Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Tue Sep 24 2019
Valid 12Z Fri Sep 27 2019 - 12Z Tue Oct 01 2019
...Much below normal temperatures expected in the West with winter
weather possible for the northern Rockies...
...Much above normal temperatures expected in the eastern third of
the nation...
...A couple potential rounds of heavy rain expected across the
central U.S...
...Pattern Overview and Guidance/Uncertainty Assessment...
Forecast confidence is a bit above average during the medium range
due to strong model consensus on the development of a highly
amplified pattern with a trough in the West and ridge in the East.
The 00Z ECMWF/06Z GFS solutions were clustered quite well within
the ensemble consensus, and a blend of these deterministic
solutions served as a basis for the forecast during days 3-5
(Fri-Sun). As upper-level shortwave energy traverses the
amplifying western trough and moves into the central U.S.,
multiple potential waves of low pressure should traverse a surface
front. The most significant of these waves looks likely to develop
Sat night-Sun in the lee of the Rockies, with a significant area
of precipitation likely north of the low from the northern Rockies
to the northern High Plains (rain and potential areas of snow). By
days 6-7 (Mon-Tue) model differences began to increase with
respect to whether the western trough moves east more quickly or
remains farther west as a slower and more cut off solution.
Ensembles remained in much better agreement than deterministic
solutions with below average spread, and thus a trend toward
heavier ECENS/NAEFS ensemble mean weighting was shown by early
next week.
Ryan
...Weather Highlights/Threats...
Expect significant precipitation over the western states to be
focused over/near the northern Rockies and into favored terrain of
the Great Basin. Northern Rockies precip should be enhanced by a
period of upslope flow. Recent forecasts have been consistent in
showing the greatest snowfall potential over western Montana.
While the heaviest snow will be confined to the mountains,
temperatures may be cold enough to bring some snow even to lower
elevations of the northern High Plains. Rain/high elevation snow
should be lighter and more scattered over northern California and
the Pacific Northwest. Some locations may see a period of
relatively stronger winds, most likely from the Great Basin into
the central Rockies.
Farther east anticipate some moisture extending across the
northern Plains while heavier rainfall is likely within an area
centered over the Midwest/Upper Mississippi Valley and western
half of the Great Lakes. A leading frontal system will likely
stall near the southern edge of the threat area and then lift
northward in response to the wave forecast to track across the
extreme northern tier. Shortwave energy ejecting from the
Southwest may enhance activity for a time as well.
Evolution of the eastern ridge aloft continues to favor a gradual
increase of moisture over Florida as easterly flow becomes
established. It is still very uncertain whether the
moisture/rainfall will be enhanced by Tropical Storm Karen.
Monitor NHC forecasts for the latest information regarding Karen.
Much below normal temperatures will settle into northern parts of
the West and northern Plains by Sat and then cover more of the
West by early next week. There will be a broad area of minus
10-25F anomalies for high temperatures and parts of Montana could
see a day or so with highs at least 30F below normal. Morning
lows will be less extreme but may still be at least 10F below
normal over some areas by Sun-Tue. Locations over the East and
even into the central/southern Plains will see plus 10-20F
anomalies for min and/or max temps. Best potential for daily
record highs will be over the Southeast, possibly extending a
little farther westward with time corresponding to motion of the
upper high. A few locations farther north could see records as
well. Morning lows may reach 20-25F above normal over parts of
the central Plains/Midwest by next Mon-Tue.
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