Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
142 AM EDT Sat Sep 11 2021
Valid 12Z Tue Sep 14 2021 - 12Z Sat Sep 18 2021
...Heavy Rain Likely Across Coastal Texas and Louisiana into mid
next week...
...Overview and Guidance Evaluation and Preferences...
A fast, quasi-zonal flow will set up across the northern tier of
the U.S. during the medium range period with frontal systems
quickly progressing from the West/central U.S. into the East. For
days 3 and 4, model guidance shows good enough agreement for a
general model compromise between the latest deterministic
solutions. For the second half of the period, models continue to
struggle both with the timing and amplification of troughing
reaching the Western U.S. next Friday-Saturday. A blend of
majority ECENS/GEFS mean with minority contributions from the
ECMWF/GFS provided a good starting point and was most consistent
with previous WPC continuity as well.
There remains quite a bit of uncertainty surrounding potential
tropical development in the western Gulf of Mexico late this
weekend and the resulting heavy rainfall threat along the
Texas/Louisiana coasts through the middle of next week. Most of
the guidance agrees that there will be heavy rainfall associated
with this system (regardless of development into a tropical
cyclone) but the bigger question is exactly where that will occur.
Both the ECMWF and GFS bring some kind of surface low along the
Texas coast Monday-Tuesday and eventually inland into either
southeast Texas (ECMWF) or Louisiana (GFS). The CMC remains well
south of the consensus bringing the low closer to Brownsville
region. The WPC QPF for this system was based largely on a blend
of the ECMWF and GFS with our in-house bias corrected model and
the NBM. Refer to the National Hurricane Centers tropical weather
outlook for additional details on development potential with this
system.
...Weather/Hazard Highlights...
The eventual track and evolution of a tropical disturbance in the
western Gulf late this weekend into the early/middle portions of
next week will likely bring multiple rounds of heavy rainfall
along the immediate Texas coast and into parts of Louisiana.
Models continue to suggest some isolated storm total amounts near
or above 10 inches are possible but it is too early to determine
the areas at greatest risk. These kinds of amounts would lead to
at least some flash and urban flooding, particularly for more
sensitive locations... urban regions and those areas that have
already seen above normal rainfall over the past couple of weeks.
Otherwise, mostly lighter rainfall will follow frontal passages
through the central and northeastern parts of the CONUS with some
potential for isolated/localized pockets of heavier amounts.
Moderate to locally heavy rainfall may also reach the Pacific
Northwest late this week with some snow possible in the highest
elevations of the Washington Cascades. Periods of above normal
temperatures will shift across the Southwest/Central U.S. into the
East ahead of a couple frontal passages, with below to much below
normal temperatures probable by late week across the Pacific
Northwest. Elsewhere, expect the western Gulf Coast to be below
normal through much of next week associated with clouds and ample
tropical moisture.
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, 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