Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun Jun 27 2021
Valid 12Z Wed Jun 30 2021 - 12Z Sun Jul 04 2021
...Historic heat wave over the Northwest is forecast to continue
into the 4th of July weekend while becoming less intense...
...Anomalous heat for New England into Wednesday...
...Heavy rain threat from southern Rockies/Plains to the Midwest
midweek should shift south to near the Gulf Coast by the weekend...
...Guidance/Predictability Evaluation...
Models and ensembles show that the highly anomalous upper ridge
across the Pacific Northwest and British Columbia will lose some
strength by midweek as a compact upper low lifts northward just
off the Pacific Northwest coast and weaken into an open wave.
There is good model agreement that the ridge will expand slowly
eastward into the northern High Plains through the latter half of
the week ahead of a positively-tilted trough initially over the
northern Plains. There continues to be increasing model signals
for the upper trough to develop into a closed low over the Great
Lakes later this week although the ensemble means tend to retain
the positively-tilted configuration. Meanwhile, a slow-moving
frontal boundary across the mid-section of the country should
begin to push slowly southward amid increasing model signals for
the upper trough/low over the Great Lakes to dip further south.
This pattern should bring the heavy rain threat progressively
southward from the mid-South to the Gulf Coast while directing
moisture toward much of the East Coast for the 4th of July weekend
following the early/mid-week heat wave across New England and the
Mid-Atlantic.
The WPC medium range product suite was derived from a composite of
the 18Z GFS/GEFS mean, 12Z ECMWF/ECENS means and some
contributions from the 12Z CMC/CMC mean in an overall pattern with
above average predictability days 3-5 (Wednesday-Friday), followed
by shifting blend focus toward the still compatible ensemble means
by the 4th of July weekend amid gradually increasing forecast
spread. This maintained fairly good continuity with the previous
WPC forecasts.
...Weather Highlights/Hazards...
A highly anomalous upper ridge over the Northwest will prolong the
dangerous and oppressive heat over the region. As the ridge is
forecast to weaken slightly, the heat should become much less
intense by Wednesday near the Pacific Northwest coast but high
temperatures across the interior sections will remain 20 to nearly
30 degrees above normal through late week with some improvement
possible by the 4th of July weekend. Little rainfall is expected
to ease this excessive heat in a pattern with growing drought and
fire threats and minimal recovery overnight will also increase the
heat stress. Conditions may improve slightly for the Pacific
Northwest later in the week as upper troughing pushes into
southwest Canada, but excessive heat will continue across interior
portions of the Northwest into next weekend.
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. Heavy rainfall axis may drop
slowly southward later next week as an upper low potentially
organizes over the Midwest states.
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 3-4/Tuesday and
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/Santorelli
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