Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
249 AM EDT Sat Aug 28 2021
Valid 12Z Tue Aug 31 2021 - 12Z Sat Sep 04 2021
...Major Hurricane Ida to bring a Heavy Rainfall/Flooding Threat
from the central Gulf Coast to the Mid-Atlantic...
...Potent Nora track into Baja CA/Gulf of CA to fuel a lead
Southwest U.S. Heavy Rain Threat mid-later next week...
...Guidance Evaluation and Preferences...
The big story continues to be impending hazardous impacts from
Hurricane Ida from the Gulf of Mexico and potent east tropical
Pacific Nora, forecast to track northward up the Gulf of
California. Please refer to products from the National Hurricane
Center and the local offices to obtain the latest information
regarding these dangerous systems.
The WPC medium range product suite was primarily derived in
conjunction with NHC tracks for Ida and Nora along with well
clustered guidance from the 12/00 UTC ECMWF, ECMWF/GEFS ensembles
and WPC continuity that seems to best fit NHC and the overall flow
pattern over the rest of the lower 48 in a pattern with average to
above average predictability.
...Pattern Overview with Weather/Hazard Highlights...
Dangerous Hurricane Ida is expected to strengthen further over the
Gulf of Mexico as it moves steadily towards the northwest with an
expected landfall along the Louisiana coast late Sunday as a major
hurricane. Ida will bring powerful and gusty winds and very heavy
rainfall to the central Gulf Coast region. Gradually weakening but
still very wet Ida is forecast to make a sharp turn to the
east-northeast spreading the heavy rainfall threat into the
Tennessee/Ohio Valleys into Tuesday/Wednesday. Considerable flash,
urban, small stream, and riverine flooding is likely with Ida.
Deep moisture feeding inland with time will also interact with a
slow moving front draped over the system, offering a lead heavy
rainfall focus into the central Appalachians and Mid-Atlantic into
Wednesday/Thursday.
Tropical Storm Nora is also forecast to become a Hurricane with
the current track guidance from NHC bringing it northward into the
Gulf of California and eventually into far northwest Mexico.
Leading deep tropical moisture ahead of Nora will likely fuel a
Southwest U.S. heavy rain threat next week. The guidance continues
to show some uncertainty in the track and certainly a track for
longer over the Gulf waters could bring a more substantial heavy
rainfall threat into the Southwest. By later next week, leading
moisture may also bring moderate to locally heavy, and terrain
enhanced, rains into the central Rockies as well.
Elsewhere, ample northern stream upper troughing will meanwhile
dig a wavy but only modestly unsettling/cooling front down from
the Northwest. Subsequent system progressions and genesis
mid-later next week out across the Rockies to the north-central
U.S. will increasingly focus an enhanced convection/rainfall
pattern that could focus locally heavy convection/rainfall as
enhanced by embedded impulses and frontal/wave reinforcements.
Moisture/instability pooling may support some locally strong to
severe convection and heavy downpours.
Schichtel
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, winter weather outlook probabilities
and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml