precipitation on the 6-hour time scale.

...Plains / Upper Midwest...
This is a pretty standard issue Great Plains convective forecast.
Expect a couple areas of concentrated upglide early in the day,
mainly Minnesota and near Omaha. By afternoon clusters of
organized storms will form in a near-surface convergence axis /
pre-frontal - from eastern Colorado to parts of the Dakotas and
Minnesota. Predicting each mesoscale cluster of rainfall will be a
low confidence endeavor, but we did our best at combining the
available model signal with known tendencies. Our QPF favored the
NAM CONUS Nest and HREF Blended Mean, while using our in-house
ensemble as a more broad scale base. The result gave us areal
average quarter inch or greater amounts over a large area, but
relatively little 1.00 inch areal average based on uncertainty as
to where to place the heavier amounts. There is no clear focus,
however, for a heavy rain event. Flash flood potential will depend
more on storm scale outflows and interactions not very well
knowable with much lead time. Certainly parts of eastern Nebraska
and also parts of Iowa and southern Minnesota are a little more
susceptible right now, per Flash Flood Guidance values.

Dewpoints remained in the upper 40s and low 50s from northern
California into the northern Great Basin during peak heating
Wednesday. Expect similar values today, and the moisture through
the column - PW - is forecast at around 2.0 standard deviations
above climatology. When combined with the development of an upper
low just off California and difluent upper flow extending inland,
we attempted to lean heavier in terms of convective coverage. We
favored the NAM CONUS Nest and HREF blended mean, while also
preserving some percentage of continuity. The degree of lift could
support cells regenerating quickly along outflow boundaries so as
to lead to some training and flash flood concerns in the mountains
terrain of northern California and adjacent NV/OR.

Warm advection becomes focused ahead of a fast-moving, low
amplitude shortwave trough this evening. Model QFPs are in good
agreement, so we kept with a consensus, with the forecast bringing
a tenth to quarter inch liquid into northern Maine. The heavier
rain may stay just outside the state to the north and east.



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