Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
205 PM EST Sun Jan 17 2021
Valid 12Z Wed Jan 20 2021 - 12Z Sun Jan 24 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 central U.S. next weekend...
...Overview...
A split upper-level flow pattern will be in place through much of
this forecast period, with a well-defined and detached upper low
just south of southern California for the middle of the week,
followed by another amplifying trough along the West Coast by next
weekend. Precipitation is forecast to spread into the Desert
Southwest while moderate to heavy rain is possible for
southeastern portions of the U.S. for the latter half of the week.
The second trough will lead to cold temperatures and precipitation
across much of the West, with multiple days of snow across higher
elevations, and there are indications that snow could spread into
northern/central parts of the Plains by next weekend. Meanwhile,
negative NAO (high pressure block over Greenland) and a negative
EPO (ridge over Alaska) should be in place, and this will
indirectly lead to quasi-zonal and broad cyclonic flow over the
north-central CONUS.
...Guidance/Predictability Assessment...
Good deterministic model agreement persisted through the 00/06Z
model cycle with the overall pattern regarding the anomalous upper
low near the Desert Southwest and and the broad upper trough over
the northeastern U.S. Wed. Some differences remain in the timing
of the low/trough ejecting eastward and getting absorbed into the
southern stream flow (likely Thu), with the 00Z UKMET still being
a fast outlier and not preferred. By the Friday to Sunday time
period, while there is good consensus on a second trough/possibly
closed low dropping southward along the West Coast, its timing and
depth remain in question. GFS runs are inconsistent with the
degree of the energy closing off at times, and the 00Z CMC
remained slower than other deterministic guidance with the
trough's movement. Thus, the WPC fronts/pressures forecast
incorporated some 06Z GEFS mean and 00Z EC mean into the blend for
the latter part of the period to minimize individual model
differences, but the 00Z and 06Z GFS and 00Z ECMWF were still
reasonable enough to be used throughout.
...Weather/Threats Highlights...
Multiple days of rainfall remain possible for the latter half of
the week across the Ark-La-Tex into parts of the Lower Mississippi
Valley and Southeast, as Gulf moisture streams into the region and
interacts with frontal boundaries. While recent model guidance has
backed off somewhat on rain amounts, perhaps due to a front
pushing southward more quickly through the region, 2-3 inches of
rain could still occur and potentially cause flooding issues where
convection repeats over the same areas. Farther west, moderate
rain can be expected for southern Arizona and southwestern New
Mexico. Then as the trough swings into the West, it will help
create precipitation, mostly in the form of moderate to heavy snow
in higher elevations as far south as the Mogollon Rim of Arizona.
The most notable snow amounts are likely to be in the Wasatch and
Central Rockies. Then by next weekend, wintry weather could spread
across the Northern/Central Plains toward the Upper Midwest ahead
of a surface low pressure system, while farther south the Lower
Mississippi Valley could get another soaking of rain.
Precipitation amounts and positioning of the heaviest totals for
next weekend are uncertain at this time. Light precipitation is
possible for the northeastern U.S. around Wed-Fri as a frontal
system moves through, with areas downwind of Lake Erie and Lake
Ontario seeing the most snow accumulation due to lake enhancement.
In terms of temperatures, readings are expected to be well above
normal across the Northern Plains Wed with highs 15 to potentially
25 degrees above normal. This will likely transition to slightly
below average readings by Fri across this region, as below normal
temperatures also expand in coverage across the western U.S. for
late week as the upper-level trough heralds a change in the
weather pattern.
Tate/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