Quantitative Precipitation Forecast
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FXUS04 KWBC 230954

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
553 AM EDT Mon Apr 23 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 23/1200 UTC thru Apr 26/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern / Southeastern U.S...

A well defined, slowly-moving closed low will bring a wet day to
parts of the southern and eastern United States. Models have done
well handling the spatial distribution of rainfall with this
system, but they have tended to over-forecast the magnitude. The
low itself, at 564 decameters at 500 mb, is barely 1.0 standard
deviation from climatology. Cloud cover hampering daytime heating,
and a mis-match between areas of strong forcing and areas of
larger CAPE, has also toned down the rainfall production. On
Monday there is seemingly a little greater chance for heavy and
excessive rainfall to occur, as the favored upslope regions in
western SC/NC will experience an extended period of strong
southeasterly low level flow beneath difluent upper flow. Models
also signal a heavy rainfall event along the Carolina coastline
where Gulf and Atlantic moisture streams will come into phase.

There are, however, still some uncertainties regarding instability
and event evolution. The NAM forecasts CAPE of greater than a few
hundred J/kg to develop not much farther north and west than Greer
and Charlotte. Sustained upslope will certainly boost rainfall
totals farther west, but the event in the mountains may be only
weakly convective. Meanwhile, at the coast, predicted cell motions
and model simulated reflectivity loops suggest that supercells
will move very slowly, and could contribute to locally intense
rainfall, but widespread forcing leading to cell mergers - and a
slow but steady progression of the synoptic system may limit
duration of the heaviest rainfall at any given location.

For QPF, WPC favored the HREF Mean and NAM CONUS Nest which were
more tempered than the aggressive WRF-ARW solutions. The National
Blend of Models also looked very good. Our forecast trended
upwards along the coast and in the mountains, and in between these
areas we depict a wet day but also a relative minimum of areal
average precipitation.

...Northern Rockies / Northern High Plains...

A relatively vigorous system with well defined mid level forcing
mechanisms had amplified over Idaho last Sunday / early Monday,
and will translate eastward today. The trough is forecast to
maintain a tight, closed mid level circulation along with a sense
of jet coupling and strong 700 mb frontogenesis, producing an
aggregate swath of forcing and associated rain/snow across
southern Montana and northern Wyoming - spilling into portions of
the Dakotas. Steep lapse rates hanging back in association with
the tail end of the trough will support some coverage of rain/snow
showers over areas farther west and south, and the arrival of deep
layer lift and a difluent upper level pattern onto the High Plains
should promote scattered convection from southeast Wyoming /
northeast Colorado into western Nebraska.

Model QPFs are clustered quite well. Therefore, WPC leaned toward
consensus and bias corrected tools such as our in-house ensemble
and the National Blend of Models. The previous forecast had a good
handle on this system, so overall change was minimal, but we did
increase amounts slightly along the swath of greatest deep layer
forcing affecting MT/WY/ND/SD.

Days 2/3...

...Mid-Atlantic and Ohio Valley to the Northeast/Great Lakes...

The mid-upper level low that is expected to enter and slow over
the Tennessee valley on Monday is forecast to gradually weaken and
begin ejecting out to the northeast ahead of a northern stream
trough amplifying over the Great Lakes early Wed.  Ongoing heavy
precipitation developing along the Southeast to southern
Mid-Atlantic region on Day 1, is expected to continue through Tue
morning, with 40-50 kt southeasterly low level inflow along with
strong upper forcing, supporting heavy precipitation across
eastern North Carolina into southeastern Virginia.  Axis of
organized precipitation is expected to shift further north into
the northern Mid-Atlantic and Northeast late Tue into early Wed.
However, the more progressive nature of the system, along with
lessening onshore moisture advection and weakening upper dynamics
is expected to hamper the threat for widespread heavy amounts.

Remnant energy associated with the weakening low is expected to
continue lifting along the Mid-Atlantic to Northeast coast Wed to
early Thu, phasing with the previously noted northern stream
trough moving into the Great Lakes.  Surface low pressure
developing along the Mid-Atlantic coast on Wed is forecast to
track into the Northeast by early Thu.  Moderate to locally heavy
rains are expected to push east ahead of the low, as light to
moderate precipitation develops further west in a deformation zone
over the Great Lakes.  At this point the models begin to diverge,
showing significant differences with respect to the interaction of
the phasing systems over the Great Lakes.  The 00 UTC guidance
generally falls into two camps, with the NCEP guidance more
progressive than the non-NCEP camp - which close off a new low
further west, in turn spreading precipitation further west into
the upper Great Lakes region late Wed into early Thu.  Despite the
clustering of the ECMWF, UKMET and Canadian, there was not a lot
of ensemble support for a solution quite as far west.  Therefore,
WPC QPF followed a compromise approach, but with a lean toward the
faster solutions.

...Great Plains...

A positively-tilted upper trough moving across the northern
Rockies Mon into early Tue is forecast to drop southeast across
the central Great Plains into the lower MS valley Tue to early
Thu.  Pooling moisture along the associated low to mid level
frontal zone, in combination with the favorable upper forcing, is
expected to support some light to moderate amounts dropping south
from the central into the southern plains on Tue, before spreading
east from the southern plains into the lower MS valley on Wed into
early Thu.  WPC QPF gave significant weight to the ECMWF through
the period.


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


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