Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Sat Oct 12 2024

Valid 12Z Tue Oct 15 2024 - 12Z Sat Oct 19 2024


...Overview...

As the medium range period begins Tuesday, upper troughing will
settle in across the eastern half of the U.S., leading to below
normal temperatures along with frost/freeze concerns for the north-
central U.S. to Northeast, along with lake-enhanced precipitation.
Meanwhile upper ridging (aside from embedded energy) will set up
atop the Interior West, and this ridge is forecast to shift
eastward into the central U.S. through the week with warmer than
average temperatures underneath it. The pattern in the West is
forecast to flip to troughing by midweek to late week with a couple
rounds of reinforcing energy. This will cool temperatures there
and lead to increasing precipitation chances, including higher
elevation snow.


...Guidance/Predictability Assessment...

Model guidance starts the medium range period in relatively good
agreement, with a broad trough across the Midwest to East while
general ridging in the West is broken up a bit by an embedded
southern stream upper low atop the central Great Basin Tuesday. The
trough in the East should gradually sweep eastward Wednesday-
Friday, with some differences in the timing. Recent models have not
indicated energy breaking off in the southern part of the trough
to form a closed low like some runs from a day or two ago, so at
least that aspect is in better agreement now. Then upstream,
ridging in the west-central to central U.S. and troughing coming
into the West behind it start to show spread in timing as soon as
Wednesday-Thursday. Generally the 12Z EC suite, including the
deterministic run and ensemble members and mean, is more
consolidated with the West trough and produces a deeper eastern
Pacific surface low (all this creating a stronger atmospheric
river), and thus is slower/farther west with the trough axis and
the ridge axis to the east. GFS runs and ensemble members show a
Western trough with shortwaves coming through at different times to
make it broader, and the ridge shifts east more quickly. Often a
middle ground solution in between the slow EC and fast GFS is
preferred, and the bulk of the 12Z AI/ML models are generally in
between with the trough and ridge positions, increasing confidence
in an intermediate solution. The 12Z CMC was in between and thus
could be utilized some. The newer 00Z GFS and EC have similar
positions as their older runs. Then by late week, some models show
an upper low closing off in the Southwest, specifically GFS and CMC
runs. The ECMWF does not show this and neither do the EC-based
AI/ML models. Will continue to monitor this uncertain potential.

The WPC forecast was based on a blend favoring the 12Z ECMWF/18Z
GFS/12Z CMC early, gradually reducing the proportion of the
deterministic runs (especially the EC and GFS) as the period
progressed in favor of the ensemble means, reaching over half means
by Day 6 and well over half by Day 7 as spread increased.


...Weather/Hazards Highlights...

Behind a surface low pressure system pulling north of New England
by Tuesday and underneath the upper trough causing cool westerly
flow, lake-enhanced and terrain-enhanced precipitation is likely
from the Great Lakes region into the Interior Northeast and into
the central Appalachians. Colder temperatures should allow for snow
especially in the higher elevations. Precipitation should taper
off Wednesday, and the East is likely to be dry for later week as a
cool surface high sets up behind a secondary cold front. The
exception could be across South Florida, which is forecast to see
return flow of moisture by late week for increasing rain and
thunderstorm chances.

The developing and reloading upper trough and multiple surface
fronts moving into the West will gradually push precipitation into
the northern/central West Coast by Tuesday and through the northern
Rockies into Wednesday-Thursday. While there is good confidence in
the overall event, unresolved specifics of the upper trough lead
to greater uncertainty with respect to the southward/eastward
extent of precipitation as well as magnitude. Expect relatively
higher totals over favored coastal areas and the Cascades. Higher
elevation snow is likely by midweek, which may be the first snow of
the season in some locations given the warm Fall in the West thus
far. While details vary, model guidance generally shows
precipitation spreading east through the Interior West/Rockies into
Thursday, and by Friday Gulf moisture may stream in just ahead of
the upper trough for some increasing rain chances in the Plains to
Upper Midwest.

An upper ridge aligned over the Rockies into Tuesday and then
shifting eastward by mid and later week will promote well above
normal temperatures in its vicinity. The biggest anomalies are
forecast across the northern Rockies and High Plains Tuesday-
Wednesday, where highs into the 70s and 80s are 15-25 degrees above
average for mid-October. These warm temperatures will shift into
much of the Plains and Midwest by Thursday-Friday. For much of the
eastern U.S., temperatures are forecast to be below average by
10-15 degrees Tuesday-Wednesday underneath cool high pressure.
Morning lows may cause frost/freeze concerns from the north-central
U.S. through the Midwest/Great Lakes and Northeast/Appalachians.
Expect some moderation there by Thursday- Saturday. Meanwhile, the
upper trough and frontal surges coming into the West will lead to
below normal temperatures spreading gradually inland for the latter
half of the week. These colder temperatures may also lead to
additional frost/freeze concerns for 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