Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Tue Jan 19 2021
Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021
***Additional heavy rainfall becoming more likely for portions of
the south-central U.S. by Sunday and Monday***
...Overview...
A quasi-zonal upper level flow pattern is expected to be in place
across the central and eastern U.S. through much of the medium
range forecast period, with a broad ridge axis extending across
the Gulf Coast region. After the departure of the strong southern
stream trough from the southwestern U.S. during the short range
period, a rather similar second trough and upper level low drops
southward along the West Coast this weekend, and likely followed
by a third shortwave disturbance by early next week. Shortwave
energy ejecting eastward from this disturbance, in combination
with a nearly stationary frontal boundary, is expected to result
in another round of active weather across portions of the
south-central U.S.
...Guidance/Predictability Assessment...
The deterministic model guidance is in above average agreement on
the evolution of the synoptic scale pattern through Sunday, and
this includes the development of the next major upper level trough
over the West Coast region, so a blend of the GFS/ECMWF/CMC along
with some of the ensemble means sufficed as a starting point in
the forecast process. By Monday, greater differences emerge with
shortwave energy pivoting around the southern Canada upper low,
with the 00Z GFS indicating a greater degree of northern stream
amplification. By Monday night and Tuesday, there are some detail
differences with the next trough to take up residence across the
West, with the 00Z CMC and ECMWF showing a slower progression.
The final two days of the forecast period incorporated a little
bit more of the ensemble means whilst still using some of the GFS
and ECMWF.
...Weather/Threats Highlights...
The storm system that will be gathering strength across the Desert
Southwest and southern Rockies is expected to reach the southern
Plains by Sunday, and the departing surface high over the Midwest
will lead to return flow from the western Gulf of Mexico and a
steady increase in low level moisture. This is expected to result
in more rain for the region, mainly from the ArkLaTex region to
the southern Appalachians. The axis of heaviest rainfall remains
uncertain for this second system on Sunday and Monday, but the
potential is there for 1 to 3 inches of rain during that time
period and locally higher, and could cause flooding concerns if
these higher amounts fall over those areas impacted by the earlier
rainfall event late this week. The upper trough amplifying over
the western U.S. will advect enough Pacific moisture and produce
enough upslope flow to result in significant snow for the Central
Rockies and Wasatch mountains. Some of this wintry precipitation
will likely spread northeastward across the northern Plains and
Upper Midwest by next weekend, but the details remain uncertain
with respect to the development of surface low pressure and
frontal interactions.
In terms of temperatures, the greatest anomalies are forecast to
be across the central and western Gulf Coast region, where the
southerly moist flow keeps readings about 10-20 degrees above
normal for late January. The most prolonged period of below
normal temperature readings are forecast to be over the western
U.S., with highs around 5-10 degrees below normal, and also across
the Upper Midwest on Friday with readings about 10 degrees below
normal.
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