Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Mon Jun 28 2021
Valid 12Z Thu Jul 01 2021 - 12Z Mon Jul 05 2021
...Historic heat wave over the Northwest becomes less intense and
continues through the 4th of July weekend...
...Heavy rain threat shifts south from southern Rockies/Plains to
near the Gulf Coast by the weekend...
...Guidance/Predictability Evaluation...
Models and ensembles agree that the highly anomalous upper ridge
currently located over the Pacific Northwest and British Columbia
will weaken slightly and shift east into the northern High Plains
during the medium range period as an upper wave/trough moves into
British Columbia and across southwest Canada. Meanwhile, ensemble
means continue to indicate a positively-tilted trough will drift
across the Great Lakes region by late this week, where
deterministic model solutions tend to call for a closed low to
form under this highly amplified synoptic pattern. There is
increasing model agreement that a slow-moving frontal boundary
across the mid-section of the country will begin to push southward
when the upper trough/low over the Great Lakes is forecast to dip
further south late this week. 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 and into early next week. However, models
continue to show considerable uncertainty regarding the details.
Solutions range from a wavy front becoming stalled across the
eastern U.S. to a more consolidated low pressure system moving
into the Northeast by the holiday weekend.
Given the uncertainties, the Day 5-7 forecasts were derived mainly
from the consensus of the 12Z EC mean, 18Z GEFS, and the 12Z CMC
mean. The 18Z GFS was not included beginning on Day 5 since it
began to place a low pressure center over the Ohio Valley that
lagged far behind its the ensemble mean position over the
Northeast. For Day 3-4, a general blend of the 18Z GFS/GEFS mean,
12Z ECMWF/ECENS means and some contributions from the 12Z CMC/CMC
mean was adopted. This maintained fairly good continuity with the
previous WPC forecasts.
...Weather Highlights/Hazards...
The historic heat wave over the Pacific Northwest is expected to
be less intense by midweek as the highly anomalous upper ridge is
forecast to weaken slightly and shift eastward. Nevertheless,
high temperatures across the interior sections will remain 15 to
25 degrees above normal through the holiday weekend and into early
next week. Little rainfall is anticipated to ease this excessive
heat in a pattern with growing drought and fire threats and
minimal recovery overnight will also increase the heat stress.
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. into midweek. 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. Episodes of
cell training and flooding are likely across some of these areas
before the heavy rainfall axis slowly drops southward toward the
Gulf Coast by next weekend.
A heat wave across the Northeast to the Mid-Atlantic into midweek
should be broken by Friday as the upper trough/low centered over
the Great Lakes directs moisture into much of the East Coast ahead
of a frontal boundary. It appears that the East Coast will enter
a period of damp/wet weather just in time for the July 4th holiday
weekend with cool daytime temperatures and warm and humid nights.
Cool days and warm nights are expected for the Deep South as well
where the heavy rain threat continues into early next week. In
contrast, dry and hot weather will return across the northern
Plains and should continue into early next week under the weakened
but still anomalous upper ridge.
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