Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Thu Jun 24 2021
Valid 12Z Sun Jun 27 2021 - 12Z Thu Jul 01 2021
...Historic and Dangerous Heat Wave with monthly/all-time record
temperatures possible over the Northwest/West...
...Heavy Rain Threat to stretch from Southern Rockies/Plains to
Ohio Valley...
18Z Update: The 12Z model guidance suite maintains good overall
continuity with the previous medium range WPC forecast with a
blocky upper level pattern, with a large upper high/ridge
remaining anchored in place over the northern Rockies, and another
ridge over the western Atlantic. This leaves a quasi-stationary
trough over the central Plains and Midwest along with a frontal
boundary with multiple perturbations along with, fueling rounds of
MCS activity that will likely produce episodes of heavy rain from
western Texas to the Ohio Valley. There was a slight downward
trend in magnitude of rainfall across the Midwest and Ohio Valley,
whilst higher totals are favored across portions of northern Texas
and into Oklahoma. The historic heatwave over the northwestern
U.S. will likely challenge some monthly June records and
potentially all-time record highs! The most impressive readings
should be across interior portions of Oregon and Washington with
some 110s possible for the lower elevations of the Columbia River
Basin. The previous 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/ECENS means and the 01 UTC National Blend of
Models in an overall pattern with above average predictability
days 3-5 (Sunday-Tuesday). Shifted blend focus toward the still
compatible ensemble means by days 6/7 amid gradually increasing
forecast spread. Recent Canadian had been quicker to suppress the
hot Intermountain West upper ridge and stronger/farther east with
the Great Lakes/Ohio Valley trough and heavy rainfall focus.
However, the 00 UTC Canadian has shifted more in line with the
rest of guidance.
...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 rainfall is expected to
ease this excessive heat this forecast period in a pattern with
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, and the Ohio Valley, Sun-Thu, Jun
27-Jul 1.
- Heavy rain across portions of the Great Lakes and the Ohio
Valley, Sun, Jun 27.
- Heavy rain across portions of the Southern Plains, Sun-Mon, Jun
27-Jun 28.
- Heavy rain across portions of Florida, Tue-Thu, Jun 29-Jul 1.
- Flooding possible across portions of the Central Plains, the
Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio
Valley.
- Excessive heat across portions of the Pacific Northwest,
California, the Northern/Central Great Basin, the Northern
Rockies, and the Southwest, Sun-Thu, Jun 27-Jul 1.
- Excessive heat across portions of the Pacific Northwest and the
Northern Great Basin, Sun-Mon, Jun 27-Jun 28.
- Excessive heat across portions of the Northern/Central Great
Basin, the Northern Rockies, and the Northern Plains, Tue-Thu, Jun
29-Jul 1.
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