FOUS11 KWBC 140950
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
450 AM EST Sun Jan 14 2024

Valid 12Z Sun Jan 14 2024 - 12Z Wed Jan 17 2024

...Prolonged Great Lake Effect and Snow Squalls in Northeast today...
Days 1-3...

A slow moving and sprawling low pressure gyre centered over
northern Ontario develops today and likely persists there into
midweek as a powerful trough axis swings over the Northeast today.
This means a prolonged WSW to W wind lake effect snow event
downwind of all five Great Lakes that have a few ebbs and flows
through this time. Cold air through the column (850mb temps <-18C)
over the still nearly unfrozen lakes will lead to more than steep
enough lapse rates for snow, but the cold air will generally favor
lower than maximum SLRs (past the DGZ on the cold end) also from
dendritic fracturing in the gusty winds today.

Snow squalls (a brief burst of snow accompanied by winds and a
real threat to overland travel) are expected from the upper trough
axis moving over much of PA/NY this morning through midday,
shifting across the northern the northern Mid-Atlantic and
southern/central New England all the way to the coast this
afternoon/evening. Snow squalls are most dangerous when
temperatures fall below freezing with the squall, causing a flash
freeze on roadways and this looks to be the case this afternoon
over eastern PA/northern NJ/southern NY and southern New England.

LES snow probs: Day 1 for at least 6 inches of snow are high

70%) over the eastern U.P. of Michigan and especially in the

single band setups east of Lakes Erie/Ontario in WSW flow
(favoring near and just south of BUF) where double-digit totals
are quite likely.

Tonight into Monday, flow becomes westerly across the western
Lakes, supporting multi-bands into much of Lower Michigan but
still some single bands into the Keweenaw Peninsula on the U.P.
where WPC probabilities for at least another 4 inches are around
50%. Intense single bands off Erie/Ontario will waver a bit and
lift northward but weaken, still allowing for several more inches
of snow with high probs for >6" in narrow bands north of Buffalo
and the Tug Hill. Westerly flow prevails Tuesday into Wednesday
with Day 3 probs moderate for additional >6" in the Keweenaw and
back over Buffalo/the Tug Hill

...Mid-South/TN Valley/Deep South/Appalachians through Northeast...Days 1-3...

A series of impulses is lined up from the Four Corners to the
OR/CA coast early this morning. These will ride a WNWly jet that
rounds the longwave trough spanning most of the CONUS. That jet
turns Wly and increases to 130-kt jet over OK to the Mid-Atlantic
tonight with the Mid-South in the right entrance region aiding
lift as these impulses/vort maxes push through. Arctic front will
reach the upper TX coast this morning and the central Gulf Coast
by this evening, bringing sub-freezing temperatures in its wake
from eastern TX through the Lower MS Valley. However, low level
ridging over the Gulf will allow return flow north over the
Arctic-sourced airmass in place, setting up a broad area of a
wintry mix of sleet and freezing rain tonight through Monday night
from east Texas across much of LA and central MS/AL. Farther north
in the deeper cold air, light to locally moderate snow will expand
eastward intermittently with the impulse arrivals from central OK
and through AR this morning through this evening before settling
on a low level frontogenetic zone from southern AR east-northeast
over northern MS/most of TN to the southern Appalachians tonight
through Monday. Along this zone, expect moderate to possibly heavy
snowfall with relatively high SLRs (~12-16:1). This focus has
shifted south a bit with the 00Z consensus tonight with the stripe
of several inches of snow now more over south-central AR and
northern MS, but still extending ENE across TN and far northern
AL. How far north the warm air aloft reaches will determine the
north area of the sleet area progresses, current thinking is the
NAMnest remains too far north into with the warm nose, especially
with the rest of 00Z guidance shifting south. Snow should be
heaviest just north of the sleet area making the warm nose
progression key to a decent forecast. Day 1.5/2 WPC probabilities
of at least 4 inches of snow are moderate (40-70%) over a stripe
from southeast AR through northern MS and most of TN and then up
the southern Appalachians into WV.

Farther south, cold surface temperatures beneath a warm nose aloft
supports an area of freezing rain/sleet starting this evening from
the TX Hill Country/South-central TX eastward across much of LA
through central MS and into AL. It will be a battle between the
arctic air beneath the advancing warmer air aloft, and the latest
guidance with potential for freezing rain accumulations to or near
the Gulf Coast given how cold the surface air is. In general, the
northern portion of the wintry mix area is more confidently going
to be sleet given the depth and magnitude of surface cold air. The
Day 1.5 WPC probabilities of at least 0.10" ice are moderate
(30-60%) from around I-35 between San Antonio and DFW eastward
into central MS (I-55) with little to no chances for 0.25" at this
time. The Day 1/2 chance of any (>0.01") icing (>10% chance)
extends from south TX across east TX nearly all of LA (outside
NOLA) through much of MS, central AL, and northwest GA into far
eastern TN.

A shortwave trough rounding the longwave pushes down the northern
Rockies tonight before shifting east over the Ohio Valley into
Tuesday and the Northeast Tuesday night. This would promote
coastal low development off the northern Mid-Atlantic late Tuesday
and past New England Tuesday night. Day 3 snow probs for >4" are
low over the northern Mid-Atlantic, but become moderate (40-60%)
over Down East Maine. Uncertainty remains with the strength of
this low, but probabilities are increasing for the first inch of
snow of the winter to some of the I-95 corridor (DC-PHL-NYC) where
the snow drought continues.

...Great Basin and Colorado Rockies... Days 1-2...

The last impulse in a series of waves moving into OR starting
Saturday cross the coast at the OR/CA border around 12Z today and
tracks inland along/south of a stationary front marking the south
border of the Arctic sourced air that invaded the Northwest.
Mountain snows continue over the Great Basin ranges, the Wasatch
and Colorado Rockies as noted by the Day 1 snow probs which are
high for >6" for the Wasatch and most CO ranges.

Ridging shifts inland behind this last impulse, cutting off precip
quickly today for the West Coast and by Tuesday for the Rockies.
However, a northern stream shortwave trough rounding the deep low
gyre developing over Ontario will shift down the northern Rockies
tonight into Monday providing lift and extending snow over the
central Rockies (with light snow extending east onto the Plains
across KS) with some low to moderate Day 2 snow probs for an
additional >6" for the central CO Rockies.

...Northwest... Day 3...

The next compact low to cut east through the Northeast
Pacific/Alaska ridge likely reaches the OR/WA coast late Tuesday
and may be joined by northern stream troughing shifting south from
BC Tuesday night into Wednesday allowing a quick shift inland to
the northern Rockies Tuesday night. With anomalously cold surface
air likely lingering over the Pacific Northwest including Seattle
and Portland metros, a wintry mix to sea level is in the cards.
Day 3 PWPF are 10-50% for >0.1" ice along the western OR/WA border
including the Portland metro and 20-60% for >6" over the WA
Cascades as well as eastern WA/northern ID and the Sawtooths
toward the Boise metro in west-central ID. This low/wave combo
looks to be the first part of a rather active winter weather
pattern for the Northwest continuing the rest of the week.

Jackson


***Key Messages for Mid-January Arctic Blast, Lake Effect, and Snow Squalls***

--Dangerously Cold Wind Chills Expected
Sub-zero wind chills will affect much of the U.S. and reach into
portions of the South. Wind chills into early this week will fall
below minus 30 degrees from the northern Rockies to the central
Plains and mid Mississippi Valley. Values will drop as low as
minus 70 degrees from Montana to North Dakota. These wind chills
will pose a risk of frostbite on exposed skin and hypothermia.
Have a cold survival kit if you must travel.

--Lake Effect Snow Persists This Weekend
Heavy snow will continue downwind of the Great Lakes into midweek.
Whiteout conditions in the lake effect snow bands are expected.
An additional 12-24 inches of snow will occur through Monday
across portions of western and northern New York.

--Snow Squalls Expected over the Northern Mid-Atlantic and
Northeast Today
Snow squalls will impact portions of the northern Mid-Atlantic and
Northeast today. Gusty winds, rapidly reduced visibility, and
flash freezing on roads will result in dangerous travel where
squalls occur.

