Quantitative Precipitation Forecast
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FXUS04 KWBC 182058

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

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

Day 1

...Lower Great Lakes---central Appalachians--northern Mid Atlantic
into southern New England...

The strong compact closed low producing late season heavy snow
Wednesday afternoon over the Upper MS Valley region will be moving
quickly eastward tonight to the south of the lower Great Lakes and
across the central Appalachians and Mid Atlantic Thursday.  There
is fairly good model agreement on a narrow axis of moderate precip
totals in the .25-.50" range in the region comprising a well
defined comma head/deformation precip band to the north of the
closed low center and an isentropic lift maxima to the northeast
of the associated surface wave.  This will support the .25-.50"
precip band from portions of the lower Great Lakes---across the
northern Mid Atlantic/southern tier of NY state into southern New
England.  Late season accumulating snows possible on the northern
edge of this precip shield from northeast OH into northern
PA---southern tier of  NY state.  See the latest QPFHSD for
additional winter weather information across this region.

...Mid Atlantic---southeast---southern Appalachians---gulf coast
and Lower MS Valley...

The trailing frontal boundary from the surface wave pushing
eastward from the lower Great Lakes---northern Mid Atlantic into
southern New England 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 ahead of this front.  This along with the
fast movement of the front and generally poor dynamics will keep
any precip on the light side from the Mid Atlantic---into the
southeast---southern Appalachians---gulf coast into the Lower MS

...Oregon---California into the Great Basin...

The next in the recent series of strong closed low to affect the
western U.S. will be pushing inland into northern to central
California tonight and into the southern Great Basin Thursday.  PW
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 from
Oregon----California and into the Great Basin.

Days 2/3

...Great Basin...

An upper-level closed 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 BC Friday night with the
vort max at the base of the trough crossing the Olympic Peninsula
late in the night per both the 12Z GFS and ECMWF. 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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