Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Tue Jan 19 2021

Valid 00Z Wed Jan 20 2021 - 00Z Fri Jan 22 2021

...Damaging winds and critical fire weather to come to an end across
California on Wednesday...

...Multiple rounds of snow showers across the Great Lakes and Northeast
with locally heavy snowfall totals downwind of the Great Lakes...

The weather pattern responsible for damaging winds and critical fire
weather across California is expected to gradually weaken by Wednesday
evening. A developing low pressure system off the Southern California
coast and strengthening high pressure from the Northern Rockies into the
Great Basin is fueling strong winds across much of the state of California
tonight and into Wednesday. High Wind warnings and advisories continue
across much of California, where wind gusts may reach 70-80 mph at higher
elevations and 50 to 70 mph in valleys and along the coast. These high
winds have the potential to produce widespread downed trees and tree
limbs, significant property damage, and numerous power outages.

In addition, the combination of high winds and low relative humidities
will continue to support a significant fire weather risk, especially along
coastal sections from San Francisco to San Diego. The only potentially
beneficial part of this weather pattern is the increased chances of
rainfall for portions of southern California into southern Arizona through
Thursday morning as the aforementioned area of low pressure pushes slowly
eastward off the northern Baja. Rain could be heavy at times across far
southeastern California and southwest Arizona, where Flash Flood Watches
have been issued.

Meanwhile, an active lake effect snow pattern is expected across much of
the Great Lakes through Thursday. Two separate systems are forecast to
cross the region over the next few days, the first will exit overnight,
while the second enters on Thursday. This will support locally heavy
snowfall accumulations mainly downwind of Lake Erie and Lake Ontario,
where Lake Effect Snow Warnings are currently in effect. Locally heavy
snow is also possible downwind of Lake Superior across portions of the
Upper Peninsula of Michigan, and downwind of Lake Michigan as well. There
will also be the potential for upslope snow showers across the central
Appalachians, with 2 to 4 inches of snowfall possible.

Snell

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Tue Jan 19 2021

Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021

...Additional heavy rainfall appears likely for portions of the
south-central U.S. by Sunday and Monday as unsettled weather
overspreads the West Coast...

...Overview...

The recent shift in the synoptic pattern toward trough
amplifications along the West Coast appears to continue through
the medium range period as model consensus forecasts multiple
vigorous shortwaves digging across the region. This pattern will
also encourage additional heavy rain to develop across the
Mid-south as these shortwaves exit into the southern Plains.

...Guidance/Predictability Assessment...

Overall, model guidance this morning was in good to very good
agreement on the evolution of the synoptic scale pattern across
the U.S. through the medium range period. The 00Z ECMWF and 06Z
GFS solutions appeared most compatible with one another through
this weekend. By early next week, both models indicate the
likelihood of a low pressure system developing in the central to
southern Plains as broad upper-level troughing dips into the
northern tier states. Both models show significant run-to-run
variability regarding the speed and intensity of this system as it
moves across the East Coast by about next Tuesday. The 00Z EC
mean and the 06Z GEFS mean show only modest development of this
system with a relatively flat upper trough. The days 6 and 7
grids were based mainly on a blend of these ensemble means with
some 06Z GFS and the least from the 00Z ECMWF. The 00Z CMC was
not as compatible with the 00Z EC/06Z GFS blend and thus was not
taken into account in the forecast package.

...Weather/Threats Highlights...

