Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
400 PM EST Tue Jan 19 2021
Valid 12Z Fri Jan 22 2021 - 12Z Tue Jan 26 2021
...Additional heavy rainfall appears likely for portions of the
south-central U.S. by Sunday and Monday as unsettled weather
overspreads the West Coast...
The recent shift in the synoptic pattern toward trough
amplifications along the West Coast appears to continue through
the medium range period as model consensus forecasts multiple
vigorous shortwaves digging across the region. This pattern will
also encourage additional heavy rain to develop across the
Mid-south as these shortwaves exit into the southern Plains.
Overall, model guidance this morning was in good to very good
agreement on the evolution of the synoptic scale pattern across
the U.S. through the medium range period. The 00Z ECMWF and 06Z
GFS solutions appeared most compatible with one another through
this weekend. By early next week, both models indicate the
likelihood of a low pressure system developing in the central to
southern Plains as broad upper-level troughing dips into the
northern tier states. Both models show significant run-to-run
variability regarding the speed and intensity of this system as it
moves across the East Coast by about next Tuesday. The 00Z EC
mean and the 06Z GEFS mean show only modest development of this
system with a relatively flat upper trough. The days 6 and 7
grids were based mainly on a blend of these ensemble means with
some 06Z GFS and the least from the 00Z ECMWF. The 00Z CMC was
not as compatible with the 00Z EC/06Z GFS blend and thus was not
taken into account in the forecast package.
An axis of heavy rain is expected to begin the medium range period
across the lower Mississippi Valley late this week near a
slow-moving frontal boundary. By Sunday, the storm system that
will be gathering strength across the Desert Southwest and
southern Rockies is expected to reach the southern Plains. A
departing surface high over the Midwest will then bring low level
moisture once again from the western Gulf of Mexico toward the
Deep South. The axis of heaviest rainfall appears to extend
mainly from the ArkLaTex region to the southern Appalachians
Sunday and Monday, with the potential of 1 to 3 inches of rain
together with locally higher amounts. This 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 addition, unsettled weather
appears likely for the Desert Southwest by this weekend and into
early next week. Wintry precipitation appears possible for the
higher elevations of Arizona while rain appears likely for the
southern half of the state into southern California.
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
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
- Heavy precipitation across portions of the Mid-Atlantic, and the
Central Appalachians, Mon, Jan
- Heavy rain across portions of the Lower/Middle Mississippi
Valley, the Central/Southern
Appalachians, the Tennessee/Ohio Valley, the Mid-Atlantic, the
Southeast, and the Southern Plains,
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 California, Mon, Jan 25.
- Heavy snow across portions of the Central Rockies, the Central
Great Basin, and the Northern
Great Basin, Fri-Sat, Jan 22-Jan 23rd and Mon, Jan 25.
- Heavy snow across portions of the Central Great Basin and the
Southwest, Sat-Mon, Jan 23-Jan 25.
- Heavy snow across portions of the Southern Rockies, the Central
Rockies, the Central Great Basin,
and the Southwest, Fri-Mon, Jan 22-Jan 25.
- Heavy snow across portions of the Great Lakes, the Upper
Mississippi Valley, and the Northern
Plains, Sat-Sun, Jan 23-Jan 24.
- Flooding occurring or imminent across portions of the Southeast.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: