Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
122 PM EST Fri Mar 12 2021
Valid 12Z Mon Mar 15 2021 - 12Z Fri Mar 19 2021
...A couple of rounds of heavy rainfall are possible across the
southeastern quadrant of the CONUS next week...
...Overview...
As a surface and upper-level low initially over the south-central
U.S. track northeast and weaken, another upper trough is forecast
to move into the Northwest Monday and track east through the week,
likely spawning another low pressure/frontal system. As periods of
Gulf moisture flow in ahead of these systems, heavy rainfall is
possible Monday-Tuesday in the Tennessee Valley/Southeast and
again Wednesday-Thursday in the Lower Mississippi/Tennessee
Valleys, Southern Appalachians, and Southeast.
...Model Guidance/Predictability Assessment...
The closed upper low over the central U.S. Monday appears
initially well-handled by model guidance, but some differences
remain with its track as it weakens and lifts out ahead of the
next trough serving as its kicker. 00Z/06Z guidance produced
generally quicker weakening of the related surface low over the
Ohio Valley by Tuesday compared to previous runs.
After a few model cycles with disagreements on the location and
potency of troughing/energy coming into the Northwest for the
early medium range period on Monday, the 00Z/06Z guidance tended
to have a better consensus with this feature, given better
alignment in the short range as that trough tracks over the
Pacific compared to yesterday's 12Z runs. Guidance indicates this
upper trough/low should drop southward into the Four Corners
region and then track eastward across the Plains midweek and
toward the Ohio Valley. This model cycle tended to show more
separation between this feature remaining the southern stream and
the northern stream, potentially through Thursday and beyond. The
strength of that upper low remains in question, with the 00Z
CMC/ECMWF stronger than GFS runs. However, its placement is not
terribly different among the deterministic guidance. The surface
low track associated with this feature aligns pretty well over the
south-central Plains Wednesday, but spread in its position
increases as it moves over the eastern U.S. and into the Atlantic
late in the workweek. The 00Z ECMWF seems to be a northern outlier
with its surface low position, though the 00Z CMC is north as
well, while the 06Z GFS clusters the best with the ensemble means,
so leaned toward the latter cluster. This surface low spread also
affects the axis and precipitation type of the associated QPF from
the Middle Mississippi Valley to the Northeast.
All considered, model agreement for this cycle appeared generally
okay, but run-to-run inconsistencies show that confidence is still
not high. The WPC forecast transitioned from a mainly
deterministic blend early in the period and trended toward the
ensemble means later given the uncertainties arising.
...Weather/Threats Highlights...
The initial low system will still support wrap-around snow on its
northwest/north side in the cold sector, with best chances across
the Corn Belt/Upper Midwest Monday. Additionally, overrunning atop
a cold surface high could lead to some light snow in higher
elevations of the Central Appalachians and into the Northeast
Monday-Tuesday. Farther south, modest to potentially heavy rain,
perhaps driven by convection, is likely across the Tennessee and
Ohio Valleys and Southeast. Confidence then continues to increase
regarding another round of rain near and ahead of the next low
pressure system, with modest to possibly heavy amounts from the
Lower Mississippi/Tennessee Valleys toward the Southern
Appalachians and Southeast Wednesday-Thursday and perhaps the East
Coast by next Friday. Precipitation is possible farther north as
well, but as mentioned, details are uncertain at this point and
vary depending on the low track.
Underneath the West trough, some precipitation is possible over
the Intermountain West toward the Rockies early in the workweek.
Snow is once again likely for the Central Rockies and some could
spread into the Central Plains Tuesday-Wednesday, but should not
be as intense as the system this weekend. Renewed precipitation is
expected for the Pacific Northwest and northern California for the
latter part of the week as another upper low spins over the
northeastern Pacific.
Temperatures will be mild ahead of the Plains system through the
Southeast, but colder than normal in the Northeast until the high
moves offshore. Somewhat cooler than average temperatures will
follow behind the lead and subsequent Plains and Northwest
systems, respectively. Temperatures may be variable across the
High Plains but within about 5-15 degrees of normal for the middle
of March.
Tate
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