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
158 AM EST Fri Feb 26 2021

Valid 12Z Mon Mar 01 2021 - 12Z Fri Mar 05 2021

...Heavy rain threat over the South continuing into early next
week and then gradually diminishing...

...Overview and Guidance Evaluation...

From the large scale perspective, guidance shows fairly
progressive northern stream and southern tier flow aloft with an
evolution toward a more amplified late-week regime featuring an
eastern Pacific trough and downstream western North American
ridge/eastern trough. Within this evolution there is considerable
disagreement over how energy may separate from a Pacific shortwave
either during the short range time frame or early in the medium
range. This flow separation issue ultimately leads to high
uncertainty/low confidence in system specifics over the eastern
Pacific and then eastward across much of the lower 48.

Guidance through the 12Z/18Z cycles essentially formed two
clusters regarding the dominant question of Pacific flow
separation. The larger ECMWF-led cluster pulled off some energy
rather early/westward with the remaining shortwave continuing into
the western U.S. (possibly closing an embedded low for a time) and
beyond. The 12Z CMC eventually strayed toward a more separated
southern solution over the East later in the period while multiple
ECMWF/ECMWF mean runs have indicated eventual surface low
development/consolidation over New England and/or eastern Canada
with contributions from the lower 48 shortwave and Canadian flow
as the overall pattern amplifies. Parallel GFS (GFSP) runs have
also been falling into this cluster in principle. On the other
hand similarly persistent GFS/GEFS mean runs have been waiting
until just offshore the West Coast to pull off a significant
portion of the Pacific shortwave energy. The resulting upper low
moves into the Southwest when the other cluster is starting to
develop a mean ridge over the West, and by late in the week the
GFS/GEFS cluster indicates a mere frontal passage across the
Northeast with minimal signal for any New England/eastern Canada
surface system. These issues will also have some effect on the
ultimate path/evolution of the compact upper low forecast to
emerge from the southern Rockies early in the week.

Compared to the wide variance of guidance for most of the period,
models and means actually come into good agreement for the
amplifying eastern Pacific upper trough late in the period. There
are still some typical detail and timing differences that are
fairly minor relative to the average spread/error for forecasts a
week out in time.

Given the relative proportion of guidance and the 18Z GFS trend to
close its eastern Pacific upper low to the east of the 12Z run,
the latest forecast went more in the direction of the ECMWF
cluster. This included some input from the 18Z GFSP as well as
the old 00Z ECMWF since the 12Z version was somewhat on the faster
edge among solutions for the shortwave crossing the lower 48.

The primary revelation in the new 00Z cycle is the CMC switching
to a variation of the GFS cluster. Other guidance is holding
serve, with the GFS/GEFS continuing its general evolution in
contrast to the ECMWF/UKMET/GFSP (albeit with some detail
differences from previous runs, as would be expected).


...Weather/Threats Highlights...

The upper low tracking out of the southern Rockies and through the
Plains early in the week should interact with a front near the
Gulf Coast, supporting one or more waves along the front and a
northward flow of Gulf moisture. This evolution will favor
another episode of locally heavy rainfall across parts of the
South during the first part of the week, with lighter
precipitation extending farther north over the southern half of
the Plains and an increasing portion of the East. There may be
some snow in the northern part of the precipitation shield. The
rest of the forecast has lower than average confidence for
specifics. One scenario--currently somewhat more likely--would
have a period of moderate rain/mountain snow over the Northwest
and vicinity at the start of the week with mostly light activity
then spreading into the Interior West/Rockies followed by
precipitation of varying intensity spreading across the East in
association with one or more surface lows/fronts (which may
eventually consolidate). The other possibility will be for a much
lighter event over the Northwest into the Rockies early in the
week and less moisture extending downstream, while a system off
California moves into the Southwest by midweek or so with moisture
continuing eastward into late week. Either way some moisture may
reach the West Coast late in the week depending on the exact
timing/strength of the upper trough amplifying over the eastern
Pacific.

The upper low emerging from the southern Rockies early in the week
will keep highs up to 10-20F below normal over the southern High
Plains on Monday followed by a rapid rebound to near normal as the
feature continues onward. Then cool high pressure building over
the East will lead to highs 5-15F below normal near the East Coast
on Tuesday, again followed by a quick recovery. The Northern
Plains region will likely be consistently warmer than average next
week with most days featuring some plus 10-20F anomalies. A
majority of the West should see near to slightly below normal
readings early-mid week followed by a warming trend as upper
ridging builds over the region. As with precipitation, some
aspects of the temperature forecast over the lower 48 will depend
closely on the uncertain details of individual systems.

