Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Sat Jul 17 2021
Valid 12Z Tue Jul 20 2021 - 12Z Sat Jul 24 2021
...Heat continues from the northern Great Basin to the northern
Rockies/northern Plains...
...Overview...
The strong upper high forecast to be centered near the central
Rockies will maintain well above normal temperatures over the
northern Rockies/Plains, with some of this heat extending into the
Upper Midwest and less extreme anomalies reaching back into the
Great Basin. An upper low forecast to track from offshore British
Columbia toward central Canada should gradually flatten the ridge
to the north of the upper high, with a cold front beginning to
erode the western side of the heat by next Saturday. Downstream
mean troughing will prevail over the East, with the northern
portion likely to trend a little deeper with time. The trough
will support a series of fronts and accompanying episodes of
showers/thunderstorms along with near to moderately below normal
temperatures. Meanwhile energy pulling away from the trough by
the start of the period early Tuesday should form a southern
Plains upper low that may weaken some later in the week but still
linger over the southern Rockies/High Plains. This feature could
support some areas of locally enhanced rainfall in its vicinity,
with clouds/rain from this system and monsoon activity farther
west keeping high temperatures below normal from the southern
Plains into the Southwest.
...Guidance/Predictability Evaluation...
Most guidance continues to offer similar ideas for the large scale
pattern along with embedded uncertainties. Within the eastern
trough aloft, at the start of the period early Tuesday the 12Z/18Z
GFS runs were somewhat faster than most other guidance with a
leading shortwave ejecting from the East Coast. The new 00Z GFS
has adjusted somewhat closer to consensus. Solutions cluster well
for a trailing midweek shortwave while some spread and variability
continue for northern Canada energy that reinforces the upper
trough later in the week. The southern Plains weakness/upper low
clusters fairly well in latest guidance and there is also decent
agreement into at least Thursday for the upper low that tracks
from offshore British Columbia into Canada. How much eastward
progression occurs after Thursday is somewhat of a question mark.
Faster trend of the 18Z GFS versus the 12Z run (and maintained in
the new 00Z run) has led to similar solutions in the GFS/ECMWF and
the ensemble means though the 00Z CMC is a bit slower by Saturday.
Toward the end of the period there is now a more pronounced
consensus advertising development of a new upper trough south of
Alaska, which should help to erode/eject the initial trough off
the West Coast to a greater degree than seen in the GEFS mean. An
operational model composite from the 12Z/18Z cycles trending to a
model/mean blend later in the period (a little more ECens relative
to GEFS) reflected the most common elements of guidance while
downplaying uncertain smaller-scale details and less confident
aspects of one solution or another depending on the feature. This
approach maintained reasonable continuity with only typical
run-to-run adjustments.
...Weather Highlights/Hazards...
The greatest northern tier temperature anomalies (plus 10-20F)
associated with the central Rockies upper high should extend from
the eastern half or more of Montana into the Dakotas and parts of
the Upper Midwest. Some locations in Montana and the Dakotas may
see one or more days with highs reaching at least 100F. Best
potential for daily records will be in the short-range period but
a few isolated records could extend into at least Tuesday. A cold
front expected to reach the northern Plains by Saturday should
bring some moderation to Montana at that time. Southern Plains
upper low/wavy front may produce some areas of heavy rainfall
Tuesday-Wednesday followed by a lighter trend as these features
weaken. Rain and thunderstorms should continue within an area of
monsoon moisture over parts of the Four Corners states, possibly
extending into parts of California/Nevada after midweek. Coverage
and intensity of rainfall will likely increase somewhat with time,
and with a potential contribution from the southern Plains
moisture/upper level energy. The unsettled pattern over the
southern Plains/Southwest will lead to highs up to 5-10F or so
below normal for one or more days.
Expect lingering rain/thunderstorms with a leading front that
settles over the Mid-Atlantic and South by Tuesday. Two trailing
fronts dropping southeastward from Canada will bring rainfall of
varying intensity, mainly to the Great Lakes/Northeast but
possibly farther southward at times. Temperatures over the East
will likely be near to moderately below normal through the period.
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