Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Sep 23 2019
Valid 12Z Thu Sep 26 2019 - 12Z Mon Sep 30 2019
...Areas of heavy rain possible over parts of the central U.S....
...Areas of rain/mountain snow and much below normal temperatures
over the West...
...Expanding area of record highs in the Southeast later in the
week through the weekend...
...Overview...
Aside from some details there is still good agreement in principle
that a very amplified western trough/eastern ridge upper pattern
will become established by the weekend and extend into the first
part of next week. Much below normal temperatures and areas of
precipitation will accompany the western trough with the northern
Rockies likely to see the highest totals of rain and higher
elevation snow. Farther east a couple waves emerging from the
West--along with energy from the upper low initially over the
extreme Southwest--will increase potential for heavy rainfall from
parts of the Midwest into the Upper Mississippi Valley/Upper Great
Lakes. The eastern ridge aloft will support a broad area of much
above normal temperatures over the East and into the
central/southern Plains with record highs most likely over and
near the Southeast. Meanwhile Tropical Storm Karen will require
monitoring as the eastern U.S./western Atlantic ridge at the
surface and aloft may direct the system toward the Bahamas late in
the period.
...Guidance Evaluation/Preferences...
Overall the full array of guidance seems to have regressed a bit
compared to the past couple days in terms of some important
details within the western trough. A couple of the more
pronounced examples: The 12Z ECMWF abruptly changed to a scenario
in which the core of upper energy reaching the Northwest by day 5
Sat quickly ejects ahead of upstream energy--which so far has
little support from the ensemble means. The new 00Z GFS/GEFS mean
and to some degree 18Z GFS take longer than established consensus
to amplify the trough, due to differences in how flow separates to
the south of the eastern Aleutians mid-late week. This issue
affects downstream flow across the northern tier U.S. as well.
Thus the favored early-period model blend transitioned to
components of the 12Z GFS/CMC and 12Z ECMWF mean/18Z GEFS mean by
the latter half of the forecast. The blend kept the forecast
fairly close to continuity though reflecting a general trend of
guidance away from some of the most extreme negative height
anomalies previously seen over or near California.
Ahead of the main trough, there is decent agreement that a leading
shortwave will support low pressure that tracks across the
northern Plains and northeast into Canada Thu onward. Another
wave appears likely to reach the northern Plains and vicinity by
next Sun-Mon with the ensemble means fairly similar to each other
and CMC/GFS runs.
For the initial Southwest upper low the full spread of individual
ensemble members remains significantly greater than usual for
being only 3-4 days out in time. The 12Z models were better
clustered and similar to continuity that had been best represented
by recent ECMWF runs. Aforementioned issues with northern stream
flow in the latest GFS lead to slower ejection in that run.
Some guidance still shows the potential for the eastern ridge to
show a 594dm height contour at 500mb for a time during the
weekend/early next week. Connected to question marks with
specifics farther west, the 12Z ECMWF was a bit east of the
guidance majority with the upper high center. Eastern
U.S./western Atlantic ridging aloft and high pressure at the
surface may influence Tropical Storm Karen as it moves north of
20N. The most likely scenario has a westward track by late in the
period but flow complexities before that time lead to fairly high
uncertainty in the forecast. Please consult the NHC forecast for
the latest on Karen.
...Weather Highlights/Threats...
Highest precipitation totals over the West should be over the
northern Rockies and possibly extending east into the northern
High Plains and south into parts of the Great Basin. Best
potential for significant snow will be over higher elevations of
western Montana, while snow levels could trend sufficiently low to
reach some valley locations. A period of low level upslope flow
may enhance precipitation as well. Other areas over the West
should see lesser amounts and with greater sensitivity to
specifics of the upper trough. Wind will increase over much of
the interior West as well, centered over the Great Basin eastward
into the central Rockies.
The overall pattern and specifically two separate waves will
promote heavy rainfall potential over areas from the Midwest into
the Upper Mississippi Valley and Upper Great Lakes. Energy
originating from the compact upper low initially over the extreme
Southwest may contribute to focused activity but with considerable
uncertainty over timing and extent.
Locations over and near Florida may see increased rainfall toward
the end of the period, whether just from moist easterly deep-layer
flow or the potential approach of T.S. Karen. Monitor NHC
forecasts for the latest information regarding Karen.
During Fri-Mon expect multiple days with highs 10-25F below normal
over parts of the West and extending into the northern Plains.
The core of most extreme anomalies should cover an area from parts
of Oregon and northeast California/northern Nevada into Montana.
Morning lows will be less extreme due to clouds and precip but
there may be pockets of min temps of at least 10F below normal.
Much of the East will see plus 10-20F anomalies for both highs and
lows, with very warm lows also covering the central-southern
Plains. Daily record highs are likely especially over the
Southeast, and at times could extend a bit northward (more so at
locations with a shorter period of record) or westward depending
on exact position of the upper ridge.
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