Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 AM EST Tue Nov 28 2023

Valid 12Z Fri Dec 01 2023 - 12Z Tue Dec 05 2023

...Overview...

An upper-level shortwave will lift from the Middle Mississippi
Valley to the Northeast Friday into Saturday, while at the same
time, a couple of short waves into the West act to amplify mean
troughing which should translate eastward with time, reaching the
East Coast by early next week. Upper ridging will build over the
East this weekend and the West by early next week. This overall
pattern would bring daily bouts of moderate to heavy rainfall to
the Southeast as well as the Pacific Northwest. Widespread but
lighter rain should spread up the East Coast as well next Monday
into Tuesday.


...Guidance Evaluation/Predictability Assessment...

The latest model guidance remains in very good agreement for the
medium range time period, though still with plenty of uncertainty
in the details. The greatest uncertainty continues with individual
shortwaves into the West, acting to reinforce a building amplified
trough over the region. There is some considerable run to run
variability with timing/location of Western U.S. energies, but
there is an overall consensus on a more amplified and active
pattern. A possible atmospheric river looks to impact the West
around Sunday-Monday time frame, and the ECMWF is by far the
strongest, indicating a possible moderate to strong AR, according
to the CW3E AR scale tool. The new 00z ECMWF came in just as
strong (if not stronger) but about 12 hours faster. At this point,
the ECMWF is a strong outlier, but the potential is there so this
should be monitored. The guidance shows similar timing as the
trough shifts into the East early next week, just with
considerable differences in the details. For tonights WPC blend, a
purely deterministic model blend was used for days 3-5, with
increasing amounts of the ensemble means by days 6-7 (still only
to about 50% of the overall blend) to account for the increased
uncertainty late period. This maintains good continuity with the
previous WPC forecast as well.


...Weather/Hazards Highlights...

Upper-level shortwave energy will move over the Mid-South while a
warm front lifts north out of the Gulf of Mexico late this week,
and an amplified trough shifts out of the West this weekend
providing a renewed surge of moisture. This will support daily
widespread moderate to heavy rainfall for the Gulf Coast and
Southeast. There is a Marginal Risk of Excessive Rainfall for this
region valid Days 4 and 5 (Friday and Saturday) as anomalous
moisture (PWs 2-3 standard deviations above normal) and
instability will be present. It is possible an embedded slight
risk will be needed at some point but there is still some
uncertainty on exact amounts. The heavy rainfall threat across the
Southeast will linger into Sunday as well, with less intense
rainfall also lifting northward up the East Coast on Monday. Out
West, precipitation will increase Thursday through the weekend as
a couple shortwaves move through the region, with some
precipitation reaching southward into California and the
Southwest. The heaviest precipitation (coastal rain and mountain
snow) will be across the Washington/Oregon coast and Cascades
Sunday into Monday, where some models indicate a potential
atmospheric river to impact the region.

The interior West should stay cooler than normal underneath
reinforced troughing through the weekend. Temperatures for the
rest of the country should be closer to normal late this week, but
trend warmer with time across the Northern Plains into the Midwest
and eventually the East by the weekend. Daytime highs could be
5-10 degrees above average across this region. The Western U.S.
may finally trend back towards normal, or above normal in some
places, early next week as the trough moves east and ridging
builds back in.


Santorelli


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