Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EST Thu Feb 25 2021

Valid 00Z Fri Feb 26 2021 - 00Z Sun Feb 28 2021

...Gusty winds and heavy snow to impact portions of the Pacific Northwest
and Northern Rockies over the next few days...

...Numerous showers and thunderstorms likely from the Southern Plains to
Southern Appalachians through this weekend...

...Light snow and wintry mix to spread throughout the Appalachians,
Northeast, and New England between Friday evening and Saturday...

A potent Pacific storm system is currently entering the Northwest and
expected to bring gusty winds and high elevation snow. Post-frontal snow
showers are forecast to linger through Friday and into the early morning
hours on Saturday. Total snowfall amounts will be measured in feet across
the Cascade Mountains of Washington and Oregon. Heavy snow will also
infiltrate inland areas, including the Blue Mountains of northeast Oregon,
as well as the Northern Rockies from northern Idaho to the Wasatch
Mountains of Utah. Wind gusts over 50 mph for much of the same region
could make visibility difficult at times due to blowing snow. In the lower
elevations, less snow is likely, but wind gusts could still make travel
difficult for high profile vehicles. Winter Storm Warnings, Winter Weather
Advisories, as well as Wind Advisories have been posted for the area. For
portions of the Northern High Plains in Wyoming and Montana, even stronger
wind gusts may mix down to the surface; therefore High Wind Warnings have
been issued.

Meanwhile, a stalled frontal boundary across the Southern Plains and
Southeast will be the focus for numerous showers and thunderstorms
beginning tonight and lasting through the weekend. Through Saturday
evening, rainfall amounts over 2 inches will be possible from northeast
Texas to the Southern Appalachians. Isolated instances of flooding will be
possible, mainly from the Lower Mississippi Valley to much of the
Tennessee Valley. Thus, WPC has issued a Marginal Risk for Excessive
Rainfall throughout the region. Isolated severe thunderstorms will also be
a concern as unseasonably warm and moist air continues to enter from the
Gulf of Mexico. SPC has issued a Marginal Risk for severe thunderstorms
that lasts through early tomorrow morning from the Southern Plains to
Lower Mississippi Valley. The severe threat then shifts slightly eastward
and into the Tennessee Valley on Friday. Large hail and damaging winds
will be the main risks with these storms.

By Friday evening and into the first half of this weekend, a wave of
showers associated with the aforementioned frontal system and an
approaching wave of low pressure is forecast to enter a colder airmass in
place from the Central Appalachians to northern New England. A few inches
of snow and a glaze of freezing rain will be possible, but little to no
travel impacts are expected. Light rain is expected for coastal regions
and major cities along the I-95 corridor during this time frame.

Snell

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Thu Feb 25 2021

Valid 12Z Sun Feb 28 2021 - 12Z Thu Mar 04 2021

...Heavy rain threat gradually diminishes across the interior Deep
South early next week...

...Overview and Guidance Evaluation...

An ongoing heavy rain event should extend across the interior Deep
South towards the southern Appalachians as the forecast period
begins on Sunday. There has been a gradual southward shift in the
heavy rain axis as additional upper level energy is forecast to
eject from the upper level trough dipping into the southwestern
U.S. On the other hand, models are trending slower with respect
to the eastward motion of the upper low itself. This could result
in keeping the heavy rain threat a bit longer in the Deep South on
Monday. Thereafter, there is a general consensus among model
guidance that the bulk of the moisture will gradually move
eastward across the Southeast ahead of the upper trough. However,
uncertainty remains quite high regarding the subsequent evolution
of this system. A blend of the 00Z ECMWF mean, 06Z GEFS and the
00Z CMC mean yielded a developing low pressure wave moving off the
Southeast U.S. coast by the middle of next week.

