Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1150 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 later in the
week through the 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 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/Midwest. 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...
Longwave pattern remains in better than average agreement across
all the ensemble systems through next weekend as the very
amplified pattern takes shape. 500mb standardized height anomalies
of -4/+2 for the western trough/eastern ridge, respectively, in
the ensemble means (!!) by the end of the period were in the
extremes for this lead time and this time of year relative to what
the ensembles can forecast. However, as is typical, the embedded
shorter wavelength features show less predictability.
In the Southwest midweek, the deterministic models still showed
timing differences for the closed low ejecting through the
Southern Plains Friday--00Z UKMET and 06Z GFS were quickest as
they have been off an on. The most consistent has been the ECMWF
over the past several days and this, along with the 00Z GFS and
Canadian, formed a good starting point for the CONUS. By Friday
into the weekend, additional uncertainty in the speed of height
falls out of British Columbia continue to persist and the 06Z GFS
again was quicker than most. Trended toward the ECMWF and GEFS
means with retained detail from the 00Z ECMWF as it was closest to
the ensemble consensus. This maintained good continuity from the
previous forecast.
Building heights (eventually a closed 594dm upper high) in the
Southeast may start to influence Tropical Storm Karen as it moves
north of 20N. That forecast is highly uncertain given the lack of
steering between the strong upper high over the lower MS Valley
and a deeper trough well east in the central Atlantic. Please
consult the NHC forecast for the latest on Karen.
...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 but lowering snow levels
could reach some valley locations. 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 as the cold front sinks south of 30N late in
the week. High elevation snow will be possible over the Northwest
and Sierra Nevada but with lower probabilities for significant
totals relative to the northern Rockies. Wind will increase of
much of the interior West as well, centered over the Great Basin
eastward into the central 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 (mid/upper 90s) 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.
Fracasso/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