Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
517 PM EDT Wed Jul 21 2021
Valid 12Z Sat Jul 24 2021 - 12Z Wed Jul 28 2021
...Locally heavy rainfall threat through the weekend in an active
Southwest/Four Corners Monsoon pattern...
...Heat will continue over the northern/central Plains into the
Upper Midwest...
1830 UTC update...
Models and ensembles continue to show very good agreement on the
large scale upper level pattern, with lingering uncertainty in the
details through the medium range period. Only minor changes were
made to the previous WPC forecast based on the newest guidance
runs. Today's update to WPCs surface and upper level charts used a
general model blend days 3-5, with more inclusion of the
ECENS/GEFS means days 6-7. See previous discussion attached below
for details regarding the overall pattern, guidance evaluation,
and the associated weather hazards.
Santorelli
Previous discussion below issued at 0700 UTC...
...Overview...
The large scale upper pattern will remain fairly similar from the
weekend into the middle of next week but there will be some
adjustments along the way. Initial ridging aloft that covers the
Great Basin/central Rockies and central/southern Plains should
consolidate into a central Plains ridge likely to strengthen by
midweek with guidance developing/expanding a 594 dm contour at
that time. This pattern will keep well above normal temperatures
over the central/northern Plains and Upper Midwest through the
period. To the south of the initial western ridge, moisture and a
compact upper low over Arizona and vicinity will extend the threat
of locally heavy monsoonal rainfall through the weekend. Then the
consolidation of upper ridging over the Plains should push the
moisture/upper energy to the west and north thereafter, bringing a
lighter trend to rainfall over the Southwest. Uncertainties over
specifics of how a Northeast Pacific trough/upper low may open up
after the weekend will affect the forecast across southern Canada
and the northern tier U.S. after the weekend. Mean troughing
should persist over the Northeast, with one shortwave departing
early in the weekend followed by one or more features that should
maintain cyclonic flow on average.
...Guidance/Predictability Evaluation...
The updated forecast based on 12Z/18Z guidance continues to
reflect a consensus model approach early in the period followed by
a trend to a model/ensemble mean combination. Details are still
in flux for the surface evolution near the southeastern coast.
Recent trends have led to clustering in the southern half of the
prior spread for initial low pressure (closer to the GFS than
ECMWF from earlier runs). Latest solutions offer the potential
for a defined surface low to reach into the Gulf of Mexico,
whether from motion of the initial low or by redevelopment, in
contrast to the wave staying near the Southeast coast. This
cycle's manual forecast attempted a conservative trend by way of a
more extensive trough while awaiting better continuity. Meanwhile
there are only typical run-to-run adjustments for the frontal
evolution from the northern Plains through the
Northeast/Mid-Atlantic during the weekend into early next week.
Fast/low amplitude nature of Pacific flow rounding the central
U.S. ridge continues to suggest low predictability for surface
details influenced by this stream. The signal persists for a wave
to emerge from Montana around the start of the week and then track
into Canada, but the models have not been consistent or agreeable
with the supporting dynamics. The energy associated with the
northeastern Pacific upper low expected to open up may also come
into play by next Tuesday or Wednesday but again guidance differs
on the strength/timing of the resulting shortwave(s).
...Weather Highlights/Hazards...
Latest consensus shows the potential for areas of heavy rainfall
within the Southwest/Four Corners Monsoon to extend into the
weekend and still with central Arizona likely to see the highest
totals from this event. Some areas will be very sensitive to
additional rain given already wet soil from prior rainfall. Rain
should trend lighter next week as upper level energy departs and
the core of greatest deep moisture/rainfall lifts northwestward
and becomes more diffuse. A combination of fronts should bring an
organized area of rain and thunderstorms from the Great Lakes into
the Northeast/Mid-Atlantic during the weekend into next week.
Some rainfall could be locally moderate to heavy. Another episode
of showers/storms could affect the Upper Midwest/Great Lakes next
week depending on uncertain wave/front details. Portions of the
Florida Peninsula may see enhanced rainfall to the south of a
surface trough/possible embedded low whose details are very
uncertain at this time.
Expect a hot pattern to continue from the northern/central Plains
into the Upper Midwest through the period with temperatures 5-15F
above normal, including at least isolated Plains locations
reaching 100F on one or more days. Additional humidity will make
temperatures feel even hotter, with heat indices reaching 105-110F
at a few locations in the southern portion of this region. Fronts
reaching the northern tier will provide a little variability from
day to day but without any meaningful cooling trend. Interior
portions of the Northwest will also see fairly hot conditions but
with anomalies tending to stay in the single digits above normal.
Clouds and rainfall over the Southwest and vicinity will keep
highs there up to 10-15F below normal through the weekend.
Temperatures over the Southwest should trend somewhat closer to
normal next week as the best moisture departs to the northwest.
Most of the eastern U.S. should see near to slightly above normal
readings except for some pockets of below normal highs over New
England during the weekend.
Rausch
Hazards:
- Heavy rain across portions of the Southern Rockies, the Central
Great Basin, and the Southwest, Sat-Tue, Jul 24-Jul 27.
- Excessive heat across portions of the Central Plains, the Upper
Mississippi Valley, and the Northern Plains, Mon-Wed, Jul 26-Jul
28.
- Much above normal temperatures across portions of California,
the Central Great Basin, the Pacific Northwest, the Northern
Rockies, and the Northern Great Basin, Sat-Mon, Jul 24-Jul 26.
- Much above normal temperatures across portions of the Central
Plains, the Northern Plains, the Southern Plains, the Middle
Mississippi Valley, the Upper Mississippi Valley, and the Great
Lakes, Sun-Wed, Jul 25-Jul 28.
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