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.
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.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: