Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat Jun 26 2021
Valid 12Z Tue Jun 29 2021 - 12Z Sat Jul 03 2021
...Historic and Dangerous Heat Wave with monthly/all-time record
temperatures probable over the Northwest/West...
...Anomalous heat for New England into Wednesday...
...Persistent Heavy Rain Threat to stretch from Southern
Rockies/Plains to the Midwest and later into the Ohio Valley...
...Guidance/Predictability Evaluation...
Models and ensembles agree that a highly anomalous upper ridge
over the Pacific Northwest will begin to weaken by around Tuesday
as a compact upper low just offshore is forecast to lift northward
as an open wave and edge closer toward the Pacific Northwest
coast. The ECMWF and CMC have been slower than the GFS regarding
the forward motion of the upper low. The 18Z and 00Z GFS runs
have slowed down the upper low to agree much better with the ECMWF
and CMC. By later next week, models generally show the weakened
upper ridge will drift further inland across the northern Rockies
into the northern High Plains but with still impressive strength.
Across the northern tier states, a positively-tilted trough will
likely drift very slowly eastward with increasing model agreement
for a closed center to form over the Great Lakes toward the end of
next week. Meanwhile, a broad and persistent southerly flow will
direct tropical moisture from the Gulf of Mexico northward through
the Deep South and interact with a slow-moving frontal boundary
across the mid-section of the country. Downstream, this leads to
an anomalously strong subtropical ridge off the East Coast to
extend northward into the Canadian Maritimes. A heat wave is
forecast for much of New England into the Mid-Atlantic into the
middle of next week before cooler air associate with the upper
trough filters into the region later next week.
Meanwhile, models continue to indicate the possibility of a
tropical wave/low to move westward from south of Bermuda to reach
the southeast U.S. coast during the early part of next week.
The pressures, 500 hPa heights, and winds were comprised of the
18Z GFS/GEFS, the 12Z ECMWF/EC mean, together with smaller
contributions from the 12Z CMC/CMC mean. A more even blend of
deterministic and ensemble mean guidance was adopted early on
before increasing ensemble mean inclusion mid to late next week to
handle the uncertainty.
...Weather Highlights/Hazards...
A highly anomalous upper ridge at peak intensity over the
Northwest early next week is forecast to weaken slowly later next
week. Temperatures will soar to 20 to locally more than 30
degrees above normal from Washington/Oregon to the northern Great
Basin/Rockies. Numerous daily and monthly high temperature
records -- including for the states of WA, OR, and ID -- are
likely to be tied or broken from the Pacific Northwest and
interior California to the north/central Great Basin and northern
Rockies, where temperatures peaking between 100F and the upper
110sF are forecast for the lower elevations. All-time record
highs are probable across portions of OR and WA. Little rainfall
is expected to ease this prolonged excessive heat in a pattern
with growing drought and fire threats.
Downstream, a positively-tilted upper trough drifting slowly
across the north-central U.S. will solidify and reinforce a wavy
and slow moving front from the southern Plains to the northeastern
U.S. through the medium-range period. Deep moisture streaming in
from the Gulf of Mexico will continue to help organize showers and
storms which 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. Drought
conditions across the Upper Midwest and Great Lakes in particular
should ease if not be outright eliminated over the next week.
Episodes of cell training and flooding are likely across some of
these areas with overall progression slowed by the blocky synoptic
pattern sandwiched between the heat wave out West and a strong
western Atlantic ridge to the east.
Under the northward displaced Bermuda High, a heat wave appears to
persist into mid-week from the Northeast to the Mid-Atlantic where
afternoon temperatures are forecast to peak well into the 90s away
from the immediate coasts. The hottest days/most anomalous
readings will be across New England from days 3-5/Monday through
Wednesday next week before a front slips southward and rain
increases in coverage and intensity. Last but not least, tropical
moisture/disturbances should move from the Atlantic into FL/the
Southeast underneath the displaced Bermuda High to fuel convection
and some gusty winds early next week.
Kong/Roth/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