441
FXUS02 KWNH 280701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sat Sep 28 2024

Valid 12Z Tue Oct 01 2024 - 12Z Sat Oct 05 2024

...Overview...

Most guidance shows progressive mean flow across the northern lower
48 during the period, pushing along a series of frontal systems.
One should push through the East around midweek while another moves
into the Northwest, with the latter front reaching the central and
eventually eastern U.S. late in the week. Another front may reach
the Northwest toward the end of the week. These fronts may produce
mostly light to moderate rainfall over some areas. Separate upper
ridges will prevail over the southern half of the West and
over/surrounding Florida, with the former in particular producing
well above normal temperatures. There is significant uncertainty
over how a potential weakness between the two ridges may eventually
draw increasing moisture northward into the Gulf Coast region,
with some signal for embedded tropical development as well.

...Guidance/Predictability Assessment...

For about the first half of the period, the most impactful
discrepancy involves the precise track of western Atlantic low
pressure and what interaction there could be between this wave and
the cold front supported by an approaching northern tier
U.S./southern Canada trough. The full array of 12Z/18Z guidance
recommended a compromise approach between the 12Z ECMWF/CMC that
were farthest west with the surface low (leading to more
interaction and higher Northeast rainfall totals) and other
solutions that were more offshore (leading to less interaction and
lower rainfall). The average of 12z machine learning (ML) models
leaned toward the more offshore scenario, and indeed the new 00Z
dynamical guidance overall has made a notable shift offshore.

The prominent uncertainties later in the week involve the details
of northern stream flow as well as the evolution over the Gulf of
Mexico and southern tier U.S.

Ensemble spaghetti plots become quite messy from the Pacific
eastward, though in a broad sense there has been somewhat better
clustering among the GFS/ECMWF, their ensemble means, and most ML
models/AIFS ensemble mean the past couple days. This favored
cluster would have an upper trough evolving over the eastern
Pacific by Friday and reaching the West Coast or inland by
Saturday, while a downstream ridge progresses into western North
America with cyclonic flow to the east. Within this cluster, ML
models include the range between the faster ECMWF runs and slower
GFS for the incoming Pacific trough by Saturday--arguing for a
blend approach. Meanwhile, CMC/CMC mean runs have been amplifying
the Pacific trough farther westward. The new 00Z CMC still exhibits
this characteristic but it does seem to be trending partially
toward the majority.

The forecast over the Gulf of Mexico and southern tier U.S. is
complicated due to the low predictability of details regarding both
the degree of tropical development (and path) reaching into the
Gulf of Mexico and upper level flow specifics over the southern
tier. Recent GFS/CMC/CMCens runs have been the most pronounced with
their combination of tropical development and faster northward
timing of a system. ML models and a majority of GEFS/ECens members
favor holding what weak low pressure there may be over the
southwestern Gulf into Saturday. At the same time there is a
general signal for a potential weakness aloft over the southern
Plains/Lower Mississippi Valley region, which could support an
increasing flow of moisture into portions of the Gulf Coast region
even in the absence of a defined surface low. This goes along with
the 12Z ML models that show relatively greater QPF focus along the
central/east-central Gulf Coast region toward the end of the week
even without much of a surface reflection--interestingly, a similar
region highlighted by the 00Z GFS/CMC/ICON in more pronounced
fashion due to their developed surface systems. Current preference
maintains continuity from yesterday`s WPC/NHC coordination
reflecting a nearly stationary weak low in the southwestern Gulf by
late week.

The updated forecast incorporated a blend of 12Z/18Z operational
runs for the early part of the period and then trended toward an
even model/ensemble mean mix, including splitting GFS/ECMWF input
between their two most recent runs to downplay low-confidence
specifics and phasing out the CMC due to disagreements with the
majority cluster.

...Weather/Hazards Highlights...

Forecasts have been showing rainfall progressing from the Great
Lakes/Appalachians/Mid-Atlantic into New England from Tuesday
through midweek, with various ideas for coverage and amounts
depending on the track of western Atlantic low pressure and how
much interaction occurs with an approaching front. 00Z model runs
are trending toward more separation of these two features, likely
leading to lower rainfall totals. Thus the Excessive Rainfall
Outlooks for Days 4-5/Tuesday-Wednesday depict no risk areas.
Expect rainfall to increase across the Gulf Coast region during the
latter half of next week, but with low confidence regarding the
details at this time due in part to guidance spread regarding
possible tropical development over the Gulf of Mexico. Monitor
forecasts over the coming days for better resolution of the
rainfall/tropical forecast across this region. A frontal system may
bring light to locally moderate rainfall to the Pacific Northwest
by Friday as well.

Temperatures will remain quite warm to hot across the southern half
of the West next week with fairly broad coverage of highs 10-15F
above average. Record highs should be most numerous through
Wednesday but there may still be a few lingering into late week.
Portions of the Plains will also be above normal one or more days
from Wednesday onward in the warm sector ahead of a front. The
Northwest will tend to be closer to normal. Temperatures over the
East will vary with frontal progression, with highs averaging out
to near normal and morning lows averaging 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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