FOUS11 KWBC 021924
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EDT Tue Apr 2 2024

Valid 12Z Tue Apr 02 2024 - 12Z Fri Apr 05 2024

...Great Lakes... Days 1-2...

...Major late season winter storm this week...

The stage is being set for a multi-day major April winter storm
that will bring about a myriad of precipitation types (heavy snow,
heavy rain, sleet, freezing rain), which combined with strong
winds from the Upper Midwest to New England will result in numerous
impacts tonight and through the end of the week. The origins of
this winter storm begins with a vigorous northern stream
disturbance diving south and phasing with a southern stream
disturbance tracking northeast into northern Illinois and southern
Wisconsin. This interaction will result in the rapid deepening of
a more consolidated 500mb low over the Upper Midwest and quick
intensification of a surface low by 06Z tonight over Lake Michigan.
The latest forecast calls for the storm to deepen by as much as
20mb over the next 18 hours. As the 500mb low rapidly deepens,
intense vertical velocities on the northwest and western flanks of
the 850mb low, induced by intense PVA aloft and exceptional
850-700mb FGEN gives rise to a potent deformation axis that will
become primarily snow this evening from northeast Iowa to central
Wisconsin. As the storm occludes overnight, the TROWAL on the
backside of the of the storm will continue to be the focus for
heavy snow over northern Wisconsin and the U.P. of Michigan. The
U.P. pf Michigan, in particular, will sport the best chances for
ripping snowfall rates of >2"/hr thanks to the lake enhanced bands
off Lake Superior and along the more elevated terrain of the
central U.P.. By 12Z Wednesday, NAEFS shows 500mb heights that,
according to NAEFS, fall below the observed CFSR database
(1979-2009) over Illinois, illustrating the highly unusual nature
of a cyclone that intense over Illinois for early April. It is on
the northern flank of the low Wednesday AM where snowfall rates
will be most significant.

Due to the upper low being cut off from the mean flow to the west and
the upper level omega block over eastern Canada and the northwest Atlantic,
the storm system will be slow to move east on Wednesday, prolonging
the period of heavy snow in northern Wisconsin and the Michigan
U.P.. Latest WPC probabilities show high probabilities (>70%) for
snowfall totals >8" from northern Wisconsin to the Michigan U.P..
Farther south, there are moderate-to-high chances (50-70%) from
just north of the I-94 corridor in southern Wisconsin up to areas
just west of Green Bay. The WSSI sports Major to even locally
Extreme Impacts for areas neighboring Green Bay and in the central
Michigan U.P. The localized Extreme Impacts are depicted along the
Huron Mountains through Wednesday afternoon. The Hurons currently
have high chances (>70%) for >24" of storm total snowfall. Snow
Amount is the primary driver in the WSSI algorithm, but the WSSI is
also showing in some parts of eastern Wisconsin and even near the
tip of Michigan's Mitt, that some Moderate Impacts as a result of
Snow Load and Blowing Snow are expected.

...Northeast... Days 2-3...

While the upper low in the Great Lakes occludes Wednesday
afternoon, farther east, an impressive IVT over the Southeast will
see some of its associated moisture stream north into the Northeast
Wednesday morning and run into an air-mass just sufficiently cold
enough to support a wintry mix of sleet and freezing rain from the
Catskills and Adirondacks to the Berkshires. This wintry mix will
translate farther north and east through the Worcester Hills, the
Green Mountains, and White Mountains by Wednesday afternoon. This
air-mass supporting the onslaught of wintry precipitation is not
expected to leave any time soon due to the upper level omega block
mentioned in the Great Lakes section that is locking in a dome of
cold Canadian high pressure over Quebec. In actuality, what this
omega block will do in part is to help keep the storm track farther
south. As the occluded front works north through the Mid-Atlantic
Wednesday afternoon, intense PVA and robust 850mb FGEN from
southern PA to the southern New England coast will rapidly
intensify an area of low pressure tracking from the Delaware Valley
Wednesday evening to along the Long Island coast by early Thursday
morning.

