Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Wed Mar 21 2018


Prelim Day 1 QPF Discussion
Valid Mar 22/0000 UTC thru Mar 23/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


Day 1

...Western U.S...

On Day 1, the models target California for the heaviest
precipitation with
several inches of liquid equivalent precipitation in the coastal
ranges of southern CA and along the spine of the CA Sierra Nevada
range. One distinct area is within the plume of deepest moisture
streaming onshore into southern CA into the southern CA Sierra
Nevada ranges. The integrated water vapor imagery is well above
normal and the slow movement of the max precipitable water values
in the streaming streaming onshore in the counties north of Los
Angeles combines with 850 mb convergence and 300 mb divergence to
produce areas of heavy rain, with ascent aided by windward
terrain.
The continuation o9f the high moisture and water vapor transport
into interior southern Ca leads to maxima along the
foothills and mountains of interior CA with heavy Sierra Nevada
snows, with several feet likely over the next day and a half.

A second area covers northern CA to the OR border as the northeast
Pacific upper low and trough drift towards
the coast...with a jet max streaming onshore and inland into the
ranges of ID and adjacent western MT.
The upper jet moving into NV Thu afternoon supports upper
divergence maxima streaming northeast from the CA Sierra across NV
into UT with a maxima near the northern Wasatch/Uintas of UT,
driving precipitation in windward terrain.

QPF was derived from a combination of the 06z high resolution
ensemble forecast mean along with the 12z runs of the wrf
ARW/nmmb/arw 2/nam/gfs. See the winter weather forecast amounts
and snow probabilities for heavy snow potential in the mountains
of CA

...Northeastern U.S...

The model indicate that as the low pressure off the coast moves
slowly northeastward overnight. the heavy snow event in the mid
Atlantic states crosses Long Island New York and southeast New
England before heading north into the Canadian maritimes.  The
850-700 mb front and convergence along it should persist as a
focus for heavier, banded precipitation, creating a swath of
maximum QPF along its path in Long Island to eastern MA this
evening and then tapering in Long Island as the front departs. 
An enhancement develops as easterly flow onshore into eastern MA
produce coastal convergence and aids synoptic scale lift tonight
in producing ascent and snow.

As the circulation continues northeast Thursday, the winds back
and then the onshore flow of mid level moisture and lift over the
850-700 mb front occurs up the coast in downeast Maine.
Soundings and surface observations in interior New York and New
England indicated very dry air in place,
which has provided for a sharp northern edge to the snow.  The dry
air counter acts the moisture fluxes to prevent heavy snow from
extending further inland into Maine. 

The models are not in total agreement yet, as the 12z nam was
heavier than other forecasts.  Given the WRF
ARW/ARW2/NMMB/Canadian regional GEM are drier, manual QPF cuts
back on NAM amounts in eastern Maine on Thu. WPC favored a QPF
blend of the HREF mean with the global model consensus.  See WPC
Heavy Snow graphics and discussion for potential amounts and
probabilities.

Petersen