Quantitative Precipitation Forecast
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FXUS04 KWBC 240953

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
552 AM EDT Sat Mar 24 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 24/1200 UTC thru Mar 27/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Mid MS Valley to the lower OH Valley, TN Valley, Southern
Appalachians, and lower mid Atlantic Region...

A vigorous, compact shortwave and attendant southern stream upper
jet streak will slide e-se from the central Plains early today
into the lower OH by late Saturday night. This southern stream
feature, which impacted central/southern CA earlier this week with
torrential rains, will continue to foster a surface low from
eastern KS early this morning into the eastern TN Valley by early
Sunday. Ahead of this system, a considerable amount of moisture
return from the Gulf of Mexico within the compact warm conveyor
belt (wcb) -- borne by robust southwesterly low-level inflow
(40-50kts at 850 mb) -- will generate 850 mb moisture flux
anomalies between 3-4 standard deviations above normal per the
latest SREF and GEFS. Meanwhile, another upper shortwave across
western Quebec will dive south and reinforce the northern stream
trough across the northeast. This in turn will sharpen the
confluence/strengthen the jet at upper levels within the trough
base across the mid-Atlantic region, providing an uptick in
northern stream forcing (upper divergence and low-level
frontogenesis) within the right-entrance region of the jet streak.
The coupled dynamical forcing from both the southern and northern
streams will allow widespread mod-heavy pcpn to blossom in a nw-se
fashion north of the surface warm front -- while the latter
(ensuing strengthened low-level frontogenesis) helps to narrow the
corridor of steep isentropic lift, leading to a tightening
moisture cutoff to the north.

The 00Z models were more tightly clustered with the axis of
heavier pcpn compared to yesterday, though WPC continued to rely
more heavily on the members that were situated on the southern
edges of the guidance spread (including the ECMWF). This given the
weaker static stability farther south, toward the instability axis
(MUCAPES at least ~200 j/kg), where 700-500 mb theta-e lapse rates
become negative per the GFS and ECMWF. As such, the heaviest 24
hour qpf (12Z Sat-12Z Sun) would set up across south-central to
southeast KY into southwest VA, eastern TN, far western NC, and
far northern GA (western slopes of the Smoky Mtns and southern
Appalachians). Highest areal-average totals of 1.5 to 2.0 inches
were noted, some of which expected to fall as snow (please refer
to the latest QPFHSD for further details). Given the lack of
deep-layer instability, evidenced by the limited variation with
the max qpf among the high-res CAMs, short term rainfall rates
will be considerably hampered. Therefore, the risk of excessive
rainfall over this region at this point remains below the marginal

...Western U.S...

A very active and wet regime will persist across the West on Sat.
A large 500mb closed low and deep upper trough offshore of the PAC
NW will sweep into the region. The main mid to upper circulation
will dig south and east toward southern OR/northern CA, while the
main thermal zone pushes into the inter-mountain West. Overall the
onshore flow will remain intact but maybe elevated in one
location. A dynamic upper jet/jet streak will focus strong PAC
moisture transport into swrn OR and northern CA for possible 1-2
inch areal avg qpf amounts, especially from the coast to terrain.
Overall WPC followed a blend of the gfs and ecmwf with the hi-res
suite for qpf amounts.

Days 2/3

...Pacific Northwest/Upper Intermountain West/Northern to Central

A myriad of shortwaves swinging from the Pacific Northwest
eastward into the northern/central Rockies will focus a persistent
period of precipitation, with snow across the higher elevations.
On Day 2, the best focus should be over northeastern Wyoming into
the Black Hills of South Dakota given favorable moist upslope
flow. A consensus of solutions favors locally heavier amounts over
this region while lighter activity prevails over the remainder of
the Intermountain West/Rocky Mountains. By Day 3, a 850-mb low
swinging through the central Rockies should spread a separate axis
of precipitation over much of central Colorado. While much of the
action shifts toward the middle of the nation, heights will build
over the eastern Pacific while sufficient moisture transport takes
aim at the Pacific Northwest coast. This should maintain a steady
period of lower elevation rain/higher elevation snow over the
western half of Washington down into northwestern Oregon. A
three-way blend of the 00Z NAM/GFS and 12Z ECMWF was utilized
throughout the western U.S.

...Great Plains/Ozarks/Middle to Upper Mississippi Valley/Great
Lakes/Ohio Valley...

An active period will unfold across the center of the country,
particularly by Day 3 as an increasing threat for flash flooding
builds from central Oklahoma northeastward into the southern half
of Missouri. Given the increasing signal noted in the 00Z
guidance, decided to introduce a slight risk area from the
Oklahoma City metro area northeastward across southwestern
Missouri along I-70 into St. Louis.

On Day 2, precipitation across the middle of the country will be
focused in two primary areas. Across the Dakotas, a north/south
oriented frontal zone extending from a wave over Manitoba will
spread a narrow axis of precipitation over the north-central U.S.
The guidance has remained in decent agreement favoring the best
action over eastern North Dakota where the strongest low-level
convergence should set up. Otherwise, a broad shield of light to
moderate precipitation is likely from Nebraska up to the Canadian
border given broad ascent with the approaching longwave trough.
Looking toward the Arklatex and Ozarks regions, loosely organized
convection should continue in close proximity to a poleward
lifting warm front. Weak upstream height falls may lead to
slightly better organization nearby the eastward ejecting surface
low and attendant frontal boundaries. Additionally, some dryline
thunderstorms are possible across central Texas although the
00Z/18Z GFS are on their own with the more robust QPF output.
Comparing instability parameters, it appears the GFS shows much
greater surface-based CAPE numbers suggesting much more pronounced
vertical motions along the dryline. Decided to downplay this
scenario favoring broad, lighter amounts. A combination of the 18Z
GFS/12Z ECMWF/18Z in-house pseudo-ensemble bias-corrected model
was utilized with more emphasis on the latter two solutions.

By Day 3, much strong forcing arrives within the southern branch
of the jet stream. 500-mb winds pick up significantly as a
shortwave rounds the base of the trough across the Four Corners
region. By Monday evening, 50 to 60 knot mid-level winds
overspread a rapidly moistening boundary layer across the Southern
Plains. The upstream trough continues to re-load over Arizona/New
Mexico which favors a continued period of southwesterly flow aloft
across the center of the country. Unidirectional low/mid-level
flow is expected to unfold across Oklahoma northeastward into
Missouri which should focus a steady period of organized, heavy
rainfall, particularly Monday evening into the overnight hours.
6-hour rainfall totals may reach the 1 to 1.50 inch range during
this period which will enhance the risk for flash flooding. Models
have come into much better agreement with many 00Z solutions
increasing amounts over the region. Decided to favor a combination
of the 00Z ECMWF/GFS/UKMET/CMC given the solid consensus noted. To
the north, additional shortwave energy is forecast to slip through
the Northern Plains/Upper Midwest. It should be a quick mover as
the north/south convergence zone accelerates downstream.

...Southeastern U.S....

An exiting frontal zone across the southeastern U.S. will favor a
period of moderate rainfall over the region early on Day 2. A wave
of low pressure is expected to push off the coast with residual
activity lingering across southern Georgia. However, with surface
ridging extending down into the eastern Gulf of Mexico, much of
the remaining precipitation would become more elevated and less
concentrated in nature. A multi-model consensus was utilized which
agreed on the heaviest amounts ending 26/0600Z.


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


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