Quantitative Precipitation Forecast
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FXUS04 KWBC 262115

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
514 PM EDT Sat May 26 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 27/0000 UTC thru May 30/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Florida into the Southeast...

No significant changes to the forecast thinking with the heavy to
excessive rainfall potential day 1 associated with the northward
movement of Alberto across the eastern portions of the Gulf of
Mexico.  Heavy rains will continue on the eastern side of Alberto
in the persistent axis of much above average pw values---2.5-3+
standard deviations above the mean---across much of FL.  Models do
show potential for a developing dry slot pushing northeastward off
the eastern Gulf and across portions of FL tonight into Sunday.
The highest pw values may focus along the east coast of FL where a
few of the hi res runs are indicating potential bands setting up
parallel to the coast late Saturday night into Sunday--with a
lessening heavy rain threat for the western portion of the
peninsula.  Farther to the northeast---the precipitation threat
will be increasing across the Southeast across southern GA into
coastal SC as the higher pw values on the east side of Alberto
continue to stream northward.  For the day 1 time period---the
potential very heavy rains near the center of Alberto will likely
remain offshore given the current official NHC track---with this
core heavy rains a day 2 and beyond threat as the low continues to
push north.

...Mid-Atlantic into the Central Appalachians...

Much above average pw values expected to remain in a east to west
axis across portions of the Mid-Atlantic---extending into the
Central Appalachians.  Initially this is well to the south of the
frontal boundary over the northeast.  With time this front will be
pressing south into the Mid-Atlantic and may become a focus for
heavy to excessive rainfall amounts Sunday.  A predecessor
rainfall band well to the northeast of Alberto is possible along
this front where favorable upper difluence in the axis of much
above average pw values will support heavy to locally excessive
precip totals Sunday.

...Great Basin into the Northern Rockies

The strong Great Basin closed low will be moving only very slowly
eastward day 1. Well defined upper difluence ahead of this closed
low will support fairly well organized comma head deformation
precipitation areas to the north and northeast of this strong
closed low.  This will support widespread moderate to locally
heavy precip totals from the Great Basin into the Northern
Rockies.  The href mean and in house hi res ensemble used
primarily to mitigate any run to run and model to model qpf detail

Days 2/3...

...Southeast and Florida Peninsula...

*** MODERATE RISK of Excessive Rainfall on Monday and Monday Night
for extreme southeast Mississippi, southern Alabama, and the
western Florida Panhandle ***

The rain associated with the center of Subtropical Storm Alberto
will begin spreading into the Southeast region in earnest Sunday,
with the center approaching the central Gulf Coast Monday per the
latest NHC forecast. The plume of tropical moisture east of the
storm center will create a mainly separate area of heavy rainfall
north from Florida to the coastal Carolinas through the Day 2-3
period. A mid-level dry slot is expected in between, as depicted
in WPC QPF through the Day 2-3 period. However, due to the
potential for repeating cells/bands in the dry slot, the Slight
Risk for excessive rainfall that spans that southeast was
maintained for both Days 2 and 3 per consultation with some
Florida WFOs. Also, the northern extent of the Slight and Marginal
Risks were shifted north a bit per 12Z guidance.

WPC QPF was based on an even blend of the 12Z GFS/NAM/ECMWF which
are in good agreement on Day 2 (when 3km NAM was used). The 12Z
ECMWF shifted the axis of heaviest rainfall east to Alabama from
Mississippi on Day 3 which is in line with the 12Z GFS (QPF max
axis placement in the 00Z ECMWF had been quite similar to the 12Z
NAM), so the final QPF also included a shift east.

The area of heavy rain around the center of Alberto may be
relatively small as it approaches the coast, per 12Z guidance
consensus of a small core circulation and limited diameter of
heavy QPF. As such, even small deviations in the track could
significantly shift the expected heavy rain and flash flooding
impacts near the center. However, there is relatively high
confidence in that flash flood threat existing near the track,
particularly given the expected slow forward motion (around 7-8
knots) hence the Moderate Risk for excessive rainfall on Day 3
(12Z Mon-12Z Tue).

Please refer to the National Hurricane Center forecasts further
information on Alberto.


A swath of heavy rain is expected to redevelop across the
Mid-Atlantic Sunday afternoon. The combination of a lingering
low-level front and theta-e gradient, along with the right
entrance region of a jet streak, should provide enough focus for
over areal average QPF of 2 to 2.5 inches from northern VA across
DC to central MD (including Baltimore City) and east across the
Delmarva Peninsula. Given the generally uni-directional flow
roughly parallel to the low-level boundary, and model consensus on
increased rainfall, a Day 2 (12Z Sun-12Z Mon) Slight Risk for
excessive rainfall was raised across this area. The maximum
guidance for QPF in Day 2 is the 12Z GEM-regional which averages
around 5 inches across this swath in what appears to be a
predecessor rainfall event on the north side of the tropical
airmass. This area will continue to be monitored, particularly
given the wet antecedent conditions over the past week as well as
ongoing activity tonight.

...Intermountain West, Northern Rockies, and Northern Plains...

An upper level low across the Intermountain West and Great Basin
should maintain a chance for scattered convection through
Sunday/Day 2, likely concentrated near terrain features, before
the low devolves into a trough and kicks into the Plains by Day 3.
QPF was based on the 12Z ECMWF/GFS/NAM. The Slight Risk for
excessive rainfall for Day 2 (12Z Sun-12Z Mon) was expanded a bit
over Wyoming and South Dakota. Strong ENE low-level inflow should
create both convergence and possibly some orographic ascent, and
models generally agree on PWATs over 1 inch being advected into
the area in this band of low-level inflow. Additionally, upper
level divergence should be enhanced by coupled jet structure just
northeast of the upper level low. Given the strong synoptic scale
forcing and PWATs around the 95th percentile for late May, there
seems to be sufficient threat of flash flooding to maintain the
Slight Risk. Over an inch of QPF is forecast over southeast
Montana and the Black Hills on Day 2.

This area shifts east to the northern plains for Day 3 (12Z
Mon-12Z Tue) and the Marginal Risk for excessive rainfall was


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


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