Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Thu Feb 21 2019

Valid 12Z Thu Feb 21 2019 - 12Z Sun Feb 24 2019


...Southwest into the Central and Southern Rockies...

Days 1/2...
A closed low over Southern CA opens up into a long wave trough
that crosses southeast CA and southern NV during Day 1. Ahead of
the mid level system, a low level southwest flow transports 0.50
inch precipitable water air over far Southern CA into AZ and far
western NM. The combination of moisture and strong lift 
from upper divergence maxima is expected to produce several inches
above 3000 feet in the mountains east of San Diego, CA. As the
trough crosses the southern Great Basin, moisture and lift
combines with long duration upslope flow to produce a couple of
feet of snowfall above 3000 feet across northern and central AZ. 
Similar amounts are expected over favored upslope areas of the San
Juan Mountains of southwest Co and adjacent northern NM.

On day 2 the upper trough moves out of the southwest across the
southern Rockies, driving the upper jet east across the
mountains/front range of CO/NM on to the Plains.  The lower
heights bring snows to the mountains east of Tuscon AZ, with a
chance of light snow occurring in the city/surrounding suburbs.
After an initial period of snow, the departure of the upper
divergence in conjunction with the eastward moving jet and drying
aloft leads to the snow tapering and the event winding down.

...Pacific Northwest into the Northern Rockies...

Day 2...
The models show the next 300 mb jet maxima streaming from the
northeast Pacific into WA/OR Fri evening, with the jet then
sinking south into OR by 12z Sat.  Snowfall commences in the WA
Olympics as the deep layer moisture and 700 mb ascent begins there
on Fri, and then as the jet extends inland, snow moves inland into
the ranges of northeast OR and central ID. An axis of 4 to 8
inches of snowfall was extended along the WA/OR Cascades, with the
highest amounts across the WA Cascades. 

Day 3...
The upper jet over OR at the end of day 2 pivots in place on day
3, becoming more west-east oriented.
The persistence of the jet core and embedded upper divergence
maxima target the OR Cascades for heavy snow on day 3 due to the
long lasting jet. Up to a foot of snow is possible.
As the jet continues inland, the favored left jet region induced
upper divergence maxima cross northern OR and southern ID. 
Locally heavy snow are likely along the axis, particularly where
it overlaps terrain favoring an orographic component of lift. The
NAM shows focused low-mid level moisture and lift over northern
OR/southern ID. Further north, a secondary frontogenesis maxima
over the northern Rockies leads to higher elevation snow showers
Sat night.

...Central Plains into the Upper MS Valley/adjacent upper Great
Lakes...

End of Day 1 through Day 3...

An increasing deep layer southwest flow transports moisture up and
over colder air near the surface in SD/ND into MN. The column
remains cold enough to support snow, with the confluent flow
leading to multiple jet maxima and differences where lift occurs
within the broad confluent zone. There is a multi model signal of
potential for a band of 3 to 6 inches of snowfall, centered over
southeast SD. A few models extend the axis further northeast into
MN than others.  Conversely, some of the GEFS members extend the
area further southwest in Nebraska.

Further southwest, strong lift associated with a long wave trough
approaching the TX Panhandle is expected to induce a coupled jet
region moving northeast out of Co from the eastern Plains and then
into southwest NE by 12z Sat. On Sat, the upper divergence maxima
and mid level frontogenesis maxima extends from NE/KS and moves
northeast across Iowa/northwest MO, southeast MN, southwest to
northern WI, and the UP of MI.  This is also the track of the 700
mb low and favored axis for heavy snow.  The models show potential
for a band of 8 to 12 inches of snow. The GFS, ECMWF show a
further west track than the NAM/Canadian Global, with the UKMET an
intermediate track.  

East of the low-mid level circulation, strong warm air advection
rides up and over retreating low level cold air from southeast NE
across central to northeast Iowa, northwest Illinois, southern to
northeast WI, and finally into northern lower MI and the eastern
UP of MI. In the strong mid level warming, snow transitions to
freezing rain.  There was a signal for measurable to 0.10+ inch
ice amounts in the aforementioned locations, with as much as a
quarter inch in northeast WI and northern MI.

...PA/NY/ New England...

Day 1...
The ongoing mixed precip event in NY/New England winds down
quickly as the 700 mb wave crosses northern New England and
departs in the morning.  The wave's departure is followed by
ascent dropping and drier air aloft advecting across the region,
bringing the event to an end. An additional couple of inches of
snow are possible in northern Maine, with light freezing drizzle
elsewhere in northern NH and Maine.

Day 3...
As a surface low passing well west of the northern Mid Atlantic
during Day 3, warm air advection occurs over much of the region.
The column will not be able to support snow with a warm nose in
place in model soundings near 750 mb. Surface sub freezing cold
air could result in a period freezing rain from central PA into
the southern tier of NY, Catskills, Adirondacks, and Berkshires. 
Ice amounts are expected to remain from measurable to 0.10 inches
with locally higher amounts.

Petersen