Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
228 PM EDT Fri Apr 12 2019

Valid 00Z Sat Apr 13 2019 - 00Z Tue Apr 16 2019

...Southern Rockies, Southern Plains into the Great Lakes...
Days 1-2...

Deepening positively tilted trough across the Southwest will tilt
negatively Saturday over northern Texas and may close off briefly
before opening and lifting northeast towards the OH VLY Sunday. An
impressive Pacific Jet of 110+ kt digging around the trough will
eventually transition into a coupled jet structure as northern jet
energy intensifies as the upper low closes. This enhanced
diffluence will produce robust synoptic ascent Saturday from the
Southern Rockies into the Southern Plains, which will then lift
towards the Great Lakes Sunday. The combination of this intense
diffluence and height falls will spawn cyclogenesis over Southern
Texas which will then lift northeast into the OH VLY and off into
Canada beyond day 2. WAA ahead of this low will spread moisture
northward, and although temperatures across most of the threat
area are marginal, snowfall is likely NW of the low.

In the Southern Rockies on Saturday, heavy snow is likely as
southerly 700mb flow impinges into the terrain coincident with
synoptic lift in the diffluent region of the developing coupled
jet. Snow levels will remain elevated, 4000-5000 ft or more, but
significant precipitation will produce heavy snow above these
levels, and WPC probabilities are moderate for 12 inches in the
Sangre De Cristos, with lesser amounts across the remaining
terrain of CO and NM likely, including as far south as the
Sacramentos.

A very difficult forecast challenge is for the TX Panhandle
Saturday. As the low begins to deepen, a significant overlap of
intense 850-600mb fgen and 700mb deformation will develop.
Temperatures initially will be warm enough for all rain, but as
the column cools on northern flow behind the low, the intense
synoptic and mesoscale forcing will likely begin to change the
precipitation over to snow. The speed at which this happens, and
the duration of snow, is still very uncertain as partial
thicknesses are marginal, suggesting it will require dynamic
cooling and intense precip rates to produce snow accumulation.
HREF probabilities for 1"/hr snowfall are above 50%, correlating
well with negative theta-e lapse rates further supporting these
intense snow rates. However, mid-April insolation and antecedent
rain will certainly be detrimental for accumulating snow. Despite
that, guidance has trended upward with its snow, especially from
AMA/LUB and points east towards the OK border where a modest
TROWAL will further support lift, and it appears there will be
enough cooling to allow snow to accumulate in very heavy rates.
While guidance is likely overdoing accumulations due to mixing and
other negative factors, a few inches of accumulation is becoming
more likely in the vicinity of AMA/LUB, and although WPC
probabilities for 4 inches are less than 30 percent, they are high
for 2 inches in a few locations.

On Sunday, as the low continues to deepen and lift northeast, the
robust ascent will persist both due to jet level diffluence and
mesoscale deformation on the NW side of this system. A stripe of
snow is likely from far NE MO northeast into lower Michigan. There
remains considerable spread in amounts however, as much of the
precipitation will start as rain before cooling to snow, and even
then temperature profiles are marginal. It will likely take
heavier rates to accumulate snow, and the NAM which is quite
robust with its snowfall, seems to be accumulating snow much too
heavily when compared to its partial thicknesses and a drying DGZ.
However, a rain/snow mix, with a gradual transition to all snow as
colder air wraps into the system is expected, and a few inches of
accumulation are likely. WPC probabilities are moderate for 4
inches, with the highest risk occurring in the upper portion of
the L.P. of Michigan.


...Pacific Northwest through the Sierra and Northern Rockies...
Days 1-3...

Two distinct upper troughs and associated moisture plumes/jet
maxima will rotate onto the Northwest coast this forecast period.
The first, a weak system which will drive a cold front inland
towards the Northern Rockies Saturday into Sunday will be
accompanied by persistent zonal flow to its south, driving
moisture into the terrain of Washington State and points east.
Snow levels will be 4000-5000 ft except in the Cascades which will
experience the cold air associated with the cold front before the
moisture shunts eastward. Snowfall is expected to be modest, with
WPC featuring a moderate risk for 8 inches in the Cascades of WA
on day 1, and then into the ranges of NW WY on day 2.

A more robust upper low will approach the Pacific NW Monday /day
3/ with a renewed surge of moisture, and better synoptic ascent in
the LFQ of a potent upper jet max. Snow levels will initially be
5000-7000 ft, warmer across the Sierra, before a cold front moves
onshore late in the period. The best forcing with this second
impulse will be displaced southward from the prior trough, and
increasing 850-700mb flow orthogonal to the N-S ranges will
enhance lift. Due to this, WPC probabilities on day 3 are highest
for 8 inches in the OR Cascades and northern Sierra.


The potential for significant freezing rain (0.25 inches) is less
than 10 percent.


Weiss