Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 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 indicate 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.
Overall, deterministic model guidance is in good agreement on the
evolution of the synoptic scale pattern across the nation through
about Monday with good ensemble mean support. By Tuesday, the 00Z
GFS remains less amplified compared to the 00Z CMC and ECMWF with
the shortwave crossing the east-central U.S., and this was also
the case with the earlier 12Z model guidance suite. More
noteworthy model differences are apparent across the western U.S.
for the second half of the forecast period as additional shortwave
energy tracks southward from the Gulf of Alaska region. By
Tuesday, the GFS is stronger and faster with the low off the
Pacific Northwest coast. Other detail differences in the
evolution of that trough/storm system make the ensemble means a
more prudent choice as a starting point in the forecast process
for the Wednesday through Thursday time period.
...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. 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
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