Quantitative Precipitation Forecast
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FXUS04 KWBC 212101

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
501 PM EDT Sat Apr 21 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 22/0000 UTC thru Apr 25/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern portions of the Southern Plains---Lower MS
Valley---Lower TN Valley---central Gulf Coast...

There is good model agreement on the overall heavy rainfall
potential day 1 across eastern portions of the Southern
Plains---Lower MS Valley...Lower TN Valley into the central Gulf
coastal region ahead of the strong closed low moving through the
Southern Plains into the Lower MS Valley.   An axis of above
average pw values...1 to 1.5+ standard deviation above the mean
expected to move along and ahead of this closed low.  This and
favorable upper difluence---strong pre-frontal to frontal
convergence will support organized heavy rains pushing eastward
across these regions----with relatively high confidence overall on
this heavy rain potential---albeit with lower confidence in some
of the shorter term details.  No significant changes made to the
excessive rainfall potential outlook for the day 1 time period
through 1200 utc Sunday April 22.  With dry antecedent conditions
and the expected overall progressive nature of the heavy rain
areas---the threat of runoff issues will remain on the low side.
A slight risk was maintained over the ARKLATEX region where model
consensus is for the greatest hourly rainfall rates---1"+---during
the 0000 to 0900 utc Sun time frame.


There is a multi-hi res model signal for locally very heavy
rainfall amounts across east central Florida tonight into early
Sunday.  The frontal boundary lying west to east across central FL
this afternoon is expected to move little during the upcoming day
1 period.  Low level southeasterly to southerly flow into this
boundary----along with pw values rising to 1.5 to 2 standard
deviations above the mean---will set the stage for locally very
heavy amounts.  Still a lot of uncertainties with some of the
details in the hi res guidance---especially some of the very high
totals such as the 10"+ in the arw...nssl wrf and nam conest and
the 14" amounts in the hrrr experimental.  While these models all
have very heavy totals---the placement differs from between 60 and
80 nm. This and the very small scale of the maxima is leading to
the low confidence.  Still---with the multi-model signal for
potentially  heavy rain amounts---a marginal risk area was
introduced in the update to the day 1 excessive rainfall potential
outlook over the east central coast of FL for localized runoff
issues especially over urbanized areas.

Days 2/3...

...Tennessee Valley and Southeast to the Ohio Valley and
Mid-Atlantic States...

Widespread moderate, to locally heavy and potentially excessive
rainfall, is expected with a dynamic upper low as it drifts
eastward through the Tennessee Valley Sunday night and then slowly
lifts northeastward through the Ohio Valley and Mid-Atlantic
states Monday and Tuesday.  The Day 2 Slight Risk of Excessive
Rainfall was maintained over portions of the Southeast and
Tennessee Valley, where models continued to show the potential for
strong moist southerly low level inflow ahead of the low to
combine with strong large-scale ascent and produce heavy,
excessive rainfall.  Also, models continued to show the potential
for rainfall to exceed relatively high flash flood guidance values
as the dynamic upper low slows while lifting through the
Mid-Atlantic states Monday into Tuesday, so the Day 3 Marginal
Risk was upgraded to a Slight Risk from coastal regions of South
Carolina to the terrain of northern Georgia and western North

...Northern Rockies to the northern/central Great Plains...

Widespread light to moderate precipitation is expected with a
well-defined shortwave trough progressing from the northern
Rockies to the northern/central Great Plains early next week, with
some locally heavy amounts expected along the favored terrain of
Montana and Wyoming.  Models continued to show quite a bit of
spread with the timing and amplitude of the shortwave and
resulting axis of precipitation across the region (especially
across the Plains on Day 3), and the WPC QPF stayed close to
continuity considering the uncertainty.  For more details on model
spread, please refer to WPC`s model diagnostic discussion and
please refer to WPC`s Winter Weather suite for details concerning
the heavy snow potential across the region.


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


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