Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 AM EDT Mon Oct 02 2023

Valid 12Z Thu Oct 05 2023 - 12Z Mon Oct 09 2023

...Heavy rain and flash flooding threat could continue in parts of
Texas Thursday, and may ramp up in the Northeast late week...


...Overview...

An amplified upper trough and an associated wavy cold front will
push through the central and eastern U.S. for the latter part of
the week, bringing rain chances ahead and below average
temperatures behind them. Rain and thunderstorms could be
particularly heavy in the southern Plains into Thursday, while by
Friday and Saturday moisture could increase in the northeastern
U.S. ahead of the front and possibly lead to heavy rain. Meanwhile
an upper ridge should overtake the western U.S. for warmer than
average temperatures and generally dry conditions, until early
next week when a trough and frontal system could push into the
Pacific Northwest.


...Guidance Evaluation/Predictability Assessment...

Model guidance is in reasonably good agreement with the large
scale pattern described above, but there are some differences in
the details of the trough and the timing of it moving eastward
through late week. One aspect that varies among guidance is the
exact evolution of the trough dependent on uncertain energies
within it. Relatively minor differences like the width of the
trough caused by shortwave positions affect the frontal positions
and the associated QPF. For example, the 12Z ECMWF and CMC had the
eastern edge of the trough and thus the front/QPF farther east
than GFS runs by Friday-Sunday. By the weekend the differences
affect if or when a closed low may form within the trough, which
GFS runs have steadily indicated. Overall the incoming 00Z
guidance shows the trough's track on the slower side, and despite
there being a closed low or not depending on the model run, there
looks to be a negatively tilted trough and room for moisture to
tuck in just east of it. The new 00Z ECMWF does eventually form a
closed low by early next week.

The WPC forecast was based on a deterministic model blend
particularly favoring the 18Z GFS and 12Z ECMWF early in the
period, transitioning to incorporate the ensemble means to half by
Day 6 and over half Day 7 as model spread increased. This blend
also worked well for the upper ridge to the west of the trough,
and for the next trough over the eastern Pacific that becomes a
factor in the forecast by late week. Continuity from the previous
forecast was reasonably well maintained, though with the 00Z
guidance looking slower with the northeastern trough, the next
shift may trend toward that, which would lead to slower QPF
movement through the Northeast this weekend.


...Weather/Hazards Highlights...

Moist and unstable inflow will meet up with the cold front pushing
through the south-central U.S. on Thursday, which along with the
right entrance region of the jet providing ample lift will lead to
a continued heavy rain threat mainly in Texas. A Slight Risk
remains in place in the Day 4/Thursday Excessive Rainfall Outlook
for this convection that could produce high rain rates. The front
should continue to push south with time and limit the areal
coverage of rainfall, but some may linger into South Texas late
week, and a Marginal Risk is delineated there in the ERO for
Friday.

Farther north, the cold front should track through the
Midwest/Upper Great Lakes and the Ohio Valley Thursday into
Friday. There is a nonzero threat of embedded heavy rain with this
activity, but hopefully the frontal progression will keep showers
and thunderstorms moving quickly enough not to cause flash
flooding issues. However, this could change by Friday and Saturday
as the tilt/placement of the trough and front may allow for
Atlantic moisture to stream in and increase heavy rainfall
potential as the front gets into the northern
Mid-Atlantic/Northeast. A Marginal Risk is planned for the Day
5/Friday ERO across the Lower Great Lakes region and parts of the
northern Mid-Atlantic and Northeast, as despite some lingering
model discrepancies heavy rain seems like a good possibility,
which could cause flooding issues especially in areas with low
Flash Flood Guidance. Heavy rain and flooding concerns will likely
continue into Saturday for New England.

Much of the CONUS should dry out by Sunday with the exception of
some lake effect rain showers in the Great Lakes region and any
moisture lingering into Maine, southern Texas, and Florida. The
Pacific Northwest though will likely see a ramp-up in
precipitation chances by Monday as a front and upper trough
approach.

The amplified trough and the cold front will lead to cooler
temperatures than average, while the upper ridges to the east and
west support above normal temperatures. The Lower Great Lakes
region into the Northeast should see warmer than average
temperatures by 10-20F on Thursday, with highs in the upper 70s to
low 80s. But as the upper trough and the primary and secondary
cold fronts march east into the latter half of the week,
temperatures will cool to below average across the central and
then east-central U.S., eventually making their way to the Eastern
Seaboard by early next week. The first frost or freeze of the
season may occur in the northern High Plains behind the cold
fronts late this week. In the eastern half of the U.S. lows will
generally be around 5-10F below average while highs should be more
like 10-15F below average, locally cooler. Meanwhile the arrival
of an upper ridge across the West will first raise temperatures
along the West Coast to 10-15F above average, with some record
warm lows possible. The shift/expansion of the ridge farther east
should lead to warmer than average temperatures across the
northern Rockies into the High Plains early next week.


Tate


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, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

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