Quantitative Precipitation Forecast
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FXUS04 KWBC 160917

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
516 AM EDT Mon Apr 16 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 16/1200 UTC thru Apr 19/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Mid Atlantic into the Northeast and Great Lakes...

As of 06z a line of heavy showers and embedded thunderstorms
stretches from southern PA into eastern NC...with this line
continuing to progress off to the northeast through the day
Monday. This remains a very dynamic system with the negatively
tilted mid/upper level trough supporting strong upper level
divergence. Instability is marginal, but anticipate approximately
100-500 J/KG of MUCAPE to be present from the Mid Atlantic into
southern and eastern New England. This should be enough to
continue supporting embedded heavier convective cores within the
broader rain shield. While the line will remain progressive off to
the northeast limiting any extreme rainfall amounts, will continue
to see some short lived south to north training of individual
cells embedded within the line. PWATs and moisture transport both
remain anomalously strong...and this short term heavy rainfall
rates remain likely.

Model agreement was very good with the evolution of this more
convective precipitation as it moves northeast. Thus WPC mainly
just blended the 0z HREF members along with the HRRR to derive QPF
totals through day 1. The mid/upper forcing becomes a bit less
pronounced by later this morning into this afternoon as the mid
level trough begins to lose some of its negative tilt and the
upper jet weakens. Thus would anticipate we will see the overall
intensity and width of the convective axis weaken some as it
progresses Northeast through the day. Still looking at a
widespread 1-2" of rain from the eastern Mid Atlantic into
southern and central New England...with locally heavier totals
into the 2-3" range. Best chance of these localized higher totals
appear to be from portions of northern NJ into southeast NY into
southern and central New England, where we are seeing some WAA
showers develop out ahead of the main convective axis...and also
where orographic impacts are maximized in the strong southerly
flow. Rainfall rates should locally peak around 0.5"-0.75" in an
hour...with up to around 1.5" in any three hour period.

Further northwest under the closed mid level low will continue to
see precipitation into the Great Lakes region. Did increase
amounts some here over our previous forecast, with the closed low
continuing to be slower to exit. Again followed close to a blend
of the 0z HREF members here.


A trough will continue moving east into the Great Basin by Monday
night. This will continue to support precipitation over much of
the west, with as much as 1-3" over western WA, around 1" in the
northern Rockies and up to 1" in the central Sierras.

Days 2/3

...West Coast/Intermountain West...

Embedded energy within an approaching long wave through will
spread showers to portions of the Pacific Northwest, Great Basin
and Intermountain West.
Strong onshore flow in the wake of the northern shortwave will
support westerly flow of 30kts to 45ks at 85-7H at or near
orthogonal to the Cascade range and though moisture is fairly
limited WPC forecast QPF are still in the range of 1.0" through
12z Thursday in Washington and .5" in Oregon. Areas of higher
terrain will have some orographic enhancement and will likely have
periods of heavy snow. Please refer to the latest suite of WPC
Winter Weather products for more information.

Deterministic models and the ensemble means are fairly clustered
for the evolution of the West Coast trough, therefore a general
model blend was preferred.

...Northern and Central Plains/Upper MS Valley...

Rapid surface cyclogenesis and rapid precipitation development is
forecast across Nebraska to Iowa Tuesday night into Wednesday,
eventually occluding late in the day and past 00z Thursday. Gulf
moisture feeding into the central low pressure will wrap
precipitation into the cold sector. Snow, possibly a wintry mix,
will spread across the Northern High Plains and Upper Mississippi
Valley.  Northeast Nebraska will likely have the highest QPF/heavy
snow as this is where model consensus shows the strongest
low-level moisture convergence and upper level divergences aligns.

The 00Z GFS persisted in being the faster solution with this
feature; however it did trend slower a bit slower for day 3.
Largely a general model blend worked well for this region with
minimal weighting of the operational GFS. At this time, it appears
a portion of the QPF will be wintry in nature (snow/ice/mix) and
likely reduce any flooding threat so no excessive rainfall area is

...Great Lakes/Northeast...

During these periods, a large closed low will continue to lift
northeast up the Eastern Seaboard with widespread showers and
thunderstorms accompanying the front as it moves through the
Mid-Atlantic and Northeast. The southerly warm conveyor belt will
shift out of the CONUS late Tuesday though the vast nature of the
upper low will support internal vorticity centers returning across
the Great Lakes into northern NY/New England Tues into Wed
increasing forcing/cooling aloft to activate some NW to SE
oriented Lake Effect Belts as well as some weak comma head return
moisture in N NH/ME by later in the Day Wed.  Still this QPF is
not substantial but please refer to WPC Winter Weather products
for more information.


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


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