Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Thu Jul 22 2021
Valid 12Z Sun Jul 25 2021 - 12Z Thu Jul 29 2021
...Locally heavy rainfall threat through Sunday in the
Southwest/Four Corners Monsoon followed by a lighter trend...
...Heat will continue over the northern/central Plains into the
Upper Midwest...
...Overview...
Expect upper ridging to consolidate and strengthen over the
central Rockies/Plains by midweek after initially extending from
California and the Great Basin into the central/southern Plains on
Sunday. An upper low drifting westward near the western
U.S.-Mexico border as of early Sunday will continue to enhance
Southwest Monsoon rainfall through the end of the weekend. This
feature should then move off the southwest U.S. into the Pacific,
leading to a lighter trend for rainfall over Arizona and vicinity.
Flow around the northern side of the ridge will feed into a broad
Northeast mean trough aloft, while an initial northeastern Pacific
low should eventually open up but with considerable uncertainty
over how/when its energy will become incorporated within the
progressive flow along/north of the U.S.-Canada border. This
overall pattern should lead to a meandering surface low/front over
the northern Plains--generally providing only modest variability
within a lengthy period of hot weather--while fronts periodically
reach the Northeast/Mid-Atlantic. Finally, guidance continues to
suggest that a surface trough with one or more embedded lows may
extend from off the Southeast/eastern Florida coast into the Gulf
of Mexico to start the week and then weaken as it edges closer to
the southeast U.S. coast.
...Guidance/Predictability Evaluation...
Guidance this morning continues to indicate noticeable spread
regarding the details of the upper-level confluence flow across
the northern tier states through much of the medium-range period.
From the larger-scale perspective the primary issue for this
stream is when/how the upper low initially over the northeastern
Pacific opens up and its energy becomes embedded within the
progressive flow. The deterministic ECMWF solution began to
deviate further and further from its ensemble mean on Day 5 so
that a cyclone is developed over the Northeast midweek. The GFS
did not exhibit such deviation for the low over the Northeast but
for the next cyclone moving into the northern High Plains midweek.
In view of these model spread, more weights from the ensemble
means were given starting on Day 5. This resulted in a blend that
yielded a lingering low pressure wave near the Great Lakes, with a
warm front lifting faster across the upper Midwest. Changes to
the remaining parts of the country were relatively minor through
the medium range period.
The WPC medium range forecast package was derived from a general
model compromise among the 06Z GFS/GEFS, 00Z ECMWF/EC mean, and
smaller contribution from the 00Z CMC/CMC mean. A greater weight
toward the ensemble means was used from Day 5 onward to handle the
uncertainty.
...Weather Highlights/Hazards...
The potential for some locally heavy monsoonal rainfall should
continue over Arizona through Sunday as a compact upper low tracks
generally westward along the southern border of the state. This
area will be sensitive to additional rain given the significant
totals already expected in the short range time frame. The
strengthening/consolidation of the Rockies-Plains upper ridge next
week should force the weakening upper feature to the west and
spread moisture to the north and west. Thus the Southwest will
see rainfall trend lighter after Sunday. Activity that extends
into California, the Great Basin, and northward over the Rockies
should be less intense than with the Arizona event.
Showers/thunderstorms will accompany a front pushing from the
Great Lakes/Plains into the East from Sunday onward. By
Tuesday-Thursday another front may bring rainfall of varying
intensity to areas from the Upper Midwest through New England.
With considerable uncertainty in the specifics, the weak surface
feature off the southeast Coast and possibly into the eastern Gulf
may enhance rainfall at some locations. Please monitor National
Hurricane Center outlooks for latest information on development
potential.
Within the persistent hot pattern from the northern/central Plains
into the Upper Midwest, expect Monday through Wednesday to feature
the greatest coverage of plus 10-15F or so high temperature
anomalies--with some locations in the Plains reaching at least
100F. A meandering surface low/front across the the northern tier
will likely provide some day-to-day variability for temperatures
but not a meaningful cooling trend. The one exception could be
over the northern High Plains on Thursday when highs may decline
to only a few degrees above normal. There is still uncertainty in
the surface front/temperature details though. From Sunday into
Monday the northern Great Basin and interior portions of the
Northwest will also see hot conditions with highs possibly
reaching 10F or so above normal. A few locations could approach
or reach daily record values. The moisture over the Southwest
will initially support highs up to 10-15F below normal followed by
a moderating trend. Northwestward progression of this moisture
will bring temperatures down somewhat over the Great Basin and
interior Northwest after Monday. Most of the eastern U.S. should
see near to somewhat above normal readings.
Kong/Rausch
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