Quantitative Precipitation Forecast
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FXUS04 KWBC 242030

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Tue Apr 24 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 25/0000 UTC thru Apr 28/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern U.S...

An active closed low steadily/gradually ejects from TN/KY to over
the s-central Mid-Atlantic today into Wednesday before shifting
offshore under increasing influence from nrn stream trough energy
digging into the Great Lakes. GOES-16 and blended TPW loop imagery
leads well into a model guidance composite that continues to
depict a plume of deep layered moisture with low latitude
connection feeding into the associated surface low/system with
lead thetae advection especially into terrain and coastal areas
along the warm front and inverted coastal trough. Expect
widespread enhanced rains/convection to spread from the Ohio
Valley/Appalachians and srn Mid-Atlantic northward across the nrn
Mid-Atlantic and Northeast with system progression. The linearity
of the forecast mass field evolution lends a WPC day1 qpf forecast
derivation mainly from a blend heavy on continuity but
incorporating latest model and radar rainfall/convective trends
that seem well represented from a WPC in-house bias corrected qpf,
HRRR and National Blend of Models. A Marginal Risk of excessive
rainfall remains centered over the central

...Central U.S...

The next in a series of fairly vigorous mid level lows will remain
well defined as it drops south to southeastward through the Plains
today to the lee of an amplifying wrn u.s. mid-upper level ridge.
Given relatively scant moisture in the wake of the lead system,
expect most precipitation will be slightly post-frontal and rooted
in strong mid level ascent from South Dakota down through Kansas,
and eventually parts of TX/OK and mid-MS Valley. There may be
quite a bit of mesoscale influence to the precipitation pattern,
given the steep lapse rates, marginal low level moisture, and
interplay between the synoptics and terrain influences. Model
guidance remains in decent overall agreement for such an
environment and WPC took a consensus/continuity approach, albeit
with modest areal coverage.

Days 2/3...

...Pacific Northwest...

Low pressure off the OR/CA border shifts east to that border
Friday which wraps Pacific moisture across far northern CA, up the
Willamette Valley, and off the western WA shore. The GFS continues
to have higher PW (around one inch) than the ECMWF (around 0.75
inch) which results in more QPF for the GFS. A general model blend
gave a Day 3 (00Z Fri-00Z Sat) areal average QPF around 0.75 inch.

...Central and Southern Great Plains to the Southeast...

A pair of shortwave troughs will swing east from the central
plains and southeast Wednesday night through Friday. An area of
one inch PW ahead of the first trough will bring an areal average
of an inch from roughly Memphis to Atlanta on Day 2 (00Z Thu-00Z
Fri). Uncertainty remains which the track of the lead trough which
is discussed in the section below.

PW is a little less for the second trough (around 0.75 inch) as it
is associated with more continental air than Gulf moisture. QPF
for Day 3 (00Z Fri-00Z Sat) is generally a quarter to half inch
from KS/OK...the Ozarks...to southern AL.

WPC QPF generally followed a general model blend including the 12Z
GEFS through both days.

...Mid-Atlantic to the Northeast...

Low pressure shifts across the northeastern CONUS Wednesday night
through Thursday. Gulf stream moisture flowing into this system
brings 1 inch PW to the New England coast which is 2 standard
deviations above normal. A pivot from an east direction from the
eastern Great Lakes Wednesday evening to northeast across interior
Maine Wednesday night allows a slightly longer period of heavy
rainfall for coastal Maine. A marginal risk for excessive rainfall
for the Day 2 ERO (12Z Wed-12Z Thu) was maintained which a slight
draw northeast per 12Z guidance consensus.

Great uncertainty persists with the Day 3 QPF forecast (00Z
Fri-00Z Sat) with NCEP guidance (NAM/GFS) much farther inland with
the shortwave trough lifting from the southeast while non-NCEP
(ECMWF/CMC/UKMET) are much farther east along the Mid-Atlantic
coast. Some narrowing of the difference was noted in the 12Z
guidance, so a general model blend with usage of the 12Z GEFS was
taken for the Day 3 QPF.


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


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