Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
510 PM EDT Wed Jun 16 2021
Valid 12Z Sat Jun 19 2021 - 12Z Wed Jun 23 2021
...Dangerous heat continues across portions of the Interior West...
...Extreme heat for the Southwest/California into the weekend
followed by a cooler trend...
...Tropical system to bring heavy rainfall and flooding to Central
Gulf Coast and Southeast Friday and this weekend...
...Overview...
Extreme heat continues over the Southwest U.S. and California
through Saturday thanks to the persistent upper level ridging over
the West. Some moderation in temperatures is expected though later
in the weekend and next week as the ridging is gradually replaced
with an upper trough forecast to approach the California coast.
Until then, some much above normal temperatures of 10F to 20F will
remain likely for the Great Basin and portions of the Northwest.
Elsewhere across the CONUS, developing troughing over the
central/east U.S. will parade a few cold fronts through the region
and could spell some locally heavy rainfall over the course of
several days. Finally, a tropical disturbance is expected to
approach the west-central Gulf Coast later this week and weekend
and will bring some significant rainfall totals to the area. This
is likely to lead to flooding along portions of the central Gulf
Coast as well as inland across portions of the Southeast.
...Guidance/Predictability Evaluation...
The model guidance has improved and clustered somewhat with the
track and timing of the potential tropical system current in the
Bay of Campeche forecast to drift northward toward the
west-central Gulf later this week. Compared to the previous
forecast cycle, only minor nudges to the east were needed for the
day 3 position and the forecast favored a GFS/ECMWF blend. The CMC
and UKMET continued to be on the slightly slower/southwestward
side of the model envelope, but that spread has lessened in the
last few cycles. The system will weaken as it moves inland over
Alabama this weekend then become absorbed in the mean flow over
the Carolinas before moving offshore. The ECMWF still wants to
hold onto a stronger low center as it lifts northeastward through
the Carolinas and Mid-Atlantic but for now, in coordination with
NHC, a weaker system is the preference.
A blend of the latest GFS/ECMWF was the preferred approach for a
consensus based forecast approach for the rest of the CONUS. The
leading shortwave crossing the Great Lakes/Northeast along with
associated front, plus the trailing upper trough that amplifies
into the central/eastern U.S. during the first half of next week,
are agreeable in concept. Shortwave energy helping to carve out
the deepening central U.S. trough will spawn a deepening low
pressure system Sunday into early next week. As it approaches the
Great Lakes, guidance all points to a sub-1000 mb low by Monday.
Its associated cold front will sweep through the central U.S. with
sprawling high pressure overtaking the central U.S. by the middle
of next week. Meanwhile, a West Coast trough will settle offshore
California and the latest model guidance shows fairly
good/agreeable support here such that a general model blend was
sufficient.
...Weather Highlights/Hazards...
Record breaking and dangerous heat should continue into Saturday
over the Southwest, California Central Valley, and Interior West
where highs well into the 100s and 110s are likely. This region
should begin to see temperatures moderating a little on Sunday and
to a greater extent during the first half of next week. Meanwhile
areas to the north will trend hotter into early next week with
some areas seeing highs reach into the 90s and possibly low 100s
as far north as eastern Washington. These readings are 10F to 20F
or so above normal and could produce a few isolated daily records.
The combination of extreme temperatures and duration will make
this a dangerous heat wave for sensitive groups. Excessive heat
warnings/watches are in place for many areas of the Southwest and
the California Central Valley through Saturday. For the central
and eastern U.S., the combination of frontal passages, potential
for scattered to numerous daily thunderstorms, and a looming
tropical system will likely keep temperatures at or slightly below
the mid/late June normals. The broadest area of cool readings will
progress from the northern Plains into the Great Lakes/Ohio Valley
Monday-Wednesday.
There is increasing concern for potentially significant flooding
impacts resulting from heavy rainfall associated with the tropical
system across the Central Gulf Coast beginning Friday and this
flooding threat then spreads inland across portions of the
Southeast this weekend. Given the much above normal rainfall seen
this spring contributing to high/wet soil moisture content, there
is concern for some significant flooding impacts.
A leading front that settles from the Plains into Mid-Atlantic
this weekend followed by strengthening Plains through Upper Great
Lakes low pressure that returns the first front northward and then
sweeps a cold front through the central/eastern states early next
week will produce multiple rounds of showers/storms. Some of this
rain may be heavy. Also worth monitoring will be to what extent
the moisture from the tropical system may interact with the second
front as it reaches the eastern U.S.
Taylor/Rausch
Hazards:
- Heavy rain across portions of the Southeast, the Southern
Appalachians, the Lower Mississippi Valley, the Southern Plains,
and the Tennessee Valley, Sat-Wed, Jun 19-Jun 23.
- Heavy rain across portions of the Middle Mississippi Valley, the
Great Lakes, and the Upper Mississippi Valley, Sun-Mon, Jun 20-Jun
21.
- Flooding possible across portions of the Southeast and the Lower
Mississippi Valley.
- Flooding possible across portions of the Southeast and the
Tennessee Valley.
- Flooding occurring or imminent across portions of the Southeast,
the Lower Mississippi Valley, the Southern Plains, and the
Tennessee Valley.
- Flooding occurring or imminent across portions of the Southeast,
the Lower Mississippi Valley, the Southern Plains, and the
Tennessee Valley.
- Flooding occurring or imminent across portions of the Southeast,
the Lower Mississippi Valley, the Southern Plains, and the
Tennessee Valley.
- Excessive heat across portions of the Central Great Basin, the
Northern Rockies, the Central Rockies, California, the Northern
Great Basin, the Pacific Northwest, and the Southwest, Sat-Wed,
Jun 19-Jun 23.
- Excessive heat across portions of the Central Great Basin,
California, and the Southwest, Sat-Mon, Jun 19-Jun 21.
- Excessive heat across portions of the Southern Plains, Sun-Wed,
Jun 20-Jun 23.
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