Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Thu Jan 21 2021

Valid 12Z Sun Jan 24 2021 - 12Z Thu Jan 28 2021

...Additional heavy rainfall likely for portions of the
south-central U.S. for the beginning of the week; heavy mountain
snow and unsettled weather moving into the western U.S....

...Weather Pattern Overview and Guidance/Predictability
Assessment...

Active amplification of upper-level troughs digging down from the
Gulf of Alaska will be a continued source of unsettled weather for
the western U.S. through the medium range period.  Meanwhile,
downstream ejections of upper-level energy into the southern
Plains will periodically interact with moisture returning from the
Gulf of Mexico to result in heavy rainfall in the south-central
U.S.  A heavy rain event should be culminating across the
Tennessee Valley as the medium range period begins Sunday night
into Monday.  Model guidance this morning has reached a better
agreement on the degree of interaction between the warm and
moisture air mass from the Gulf of Mexico and broad troughing
sliding across the northern tier states.  The GFS is now showing
more northern stream cyclogenesis in better agreement with the
ECMWF while the ECMWF shows slightly less robust cyclogenesis. 
Consensus of the 00Z EC mean and the 06Z GEFS indicates a
northward shift in the heavy rainfall axis across the Tennessee
Valley.  Meanwhile, a swath of wintry precipitation appears likely
from the Midwest to the Mid-northern Atlantic and into southern
New England Monday into Tuesday morning as the system moves across
and then off the Mid-Atlantic.  By the middle of next week, model
consensus indicates another enhanced precipitation event will
develop along the Gulf Coast and then head northeastward into the
East Coast.

Over the Southwest, models are in good agreement on the timing of
another Pacific system moving through the area Sunday through
Tuesday.  Thereafter, models continue to show significant
divergence on the timing and intensity of the next system dipping
from the Gulf of Alaska.  The GFS continues to indicate a more
robust closed off system dipping farther south toward the West
Coast relative to the ECMWF.  The CMC was somewhere in between. 
The last couple of runs of the ECMWF did develop a better defined
system off the west coast of Canada in better agreement with the
GFS.

The WPC medium range forecast package was based on the consensus
of the 06Z GFS/GEFS, the 00Z ECMWF/EC mean, and smaller
contributions from the 00Z CMC and 00Z NAEFS.  More of the
ensemble means were used on Days 6 and 7 to handle the higher
forecast uncertainty.

...Weather/Threats Highlights...

Another round of heavy rain will likely make weather headlines
across portions of the south-central U.S. by late Sunday and into
early Tuesday as the southwestern storm system emerges over the
Gulf Coast region.  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
southern Arkansas to the southern Appalachians, and this may be
enough to produce some episodes of flooding if high rainfall rates
persist for any given area.  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.  The next episode of
enhanced rainfall and interior wintry precipitation will appear to
develop near the Gulf Coast and progress toward the East Coast
during the middle of next week.

Meanwhile, heavy snow is becoming more likely for the central and
southern Rockies, the Wasatch Range, and the Sierra Nevada as
multiple upper level troughs pass through the region.  Heavy rain
can also be expected for the lower elevations of central and
northern California as copious Pacific moisture is advected
towards the coast.

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 will make it feel more like spring, with
highs about 10-20 degrees above normal, 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.  Most of the
central U.S. should have temperatures close to average for late
January.

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 Southern California, the
Central and Southern
Appalachians, the Mid-Atlantic, the Great Lakes, and the Ohio
Valley, Mon-Tue, Jan 25-Jan 26.
- Heavy precipitation across portions of the Central Plains and
the Middle Mississippi Valley, Mon,
Jan 25.
- Heavy rain across portions of California, Wed-Thu, Jan 27-Jan 28.
- 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 the Pacific Northwest and the
Northern Great Basin, Sun, Jan 24.
- Heavy snow across portions of California, the Central Great
Basin, the Pacific Northwest, and the
Southwest, Sun-Mon, Jan 24-Jan 25 and Wed-Thu, Jan 27-Jan 28.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Mon-Tue, Jan 25-Jan 26.
- Severe weather across portions of the Lower Mississippi Valley
and the Southern Plains, Sun, Jan
24.

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