Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
153 PM EDT Fri Jun 25 2021
Valid 12Z Mon Jun 28 2021 - 12Z Fri Jul 02 2021
...Historic and Dangerous Heat Wave with monthly/all-time record
temperatures probable over the Northwest/West...
...Anomalous heat for New England Monday into Wednesday...
...Persistent Heavy Rain Threat to stretch from Southern
Rockies/Plains to Ohio Valley and eventually the Northeast...
...Guidance/Predictability Evaluation...
Models and ensemble continue to advertise a highly anomalous upper
ridge to reach peak intensity over the Pacific Northwest early
next week while a compact upper low in the Pacific is forecast to
move toward the northern coast of California before opening up.
All models agree that the upper ridge will then gradually
weaken/flop over across southern Canada toward the latter half of
next week as the compact upper low lifts northward and edges into
the Pacific Northwest as an open wave.
An amplifying positively-tilted trough with an embedded closed
center moves little across the Midwest, Plains, into the Great
Lakes during this period due to the orientation of the upstream
ridge -- thought the 00z ECMWF was too far east with this feature
and that the 06z GFS/weaker form of the 00z Canadian was a better
way to go, model wise. 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 north and strong
subtropical ridge off the East coast, which leads to anomalous
heat in Maine/New England and 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 a blend of
the 06z GFS/00z Canadian/00z UKMET/00z ECMWF early on before
increasing ensemble mean inclusion mid to late next week. The
remainder of the grids showed a more even blend of deterministic
and ensemble mean guidance through the period, as always.
...Weather Highlights/Hazards...
A highly anomalous upper ridge is forecast to reach peak intensity
over the Northwest early next week before showing signs of
weakening later next week. Temperatures will soar to 20-30
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.
Downstream, a positively-tilted 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 southern Plains to the
northeastern U.S. through the medium-range period. This will
invite deep moisture to continue streaming in from the Gulf of
Mexico. 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 -- 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
develop and persist from the Northeast to the Mid-Atlantic where
afternoon temperatures are forecast to peak well into the 90s away
from the immediate coasts during next week. 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 next week.
Roth/Kong/Schichtel
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