096
FXUS02 KWBC 180709
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Fri Oct 18 2024

Valid 12Z Mon Oct 21 2024 - 12Z Fri Oct 25 2024

...Overview...

An anomalous cut-off low over the Great Plains will produce some
scattered shower and thunderstorm activity across the region on
Monday, before phasing with a northern stream trough and amplifying
over the East Coast by mid-week. This will be succeeded by a ridge
across the western and central CONUS through next week. A stalled
trough in the eastern Pacific will contribute to a weak
atmospheric river event across the Pacific Northwest early next
week, followed by a closed low system and renewed potential for
precip over the Pacific Northwest toward the end of the week. An
anomalous warm airmass over the northern tier will shift into the
Northeast through mid-week. High elevation snow possible over
portions of the Pacific Northwest early and late next week.


...Guidance/Predictability Assessment...

Guidance is in reasonably good agreement regarding the overall
synoptic pattern across the lower 48. A general model blend
consisting of equal parts 12z ECMWF/CMC/UKMET and 18z GFS were
used on days 3 and 4. The GFS trended faster than the 12z guidance with
the ejection of the cut-off low over the Great Plains. The 18z
GEFS and 12z ECENS were introduced to the blend on day 5 to account
for deterministic variance in the orientation of the phasing
troughs over the Great Lakes and East Coast. The GFS and EC are
favored in this blend.

The CMCE is introduced to the blend on day 6
while the deterministic GFS is removed altogether due to its
insistence of yet another cut-off low over the East Coast. The
ensemble means and deterministic Euro appear to resolve the eastern
Pacific system similarly with the ensembles carrying a reasonable
amount of spread for a day 6 forecast. An 18z GEFS favored day 7
blend is used with the 12z ECE/CMCE also incorporated.



...Weather/Hazards Highlights...


A Four Corners cut-off low will shift into the Central Plains by
Monday, marking the end of an active weekend across the region.
Some lingering showers and thunderstorms will impact parts of the
Central/Southern Plains and Middle-Mississippi Valley on Monday.
The mid-level low will lift far enough north that it will avoid
tapping moisture from the Gulf of Mexico, which will keep PWATs,
moisture advection and ultimately QPF into the mid-section of the
country relatively low. Temperatures will rise 15-25 degrees above
normal over portions of the Northern Plains and Upper Midwest on
Monday due to ridging out ahead of the system in the Plains.

A positively tilted northern stream trough will promote rain
showers across the Upper Great Lakes on Tuesday and Wednesday
while also dragging a plume of moisture toward the Pacific
Northwest upstream of that. Things are relatively quiet on Thursday
before the arrival of a Pacific system that is forecast to produce
showers and potentially high elevation snow across the Pacific
Northwest on Friday. Upper ridging returns over the Great Plains
later in the week, which will support above average temperatures
throughout the mid-section of the country heading into the next
weekend.


Kebede


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







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