Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Sun Feb 28 2021
Valid 12Z Wed Mar 03 2021 - 12Z Sun Mar 07 2021
...Overview and Guidance Evaluation...
At least through the 12Z/18Z cycles, guidance was showing some
favorable converging trends with respect to the evolution of an
upper low that forms offshore California during the short range
and then progresses inland, along with how developing eastern U.S.
upper troughing will eventually influence what remains of this
feature later in the period. At the same time the models and
means have consolidated fairly well for the upper trough
amplifying off the West Coast into early Friday, but then continue
to diverge with specifics of the leading shortwave as an upstream
feature amplifies into the overall mean trough next weekend.
Among data reviewed for this forecast the 18Z GFS and 12Z
ECMWF/CMC ended up clustering fairly well for the upper low
tracking through the Southwest and into the Plains. The 18Z GFS
represented a slower trend from the 12Z run. The 12Z UKMET was
the slow extreme. The 18Z GFS/12Z ECMWF and their means also
provided a decent basis for the preferred blend for eastern U.S.
upper trough that develops in response to the upstream pattern and
ultimately suppresses the Southwest/Plains system toward the Gulf
Coast before strengthening it over the western Atlantic next
weekend. At the moment it looks like a close call regarding how
much of an effect this system may have along central and northern
parts of the East Coast but with greater likelihood of minimal
influence. Specifics of this system by then will depend on exact
details of northern stream trough energy which so far has differed
among guidance and consecutive runs. Convergence for the eastern
U.S. and vicinity has been by way of the GFS trending more
amplified with the eastern trough and the ECMWF backing off from
its prior depth somewhat.
As upper troughing amplifies off the West Coast mid-late week,
latest consensus represents an eastward trend in the ECMWF over
the past couple days and slightly slower adjustment in the GFS.
Then after early Friday the GFS/GEFS mean have consistently been
ejecting the leading shortwave quickly inland as upstream energy
drops into the mean trough, in contrast to the much slower and
more amplified/sharper ECMWF. The 12Z CMC was a compromise though
on the fast side with the upstream feature. Some aspects of each
solution could have some merit. Typical GFS biases suggest it
could be too fast but at that time frame there has been a modest
faster trend with this feature in most guidance recently. Also
the idea of a flatter shortwave ejecting into the downstream ridge
could have some merit versus the ECMWF/ECMWF mean that bring more
height falls into the ridge. Thus a blended approach seems best
for the purposes of a single deterministic forecast.
New 00Z model runs seem to continue the theme of volatility from
recent days. The GFS/CMC/ECMWF have all trended westward with the
upper low offshore California, joining the UKMET. The 00Z cluster
remains slower than the 12Z ECMWF/18Z GFS as the system tracks
through the Southwest but gradually catches up over the Plains on
Friday. However this slower trend is enough to lead to a more
suppressed surface pattern farther eastward as the eastern upper
trough has more time to amplify. Meanwhile solutions have
remained in their respective clusters near the West Coast during
the latter half of the period.
...Weather/Threats Highlights...
On Wednesday expect precipitation over the Southeast/southern
Mid-Atlantic to taper off as a wave initially along the
southeastern coast continues into the Atlantic. Some rainfall
could still be locally heavy early in the day depending on timing
of this system. The upper low closing off to the west of
California during the short range period should move through the
Southwest and into the Plains during the latter half of the week.
This system will bring some rain into parts of southern California
and rain/higher elevation snow to portions of the Southwest and
southern Rockies. Then rainfall over the Plains (possibly
moderate to heavy in places) should stay suppressed at least as
far south as the southeastern quadrant of the lower 48 as the
surface system drops toward the Gulf Coast or even into the Gulf,
possibly continuing over the western Atlantic. Confidence is
improving for the general path of the upper low through the
Southwest into the Plains but timing is still a question. However
confidence remains lower than desired for precipitation coverage
and intensity over the eastern half of the country due to the
combined uncertainties of the upper low's timing and relation to
the upper trough that develops over the East. 00Z model trends
toward slower upper low timing could allow for sufficient eastern
U.S. trough amplitude to prevent most of the moisture from
extending eastward from the Plains. Currently expect most effects
to remain offshore the East Coast if low pressure develops over
the western Atlantic next weekend.
Precipitation ahead of the amplifying eastern Pacific trough aloft
will initially focus primarily over Vancouver Island and into the
Olympics, with some moderate to heavy totals possible. Expect
rain and mountain snow of varying intensity to extend southward to
the central West Coast by late week and the weekend. Some light
precipitation will be possible at times during the period over the
northern Rockies, and possibly over parts of the Interior
West/central Rockies next weekend depending on the details of
shortwave energy aloft.
The forecast is very consistent in showing above normal
temperatures over the Northern Plains and vicinity through the
period, with decent coverage of plus 10-25F anomalies for highs
and plus 10-20F anomalies for morning lows. Gradual amplification
of the large scale pattern will bring a warming trend to the West.
The warmest days should be around Friday-Saturday with the Great
Basin the most likely to see readings up to 10-15F above normal.
On the other hand areas near the East Coast will see a cooling
trend that could bring highs down to 5-15F below normal by next
weekend.
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