--Hazardous Cold Weather to Persist and Redevelop
A new surge of Arctic air will drop south over the northern Plains
and Midwest midweek, reaching the Deep South by the end of the
week. This will reinforce dangerously low temperatures and wind
chills.


***Key Messages for Western U.S. to Mid-South Winter Storm***
--Heavy Snow Impacting the Intermountain West
Heavy snow will taper down along the West Coast this morning and
focus over the Wasatch of Utah and the Colorado Rockies today
where continued significant impacts are expected.

--High Avalanche Danger Continues for Portions of the Sierra
Nevada and the Rockies
Heavy snowfall and strong winds have created dangerous avalanche
conditions across portions of the Sierra Nevada and the northern
to central Rockies. Travel in, near, or below avalanche terrain is
not recommended.

--Snow and Ice to Reach the Southern Plains Later Today and the
Mid-South Tennessee Valley Tonight and Monday
Areas of snow, sleet, and freezing rain are likely to develop and
spread across portions of the southern Plains, through the
ArkLaTex, and into the Tennessee Valley today through Monday.
Several inches of snow are likely. Accumulating ice is expected
from portions of central and southern Texas through the Lower
Mississippi Valley into portions of the Tennessee Valley. With
Arctic air continuing to filter south this week, impacts from
wintry weather may last for several days, resulting in a prolonged
period of hazardous travel.

$$
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FOUS11 KWBC 151012
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
511 AM EST Mon Jan 15 2024

Valid 12Z Mon Jan 15 2024 - 12Z Thu Jan 18 2024

...East Texas, Lower MS Valley, Deep South, TN Valley,
Appalachians through Northeast... Days 1-3...

A vort max over the TX Panhandle will zip east today on the tail
of a potent WSWly jet which will promote further low level
moisture flow from the western Gulf and produce light precip over
south-central and East TX through the Deep South. Surface cold air
advection will continue to bring these areas below freezing with a
light glaze likely through this swath and 20% probs for >0.1" ice
over east TX/northern Louisiana/central MS as well as far
northeast AL/northwest GA.

To the north, the arctic air will be much deeper, supporting all
snow from eastern AR eastward across northern MS and through much
of TN and the southern Appalachians. Relatively high SLRs
(~12-16:1) would favor several inches of snow that will continue
to be enhanced by frontogenetical banding as the entire system
shifts eastward ahead of a sharpening trough axis rounding the
sprawling low centered up over Ontario. Day 1 snow probs for >4"
are 40%-70% from eastern TN up far western VA into central WV with
lower probs over the northern VA Blue Ridge and north-central MD.

Coastal low development off the Mid-Atlantic coast begins late
tonight which quickly rides north in the right entrance to the
intensifying SWly jet just inland of the Northeastern Seaboard.
Day 1.5 snow probs for >4" are around 20% for eastern PA/northern
NJ/southern NY into southwest New England and then they increase
farther north for Day 2 probs with northern and eastern Maine
generally 40-60% where the coastal low is rapidly deepening as it
moves north just off the New England coast.

...Colorado Rockies onto Central High Plains... Day 1...

Shortwave trough rounding sprawling/deep low centered over
northern Ontario shifts southeast over the CO Rockies today before
swinging east over the central Plains this evening. 1050mb surface
high pressure is on its heels this morning over northern MT. Snow
continues through this afternoon over the CO Rockies with light to
locally moderate snow bands shifting east in the strengthening Wly
jet across KS/MO/IL/IN today through tonight. Day 1 snow probs are
40-60% for an additional >6" on the central CO Rockies with low
probs for >2" east from the Palmer Divide across KS.

...Prolonged Great Lake Effect Snow... Days 1-3...

Expansive upper low established itself over far northern Ontario
today and lingers through Wednesday. A wrapping vort/wave crosses
the Great Lakes from west to east this evening through Tuesday.
The prolonged cyclonic flow across the Great Lakes maintains the
multi-day lake effect snow with NWly flow and multi-bands over the
western Lakes and Wly to WSWly flow and single bands for the
eastern Lakes. Robust single bands off Lakes Erie/Ontario continue
for nearly all typical lake belts, with minor variations
continuing as well as some disruption from the vort passage
Tuesday morning. PWPF do not handle the small phenomena of LES
well, but for the Erie/Ontario single bands, Day 1 is generally
north of Day 2 as the flow veers more from WSWly to Wly. Day 3 is
back north a bit and much more intense with high probs for >8" off
both. Otherwise, it's the Keweenaw Peninsula that gets enough snow
Days 1/2 to show up in the PWPF.

