Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Thu Mar 04 2021
Valid 12Z Sun Mar 07 2021 - 12Z Thu Mar 11 2021
...Overview and Guidance Evaluation...
The dominant feature of interest from Sunday through most of next
week will be an amplified eastern Pacific mean trough aloft that
gradually makes its way into the western U.S. This trough will
support a broad area of rain and higher elevation snow across the
West along with below normal temperatures. Meanwhile by
Tuesday-Wednesday a fairly vigorous shortwave expected to eject
from the trough is likely to produce a significant Northern Plains
storm system that should spread a broad area of precipitation
across the central/east-central U.S. Ahead of this evolution, an
amplified East Coast upper trough will depart after Sunday and
eastern heights will continue to rise through the period as
ridging moves in from the central U.S. and an upper high builds
from Mexico into the Gulf around midweek.
There is decent consensus for the overall evolution within the
East Pacific into western U.S. upper trough. The most likely
scenario has an upper low drifting southward off the Pacific
Northwest while energy flowing around the western/southern side of
this low reaches the West by Tuesday and then continues
northeastward to provide the dynamic support for the midweek
Northern Plains storm. Meanwhile energy from the upper low off
the Northwest should drop southeastward and possibly combine with
upstream flow to yield an upper trough that reaches near the Great
Basin/Southwest by day 7 Thursday.
The primary forecast challenges involve the timing and evolution
of the ejecting energy that supports the Plains storm as well as
the overall amplitude of flow to the southeast--affecting
specifics of the surface system and how far southeastward the
trailing front extends. Overall preference is to stay closest to
the ensemble means for timing later in the period, given
reasonable continuity thus far and intermediate position between
faster GFS runs and slower ECMWF runs. The new 00Z ECMWF has
adjusted favorably faster for the northern tier storm. GFS runs
through the 18Z cycle had been the most eager to close off an
upper low (though the 12Z ECMWF did the same but later), and while
the new 00Z GFS keeps the energy as an open wave it continues the
theme from previous runs of extending height falls and the cold
front farther southeastward than most other guidance. It will
likely take additional time to resolve the specifics of how this
energy evolves given its medium to smaller scale lending itself to
lower predictability. With the 18Z parallel GFS (GFSP) comparing
better to the means in principle versus operational GFS runs, the
initial blend followed a 12Z model consensus with the 18Z GFSP for
the GFS contribution early-mid period and then transitioned to
about half ensemble means and the rest weighted more to the GFSP
than 12Z ECMWF.
...Weather/Threats Highlights...
Expect a large portion of the West to see at least some
rain/higher elevation snow during the period. Highest totals
should extend from southwestern Oregon through favored terrain in
California, perhaps maximizing along the Sierra Nevada.
Meaningful though more moderate precipitation is likely across the
Interior West and Rockies. Energy ejecting into the Plains should
produce a fairly strong storm system that tracks across northern
tier locations Tuesday onward, with a broad area of precipitation
extending across the northern/eastern Plains and into the
east-central U.S. Some wintry weather may be possible over
extreme northern locations depending on exactly how the storm
evolves. Farther south there may be locally moderate to heavy
rainfall with the Middle Mississippi Valley and Lower Ohio Valley
currently the most likely to see highest rainfall totals. Also
the Storm Prediction Center is monitoring the potential for some
strong to severe thunderstorms. Check future outlooks as the
system's details come into clearer focus. Around the end of the
period and beyond, the cold front anchored by the surface low
could stall with continued Gulf inflow helping to add to the
rainfall potential. There is currently enough spread to temper
confidence in this scenario somewhat but it requires monitoring.
Much above normal temperatures will persist over the Northern
Plains/Midwest and expand into the Great Lakes through the first
part of next week with readings generally 10-25F above normal.
Daily records should be more numerous for warm lows than daytime
highs. Development/progression of Northern Plains low pressure
and the trailing front Tuesday onward will finally push this
warmth more into the eastern half of the country by mid-late week.
At that time morning lows should remain as warm as 15-25F above
normal while highs will likely be somewhat less extreme. The West
will see a cooling trend spread across the region from west to
east during the period with highs tending to drop to 5-10F or so
below normal. On the other hand the East Coast states will start
out chilly with similar negative anomalies on Sunday followed by a
steady warming trend.
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