Quantitative Precipitation Forecast
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FXUS04 KWBC 160942

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

Final 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 08z 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. While mid/upper level forcing will persist through the
day, would appear the overall dynamics become a bit less
impressive with time...as the shortwave responsible for the
increasingly negative tilt to the trough weakens along with a
weakening upper jet. Also, anticipate CAPE will be harder and to
come by as this area of rain moves northeast, with it taking on an
increasingly stratiform nature with time. Some limited MUCAPE
should however allow for continued embedded heavier convective
cores within the rain shield.

In general, would anticipate these factors to result in decreasing
rain rates after 12z, although localized rates peaking in the
0.5"-0.75" in an hour range...and up to around 1.5" in any three
hour period...will remain possible given the impressive PWATs and
moisture transport in place. It appears likely that as the system
occludes we actually see a focused, and slightly more persistent,
850 mb moisture transport axis across portions of southeast NY
into adjacent MA/CT. This should result in a bit longer duration
of the moderate rainfall rates...resulting in storm total amounts
at least locally getting into the 2-3" range. These 1-3" amounts
should stretch from southeast NY across southern New England and
into eastern NH/ME...with the potential for a few locally heavier
amounts, especially where orographic impacts may be maximized in
the strong southerly flow.

Will continue to carry a Marginal risk for these areas. Think
rainfall rates will generally not be high enough to exceed 1 or 3
hour FFG. However, rainfall of the magnitude forecast may still be
enough to cause some urban and small stream flooding concerns.

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.

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. WPC QPF
blended the 12z ECMWF and the 0z HREF members.

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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