Quantitative Precipitation Forecast
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FXUS04 KWBC 202010

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

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

Day 1

...Lee of the Central Rockies---Central to Southern Plains...
A welcome widespread moderate to heavy precipitation event is
likely from the lee of the central Rockies southeast through large
portions of the Central to Southern Plains.  An incoming closed
mid- to upper-level low will help draw increasingly anomalous
moisture northward due to strengthening low level southerly to
southeasterly flow into the area.  Strong upper difluence ahead of
this closed low in this increasingly anomalous precipitable water
(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
the new rendition trending drier per recent radar trends and 12z
ECMWF/12z GFS/18z in-house experimental bias corrected QPF runs.
With recent dry conditions and the overall expected progressive
nature of the organized heaviest precip---do not believe there
will be any runoff issues.  Areal average 1-2" totals expected
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 where instability
should be lower.

...Pacific Northwest into the Northern Rockies...
A fast-moving shortwave 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 cold low will continue to progress across 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 runs 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 difluence 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 bulls eyes 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 advances 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 PW values in excess of 1.5
inches near and south of the front is expected to increase
standardized PW 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 later high
resolution 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 PWs 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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