Quantitative Precipitation Forecast
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FXUS04 KWBC 160653

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Fri Mar 16 2018

Prelim Day 1 QPF Discussion
Valid Mar 16/1200 UTC thru Mar 17/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1


A compact, fairly vigorous low will drift east across the Central
Plains, bringing rain, ice, and snow. The system will begin to
weaken by Friday night as it has limited real estate with the
large, stable trough still occupying the eastern United States.
Within the eastern trough over New England and parts of OH/PA/WV,
continued northwesterly flow and a sufficiently moist boundary
layer / steep lapse rates will yield additional rain/snow showers
to little effect in terms of areal average liquid equivalent.
Meanwhile out West a 540 decameter closed low - not particularly
anomalous - will lumber its way inland through California, Oregon,
and the western Great Basin.

...Western U.S...

Sub-tropical moisture is suppressed, but the trough along the West
Coast will make the most of PW values peaking around a 0.50-0.60
inches, with strong 700 mb flow directed orthoginally into the
Sierras. At lower levels, 850 mb, the models forecast a slight
increase of onshore flow wind speeds into Southern California by
late afternoon and evening, along with moisture convergence / a
slight spike to PW values. This area will sit favorably along the
southern periphery of strong height falls associated with the
upper low, yielding a combination of increased westerly flow and
some deep layer ascent. Thus, we leaned toward heavier guidance
such as the 00z WRF-ARW over Southern California. Elsewhere
throughout the West, cold air aloft and the broad scale cyclonic
flow will support a showery regime over much of Oregon, and some
semblance of a renewed frontal zone / secondary Pacific front
behind the one which entered the Great Plains - will help to focus
precipitation in Nevada and Idaho. WPC QPF here was derived from a
blend of the 00z HREF Mean, WRF-ARW, our in-house ensemble
summarizing the larger scale guidance, and 20 percent continuity
from the previous forecast.

...Central / Eastern U.S...

Compact cyclogenesis will favor a swath of at least half  inch to
one inch areal average liquid equivalent, and locally greater
amounts over parts of NE/SD/IA, and especially focused in the
daytime hours. Models are relatively similar, but they do vary in
the orientation of the precipitation axis. Models appeared a
little bit split on whether to favor the 850-700 mb ascent which
fans out northward over time, or to favor the 925-850 mb
convergence that will occur farther south near the surface frontal
zone. The WPC forecast smooths out this choice a bit, but with a
seeming continued shift southward in the guidance over NE/IA, some
tweaks are possible in the final 0830z WPC forecast which could
reduce precipitation amounts in far southeast SD and northwest IA.
Will have to monitor observational trends. Otherwise, we attempted
to keep continuity while incorporating the 00z WRF-ARW and HREF.
This helped latch onto some of the trend over NE/IA, and we also
favored the GFS which was more generous with some of the shallow
convection that is likely to take place under the compact, stacked
system later today in KS/NE/MO.

From the Plains system heading northwest, vorticity pinches off
and a definable shortwave is expected to lift through eastern
Montana, resulting in a local maximum of precipitation there.
Farther east, warm advection and upglide on the 295-300 K surfaces
will support rain arcing out through the lower Ohio Valley /
Tennessee Valley and lower Mississippi Valley. A consistent signal
has been for a more concentrated rainfall over parts of LA/MS,
supported by a southern stream jet streak and proximity to deeper
moisture seen on the evening soundings over south Texas. The SREF
6-hourly rainfall probabilities were useful, as was the previous
WPC forecast which seemed to have a good handle.



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