Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Tue Sep 24 2019
Valid 12Z Fri Sep 27 2019 - 12Z Tue Oct 01 2019
...Tropical Storm Karen over the Caribbean...
...Much below normal temperatures over the West with rain and
areas of heavy mountain snow...
...Heatwave for the South/Southeast U.S...
...Heavy thunderstorms/rain over parts of the Midwest/Great
lakes...
...Pattern Overview and Guidance/Uncertainty Assessment...
After about a day of some guidance straying from each other and/or
continuity for some important details, latest solutions (minus the
new 00Z CMC) have returned to a more agreeable consensus
representing the evolution of an increasingly amplified western
trough/eastern ridge upper pattern. This flow regime will support
contrasting extremes of much below normal temperatures over the
West into the northern Plains and unusually warm to hot conditions
for this late in the year farther to the east. Over the West
expect highest precipitation totals to be centered over/near the
northern Rockies with best potential for heavy higher elevation
snow over western Montana. At the same time a heavy rainfall
threat will exist over parts of the Midwest/Upper Mississippi
Valley/Great Lakes. The eventual path of Tropical Storm Karen
remains very uncertain, with dependence on sensitive details of
steering flow and the ultimate strength of the system. Monitor
NHC forecasts for the latest information regarding Karen.
Overall the 18Z GFS, 12Z ECMWF/CMC, and 18Z GEFS/12Z ECMWF means
provided a good starting point for the updated forecast and
yielded only modest adjustments in continuity. Consensus shows a
leading upper low reaching the Northwest by day 4 Sat with
upstream energy dropping down from western Canada then serving to
maintain the upper low and perhaps further amplify the overall
trough which may finally begin to drift east a bit on day 7 Tue.
The majority cluster is also similar in having the eastern ridge's
upper high center retrograde close to or over the Lower
Mississippi Valley by day 6 Mon. Guidance agreement for the
overall pattern leads to fairly high confidence for that part of
the forecast but some embedded uncertainties remain, and it is
certainly possible that guidance might diverge again for some
details as was seen 24-36 hours ago. As for current
uncertainties, by the latter half of the period the GFS/ECMWF
trended faster than the means with the wave emerging over the
northern Plains and then tracking northeast into Canada. An
intermediate solution looked best given that flow around the
eastern ridge could at least support somewhat faster wave
progression than seen in the means. Earlier in the period there
was still some lingering spread for timing the ejection of the
short range Southwest upper low whose forecast has proven to be
challenging over recent days--with solutions tending toward the
slower half of prior spread in response to the western trough
taking a bit longer to amplify.
...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