Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
341 PM EDT Wed Sep 06 2023
Valid 12Z Sat Sep 09 2023 - 12Z Wed Sep 13 2023
...Hazardous Heat from the south-central U.S to the Southwest...
19Z Update: The 12Z model guidance suite is in good overall
agreement on the synoptic scale through the upcoming weekend
across the U.S., and differences first become apparent across the
Pacific Northwest early next week with the timing of the next
storm system. There are also some minor differences with the
strength and longitudinal placement of the trough over the Great
Lakes and Ohio Valley region going into the middle of the week.
Overall, a multi-deterministic blend works well as a starting
point in the forecast process through early Monday, followed by up
to 30% of the ensemble means by next Wednesday. /Hamrick
...Guidance Evaluation/Predictability Assessment...
The WPC medium range product suite was primarily derived from a
broad composite blend of reasonably well clustered mid-larger
scale guidance of the 18 UTC GFS/GEFS mean and the 12 UTC
ECMWF/UKMET/ECMWF ensemble mean along with the 01 UTC National
Blend of Models in an overall pattern with seemingly above normal
predictability. The blend tends to mitigate much of the less
predictable smaller scale variances consistent with uncertainty.
WPC product continuity is well maintained in this manner and
latest guidance from the 00 UTC model cycle generally remains
consistent.
...Weather/Hazards Highlights...
Amplified upper troughing will settle over the East through the
weekend into early next week as anchored by an upper low over
Hudson Bay and as a series of weaker impulses ride over the top
then to the lee of a transitioning upper ridge as it strengthens
over the South and expands westward into the Southwest. With this
ridge in place, hot late summer temperatures will linger across
the south-central U.S. and the Southwest through this weekend.High dewpoints and humidity especially near the western Gulf Coast
will lead to dangerous heat indices as high as 115 degrees. Above
normal temperatures by about 10-15 degrees should persist into the
weekend broadly across the south-central U.S. and into the
Southwest. Many daily record high minimum and maximum temperatures
are likely to be set, with perhaps even a few monthly records, in
what has proven to be a sweltering summer. Upper-level energy
trending southward should help to reduce the strength of the ridge
and begin to bring temperatures to still hot but more reasonable
levels next week.
Meanwhile in this pattern, the northern Mid-Atlantic and the
Northeast offer a main area for precipitation to focus, given
higher precipitable water values than average and instability
pooling ahead of the upper trough and wavy surface frontal system.
While rain amounts should be mainly moderate, the ingredients will
be in place for localized heavier rain totals especially because
storms may be slow-moving and relatively parallel to the front,
allowing for potential training of storms. The WPC Day 4/5
Excessive Rainfall Outlooks (EROs) denote Marginal risk areas for
this weekend from the northern Mid-Atlantic into the Northeast as
convection looks to have a relatively better focus near/ahead of
the front and favorable right entrance region upper jet support.
The Marginal Risks encompass lower FFGs mainly from Pennsylvania
into New York and includes favored terrain of the Poconos and
Catskills to the Adirondacks. Rain chances should gradually
decrease into next week along the Eastern Seaboard as the fronts
push east off the coast. For the 19Z update, the Day 4 Marginal
Risk area is planned to be extended to the southwest to basically
cover the spine of the Appalachians from western NC to Vermont,
where a better deterministic model signal exists in the 12Z
guidance for patchy areas of heavy QPF maxima. For Day 5, the
existing Marginal Risk is planned to cover most of New England
except for the coastal areas.
Upstream, a series of shortwaves look to work from the Pacific
through the West/Northwest and inland across the Intermountain
West, albeit in a pattern with limited precipitation this weekend
into early next week. Convection may start to become more
widespread and better organized in the northern Rockies/Plains by
Saturday before working down more earnestly through the Plains
Sunday, where some heavy rain totals are expected locally. A
guidance signal is growing for this convective potential over the
central Plains for the WPC Day 5/Sunday ERO where a "Marginal"
risk area is shown. Forecast uncertainty is greater, but it seems
the main rainfall focus in this pattern may shift more to the
southern Plains and possibly the Upper Midwest early next week
with system emergence and flow translation.
Schichtel/Hamrick
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.phpHazards:
- Heavy rain across portions of the Central Plains and the
Southern Plains, Sun, Sep 10.
- Heavy rain across portions of the Mid-Atlantic, the Northeast,
and the Central Appalachians, Sat,
Sep 9.
- Flooding occurring or imminent across portions of the Southeast
and the Mid-Atlantic.
- Flooding likely across portions of the Southeast.
- Hazardous heat across portions of the Southern Rockies, the
Southern Plains, the Lower
Mississippi Valley, California, and the Southwest, Sat-Sun, Sep
9-Sep 10.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:
