Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Mon Jul 19 2021
Valid 12Z Thu Jul 22 2021 - 12Z Mon Jul 26 2021
...Heat will continue over the northern Plains and extend into the
Upper Midwest...
...Overview...
Latest guidance continues to show a persistent mean pattern
featuring an upper high situated over the central Rockies/High
Plains while a combination of energy from northern/eastern Canada
and flowing around the northern side of the upper high will feed
into an eastern U.S. upper trough. Northeastern Pacific troughing
will be in the process of reloading at the start of the period as
an ejecting upper low/trough reaches western Canada as of early
Thursday. This upper low should continue into central Canada and
possibly merge with northern Canada flow. The upstream trough
should approach the Pacific Northwest late in the period. This
pattern will maintain well above normal temperatures over the
northern Plains and vicinity, with a front anchored by the
Canadian system likely bringing only a modest cooling trend.
Fronts reaching the East will bring episodes of showers/storms and
keep temperatures near to slightly below normal. Farther south, a
weak upper low over the southern Plains and an active
Southwest/Four Corners monsoon will promote periods of
rain/thunderstorms and below normal high temperatures over those
areas.
...Guidance/Predictability Evaluation...
Within the agreeable mean pattern there is still a struggle to
resolve embedded details whose smaller scale leads to low
predictability multiple days out in time. One persistent
uncertainty is the details of short-range energy from Hudson Bay
and northward reaching the Northeast U.S./eastern Canada by
Friday-Saturday. Models have been quite divergent and
inconsistent in the path of a compact upper low which could either
remain at high latitudes or drop into or just north of Maine. At
the moment the GFS is the only model to drop it southward--the
opposite of 24 hours ago when the majority brought down the low
versus a mere shortwave in the GFS. Upstream, a model/mean
approach tones down run-to-run variability for timing of the
Canadian system and trailing front that extends into the northern
U.S. There has been a signal for leading low pressure to track
approximately from the northern Plains through the Upper Great
Lakes into the Northeast U.S./southeastern Canada but with a lot
of spread and variability for specifics. The 12Z/18Z GFS runs
looked particularly questionable with how strongly this low
developed by early next week, while the new 00Z run appears more
reasonable. Meanwhile guidance displays various ideas for the
path of the weak upper low over the southern Plains. The 12Z
ECMWF was on its own with a more eastward drift. GFS/CMC runs
offer potential for it to retrograde toward the Southwest U.S.
late in the period. By next Monday the 18Z GFS was on the strong
side with various impulses reaching the Southwest/Great Basin with
the ridge adjusting farther east into the Plains than consensus.
The various forecast uncertainties led to a blended/mean approach
to represent the most agreeable aspects of the pattern and an
intermediate/majority scenario for the contentious details.
Guidance weight began with mostly operational model emphasis on
Thursday and trended somewhat more to the 18Z GEFS/12Z ECMWF means
late.
...Weather Highlights/Hazards...
Portions of the northern Plains into Upper Midwest will likely see
highs up to 10-15F above normal on multiple days. Highest
anomalies and best potential for parts of eastern Montana and the
Dakotas to reach at least 100F should be during Thursday-Friday,
when plus 10F anomalies for highs may also extend into the central
Plains. A front extending from Canadian low pressure will likely
bring temperatures down a little from west to east into the
weekend but perhaps by only a few degrees or so. Northern High
Plains temperatures could rebound some next Monday as another
upper trough approaching the Pacific Northwest could give a
northward push to the front stalled over the northern Rockies/High
Plains.
Another period of enhanced monsoonal rainfall appears likely over
the Four Corners states from late this week onward. Some moisture
could reach a little farther northward and westward as well.
Coverage and intensity of rainfall will depend in part on
specifics of low-predictability impulses tracking around the
central Rockies upper high, including possibly the small upper low
initially over the southern Plains. Rain/thunderstorms could be
heavy at times with highest totals during the period currently
expected over central Arizona. The unsettled regime will keep
highs up to 5-10F or so below normal over the southwestern U.S.
Over Texas expect rainfall to continue trending lighter and more
scattered with time while below normal highs trend somewhat closer
to normal.
Lingering moisture along and south of a dissipating southern tier
front may provide localized enhancement to diurnal
showers/thunderstorms into the end of the week. Locations from
the Upper Midwest through the Great Lakes/Ohio Valley into the
Northeast/Mid-Atlantic should see at least a couple episodes of
rain and thunderstorms with fronts that reach the eastern U.S.
Models are providing a general signal that some of this activity
could be locally moderate to heavy but continue to vary
considerably for the details which may take into the short-range
time frame to come into better focus. The upper trough over the
Northeast will keep highs over that region moderately below
normal, especially late this week into the weekend. The remainder
of the East should see readings within a few degrees on either
side of normal.
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