Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
216 PM EDT Thu Oct 3 2024

Valid 12Z Sun Oct 06 2024 - 12Z Thu Oct 10 2024

...Anomalous heat will persist across the Southwest into early next
week...

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


...Overview...

Latest models and ensembles still show the upper pattern settling
into a slower-evolving pattern dominated by large scale features.
Expect an upper ridge covering the western-central U.S. to produce
a broad area of much above normal temperatures over that part of
the country while a trough settling over the East (anchored by an
upper low that may track over or near New England) will bring a
cooler trend. The leading edge of a Pacific trough will produce
gradual height falls over the Pacific Northwest and bring some
precipitation to the northern parts of the region. 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. Farther south, expect the heaviest rainfall over
the Lower 48 to be over parts of Florida, due to the combination of
abundant moisture in addition to one or more Gulf of Mexico
surface features and a front draped across the peninsula.

...Guidance/Predictability Assessment...

Guidance continues to show typical detail differences for the
evolution of the upper trough settling over the East and the
anchoring upper low. General trends of the GFS/ECMWF and their
means over the past couple days have been unevenly a little slower
with the upper low, suggesting a middle ground solution. By the end
of the period the full array of dynamical and machine learning
(ML) guidance would recommend a farther east position (over or east
of northern Maine) versus the slower 00Z CMC. However, the 06Z GFS
was on the quickest side esp vs its mean. Therefore, an average of
the GFS/ECMWF and their means had good support from the average of
ML models by late in the period, but with a bit less emphasis on
the GFS.

Some aspects of the forecast over and near the Gulf of Mexico
still 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 multiple impulses aloft. Lately there has
been some improvement in the signal for these features to phase
early next week as they approach and interact, with this
potentially messy evolution involving fronts and moisture gradients
possibly yielding a more conventional wavy front that sags just
south of the Florida Peninsula by the end of the period. Per
coordination with the National Hurricane Center, will maintain a
separate area of low pressure from the frontal boundary just to its
north through the period but uncertainty is quite large. Please
consult the NHC outlooks for the latest information on features of
interest over the Gulf.

...Weather/Hazards Highlights...

The majority of the Lower 48 should see dry weather for most or
all of the Sunday-Thursday period, with most precipitation focus
around the corners. By far the heaviest rainfall potential should
be over the Florida Peninsula, with slow-moving low pressure over
the Gulf of Mexico along with a wavy front draped over the state
and a broad area of anomalous moisture extending over/east of the
Gulf. There is still a fair amount of uncertainty for details
though. During Day 4 Sunday, most dynamical models/ensemble
probabilities keep the highest rain rates just offshore the western
peninsula but machine learning models show faster progression of
Gulf low pressure and leading enhanced rainfall. For now will
maintain no risk area in the Day 4 Excessive Rainfall Outlook until
a dynamical model trend toward the ML models becomes evident. On
Monday there is better consensus that enhanced moisture and
potentially heavy rainfall will extend farther eastward across at
least the southern half of Florida, meriting the continuation of a
Marginal Risk area for the Day 5 ERO introduced overnight last
night. Southern Florida may continue to see periods of significant
rainfall for another day or so, with the axis of highest totals
sagging south with time along with the focusing wavy front.

Elsewhere, the frontal system crossing the eastern Great Lakes and
Northeast Sunday-Monday may generate rainfall of varying
intensity. This region may then see showery conditions at times as
shortwave impulses flow around an upper low expected to track over
or near northern New England and the Canadian Maritimes. 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 light to locally moderate rain and non-zero chances of
snow in the highest elevations.

The strong and persistent western-central U.S. upper ridge will
promote an extended period of well above normal temperatures over
that part of the country. Gradual eastward drift of the ridge axis
will shift the highest anomalies from the West on Sunday, with some
highs at least 10-15F above normal, more into the Plains by
Tuesday-Thursday when some northern locations could reach 20-25F
above normal. Expect numerous daily records for warm minimum and
maximum temperatures, especially over the Southwest through Sunday-
Monday. Height falls slowly pushing in over the West Coast will
support a trend toward near normal temperatures there by Wednesday
or Thursday. Moderately above normal temperatures over most of the
East as of Sunday should then 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.

Fracasso/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