Quantitative Precipitation Forecast
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FXUS04 KWBC 130653

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Tue Mar 13 2018

Prelim Day 1 QPF Discussion
Valid Mar 13/1200 UTC thru Mar 14/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...


The heavy snow event across New England will continue through the
day Tuesday as an intensifying area of low pressure moves
northward offshore. The setup continues to favor a broad area of
moderate snowfall with the likelihood of a heavy snow band given
the impressive frontogenesis forecast from eastern MA into
portions of NH and ME. Some model differences were still noted.
The 0z NAM and the 0z HREF were stronger and further west with the
low/mid level low...resulting in heavier QPF totals both across
eastern and western New England. Meanwhile the majority of the
global models (GFS/ECMWF/UKMET) and the GEM regional were slightly
weaker/east...resulting in a further east QPF distribution. At
this point the feeling was that both camps had some aspects of the
forecast more correct. Thus WPC QPF was close to a 50/50
compromise of the latest global model mean and the HREF mean. This
does result in an increase in amounts across much of New
England...but not as high as most of the 0z high res runs would
suggest. Though the day 1 period (06z Tue- 12z Wed) this brings
the 1.5" QPF line into coastal MA/NH/ME with some 2"+ amounts
possible over far southeast MA and Cape Cod...while also expanding
the 1" line further west into central New England.

Another well defined shortwave will dive southeast on the southern
periphery of the closed northeast mid/upper level low. This should
result in increased snow shower coverage across portions of the
Great lakes into the OH valley and the Appalachians today into
tonight. Overall kept pretty good continuity here form our
previous forecast...with the high res guidance more leaned on than
the global solutions.


Strong mid/upper level forcing will overspread the west coast
today. Shortwave energy within the longwave trough...and strong
upper level divergence in the left exit region of an approaching
upper jet...will provide the synoptic ascent for precipitation
through the period. PWATs will generally be running around the
climatological 90th percentile and integrated moisture transport
is closer to the 97th percentile. Also given the falling
heights...would anticipate the development of weak instability
through the day.

The combination of the strong forcing, moderately anomalous
moisture/IVT and weak instability supports the idea of pretty good
rainfall rates today over CA...primarily over the Sierras and the
western slopes of the terrain where orographic impacts will be
maximized. Rainfall rates of 0.25"-0.5" within an hour are
expected. Given the weak instability...the high res guidance
supports the idea of convective elements embedded within the
broader rain shield...with localized rainfall rates potentially
exceeding 1" in an hour in the eastern valley and western slopes
of the Sierras. These enhanced rates should not last all that
long...as the enhanced moisture transport axis is rather
progressive. Behind it the cooling temperature aloft and
persistent upslope flow continues to support showers into the
terrain...however dropping PWATs and forcing by this time should
result in decreasing rates.

Thus overall appears like a longer duration of steady showers
through the day 1 period...with a window for a shorter period of
more intense rates. QPF totals were increased some from our
previous forecast with 1-3" expected where orographic enhancement
is maximized west of the Sierras...with 0.25"-1" across the
Valleys and around 1" across the favored coastal terrain. WPC QPF
used a multi model blend...weighted towards the 0z HREF mean.



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