An axis of heavy rain is expected to begin the medium range period
across the lower Mississippi Valley late this week near a
slow-moving frontal boundary. By Sunday, the storm system that
will be gathering strength across the Desert Southwest and
southern Rockies is expected to reach the southern Plains. A
departing surface high over the Midwest will then bring low level
moisture once again from the western Gulf of Mexico toward the
Deep South. The axis of heaviest rainfall appears to extend
mainly from the ArkLaTex region to the southern Appalachians
Sunday and Monday, with the potential of 1 to 3 inches of rain
together with locally higher amounts. This could cause flooding
concerns if these higher amounts fall over those areas impacted by
the earlier rainfall event late this week. The upper trough
amplifying over the western U.S. will advect enough Pacific
moisture and produce enough upslope flow to result in significant
snow for the central Rockies and Wasatch mountains. Some of this
wintry precipitation will likely spread northeastward across the
northern Plains and Upper Midwest by next weekend, but the details
remain uncertain with respect to the development of surface low
pressure and frontal interactions. In addition, unsettled weather
appears likely for the Desert Southwest by this weekend and into
early next week. Wintry precipitation appears possible for the
higher elevations of Arizona while rain appears likely for the
southern half of the state into southern California.

In terms of temperatures, the greatest anomalies are forecast to
be across the central and western Gulf Coast region, where the
southerly moist flow keeps readings about 10-20 degrees above
normal for late January. The most prolonged period of below
normal temperature readings are forecast to be over the western
U.S., with highs around 5-10 degrees below normal, and also across
the Upper Midwest on Friday with readings about 10 degrees below
normal.

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 the Mid-Atlantic, and the
Central Appalachians, Mon, Jan
25.
- Heavy rain across portions of the Lower/Middle Mississippi
Valley, the Central/Southern
Appalachians, the Tennessee/Ohio Valley, the Mid-Atlantic, the
Southeast, and the Southern Plains,
Sun-Mon, Jan 24-Jan 25.
- Heavy snow across portions of California, the Pacific Northwest,
and the Northern Great Basin,
Sun, Jan 24.
- Heavy snow across portions of California, Mon, Jan 25.
- Heavy snow across portions of the Central Rockies, the Central
Great Basin, and the Northern
Great Basin, Fri-Sat, Jan 22-Jan 23rd and Mon, Jan 25.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Sat-Mon, Jan 23-Jan 25.
- Heavy snow across portions of the Southern Rockies, the Central
Rockies, the Central Great Basin,
and the Southwest, Fri-Mon, Jan 22-Jan 25.
- Heavy snow across portions of the Great Lakes, the Upper
Mississippi Valley, and the Northern
Plains, Sat-Sun, Jan 23-Jan 24.
- Flooding occurring or imminent across portions of the Southeast.

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
304 AM EST Tue Jan 19 2021

Valid 00Z Wed Jan 20 2021 - 00Z Wed Jan 27 2021

The unsettled weather pattern that is currently in place across
the Hawaii domain is expected to improve by late Wednesday and
into Thursday, as the surface trough and upper level low slowly
weaken and move westward. By the end of the week and into the
weekend, a strong surface high builds back in to the north and
northeast of the state. This will tend to strengthen the trades
with a mainly easterly component to the flow, and then veers to
the east-southeast by Sunday. This is in response to another
inverted surface trough that should 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.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
730 PM EST Tue Jan 19 2021

Day 1
Valid 16Z Tue Jan 19 2021 - 12Z Wed Jan 20 2021

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

Jackson

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 PM EST Tue Jan 19 2021

Day 2
Valid 12Z Wed Jan 20 2021 - 12Z Thu Jan 21 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN CALIFORNIA...

...Southern California/Arizona...
A cut off low off the Southwest coast will slowly migrate inland
over southern California and the northern Baja region through the
forecast period (Wednesday into early Thursday). In response,
deep southerly flow will transport the moisture rich environment
north just ahead of the aforementioned low into southern CA/AZ.
This is evident by the precipitable water values climbing to over
0.75 inches, possibly reaching 1 inch in far southern AZ, which is
2 to 3 standard deviation above the mean. The moisture flux will
be aided by 25 knot low level south/easterly flow helping to
promote upslope along the terrain, most notable the Peninsular
Ranges. As a result, the precipitation will be enhanced by the
upslope component. In addition, with lapse rates are expected to
steepen as cooler air aloft moves in from the west allowing the
instability to increase (albeit weak). Therefore, rain rates may
exceed 0.75 inches per hour at times.

Areal average precipitation will range between 2-3+ inches along
the Peninsular Ranges and 1-2+ across the deserts and upslope
terrain in southern CA/AZ. This combined with the aforementioned
rain rates and QPF on Day 1 (through Wednesday morning), there is
the increased potential for flash flooding within this region.
Given the orographic effects will be most robust along the
Peninsular Range with multiple rounds of showers and isolated
thunderstorms, a Slight Risk was issued for this location. The
rest of the region will see moderate to heavy rain, though the
impacts should be localized given the progressive nature of the
precipitation activity and less dynamic forcing and instability.

Pagano




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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
304 PM EST Tue Jan 19 2021

Day 3
Valid 12Z Thu Jan 21 2021 - 12Z Fri Jan 22 2021

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

...Lower Mississippi Valley...
A shortwave over the Desert Southwest will transition quickly east
while a weak northern stream trough moves through the Great Lakes
and into the Northeast. The southern and northern stream will not
phase resulting in this aforementioned shortwave energy shifting
east across the Lower MS Valley and into the Southeast through the
forecast period (Thursday into early Friday). While the dynamics
associated with this system are not robust, there will be a period
of time across the region where training of moderate to heavy rain
could result in localized flash flooding. Precipitable water
values will increase to around 1.5 to 1.75 inches, which is 2-3
standard deviations above the mean for this time of year. This
will be aided by 30-40 knot low level southern flow with a
pronounced low level nocturnal jet developing Thursday night
helping to enhance rain rates along and south of the surface
front. A key feature that may limit rain rates will be the weak
instability. Despite this however, the propagation vectors should
align with the mean wind to result in training of precipitation as
activity moves from west to east along the front. This will
increase hourly precipitation totals and thus whittle down the
high flash flood guidance that is currently in place.

Areal average precipitation will range between 1-3+ inches, with a
narrow corridor of 2.5-3.5 inches likely. The exact placement of
the heavier footprint of QPF will depend on the position of the
surface front and progression of the surface low along this
boundary. While the antecedent conditions are dry, the training
as mentioned above could be enough to produce nuisance/localized
flash flooding issues. Therefore, a Marginal Risk was introduced.

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
339 AM EST Tue Jan 19 2021

Valid 12Z Tue Jan 19 2021 - 12Z Fri Jan 22 2021


...Great Lakes...
Days 1-3...
Cold westerly flow will continue to support lake effect snows east
of Lake Erie, with additional heavy accumulations possible across
the western Southern Tier of New York on Tuesday, with WPC PWPF
indicating high probabilities for additional accumulations of
6-inches or more for the region during the Day 1 period. A
redeveloping band farther to the south may reintroduce the threat
for heavy amounts back into Northwest Pennsylvania Tuesday night.
Meanwhile, lake effect snows are expected to develop and intensify
east of Lake Ontario into the Tug Hill Region on Tuesday, where
WPC PWPF indicates locally heavy amounts of 8-inches or more are
likely. The threat for heavy snow is expected to diminish on
Wednesday, with snow bands becoming more transient as the veering
winds direct them farther to the south before diminishing as a
shortwave ridge moves across the region. Snows are expected to
return to portions of the region on Thursday as a low moves north
of the lakes across southern Ontario into Quebec. Westerly winds
behind the trailing cold front are expected to help reignite lake
effect snows east of Ontario, with locally heavy amounts possible
once again across the Tug Hill Region.

...Southern Rockies...
Day 1...
Favorable upper forcing along with low level easterly flow will
continue to support snows through the morning hours across the
southern Colorado and northern New Mexico mountains, with
additional heavy accumulations possible across the San Juan and
Sangre de Cristo mountains. As the supporting upper low continues
to retrograde west across Southern California, expect snow to
quickly diminish during the afternoon.

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

Pereira

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Tue Jan 19 2021

Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021

...Additional heavy rainfall appears likely for portions of the
south-central U.S. by Sunday and Monday as unsettled weather
overspreads the West Coast...

...Overview...

