Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Thu Jan 21 2021

Valid 00Z Fri Jan 22 2021 - 00Z Sun Jan 24 2021

...Heavy snow for the Sierra Nevada Mountains, mountains of the Great
Basin, and the Central Rockies Friday through Saturday...

...Heavy lake effect snow downwind from Lakes Erie and Ontario through
Friday...

...Snow to develop over parts of the Upper Midwest Saturday...

A front extending across the interior Gulf Coast States will have moisture
pooling along the boundary and will produce rain from parts of the
Southern Plains eastward across the Southeast through Friday.

Meanwhile, a reinforcing cold front will sweep across the Great Lakes
tonight, producing favorable cold westerly to northwesterly flow across
the unfrozen Great Lakes. Lake-effect snow showers will increase in
coverage and intensity this evening and continue through Friday.
Locally-heavy snow accumulations are likely with the heaviest totals
expected over western and northwestern portions of New York State,
downwind from Lakes Erie and Ontario; northern portions of the Lower
Peninsula of Michigan, downwind of Lake Michigan; and along the north
coast of the Upper Peninsula of Michigan, downwind of Lake Superior
through Saturday.

A front over the Pacific Northwest will advance southeastward to the
Southwest through Friday and into the Southern Rockies Saturday. The
system will produce coastal rain and higher elevation snow over parts of
southern Oregon through California and snow across the Intermountain
Region into parts of the Northern Rockies through Friday. Later Friday,
higher elevation snow will develop over parts of Northern/Central
California. The higher elevation snow will expand across the terrain of
the Great Basin, Southwest, and Central/Southern Rockies along with lower
elevation rain. The rain will also shift into Southern California on
Friday evening. By Saturday morning, the rain and snow will end over the
Pacific Northwest and most of Northern California, yet continue over
Southern California, the Sierra Nevada Mountains, and the Great Basin into
the Central/Southern Rockies.

Overnight Friday through Saturday, a front over South-Central Canada will
move southward into the Upper Midwest. In conjunction with upper-level
dynamics over the Central/Southern Rockies, snow will develop over parts
of the Northern/Central Plains into parts of the Upper/Middle Mississippi
Valley.

Ziegenfelder

Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Thu Jan 21 2021

Valid 12Z Sun Jan 24 2021 - 12Z Thu Jan 28 2021

...Additional heavy rainfall likely for portions of the
south-central U.S. for the beginning of the week; heavy mountain
snow and unsettled weather moving into the western U.S....

...Weather Pattern Overview and Guidance/Predictability
Assessment...

Active amplification of upper-level troughs digging down from the
Gulf of Alaska will be a continued source of unsettled weather for
the western U.S. through the medium range period. Meanwhile,
downstream ejections of upper-level energy into the southern
Plains will periodically interact with moisture returning from the
Gulf of Mexico to result in heavy rainfall in the south-central
U.S. A heavy rain event should be culminating across the
Tennessee Valley as the medium range period begins Sunday night
into Monday. Model guidance this morning has reached a better
agreement on the degree of interaction between the warm and
moisture air mass from the Gulf of Mexico and broad troughing
sliding across the northern tier states. The GFS is now showing
more northern stream cyclogenesis in better agreement with the
ECMWF while the ECMWF shows slightly less robust cyclogenesis.
Consensus of the 00Z EC mean and the 06Z GEFS indicates a
northward shift in the heavy rainfall axis across the Tennessee
Valley. Meanwhile, a swath of wintry precipitation appears likely
from the Midwest to the Mid-northern Atlantic and into southern
New England Monday into Tuesday morning as the system moves across
and then off the Mid-Atlantic. By the middle of next week, model
consensus indicates another enhanced precipitation event will
develop along the Gulf Coast and then head northeastward into the
East Coast.

Over the Southwest, models are in good agreement on the timing of
another Pacific system moving through the area Sunday through
Tuesday. Thereafter, models continue to show significant
divergence on the timing and intensity of the next system dipping
from the Gulf of Alaska. The GFS continues to indicate a more
robust closed off system dipping farther south toward the West
Coast relative to the ECMWF. The CMC was somewhere in between.
The last couple of runs of the ECMWF did develop a better defined
system off the west coast of Canada in better agreement with the
GFS.

