Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
154 AM EST Wed Jan 27 2021
Valid 12Z Sat Jan 30 2021 - 12Z Wed Feb 03 2021
...Significant precipitation event likely along the central West
Coast from late weekend into early next week...
...Plains to western Atlantic system will spread a broad area of
rain and wintry weather across the eastern half of the lower 48...
...Weather Pattern Overview and Guidance/Predictability
Assessment...
Models and ensembles agree that two dominant features will
highlight the Sat-Wed period. One will be an eastern Pacific
upper trough gradually sharpening during the weekend and then
moving into the western U.S./northern Plains by Tue-Wed. The
upper trough and associated surface systems should initially focus
precipitation over the Pacific Northwest followed by a more
concentrated emphasis near the central West Coast. Then moisture
will spread across much of the West with somewhat less
precipitation intensity as the upper trough moves inland. Farther
east, low pressure emerging over the northern Plains early Sat
will continue into the East before giving way to coastal low
development whose center is most likely to track just off the
Mid-Atlantic/New England coast. Guidance continues to suggest
potential for a band of significant snowfall/wintry weather where
a sufficient combination of moisture and cold air exists, while
farther south some areas of moderate to heavy rainfall are
possible.
For the eastern Pacific into the western U.S. upper trough, the
primary guidance difference involves the amplitude and timing
around Tue-Wed. GFS/UKMET runs have been sharper/more amplified
and slower in contrast to the CMC/ECMWF. Over the past day the
ECMWF mean has nudged a bit slower toward the GEFS mean (which
predictably hints at the GFS scenario), but the new 00Z GFS has
adjusted to a more intermediate solution versus some earlier GFS
runs. Interestingly these various solutions converge in principle
by day 7 Wed as the trough moves into the West. Persistence of
the two camps along with the occasional adjustment of one solution
or another toward the middle of the spread would favor a general
blend approach at this time.
The evolution affecting the eastern half of the U.S. will have
some complexity in terms of contributions from individual features
aloft but guidance is gradually coming closer together for the
overall evolution. The GFS/GEFS mean are a bit on the fast/strong
side with the core of the leading shortwave that crosses the
Plains on Sat. This leading impulse may settle near the Lower
Great Lakes while upstream energy feeds into an eastern U.S.
trough that takes shape as an upstream ridge amplifies. Then the
amplifying ridge may help another bundle of energy to drop into
the eastern trough. By around early Tue there is decent
clustering toward an upper low centered over the northern
Mid-Atlantic coast but it remains to be seen whether the latter
energy merely flows around the established upper low (as in the
00Z GFS) or leads to a new low on its own (00Z CMC). The upper
low should track northeastward thereafter. For surface low
pressure, issues with the GFS/GEFS aloft lead to the initial
Plains to Ohio Valley system straying somewhat to the northeast
side of the spread. With the coastal development by early Mon the
ECMWF/ECMWF mean are a bit south of other solutions though they
and the 18Z/00Z GFS cluster fairly well by early Tue--farther
west/southwest than latest GEFS means. Multi-day trends have
generally favored a farther southward track, though this trend
appears to have stabilized over the past 24 hours. Both eastern
events this week fit into the theme of colder air winning out to
support a farther south low track/precipitation distribution.
These considerations favored tilting the forecast somewhat (but
not completely, due to the various detail uncertainties) toward
the ECMWF/ECMWF mean in principle. Ultimately a 12Z ECMWF/18Z
GFS/12Z UKMET blend in order from more to less weight Sat into
early Mon, followed by a blend of ECMWF/GFS runs and their means
with a little more ECMWF/ECMWF mean versus GFS/GEFS, led to a
coastal low position closest to the ECMWF early Mon followed by a
track toward an average of the 12Z ECMWF/12Z ECMWF mean/18Z GFS by
early Tue.
...Weather/Threats Highlights...
Precipitation coming into the West during most of this weekend
will primarily focus over the Pacific Northwest with some areas of
moderate to heavy rain and mountain snow. Lesser amounts may
reach down into northern California. Amplification/sharpening of
the offshore upper trough will likely begin to support a heavier
precipitation focus over or near southwestern Oregon and northern
California around late Sunday or Sunday night, most likely
continuing into Tuesday. This general scenario has been fairly
consistent in the guidance but there is still some fine-tuning to
be done for specifics of the upper trough and associated
rain/snow. Heading into midweek expect progression of the upper
trough to spread moisture eastward/southeastward across the West
into the Rockies. Precipitation should be less intense than along
the West Coast but some localized enhancement along favored
terrain is possible.
With the system tracking eastward from the Plains and then
redeveloping near the Mid-Atlantic/New England coast,
probabilistic winter weather guidance shows the best potential for
meaningful snow extending from the Midwest through parts of the
Ohio Valley and then through the Mid-Atlantic and Northeast.
There is still some uncertainty in specifics even west of the
Appalachians. Meanwhile the potential complexity of the evolution
aloft over the East Coast further tempers confidence in specifics,
especially given how sensitive precipitation
location/type/intensity will be to important details that will
likely take into the short range time frame to resolve. There may
be a small margin for error between a significant snowfall and a
modest one. Farther south expect the highest rainfall totals to
extend from near the east-central Plains/Mid-Mississippi Valley
into North Carolina or vicinity. Some of this activity may be
moderate to heavy but progression should be sufficient to keep
rainfall totals from becoming too extreme.
Expect the warmest temperature anomalies during the period to be
over the central U.S., supported by flow ahead of the initial
Plains system and then again ahead of the system reaching into the
West/northern Plains by next Wednesday. In general the anomalies
for morning lows should be greater than those for daytime
highs--the former seeing broad coverage of plus 10-20F or even
greater versus normal on some days in contrast to smaller areas of
highs 10-20F above normal. The eastern U.S. will tend to be near
to below normal during the period with coldest readings likely
over the Northeast into Mid-Atlantic early in the weekend. Cool
highs over the southwestern U.S. should moderate some from the
weekend into Monday. Then expect a cooling trend over much of the
West by midweek, with some highs next Wednesday possibly 10F or
more below normal over the southern half of the West.
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