The recent shift in the synoptic pattern toward trough
amplifications along the West Coast appears to continue through
the medium range period as model consensus forecasts multiple
vigorous shortwaves digging across the region. This pattern will
also encourage additional heavy rain to develop across the
Mid-south as these shortwaves exit into the southern Plains.

...Guidance/Predictability Assessment...

Overall, model guidance this morning was in good to very good
agreement on the evolution of the synoptic scale pattern across
the U.S. through the medium range period. The 00Z ECMWF and 06Z
GFS solutions appeared most compatible with one another through
this weekend. By early next week, both models indicate the
likelihood of a low pressure system developing in the central to
southern Plains as broad upper-level troughing dips into the
northern tier states. Both models show significant run-to-run
variability regarding the speed and intensity of this system as it
moves across the East Coast by about next Tuesday. The 00Z EC
mean and the 06Z GEFS mean show only modest development of this
system with a relatively flat upper trough. The days 6 and 7
grids were based mainly on a blend of these ensemble means with
some 06Z GFS and the least from the 00Z ECMWF. The 00Z CMC was
not as compatible with the 00Z EC/06Z GFS blend and thus was not
taken into account in the forecast package.

...Weather/Threats Highlights...

An axis of heavy rain is expected to begin the medium range period
across the lower Mississippi Valley late this week near a
slow-moving frontal boundary. By Sunday, the storm system that
will be gathering strength across the Desert Southwest and
southern Rockies is expected to reach the southern Plains. A
departing surface high over the Midwest will then bring low level
moisture once again from the western Gulf of Mexico toward the
Deep South. The axis of heaviest rainfall appears to extend
mainly from the ArkLaTex region to the southern Appalachians
Sunday and Monday, with the potential of 1 to 3 inches of rain
together with locally higher amounts. This could cause flooding
concerns if these higher amounts fall over those areas impacted by
the earlier rainfall event late this week. The upper trough
amplifying over the western U.S. will advect enough Pacific
moisture and produce enough upslope flow to result in significant
snow for the central Rockies and Wasatch mountains. Some of this
wintry precipitation will likely spread northeastward across the
northern Plains and Upper Midwest by next weekend, but the details
remain uncertain with respect to the development of surface low
pressure and frontal interactions. In addition, unsettled weather
appears likely for the Desert Southwest by this weekend and into
early next week. Wintry precipitation appears possible for the
higher elevations of Arizona while rain appears likely for the
southern half of the state into southern California.

In terms of temperatures, the greatest anomalies are forecast to
be across the central and western Gulf Coast region, where the
southerly moist flow keeps readings about 10-20 degrees above
normal for late January. The most prolonged period of below
normal temperature readings are forecast to be over the western
U.S., with highs around 5-10 degrees below normal, and also across
the Upper Midwest on Friday with readings about 10 degrees below
normal.

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 the Mid-Atlantic, and the
Central Appalachians, Mon, Jan
25.
- Heavy rain across portions of the Lower/Middle Mississippi
Valley, the Central/Southern
Appalachians, the Tennessee/Ohio Valley, the Mid-Atlantic, the
Southeast, and the Southern Plains,
Sun-Mon, Jan 24-Jan 25.
- Heavy snow across portions of California, the Pacific Northwest,
and the Northern Great Basin,
Sun, Jan 24.
- Heavy snow across portions of California, Mon, Jan 25.
- Heavy snow across portions of the Central Rockies, the Central
Great Basin, and the Northern
Great Basin, Fri-Sat, Jan 22-Jan 23rd and Mon, Jan 25.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Sat-Mon, Jan 23-Jan 25.
- Heavy snow across portions of the Southern Rockies, the Central
Rockies, the Central Great Basin,
and the Southwest, Fri-Mon, Jan 22-Jan 25.
- Heavy snow across portions of the Great Lakes, the Upper
Mississippi Valley, and the Northern
Plains, Sat-Sun, Jan 23-Jan 24.
- Flooding occurring or imminent across portions of the Southeast.

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