Quantitative Precipitation Forecast
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FXUS04 KWBC 192051

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
448 PM EDT Thu Apr 19 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 20/0000 UTC through Apr 23/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

The precipitation pattern forecast for tonight and Friday
generally resembles a compromise of the 12z GFS, 12z ECMWF, and
18z in-house experimental bias-corrected QPF.  This led to slight
increases from continuity mainly across the High Plains, WY/UT
border, and across southwest UT.

...Great Basin into the central to southern Rockies
The strong closed mid-level low will move steadily eastward
through the Great Basin this evening/overnight and into the
central to southern Rockies on Friday.  Precipitable water (PW)
values ahead of this closed low across the Great Basin will be
near or slightly above the mean.  This should be a limiting factor
for widespread heavy precipitation totals with model consensus of
.10-.25" areal average totals across the Great Basin and into the
southern to central Rockies.  Locally heavier totals likely in the
favored upslope regions of the Wasatch of UT and into the central
to southern Rockies from south central WY---through CO and into
north central AZ.  Heavy early spring snows likely through the
southern Wasatch and through the southern to central Rockies.  See
the latest QPFHSD/Heavy Snow and Icing Discussion for additional
winter weather details here.

...Southern to Central High Plains...
PW values expected to become more anomalous to the east of the
Rockies into the central to southern High Plains where low-level
south-southeasterly flow is expected to strengthen significantly
Friday ahead of the upstream closed low.  Increasing upper
difluence ahead of the Great Basin/Rockies closed low within the
increasingly anomalous PW axis will support increasing precip
coverage Friday through large portions of the central to southern
High Plains.  Areal average amounts of .25 to .50"+ with locally
heavier totals are possible where meridionally-oriented
precipitation bands train for a short period of time.

...Western to northern NY state into northern New England...
Surface low pressure moving to the south of New England this
afternoon will be pushing into the Canadian Maritimes tonight into
Friday with the western peripheries of the associated comma head
precipitation area possibly affecting far eastern Maine tonight.
In the wake of this low, deep layer cold cyclonic flow will
persist from the eastern Great Lakes into New England.  This
supports scattered snow showers for portions of western to
northern NY state and into northern New England.  Light to locally
moderate precipitation/snowfall totals are possible across these
areas.  See the latest QPFHSD/Heavy Snow and Icing Discussion for
additional winter weather details here.

Days 2/3

...Central-Southern Plains...Lower Mississippi Valley...Western
Tennessee Valley...
Compact, closed mid-upper level low will migrate slowly eastward
through the southern Plains and lower MS Valley during days 2-3
(Friday night-Sunday). This feature is essentially cut off from
the northern stream, i.e. within an otherwise broad northern
stream ridge, and as such given the Rex Block characteristics,
believe the eastern progression will be hindered somewhat. Exit
region upper level dynamics ahead of the southern stream trough
and associated upper jet streak approaching the trough base will
be fairly robust, albeit focused given the compact nature of the
low, owing to the stream curvature and degree of upper difluence.
Moreover, this will also favor a strong southerly ageostrophic
response at low levels, bolstering the deep-layer warm air
advection/theta-e advection, evidenced by the uptick in PW and
850-700 mb moisture flux/transport (albeit not overly anomalous).
At the same time, limiting factors for a widespread heavy
(potentially excessive) rainfall event will be the compact nature
of the upper low and the absence of upper jet forcing to the north
(which would enhance low-level frontogenesis). Deep-layer
instability within the warm conveyor belt (WCB) will be modest at
best, with MUCAPES peaking around 1000-1500 j/kg per the GFS/ECMWF
during the day on Sat-Sun.

In terms of the QPF, WPC stayed fairly close to a blend of the
ECMWF-GFS along with the NBM and WPC in-house bias corrected
ensemble QPF, which yielded a broad area of 1-1.5+ inches across
portions of the southern Plains in to the western TN Valley. In
terms of the day 3 excessive rainfall outlook, WPC opted to pull
the MARGINAL outlook farther south toward the TX-LA border coast,
while extending the SLIGHT a little farther west to northwest to
encompass the pivoting/back-building WCB across northeast TX and
southeast OK.


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

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