628
FXUS02 KWBC 300659
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Sep 30 2024

Valid 12Z Thu Oct 03 2024 - 12Z Mon Oct 07 2024

...Overview...

The majority of guidance still suggests a moderately progressive
pattern from the eastern Pacific through the northern lower 48 and
southern Canada, eventually transitioning toward a larger scale
western ridge/eastern trough by next Monday. For the most part,
fronts pushed along by northern tier dynamics should bring only
some areas of light rain at times. Meanwhile a persistent upper
ridge over the Southwest/Four Corners, at times connecting with
ridging to the north, will keep the southern half of the West
unseasonably warm/hot with some of the heat reaching the Plains.
Meanwhile the guidance seems to be no closer to resolving
significant differences within a general evolution favoring low
pressure/heavy rainfall potential over the Gulf of Mexico and
potentially extending into some areas along the Gulf Coast. There
may be multiple surface features to focus rainfall, including one
lifting from the northwestern Caribbean into the Gulf and another
that may be over the western Gulf.


...Guidance/Predictability Assessment...

The most significant forecast problem in the northern stream has
been the evolution of trough energy reaching the northeastern
Pacific by early Friday. Over recent days there has been a decent
cluster in principle among the dynamical ensemble means, more than
half of the models, and the majority of machine learning (ML)
models plus ECMWF AIFS ensemble mean, toward inland progression of
the shortwave by Saturday and then some trough amplification into
the East by early next week as an upper ridge builds into western
North America. However the ensemble spaghetti plots have been (and
still are) quite messy after Friday, while different models on
different days have differed from this majority cluster. In the new
00Z cycle, it is the GFS that strays by holding the Pacific
shortwave back--which some earlier CMC/UKMET/ICON runs had done,
but now those latter models have joined the more consistent ECMWF
cluster. The 18Z GFS also compared well to this majority. The
latest cycle maintains recent trends among the majority for a
somewhat flatter depiction of the shortwave as it crosses western
North America, maintaining warmer temperatures over more of the
West.

The forecast continues to be very uncertain over and near the Gulf
of Mexico. There have been some indications that one feature may
track from the northwestern Caribbean into the central/eastern Gulf
and possibly affect Florida. Relative to 12Z/18Z guidance
available for this forecast update, yesterday`s WPC/NHC
coordinated track was closest in principle to the 18Z GFS and thus
a track close to continuity was maintained. There may be a
separate feature over the western Gulf, which from coordinated
continuity is currently depicted as a trough near the western Gulf
Coast. Overall the new 00Z guidance (minus the GFS) seems to be
suggesting somewhat later arrival of moisture into Florida.
Continue to monitor outlooks from the National Hurricane Center for
the latest information on features of interest.

The updated forecast started with a 12Z/18Z operational model
composite early in the period, followed by a transition toward
30-50 percent total input of the 18Z GEFS/12Z ECens means with the
18Z GFS/12Z ECMWF. The forecast over the Gulf adjusted to
yesterday`s WPC/NHC coordinated depiction.


...Weather/Hazards Highlights...

Guidance differences over the Gulf of Mexico and Gulf Coast region
continue to result in low confidence for resolving where heavy rain
may affect coastal and/or inland areas during the period. During
the Thursday-Friday period covered by the Days 4-5 Excessive
Rainfall Outlooks, ML models showed some support for latest GFS
runs and recent ECMWF runs with some potential focus from
southeastern Louisiana into the western Florida Panhandle, with a
little eastward expansion with time. However other models/ensemble
probabilities are less enthusiastic over this region, while also
showing some enhanced rainfall potential along parts of the Texas
coast. Flash flood guidance values are high throughout the Gulf
Coast region, though Florida would be more sensitive from recent
wetness. Currently the Days 4-5 EROs do not depict any risk areas
given the wide dispersion of guidance, while awaiting a more
coherent signal for rainfall focus. Recent indications have been
for increasing rainfall to reach into Florida by the weekend,
though latest runs hint at some delay in the moisture`s arrival. In
addition, there has been somewhat of a more suppressed trend for
the northern periphery of rain that has been forecast to extend
into the Southeast.

Elsewhere, anticipate lighter rainfall along portions of the
fronts supported by northern stream dynamics. One front may produce
areas of light rain over the Great Lakes/Northeast late this week.
Another front reaching the Pacific Northwest around Friday may
produce some light rain, with trends over the past couple days
leaning toward lower amounts. Rainfall may expand along this front
when it reaches into the East by next Sunday/Monday.

Latest guidance continues the recent trend toward more persistent
upper ridging over the West. As a result, there may be decent
coverage of highs 10-15F or so above average extending all the way
into early next week with numerous daily record highs, albeit
becoming a little more confined to the Desert Southwest toward
Monday. The heat will extend into the Plains at times, with
Saturday likely to feature the highest temperatures over that
region (plus 10-20F anomalies), including some daily records over
the central High Plains. Temperatures over the East will vary with
frontal progression, with highs averaging out close to normal and
morning lows leaning more above normal.

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
















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