Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
140 PM 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...
...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/Tennessee
Valley...
...Guidance/Predictability Evaluation...
Models and ensembles continue to show very good agreement that a
highly anomalous upper ridge will remain parked over much of the
West through next week. The ridge may slowly shift eastward with
time (though still maintain impressive strength) in response to a
compact upper low lifting along the Pacific Northwest coast and
eventually pushing into British Columbia as an open wave. Models
show some minor timing/strength variability with this feature,
though a general blend should help to mitigate those differences.
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. The ECMWF/GFS have been most consistent in showing this
feature drifting southward into the Midwest, with some timing
differences later in the period. The 00z CMC however drags this
low more westerly into the Northern Plains, which is inconsistent
with the rest of the guidance. 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 associated with the upper trough filters into the
region later next week.
The WPC forecast used a blend of the deterministic models (with
more emphasis towards the 00z ECMWF/06z GFS) for days 3-4. Removed
the CMC thereafter in favor of increasing contribution from the
ensemble means to help smooth out some of the less predictable
detail differences. This maintained fairly good continuity with
the previous WPC forecast as well and only minor changes were
needed to the surface progs.
...Weather Highlights/Hazards...
A highly anomalous upper ridge over the Northwest will prolong the
dangerous and oppressive heat over the region. Temperatures will
soar 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 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.
Santorelli/Kong
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