Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
531 PM EDT Mon Aug 23 2021
Valid 12Z Thu Aug 26 2021 - 12Z Mon Aug 30 2021
...Upper Midwest heavy rain threat Friday into the weekend...
...Overview...
The latest model guidance shows average to above average agreement
for the upper level pattern and trended well from continuity. This
adds forecast confidence to the period. Shortwave troughing over
the Pacific Northwest will gradually extend eastward toward the
Northern Plains and Upper Midwest by late this weekend into early
next week, around the north side of a broad mean ridge that
extends from the Atlantic through eastern/southern parts of the
lower 48. Periodic impulses moving through the flow will support
episodes of potentially heavy rainfall across the Upper Midwest
beginning Friday through the weekend. Areas to the south of the
fronts will tend to be humid and very warm to hot with some
diurnally favored showers and thunderstorms. Possible tropical
development over the northwestern Caribbean and southwestern Gulf
of Mexico may bring some enhanced moisture to the western half of
the Gulf Coast region by Sunday-Monday.
...Guidance Evaluation and Preferences...
The model guidance has clustered together somewhat for the
expected upper level pattern late this week into early next week,
particularly with the progression and impulses of shortwave
troughs through the Northwest to Upper Midwest. The first wave
looks to arrive late Friday with the deterministic guidance
agreeing well. Perhaps some minor timing differences but all the
models then advertise upstream energy carving out a deeper trough
over the Northwest/Northern Rockies Saturday/Saturday night.
Beyond that, as that wave ejects into the Upper Midwest, there is
some amplitude and timing issues, with the CMC notably stronger
while the ECMWF a bit flatter/progressive. The 00Z GFS was on the
slower side of the model spread. A consensus approach, favoring
the GFS and ECMWF, were chosen. Another shortwave trough or closed
low will approach the Pacific Northwest early next week with some
of the typical model biases seen. A tropical wave across the
southwest Caribbean will be watched closely by the National
Hurricane Center for possible development this weekend into early
next week as the system is forecast to track into the far western
Gulf. For now, the guidance is reasonably clustered for day 6 and
day 7 with the coordinated position near the model consensus.
Overall, preferences followed an operational model blend early and
then transitioned to a model/mean mix including the 06Z GFS/GEFS
mean and 00Z ECMWF/ECMWF mean/CMC. This led to reasonable
continuity in principle while reflecting some adjustments as
reflected in the majority of latest guidance and downplaying
details that have the lowest confidence.
...Sensible Weather/Hazard Highlights...
A strong heavy rainfall signal exists for portions of the Upper
Midwest / Upper Mississippi River Valley Friday into the weekend
as several impulses move through the area at the nose of a
warm/moist air mass. Multiple episodes and possible training
storms could lead to some significant rainfall totals. Despite
some antecedent dryness to the soils for some parts of the area,
the multiple day event and potential higher end rainfall totals in
a short duration is likely to overcome that and the repeated
nature is likely to become problematic over some areas. Well in
the warm sector across the central/southern parts of the East,
more typical summer-time diurnally-favored convection within the
hot and humid airmass. A front settling into the Mid-Atlantic
during Friday-Sunday may provide some added focus for rainfall,
while upper level impulses could enhance convection over parts of
the South and/or southern Mid-Atlantic. Expect rainfall to
increase along the western half of the Gulf Coast Sunday-Monday as
possible tropical development reaches the southwestern Gulf of
Mexico. Daily convection over the southern Rockies and Arizona
may gradually increase in coverage and intensity over the course
of the period.
Rausch/Taylor
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
Hazards:
- Heavy rain across portions of the Southern Plains, Sun-Mon, Aug
29-Aug 30.
- Heavy rain across portions of the Upper/Middle Mississippi
Valley, the Great Lakes, and the
Northern Plains, Thu-Sat, Aug 26-Aug 28.
- Flooding possible across portions of the Mid-Atlantic and the
Northeast.
- Flooding occurring or imminent across portions of the Northeast
and the Great Lakes.
- Flooding likely across portions of the Mid-Atlantic.
- Excessive heat across portions of the Middle/Lower Mississippi
Valley, the Ohio Valley, and the
Tennessee Valley, Thu, Aug 26.
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