Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Wed Mar 03 2021

Valid 12Z Wed Mar 03 2021 - 12Z Fri Mar 05 2021

...A low pressure system will spread precipitation across the southwestern
to south-central CONUS...

...Elevated to Critical Risks of fire weather are in place for portions of
the Southwest to Southern High Plains...

A surface low pressure system along with an upper-level low are forecast
to move across Southern California into the Four Corners states through
Thursday and into the Southern Plains early Friday. Along this track,
precipitation is forecast to spread across these regions, with rain in the
lower elevations and snow in higher elevations. Snow could accumulate to a
few inches in the mountains of Southern California, the Mogollon Rim, and
Wasatch, while the Central Rockies and San Juan Mountains could see 6-12
inches of snow through Friday morning. Showers and a few thunderstorms are
forecast for the Southern/Central Plains late Thursday into Friday.

Another threat with this system will be fire danger. An Elevated Risk of
fire weather as well as Red Flag Warnings are in effect for much of
Arizona today. On Thursday, as winds increase and humidity dries out
behind the cold front, a Critical Risk of fire weather is in place for the
Southern High Plains.

Elsewhere, the Southeast should see rain and thunderstorms ending today
after a low pressure system moves away into the Atlantic. Light snow is
possible in the interior Northeast through Thursday, with snow
accumulating to a few inches possible in higher elevations. The Pacific
Northwest can expect precipitation chances to increase late Thursday into
Friday as an upper-level low and frontal system approach.

Temperature-wise, the Northern Plains should continue to see rather mild
temperatures with highs nearing or exceeding 60 degrees over the next
several days, about 20-30 degrees above average. The Southeast can expect
cooler than normal temperatures today, while Thursday will be chillier
than average in the Northeast.

Tate

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Wed Mar 03 2021

Valid 12Z Sat Mar 06 2021 - 12Z Wed Mar 10 2021

...Overview and Guidance Evaluation...

Eastern Pacific mean troughing aloft will gradually expand
eastward over the course of the period, eventually including
portions of the western U.S. Features within and ejecting from
this trough will focus multiple episodes of precipitation along
the West Coast with somewhat more moderate totals over the
Interior West and Rockies. By next Tuesday-Wednesday one ejecting
shortwave may generate a significant Plains system along with a
broad precipitation area over the central into east-central U.S.
Meanwhile a shortwave entering the West to start the weekend will
become more diffuse as it traverses the lower 48 though a mean
ridge, temporarily flattening flow slightly which will help push
along an amplified upper trough over the East Coast during the
weekend. Expect a mean ridge to reach the East by next Wednesday.

As for guidance details, the 12Z model consensus suddenly jumped
toward the somewhat faster GFS for the shortwave entering the West
by the start of the period early Saturday and the new 00Z runs are
holding fairly close to that idea. The average of guidance is
still a little slower than the GFS but differences are trending
closer to the noise range. Upstream there is reasonable
clustering for a deep-layer low to settle just off the Pacific
Northwest from about late weekend into the first half of next
week, with gradually increasing uncertainty for the eastward
extent of the low and details of a possible reinforcing feature
late in the period. Recent GFS runs have been a bit on the
fast/strong side with shortwave energy ejecting into the West and
Plains early-mid week. Based on recent operational model runs
there is still at least a moderate signal for what could be a
fairly strong system over the Northern Plains but with a fair
amount of variability/spread for specifics. Overall an
operational model blend trending to a model/mean mix (including
some 18Z parallel GFS to tone down some operational run specifics
especially late in the period) represented consensus evolution
well. This yielded a solution perhaps slightly on the
conservative side for the late period Plains system while awaiting
improved agreement/stability on specifics.


...Weather/Threats Highlights...

Systems carried within the mean trough aloft near the West Coast
will support an unsettled period with multiple episodes of rain
and higher elevation snow from the Pacific Northwest into northern
California and Sierra Nevada. Precipitation could be moderate to
locally heavy at times with highest five-day totals (perhaps
reaching several inches) most likely over southwest Oregon into
northern California. The Interior West and Rockies should also
see periods of precipitation though with less extreme totals.
Expect precipitation to expand across the Plains and east-central
U.S. with a potential Tuesday-Wednesday system, with rain and
thunderstorms in the warm sector and some wintry weather possible
over the extreme northern tier depending on how the system evolves.

Weakening Southern Plains upper low energy dropping toward the
Gulf Coast by early Saturday and then becoming absorbed into the
longwave East Coast trough will support fast moving Gulf of Mexico
into Atlantic low pressure. Most associated rainfall should be
over the Florida Peninsula with moderate to perhaps locally heavy
rainfall possible over northern/central areas with the best
interaction of daytime heating, available moisture, and lift.
This wave will then deepen and move well offshore while cool high
pressure builds over the eastern U.S.

Much above normal temperatures over the Northern Plains this
weekend, reaching 10-25F above normal, will gradually extend
eastward into the Upper Midwest and Great Lakes with less extreme
warmth also reaching into the Northeast by late in the period.
Some daily records for highs and/or warm lows will be possible.
By next Tuesday-Wednesday well above normal readings for morning
lows in particular will extend well southward over the Plains and
Mississippi Valley as Gulf inflow increases. Elsewhere, a cooling
trend toward moderately below normal readings will spread across
the western U.S. during the period. East Coast states will be
chilly during the weekend with highs 5-10F or so below normal and
then trend warmer.

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
242 AM EST Wed Mar 03 2021

Valid 00Z Thu Mar 04 2021 - 00Z Thu Mar 11 2021

The consensus of latest guidance displays good continuity for the
overall pattern evolution during the period. Trades will be
fairly brisk through the period, though with some relative
weakening mid-late week as a high northwest of the state weakens
and a modest surface trough develops over portions of the state.
An upstream high building in to the north and then northeast may
support a rebound in wind speeds from the weekend into next week.
Expect windward/mountain focused showers to be on the lighter half
of the spectrum through Thursday with precipitable water values
below climatology. Moisture will increase late week into the
weekend as a front and shortwave energy (GFS a bit deeper than
most other solutions) drop into the area, with the best guidance
signal for rainfall enhancement over the western islands. Then
during the first half of next week guidance has been consistent in
showing greater moisture moving in gradually from the
south/southeast, and also consistent with the spread in how much
precipitable water values increase. Thus by that time continue to
expect an increase in rainfall focus over the Big Island and
vicinity but with ongoing uncertainty for magnitude. Upper
troughing over the area will gradually fill with time but enough
of a weakness may persist through midweek to provide some
enhancement to the rainfall.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Wed Mar 03 2021

Day 1
Valid 12Z Wed Mar 03 2021 - 12Z Thu Mar 04 2021

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

Chenard

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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 AM EST Wed Mar 03 2021

Day 2
Valid 12Z Thu Mar 04 2021 - 12Z Fri Mar 05 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
309 AM EST Wed Mar 03 2021

Day 3
Valid 12Z Fri Mar 05 2021 - 12Z Sat Mar 06 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
328 AM EST Wed Mar 03 2021

Valid 12Z Wed Mar 03 2021 - 12Z Sat Mar 06 2021

...Four Corners...
Days 1-2...

A mid-level closed low emerging from Southern California this
evening will move eastward across the Four Corners Thursday before
ejecting into the Southern Plains on Friday. Enhanced mid-level
divergence, height falls, a LFQ jet streak divergence maxima, and
intensifying southerly WAA will combine to produce moderate to
heavy snow in the terrain of the Four Corners mountains this
evening and Thursday. Snow levels are expected to climb as high as
6000-8000 ft during the heaviest precip, confining the significant
snow to these elevations. The heaviest snow through Thursday is
likely to be in the San Juans which will benefit from upslope
enhancement due to the orthogonal southerly flow, and WPC
probabilities are high for 6 inches each of the two days.
Otherwise, WPC probabilities are moderate for 6 inches on D1 in
the southern Wasatch, extending into most of the CO Rockies on D2.
Lighter snows accumulating to a few inches are likely D2 across
the Palmer Divide.


...Pacific Northwest...
Days 2-3...

