Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Wed Jun 23 2021
Valid 12Z Sat Jun 26 2021 - 12Z Wed Jun 30 2021
...Dangerous Heat Wave to build with Daily to Monthly Record
Temperatures across the Northwest/West...
...Heavy Rain Threat to stretch from Southern Rockies/Plains to
Midwest/Mid-South, Ohio Valley and the Northeast...
18Z Update: Similar to yesterday's 12Z guidance, the
deterministic models and the ensemble means are in above average
agreement through Monday, with the exception of the CMC that is
more suppressed with the Intermountain West upper ridge and
stronger/farther east with the Great Lakes/Ohio Valley trough.
Therefore, the CMC breaks down the heatwave across the
northwestern U.S. quicker than the model consensus. There was a
bit of an upward trend in rainfall amounts across the central
Plains compared to the prior forecast given the presence of a slow
moving surface low/front intercepting deep moisture from the
western Gulf of Mexico. The forecast was primarily derived from a
blend of the 00Z ECMWF/00Z UKMET/6Z GFS along with some previous
WPC continuity, and some use of the GEFS/EC means by early next
week owing to more model differences on individual shortwave
impulses. The previous forecast discussion is appended below.
/Hamrick
...Guidance/Predictability Evaluation...
The WPC medium range product suite was primarily derived from a
composite of best clustered solutions of the 18 UTC GFS/GEFS mean,
12 UTC UKMET/ECMWF, 12 UTC NAEFS/ECENS means, the 01 UTC National
Blend of Models and WPC continuity in an overall pattern with
above average continuity and predictability. Forecast spread and
run-to-run variability has improved compared to the last few days
and a composite seems to mitigate much of the lingering and less
predictable small-mid scale variances. Newer 00 UTC guidance so
far remains in line with the WPC composite. However, recent
Canadian runs are displaced east of most guidance with placement
of the central U.S. trough and western Atlantic ridge, so they
remain discounted.
...Weather Highlights/Hazards...
A highly anomalous upper ridge will build inland over the
Northwest this weekend and the heat wave will expand over the West
into next week. Temperatures will soar to 20-30 degrees above
normal from Washington/Oregon to the northern Great Basin/Rockies.
Numerous daily to some monthly records are likely from the Pacific
Northwest and interior California to the north/central Great Basin
and Northern Rockies, with temperatures peaking over 100F to 110+F
for interior sections of the West. Little to no rainfall is
expected to ease this excessive heat this forecast period, growing
drought and fire threats.
Downstream, a robust upper trough will dig and settle into the
north-central U.S. to solidify and reinforce a wavy and slow
moving front to stretch from the south-central to northeastern
U.S. this period. This will invite deep moisture return and
pooling. Well organized showers and storms will significantly
increase rainfall and runoff potential along and ahead of the
front with an elongated and focused zone from the Southern
Rockies/Plains northeastward across the Midwest/Mid-South, Ohio
Valley and the Northeast. Episodes of cell training and flooding
are likely across some of these areas with overall progression
slowed as broadly sandwiched between the heat wave out West and a
strong western Atlantic ridge to the east. Also, there is also
some signal for tropical moisture to feed into southern Florida to
fuel convection early-mid next week.
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
Hazards:
- Heavy rain across portions of the Central/Southern Plains, the
Middle/Lower Mississippi Valley, the Great Lakes, and the Ohio
Valley, Sat, Jun 26.
- Heavy rain across portions of the Middle/Lower Mississippi
Valley, the Great Lakes, the Ohio Valley, and the Tennessee
Valley, Sat-Tue, Jun 26-Jun 29.
- Heavy rain across portions of the Lower Mississippi Valley and
the Southern Plains, Sat-Sun, Jun 26-Jun 27.
- Heavy rain across portions of Florida, Tue-Wed, Jun 29-Jun 30.
- Flooding possible across portions of the Middle/Upper
Mississippi Valley, the Great Lakes, and the Ohio Valley.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley.
- Excessive heat across portions of the Pacific Northwest,
California, the Northern Great Basin, and the Northern Rockies,
Sat-Wed, Jun 26-Jun 30.
- Excessive heat across portions of the Pacific Northwest and the
Northern Great Basin, Sat-Mon, Jun 26-Jun 28.
- Excessive heat across portions of the Pacific Northwest,
California, the Northern/Central Great Basin, the Northern
Rockies, and the Southwest, Sun-Wed, Jun 27-Jun 30.
- Excessive heat across portions of the Northern Plains, the
Northern Rockies, and the Northern Great Basin, Tue-Wed, Jun
29-Jun 30.
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