Quantitative Precipitation Forecast
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FOUS11 KWBC 062004

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Mon Apr 06 2020

Valid 00Z Tue Apr 07 2020 - 00Z Fri Apr 10 2020

...Southwest U.S....
Days 1-3...

A vertically stacked and slow moving upper low will pinwheel
slowly down the CA coast just offshore before ejecting eastward
near Baja Wednesday evening, and then lift slowly northeastward
towards the Four Corners Thursday /D3/. The GFS remains a fast
outlier and the forecast is comprised preferentially of the
non-NCEP members which show a slower evolution.

As the low pivots southward, persistent and anomalous moisture
will drive on confluent 700-500mb flow into CA and the Southwest
with a tropical moisture tap driving heavy precipitation from CA,
through the Great Basin, and into the Four Corners. Snow levels
will generally be around 5000 ft, and exceptionally heavy snow is
likely in the highest terrain of the Sierra, Transverse ranges of
Southern CA, and into the higher peaks around Las Vegas. WPC
probabilities are high for 12 inches on D1 in the San Bernadinos
and on Mt Charleston, with more expansive risks for 12 inches to
include the Sierra on D2. By D3 as the low begins to pivot
northeast, the intensity of snow is likely to begin to ease and
shift east, with moderate probabilities for 12 inches in the
southern Wasatch.

Day 3...

Low pressure moving through Ontario will drape a warm front into
the Mid-Atlantic Thursday evening. WAA precipitation will
overspread the northeast associated with this warm front, and a
secondary low is likely to develop east of Massachusetts as this
warm front lifts northward. At the same time, height falls
associated with a large and deep upper low becoming negatively
tilted will combine with intensifying jet level diffluence on the
LFQ of a 170+ kt jet streak to drive robust intense ascent. This
synoptic forcing on top of a sharp baroclinic zone will likely
lead to rapid intensification of the surface low as it moves into
the Gulf of Maine. Cold surface high pressure slowly exiting to
the east will attempt to wedge back into New England during this
time, and as the low deepens, isallobaric drainage should keep
cold air locked in place across much of NH and ME. There is still
a large latitudinal spread in the placement of the surface low and
its associated warm air lifting northward, but there is increasing
potential for heavy snow from eastern VT through northern NH and
central/northern ME. WPC probabilities on D3 are moderate for 4
inches in the White Mountains of NH and ME, with lower
probabilities further to the northeast.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.



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