Meanwhile, deterministic models and ensembles continue to struggle
with both the timing and evolution of the next trough approaching
the West Coast from the Gulf of Alaska/North Pacific region as
early as Sunday night. The GFS and to a lesser extent the GEFS
mean continue to indicate the development of a closed upper level
low that lingers well offshore, while the ECMWF keeps the upper
trough largely over land as it digs down the West Coast early next
week. The latest (12Z) ECMWF is even more progressive than the
00Z run. The CMC is somewhere in between. These differences
have implications for the southern tier states for the latter part
of next week. Forecast confidence is below average for this part
of the country from Day 5 onward.

The WPC medium-range fronts and pressures were derived based on
the consensus of the 00Z ECMWF/00Z EC mean and the 06Z GFS/GEFS
for Days 3 to 4. Beginning on Day 5, more of the EC mean and 06Z
GEFS together with the 00Z CMC mean were used due to the above
normal forecast uncertainty.


...Weather/Threats Highlights...

The upper level trough forecast to move across the southern
Rockies and then the southern Plains will allow Gulf moisture to
lift northward as multiple waves of low pressure develop along a
slow-moving front. This setup will favor rounds of locally heavy
rainfall late this weekend into early next week from the Arklatex
to the southern Appalachians. The heavy rain threat could linger a
bit longer in the Deep South on Monday but should gradually
diminish by Tuesday as the bulk of moisture should slide eastward
into and then off the Southeast afterwards. Meanwhile,
uncertainty is quite high regarding the specific details across
the Southwest to the Plains. Mountain snows are possible for the
central/southern Rockies as the upper trough moves through. Some
precipitation could be moving into the central Plains toward the
latter part of next week but uncertainty remains much higher than
normal.

High temperatures will likely be about 5 to 15 degrees above
average across the eastern U.S. on Sunday ahead of the front, and
overnight lows will likely be even more above average across
portions of the Deep South owing to southerly flow from the Gulf
of Mexico and widespread cloud cover. These readings should
return closer to late February/early March averages as the frontal
boundary settles south of the Gulf Coast by the middle of the
week. Meanwhile, readings should be slightly above average for
much of the central and northern Plains for much of this forecast
period.

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 rain across portions of the Lower Mississippi Valley, the
Central/Southern Appalachians,
the Tennessee Valley, the Mid-Atlantic, the Southeast, the
Southern Plains, and the Ohio Valley,
Sun-Mon, Feb 28-Mar 1.
- Flooding possible across portions of the Lower Mississippi
Valley, the Southern Appalachians, the
Ohio Valley, and the Tennessee Valley.
- Flooding occurring or imminent across portions of the Southeast
and the Mid-Atlantic.
- Flooding likely 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
244 AM EST Thu Feb 25 2021

Valid 00Z Fri Feb 26 2021 - 00Z Fri Mar 5 2021

The ensemble means are in good overall synoptic scale agreement
across the central Pacific through the upcoming week. A tight
pressure gradient early in the period from a very strong surface
high northeast of Hawaii will promote the development of windward
favored precipitation late this week into this weekend, while
aloft there should continue to be weak upper level troughing in
place. By Sunday, moisture is expected to increase from the
southeast, and this could favor enhanced precipitation across
portions of the eastern Islands. The latest guidance suggests a
weak front to approach the state early to mid next week, bringing
slightly drier air and stronger easterly flow.

Hamrick

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
735 PM EST Thu Feb 25 2021

Day 1
Valid 01Z Fri Feb 26 2021 - 12Z Fri Feb 26 2021

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

However, some locally moderate to heavy rainfall is possible later
tonight around the Arklatex as an area of generally light rain
becomes better organized. The models have been advertising good
coupling of low level moisture and upper level divergence within
the right entrance region of the 250mb jet, and a quick look at
the 26/00Z upper air maps generally supports this scenario. A
forecast of 2" areal averaged rain is often enough to consider at
least a Marginal risk. However, only weak instability in place
across the area probably will not support the rainfall rates
necessary to result in flash flooding. Given antecedent
conditions and near normal streamflow across the area, still do
not believe a Marginal Risk area was warranted at this point.

Bann


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Thu Feb 25 2021

Day 2
Valid 12Z Fri Feb 26 2021 - 12Z Sat Feb 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...CUMBERLAND
PLATEAU...SOUTHERN APPALACHIAN MOUNTAINS...AND THE CAROLINA
PIEDMONT...

