Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
138 AM EDT Wed Jun 23 2021
Valid 12Z Sat Jun 26 2021 - 12Z Wed Jun 30 2021
...Dangerous Heat Wave to build 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...
...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
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