Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Oct 14 2024
Valid 12Z Thu Oct 17 2024 - 12Z Mon Oct 21 2024
...Overview...
Upper troughing and reinforcing energy in the West will lead to a
cool and wet pattern there, including higher elevation snow. Rain
chances could increase in the southern/central Plains by the
weekend. Upper ridging ahead of this trough will promote warmer
than average temperatures in the northern/central Plains to Midwest
as it gradually moves east, eventually displacing the chilly upper
trough and surface high in the East by the weekend.
...Guidance/Predictability Assessment...
Model guidance is rather agreeable on the large scale through much
of the period. The upper pattern as the period begins Thursday
will consist of a broad trough atop the West, a ridge axis aligned
atop the Mississippi Valley and Midwest, and lingering troughing
atop the Eastern Seaboard. The ridge and Eastern trough should
shift slowly eastward late week, while the trough in the West is
forecast to dig and create a closed upper low over the Southwest by
late Friday-Saturday. Recent ECMWF runs from 12Z and the newer 00Z
runs are finally on board with forming this upper low, after
earlier runs maintained a phased upper trough. There is some spread
in placement with the closed low as it meanders over the weekend
and may slowly eject northeast by Day 7/Monday, but relatively
minor spread for a feature like this in the late medium range
period. Among the 12/18Z model cycle, the CMC becomes a
slow/southwest outlier by the end of the period. The 12/18Z GFS and
12Z ECMWF and most AI/ML models were more agreeable, but
interestingly the 12/18Z AIFS was the main outlier taking the upper
low farther east of consensus as early as Saturday. Also
interestingly though, the new 00Z CMC is more like these AIFS runs
with an eastward/fast trend. Prefer the majority cluster farther
west, but this will continue to be monitored. Guidance shows upper
ridging retrograding a bit by early next week in response to this
cutoff low.
The WPC forecast used a multi-model blend of the deterministic
guidance early in the forecast period. Given the relatively good
model agreement even into early next week, was able to maintain a
majority of deterministic guidance through the period though with
some GEFS and EC ensemble means to smooth out the minor
differences. Since recent models are more agreeable with a cutoff
upper low forming and moving slowly through the Southwest, QPF
trended up from the previous forecast in the south-central Rockies
and High Plains, a westward shift from where in the Plains the
heavier totals were previously.
...Weather/Hazards Highlights...
The upper trough/low pushing across the West later week will push
precipitation from the Pacific Northwest into the Intermountain
West/Rockies later week, with enhanced precipitation totals in
terrain. Higher elevation snow is likely, which will be the first
snow of the season in some locations given the warm Fall in the
West thus far. Some rain and snow could continue across the
central/southern Rockies into the weekend. Meanwhile, Gulf moisture
may stream in just ahead of the upper low for increasing rain
chances in the central/southern High Plains late week into the
weekend. While forecast details vary, recent models show some
moderate to locally heavy rain totals there, but with dry
antecedent conditions and rain rates not forecast to be extreme,
this may not cause an excessive rain threat. Lighter rain may
spread into the north-central U.S. late week too. More rounds of
precipitation are likely for the Pacific Northwest into the weekend
with additional impulses coming through.
Meanwhile, the eastern U.S. is forecast to be generally dry late
week into early next week, with the exception of parts of the
Florida Peninsula. There, return flow of moisture could increase
rain and thunderstorm chances especially for South Florida by
Friday and perhaps spreading northward into the weekend. Models
continue to vary on how much rain may fall onshore versus offshore,
so no area is currently delineated in the Day 5/Friday ERO, but
there may be a nonzero chance for flash flooding especially if
heavy rain rates occur in the sensitive urban areas.
An upper ridge aligned over the Mississippi Valley Thursday and
shifting into the East Friday will promote well above normal
temperatures in its vicinity. The biggest anomalies are forecast
across the north-central Plains on Thursday, where highs into the
70s and 80s are 15-25 degrees above average for mid-October. Cooler
than average temperatures by 10-15 degrees are expected to
continue across the eastern third of the U.S. into Thursday under
cool high pressure. Another round of chilly morning lows may cause
frost/freeze concerns from the Great Lakes region into the
Northeast/Appalachians. Gradual moderation/warming is likely there
Friday and even more so into the weekend when upper ridging comes
overhead, for temperatures around 5-10 degrees above normal in the
northeastern tier. Then the ridge rebuilding over the central U.S.
early next week will lead to warmer than average temperatures in
the northern Plains to Upper Midwest once again. On the other hand,
the upper trough and frontal surges in the West will cause below
normal temperatures spreading across the West Coast to Great
Basin/Intermountain West and Southwest particularly through late
week. These colder temperatures may also lead to additional
frost/freeze concerns for any still susceptible growing areas.
Tate
Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:
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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw
