Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Wed Oct 2 2024

Valid 12Z Sat Oct 05 2024 - 12Z Wed Oct 09 2024

...Anomalous heat will persist across the Southwest at least into
the weekend...

...Chances for heavy rainfall across the Florida Peninsula will
increase into next week...


...Overview...

Guidance continues to show the expected pattern transition from
progressive northern tier flow extending into the weekend toward
an amplifying eastern U.S./Atlantic Coast trough early next week
(with possible New England or vicinity upper low) while a mean
ridge covering much of the West extends into the Plains, producing
an extended period of unseasonably warm to hot weather. Ahead of
the dynamics that will ultimately establish the eastern upper
trough, a front crossing the Great Lakes and Northeast should
produce some organized rain, and possible surface low development
could enhance amounts for parts of New England. Meanwhile, details
remain uncertain within the general Gulf of Mexico evolution
favoring low pressure and heavy rainfall potential. Most guidance
suggests heaviest rainfall will initially stay over the Gulf but
then eventually some of it may spread eastward into Florida. There
may be multiple surface features to focus rainfall, including
combining tropical energies and frontal boundaries in the vicinity.


...Guidance/Predictability Assessment...

Aside from typical smaller-scale details, the primary question
mark with the northern stream flow is how upper troughing will
evolve once it reaches eastern North America during the early to
middle part of next week. Although not unanimous, there has been a
majority cluster suggesting that an upper low will close off
somewhere over eastern Canada or New England. This is certainly
plausible with the overall pattern looking more blocky as an upper
ridge strengthens over/near Greenland. CMC runs have been leaning
to the western side of the spread for the overall trough/upper low,
especially in the newer 12Z run, and the newly arriving 12Z ECMWF
has actually flipped to an upper low spinning near and just north
of the Great Lakes as energy digs southeast from the Hudson Bay.
EC-based AI/ML runs for the 00Z cycle tend toward a New
England/Canadian Maritimes upper low on average, though with
spread, and it will be interesting to see if the 12Z versions are
similar or lean more like the 12Z operational EC.

The forecast over and near the Gulf of Mexico continues to have
low predictability due to a combination of smaller-scale features.
One wave may track into the central/east-central Gulf from the
northwestern Caribbean while another feature may evolve over the
western Gulf, with the surface/QPF details also potentially
affected by energy aloft near the western Gulf Coast. These
features are likely to phase early next week as they approach and
interact. Guidance shows the system(s) could linger and meander
over the Gulf even through the first half of next week, though
perhaps beginning to drift eastward with time as guidance shows
some progression of a positively tilted upper trough. This appears
to be a messy evolution as fronts and moisture gradients are also
evident nearby, and some lows may be or become frontal. Continue to
monitor outlooks from the National Hurricane Center for the latest
information on features of interest.

The updated forecast started with a 00Z/06Z operational model
composite early in the period and then transitioned toward a blend
of the 06Z GFS/00Z ECMWF and their means late, as the CMC seemed
like an outlier over the east, though the newer 12Z ECMWF
complicates this thinking.


...Weather/Hazards Highlights...

The rainfall forecast for areas along the Gulf Coast during the
weekend continues to be in the proverbial holding pattern, with a
relative majority of guidance keeping most of the significant
totals over the Gulf of Mexico, especially on Saturday. The
guidance messages become more mixed by Sunday. Many models continue
to show potential for heavy rainfall to reach western Florida, with
some trend toward a possible rainfall axis from the Tampa Bay area
south. But this is certainly not unanimous as it may take
additional time for this activity to reach Florida. At the very
least, reasonable consensus of the GFS/ECMWF and their means would
have moisture anomalies increasing over the state as an east-west
front becomes aligned across the peninsula, so some increase in
rainfall on Sunday relative to Saturday seems reasonable. The
question is to what degree. The Days 4-5 Excessive Rainfall
Outlooks for Saturday-Sunday maintain no risk areas for now, but
with an eye to parts of the Florida Gulf Coast region for possible
introduction of an area on Day 5 if there is further
clustering/continuity in future cycles. The pattern continues to
look favorable for enhanced rainfall over Florida during the first
half of next week with a general area of low pressure expected to
drift into the eastern Gulf and a potentially wavy front still
draped across the peninsula.

Elsewhere, the frontal system reaching the Great Lakes and
Northeast by Sunday-Monday and possible low development near New
England and the Canadian Maritimes thereafter may generate rainfall
of varying intensity. There is still a fairly wide range of
possible QPF totals from the eastern Great Lakes into New England
depending on exactly how this system evolves. A mean frontal zone
with one or more embedded waves should reach near the northern
Pacific Northwest by early-mid week, producing some periods of
mostly light rain and nonzero chances of snow in the highest
elevations.

The strong and persistent upper ridge covering the West and
extending into the Plains will support a lengthy period of well
above normal temperatures over many parts of the western/central
U.S. Highs 10-20F or so above normal will be common, and numerous
daily records for warm minimum and maximum temperatures appear
likely from the southern half of the West into parts of the High
Plains, especially during the weekend but also lingering into early
next week to some extent. Best potential for some highs 20-25F
above normal will be over the central Plains and Midwest on
Saturday and northern Plains by midweek. Above normal temperatures
over most of the East during the weekend should decline to near or
somewhat below normal as upper troughing sets up near the East
Coast. This may lead to frost/freeze concerns across portions of
the Upper Great Lakes region and interior Northeast, with the
timing around the median to a bit late for the first frost/freeze
of the season.


Tate/Rausch


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