...Northwest... Days 2/3...

A one-two punch of mid-level waves reaches the Pacific Northwest
late Tuesday and on Wednesday. The first is a southern stream
compact low that slices east through the northeastern Pacific
ridge and the second is a sharp northern stream trough that slowly
shifts south down the Alaskan Panhandle tonight/Tuesday and the BC
coast Tuesday night/Wednesday before shifting southeast down the
northern Rockies Wednesday night. Ample moisture plume leading the
compact low surges inland across the PacNW coast Tuesday to the
northern Rockies Tuesday night with snow levels shooting up to
8000ft by Tuesday evening, though these quickly drop below 5000ft
under the low Tuesday night.

However, the Arctic-sourced air that remains over northern
OR/western WA will persist under the milder air aloft move into
the region Tuesday. A wintry mix of snow/sleet to freezing rain is
likely over the lower Columbia area including the Portland metro
where Day 2 ice probs for >0.1" ice are 30-60% along with 10-20%
probs for >0.25" ice.

Farther inland, deep cold air will remain entrenched which favors
higher SLRs with Day 2 snow probs for >6" 10-40% over the WA
Cascades and the Sawtooths near Boise. Then the second wave moving
into the interior Northwest late Wednesday will further aide the
inland surge of Pacific moisture with widespread mountains snows
over the Cascades and northern/central Rockies where day 3 snow
probs for >6" are 40-60%. Day 3 probs for >12" are 50-90% for the
WA Cascades and the Bitterroots.

Jackson


***Key Messages for Southern and Eastern Winter Storm***

--Southern Wintry Mix Continues Today
Areas of sleet and freezing rain will continue to impact areas
from south-central Texas through the lower Mississippi Valley and
into eastern Tennessee. With arctic air in place hazardous travel
can be expected.

--Snow over the Southern Appalachians through Mid-Atlantic today
into Tuesday
Areas of snow will continue over Tennessee, the southern
Appalachians and Mid-Atlantic today into Tuesday. Plan on slippery
roads and difficult travel conditions.

$$
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FOUS11 KWBC 250813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Mon Mar 25 2024

Valid 12Z Mon Mar 25 2024 - 12Z Thu Mar 28 2024

...Southern Rockies... Day 1...

An inverted 500mb trough over the Southern Rockies will be the
focus for additional heavy snow today. The Sangre De Cristo and
eastern San Juans both sport high chances (>70%) for additional
snowfall totals >8" with the tallest peaks of the Sangre De Cristo
in northern New Mexico even having moderate-to-high chances
(50-70%) for >12" snowfall accumulations through Monday afternoon.
The axis of moisture and lift associated with this upper trough
will even result in heavy snow over the Gila Mountains in eastern
Arizona. WPC PWPF shows moderate chances (40-60%) for snowfall
accumulations >8" through Monday evening. The 500mb trough will
eject east into the Southern Plains Monday night, thus all but
concluding the lingering periods of snow by Tuesday morning.

...Northern Plains & Upper Midwest... Days 1-2...

The major winter storm will be well underway with the worst impacts
being felt from the central High Plains on north to north-central
Minnesota. The deformation axis on the northern and western flank
of the storm will be capable of producing 1-2"/hr snowfall rates to
go along with wind gusts that are exceeding 40 mph in northwest
Kansas, central Nebraska, and southeast South Dakota. Farther east,
the impressive 850mb WAA in the Upper Mississippi Valley that was
in part responsible for the heavy snowfall overnight will lead to
a burgeoning warm nose at low levels that causes snow to
changeover to a wintry mix this morning in southern Minnesota,
northern Wisconsin, and the Michigan U.P. By this afternoon, any
lingering wintry mix is expected to transition over to plain rain.
Meanwhile, the storm will continue its trek northeast into the
Upper Mississippi Valley Monday evening with heavy snow enveloping
eastern South Dakota, northern Minnesota, and the Minnesota
Arrowhead.

