Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
154 PM EST Mon Jan 18 2021
Valid 12Z Thu Jan 21 2021 - 12Z Mon Jan 25 2021
...Multiple days of rain expected for the Lower Mississippi Valley
to Southeast...
...Colder temperatures and increasing precipitation in the West
could spread into the north-central U.S. over the weekend...
...Overview...
A split upper-level flow pattern across the CONUS at the start of
the medium range period is expected to transition to primarily
troughing over the interior West and southwest flow over the
central U.S., which will likely result in a parade of storm
systems toward the latter half of the week and next weekend.
Rounds of moderate to locally heavy rainfall will be possible
across portions of the south-central to southeast U.S. while a
secondary trough ushers in much colder air for the northern tier
and could bring snow from the Rockies eastward toward the Upper
Midwest by next weekend, though snow amounts remain uncertain.
...Guidance/Predictability Assessment...
Deterministic model guidance continues to show fairly good
agreement through much of the period regarding the large scale
pattern, with an upper low near southern California Thu getting
quickly absorbed as it tracks eastward, and as troughing/energy
drop southward along North America's West Coast Thu-Fri and then
east into the West over the weekend. The 00Z CMC was on the
stronger side with the West upper trough around Sat, while the 00Z
UKMET was on the weaker side, but they balanced well. A
multi-model deterministic blend sufficed for the first part of the
period for the fronts/pressures forecast. Larger differences arise
with the evolution of the trough early next week, in particular
with the degree of separation from the northern stream and how
troughing and energy interact with an upper low persistently
spinning over central Canada. There are large run-to-run and
model-to-model inconsistencies with this, and it affects surface
low positions as well. At this point, the 06Z GFS appeared
different enough with its pattern and too fast/northeast with its
surface low to exclude, so used a component of the 00Z GFS by day
7 (along with the 00Z ECMWF), but generally favored the 00Z EC and
06Z GEFS means by the end of the period, which were reasonably
well aligned.
...Weather/Threats Highlights...
Waves of low pressure interacting with a surface boundary over the
Lower Mississippi Valley and a steady stream of Gulf moisture will
promote the development of a couple of rounds of moderate to heavy
rainfall. The first is likely around Thu-Fri this week and should
focus from the Lower Mississippi Valley into the Deep South. Then,
as another frontal system ejects into the Plains early next week,
more rain is expected for the region and for portions of the
Tennessee and Ohio Valleys. The axis of heaviest rainfall remains
uncertain especially for this second system. But the potential is
there for a few inches of rain during the period, while more is
possible if the rounds overlap over the same area, and could cause
flooding concerns. As an upper trough digs over the Rockies/West,
plenty of moisture and upslope flow will lead to some significant
snow potential for the Central Rockies and Wasatch mountains. Some
of this winter precipitation could spread across the Central
Plains and Upper Midwest by next weekend, but details remain
uncertain with respect to the development of low pressure and
frontal interactions. In terms of temperatures, the greatest
departures from normal are forecast to be across the TX/LA Gulf
Coasts, where the southerly moist flow holds up nighttime lows
well above normal, by 15-20 F. The most prolonged below normal
temperature readings are forecast to be over the Northern Rockies
during the period, around 5-10 F below normal for lows and 10-15 F
below normal for highs.
Tate/Taylor
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