Quantitative Precipitation Forecast
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FXUS04 KWBC 210718

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

Prelim Day 1 QPF Discussion
Valid Mar 21/1200 UTC thru Mar 22/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Eastern U.S...

On the synoptic scale it is becoming less common in the modern age
to reach Day 1 and have trouble finding a QPF consensus. Yet this
early spring Nor`easter remains an enigma. The forecast leading
into this storm has shown remarkable sensitivity to small scale
interaction among shortwaves embedded in the unusually energetic
yet low of amplitude - flow over the southern U.S. The overall
trend had been toward more of a major snowstorm with 1 to 2 inch
liquid equivalent amounts from Baltimore northeastward. The trends
from one forecast cycle to the next, however, have been so
inconsistent whether considering one model or all of them, that we
are largely left with a manual forecast effort, relying on
conceptual models more than any specific piece of guidance
(especially where QPF is concerned).

The model mass fields seem to have finally settled down, with a
nicely depicted track of a low deepening off the Mid-Atlantic
coast, down to about 990 mb as it then slowly lifts northeast. The
850-700 mb front / inverted trough structure north of the low
track would appear to have set up as was anticipated from the
MD/PA border region up through NYC / Long Island and beyond. This
should become a focus for heavier, banded precipitation, creating
a swath of maximum QPF in the vicinity of this region. Farther
south the easterly onshore flow and deep layer ascent will be less
long lived, but still substantial in the early hours of the day.

Watching animations of the model fields, the focus and pivot in
the 850-700 mb layer should occur near NYC / Long Island, acting
to maximize snow and liquid equivalent amounts there. Most
guidance actually does agree to this concept, all but the 00z
ECMWF whose QPF axis was inexplicably offshore despite little
change to the mass field solution.

Working initially with the trends in NCEP guidance, and supported
a fair amount by the UKMET, WPC favored a QPF blend of the HREF
mean and a version of our in-house ensemble which gives
significant weight to the hi-res model suite. This yielded a
similar precip pattern, but with heavier amounts, up to a half
inch heavier liquid amount over southern NY and southern New
England. This may end up being a bit heavy handed, especially as
the low is not bombing out. Deep layer height falls are somewhat
delayed in coming into sync with the 850-700 mb frontal zone, and
perhaps this will hamper precipitation intensity. We may tone it
down slightly for the final issuance, but do feel confident in
placement just based on synoptic reasoning...with a forecast that
still supports heavy snowfall along the east coast - see WPC Heavy
Snow Discussion.

...Western U.S...

On Day 1 into Day 2, the closed low well offshore of California
phases with and is kicked by the northern stream flow, creating a
full latitude trough with a pronounced southern stream component
at its base. The pattern as a whole makes some progress into the
Western U.S. on Wednesday, bringing widespread rain/snow as far
inland as the northern Rockies down to the lower Great Basin /
lower Colorado River basin. By far, however, this event will be
known as a California storm, with heavy Sierra snows and heavy
rainfall along the coast. The inherited WPC forecast was ratcheted
up about as high was was necessary, especially as some of the
heavy coastal rain into southern California will spread over into
the Day 2 period. Therefore, we started with the previous
forecast, and looked to just make minor adjustments to keep up
with trends. For one, we trimmed back on the coverage of quarter
inch amounts in the rain shadow areas of eastern WA/OR, based on
hi-res model consensus, NDFD, and the GEFS Re-forecast data based
on analog events. Secondly, there has been a continued trend for
the heaviest Day 1 rainfall (through 12z Thursday) to back up
northward through San Luis Obispo County and Monterey County.
There will certainly be heavy rainfall reaching down into Ventura
County and possibly the L.A. basin, but overall we nudged the Day
1 centroid farther north up the coast. This will mean that the
risk of flash flooding and debris flows associated with some of
the largest fire burn scars will certainly extend into the Day 2
period as well, as the atmospheric river then makes more eastward

QPF was derived from a high percentage previous WPC, combined with
influence from the 00z NAM CONUS Nest and our in-house ensemble.
The WRF-ARW did not follow the trend, and was more aggressive with
heavy rain extending into Los Angeles suburbs. This is its
tendency nationwide (south and east of consensus), but it is
sometimes right. The Excessive Rainfall Outlook will remain
generous with its coverage given this possibility.



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