Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Fri Feb 26 2021
Valid 12Z Mon Mar 01 2021 - 12Z Fri Mar 05 2021
...Heavy rain threat over the South continuing into early next
week and then gradually diminishing...
...Overview and Guidance Evaluation...
From the large scale perspective, guidance shows fairly
progressive northern stream and southern tier flow aloft with an
evolution toward a more amplified late-week regime featuring an
eastern Pacific trough and downstream western North American
ridge/eastern trough. Within this evolution there is considerable
disagreement over how energy may separate from a Pacific shortwave
either during the short range time frame or early in the medium
range. This flow separation issue ultimately leads to high
uncertainty/low confidence in system specifics over the eastern
Pacific and then eastward across much of the lower 48.
Guidance through the 12Z/18Z cycles essentially formed two
clusters regarding the dominant question of Pacific flow
separation. The larger ECMWF-led cluster pulled off some energy
rather early/westward with the remaining shortwave continuing into
the western U.S. (possibly closing an embedded low for a time) and
beyond. The 12Z CMC eventually strayed toward a more separated
southern solution over the East later in the period while multiple
ECMWF/ECMWF mean runs have indicated eventual surface low
development/consolidation over New England and/or eastern Canada
with contributions from the lower 48 shortwave and Canadian flow
as the overall pattern amplifies. Parallel GFS (GFSP) runs have
also been falling into this cluster in principle. On the other
hand similarly persistent GFS/GEFS mean runs have been waiting
until just offshore the West Coast to pull off a significant
portion of the Pacific shortwave energy. The resulting upper low
moves into the Southwest when the other cluster is starting to
develop a mean ridge over the West, and by late in the week the
GFS/GEFS cluster indicates a mere frontal passage across the
Northeast with minimal signal for any New England/eastern Canada
surface system. These issues will also have some effect on the
ultimate path/evolution of the compact upper low forecast to
emerge from the southern Rockies early in the week.
Compared to the wide variance of guidance for most of the period,
models and means actually come into good agreement for the
amplifying eastern Pacific upper trough late in the period. There
are still some typical detail and timing differences that are
fairly minor relative to the average spread/error for forecasts a
week out in time.
Given the relative proportion of guidance and the 18Z GFS trend to
close its eastern Pacific upper low to the east of the 12Z run,
the latest forecast went more in the direction of the ECMWF
cluster. This included some input from the 18Z GFSP as well as
the old 00Z ECMWF since the 12Z version was somewhat on the faster
edge among solutions for the shortwave crossing the lower 48.
The primary revelation in the new 00Z cycle is the CMC switching
to a variation of the GFS cluster. Other guidance is holding
serve, with the GFS/GEFS continuing its general evolution in
contrast to the ECMWF/UKMET/GFSP (albeit with some detail
differences from previous runs, as would be expected).
...Weather/Threats Highlights...
The upper low tracking out of the southern Rockies and through the
Plains early in the week should interact with a front near the
Gulf Coast, supporting one or more waves along the front and a
northward flow of Gulf moisture. This evolution will favor
another episode of locally heavy rainfall across parts of the
South during the first part of the week, with lighter
precipitation extending farther north over the southern half of
the Plains and an increasing portion of the East. There may be
some snow in the northern part of the precipitation shield. The
rest of the forecast has lower than average confidence for
specifics. One scenario--currently somewhat more likely--would
have a period of moderate rain/mountain snow over the Northwest
and vicinity at the start of the week with mostly light activity
then spreading into the Interior West/Rockies followed by
precipitation of varying intensity spreading across the East in
association with one or more surface lows/fronts (which may
eventually consolidate). The other possibility will be for a much
lighter event over the Northwest into the Rockies early in the
week and less moisture extending downstream, while a system off
California moves into the Southwest by midweek or so with moisture
continuing eastward into late week. Either way some moisture may
reach the West Coast late in the week depending on the exact
timing/strength of the upper trough amplifying over the eastern
Pacific.
The upper low emerging from the southern Rockies early in the week
will keep highs up to 10-20F below normal over the southern High
Plains on Monday followed by a rapid rebound to near normal as the
feature continues onward. Then cool high pressure building over
the East will lead to highs 5-15F below normal near the East Coast
on Tuesday, again followed by a quick recovery. The Northern
Plains region will likely be consistently warmer than average next
week with most days featuring some plus 10-20F anomalies. A
majority of the West should see near to slightly below normal
readings early-mid week followed by a warming trend as upper
ridging builds over the region. As with precipitation, some
aspects of the temperature forecast over the lower 48 will depend
closely on the uncertain details of individual systems.
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