Quantitative Precipitation Forecast
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FXUS04 KWBC 150935

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
534 AM EDT Thu Mar 15 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid Mar 15/1200 UTC thru Mar 18/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Western U.S...

Flow was generally onshore into California, with steep lapse rates
supporting showers farther north along the Pacific Northwest
coast. Other rain and snow showers were sweeping gradually through
the intermountain region as broad cyclonic flow had taken up
residence in the West. The trough had carved out enough territory
that the sub-tropical jet was suppressed well south, so
precipitation was not especially heavy, but will remain fairly
widespread through Thursday. One stripe of some of the more
hydrologically significant precipitation will lift from Wyoming up
into Montana early in the day, associated with a well defined
shortwave. Heavier, upslope-enhanced precipitation will occur
through the daytime hours before winds switch to downslope as the
feature lifts north of the Canadian border and is caught wheeling
around the gyre / upper low over the Pacific Northwest coast.

The frontal zone trailing the Montana shortwave and marking the
leading edge of the larger scale trough as it undercuts Northern
Plains ridging, will promote orographically enhanced precipitation
generally under a half inch liquid, but spiking above a half inch
in some of the highest terrain in Utah / Colorado / northern New

Meanwhile, back over California the upper low coming ashore will
bring stronger gradients and some sense of a new surface frontal
structure. The combination of difluence east through north of the
low and enhanced lower level onshore flow should yield some
healthy precipitation totals, both rain and snow. Precipitable
water values were fairly low, under a half inch as of the 00z
soundings. PW is forecast to increase above a half inch, but given
the the cold air aloft / steep lapse rate environment, this event
could be rather showery with shallow convection, leading to more
variability in local precipitation amounts. This might perhaps
argue for lesser areal average QPF. Still, the high terrain and
westward facing terrain should be favored for steadier
precipitation, and guidance was in such strong agreement that our
blend resulted in the same or slightly heavier QPF in most areas.
Throughout the West, WPC QPF on Day 1 was a 40/30/30 blend of the
HREF blended mean / 00z GFS / latest in-house ensemble pre-bias

...Central Plains...

With a blocky pattern downstream toward the Atlantic, the trough
and Pacific front cresting over the Rockies will have a limited
playing field, resulting in a very concentrated Central Plains
storm beginning in the Day 1 period. Model agreement on the large
scale is pretty strong, and the QPF agreement is good aside from
the ECMWF which is locked more onto the terrain with its higher
amounts west of Cheyenne, WY. It would seem more likely that the
heaviest precipitation will fall along and inverted trough axis
that pushes out onto the Plains in tandem with the developing
surface low. The majority of guidance, especially the NCEP models
and hi-res suite place the bulls-eye over southeast WY / northwest
NE / southwest SD through Thursday night, with strong low level
convergence and moist easterly inflow to the north of the
developing cyclone. Upglide / warm advection should cause rain to
expand east and southeast through eastern NE/KS and much of
Missouri as well, and a jet axis with embedded shortwave farther
south will likely yield some spotty rainfall across Texas, and
more concentrated rain breaking out early Friday morning over the
lower MS Valley. Precip type will be an issue across the Central
Plains, but some ice and locally heavy snow are possible - see WPC
Heavy Snowfall Discussion for details.

Here, as was the case in the West, we used a 40/30/30 blend of the
HREF blended mean / 00z GFS / latest in-house ensemble pre-bias


Northwesterly flow will favor certain terrain and lake-enhanced
areas of New York and New England for additional 0.10 inch or
greater liquid equivalent on Thursday. Generally, the trough that
spawned the unsettled weather is spent and is no longer a threat
for heavy precipitation, but daytime heating and steep low level
lapse rates will support rain and snow showers all the way to the
coast and as far south as PA/NJ. We used a number of hi-res models
to derive the forecast, but areal average amounts should be

Days 2/3...

WPC QPF was based on a blend of the 00Z GFS/ECMWF with some
consideration of the 00Z NAM/UKMET.

...West Coast to the Rockies...

The center of a deep upper-level low spanning the entire west,
will drift east along the CA/OR border Friday into Saturday, and
slowly fill as the system occludes. Circulation around the
upper-level low with a series of vort lobes rounding the main low
will stream half inch PW in from the Pacific across central CA
Friday with 1.5 inch QPF maxima in the Sierra. A secondary maxima
around a half inch QPF will be farther south in the Santa
Barbara/Ventura area as a low level trough sets up. Rain rates do
not look excessive in southern CA. Low snow elevations under the
low will keep much of the precip snow in the Sierra Nevada. A
baroclinic zone associated with the surface low will allow
moderate precip across eastern NV into eastern ID Friday night
into Saturday with half inch average QPF before the zone weakens.

...Central/Northern High Plains to the Ohio/Tennessee Valleys and

Short wave energy ejected will shift east over southern CO
Thursday night and allow surface low development over KS to be in
a mature phase by Friday morning. This low will shift east into
the ridge over the Midwest and weaken through Saturday as it
tracks east to the OH Valley. 0.75 inch PW will wrap around the
low on a low-level flow of 45 to 50 knots Friday, weakening
through Friday night as the low weakens. Frontogenesis along the
north side of the low/baroclinic zone will enable locally heavy
precip with the heaviest swath of up to one inch stretching from
western IA to the SD/NE border per 00Z guidance consensus.

The GFS remains the heaviest with precip east of the central
Appalachians Saturday due to a stronger low. The 00Z ECMWF came in
with a similar coverage of precip in the central Mid-Atlantic, but
at a much lower magnitude. A compromise between these two was
drawn for Saturday.

...Gulf Coast...

Weakening low pressure pushing east across the OH Valley Saturday
with an associated front stalling near the Gulf Coast. 1.25 inch
PW along the Gulf Coast along with strengthening bulk shear and
MLCAPE over 1000 j/kg will allow any activity that develops to be
sustained. However, a general lack of forcing should limit
coverage with areal averages around a quarter inch.


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


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