Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Wed Feb 20 2019

Valid 00Z Thu Feb 21 2019 - 00Z Sun Feb 24 2019


DAYS 1 THROUGH 3...

...Southwest into the Central and Southern Rockies...
A long wave trough crossing from Southern CA into the Southwest
states interacts with Pacific moisture to produce heavy snowfall
amounts over the higher terrain of Southern CA, southern NV into
AZ and western NM. There was generally good model agreement with
the timing of the mid level system, as well as the placement of
the highest snowfall amounts (though the 12z GFS was a tad slower
than the consensus). Based on this, the thermal portion of the
forecast was based mainly on a blend of the 12z NAM/00z ECMWF. The
QPF portion of the forecast was based on the most recent WPC QPF.

Day 1...
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 much of Southern CA into AZ and
far western NM. The combination of moisture and strong lift
associated with the mid level system is expected to produce 6 to
10 inches above 3000 feet over the Transverse Range in Southern
CA. As the trough crosses the southern Great Basin during the
second half of the period, moisture and lift produces 12 to 24
inches of snowfall above 3000/4000 feet across northern and
central AZ. Similar amounts are expected over favored upslope
areas of the San Juan Mountains. These amounts were supported by
members of the most recent ECMWF ensemble output showing 12+
inches in both locations.

Day 2...
The mid level trough crosses the Southern Rockies and Southwest
states during Day 2. The moisture source is not as robust as Day
1, but the combination of upslope flow and strong synoptic scale
ascent should be sufficient to produce an axis of 12 to 24 inches
of snow above 3000/4000 feet across much of central and southern
AZ, with a larger swath of 8 to 12 inches of snow across much of
the remainder of the higher terrain of eastern AZ into western NM.
These amounts were supported by members of the latest ECMWF and
the 20/09z SREF showing members with 12+ inches over much of the
higher terrain.


...Pacific Northwest into the Northern Rockies...
A strong short wave tracking from west of British Columbia on Day
2 into the Northern Rockies during Day 3 provides enough synoptic
scale ascent to produce heavy snowfall across the higher terrain
each day. There was generally good model agreement with the timing
of the short wave (though the 12z GFS was a bit slower than the
consensus), so the thermal portion of the forecast was based
mainly on a blend of the 12z NAM/00z ECMWF. The QPF portion of the
forecast was based on the most recent WPC QPF.

Day 2...
The strong short wave moves from western of the British Columbia
coast to northern WA during Day 2. Ahead of the short wave, a low
level west southwest flow transports 0.50 inch precipitable water
air to the coasts of WA and OR. Snow levels fall as the short wave
approaches, reaching near the valley floors by the end of the
period. An axis of 4 to 8 inches of snowfall was extended along
the WA Cascades, with the highest amounts across the northern end.
These amounts were well supported by members of both the most
recent ECMWF ensembles and 20/09z SREF showing
4+ inches of snowfall, generally above 4000 feet.

Day 3...
The short wave crosses WA early on Day 3, reaching the Northern
Rockies before 24/00z. In the low level westerly flow, moisture is
transported to the favored upslope areas of WA/OR, as snow levels
remain close to 2000/3000 feet over western WA/western OR. There
was a strong model signal for an axis of 6 to 10 inches of snow
along the axis of the Cascades Range, with the highest amounts
over the central OR Cascades (where the best moisture transport is
expected). Again, These amounts were well supported by members of
both the most recent ECMWF ensembles and 20/09z SREF showing 4+
inches of snowfall, generally above 3000 feet.


...Central Plains into the Upper MS Valley...
Short wave energy in the deep low to mid level southwest flow
provides sufficient synoptic scale ascent for snow and ice of
portions of the Central Plains into the Northern Plains during Day
2. Lift associated with a long wave trough approaching the
Southern Rockies on Day 3 aids in spinning up surface low pressure
over southeast CO, posing the threat for heavy snowfall over
portions of the Central Plains, with ice possible over the Upper
Great Lakes. For the most part, there was generally good model
agreement with the overall setup, so the thermal portion of the
forecast was based on a multi model blend. The QPF portion of the
forecast was based on the most recent WPC QPF.

Day 2...
An increasing low level south southwest flow transports moisture
up and over colder air near the surface during the second half of
Day 2. Across SD/ND into western MN, the column remains cold
enough to support snow through the event, with a multi model
signal painting an axis of 4 to 6 inches of snowfall, centered
over southeast SD (these amounts were supported by members of the
latest ECMWF/SREF ensemble output). Further south into eastern NE,
the strong warm air advection robs the column of its ability to
make snowflakes, with moisture falling into a subfreezing layer
near the surface. The change occurs at the southern and eastern
edge of the precipitation shield, with light icing expected.

Day 3...
Moisture on isentropic ascent associated with the short wave
crossing the Upper MS Valley into the Upper Great Lakes is
expected to produce an area of 3 to 6 inches of snowfall, with the
highest amounts over northeast MN. These amounts were consistent
with ensemble output from the latest ECMWF before the short wave
exits into Ontario.

Further southwest, strong lift associated with a long wave trough
approaching the TX Panhandle is expected to induce surface
cyclogenesis over this area early in the period. The surface low
deepens as it tracks from here to the Mid and Upper MS Valley by
24/00z. Ahead of the short wave, strong warm air advection rides
up and over retreating low level cold air from the Central Plains
into WI/MI. In the strong mid level warming (with model soundings
showing a pronounced warm nose near 750 mb across southeast MN
into central WI), snow transitions to freezing rain as the column
can no longer support snowflake production. There was a sold model
signal for local 0.10+ inch ice amounts in the abovementioned
locations, with lesser amounts stretching back into eastern NE.

As the surface low deepens, colder air on the back side of the
circulation becomes deep enough to support snow across western KS,
where there was a multi model signal for a small area of 4 to 8
inches of snow. These amounts were supported by several members of
the ECMWF ensemble output showing 8+ inches of snowfall here.
Further east across central KS into southeast NE, rain changing to
snow could support an area of 3 to 6 inches of snowfall before
24/00z.


...Mid Atlantic into New England...
Surface low pressure tracking west of the region results in a
mixed precipitation event, with snow across New England and ice
extending from the central Appalachians into central and southern
New England during Day 1. Another icing event is possible over
central PA into portions of southern NY during Day 3.


Day 1...
Cold air in place ahead of surface low pressure tracking into
Ontario/Quebec is overrun by strong warming above 750 mb,
resulting in a large area of snow and ice. The column remains cold
enough for mostly snow across northern New England, where a large
area of 4 to 6 inches was placed. A transition from snow to
freezing rain (and possibly rain) occurs central New England
between 20/00z and 20/06z, with snowfall amounts generally less
than 3 inches, and ice amounts below 0.10 inches. Over eastern
PA/northwest NJ into southern NY state and southern New England,
the column becomes too warm to support snow, but cold air wedged
up against the higher terrain supports local 0.10+ inch ice
amounts central PA into portions of western MA, before the
precipitation tapers off before 21/12z.

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.
While the column will not be able to support snowflakes here (with
a warm nose in place in model soundings near 750 mb). surface cold
air wedged in along the terrain could result in a period freezing
rain from central PA into the southern tier of NY. Ice amounts are
expected to remain below 0.10 inches.

Hayes