Quantitative Precipitation Forecast
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FXUS04 KWBC 260859

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

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 26/1200 UTC thru May 29/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Tropics / Gulf...
Sub-tropical storm Alberto is expected to become more organized
and strengthen. Most of the heavy precipitation will affect the
open waters of the Gulf, but also Cuba through Saturday night.
Some heavy rain will fall, however, over the Florida Keys and
south Florida. Alberto is embedded with what is still a weakly
baroclinic, broad scale trough, and the warm conveyor belt /
southerly flow is forecast to migrate out from the center toward
Florida by afternoon/evening. This should enhance convergence
within the very moist and moistening atmosphere that was in place
there. Using the official NHC track for Alberto as a guide, WPC
QPF favored the ECMWF, SREF, and continuity in this area. The
WRF-ARW2 was one of the more useful hi-res models. The NCEP
guidance was generally a little drier than the new 00Z ECMWF,
causing us to lower 24-hour totals a tad in the FL Keys.
Nevertheless, some tropical downpours are likely, and the
potential for focused bands and training of echoes should increase
as the system takes shape. The QPF will be revisited and may be
massaged upwards for the final 0930Z package.

...Eastern U.S...
Sea breezy activity along the Gulf Coast and then a generally
supportive mix of May sun angle and plentiful moisture should
yield large coverage of measurable rainfall today everywhere east
of a Dallas to Green Bay line. Focus for more greatly organized
thunderstorms is expected to exist with a shortwave trough over
the Ohio Valley, within a confluent flow regime with strong
moisture advection over the Mid-Atlantic states, and along a
northern stream frontal boundary in New England. WPC QPF was
derived primarily from the NAM, NAM CONUS Nest, and WRF-ARW2, as
these were more generous with the more moderately forced regimes,
especially that in the mid Atlantic.

...Western U.S...
The upper low over Nevada will move eastward into areas that quite
a bit drier in the low levels, so much of the diurnal thunderstorm
activity on Saturday will occur in the post frontal environment
back across Nevada and up through the northern Rockies. The models
also hint strongly at MCS development over eastern Montana to the
north of a developing warm front this evening. WPC QPF favored the

Days 2/3...

...Southeast and Florida Peninsula...

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

The rain associated with Subtropical Storm Alberto will begin
spreading into the Southeast region in earnest during the Day 2
period (Sunday and Sunday Night), with the storm`s center
approaching the central Gulf Coast on the Day 3 period. WPC QPF
for the Day 2-3 period includes more than 1 inch over much of the
Southeast region from North Carolina and the southern tip of the
Appalachians, to near Tuscaloosa, to near New Orleans, and all
areas to the southeast.

A somewhat bifurcated QPF pattern is anticipated due to a
prominent mid-level dry slot expected to wrap around the south and
east side of Alberto`s center. One QPF maximum is expected near
(and perhaps displaced just east) of the track of the center,
where a bubble of higher PWATs should remain relatively
undisturbed. Another should be situated in the warm conveyor belt
from the Florida Peninsula north into the coastal Carolinas. The
model that most closely approximated these expectations, as well
as the official NHC forecast track, was the 00Z ECMWF. Therefore,
WPC QPF was weighted most substantially toward the ECMWF, combined
with an internal multi-model ensemble. The QPF distribution was
modified slightly to focus heavier QPF more closely to the center
overnight, and away from the center during the day as convective
instability grows over land areas -- a typical diurnal oscillation
with tropical cyclones.

WPC QPF is generally higher than the NBM near the track of the
center, with the maximum QPF swath displaced east from the NBM
position. The NBM appeared to be incorporating more models that
have a surface low position west of the NHC track. WPC QPF was
also higher than the NBM over the coastal Carolinas.

For the Excessive Rainfall Outlooks, the majority of the heavy
rainfall threat was addressed with broad Slight Risks over much of
the region. This struck a balance between the favorability of the
environment with deep tropical moisture, and uncertanties
regarding the specific location of mesoscale banding. Two areas of
particular concern on the Day 3 outlook (Monday and Monday Night)
were considered for a Moderate Risk. (1) Near the track of Alberto
on the central Gulf Coast, where the decision was ultimately made
to upgrade in collaboration with WFOs MOB and TAE. And (2) along
the coastal Carolinas in a region of jet coupling (left exit
region of dry slot jet streak; right entrance region of Mid
Atlantic jet streak). PWATs there will be above 2 inches, with the
potential for a coastal front to aid in focusing convection, but
lingering uncertainties about mesoscale details precluded an
upgrade for now.

The area of heavy rain around the center of Alberto may be
relatively small as it approaches the coast, as most models show a
relatively small core circulation and limited diameter of heavy
QPF. This also shows up in the 3km NAM and HWRF. 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). Therefore, the decision
was made to go with a Moderate Risk to highlight the expected
threat, with the possibility that the exact location may shift
with future adjustments to the NHC forecast track.

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


A swath of heavy rain is expected across the Mid Atlantic into the
Day 2 period, likely as a continuation from the Day 1 period. The
combination of a lingering low-level front and thetae gradient,
along with the right entrance region of a jet streak, should
provide enough focus for over 0.5 inches of QPF from Nrn VA and MD
into the Delmarva Peninsula. Given the generally uni-directional
flow roughly parallel to the low-level boundary, some possibility
of localized heavier totals and flash flooding should exist, and a
Marginal Risk was maintained on the Day 2 Excessive Rainfall

...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 the Day
2 period, likely concentrated near terrain features, before the
low devolves into a trough and kicks into the Plains by Day 3.
Generally followed a mixture of the NBM and WPC internal
multi-model ensemble in this region. Some of the southward extent
of QPF was limited (if not in location, then at least in
magnitude), due to the possibility of a fairly robust EML plume,
particularly on Day 2. The Slight Risk was maintained on the Day 2
Excessive Rainfall Outlook over southeast Montana and northern
Wyoming. 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


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


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