Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Wed Jun 30 2021
Valid 12Z Sat Jul 03 2021 - 12Z Wed Jul 07 2021
...Triple-digit heat continues over the interior Northwest while
spreading into portions of the northern Plains through the 4th of
July weekend...
...Threat of heavy rain lingers near the Gulf Coast through the
weekend into next week...
...Overview and Guidance/Predictability Evaluation...
Model and ensemble guidance continues to show the large scale
pattern settling into a more typical summer regime after this
weekend, with the westerlies confined to southern Canada and
northern tier of the U.S., after an amplified upper trough over
the East Coast states on Saturday lifts away to the northeast.
Elsewhere, a seasonably strong upper ridge will prevail over the
Southwest through the period and a weakness with possible embedded
low should evolve over the southern Plains into northern Mexico
during Monday-Wednesday. In the tropical Atlantic a leading wave
poised to enter the eastern Caribbean should eventually weaken.
However there appears to be increasing potential for a trailing
feature now near 40W to strengthen over the coming days and track
into or near the eastern Gulf of Mexico next week.
The updated forecast incorporated a composite of 00Z/06Z
operational models early in the period and then transitioned
toward somewhat more 06Z GEFS/00Z ECMWF mean input relative to the
past two ECMWF runs and 06Z GFS by day 7 Wednesday. This cycle
furthered the recent model/ensemble mean trend toward faster
ejection of the East Coast upper trough, with the operational
model blend fitting well within the ensemble mean template.
Latest consensus including the new 12Z guidance suggests an upper
low should track near Maine/Nova Scotia by day 4 Sunday and into
the Canadian Maritimes thereafter. Trends aloft support
associated low pressure tracking farther offshore and the trailing
cold front pushing farther southeast over the western Atlantic.
The low amplitude and moderately progressive nature of upper flow
along the U.S.-Canadian border still suggest low predictability
for specifics several days out in time. The most common theme in
the guidance is a leading shortwave (12Z GFS possibly becoming too
amplified by Tuesday) that will bring a front across the northern
tier states while a modest mean trough sets up close to the
Pacific Northwest coast. One or more shortwaves emerging from
this mean trough may support waviness along the northern tier
front next week. The blended approach accounts for these ideas
while downplaying lower-confidence details.
Farther south, the model/mean blend has been fairly stable for the
upper weakness/low over the southern Plains/northern Mexico. On
the other hand the tropical forecast for next week relative to the
wave initially near 40W has been rapidly evolving and very
uncertain. At the time of forecast preparation only GFS runs and
the 00Z UKMET (reaching the southeast corner of the Gulf at the
end of its run) showed a defined surface feature reaching the
eastern Gulf. ECMWF runs through 00Z have not shown even a hint
of a surface low. Ensembles diverge considerably with the
resulting means not showing any system. In the 12Z cycle the CMC
has joined the GFS/UKMET in depicting a system but the ECMWF still
has none. Today's NHC-WPC coordination yielded a nod toward more
definition than continuity and this will be reflected in the
surface forecast update sent by 22Z.
...Weather Highlights/Hazards...
The historic heat wave over the Pacific Northwest will be less
intense than it has been over recent days but triple-digit heat is
still likely each afternoon across some interior areas through the
holiday weekend and into early next week. A portion of this heat
will spread into the northern Plains, continuing into next week to
the south of a front that reaches the northern tier. Best
potential for afternoon temperatures to reach the century mark at
some locations in Montana and the Dakotas should be during the
weekend. Also expect some scattered daily record highs/warm lows
over the Northwest-northern Plains on Saturday-Sunday. The
moderating trend next week will gradually reduce the coverage of
high temperatures that are 10F or more above normal. Some rain
showers over the northern Rockies may offer slight relief to the
heat but they will do little to ease the growing drought concern
in these areas. One or more northern tier frontal waves could
help to focus areas of heavier rainfall next week but confidence
in specifics is below average at this time. Farther south, some
late-day monsoonal type showers and thunderstorms are likely each
day over the central to southern Rockies through the medium range
period.
The cold front expected to reach near the Gulf Coast this weekend
and then stall/dissipate may be a focus for areas of moderate to
heavy rainfall. With time the heaviest rainfall should gravitate
toward the western half of the Gulf Coast and nearby areas due to
the upper weakness/low likely to develop over/near Texas.
Meanwhile interests near the eastern Gulf should monitor the
latest National Hurricane Center products for the latest
information regarding the feature that could eventually affect the
region by next week. Farther north, the upper trough near the
East Coast this weekend should tend to promote areas of
diurnally-favored rainfall, with recent faster trends for the
upper trough leading to lighter amounts and less coverage. Clouds
and rainfall over the southern half of the Plains will keep that
region below normal for highs during the period with some minus
5-10F or so anomalies. High pressure behind the Gulf Coast front
should provide moderately below normal morning lows to portions of
the eastern half of the country during the weekend.
Rausch/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