Snow will continue to fall heavily over north-central Minnesota
Monday night with some residual snow as far south as eastern
Nebraska and northern Kansas as a TROWAL pivots overhead. By
Tuesday morning, the low will be tracking into northern Wisconsin
with heavy snow across northern Minnesota and a mix of heavy
snow/wintry mix along the Minnesota Arrowhead. The storm will then
move over the U.P. of Michigan Tuesday afternoon, then finally into
southern Ontario Tuesday night. Snow will continue to fall across
northern Minnesota and portions of far northern Wisconsin and the
western Michigan U.P. Heavy snow will effectively come to an end by
Wednesday morning with just lingering snow showers along the
Minnesota/Canada border and along the northern coast of the
Michigan U.P..

WPC PWPF shows moderate-to-high chances (50-70%) for additional
snowfall >6" from north-central Nebraska and eastern South Dakota
to far southeast North Dakota, northern Minnesota, and the
Minnesota Arrowhead. South-central South Dakota and the Minnesota
Arrowhead even show low chances (10-30%) for additional localized
snowfall amounts >8". Farther southwest, WPC PWPF shows moderate-
to-high chances (50-70%) for additional snowfall totals >4" in
southwest Nebraska, northwest Kansas, and southeast Colorado.
Nebraska and northwest Kansas, in particular, could contend with
blizzard conditions due to prolonged high wind gusts and
significantly reduced visibilities.

...Pacific Mountains Ranges... Days 2-3...

Outside of some diurnally driven snow showers and weak onshore
flow in the Pacific Northwest through Tuesday night, little in the
way of heavy snowfall is likely to occur for many of the mountains
ranges along the West Coast. That finally changes by early
Wednesday morning as the next Pacific storm system approaches and
directs a slug of Pacific moisture first at the Pacific Northwest,
then into the Northern California ranges and interior Northwest
ranges Wednesday night. The IVT is a quick mover but a fairly
potent IVT sporting values >400 kg/m/s just off the California
coast. Overall, the peak of the IVT tracking into northern
California Wednesday afternoon is likely to top the 97.5
climatological percentile, supplying not only abundant moisture to
the region but strong upslope enhancement (especially in the Sierra
Nevada). Snow levels in the Sierra Nevada will initially start out
as high as 6,500ft, but gradually fall to 5,000ft by Thursday
morning as the cold front pushes through. Farther north, snow
levels will start out around 5,000ft in the northern California
ranges (Salmon/Siskiyou/Trinity) and closer to 4,500ft in the OR/WA
Cascades, but those too will fall an additional 2,000ft by
Thursday AM. Latest WPC PWPF shows high chances (>70%) for snowfall
amounts >8" in the Northern Sierra Nevada >6,000ft and in the
northern California ranges, while only the volcanic peaks of
southern Washington and the Olympics sport similar high chances
for >8" of snowfall through early Thursday morning. Some snow will
spill over into the Blue Mountains and the Boise/Sawtooth late
Wednesday night into Thursday with low-to-moderate chances (30-50%)
for snowfall totals >6" through early Thursday morning.

Mullinax


***Key Messages for Plains-Upper Midwest Winter Storm***

--A high-impact winter storm unfolding

A powerful storm system will impact the Northern/Central Plains
into the Upper Midwest through Tuesday. Periods of snow and gusty
winds will continue from the Central Plains to northern Minnesota,
along with some sleet and freezing rain in parts of the Upper
Mississippi Valley.

--Widespread heavy snow expected

Snow accumulating at rates of 1-2"/hr in heavier bands are
expected from central Nebraska and eastern South Dakota to northern
Minnesota. Additional snowfall totals between 6-12" are forecast
from central Nebraska to northeastern Minnesota.

--Hazardous impacts due to snow, snow load, and wind

Heavy snow and gusty winds approaching 50 mph will produce
blizzard conditions with near zero visibility into Tuesday morning.
Travel could be nearly impossible at times. Power outages and tree
damage are likely in some areas due to the heavy and wet snow
combined with icing and strong winds.


$$
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