Quantitative Precipitation Forecast
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FXUS04 KWBC 240746

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Thu May 24 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 24/1200 UTC thru May 27/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

The pattern looks like May, but overall, considering the full
CONUS, it is quite a bit warmer than climatology and the mid level
flow is generally weaker than usual, at least where it is
precipitating. A shear axis and embedded circulation centered
around 700 mb will provide some level of organization to diurnal
thunderstorms over Georgia and South Carolina. A better defined
northern stream trough will bite into the ridge over the northern
Plains and upper Midwest today, producing thunderstorm clusters
from the Dakotas to Kansas and eastward...with more widely
scattered activity over the southern Plains and the rest of the
Southeast. In much of the central and southeast United States the
convection will be driven as much or more by the available
instability / strong May sun angle and plentiful surface moisture
as it will be driven by the mid level flow regime. Meanwhile an
upper low poking into California will provide lift enough to yield
organized thunderstorms from the Sierras to southern Oregon.

...Southern / Southeast U.S...
With slow cell movement but also a lack of deep layer shear this
afternoon, the greatest areal average rainfall should occur where
CAPE and low level inflow are maximized. This especially describes
central to eastern Georgia and western South Carolina, where low
level inflow will be slightly enhanced by the presence of the 700
mb circulation over central GA. Models also signal heavy rain
around the southern periphery of the low in somewhat a repeat of
what occurred Wednesday. Precipitable water values of 1.50 inches
and greater provided a pretty good trace of where organized
diurnal convection took place on Wednesday. For Thursday this
threshold led us to favor the WRF-ARW2 and the HREF blended mean.
We combined these sources with 30 to 40 percent of continuity to
derive the updated forecast. The ECMWF ensemble QPF probabilities
were also very useful at separating some of the mesoscale areas of
precipitation on the 6-hour time scale.

...Plains / Upper Midwest...
This is a pretty standard issue Great Plains convective forecast.
Expect a couple areas of concentrated upglide early in the day,
mainly Minnesota and near Omaha. By afternoon clusters of
organized storms will form in a near-surface convergence axis /
pre-frontal - from eastern Colorado to parts of the Dakotas and
Minnesota. Predicting each mesoscale cluster of rainfall will be a
low confidence endeavor, but we did our best at combining the
available model signal with known tendencies. Our QPF favored the
NAM CONUS Nest and HREF Blended Mean, while using our in-house
ensemble as a more broad scale base. The result gave us areal
average quarter inch or greater amounts over a large area, but
relatively little 1.00 inch areal average based on uncertainty as
to where to place the heavier amounts. There is no clear focus,
however, for a heavy rain event. Flash flood potential will depend
more on storm scale outflows and interactions not very well
knowable with much lead time. Certainly parts of eastern Nebraska
and also parts of Iowa and southern Minnesota are a little more
susceptible right now, per Flash Flood Guidance values.

Dewpoints remained in the upper 40s and low 50s from northern
California into the northern Great Basin during peak heating
Wednesday. Expect similar values today, and the moisture through
the column - PW - is forecast at around 2.0 standard deviations
above climatology. When combined with the development of an upper
low just off California and difluent upper flow extending inland,
we attempted to lean heavier in terms of convective coverage. We
favored the NAM CONUS Nest and HREF blended mean, while also
preserving some percentage of continuity. The degree of lift could
support cells regenerating quickly along outflow boundaries so as
to lead to some training and flash flood concerns in the mountains
terrain of northern California and adjacent NV/OR.

Warm advection becomes focused ahead of a fast-moving, low
amplitude shortwave trough this evening. Model QFPs are in good
agreement, so we kept with a consensus, with the forecast bringing
a tenth to quarter inch liquid into northern Maine. The heavier
rain may stay just outside the state to the north and east.

Days 2/3...

...Southeast/eastern to central Gulf Coast/FL...

Deep tropical moisture within confluent flow combines with diurnal
heating/destabilization to produce widespread showers/storms in
eastern GA/southern SC east of a remnant 850 mb circulation. Above
normal moisture leads to a continuation of a Marginal risk of
excessive rain.

The models also depicted persistent, deep flow of moisture from
the Bahamas into southeast FL
with peak precipitable water values near 2 inches so showery
conditions are expected along the southeast coast. The pattern
favors inland penetration of the sea breeze from the east coast so
peak afternoon convection should occur over interior portions of
the FL peninsula to the west coast. Lapse rates are not as high as
usual due to warm mid level temps, so bands of larger scale lift
are needed to trigger FL convection. Convective activity into
southern FL warrants a Marginal risk for excessive rainfall for
Day 2 and 3 given recent wet conditions.

On day 3 the models diverge as to the track and intensity of a low
forecast to develop over the Gulf of Mexico.  The GFS. Canadian
global,and UKMET are east of the ECMWF/NAM/ECMWF Ensemble Mean,
which have better continuity.  With the GFS and Canadian global
trending west, much more weighting was given to the 12-00z
ECMWF/ECMWF Ensemble mean and 18-00z NAM QPF.

...Pacific Northwest/Great Basin/Northern Rockies...

A closed mid-upper level low drifts onshore and across CA Fri to
NV through Saturday. Southerly flow ahead of the circulation
increases moisture.  On Friday the deep layer circulation drifts
onshore in CA with the upper trough drifting east across OR.  Well
defined coupled 700 mb convergence and 300 mb divergence cross
from northern Ca and NV into OR, with better than average cluster
of higher QPF across southern to northeast OR as a result.  A
Marginal risk for excessive rainfall is in effect for impacted
areas of OR. Good agreement among global guidance led a consensus
based approach of the 00z ECMWF/NAM/UKMET. Less weighting was
given to the lighter GFS and heavier 12z ECMWF.

On Sat., the 700 mb circulation moves northeast out of NV towards
the WY/UT border. Continuing 700 mb convergence along and
northwest of the low track promotes showers across northeast NV
across southern ID and much of MT.  With low pressure developing
on the northern high Plains, low level convergence leads to
showers developing in the northern Plains Sat night.  The 00z NAM
shows heavier amounts than the other models, with what might be a
spurious maxima in 700 mb convergence near the MT/ND border
centered on 06z Sun. A more conservative approach using the
multi-model consensus was used.

...Central Plains to the mid-upper MS Valley, Great Lakes, Ohio
Valley, New York and New England...

Activity on Fri will focus ahead of a low level front and
prefrontal moisture plume with low-mid level convergence on the
nose of the low level jet from the central Plains towards the
mid-upper MS Valley.
The 00z NAM depicts what appears to be a case of convective/grid
scale feedback, showing anomalous low pressure and forced
convergence and strong lift from southeast KS across OK.
Less weighting was given to the 00z NAM as a result of low
confidence in its high QPF amounts.

Activity on Sat-Sat night continues to move east across the Great
Lakes, Ohio Valley, TN Valley, and central Appalachians.  The
parent circulation crosses Ontario to the Quebec border and
downstream 700 mb warm/moist convergence produces rain crossing
western and northern New York.  A weak frontal wave crossing the
mid Atlantic into the nearby coastal waters is predicted by the
ECMWF/GFS/NAM to produce rain Sat night in New York City across
Long Island, NY.

Manual QPF was derived by blending the 00Z GFS/ECMWF/NAM/UKMET.


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


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