The WPC medium range forecast package was based on the consensus
of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, and smaller
contributions from the 00Z CMC and 00Z NAEFS. More of the
ensemble means were used on Days 6 and 7 to handle the higher
forecast uncertainty.

...Weather/Threats Highlights...

Another round of heavy rain will likely make weather headlines
across portions of the south-central U.S. by late Sunday and into
early Tuesday as the southwestern storm system emerges over the
Gulf Coast region. This will support a surface low along a
quasi-stationary frontal boundary, with copious Gulf moisture
being advected northward across interior portions of the Deep
South and Tennessee River Valley. The potential exists for 1 to 3
inches of rainfall with locally higher amounts, mainly from
southern Arkansas to the southern Appalachians, and this may be
enough to produce some episodes of flooding if high rainfall rates
persist for any given area. As this storm system reaches the East
Coast late Monday and into Tuesday, there should be enough cold
air in place to support either snow or a wintry mix for portions
of the northern Mid-Atlantic and Northeast. The next episode of
enhanced rainfall and interior wintry precipitation will appear to
develop near the Gulf Coast and progress toward the East Coast
during the middle of next week.

Meanwhile, heavy snow is becoming more likely for the central and
southern Rockies, the Wasatch Range, and the Sierra Nevada as
multiple upper level troughs pass through the region. Heavy rain
can also be expected for the lower elevations of central and
northern California as copious Pacific moisture is advected
towards the coast.

In terms of temperatures, the greatest anomalies are forecast to
be from Texas to the Deep South and Gulf Coast region, where the
humid southerly flow will make it feel more like spring, with
highs about 10-20 degrees above normal, and up to 25 degrees above
normal for overnight lows. This will particularly be the case
from Sunday into Tuesday in the warm sector of the low pressure
system. The most prolonged period of below normal temperature
readings are forecast to be over the western U.S., with highs up
to 15 degrees below normal, and also across portions of New
England, with readings about 10 degrees below normal. Most of the
central U.S. should have temperatures close to average for late
January.

Kong/Hamrick


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

Hazards:
- Heavy precipitation across portions of Southern California, the
Central and Southern
Appalachians, the Mid-Atlantic, the Great Lakes, and the Ohio
Valley, Mon-Tue, Jan 25-Jan 26.
- Heavy precipitation across portions of the Central Plains and
the Middle Mississippi Valley, Mon,
Jan 25.
- Heavy rain across portions of California, Wed-Thu, Jan 27-Jan 28.
- Heavy rain across portions of the Lower Mississippi Valley, the
Central Appalachians, the
Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic,
the Southern Appalachians, the
Southeast, the Southern Plains, and the Ohio Valley, Sun-Mon, Jan
24-Jan 25.
- Heavy snow across portions of the Pacific Northwest and the
Northern Great Basin, Sun, Jan 24.
- Heavy snow across portions of California, the Central Great
Basin, the Pacific Northwest, and the
Southwest, Sun-Mon, Jan 24-Jan 25 and Wed-Thu, Jan 27-Jan 28.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Mon-Tue, Jan 25-Jan 26.
- Severe weather across portions of the Lower Mississippi Valley
and the Southern Plains, Sun, Jan
24.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
203 AM EST Thu Jan 21 2021

Valid 00Z Fri Jan 22 2021 - 00Z Fri Jan 29 2021

The overall weather pattern across the Hawaii domain will be
featured with a strong mid-latitude high building in around 40
degrees north through the upcoming weekend. This will tend to
strengthen the trades with a mainly easterly component to the
flow, and then veers more towards the southeast by Sunday.

By Saturday night, an inverted surface trough is progged to
develop near 170 degrees west in response to a weak upper low,
with most of the rain from this event expected to be west of the
Islands. However, locally heavy trade wind based showers are
likely across the favored terrain of the Big Island early next
week. By Wednesday, there are signs of a larger scale trough well
to the northwest of the state, but the specifics are more
uncertain with the GEFS mean well to the east of the EC mean.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
801 PM EST Thu Jan 21 2021

Day 1
Valid 01Z Fri Jan 22 2021 - 12Z Fri Jan 22 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

...Central Louisiana through southern Mississippi and into
southwestern Alabama...

Areas of moderate rain have developed this evening along a
stationary front over central LA and south-central MS. Ample
moisture is converging on this front with precipitable water
values of 1.6 to 1.8 inches which is about 2.5 standard
deviations above normal and near climatological max values for
late January. However, limiting factors include a lack of
instability (MUCAPE around 250 J/Kg) and rather dry antecedent
conditions (7-day rainfall totals are near zero and soil
saturation only about 50% per the HRRR). So this will mainly be a
beneficial rain, but should repeating activity late tonight go
over areas that receive around 2 inches per the 23Z HRRR, there
may be localized runoff issues. Narrowed the Marginal Risk area a
little on the north side given regional radar trends and 12hr QPF
overnight from recent CAMs.


Jackson


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Thu Jan 21 2021

Day 2
Valid 12Z Fri Jan 22 2021 - 12Z Sat Jan 23 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
322 PM EST Thu Jan 21 2021

Day 3
Valid 12Z Sat Jan 23 2021 - 12Z Sun Jan 24 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano


Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Thu Jan 21 2021

Valid 00Z Fri Jan 22 2021 - 00Z Mon Jan 25 2021

...Great Lakes and Northeast...
Days 1-2...

A continuation of moderate to heavy lake effect snow is likely
downwind of Lake Ontario and Lake Erie into the weekend. Broad
cyclonic flow across the northeast persists, with at least 2
embedded shortwaves moving through the flow - one tonight and
another Friday evening. While each shortwave will have the
potential to produce some light WAA snow ahead of a cold front due
to some modest ascent, the primary heavy snow producer will be
again due to LES as flow becomes aligned to Ontario and Erie
behind each front on CAA. For Ontario, heavy LES should begin
early Friday morning and then persist into the evening, with a
slow SE push of the trajectories likely into early D2. This should
produce bursts of heavy snow rates of >1"/hr at times, and WPC
probabilities for 6" are high on D1 in the Tug Hill Plateau, with
high probabilities for 4" on D2 shifting to a more SE trajectory
off the Lake. For Lake Erie, the heaviest snow is expected D1 on
W/SW flow beneath the inversion, producing long fetch across this
lake as well. Total snowfall may be less here than east of Ontario
due to slightly less favorable fetch and upslope, but WPC
probabilities also indicate a high chance for 4" on D1 here with
some Huron moisture likely aiding in this snow band.

While not LES, there is potential for scattered snow squalls or
heavy snow showers across Upstate New York and Northern/Central
New England on D1, potentially lingering into early D2. Increasing
instability behind the cold front with continued mid-level ascent
could produce bursts of snowfall with rates >1"/hr for a short
duration. Accumulations within any of these showers/squalls should
be light, but could produce brief periods of limited visibility
and difficult travel.


...Central Rockies to Great Basin and Southwest...
Days 2-3...

A closed low deepening over Southern California will drop
southeast Saturday and then gradually begin to open as it moves
atop the Four Corners region on Sunday. This feature will produce
height falls and mid-level divergence, with modest upper
diffluence also present for ascent. Downwind of this trough, a
Pacific jet streak will intensify, while 700-500mb flow becomes
oriented to the SW. These features together will cause an increase
in PW evident by saturation in the 700-500mb RH fields, creating
an environment favorable for heavy precipitation. While the
mid-level low opens and begins to eject quickly, pronounced
synoptic ascent in the presence of this deeper moisture will allow
for heavy snowfall to spread from the Sierra eastward through the
Utah ranges, and into the CO Rockies and San Juans D2, shifting
further south towards the Mogollon Rim and White Mtns of Arizona
on D3. WPC probabilities are high for 8 inches on D2 in the Uintas
and San Juans, as well as some of the higher isolated peaks in the
Great Basin. By D3, the remaining high probabilities for 8 inches
are confined to the Mogollon Rim, White Mtns, and continue into
the San Juans. While less impressive, snow levels which may drop
as low as 3000 ft into Southern CA will produce moderate snow as
far south as the San Gabriels and San Bernadinos, as well as light
accums possible even into the somewhat lower elevations east of
Los Angeles and east of Tucson.


...Northern Plains to Upper Midwest...
Days 2-3...

A piece of energy shedding from a closed low over the Great Basin
will eject northeast and embed within confluent mid-level flow as
it traverses into the Plains and Midwest this weekend. As this
wave moves northeast, it will be accompanied by moist advection
within the aforementioned confluent flow from the Pacific, while
low-level southerly flow intensifies out of the Gulf of Mexico
driving warm and moist advection northward. At the same time,
modest diffluence from the LFQ of an upper jet streak will aid in
ascent, and a fast moving but potent swath of precipitation is
likely to spread eastward from the High Plains of SD eastward
through the Western Great Lakes. While this system is fast moving,
it does have plentiful moisture with which to work, and forcing is
briefly robust. Additionally, regional soundings indicate an
environment favorable for very high SLRs, possibly approaching
20:1, due to a deepening DGZ, a nearly isothermal layer within the
WAA, and a cold overall column with temps in the teens near the
surface. These high SLRs should squeeze out heavy snow despite
modest QPF, and WPC probabilities for 4" are 40-60% from central
SD eastward across southern MN and into Wisconsin. Local maxima in
excess of 6" are likely in southern MN which has the best overlap
of high SLR, moisture, and forcing.


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

A shortwave dropping out of the Gulf of Alaska will be accompanied
by the LFQ of a modest Pacific jet streak to drive moisture and
ascent into the Pacific Northwest the latter half of D3.
Increasing moisture and ascent will spread precipitation down the
coast from WA through northern CA. An elongated trough across the
West ahead of this feature will lead to lowering snow levels, with
further reduction likely as the shortwave drops along the coast.
Snow levels may fall below 1000 ft as far south as Portland, OR,
and between 1000-2000 ft into northern CA. The heaviest
precipitation is likely into the OR Cascades with this impulse,
but WPC probabilities for 4 inches are high from the Siskiyous
northward along the Cascades of OR and WA and into the Olympics.
With the very low snow levels and the potential for easterly winds
due to the pressure gradient across OR/WA, light snow accums are
possible into the Columbia Gorge and some of the other surrounding
lowlands on Sunday, but looks to remain just outside the major
metro areas of Seattle and Portland at this time.


For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Thu Jan 21 2021

Valid 12Z Sun Jan 24 2021 - 12Z Thu Jan 28 2021

...Additional heavy rainfall likely for portions of the
south-central U.S. for the beginning of the week; heavy mountain
snow and unsettled weather moving into the western U.S....

...Weather Pattern Overview and Guidance/Predictability
Assessment...

Active amplification of upper-level troughs digging down from the
Gulf of Alaska will be a continued source of unsettled weather for
the western U.S. through the medium range period. Meanwhile,
downstream ejections of upper-level energy into the southern
Plains will periodically interact with moisture returning from the
Gulf of Mexico to result in heavy rainfall in the south-central
U.S. A heavy rain event should be culminating across the
Tennessee Valley as the medium range period begins Sunday night
into Monday. Model guidance this morning has reached a better
agreement on the degree of interaction between the warm and
moisture air mass from the Gulf of Mexico and broad troughing
sliding across the northern tier states. The GFS is now showing
more northern stream cyclogenesis in better agreement with the
ECMWF while the ECMWF shows slightly less robust cyclogenesis.
Consensus of the 00Z EC mean and the 06Z GEFS indicates a
northward shift in the heavy rainfall axis across the Tennessee
Valley. Meanwhile, a swath of wintry precipitation appears likely
from the Midwest to the Mid-northern Atlantic and into southern
New England Monday into Tuesday morning as the system moves across
and then off the Mid-Atlantic. By the middle of next week, model
consensus indicates another enhanced precipitation event will
develop along the Gulf Coast and then head northeastward into the
East Coast.

Over the Southwest, models are in good agreement on the timing of
another Pacific system moving through the area Sunday through
Tuesday. Thereafter, models continue to show significant
divergence on the timing and intensity of the next system dipping
from the Gulf of Alaska. The GFS continues to indicate a more
robust closed off system dipping farther south toward the West
Coast relative to the ECMWF. The CMC was somewhere in between.
The last couple of runs of the ECMWF did develop a better defined
system off the west coast of Canada in better agreement with the
GFS.

The WPC medium range forecast package was based on the consensus
of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, and smaller
contributions from the 00Z CMC and 00Z NAEFS. More of the
ensemble means were used on Days 6 and 7 to handle the higher
forecast uncertainty.

...Weather/Threats Highlights...

Another round of heavy rain will likely make weather headlines
across portions of the south-central U.S. by late Sunday and into
early Tuesday as the southwestern storm system emerges over the
Gulf Coast region. This will support a surface low along a
quasi-stationary frontal boundary, with copious Gulf moisture
being advected northward across interior portions of the Deep
South and Tennessee River Valley. The potential exists for 1 to 3
inches of rainfall with locally higher amounts, mainly from
southern Arkansas to the southern Appalachians, and this may be
enough to produce some episodes of flooding if high rainfall rates
persist for any given area. As this storm system reaches the East
Coast late Monday and into Tuesday, there should be enough cold
air in place to support either snow or a wintry mix for portions
of the northern Mid-Atlantic and Northeast. The next episode of
enhanced rainfall and interior wintry precipitation will appear to
develop near the Gulf Coast and progress toward the East Coast
during the middle of next week.

Meanwhile, heavy snow is becoming more likely for the central and
southern Rockies, the Wasatch Range, and the Sierra Nevada as
multiple upper level troughs pass through the region. Heavy rain
can also be expected for the lower elevations of central and
northern California as copious Pacific moisture is advected
towards the coast.

In terms of temperatures, the greatest anomalies are forecast to
be from Texas to the Deep South and Gulf Coast region, where the
humid southerly flow will make it feel more like spring, with
highs about 10-20 degrees above normal, and up to 25 degrees above
normal for overnight lows. This will particularly be the case
from Sunday into Tuesday in the warm sector of the low pressure
system. The most prolonged period of below normal temperature
readings are forecast to be over the western U.S., with highs up
to 15 degrees below normal, and also across portions of New
England, with readings about 10 degrees below normal. Most of the
central U.S. should have temperatures close to average for late
January.

Kong/Hamrick


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

Hazards:
- Heavy precipitation across portions of Southern California, the
Central and Southern
Appalachians, the Mid-Atlantic, the Great Lakes, and the Ohio
Valley, Mon-Tue, Jan 25-Jan 26.
- Heavy precipitation across portions of the Central Plains and
the Middle Mississippi Valley, Mon,
Jan 25.
- Heavy rain across portions of California, Wed-Thu, Jan 27-Jan 28.
- Heavy rain across portions of the Lower Mississippi Valley, the
Central Appalachians, the
Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic,
the Southern Appalachians, the
Southeast, the Southern Plains, and the Ohio Valley, Sun-Mon, Jan
24-Jan 25.
- Heavy snow across portions of the Pacific Northwest and the
Northern Great Basin, Sun, Jan 24.
- Heavy snow across portions of California, the Central Great
Basin, the Pacific Northwest, and the
Southwest, Sun-Mon, Jan 24-Jan 25 and Wed-Thu, Jan 27-Jan 28.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Mon-Tue, Jan 25-Jan 26.
- Severe weather across portions of the Lower Mississippi Valley
and the Southern Plains, Sun, Jan
24.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml