Weather Prediction Center

National Oceanic and Atmospheric Administration

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

Valid 12Z Thu Jan 21 2021 - 12Z Sat Jan 23 2021

...Heavy rain and possible flash flood threat decreasing across portions
of the Desert Southwest Thursday morning...

...Heavy rain developing across the Lower Mississippi Valley/Southeast on
Thursday...

...Much need snows expected across the Sierra and Wasatch Friday into
Saturday...

...An active Lake effect snow pattern begins Thursday and continues
through Friday downwind of the Great Lakes...

A strong mid to upper level low will be pushing eastward along the
Southwest U.S./northwest Mexico border region on Thursday and then
accelerate across the Southern Plains and into the Southeast on Friday.
This system is currently producing heavy rains over portions of the Desert
Southwest with this heavy rain and flash flood threat continuing into
Thursday morning before diminishing by Thursday afternoon.

While conditions improve over the Southwest, they will become increasingly
wet farther to the east from portions of the Southern Plains, into the
Lower Mississippi Valley and Southeast Thursday into Friday. Widespread
rains are expected across these areas with the heaviest totals in a stripe
from far eastern Texas, across central to northern Louisiana, central to
southern Mississippi, southern Alabama into southern Georgia. Aside from
eastern Texas into far northwest Louisiana, much of this region has had
below average precipitation this month resulting in stream flows at or
below normal levels. This should reduce the threat of flash flooding with
only a marginal risk depicted across portions of this region Thursday into
Friday.

Another storm system is expected to push into northern California on
Friday and move eastward into the Great Basin on Saturday. This system
will bring some much needed heavy snows to the Sierra of California and
Wasatch of Utah. Much of California into the Great Basin is well below
average in precipitation this season, experiencing severe to exception
drought levels.

Two cold fronts will be sweeping across the Great Lakes Thursday into
early Friday producing favorable cold westerly to northwesterly flow
across the unfrozen Great Lakes. Lake effect snow showers will increase
in coverage and intensity Thursday and continue through Friday. Locally
heavy snowfall accumulation are likely with the heaviest totals expected
over western and northwestern portions of New York State, downwind of
Lakes Erie and Ontario, northern portions of the Lower Peninsula of
Michigan, downwind of northern Lake Michigan and along the north coast of
the Upper Peninsula of Michigan, downwind of Lake Superior.

Oravec

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 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 indicate 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.

Overall, deterministic model guidance is in good agreement on the
evolution of the synoptic scale pattern across the nation through
about Monday with good ensemble mean support. By Tuesday, the 00Z
GFS remains less amplified compared to the 00Z CMC and ECMWF with
the shortwave crossing the east-central U.S., and this was also
the case with the earlier 12Z model guidance suite. More
noteworthy model differences are apparent across the western U.S.
for the second half of the forecast period as additional shortwave
energy tracks southward from the Gulf of Alaska region. By
Tuesday, the GFS is stronger and faster with the low off the
Pacific Northwest coast. Other detail differences in the
evolution of that trough/storm system make the ensemble means a
more prudent choice as a starting point in the forecast process
for the Wednesday through Thursday time period.


...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. 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

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
1058 AM EST Thu Jan 21 2021

Day 1
Valid 16Z Thu Jan 21 2021 - 12Z Fri Jan 22 2021

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

...Louisiana into Mississippi and southwestern Alabama...

Rainfall over the Lower Mississippi Valley will increase and
expand especially after 00Z tonight as an in situ stationary front
weakens across Louisiana. Shortwave exiting Texas will skirt
across the region as another cold front drops in from the north,
increasing moisture convergence in a west-to-east band generally
between I-10 to the south and I-20 to the north. Precipitable
water values are forecast to increase to about 1.6-1.8 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 and rather dry antecedent
conditions (7-day rainfall totals are near zero and soil
saturation only about 50% per the HRRR). 12Z HREF probabilities
show ~0% chance of 1"/hr rates but ~50% chance of >0.50"hr
suggesting a rather low threat of flash flooding (taken at face
value). Guidance still varied in N-S axis displacement and timing
from west to east, but consensus still lied near the 13Z NBM and
WPC preference with perhaps a very slight shift south (within the
noise). 12-hr QPF (00Z-12Z) may total 1.5-2.5" in/near the Risk
area with local amounts near 3" possible. Maintained the Marginal
Risk area despite the limiting factors as any underforecast
instability could contribute to higher rainfall rates than
currently advertised by the guidance.


Fracasso


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
307 AM 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.