2030 UTC Update -- Have hoisted a Marginal Risk over the area,
based on the latest guidance trends, particularly late in the
period (00-12Z Sat) as the warm front lifts slowly northward
Fri-Sat. PWs Friday night climb to 1.25+ inches, which is about 2
standard deviations above normal. This as southwesterly low-level
inflow increases to 50-55 knots, becoming nearly parallel while
approaching the magnitude of the mean 850-300 mb low -- setting
the stage for upwind propagation and cell training with elevated
convective bands north of the surface warm front Friday night.
Instability looks to be on the marginal side, with MUCAPEs
generally averaging between 250-500 j/kg, which for the most part
will cap 3 hourly rainfall rates between 1-1.5 inches.

Previous discussion..

The latest guidance continues to show Gulf moisture being
transported northward within an increasing low level flow. The
best mid-upper level lift will be present with the best lift
occurring 00Z-12Z near northern Mississippi/western and central
Tennessee as the associated front advances. Periods of moderate to
locally heavy rainfall will be supported within this environment;
with a couple of the hi-res guidance showing rain rates of 0.50
inch/hour across parts of Arkansas into Mississippi and western
Tennessee during the 12-18Z period. This is also where the swath
of higher QPF is expected to occur, with WPC forecasting 1 to 2
inches. During this time isolated locations may experience ponding
or rapid runoff but the aerial coverage of flooding is not
expected to be widespread. It will however increase soil
saturation across the region and lower the FFG from the Southern
Plains/Lower Mississippi Valley to the Tennessee Valley.

Hurley/Campbell


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Thu Feb 25 2021

Day 3
Valid 12Z Sat Feb 27 2021 - 12Z Sun Feb 28 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE
SOUTHERN PLAINS INTO PORTIONS OF THE LOWER-MID MISSISSIPPI VALLEY,
THE NORTHERN TENNESSEE VALLEY, SOUTHERN OHIO VALLEY, AND WESTERN
SLOPES OF THE CENTRAL APPALACHIANS...

2030 UTC Update -- Nudged the bottom portion of the Marginal Risk
area a bit farther northward based on the latest trends in the
guidance, while expanding the outlook area slightly downstream to
include more of the lower OH Valley and western WV. Elevated
instability north of the warm front at this appears modest at
best, averaging ~250-500 j/kg over the outlook area, with pockets
closer to 1000 j/kg. This should be more than sufficient to
support 1.5+ inches of rainfall within a 3 hour period, especially
with some training likely Sat night given the uptick in
southwesterly 1000-850 mb flow into a strengthening low-level
frontogenetic zone within the right entrance region of an upper
level jet streak.

Previous discussion below...

Scattered to widespread rain will persist during the day 3 period
after spreading 1 to 2 inches across the region the day prior.
This antecedent moisture will have likely lowered flash flood
guidance across the Lower Mississippi and Tennessee Valleys where
an additional 1 to 3 inches are forecast. Although the hourly
rates across this region are expected to be less than 0.50
inch/hour, precipitation of this magnitude will lead to an
increased threat for flash flooding to develop. In coordination
with the local forecast offices from far northeast Texas to
eastern Kentucky/Tennessee a Marginal Risk area was hoisted. This
area will continue to be monitored as the threat of moderate to
heavy rainfall continues into the day 4 period.

Hurley/Campbell


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
325 PM EST Thu Feb 25 2021

Valid 00Z Fri Feb 26 2021 - 00Z Mon Mar 01 2021


...Pacific Northwest to the Northern Rockies...
Days 1-2...
Strong northwesterly flow, with embedded shortwave energy aloft,
will continue to support periods of heavy snow across the
Northwest into the northern Rockies through Friday and into early
Saturday. Through late Saturday, widespread additional
accumulations of 8-12 inches are likely across the Olympics, the
Washington and Oregon Cascades, the Blue Mountains, as well as the
Rockies from northern Idaho and western Montana to eastern Idaho
and western Wyoming. Within these areas, local accumulations of
2-feet or more are likely across the higher elevations of the
Cascades, the Blue Mountains and the Bitterroot Range. Drier
weather is expected by late Saturday and continuing into Sunday as
a strong ridge shifts east across the region.

