Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
125 AM EST Mon Feb 15 2021
Valid 12Z Thu Feb 18 2021 - 12Z Mon Feb 22 2021
...Much below temperatures across the Central U.S. will continue
this week but should finally moderate back towards normal by next
weekend...
...Moderate to heavy precipitation expected across much of the
East Thursday into Friday...
...Overview and Guidance Evaluation...
An amplified pattern across the CONUS the first half of the medium
range period will be replaced by a welcomed change to a flatter
more zonal upper level pattern by next weekend. This pattern shift
would allow for a moderation in temperatures across much of the
lower 48 and less active storm systems. In the meantime though, a
prevailing amplified mean trough over the Central U.S. on Thursday
will shift into the Eastern U.S. by Friday. The associated low
pressure/frontal system will emerge from the Gulf and race into
the East. A widespread area of moderate to heavy precipitation
(snow to the north, rain to the south) is likely from the central
Gulf Coast into the Northeast Thursday into Friday. A series of
fronts should also keep the Northwest active through much of the
period.
The troughing entering the East by Friday shows very good
agreement in the models with only minor timing differences (UKMET
slower than the consensus) so a blend of the deterministic
GFS/ECMWF/CMC sufficed. The next system entering the Pacific
Northwest this weekend exhibits a bit more variability in the
guidance, especially as the shortwave shifts into the central U.S.
early next week. The ECMWF is on the faster side of the spread,
with the CMC a bit more amplified/slower. The latest run of the
GFS lies somewhere in the middle, and has support from the
generally agreeable (albeit a bit washed out) ensemble means.
Therefore, WPC prefers a majority blend of the ensemble means
beyond day 5, with smaller contributions from the GFS just to
maintain some degree of system definition. This also maintains
good continuity with the previous shift.
...Weather/Threats Highlights...
Heavy precipitation will accompany the East system as it lifts
from the Gulf into the Northeast late this week. Wintry
precipitation on the northern side of the precipitation shield
will be ongoing as the medium range period begins Thursday, with
the greatest chance for significant snowfall Thursday into Friday
from the Ohio Valley into the interior Northeast. To the south,
moderate to heavy rainfall is likely across portions of the
Southeast and southern Mid-Atlantic, with some ice/sleet possible
in between the rain and snow areas. Of course, the
rain/snow/wintry mix transition zones will continue to be refined
in the coming days. Also to note, SPC continues to mention the
severe weather potential, particularly for portions of the central
Gulf Coast where moisture and instability parameters are expected
to be most favorable.
Behind this system, another couple fronts reaching the Northwest
between Thursday and Saturday should bring the potential for
additional focused rainfall along coastal portions of
Washington/Oregon with heavy mountain snows in the
Washington/Oregon Cascades and northern Rockies. Meaningful
rain/high elevation snow may extend into parts of northern
California as well.
For temperatures, much below normal temperatures are expected to
persist through this week from the southern Plains and
northeastward, but finally should moderate back towards normal by
early next week as a less amplified pattern settles in across the
contiguous U.S.. Out West, temperatures should also trend warmer,
especially over the Southwest where highs may reach 5-10F or so
above normal Sunday into Monday, with above normal temperatures
also possible across parts of the Northern Plains. Warmer than
average temperatures across Florida/the Southeast will continue
into Thursday, but below normal temperatures should eventually
spread into the Sunshine State by Friday/Saturday after passage of
the front anchored by the system affecting much of the East.
Santorelli
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