The key to this forecast lies with when the surface low takes over.
Most of the interior Northeast will be dealing with a wintry mix
due to the >0C warm nose in the 800-750mb layer. However, once the
850mb low forms, winds will shift more out of the E-NE, reducing
the warm nose aloft and leading to a sudden changeover to snow.
Latest guidance shows this happening somewhere in the 03-09Z
Thursday timeframe, which given this coincides with snow falling
overnight, will maximize the opportunity for rapid accumulations on
all surfaces. By 12Z Thursday, just about everyone from Upstate New
York and interior New England to even the coast of New Hampshire
and Maine can expect to be all snow.Snow fall rates late Wednesday
night and through Thursday morning are likely to be between 1-2"/hr
with wind gusts topping 40mph in many cases, especially along the
New England coast and in the higher elevations. The storm looks to
occlude off the eastern Massachusetts coast with the cold conveyor
belt (CCB) of snow to the north of the low lasting over the
northern Appalachians and much of Maine through Thursday evening.
Due to the upper level omega block still in place, the upper low
over the Northwest will continue to keep periods of snow in the
forecast in the Green and White Mountains, but now with the air-
mass modifying and gradually diminishing upper level support, a mix
of rain and snow showers will be possible through Friday afternoon.
Last but not least, upslope enhancement in the central Appalachians
of eastern West Virginia will also ensue Thursday evening and into
the day on Friday with elevations >3,000ft most likely to see heavy
snow through the end of the work-week.

In terms of impacts, this will be an exceptional one for the
Northeast given not only the heavy, wet snow that is expected, but
the prolonged round of strong winds combined with highly saturated
soils in the Northeast. The WSSI shows Major Impact potential
(considerable disruptions to daily life, widespread closures and
disruptions) in parts of the Adirondacks, the White Mountains, and
as far east as the Kennebec Valley of central Maine. Moderate
Impacts (hazardous driving conditions, closures and disruptions
possible) are higher confidence in the areas expecting Major
Impacts, but are also possible in parts of the Catskills, Green
Mountains, Berkshires, Worcester Hills, and in parts of the central
Maine Highlands and along the central Maine Coast. Snow Amount is
the primary driver in the WSSI algorithm, but a combination of Snow
Load and Blowing Snow is also included with Moderate Impacts
possible. With the expected impacts from Blowing Snow correlated to
strong winds and the Snow Load component present as well, the
exceptionally saturated soils throughout the region is leading to
increased concerns for extensive tree damage and power outage
potential. WPC PWPF sports high chances (>70%) for snowfall totals

12" in the Adirondacks, as well as the Green and White Mountains,

and into portions of central Maine. There are even some moderate-
to-high chance probabilities (50-70%) for >24" in parts of the
White Mountains. Please see our Key Messages below for the

...Western U.S.... Days 2-3...

An upper level trough originating in the Gulf of Alaska will plunge
south and be located off the Pacific Northwest coast by Wednesday
afternoon. Pacific moisture streaming out ahead of the upper trough
will result in some mountains snow along the Cascade Range and over
the Olympics during the day on Wednesday. By Wednesday evening, a
mid-upper level frontal band will focus a heavier swath of
precipitation from northern California to the Northern Rockies
where snow will fall from the Trinity/Shasta and the Blue Mountains
of eastern Oregon to the Boise/Sawtooth and Bitterroots overnight.
As the upper trough continues to dived south off the West Coast,
the current of Pacific moisture will work its way south along the
spine of the Sierra Nevada where upslope enhancement will prompt
heavier snowfall rates over the central and southern Sierra Nevada
throughout the day. The enhanced snowfall rates are also a
byproduct of a compact and robust 500mb low at the base of the
upper trough moving into central California. This leads to falling
snow levels that support heavier snowfall totals occurring as low
as 5,000ft. By Thursday night, 500mb and 700mb heights over central
California are forecast by NAEFS to be below the 0.5
climatological percentile and will even allow for some locally
heavy snowfall amounts in the Transverse Ranges through Friday.
Snowfall rates will back off some across most of the Pacific
Mountains and Great Basin on Friday, but modest lift and steep
lapse rates will still keep mountain snow in the forecast from as
far south as the Peninsular Range to as far north as the
Bitterroots.

Latest WPC PWPF shows high chances (>70%) for snowfall
accumulations >12" in portions of the Blue Mountains and both the central
and southern Sierra Nevada above 6,000ft, while similar high
chance probabilities for >8" of snow are present in the
Trinity/Shasta, the Oregon Cascades, and into parts of the
Boise/Sawtooth mountains. The WSSI-P sports moderate-to-high
chances (50-70%) for Moderate Impacts in the central and southern
Sierra Nevada, the Blue Mountains, and the central Great Basin in
central Nevada through Friday afternoon.

Mullinax

***Key Messages for Early April Nor'easter***

---Western Great Lakes heavy snow Tue night

Heavy snow will develop over Wisconsin and expand into Upper
Michigan overnight. Heavy snow will continue over much of the
region through Wednesday, with additional heavy snow across
portions of Upper Michigan, before ending on Thursday. As much as
1-2 feet of snow is expected in parts of northern Wisconsin and the
U.P. of Michigan.

---Western Great Lakes heavy snow Tue night

Heavy, wet snow and some sleet will envelope the Northeast
Wednesday afternoon through Friday. Significant snow accumulations
over 12 inches are likely over northern New York and central New
England.

---Significant impacts from heavy snow and wind

The combination of heavy snow rates and gusty winds will result in
hazardous travel due to whiteout conditions and snow-covered
roads. The wet snow and high snow load combined with strong wind
gusts may also cause tree damage and power outages.

---Moderate coastal flooding in the Northeast

Prolonged onshore flow late Wednesday and continuing through
Thursday will result in moderate coastal flooding for portions of
the Northeast coast. Impacts include widespread roadway flooding,
coastal and bayside flooding, impassable roads, and some damage to
vulnerable structures.

$$
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FOUS11 KWBC 030855
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 AM EDT Wed Apr 3 2024

Valid 12Z Wed Apr 03 2024 - 12Z Sat Apr 06 2024

...Great Lakes/Northeast... Days 1-3...

...Major late season winter storm this week...

As expected, phasing northern and southern streams are supporting
the development of a deep closed over the western Great Lakes
region this morning. Its associated surface low is now centered
near southern Lake Michigan where it is forecast to linger for the
next several hours before drifting to the southeast later today.
Snow will continue to spread across much of Wisconsin, as well as
portions of southeastern Minnesota, eastern Iowa, and northern
Illinois. Additional accumulations for much of this area are
expected to be an inch or two, with pockets of locally heavier
amounts. Much heavier amounts are likely to the north across
portions of Upper Michigan where persistent easterly to
northeasterly winds off of Lake Superior will contribute to several
more hours of heavy, wet, lake-enhanced snow. The heaviest amounts
are forecast to center over Marquette and Baraga counties, where
WPC PWPF show high probabilities (70 percent or greater) for
additional accumulations of a foot or more today. Moderate (40
percent or greater) to high probabilities for additional
accumulations of 6 inches or more extend further west into far
northeastern Wisconsin. Snow is expected to continue over Upper
Michigan and northern Wisconsin through the overnight, but diminish
by early Thursday as the low moves east of the region.