Bann


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM 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.

Bann


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
314 AM EST Thu Jan 21 2021

Valid 12Z Thu Jan 21 2021 - 12Z Sun Jan 24 2021

...Great Lakes...
Days 1-3...

The heaviest lake effect snow on Day 1 is forecast to be in the
Tug Hill of NY, as an initial burst of snow showers from a
combination of warm advection and upslope flow is followed by
winds veering towards the west tonight, supporting long cross-lake
trajectories that maximize moisture fluxes and support snow
developing in windward terrain.
The low level westerly flow persists into the first few hours of
Day 2 Fri., with the primary band likely continuing.
WPC probabilities are high both on D1 for 4/8 inches, with local
totals exceeding 12" possible combined over the two days.

In the upper lakes, the primary lake effect occurs on Day 1 as a
secondary front progresses across the Upper Peninsula of MI and
then northern lower MI tonight. Low level frontal convergence is
followed by the frontal passage and then northwest flow that
provides lee shore convergence and lift along the south shore of
Lake Superior and in the Traverse Bay portion of northwest lower
MI and adjacent terrain.

The cold front driving through the lower lakes then forces more
veering, winds turning northwest, and the band relocation further
south Fri night.
The northwest flow crossing Lake Erie spawns lee shore convergence
areas with several inches of snow at the end of Day 1 through Day
2.

On Day 3 Saturday, low-mid level warm advection in advance of a
synoptic scale wave provides ascent and moisture return, with snow
developing in the western UP of Michigan.
Probabilities are low for heavy snow given the short duration,as
snow is confined to late Sat night.


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

A long duration snow is expected over the mountains of western WY
as an upper level west-east jet persists with embedded divergence
maxima aloft. The column moistens and low-mid level ascent
resulting from convergence maxima continues tonight into Friday.
Up to a foot is possible in the Tetons from this event.

Moisture streams northeastward from the Pacific Ocean along a 300
mb jet maxima. This is likely to produce widespread snow from the
Sierra eastward across the ranges of NV, northern UT and western
CO on Day 2. Vorticity lobes swinging through the flow may enhance
ascent and snowfall at times, and there is likely to be a 36 hour
window of off-and-on snow across the region. WPC probabilities for
6" are high D3 across many of the ranges of Utah including the
Wasatch and Uintas, as well as into the CO Rockies and San Juans.
Local snowfall may exceed 12 inches in the highest peaks.

The models have indicated potential for a 700 mb low to close off
in the southwest Saturday. The 300 mb jet moves across AZ and NM,
with embedded difluent flow supporting ascent in windward terrain
of the Mogollon Rim of AZ, and then the San Juan Mountains of
northern NM and southwest CO. Locally heavy snow is expected in
windward terrain of these ranges on Day 3. WPC probabilities are
low to moderate for 8 inches Saturday/Saturday night.


...Northern Plains to Upper Midwest...
Day 3...

The models show an 850-700 mb shortwave moving east across the
northern Plains towards the Upper MS Valley. Confluent flow
produces 700 mb theta-e advection convergence. The moisture
return combined with warm advection leads t snow developing.
About a quarter to four tenths of an inch of liquid equivalent QPF
is forecast across portions of eastern SD, southern MN, northern
Iowa, and western Wisconsin. An area of 4-7 inches of snow is
forecast across this region.

While the forcing for ascent is likely to be transient from west
to east, it could be intense, and forecast profiles indicate a
deepening saturated dendritic growth zone to promote snow growth
and high snow to liquid ratios. A period of moderate to at times
heavy snow is becoming more likely Friday night and Saturday. WPC
probabilities for 4" are as high as 60% across eastern SD into
southern MN .

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

Petersen

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 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 indicate 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.

Overall, deterministic model guidance is in good agreement on the
evolution of the synoptic scale pattern across the nation through
about Monday with good ensemble mean support. By Tuesday, the 00Z
GFS remains less amplified compared to the 00Z CMC and ECMWF with
the shortwave crossing the east-central U.S., and this was also
the case with the earlier 12Z model guidance suite. More
noteworthy model differences are apparent across the western U.S.
for the second half of the forecast period as additional shortwave
energy tracks southward from the Gulf of Alaska region. By
Tuesday, the GFS is stronger and faster with the low off the
Pacific Northwest coast. Other detail differences in the
evolution of that trough/storm system make the ensemble means a
more prudent choice as a starting point in the forecast process
for the Wednesday through Thursday time period.


...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. 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

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