Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Thu Oct 10 2024

Valid 12Z Sun Oct 13 2024 - 12Z Thu Oct 17 2024


...Overview...

Most models and ensembles maintain the theme of an amplifying
ridge-trough pattern during the first part of next week, followed
by somewhat more progression as a trough moves into the West by
next Thursday as downstream ridging crosses the central U.S. and
eastern troughing relaxes. Much below normal temperatures and
frost/freeze may be a concern over some areas from the northern
Plains into the East, while the Rockies and portions of the Plains
will be the main focus for above normal temperatures. Some daily
record highs will be possible over the far southern tier early in
the period. Expect the majority of precipitation to occur over
three areas: the Great Lakes/Northeast in association with
developing low pressure and chilly flow in its wake, southern
Florida early in the week with a lingering front, and over parts of
the West with the approach/arrival of a Pacific upper trough and
leading front.

...Guidance/Predictability Assessment...

In terms of the larger-scale sensible weather effects, the most
contentious part of the forecast involves what happens with energy
in the southern part of the eastern U.S. upper trough from around
Tuesday onward. The majority of guidance, including recent runs of
machine learning (ML) models, have generally favored maintaining an
open trough aside from perhaps a transient upper low forming near
the East Coast and then continuing into the Atlantic. This scenario
would support surface high pressure and dry conditions over the
East. However CMC runs have been persistent in closing an upper
low (but trending eastward over the past couple days) and sporadic
GFS runs have as well--most notably the new 12Z version that is
like the 00Z CMC. Such a closed low would develop low pressure and
rainfall over parts of the East. Among the ensemble systems, CMC
members unsurprisingly show a greater closed low potential though
some ECMWF members have that idea as well, while that solution is
much more in the minority among GEFS members. The combination of ML
models, overall dynamical guidance majority, and multiple days of
CMC runs eventually trending more toward consensus when depicting
stray solutions several days out, favor the open trough solution
for a single deterministic forecast. There is enough of the
minority signal to require monitoring for potential future changes
though.

As for more localized but still important detail differences,
guidance still shows some spread for specifics of Northeast low
pressure development by Monday. Latest ensemble means and most ML
models support a primary low track in the middle/western part of
the overall spread in contrast to the coastal track of the 12Z
GFS/UKMET/CMC. Meanwhile guidance shows better than average
agreement in principle for the upper trough reaching the West by
next Thursday, though some low-predictability shortwave detail
differences arise and will affect the coverage and amounts of
associated precipitation.

Based on 00Z/06Z guidance, a blend incorporating more 06Z GFS/00Z
ECMWF relative to the 00Z UKMET/CMC represented preferences for
significant features during the first half of the period. Then the
blend transitioned to mix of the GFS/ECMWF and their corresponding
ensemble means, with the latter reaching 50 percent total weight by
the end of the period next Thursday.

...Weather/Hazards Highlights...

The lower 48 should see fairly limited precipitation coverage
overall. One area of organized focus will be over the Great Lakes
and Northeast this weekend into the first half of next week, from
a combination of the strengthening Sunday-Monday surface system as
upper troughing begins to amplify and then lake effect/terrain-
enhanced precipitation behind the system and underneath the upper
trough. After system passage there may be some pockets of snow over
high elevations and perhaps other locations during the colder
overnight hours. Over the past day, guidance has trended lower
with max QPF totals over the Northeast on Sunday in association
with surface low development (with instability still lacking),
further supporting no risk area in the Day 4/Sunday Excessive
Rainfall Outlook.

Farther south, a front lingering near south Florida will be
accompanied by anomalous moisture and some instability Sunday-
Monday, potentially leading to some areas of enhanced rainfall over
the southern part of the peninsula. Guidance signals are mixed as
to how much of the heavier rain may fall over land versus offshore,
so for now will maintain no risk area in the Days 4-5 EROs.
However will monitor future guidance updates for refinements in
determining potential for localized flash flood threats, keeping in
mind initially wet ground conditions.

The upper trough and leading surface front moving into the West
will gradually push precipitation into the northern/central West
Coast through the northern Rockies. 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. Some
higher elevation snow will be possible by midweek.

An upper ridge aligned over the Rockies into early next week and
then shifting eastward by midweek will promote well above normal
temperatures in its vicinity. Plus 10-20F anomalies for highs will
focus over the Interior West/Rockies into early next week and
expand into the northern High Plains by Tuesday, followed by more
emphasis over the northern/central Plains by next Thursday. Highs
could locally exceed 20F above normal over the northern Plains
around midweek. The southern Plains should remain unseasonably warm
(at least plus 10-15F anomalies) into the start of next week
before northern stream troughing and a cold front suppress above
normal readings to the south. Daily records will be possible from
the Southwest into the Lower Mississippi Valley Sunday-Monday.
Meanwhile, the central/southern parts of the East will start out
above normal but then trend colder with time toward highs 5-15F
below normal next Monday-Wednesday underneath amplified upper
troughing. Morning lows next week may be a tad less anomalous but
still cold enough to raise some frost/freeze concerns from the
north-central U.S. through the Midwest/Great Lakes and
Northeast/Appalachians. Expect some moderation by next Thursday.
Arrival of a Pacific upper trough will likely bring below normal
temperatures to the West by next Thursday.

Rausch/Santorelli


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