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Quantitative Precipitation Forecast (QPF) Discussion
(Caution: Version displayed is not the latest version. - Issued 0941Z Apr 24, 2018)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

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

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

Day 1

...Eastern U.S...

The slow moving, stacked cyclone affecting the eastern United
States will bring a wet, showery day to areas from Ohio to eastern
Tennessee and toward North Carolina and the Mid-Atlantic states.
The tail end of the frontal zone will also catch south Florida
with rainfall. Rain will spread northward into New York and
southern New England Tuesday night. The model biases with this
system have been very consistent. The ECWMF is too confined with
its QPF output, especially around the northern periphery. The
hi-res models are generally over-estimating the precipitation
magnitude, and also failing to pick up on diurnally driven swaths
of convection that occur in the narrow instability axis near /
south of the warm front. The GFS, while not having an ideal deep
layer solution for the mass fields, at least does a better job of
spreading synoptically forced precipitation downstream to the east
and north.

All of these biases are readily apparent again this morning.
Perhaps the good news is that the hi-res models were less bullish
with amounts today, and they are more unanimous in keeping the
largest swath of deep convection just offshore or grazing the
Outer Banks.  So sources like the WRF-ARW and HREF Mean looked
more usable. WPC leaned toward a version of our in-house ensemble
which incorporates the hi-res models, melding their output with
the global model consensus - which was advantageous given what was
described above about the complimentary GFS/ECMWF strengths and
weaknesses for this system. Our manually derived QPF ends up
looking also very similar to the National Blend of Models, which
has been performing well the past couple of days.

A Marginal Risk of excessive rainfall is maintained over parts of
the eastern U.S. Any truly heavy rainfall will probably occur on
small scales, with no real stand-out signal on the large scale.
Our primary concern is with parts of North Carolina where
antecedent rain this morning will lower flash flood guidance
values before late afternoon convection ramps up. And
upslope-enhnaced rainfall over Virginia. The big cities / I-95
corridor is more of a question mark, being located north of the
surface low and occlusion. The literal FFG numbers requiring 1.50
to 2.0 inches in 3 hours will have a low chance of being exceeded,
but there may be enough local convective enhancement, especially
Tuesday night as the upper low approaches, that the combined
effects of daytime and nighttime rainfall could eventually lead to
some isolated issues with high water.

...Central U.S...

A fairly vigorous mid level low will remain well defined as it
drops south to southeastward through the Plains / High Plains
today. Given relatively scant moisture, 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.
Some additional convection may act as surface-based during peak
heating over Colorado and west Texas. 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. That being said, however,
the model guidance was in unusually good agreement for such an
environment, and WPC took a consensus approach while leaning also
on the SREF 6-hour QPF probabilities and deterministic output from
the WRF-ARW and WRF-ARW2.

Days 2/3...

...Northern Mid-Atlantic to the Northeast/Great Lakes...

A negatively-tilted shortwave trough, associated with the remnants
of the upper low presently centered over the Tennessee valley, is
forecast to continue lifting north along the Mid-Atlantic to the
Northeast coast Wed into early Thu.  There it is expected to phase
with a northern stream trough pivoting across the Great Lakes.  A
newly formed mid-upper level low closing off over the Great Lakes
is expected to move into the Northeast on Thu, continuing into the
Canadian Maritimes by early Fri.  Surface low pressure organizing
along the Mid-Atlantic coast on Wed is forecast to track north
into New England Thu morning.  Strong, moist southerly flow ahead
of the low, along with favorable forcing aloft and marginal
instability, is expected to support some moderate to heavy amounts
across New England, particularly along coastal Maine.  These
resulting amounts, along with snow and ice melt, may result in
local runoff concerns.  Therefore, a 'marginal' risk for excessive
was introduced to the Day 2 Excessive Rainfall Outlook. 
Meanwhile, a developing deformation zone is expected to support
light to moderate precipitation shifting east from the Great Lakes
into the Northeast.  By late Thu, with the low beginning to lift
out to the northeast, expect precipitation to begin to wane -
limited to some light precipitation wrapping around the backside
of the departing low. 

Guidance have been moving into better agreement with the evolution
and timing of this system.  While differences remain, WPC QPF
followed a compromise solution, represented by a blend of the
ECMWF and NAM.  

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

A positively-tilted shortwave trough/compact mid-upper level low
is expected to pivot from the central plains to the lower
Mississippi valley Wed into early Thu.  Deepening moisture along a
low-mid level wave/frontal band, interacting with the favorable
forcing aloft, is expected to support areas of precipitation
moving southeast from the plains into the lower Mississippi valley
late Wed into early Thu.  This shortwave is expected to swing
progressively from the lower Mississippi valley into the Southeast
on Thu, before lifting further north into the Mid-Atlantic ahead
of an upstream trough digging across the Mississippi valley on
Fri.  This is expected support light to moderate amounts across
the region.

WPC QPF generally followed a blend of the GFS and ECMWF through
the period.


Graphics available at