Meanwhile, warm advection precipitation will continue to spread
into the Northeast this morning, with it expected to begin as or
quickly mix with sleet across a good portion of eastern Upstate New
York and central New England during the afternoon. As the upper
low begins to interact with additional southern stream energy
lifting out of the South, a triple-point low will begin to develop
and deepen over the Mid-Atlantic this afternoon. Models show this
feature continuing to deepen as it moves northeast toward Long
Island and southern New England overnight. As the low approaches,
strong vertical ascent afforded in part by left-exit region upper
jet forcing will support increasing precipitation rates, with snow
likely to become the predominant p-type across much of northern New
York into interior central and northern New England. By this
evening and continuing into the overnight, HREF guidance shows
snowfall rates of 1-2 inches spreading from Upstate New York into
New England. By daybreak, the heaviest snows are expected to
center over New Hampshire and western Maine. As the coastal low
tracks into and then lingers near the Gulf Maine, snow will
spread east across Maine on Thursday, while continuing to impact
the remainder of northern New England and parts of northern New
York through Friday. WPC PWPF shows storm total amounts likely
exceeding a foot over portions of the Adirondack, Green, and White
mountains, as well as much of Maine. Some parts of the region may
see over two feet, with the PWPF indicating moderate or higher
probabilities for reaching these amounts over the White Mountains
into western Maine.

...Central and southern Appalachians.... Days 1-3...

Shortwave energy embedded within deep cyclonic flow will support
showers across the region beginning Thursday, with thermal profiles
supporting snow across the higher elevations. The heaviest
accumulations are expected to fall along the Allegheny Mountains
in West Virgina, where several inches are possible by the end of
the period. WPC PWPF shows probabilities for accumulations of 4
inches or more climbing above 70 percent across this region during
the Day 2 period (ending 12Z Friday).

...Western U.S.... Days 1-3...

An amplified upper trough associated with a low dropping south from
the Gulf of Alaska will begin to dig into the northwestern U.S.
today Wednesday. Backing flow ahead of the advancing trough will
support deepening moisture and the increasing potential for snow
along a low-to-mid level front extending northeastward from the
southern Cascades. While widespread heavy accumulations are not
expected, locally heavy amounts are possible from the southern
Cascades to the Blue Mountains late today into early Thursday.

By early Thursday, models show a deep upper low centered along the
Northwest coast that will then dig further south into northern
California and Nevada by early Friday. This will bring heavy snows
into the Sierra Nevada and parts of central and northern Nevada.
Meanwhile, energy lifting east of the low and a lingering frontal
boundary will support additional heavy amounts across northeastern
Oregon.

From Friday into Saturday, the upper low is forecast to lift
northeast across Nevada into southern Idaho, bringing additional
snows to portions of northern Nevada, eastern Oregon, and southern
to central Idaho. While the mountains will likely see the heaviest
amounts, this will be an anomalously deep system that will bring
snow levels well down into many of the valleys.

Meanwhile, snow with locally heavy totals will also extend south
into the southern California mountains and along the Mogollon Rim,
and east into portions of the central Rockies.

Regarding three day totals, WPC PWPF shows high probabilities for
accumulations of 8 inches covering much of the Sierra Nevada, the
Oregon Cascades, and the Blue Mountains. Locally high probabilities
these amounts also cover the central and Nevada mountains into
southeastern Oregon and southwestern Idaho, as well as the central
Idaho ranges.

Pereira

***Key Messages for Early April Nor'easter***

---Western Great Lakes snow through today

Snow will continue throughout the region today, with additional
heavy snow likely across portions of Upper Michigan and northern
Wisconsin. Before ending on Thursday, snowfall accumulations over 2
feet are likely in parts of Upper Michigan.

---Northeast snow and wind through Friday

Heavy, wet snow and some sleet will spread into the Northeast
Wednesday today and continue through Friday. Snowfall
accumulations of 1-2 feet are likely across portions of northern
New York and New England.

---Significant impacts from heavy snow and wind

Heavy snowfall rates and gusty winds will result in dangerous
travel conditions, with whiteout conditions and snow-covered roads.
The combination of wet snow, a high snow load, and strong wind
gusts could also cause tree damage and power outages.

---Moderate coastal flooding in the Northeast

Prolonged onshore flow late today and continuing through Thursday
will result in moderate coastal flooding for portions of the
Northeast coast. Impacts include widespread roadway flooding,
coastal and bayside flooding, impassable roads, and some damage to
vulnerable structures.

$$
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