Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun Sep 22 2019
Valid 12Z Wed Sep 25 2019 - 12Z Sun Sep 29 2019
...Areas of heavy rain possible over parts of the central U.S....
...Rain and mountain snow expanding over the West late
week/weekend...
...Expanding area of record highs in the Southeast late next week
through next weekend...
...Overview...
The overall forecast is consistent, with guidance unanimous in
showing the development of a very amplified West Coast
trough/eastern U.S. ridge upper pattern by next weekend. The
western trough will support much below normal high temperatures
along with a broad area of precipitation including some higher
elevation snow, while the ridge will bring much above normal
temperatures to most of the eastern half of the lower 48 including
potential for numerous record highs over the Southeast. Between
the two extremes there will likely be one or more areas of heavy
rainfall over the Plains/Upper Mississippi Valley. A compact
upper low expected to be near the western U.S.-Mexico border as of
early Wed still provides some uncertainty in terms of when it
ejects northeastward ahead of the developing western trough.
...Guidance Evaluation/Preferences...
Most models and means through the 12Z/18Z cycles were generally
within typical error range for the large scale evolution over the
course of the period but there were some details that led to
downplaying a particular model solution for parts of the forecast.
For the upper low initially near the Southwest, the GFS/ECMWF
were reasonable (and close to previous preference) while the
questionably fast 12Z CMC timing required that run to be excluded
from the blend Wed-Fri. Among new 00Z guidance thus far the CMC
has adjusted back to the GFS/ECMWF while the UKMET appears fast.
Individual ensemble members are still very diverse so the majority
consensus could change at some point. Over the Northeast, the 12Z
GFS strayed from consensus Fri-Sun with excessive troughing around
a slow moving Canadian Maritimes upper low so that run had to be
dropped after Thu. Within the amplifying western trough both the
12Z and 18Z GFS runs were somewhat faster than consensus to bring
the embedded upper low into the Northwest late in the week. The
new 00Z GFS is slower but still a tad ahead of the
ECMWF/UKMET/CMC. On the other hand it remains to be seen if the
upper low tracks quite as far southwest as in the past couple
ECMWF runs. The ECMWF path would lead to some 500mb heights
reaching at least 5 standard deviations below normal over/near
California. As it is other models still suggest some minus 3-4.5
standardized anomalies.
Days 3-4 Wed-Thu started with primary emphasis on the 12Z ECMWF,
12-18Z GFS, and 12Z UKMET in order from more to less weight with
minor editing as appropriate. Later in the period the blend
transitioned to a combination of 12Z ECMWF/18Z GFS/12Z CMC and the
12Z ECMWF/18Z GEFS means. Operational guidance still held a
slight majority weight over the means through day 7 Sun given
their definition with the western trough and eastern ridge (whose
core should exceed 594dm Sat-Sun as it builds from the Southeast
into the Lower Mississippi Valley).
...Weather Highlights/Threats...
The deepening West Coast trough aloft will expand precipitation
coverage over the western states. The most agreeable area of
highest totals currently includes the northern Rockies and
northern Great Basin. Higher elevations of western Montana have
the best potential for meaningful snow. In addition the eastern
side of the northern Rockies may see a period of upslope flow that
would enhance rain/snow activity. Precip amounts closer to the
coast will be more sensitive to the exact position of the upper
trough/embedded low. Some moisture may reach as far south as
southern California. High elevation snow will be possible over
the Northwest and Sierra Nevada but with lower probabilities for
significant totals relative to the northern Rockies. Well ahead
of the upper trough, the compact upper low initially near the
Southwest should generate some rainfall into midweek.
Latest guidance is generally suggesting two episodes of enhanced
rainfall over the central U.S.--one over the south-central Plains
early in the period as a front reaches the region and then a
second centered over the Upper Mississippi Valley/Upper Great
Lakes late week into the weekend. This latter area of rainfall
may be enhanced by the upper low energy ejecting from the
Southwest. It also remains consistent with where teleconnections
support precip based on the amplified trough/ridge pattern aloft.
The dramatic cooling trend over the West will start with some
highs 10F or so below normal over the Northwest on Thu. Then
expect a broad area with minus 10-25F max temp anomalies from the
West Coast states into Montana during Fri-Sun. A few record cool
highs may be possible. Clouds and precip will lead to less
extreme min temps but some morning lows may still reach 5-10F
below normal by the weekend. Meanwhile the East and to some
degree the central/southern Plains should see an expanding area of
highs and/or lows 10-20F above normal. Highs over the Southeast
could approach daily records at a few places as early as Wed, with
records likely becoming more numerous Thu into the weekend.
Hottest temperatures may push a little westward over the course of
the weekend as the upper high center retrogrades toward the Lower
Mississippi Valley. This pattern of extreme contrasts should
persist beyond the weekend based on the CPC day 6-10 and 8-14 day
outlooks.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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