Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
411 PM EST Tue Jan 05 2021
Valid 12Z Fri Jan 08 2021 - 12Z Tue Jan 12 2021
...Overview...
A progressive synoptic pattern will prevail over the CONUS through
the period with a tendency for mean flow aloft to evolve toward a
larger scale West Coast (or a little inland) ridge and eastern
Canada into southern Plains trough by next Tue. Primary systems of
interest will affect the Northwest, South, and Southeast. Eastern
Pacific systems should track into Canada with their fronts
producing daily light to moderate precipitation for especially
coastal Washington and Oregon. Over northern areas the amounts
could trend a little heavier toward the end of the period. Lead
system in the Southeast Friday will move off the coast and out to
sea as a southern Rockies system heads for the Texas coast. The
second system may bring moderate to heavy rain to areas near the
Texas/Louisiana coast. Its front will move eastward through the
Gulf and perhaps off the Southeast/Mid-Atlantic coast by next
Mon-Tue. Pacific origin of most flow reaching the lower 48 will
keep any arctic air in Canada for the next week but the mean
trough aloft extending into the southern Plains and succession of
southern tier systems will promote cooler than normal temperatures
over much of the southern U.S.
...Guidance/Predictability Assessment...
Ensemble means continue to provide good agreement and continuity
for the overall pattern evolution but recent operational model
runs show a lot of uncertainty for a number of embedded features,
tempering confidence in specifics--especially by next week. Even
earlier in the period there has been some disagreement over
eastern Canada flow that could drop a compact upper low into
northern New England, though seemingly with fairly modest
influence on specifics of the leading system tracking into the
Atlantic. As for the evolution near the East Coast by next Tue,
recent ECMWF runs have been the most stable even in spite of some
pronounced variability in details farther west (seen in other
models as well). Perhaps not a surprise, the new 12Z ECMWF has
changed its details yet again with pronounced effects at the
surface by next Tue. Fast GFS runs have been trending slower over
the past 12 hours with the new 12Z run getting closer to the 00Z
ECMWF timing. Very ill-defined surface reflection in the ensemble
means thus far and 12Z CMC adjustment of dampening this low in
favor of a low well to the north further highlight the continued
uncertainty. Over the West the primary notes are a recent CMC
tendency to exaggerate its amplitude of shortwave energy and then
late in the period a 12Z GFS adjustment away from the amplified
shortwaves seen in the 00Z/06Z runs. Otherwise, resolving the
character of shortwaves encountering the western ridge aloft is a
low-confidence endeavor.
Guidance comparisons led to starting with an operational model
blend early followed by a steady increase of 06Z GEFS/00Z ECMWF
mean input, reaching 60 percent total by day 7 Tue. At that time
operational model weight leaned more toward the past two ECMWF
runs. The resulting forecast provided good continuity from the
previous cycle.
...Weather/Threats Highlights...
The Pacific Northwest will continue to see periods of
precipitation about every day or so Fri-Tue. Current guidance is
not yet unanimous but there is a decent signal that heaviest
rain/high elevation snow will fall next week. Low pressure near
the southeastern coast late this week will turn northeastward and
spread some precipitation over the Carolinas and southern
Mid-Atlantic region Fri-Sat. The northern/western part of the
moisture shield may contain snow. The Winter Weather Outlook
probabilities depict the most likely area of meaningful snowfall,
though potential is on the lower side of the spectrum in absolute
terms. The most likely track of this system would keep
precipitation away from New England except for perhaps far eastern
areas. Next system out of the southern Rockies will tap the Gulf
for moisture reaching into the Texas and Louisiana coasts around
late Sat through at least Sun night. This could lead to a heavy
rain threat for east-central Texas to the central Gulf Coast
though the heaviest rainfall may be just offshore. To the north,
temperatures may be cold enough for some light wintry weather near
the ArkLaTex. Confidence is low for precipitation coverage and
type over the East toward the end of the period.
Northern tier locations, especially from Montana into Minnesota,
will be consistently well above normal for temperatures. Expect
some plus 10F or greater anomalies each day. Morning lows should
generally be farther above normal than the daytime highs and plus
20F or greater anomalies are possible for Fri-Sat lows. California
will tend to be above normal but with only single-digit anomalies.
The South will likely see below normal highs most of the period,
with one area of minus 5-10F or so anomalies for highs behind the
first system and then another from the central Rockies through the
southern Plains/South in association with the second system.
Rausch/Fracasso
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 precipitation across portions of the Pacific Northwest and
the Northern Great Basin,
Mon-Tue, Jan 11-Jan 12.
- Heavy rain across portions of the Lower Mississippi Valley and
the Southern Plains, Sun, Jan 10.
- Heavy snow across portions of the Central Rockies, the Central
Plains, and the Southern Plains,
Sat, Jan 9.
- Flooding occurring or imminent across portions of the Southern
Plains, the Mid-Atlantic, and the
Pacific Northwest.
- Flooding likely across portions of the Mid-Atlantic.
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