Quantitative Precipitation Forecast
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FXUS04 KWBC 182105

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
501 PM EDT Wed Apr 18 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 19/0000 UTC through Apr 22/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

The QPF pattern composed resembles a compromise of the 12z GFS/12z
ECMWF guidance, which matches well with the 12z experimental
in-house bias corrected QPF and WPC continuity.

...Lower Great Lakes---central Appalachians--northern Mid Atlantic
into southern New England...
A strong compact closed low producing late season heavy snow
Wednesday afternoon over IA/MN/WI will move quickly eastward to
the south of the Great Lakes and across the central Appalachians
and Mid Atlantic Thursday.  There is agreement on a narrow axis of
moderate precip totals within its low- to mid-level frontogenetic
band and within a well-defined comma head/deformation axis to the
north of the closed low center, though spread remains concerning
its latitude.  See the latest QPFHSD/Heavy Snow and Icing
Discussion for more winter weather detail here.

...Mid Atlantic---southeast---southern Appalachians---gulf coast
and Lower MS Valley...
A trailing frontal boundary is not expected to have much in the
way of precipitation along and ahead of it.  The inflow off the
Gulf of Mexico ahead of this front is not expected to have a
significant southerly component---limiting the potential for above
average pw values being entrained.  Along with the fast movement
of the front and generally poor dynamics, precipitation is
expected to be on the light side from the Mid Atlantic into the
Southeast, southern Appalachians, Gulf coast, and the Lower MS

...Oregon---California into the Great Basin...
The next in the series of strong closed lows to affect the western
U.S. will push inland into northern to central California tonight
and into the southern Great Basin Thursday.  Precipitable water
values are not expected to be anomalous---mostly at or below
seasonal norms.  This will limit precipitation potential with this
system---with mostly light to moderate totals expected across
Oregon, California, and into the Great Basin near the best 850-650
hPa frontogenesis at the mid-level cyclone`s edge.

Days 2/3

...Great Basin...
An upper-level closed low pushes east from the Great Basin
Thursday evening to the OK panhandle (per guidance consensus)
Friday night. Desert inflow with PW around 0.25 inch will limit
precip over the Great Basin to light to locally moderate on
preferred ridges through Thursday night. Guidance consensus was
used for this portion of WPC QPF.

...South/Central Rockies and Texas to South Dakota...
The upper-level low will draw moisture (PW 0.5 to 1.5 inch) from
both the Pacific and the Gulf of Mexico across west Texas and up
the High Plains into WY. Snow will be limited to the higher
elevations of CO/NM Rockies late Thursday night into Friday night
as the low passes.

Surface low pressure passes the TX panhandle Friday night,
shifting the focus to the southern into central Great Plains with
a bulls eye from north TX into KS. This is merely one standard
deviation above normal, but the slow nature of the closed upper
low will result in heavy rainfall over this bull eyes of 2 areal
averaged inches Saturday.

The 12Z GFS remained slightly more progressive than the well
agreed 12Z ECMWF/NAM/UKMET which were preferred for WPC QPF.

...Lower Rio Grande Valley...
Showers and thunderstorms will develop on the Mexican side of the
Lower Rio Grande Valley where moisture convergence sets up along
the Mexican Plateau Thursday night into Friday. Flow from the Gulf
will pool along the terrain, aiding in persistent convection
through Friday. Little precipitation is expected in south TX from
this system due to lack of lift and instability.

Surface low pressure shifts north across Nova Scotia Thursday
night. The western edge of moderate precip around the low center
looks to merely clip the eastern end of Maine Thursday night with
light wrap around precip across northern New England and the
Adirondacks through Friday per guidance consensus.

...Pacific Northwest...
A northern stream trough shifts east into British Columbia Friday
night with the vorticity maximum at the base of the trough
crossing the Olympic Peninsula late in the night per both the 12Z
GFS and ECMWF. The 0.75 inch PW ahead of the trough axis will
allow moderate rainfall to the western slopes of the Olympics and
northern WA Cascades late Friday night into Saturday morning. The
progressive nature of the trough looks to limit the QPF max to
0.75 inch on areal average.


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

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