Quantitative Precipitation Forecast
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FXUS04 KWBC 251016

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
615 AM EDT Fri May 25 2018

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

Day 1

The flow pattern is not too exciting except for a closed low
entering the west coast and a broad area of anomalously low
heights in the Gulf of Mexico / northern Caribbean where the
National Hurricane Center places high probabilities of subtropical
or tropical depression development in the near future. A shortwave
digging into the back side of the Gulf trough early this morning
will help induce inverted troughing and enhanced southerly flow of
very moist air downstream over south Florida today. Elsewhere
northern stream waves will promote a few mesoscale thunderstorm
clusters over the upper Midwest / Great Lakes and New England.
Diurnal convection should again become fairly widespread in
coverage over the southern tier, and the low along the west coast
will yield organized rainfall, especially northern California and
southern to eastern Oregon. WPC QPF was derived from a near 50/50
blend of our in-house ensemble and the HREF blended mean. Six-hour
QPF probabilities from the GEFS were also helpful.

...Western U.S...
Models are very generous with rainfall in the difluent region
north to northeast of the closed low center. Although the low is
forecast to open up late in the Day 1 period there should be a
lengthy duration of favorable ascent profiles and southeasterly
low level inflow from northern California into Oregon, and
catching part of northwest Nevada. The event may begin with some
convective enhancement and then transition to longer duration
stratiform rainfall behind the surface boundary. An environment
more supportive of deep convection and/or more repetitive
convection should exist farther east, from southeast Oregon into
parts of northern Nevada and much of Idaho. Instability drops off
with eastward extent through NV/UT given drier air in that region,
but thunderstorms should be able to make some eastward progress
before drying out. WPC favored the NAM CONUS NEST for its more
expansive coverage, but perhaps the HREF mean for more toned down
amounts. Still, areal average rainfall greater than an inch is
forecast in parts of CA/OR. The odds of training cells appears
lower today, but hourly rain rates will likely exceed a half an
inch locally during the stronger individual thunderstorms.

With a shortwave trough dropping in to strengthen the broad upper
trough over this region, the models develop an inverted trough
axis over south Florida along with upwind convergence in the
strengthening and very moist southeasterly flow coming off the
Caribbean. There is an increased model signal for widespread 1 to
2 inch areal average rainfall, and the potential for locally more
intense amounts as the pattern remains locked in, allowing
repeated tropical convection into the Keys and parts of south

...Central / Eastern U.S...
The small vort max that has aided in rainfall production over the
southeast will lift a bit and begin to shear toward the Carolinas.
On the broader scale, the response to progression in the northern
stream will start to draw higher octane air back into the eastern
Tennessee and upper Ohio Valleys. The coverage of measurable
precipitation, therefore, is expected to increase or at least
expand toward the north compared to recent days. The pattern
overall still lacks a lot of focus, however, with very weak winds
in much of the column over the eastern and southern states. Some
stronger shear profiles and forcing could lead to more
organization over the upper Midwest, and early day warm advection
followed by strong heating will again allow for a few clusters of
pseudo-organized convection in the lower Missouri Valley and
southern Plains. WPC favored many of the details in the NAM CONUS
Nest, although the lack of synoptic control on the forecast does
give us less confidence, and for QPF we mainly relied on a blend
of low-res and high-res ensemble output, using our in-house
ensemble and the HREF blended mean. The result was areal average
rainfall of a quarter inch or more, but not exceeding one inch,
over broad areas from the upper peninsula of Michigan to Oklahoma
and over across the southern states.

Days 2/3...

...Southeast and Florida Peninsula...

Heavy rainfall is likely this weekend across a wide swath of the
Southeast and Florida. Much of this will be related to the
developing low in the Gulf of Mexico that may become a subtropical
or tropical cyclone. For the latest on the tropical development,
refer to National Hurricane Center forecasts.

The primary uncertainties with the QPF are related to the timing
and track of the low, as that will have a large influence on the
eventual distribution of rainfall. The preference was to follow a
model blend closest to the 00Z ECMWF, with some 00Z CMC
incorporated as well. The CMC is further east, but represents a
reasonable possibility if the surface low gets pulled east by
large, sheared convective clusters. The timing of these models
(and most available deterministic models and ensemble members)
generally keeps the surface low offshore of the central Gulf Coast
by the end of the Day 3 period (12Z Monday). Therefore, most of
the Day 2-3 QPF will be from scattered convection ahead of the
advancing low, or outer convective bands in the warm conveyor belt
to the east.

Models all generally show an evolution to a more subtropical
configuration of the low by the Day 3 period, with a prominent
mid-level dry slot wrapping around the south, and eventually east,
side of the circulation. This may bifurcate the heavy rainfall,
with one concentrated area very close to the low center, and
another to the east in the undisturbed PW plume associated with
the warm conveyor belt.

The largest differences in WPC QPF relative to the NBM are a
decrease in QPF associated with the initial northward push of rain
on the north and northeast side of the circulation (indicating a
slower progression), and an increase in QPF over the Florida
Peninsula given the possibility of a slightly more eastward track.
WPC QPF also was higher than the NBM over the coastal Carolinas on
Day 3 given a strong signal for the right entrance region of a 60
knot upper level jet interacting with the northward-advancing
tropical moisture plume. Overall, though, WPC QPF remained fairly
consistent with the previous forecast.

A Slight Risk of Excessive Rainfall was introduced on Day 2 in the
southern Florida Peninsula as the cyclonic circulation begins to
advance into the state with PWs climbing well above 2 inches. A
more expansive Slight Risk was introduced on Day 3 for much of the
Southeast as the extent of the moisture plume becomes more broad.
Refrained from more elevated risk categories at this time given
(1) the expectation that the core of the low should still be
offshore at the end of the Day 3 period, and (2) uncertainty about
the placement of outer rain bands to the east. Most models show a
gradual eastward drift to the predominant rain band over Florida,
suggesting that the rain band may not lock in over a particular
area for an extended period of time. Nevertheless, given the
tropical environment with deep, saturated profiles and PWs
approaching 2.5 inches, any convective rain bands should be
capable of producing significant rain rates and therefore flash


WPC QPF in the Mid Atlantic was largely consistent with the NBM
for the Day 2-3 period, although amounts were increased from
southeast Pennsylvania into New Jersey on Saturday Night. Several
rounds of rain appear likely, one with a front (and broad trough
aloft) advancing into the region on Saturday, and the next with a
wave developing along the stalling front on Sunday. The
environment along and ahead of the front should be fairly moist,
with PWs on the order of 1.7 to 2.0 inches in some places. These
PWs would be above the 99th percentile for late May from the
central Appalachians into the coastal Mid Atlantic. Given the
anomalously high moisture values and unidirectional flow (that may
promote training of convection), a marginal risk was introduced on
both Day 2 and Day 3. If confidence increases on placement of more
organized convection, an upgrade to Slight Risk will be possible
in future outlooks.

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

An upper level low will slowly lift northeast through the
Intermountain West in the Day 2-3 period, focusing some scattered
convective rainfall through the region and just downstream into
the Northern Rockies. In general, did not deviate far from a
broad-based model blend and the NBM. Ridging downstream into the
Plains should be quite pronounced, with a substantial EML
extending well up into the Dakotas. Potential exists for one or
more convective clusters to emerge from the higher terrain in
Montana and Wyoming and then track east through the Dakotas.
Adjusted QPF to fit generally north of model consensus +11C to
+12C isotherms at 700mb, and increased the QPF slightly given the
expectation that convection may be more focused into organized
clusters of thunderstorms on the nose of the thermal ridge.


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


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