Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
254 AM EDT Fri Jul 16 2021
Valid 12Z Mon Jul 19 2021 - 12Z Fri Jul 23 2021
...Hot temperatures from the northern Great Basin to the northern
Rockies/northern Plains...
...Overview...
A strong upper high anchored over or near southern Wyoming will
dominate the pattern next week. This high will support well above
normal temperatures from the northern Great Basin through the
northern Plains and to some degree into the Upper Midwest, while
downstream upper troughing will bring a series of fronts into the
East along with near to moderately below normal temperatures.
Some energy within the western side of the trough could form an
upper low that reaches the southern Plains/Rockies by mid-late
week before weakening--resulting in some locally enhanced rainfall
and below average high temperatures. An upper low off the coast
of British Columbia/Haida Gwaii should eject into western Canada
during the latter half of the week, with the trough to its south
possibly lowering heights over the Northwest slightly.
...Guidance/Predictability Evaluation...
The relative agreement and continuity of models/ensembles for the
overall pattern would suggest better than average predictability
but of course confidence is lower for specifics of smaller-scale
shortwaves. For the updated forecast based on 12Z/18Z guidance,
an operational model composite early followed by a trend toward a
model/model ensemble mean blend represented the most common ideas
of guidance while representing an intermediate approach where
differences existed. The 12Z CMC strayed east of consensus with
the Rockies upper ridge for the latter half of the period so the
forecast phased out its already minimal weight at that time.
Toward the end of week solutions diverge for how much troughing
remains off the West Coast depending on the details of upstream
flow. The GFS/GEFS runs through the new 00Z cycle maintain a
deeper trough while the 12Z ECMWF mean is noticeably weaker and
the 12Z ECMWF even develops a shortwave ridge. The 12Z CMC mean
is conveniently between the extremes. This difference has
persisted for multiple runs so an average of these scenarios
appears best for now. Within the eastern mean trough, latest GFS
runs could be a bit fast with leading energy over the Northeast as
of early Monday. By day 7 Friday the 00Z CMC trended toward
latest GFS runs and the 12Z ECMWF for what may be the strongest
energy of the period to reach southeastern Canada and the
northeastern U.S. However the new 00Z ECMWF keeps the feature
farther north.
...Weather Highlights/Hazards...
The potent upper ridge likely to be centered near southern Wyoming
for much of next week will sustain very warm to hot temperatures
from the northern Great Basin through the northern Rockies/Plains
with highs in the 90s up to at least lower 100s at lower
elevations, which are 10-20 degrees above normal. Such readings
may approach or exceed daily records especially early in the week.
In less extreme fashion some of this heat will extend eastward
into Minnesota and Wisconsin. On the other hand daytime highs in
the Southern Tier/Southwest will be somewhat below normal (by up
to 5-10F on one or more days) due to increased cloud cover and
higher rainfall chances. The combination of a wavy front and
possible upper low reaching the southern Plains and vicinity could
produce some areas of locally heavy rainfall. Monsoon activity
over Arizona and some neighboring areas should continue, with an
increase in rainfall amounts possible late in the week depending
on how far southern Plains moisture and upper level energy
retrograde.
A leading wavy and slow-moving front will extend from near the New
England coast through the Mid-Atlantic into the South early in the
week, accompanied by scattered to numerous showers and
thunderstorms with locally heavier amounts where thunderstorms
locally train. Additional fronts should reach the Great
Lakes/Northeast around Tuesday-Wednesday and then Thursday-Friday.
In both cases lower-predictability shortwave specifics and
frontal waves will play a role in the existence/placement of any
locally heavier rainfall. The mean trough aloft and series of
fronts will keep temperatures over the East near to somewhat below
normal next week.
Rausch/Fracasso
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