Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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 forecast to be 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 weaken and progress around the western periphery of
the building Rockies/Plains ridge, 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 lifts
northwestward.
...Guidance/Predictability Evaluation...
Once again the full array of guidance provides above average
confidence in the large scale pattern but below average confidence
in the details, especially with shortwave specifics around the
northern periphery of the Rockies/Plains upper ridge. 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. Most models from the 12Z/18Z cycles showed some splitting
of the energy while the majority was slower than the GFS runs for
the overall evolution. This consideration, along with the fact
that the GFS strayed to the weak side of guidance with the
Plains/Rockies ridge late in the period, led to a forecast
approach that started with mostly operational model focus early
and then trended toward the 18Z GEFS/12Z ECMWF means and lingering
operational input from the 12Z ECMWF/CMC. Even with the southern
Canada details looking more ambiguous as a whole after Monday, the
blend still yielded some elements of continuity. Among new 00Z
runs the GFS is still a bit on the fast side with the Northeast
Pacific upper low but the CMC has trended faster to make the GFS
appear less extreme. The ECMWF is still fairly slow with the low
and also eventually builds the downstream ridge more into Canada
than consensus/continuity. Meanwhile the 00Z GFS has adjusted
stronger toward consensus for the Rockies/Plains upper ridge by
the end of the period. Farther south the small scale of the
feature initially expected off the southeastern coast/Florida will
likely lead to low predictability until the short range time
frame. The National Hurricane Center's 06Z Tropical Weather
Outlook shows a 30 percent chance of development over the next
five days.
...Weather Highlights/Hazards...
The potential for some locally heavy monsoonal rainfall should
continue over Arizona through Sunday as a compact upper low tracks
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. 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.
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