Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
222 AM EDT Sat Mar 20 2021
Valid 12Z Tue Mar 23 2021 - 12Z Sat Mar 27 2021
...Overview...
The guidance continues to show a broad mean trough aloft settling
over the Central U.S. through much of the medium range period with
several embedded quick moving shortwaves. Ahead of this mean
trough, an active weather pattern is expected with heavy rainfall
potential from the Gulf Coast into the East. Shortwave energy
digging into the western side of the trough should also bring
periods of rain and mountain snow across parts of the West. Models
and ensembles continue to agree on the overall large scale
pattern, but differ on some meaningful timing and detail issues.
...Model Guidance/Predictability Assessment...
A vigorous shortwave ejecting out into the Upper Midwest on
Tuesday, will support surface low pressure that tracks northeast
from the Central Plains. Models have come into better agreement
regarding strength and timing of this system so a multi-model
blend should suffice. Meanwhile, another shortwave will dig
through the West with a possible closed low over the Four Corners
region early Wednesday, before opening back up as it lifts to the
northeast thereafter. The GFS is a notable outlier, now lagging
well behind the majority of the guidance. A blend away from the
GFS is preferred. This does result in a faster progression of the
cold front across the Eastern U.S. compared to previous
continuity, but this is supported by recent trends in both the
ECMWF and the CMC (as well as ensemble means). Finally, amplifying
shortwave energy into the Southwest U.S. late week continues to
offer some degree of uncertainty. Interestingly enough, the GFS is
quite a bit faster/weaker than the ECMWF/CMC and means so again, a
blend away from the GFS is a good starting point.
Overall, guidance preferences led to an early-period blend leaning
mostly on the deterministic guidance (away from the GFS). After
day 5, increasing weighting of the ensemble means was used to
mitigate the late period timing and detail differences, while
still including some deterministic ECMWF/CMC just for better
feature definition.
...Weather/Threats Highlights...
The developing mean trough aloft with periodic reinforcement will
bring below normal temperatures over much of the West into the
Rockies during most of next week. Expect coolest temperatures to
be Tuesday-Wednesday with highs 10-15F below normal over parts of
the Great Basin and central-southern Rockies. Many areas will see
a moderating trend after midweek but another cold front may
reinforce minus 5-10F anomalies for highs from the Northwest into
central Rockies. Incoming shortwaves/leading surface fronts will
bring periods of rain and mountain snow to the Pacific Northwest
and extending eastward/southeastward through the West into the
Rockies as the shortwaves amplify. Most of this activity should be
of light to moderate intensity but some localized heavier areas
will be possible, especially across parts of the Colorado Rockies
Tuesday into Wednesday.
The upper dynamics ejecting from the Rockies will support an
organized system lifting through the Upper Midwest early to
mid-week dragging a cold front eastward. North and west of the
surface low, some light snowfall could fall in the sufficiently
colder airmass. To the south, improving Gulf of Mexico inflow
should promote areas of heavy rainfall from the Lower Mississippi
Valley/Gulf Coast with lighter rainfall extending northeastward.
Recent guidance continues to highlight the potential for highest
rainfall totals near the central/east-central Gulf Coast around
midweek as the front trailing from the Upper Midwest system
decelerates/stalls while awaiting the next western U.S. shortwave.
This latter ejecting western energy should generate another late
week system that lifts northeastward over the east-central U.S.,
spreading areas of enhanced rainfall into a greater portion of the
East.
Warmest temperature anomalies will progress from the
Midwest/western Great Lakes into the Northeast over the course of
next week. These areas should see one or more days with highs of
10-20F above normal. Morning lows will be somewhat more above
normal with greater southward extent of plus 10F and greater
anomalies over the eastern half of the country, aided by the moist
flow from the Gulf.
Santorelli/Rausch
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