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


Prelim Day 1 QPF Discussion
Valid Mar 15/1200 UTC thru Mar 16/1200 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
Mexico.

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
correction.


...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
correction.


...Northeast...

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
minimal.

Burke