A closed mid-level low is likely to move slowly west of the coast
Thursday and Friday, with pronounced southerly 700-500mb flow
drawing tropically sourced moisture northward along the coast of
northern CA and into WA/OR. As the low slowly migrates eastward, a
surface cold front will move onshore, but not until late D3
/Friday evening/, which will finally bring a slow wane to the
precipitation. However, a prolonged period of warm/moist advection
ahead of this front will drive significant precipitation across
the Coastal Ranges and into the WA/OR Cascades. Snow levels will
rise on this warm flow, and are expected to generally be above
4000 ft in WA/OR, before falling to 2500-3000 ft on D3. WPC
probabilities D2 are high for 6 inches in the Olympics, where
isolated totals approaching 2 ft are possible. By D3, there will
be enough of an eastward push in the flow to drive heavy snow
across the northern Sierra, Siskiyous/Shastas/Trinities, and into
the OR/WA Cascades, as well as continuing in the Olympics. WPC
probabilities are moderate to high for 6 inches in these ranges,
with local maxima above 12" likely.


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
156 AM EST Wed Mar 03 2021

Valid 12Z Sat Mar 06 2021 - 12Z Wed Mar 10 2021

...Overview and Guidance Evaluation...

Eastern Pacific mean troughing aloft will gradually expand
eastward over the course of the period, eventually including
portions of the western U.S. Features within and ejecting from
this trough will focus multiple episodes of precipitation along
the West Coast with somewhat more moderate totals over the
Interior West and Rockies. By next Tuesday-Wednesday one ejecting
shortwave may generate a significant Plains system along with a
broad precipitation area over the central into east-central U.S.
Meanwhile a shortwave entering the West to start the weekend will
become more diffuse as it traverses the lower 48 though a mean
ridge, temporarily flattening flow slightly which will help push
along an amplified upper trough over the East Coast during the
weekend. Expect a mean ridge to reach the East by next Wednesday.

As for guidance details, the 12Z model consensus suddenly jumped
toward the somewhat faster GFS for the shortwave entering the West
by the start of the period early Saturday and the new 00Z runs are
holding fairly close to that idea. The average of guidance is
still a little slower than the GFS but differences are trending
closer to the noise range. Upstream there is reasonable
clustering for a deep-layer low to settle just off the Pacific
Northwest from about late weekend into the first half of next
week, with gradually increasing uncertainty for the eastward
extent of the low and details of a possible reinforcing feature
late in the period. Recent GFS runs have been a bit on the
fast/strong side with shortwave energy ejecting into the West and
Plains early-mid week. Based on recent operational model runs
there is still at least a moderate signal for what could be a
fairly strong system over the Northern Plains but with a fair
amount of variability/spread for specifics. Overall an
operational model blend trending to a model/mean mix (including
some 18Z parallel GFS to tone down some operational run specifics
especially late in the period) represented consensus evolution
well. This yielded a solution perhaps slightly on the
conservative side for the late period Plains system while awaiting
improved agreement/stability on specifics.


...Weather/Threats Highlights...

Systems carried within the mean trough aloft near the West Coast
will support an unsettled period with multiple episodes of rain
and higher elevation snow from the Pacific Northwest into northern
California and Sierra Nevada. Precipitation could be moderate to
locally heavy at times with highest five-day totals (perhaps
reaching several inches) most likely over southwest Oregon into
northern California. The Interior West and Rockies should also
see periods of precipitation though with less extreme totals.
Expect precipitation to expand across the Plains and east-central
U.S. with a potential Tuesday-Wednesday system, with rain and
thunderstorms in the warm sector and some wintry weather possible
over the extreme northern tier depending on how the system evolves.

Weakening Southern Plains upper low energy dropping toward the
Gulf Coast by early Saturday and then becoming absorbed into the
longwave East Coast trough will support fast moving Gulf of Mexico
into Atlantic low pressure. Most associated rainfall should be
over the Florida Peninsula with moderate to perhaps locally heavy
rainfall possible over northern/central areas with the best
interaction of daytime heating, available moisture, and lift.
This wave will then deepen and move well offshore while cool high
pressure builds over the eastern U.S.

Much above normal temperatures over the Northern Plains this
weekend, reaching 10-25F above normal, will gradually extend
eastward into the Upper Midwest and Great Lakes with less extreme
warmth also reaching into the Northeast by late in the period.
Some daily records for highs and/or warm lows will be possible.
By next Tuesday-Wednesday well above normal readings for morning
lows in particular will extend well southward over the Plains and
Mississippi Valley as Gulf inflow increases. Elsewhere, a cooling
trend toward moderately below normal readings will spread across
the western U.S. during the period. East Coast states will be
chilly during the weekend with highs 5-10F or so below normal and
then trend warmer.

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