Quantitative Precipitation Forecast
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FXUS04 KWBC 221027

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
626 AM EDT Sun Apr 22 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 22/1200 UTC thru Apr 25/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Southeast U.S...

A closed low of moderate strength and maintaining itself along its
slow eastward march - characteristically a very April system -
will spread rain and thunderstorms throughout areas from the lower
Ohio Valley to the Gulf Coast. In Florida easterly onshore flow,
heating, and gradually increasing deep layer ascent will also
promote widespread coverage of rainfall.

Model guidance is in decent agreement, and the notion of
widespread 1 to 3 inch rainfall is supported even by the GEFS
Reforecast data based on analog events. The main question is
whether rain rates will be intense enough or persistent enough
over any given location to yield much in the way of flash
flooding. Generally the most persistent forcing and better
alignment of lower and upper level flow should occur during the
day from northern Mississippi into western Tennessee. This is
perhaps where training will be most prevalent, but instability
will be more marginal. Areas farther south should see somewhat
greater instability, but with mid level flow cutting across from
west to east, the mean 0-6 km winds are at a sharp angle to
Corfidi vectors, suggesting training will not be too common. Given
flash flood guidance values are also much higher, and will be
difficult to surpass with southward extent, WPC trimmed back a bit
on the inherited Slight Risk area for the 12z Sun- 12z Mon period.
Otherwise, Slight Risk is maintained in areas of lower FFG across
northern MS/AL into southern TN, and including the Atlanta Metro
to the east, as well as the more favorable training environment in
western TN.

Throughout these areas areal average precipitation is forecast at
1.5 to around 3.0 inches for the 24-hour period, and judging by
moisture availability and hi-res model solutions, hourly rates
could peak around 1.50 inches, leading to at least some risk of
flash flooding. For QPF there was no ideal solution, but the
models were well clustered. WPC began with a blend of the 00z GFS,
WRF-NMM, and the non bias corrected version of our in-house
ensemble (more generous with areal coverage). We expanded coverage
northward, resulting in quite a bit more QPF over northern TN into
KY compared to the previous cycle, and toned down QPF along the
east coast of Florida, as low level convergence and cell motions
appear erratic enough to spread the convection out.

...Northern Rockies...

In the immediate wake of a lead shortwave - which deposited a
frontal zone into the northern Rockies - a stronger shortwave
trough will amplify Sunday over Idaho and Montana, bringing a
period of strengthened frontogenesis and well defined upper jet
forcing. Weakly convective rain/snow will likely result. The
previous WPC forecast had a great handle on this event, and we
made only minor tweaks using the 00z GFS and our in-house ensemble

Days 2/3...

...Tennessee and Ohio Valleys to the Eastern U.S....

A northern stream trough moving off of the Northeast coast Monday
morning is forecast to give way to an amplifying ridge along the
Eastern Seaboard.  This will slow the forward progression of a
compact mid-upper level low moving from the lower Mississippi into
the Tennessee valley on Mon.  This will set the stage for
prolonged period of moderate to heavy rains along the Southeast
coast back into the southern Appalachians Mon into early Tue.
Heaviest accumulations during the Day 2 period (ending 12 UTC Tue)
are expected to center from the South Carolina shores and the Cape
Fear region of North Carolina back into the southern Blue Ridge.
Deep moisture, on the order of 2 standard deviations above normal
afforded by 30-40 kt southeasterly low level inflow, interacting
with favorable upper jet forcing, is expected to support heavy
totals on the order of 1 to 3 inches, with potentially heavier
amounts, along the coast.  Although removed from the deepest
moisture and better instability, there remains a good model signal
for a secondary max further west, where good orographic ascent
should support heavier totals from the Piedmont back into the
southern Blue Ridge.  For the Day 2 Excessive Outlook, a `slight`
risk was maintained across these areas.

For Day 3 (12 UTC Tue to 12 UTC Wed), the heavy rainfall threat is
expected to wane some as the low begins to weaken and lift out to
the northeast.  Light to moderate rains are expected to spread
north through the Mid-Atlantic region into the Northeast.  Again,
the potential for heavy amounts is expected to lessen as the
onshore flow and the upper forcing weakens.

WPC QPF gave significant weight to the GFS, NAM and recent runs of
the ECMWF through the Day 2 period.  By Day 3 however, less weight
was given to the ECMWF, relying more heavily on the GFS and NAM as
the 00 UTC ECMWF becomes a more southerly and a relative outlier
with the position of the low.

...Northern Rockies to the Great Plains...

A well-defined shortwave trough is expected to support light to
moderate precipitation, including mountain snows, from the
northern and central Rockies to the adjacent plains Mon into early
Tue.  While the models show fairly good clustering through Day 2,
there remains significant spread thereafter, with the GFS showing
more interaction with a shortwave trough moving across central
Canada, carrying a more amplified trough further east into the
central Great Plains and mid Mississippi valley Wed morning. With
more separation in the flow, the ECMWF drops this energy further
south along the central High Plains.  With support from the UKMET
and the Canadian Global, leaned closer to the ECMWF solution, with
WPC QPF holding developing precipitation across the central and
southern Great Plains further west late Tue into early Wed.


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


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