Quantitative Precipitation Forecast
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FXUS04 KWBC 171952

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Tue Apr 17 2018

Prelim Day 1 QPF Discussion
Valid Apr 18/0000 UTC thru Apr 19/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Northern to Central Plains---Upper MS Valley toward the lower
Great Lakes...
The next in the series of mid-level shortwaves moves through the
Rockies-Great Basin into the northern/central high plains late
this afternoon and evening engulfing the region tonight before
moving into the Upper MS Valley on Wednesday.   Precipitation
concentrated along the meridionally-oriented inverted trough over
western Northern Plains early on will shift southeast early
Wednesday across the lower portions of the MO River Valley/Upper
MS Valley as the shortwave strengthens into a mid-level closed
low.  The enhancing area of overrunning spreads east-northeast
across the Upper MS Valley toward the lower Great Lakes Wednesday
afternoon within a region of low- to mid-level frontogenesis.
Snow will be the primary precipitation type across these
areas---with the best chance of late season heavy snows from
northeast NE...far southeast SD---central to northern Iowa---far
northwest IL into southern WI.  The precipitation forecast relied
upon the in house experimental bias corrected ensemble mean and
12z ECMWF to mitigate recent model spread.  See the latest
QPFHSD/Heavy snow and icing discussion for additional winter
weather details.

...Northern NY state into northern New England...
The deep layer cyclone over northern New England-southeast Canada
lifts slowly northeast into Atlantic Canada.  Scattered rain and
snow showers possible underneath the upper trough, with the best
chances of any measurable precip from northern NY state into
northern New England.

...Northern California into the Pacific Northwest...
The third in the series of strong systems to affect the western
U.S. will push from the northeast Pacific and toward Northern
California Wednesday afternoon.  The expected timing of this
system should keep the most organized precipitation offshore until
Wednesday night when a greater inland push is likely across
California into the Great Basin.  Mostly light precipitation
amounts are expected from the terrain of western OR and WA into
northern California.  The precipitation forecast here trended
towards a 12z GFS/12z ECMWF compromise.

Days 2/3

...California to Desert Southwest...

An upper-level trough with a closing low off the northern CA coast
Wednesday evening will deepen and shift southeast through the
Great Basin the turn east over the Four Corners by Friday evening.
Inflow from northern Mexico into this system will limit moisture
with PW around 0.25 inch. Light shower activity is expected over
CA/NV/UT/AZ Wednesday night through Thursday night with max QPF
around a quarter inch for preferred higher elevations in the
cyclonic flow. The slower 12Z ECMWF was preferred in WPC QPF with
some 12Z NAM and a little 12Z GFS (which seems too fast given the
closed low) included.

...South-Central Rockies and South-Central High Plains...

A southerly fetch ahead of the closing low moving east from CA
Thursday night will draw a mix of Pacific and western Gulf
moisture into the southern high plains. PW  of 0.5 to 1.25 inches
will wrap into the system, producing light to moderate qpf along
the Rockies in CO/NM late Thursday night, spreading into the
adjacent high plains (which are quite parched) Friday. Despite the
depth of the low, thicknesses remain great enough to limit snow to
high elevations. The slower 12Z ECMWF was preferred in WPC QPF
with some 12Z NAM and a little 12Z GFS (which seems too fast given
the closed low) included.

...Lower Rio Grande Valley...

Deep tropical moisture from the western Gulf flows across the
lower Rio Grande Thursday with terrain enhanced precip west of the
river both Days 2 and 3. Little precip is expected in south TX
from this system due to lack of lift.

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

A quick moving low pushes through the mid-CONUS ridge Wednesday
which weakens the system over the upper Midwest until it merges
with a lobe of the exiting low over the eastern Great Lakes
Thursday and redevelops over the northeast through Friday. Expect
diminishing QPF rates Thursday evening as the low moves from WI to
MI then increasing to moderate in places over NY state and New
England Thursday into Thursday evening as 0.75 inch PW air flows
into the system. Wrap around northwesterly flow will also promote
lake effect precip off the eastern Great Lakes. Good model
agreement with this low with a non-CMC blend used for WPC QPF that
favored the 12Z ECMWF which came in a little lighter than the 00Z



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