Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
244 AM EDT Sat Mar 17 2018


Prelim Day 1 QPF Discussion
Valid Mar 17/1200 UTC thru Mar 18/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Overview...

The large scale features cold air and northwest flow locked in
place over the Northeast / Mid-Atlantic. Mean large scale
troughing is anchored along the West Coast. In between, the flow
is semi-progressive with occasional lee-of-the-Rockies
cyclogenesis. The cyclone that formed Friday will shear as it
meets the blocked and confluent flow along the East Coast. Still,
it will squeeze out a light wintry mix in parts of the mountains
and Piedmont, and a swath of rain farther south, including some
enhancement near a warm front in North Carolina and southern
Virginia. The quasi-stationary front trailing this upper air
system will linger over north Texas, and the gradual approach of
renewed height falls and jet energy from the West should
contribute to a few stronger thunderstorms breaking out near the
Texas front and dryline this afternoon.


...Western U.S...

Another spoke will rotate around the base of the mean Western U.S.
trough, swinging into Arizona by Sunday morning. A separate
shortwave will lift into Idaho and western Montana early in the
day, and very cold upper air temperatures / cyclonic flow will
remain in place over Oregon and Washington. The weather system as
a whole remains separated from any substantial deep layer
moisture. Locally respectable height falls, terrain enhancement,
and steep lapse rates will, however, yield fairly wide coverage of
measurable rain/snow west of the continental divide. The locations
particularly favored for greater amounts, a half inch liquid or
more, appear to be central Idaho near the lifting shortwave, and
northern Arizona ahead of the shortwave swinging through the base.
Still, we tried to err on the conservative side given PW values
near or below climatology most areas. WPC QPF leaned toward the
00z WRF-NMMB...HREF blended mean...and held onto 40 percent
continuity from the previous forecast.


...Eastern / Southern U.S...

Precipitation should not be too well focused on the large scale,
but there are some mesoscale QPF signals from the models, showing
up in the upslope enhanced parts of the Virginias, along the warm
front in NC/VA, and associated with the combination warm advection
and surface-based deep convection event expected in parts of
Texas. The SREF 24-hour QPF probabilities have done a little
better job depicting some of these details than the GEFS and ECMWF
ensemble have in the early spring season, at least on Day 1. We
used these SREF probabilities as a guide, and this pointed us
toward a QPF blend using the HREF and NAM CONUS Nest among other
things east of the Tennessee Valley and along the Gulf Coast. For
Texas we gave the WRF-NMMB, WRF-ARW, and GFS more weight,
expecting that thunderstorms will form initially ahead of the
slight dryline bulge in the Hill Country and then expand / evolve
northeast from there.

Burke