WPC Medium Range Archive
Medium Range Products Valid On:
(Day 7 PMDEPD: Valid 07/23/2021 to 07/27/2021)
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
518 PM EDT Tue Jul 20 2021
Valid 12Z Fri Jul 23 2021 - 12Z Tue Jul 27 2021
...Locally heavy rainfall threat in an active Southwest/Four
Corners Monsoon pattern...
...Heat will continue over the northern/central Plains into the
1800 UTC Update... The latest 00z/06z guidance for today continues
to show good agreement on the overall long wave pattern for the
medium range period and typical smaller scale differences which
are hard to resolve at this time scale and should become more
clear with time. No major changes were needed to the previous WPC
forecast, which is valid for the same time period. This forecast
cycle used a general blend of the 00z ECMWF/CMC/UKMET with the 06z
GFS for days 3 and 4, with some modest (but still less than
majority) contributions from the 00z ECENS/06z GEFS means by late
period. See the previous discussion below for additional details
on the pattern and associated weather hazards.
Previous Discussion below issued at 0700 UTC...
During the Friday-Tuesday period expect only modest variations of
a similar theme for the mean pattern. Upper ridging should remain
centered over the central Rockies/Great Basin into the weekend. A
separate Gulf of Mexico ridge drifting northwestward may
ultimately merge with the Rockies ridge to yield a high center
over or near the central Plains by next week. Mean troughing will
likely prevail over the Northeast Pacific and eastern North
America. Features ejecting from the Pacific trough and rounding
the northern periphery of the ridge will bring a couple fronts
into the northern tier states but with only modest impact on the
persistent heat over the region. To the south of the initial
upper high, a weak upper low over the southern High Plains/Rockies
along with other impulses should provide occasional enhancement of
monsoonal rainfall over the Four Corners/Southwest and nearby
Confidence remains above average for the expected large scale
pattern while there are ongoing uncertainties/trends for some of
the medium to smaller scale details that have lower
predictability. An operational model blend of 12Z/18Z solutions
provided a good representation of consensus Friday into the
weekend. Guidance has finally converged better for details of the
moderately deep trough over the Northeast on Friday and then
lifting away--ultimately leading to greater progression of the
main surface front into the Atlantic over the past couple days of
runs. Meanwhile there is an ongoing trend for better defined low
pressure near the southern Mid-Atlantic and/or Southeast coast but
for now there is wide north-south spread (intermediate position
favored at this time). Consensus has been more stable for the
system crossing central Canada and the trailing front extending
into the northern tier of the U.S.
During the first half of next week there is decent model signal
for an ejecting Pacific shortwave to support a southern Canada
wave that brings another front into the northern Plains. The
weak/progressive nature of the feature leads to a fair degree of
model spread and run-to-run variability thus far, favoring an
average of 12Z/18Z GFS and 12Z/old 00Z ECMWF runs along with the
18Z GEFS/12Z ECMWF means. The 12Z ECMWF strayed to the fast side
of the envelope (new 00Z run slower like consensus) while the 12Z
CMC was on the slow side. Meanwhile energy rounding the initial
Rockies ridge will reach the eastern North America mean trough
next week, bringing the associated cold front into the eastern
states while a leading wave/front cross the northeastern quadrant
of the lower 48. After a lot of spread/variability for the
leading wave in recent days, guidance has lately stabilized upon a
fairly modest depiction but differences persist for specifics. A
GFS/ECMWF model/mean combination provides reasonable continuity
for the main cold front.
Some areas of heavy rainfall will be possible on one or more days
within the Southwest/Four Corners Monsoon. Currently expect the
best potential for highest totals to be over central Arizona late
this week. There is still uncertainty over day-to-day specifics
given the low predictability of small scale impulses that will be
important for resolving the location and timing of significant
rainfall. Some areas will be very sensitive to additional rain
given already wet soil from prior rainfall. Overall expect the
core of greatest deep moisture and best rainfall potential to lift
northwestward early next week. Elsewhere, an organized area of
rain and thunderstorms should progress from the Upper Midwest
through the Great Lakes and into the East, ahead of a
Canadian/northern tier front and leading wave/frontal system.
Some rainfall could be locally moderate to heavy. Areas of rain
may accompany a wavy frontal system over the southern
Mid-Atlantic/Southeast coast late this week into the weekend while
Florida should see multiple episodes of showers/thunderstorms
during the period.
Northern/central portions of the Plains into the Upper Midwest
will likely see persistent heat 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 105-110 for a few locations. Fronts reaching the northern
tier will provide a little variability from day to day but without
any meaningful cooling trend. On the other hand the clouds and
rainfall over the Southwest and vicinity will tend to keep highs
there 5-15F below normal, with a slight northwestward progression
of coolest anomalies possible by early next week. The upper
trough over the Northeast late this week into the weekend may keep
highs 5F or more below normal. Otherwise expect temperatures over
most the East to be within a few degrees on either side of normal.
- Heavy rain across portions of the Southern Rockies, the Central
Great Basin, and the Southwest, Fri-Tue, Jul 23-Jul 27.
- Heavy rain across portions of the Great Lakes, Sat, Jul 24.
- Excessive heat across portions of the Middle Mississippi Valley,
the Lower Mississippi Valley, the Southern Plains, the Ohio
Valley, and the Tennessee Valley, Sun-Tue, Jul 25-Jul 27.
- Excessive heat across portions of the Northern Plains, Thu, Jul
23 and Mon, Jul, 26.
- Excessive heat across portions of the Central Plains, the Upper
Mississippi Valley, and the Northern Plains, Fri, Jul 23 and Mon,
- Excessive heat across portions of the Central Plains and the
Middle Mississippi Valley, Mon-Tue, Jul 26-Jul 27.
- 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 Great Lakes, the Northern
Rockies, the Middle Mississippi Valley, the Upper Mississippi
Valley, and the Southern Plains, Fri-Mon, Jul 23-Jul 26.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: