Quantitative Precipitation Forecast
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FXUS04 KWBC 210858

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

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 21/1200 UTC thru Apr 24/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Southern Plains into the Southeast...

A closed and not quite stacked cyclone will make its way slowly
and steadily out of the Rockies, through the southern Plains into
the lower Mississippi Valley. Accumulating snow will come to an
end this morning as the system pulls away from the high elevations
in Colorado. Attention turns entirely to the rainfall pattern. At
least some of the D3 to D4 drought areas in the southern High
Plains received some rainfall to finally kick off the spring
season. The parent weather system, itself, carries seasonably
strong wind fields, but is marked by 500 mb height anomalies only
one standard deviation below climo, and instability is somewhat
difficult to find given persistent dry ridging along the Gulf
Coast. Thus, initially moisture is taking the scenic route up
through West Texas. The moist plume will broaden today, but
ongoing rain/storms north of an advancing warm front will curtail
the northward extent of surface-based instability while the
compact nature of the height falls will mean only a narrow zone of
steep lapse rates to the east of the cyclone. Combine all this
with a jet streak cutting straight across central Texas during
peak heating, and the heavy rain potential appears confined to
areas just north of the jet streak, and then more briefly farther
south and east where convection will be more linearly forced in
the low levels.

Given the limiting factors, WPC QPF is more conservative than the
spotty 3 and 4 inch amounts from some of the hi-res models, but
there will be a good soaking rain of areal average 1.00 to 2.25
inches from the ARKLATEX into easertern Oklahoma, with some
potential for excessive rainfall on the local level, especially in
the more unstable areas across east Texas.

Overall, the guidance trend was to contain the heavier rates
closer to the mid level cyclone, given the instability-based
factors discussed above. This resulted in a westward shift of WPC
QPF for Day 1, meaning lesser amounts creeping into eastern AR/LA
and adjacent TN/MS. Still, warm advection should yield increasing
coverage of rain to parts of the Southeast by Saturday night.


Easterly onshore flow deepens today, and the moisture depth /
magnitude increases. Models certainly signal a fair amount of
coverage of convective showers. Some of the hi-res models indicate
spotty heavy rainfall. We were less confident in the intensity
given the looks of the observed and forecast soundings. Upper flow
becomes more supportive and difluent by Saturday night, but mainly
at high levels, perhaps not tapping into the rich low level moist
layer. Convection may instead by governed by diurnal boundary
layer influences and outflows. The mid level lapse rates do not
seem too supportive of organized heavy rain, but we will have to
watch out for small scale organization along Florida`s east coast.
WPC QPF leaned toward a global model consensus while using the
WRF-ARW2 for some of the mesoscale details.

Days 2/3...

...Lower MS and TN valleys to the Southeast and southern

Widespread moderate, to locally heavy and potentially excessive
rains, are expected to move east across the region as a upper low
drifts east from the lower MS to the TN valley this period.

Overall, models are in good agreement with the larger scale
features - showing a compact mid-upper level low moving east from
the southern Great Plains into the lower MS valley on Sun.  Strong
moist southerly low level inflow ahead of the low, interacting
with strong large-scale ascent afforded in part by favorable upper
jet forcing, is expected to support widespread moderate to locally
heavy rains from lower MS into portions of the TN valley and
Southeast Sun to early Mon.  While models continue to show typical
differences with respect to the finer details, there remains a
general model signal for 1 to 3 inch totals, with locally heavier
amounts, extending from western TN to western GA during the Day 2
period (ending Mon 12 UTC).  With these amounts causing potential
runoff concerns, a `slight` risk for excessive was maintained
across this area in the Day 2 Excessive Rainfall Outlook.

By early Mon, models show a northern stream trough moving off of
the Northeast/Mid-Atlantic coast, allowing an upper ridge to build
along the East Coast and causing the upper low to slow over the TN
valley.  Strong southerly-southeasterly low level winds sandwiched
between the low and high pressure anchored off of the New England
coast will channel deep moisture from the Southeast coast back
into the southern Appalachians.  As on Day 2, there is plenty of
uncertainty with respect to the finer details of forecast.
However, there is pretty good model signal for heavy amounts on
the order of 2-4 inches along portions of the SC coast during the
Day 3 period.  It is uncertain to what degree convection
developing offshore and along the coast may hinder moisture
advection further west, however most of the overnight guidance
show a secondary max along the favored terrain of western
Carolinas and northern GA.  Given the forecast uncertainty and the
antecedent dry conditions across portions of the region, only a
`marginal` risk for excessive was introduced into the Day 3
Excessive Outlook (ending 12 UTC Tue) at this time.  However, an
upgrade to a `slight` for portions of the region may need to be
considered in future updates.

WPC QPF generally followed a blend that gave significant weight to
00 UTC GFS and recent runs of the ECMWF through the period.

...Northern Rockies to the Northern Great Plains...

A well-defined shortwave trough is forecast to move across the
northwestern U.S. into the northern Great Plains this period.
This is expected to produce widespread light to moderate
precipitation from the northern Rockies to the plains, with some
locally heavy mountain snows across southwest MT and northern WY.
WPC QPF gave more weight to recent runs of the ECMWF and NAM.  The
GFS is more progressive and amplified than the model consensus
with a leading piece of energy, producing heavier precipitation
further east across the plains on Day 3.  Refer to the WPC 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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