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
255 PM EST Thu Feb 25 2021

Valid 12Z Sun Feb 28 2021 - 12Z Thu Mar 04 2021

...Heavy rain threat gradually diminishes across the interior Deep
South early next week...

...Overview and Guidance Evaluation...

An ongoing heavy rain event should extend across the interior Deep
South towards the southern Appalachians as the forecast period
begins on Sunday. There has been a gradual southward shift in the
heavy rain axis as additional upper level energy is forecast to
eject from the upper level trough dipping into the southwestern
U.S. On the other hand, models are trending slower with respect
to the eastward motion of the upper low itself. This could result
in keeping the heavy rain threat a bit longer in the Deep South on
Monday. Thereafter, there is a general consensus among model
guidance that the bulk of the moisture will gradually move
eastward across the Southeast ahead of the upper trough. However,
uncertainty remains quite high regarding the subsequent evolution
of this system. A blend of the 00Z ECMWF mean, 06Z GEFS and the
00Z CMC mean yielded a developing low pressure wave moving off the
Southeast U.S. coast by the middle of next week.

Meanwhile, deterministic models and ensembles continue to struggle
with both the timing and evolution of the next trough approaching
the West Coast from the Gulf of Alaska/North Pacific region as
early as Sunday night. The GFS and to a lesser extent the GEFS
mean continue to indicate the development of a closed upper level
low that lingers well offshore, while the ECMWF keeps the upper
trough largely over land as it digs down the West Coast early next
week. The latest (12Z) ECMWF is even more progressive than the
00Z run. The CMC is somewhere in between. These differences
have implications for the southern tier states for the latter part
of next week. Forecast confidence is below average for this part
of the country from Day 5 onward.

The WPC medium-range fronts and pressures were derived based on
the consensus of the 00Z ECMWF/00Z EC mean and the 06Z GFS/GEFS
for Days 3 to 4. Beginning on Day 5, more of the EC mean and 06Z
GEFS together with the 00Z CMC mean were used due to the above
normal forecast uncertainty.


...Weather/Threats Highlights...

The upper level trough forecast to move across the southern
Rockies and then the southern Plains will allow Gulf moisture to
lift northward as multiple waves of low pressure develop along a
slow-moving front. This setup will favor rounds of locally heavy
rainfall late this weekend into early next week from the Arklatex
to the southern Appalachians. The heavy rain threat could linger a
bit longer in the Deep South on Monday but should gradually
diminish by Tuesday as the bulk of moisture should slide eastward
into and then off the Southeast afterwards. Meanwhile,
uncertainty is quite high regarding the specific details across
the Southwest to the Plains. Mountain snows are possible for the
central/southern Rockies as the upper trough moves through. Some
precipitation could be moving into the central Plains toward the
latter part of next week but uncertainty remains much higher than
normal.

High temperatures will likely be about 5 to 15 degrees above
average across the eastern U.S. on Sunday ahead of the front, and
overnight lows will likely be even more above average across
portions of the Deep South owing to southerly flow from the Gulf
of Mexico and widespread cloud cover. These readings should
return closer to late February/early March averages as the frontal
boundary settles south of the Gulf Coast by the middle of the
week. Meanwhile, readings should be slightly above average for
much of the central and northern Plains for much of this forecast
period.

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 rain across portions of the Lower Mississippi Valley, the
Central/Southern Appalachians,
the Tennessee Valley, the Mid-Atlantic, the Southeast, the
Southern Plains, and the Ohio Valley,
Sun-Mon, Feb 28-Mar 1.
- Flooding possible across portions of the Lower Mississippi
Valley, the Southern Appalachians, the
Ohio Valley, and the Tennessee Valley.
- Flooding occurring or imminent across portions of the Southeast
and the Mid-Atlantic.
- Flooding likely 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