Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
244 PM EST Wed Jan 20 2021
Valid 12Z Sat Jan 23 2021 - 12Z Wed Jan 27 2021
***Additional heavy rainfall appears likely for portions of the
south-central U.S. by Sunday and Monday as unsettled weather
overspreads much of the western U.S.***
...Weather Pattern Overview...
Active shortwave amplifications along the West Coast will continue
to be the highlight through the medium range period. The next
around of unsettled weather will likely expand through the West
Coast on Sunday before overspreading across the Desert Southwest
and then the central/southern Rockies early next week. Meanwhile,
upper-level energies ejecting eastward into the Plains will likely
interact with Gulf moisture to result in a heavy rain event across
portions of the south-central states. Cold air and upper-level
broad troughing sliding across the northern tier states could
support a swath of wintry precipitation across the Midwest to the
northern Mid-Atlantic early next week although model uncertainty
remains high. Another deepening trough could reach the West Coast
by the middle of next week with noticeable model uncertainty.
...Guidance/Predictability Assessment...
Overall, deterministic model guidance was in above average
agreement on the evolution of the synoptic scale pattern across
the nation through about Monday with good ensemble mean support.
Thereafter, solutions begin to diverge regarding the degree of
interaction between the broad upper troughing across the northern
tier states and a low pressure system moving from the Deep South
toward the eastern U.S. The GFS offers a flatter and more
progressive solution while the ECMWF allows a greater degree of
interaction between the two streams and thus a greater chance of
cyclogenesis as the system is forecast to move across the
Mid-Atlantic on Tuesday. However, run-to-run variability remains
noticeable with this system, with the ensemble means favoring a
southern track. Toward the middle of next week, another Pacific
system could reach the West Coast but with noticeable model
spreads. The GFS and GEFS show robust development of a closed low
dipping down from the Gulf of Alaska to a position off the West
Coast by next Wednesday. The ECMWF and EC mean were less certain
about this scenario. The CMC was somewhere in between. This
morning's WPC medium-range package was based on a consensus of the
00Z ECMWF/EC mean, 06Z GFS/GEFS, as well as a smaller contribution
from the 00Z CMC and the 00Z NAEFS. More of the ensemble means
were used to handle the uncertainties on Days 6 and 7.
...Weather/Threats Highlights...
Another round of heavy rain will likely make weather headlines
again across portions of the south-central U.S. by late Sunday and
into Monday as the southwestern storm system emerges over the
southern Plains. This will support a surface low along a
quasi-stationary frontal boundary, with copious Gulf moisture
being advected northward across interior portions of the Deep
South and Tennessee River Valley. The potential exists for 1 to 3
inches of rainfall with locally higher amounts, mainly from
Arkansas to the southern Appalachians, and this may be enough to
produce some episodes of flooding if it happens over areas
affected by the antecedent rainfall forecast for later this week.
As this storm system reaches the East Coast late Monday and into
Tuesday, there should be enough cold air in place to support
either snow or a wintry mix for portions of the northern
Mid-Atlantic and Northeast. Moderate to heavy snow is likely for
the central and southern Rockies, as well as the Sierra Nevada, as
the upper level trough/low pass through the region.
In terms of temperatures, the greatest anomalies are forecast to
be from Texas to the Deep South and Gulf Coast region, where the
humid southerly flow keeps readings about 10-20 degrees above
normal for highs, and up to 25 degrees above normal for overnight
lows. This will particularly be the case from Sunday into Tuesday
in the warm sector of the low pressure system. The most prolonged
period of below normal temperature readings are forecast to be
over the western U.S., with highs up to 15 degrees below normal,
and also across portions of New England, with readings about 10
degrees below normal.
Kong/Hamrick
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 Central Appalachians,
the Mid-Atlantic, California,
the Southern Appalachians, the Great Lakes, and the Ohio Valley,
Mon-Tue, Jan 25-Jan 26.
- Heavy precipitation across portions of the Central Great Basin
and California, Wed, Jan 27.
- Heavy rain across portions of the Lower Mississippi Valley, the
Central Appalachians, the
Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic,
the Southern Appalachians, the
Southeast, the Southern Plains, and the Ohio Valley, Sun-Mon, Jan
24-Jan 25.
- Heavy snow across portions of California, the Pacific Northwest,
and the Northern Great Basin,
Sun, Jan 24.
- Heavy snow across portions of the Central Great Basin,
California, and the Southwest, Sun-Mon,
Jan 24-Jan 25.
- Heavy snow across portions of the Central Rockies, the Central
Great Basin, the Northern Plains,
the Southern Rockies, the Upper Mississippi Valley, and the
Southwest, Sat-Sun, Jan 23-Jan 24.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Mon, Jan 25.
- Heavy snow across portions of the Southwest, Mon-Tue, Jan 25-Jan
26.
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