Quantitative Precipitation Forecast
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FXUS04 KWBC 201938

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
338 PM EDT Fri Apr 20 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 21/0000 UTC thru Apr 24/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Lee of the Central Rockies---Central to Southern Plains...

A much needed widespread moderate to heavy precipitation event
likely day 1 from the lee of the central Rockies---southeastward
through large portions of the Central to Southern Plains.  Strong
height falls moving east from the four corners region this
afternoon--across the central to southern Rockies evening and into
the Central to Southern Plains tonight into Saturday will help
draw increasingly anomalous pw values northward as low level
southerly to southeasterly flow strengthens across the Central to
Southern Plains.  Strong upper difluence ahead of this closed low
in this increasingly anomalous pw axis will support the likelihood
of widespread moderate to heavy totals.  There is some latitude
differences with the max precip amounts.  WPC qpf did favor the
farther south axis of the CMC GEM and hi-res ARW across southern
OK and north TX---closer to the west to east oriented frontal
boundary across north TX.  With recent dry conditions and the
overall expected progressive nature of the organized heaviest
precip---do not believe there will be any runoff issues at the
moment.  Areal average 1 to 1.5 totals depicted in the max precip
area over much of OK into north TX---with lighter totals in the
.25-.50"+ range extending north and northwest of this into the
Central Plains and lee of the Central Rockies.

...Pacific Northwest into the Northern Rockies...

A fast moving area of strong northeast Pacific height falls will
be moving inland along the Washington---B.C.---Idaho border region
tonight/early Saturday and into southern Alberta later Saturday.
An area of organized precip along and ahead of the associated
frontal boundary will support moderate to heavy precip totals over
the northern Washington Cascades---Olympic Range and into far
southwest B.C. and Vancouver Island.  Lighter precip totals
expected south into the Southern Washington-Oregon Cascades---west
to the coast ranges and into the Northern Rockies from far
northern ID into northwest MT.

...Northern NY state into northern New England...

The broad closed low moving into the Canadian Maritimes this
afternoon will continue to progress across New Newfoundland
tonight and over the far northwest Atlantic during Saturday.  The
persistent cyclonic flow in its wake will support scattered snow
showers from northern NY state into northern New England.  Any
precip/snow amounts are expected to be very light.

Days 2/3...

...Central-Southern Plains to the Southeastern U.S....

While there are plenty of differences in the details, there
remains a good model signal for widespread moderate to heavy and
potentially excessive rains to develop and move east across the
region as a compact mid-upper level low tracks east-southeast from
the central Great Plains on Saturday to the lower Mississippi and
Tennessee valleys by Monday morning. Ensemble trends of the 500 mb
closed low have been somewhat slower over the past several as the
system reaches the Mississippi valley, with a 12Z GFS/00Z ECMWF
blend representing the greatest ensemble clustering and near the
ensemble means. The 12Z NAM was considered too far north with its
mass fields and was not used for the QPF generation.

Convection is expected to be ongoing at the start of the Day 2
period (00Z/22) over the ArkLaTex, and perhaps heaviest just north
of a returning warm front over the west-central Gulf Coast at the
start of the period. The position of the upper low and jet
placement support strong lift within the left exit region of a 110
kt jet core and strong upper level diffluence over the lower
Mississippi valley. However, 700-500 mb lapse rates remain rather
weak...less than 7 C/km...which may only support elevated
instability up to 1000 J/kg per a consensus of available model

The closed nature of the low and PWATs in excess of 1.5 inches
with 20-40 kt of 850 mb inflow should help to support heavy
rainfall rates. However, the forecast shows the upper low to
steadily progress toward the east which should mitigate rainfall
totals to some degree. Another concern with some of the latest
model guidance was the potential for precipitation to be displaced
south of the global model QPF, given hi-res support farther south
through 12Z/22. The 12Z ECMWF also supported higher QPF to the
south of many of the deterministic guidance, which is often true
and better co-located with forecast available instability.

While continuity was adjusted toward a GFS/ECMWF blend, WPC QPF
adjustments were made to move maxima south and to reduce apparent
feedback bullseyes noted in the GFS and ECMWF across the
Southeast. Across the Southeast, a coastal convergence boundary is
expected to setup on Monday with any significant instability along
the coast or offshore. Overrunning of low level moist flow should
support locally heavy rainfall in the vicinity of the GA/SC coast
for Day 3 (00Z 23-24), with a secondary maxima just ahead of the
mid-level low (greater mid-level lapse rates) and with a strong
upslope component into the southern Appalachians.

Future outlooks will likely be needed beyond the Day 3 Excessive
Rainfall Outlook (12Z/24+), but marginal risks were drawn for the
Day 2 and Day 3 Excessive Rainfall Outlook to cover the model
spread from eastern TX/OK into the TN valley and Florida
Peninsula, with a Slight Risk on Day 2 straddling the TX/LA border
and from northern/eastern Alabama into western Georgia for Day 3.


As the upper trough described in the above section moves east, a
front is expected to initially lie east to west across the
Peninsula, drifting north as a warm front Sunday night into
Monday. A somewhat narrow axis of PWAT values in excess of 1.5
inches near and south of the front is expected to increase
standardized PWAT anomalies to near +2 on Sunday. Southeasterly
925-850 mb flow of 20-30 kt early Sunday morning may focus
localized cores of heavy rain with areas that are slow moving or
regenerating over the same locations in the vicinity of low level
convergence along the coast. Flow veers to a more southerly
direction late Sunday, which is likely to shift any heavy rainfall
focus offshore. The 12Z HRW-ARW, NSSL and NAM_NEST support the
idea of 3-6 inch totals along the central and northern east coast
of the Florida Peninsula for the 24 hour period ending 12Z/23. No
additions were made on the Day 2 Excessive Rainfall Outlook at
this time given low confidence in the details, but future access
to nearer term hi-res model guidance may allow for a narrow
marginal risk in future outlooks.

...Northern Rockies into the northern High Plains...

After the departure of a strong shortwave into south-central
Canada Saturday night, quasi-zonal flow in its wake will bring the
passage of another shortwave, albeit weaker, across the West Coast
during the day on Sunday. A 700 mb trough axis will likely amplify
across the interior northwestern U.S. on Sunday, developing into a
closed low near the ID/MT/WY border Monday morning. Model
agreement on placement was generally agreeable, but the 12Z NAM
was a deterministic outlier with taking energy from the base of
the trough into the Southeast for Monday leaving less strength to
the trough over the interior Northwest. WPC QPF was led largely by
a 12Z GFS/00Z ECMWF blend.

Forecast moisture anomalies are fairly minimal on Sunday across
the Northwest, but as the low becomes better defined with eastward
progression into Monday, 700 mb wind speeds will increase into the
30-40 kt range ahead of the closed low. Forecast guidance does
show the low opening back up into a positively tilted trough axis
during the day on Monday but weak moisture return from the Plains
will join with the system, reaching the northern Plains and
allowing for PWATs to peak near 0.75 inches. Increasing
overrunning north of a quasi-stationary front extending from
northern Wyoming into the Dakotas on Monday along with low level
upslope flow into the High Plains with some enhancement to lift in
the right entrance region of a 70-90 kt 250 mb jet streak should
all combine to aid in producing heavier liquid equivalents from
the northern Rockies into the eastern MT/WY border.


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


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