Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Thu Apr 18 2019

Valid 00Z Fri Apr 19 2019 - 00Z Mon Apr 22 2019

...The West...
Days 2-3...

A shortwave dropping slowly along the WA/OR coast Saturday will
drift onshore California and into the Great Basin on Sunday.
Associated with this feature, increasing diffluence within the LFQ
of an accompanying upper jet, and moist advection rotating
cyclonically into the West will produce lift and increasing snow
chances in the high terrain. Snow levels are expected to be quite
high, 7000 to 9000 ft, falling slightly from NW to SE on Sunday.
However, the greatest accumulation will remain only in the highest
terrain. The best overlap of moisture and synoptic ascent is
expected in the Northern Rockies, as well as ranges of NW WY, ID,
and SW MT where WPC probabilities are moderate for 8 inches.
Otherwise, WPC probabilities suggest a good chance for 4 inches
along the Front Range of CO, the Uintas of UT, and some of the
mountains of the Great Basin in NV.


...Southern Appalachians...
Day 2...

Anomalously strong upper low characterized by 500mb heights of 2-3
standard deviations below the climo mean will lift slowly
northward Saturday before pivoting slowly away to the northeast
Sunday. On the back side of this system, cold air will be dragged
southward, and a bubble of sub-0C 850mb temps is progged to rotate
into the Southern Appalachians as evidenced by most available
guidance. At the same time, mid-level deformation and the leading
edge of a maturing TROWAL/theta-e gradient will push across the
area coincident with increasing W/NW upslope flow. Forcing looks
to be modest much of the period, but may briefly intensify at
times enough to overcome marginal thermal profiles. With snow
levels progged to remain around 4000 ft or higher, only the
highest terrain on the windward side of the mountains in NC/TN
should see any snow accumulation. Although WPC probabilities for 4
inches are zero, they are moderate for 2 inches, and a few inches
of snow here in late April is noteworthy enough for a brief
mention.


The probability of significant freezing rain is less than 10
percent.

Weiss