Rausch


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
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
158 AM EST Fri Feb 26 2021

Valid 12Z Mon Mar 01 2021 - 12Z Fri Mar 05 2021

...Heavy rain threat over the South continuing into early next
week and then gradually diminishing...

...Overview and Guidance Evaluation...

From the large scale perspective, guidance shows fairly
progressive northern stream and southern tier flow aloft with an
evolution toward a more amplified late-week regime featuring an
eastern Pacific trough and downstream western North American
ridge/eastern trough. Within this evolution there is considerable
disagreement over how energy may separate from a Pacific shortwave
either during the short range time frame or early in the medium
range. This flow separation issue ultimately leads to high
uncertainty/low confidence in system specifics over the eastern
Pacific and then eastward across much of the lower 48.

Guidance through the 12Z/18Z cycles essentially formed two
clusters regarding the dominant question of Pacific flow
separation. The larger ECMWF-led cluster pulled off some energy
rather early/westward with the remaining shortwave continuing into
the western U.S. (possibly closing an embedded low for a time) and
beyond. The 12Z CMC eventually strayed toward a more separated
southern solution over the East later in the period while multiple
ECMWF/ECMWF mean runs have indicated eventual surface low
development/consolidation over New England and/or eastern Canada
with contributions from the lower 48 shortwave and Canadian flow
as the overall pattern amplifies. Parallel GFS (GFSP) runs have
also been falling into this cluster in principle. On the other
hand similarly persistent GFS/GEFS mean runs have been waiting
until just offshore the West Coast to pull off a significant
portion of the Pacific shortwave energy. The resulting upper low
moves into the Southwest when the other cluster is starting to
develop a mean ridge over the West, and by late in the week the
GFS/GEFS cluster indicates a mere frontal passage across the
Northeast with minimal signal for any New England/eastern Canada
surface system. These issues will also have some effect on the
ultimate path/evolution of the compact upper low forecast to
emerge from the southern Rockies early in the week.

Compared to the wide variance of guidance for most of the period,
models and means actually come into good agreement for the
amplifying eastern Pacific upper trough late in the period. There
are still some typical detail and timing differences that are
fairly minor relative to the average spread/error for forecasts a
week out in time.

Given the relative proportion of guidance and the 18Z GFS trend to
close its eastern Pacific upper low to the east of the 12Z run,
the latest forecast went more in the direction of the ECMWF
cluster. This included some input from the 18Z GFSP as well as
the old 00Z ECMWF since the 12Z version was somewhat on the faster
edge among solutions for the shortwave crossing the lower 48.

The primary revelation in the new 00Z cycle is the CMC switching
to a variation of the GFS cluster. Other guidance is holding
serve, with the GFS/GEFS continuing its general evolution in
contrast to the ECMWF/UKMET/GFSP (albeit with some detail
differences from previous runs, as would be expected).


...Weather/Threats Highlights...

The upper low tracking out of the southern Rockies and through the
Plains early in the week should interact with a front near the
Gulf Coast, supporting one or more waves along the front and a
northward flow of Gulf moisture. This evolution will favor
another episode of locally heavy rainfall across parts of the
South during the first part of the week, with lighter
precipitation extending farther north over the southern half of
the Plains and an increasing portion of the East. There may be
some snow in the northern part of the precipitation shield. The
rest of the forecast has lower than average confidence for
specifics. One scenario--currently somewhat more likely--would
have a period of moderate rain/mountain snow over the Northwest
and vicinity at the start of the week with mostly light activity
then spreading into the Interior West/Rockies followed by
precipitation of varying intensity spreading across the East in
association with one or more surface lows/fronts (which may
eventually consolidate). The other possibility will be for a much
lighter event over the Northwest into the Rockies early in the
week and less moisture extending downstream, while a system off
California moves into the Southwest by midweek or so with moisture
continuing eastward into late week. Either way some moisture may
reach the West Coast late in the week depending on the exact
timing/strength of the upper trough amplifying over the eastern
Pacific.

The upper low emerging from the southern Rockies early in the week
will keep highs up to 10-20F below normal over the southern High
Plains on Monday followed by a rapid rebound to near normal as the
feature continues onward. Then cool high pressure building over
the East will lead to highs 5-15F below normal near the East Coast
on Tuesday, again followed by a quick recovery. The Northern
Plains region will likely be consistently warmer than average next
week with most days featuring some plus 10-20F anomalies. A
majority of the West should see near to slightly below normal
readings early-mid week followed by a warming trend as upper
ridging builds over the region. As with precipitation, some
aspects of the temperature forecast over the lower 48 will depend
closely on the uncertain details of individual systems.

Rausch


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