Quantitative Precipitation Forecast
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FXUS04 KWBC 250927

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
526 AM EDT Wed Apr 25 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 25/1200 UTC thru Apr 28/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Northeast U.S...

Some upper level phasing will occur over the Northeast today as
the long-lived meandering low in the southern stream lifts and
joins a more progressive northern stream trough. The mean
resulting trough takes on a slightly negative tilt, with one lobe
of stronger forcing over the eastern Great Lakes and the other
pushing toward Long Island and southern New England. Certainly
this will lead to widespread rainfall lifting out of the
Mid-Atlantic states this morning. Heavy rainfall, however, will be
a little difficult to find, given the vorticity center over
upstate New York / eastern Great Lakes is farther removed from
rich moisture and any instability, and along the coast a surface
occlusion is forecast to remain just offshore. The warm conveyor
belt will promote relatively heavier totals, likely 1 to 2 inch
24-hour amounts, from Connecticut to southern Maine, but expect
this to mainly be a stratiform event. The phased upper trough
should reach its peak efficiency around the time the conveyor is
directed into Down East Maine, such that our 24-hour amounts are
locally maximized there, using the the HREF Mean, WRF-NMM, and the
00z NAM as a guide.

It will be interesting to see if any deeper convection can occur
in southern New England, CT/RI/MA, this afternoon in the wake of
the warm advection / conveyor precip, as a few hundred J/kg MUCAPE
is forecast to form. If this does occur, the duration over any one
location would be brief, but when added atop the morning rainfall
in an urban corridor there could be some flashy surface runoff.

The same models mentioned above were used to depict the few
diurnally driven showers expected in the Mid-Atlantic and North
Carolina today, as well as the northwesterly flow upslope into the

...Central/Southern U.S...

A compact mid level low will drop from the central Plains down
through Oklahoma and Arkansas today. At 06z the leading edge of
height falls and associated frontal zone was crossing paths with a
narrow instability axis in southwest Texas, giving rise to a few
thunderstorms. Otherwise, precipitation was more greatly rooted in
the mid and upper levels near the tight 700 mb frontogenetical
zone on the periphery of the upper low - namely over Kansas and
northwest Oklahoma. This zone of strong mid and upper level
forcing is forecast to stay pretty much intact today, producing a
swath of rainfall heading southeastward across parts of
OK/TX/MO/AR. Lower level moisture return is somewhat inhibited by
a recent frontal intrusion into the Gulf and residual surface
ridging there. During peak heating, however, some quasi-surface
based convection is possible over central and northeast Texas,
eventually forming toward parts of Arkansas and Louisiana in the
evening as well.

A more robust MCS is possible within the Rio Grande Valley where,
per the Storm Prediction Center Day 1 Outlook, the bulk of
activity will be on the Mexican side of the border, but forecast
MUCAPE is supportive of storms reaching or forming in the vicinity
of Del Rio and Laredo, with activity working its way southward
over time.

There is a fair amount of agreement among the model QPFs over this
region. WPC favored the HREF blended mean (average of its ensemble
mean and probability matched mean). The WRF-NMMB was used for a
few details as well, given that it was a bit more generous with
areal coverage of QPF in the few pockets of surface based
instability that are expected to develop over parts of TX, LA, AR.
The flash flood risk appears very low today given that most
activity will have roots in the mid levels and should display a
progressive character. Slower cell motion and stronger rain rates
could occur near the Rio Grande during the early stages of the
afternoon convective event.

Days 2/3...

...Tennessee Valley and Southeast to the Northeast...

A compact mid-upper level low moving out of the lower Mississippi
valley Thu morning is expected to support a swath of light to
moderate amounts from the Tennessee valley eastward into Georgia
and the Carolinas Thu into early Fri.  Precipitation is then
expected to shift northward to the northern Mid-Atlantic and along
the Northeast coast Fri into early Sat as the system weakens and
lifts north ahead of an amplifying trough over the Midwest.
Marginal moisture and the progressive nature of this system are
expected to hamper the potential for any widespread heavy amounts.
 WPC QPF relied heavily on a blend of the NAM and recent runs of
the ECMWF.   As the system begins to lift out of the Southeast
early Fri, the GFS becomes a deep outlier, spreading precipitation
much further west from the central Appalachians and into the
Northeast Fri into early Sat.  Therefore little weight was given
to its solution beyond late Thu.

...Pacific Northwest/Northern California...

An expanding area of precipitation, with increasing amounts, can
be expected across Oregon into southern Washington and northern
California as an upper ridge gives way to an upper low on Fri.
While widespread heavy amounts are not expected, ample forcing and
instability may be sufficient for some locally heavy accumulations
across portions of the region.  WPC amounts leaned closer to a
blend of the NAM, GFS and UKMET, with less weight given to the
ECMWF - which was a relatively dry outlier.

...South Florida...

A shortwave trough digging into the north-central Gulf of Mexico
may support heavy amounts developing across the Keys and the
southern peninsula as southwesterly flow ahead of the wave begins
to draw deep moisture across the region.  As this moisture
interacts with favorable mid-upper level forcing, some of the
overnight guidance, including the NAM, UKMET and ECMWF show
moderate to locally heavy amounts developing across the region Fri
night into Sat morning.  The GFS which is faster with the trough
aloft, pushes the deeper moisture axis further south and develops
heavy precipitation mainly out over the Florida Straits.  Given
its outlier position aloft, leaned away from the GFS and more
toward the NAM, UKMET and ECMWF.


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


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