Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sun Aug 22 2021
Valid 12Z Wed Aug 25 2021 - 12Z Sun Aug 29 2021
...Upper Midwest heavy rain potential late week into the weekend...
...Overview...
Guidance maintains a fairly typical late-summer pattern during the
forecast period. Flow across southern Canada and the northern
contiguous U.S. will push along a couple rainfall-focusing
waves/frontal systems, with the Upper Midwest region likely to see
the heaviest activity. Meanwhile a broad upper ridge over the
South and parts of the East will maintain hot and humid
conditions. The most oppressive heat/humidity combination is
likely to continue into midweek or so over the Mississippi Valley
and vicinity. Recent consensus has been showing a West
Coast/Northwest upper troughing departing toward the Plains,
allowing a southern tier ridge to bridge across California and
join with an East Pacific ridge--supporting another West Coast
heat wave from late week through the weekend. Some new 00Z
guidance is casting some doubt on this scenario though. At the
same time slow retrogression of a western Atlantic ridge may
ultimately reinforce ridging over the East.
...Guidance Evaluation and Preferences...
Most models and the ensemble means have been identifying two
primary features within the southern Canada/northern U.S. flow but
with some detail differences and run-to-run variability. For the
leading system tracking eastward from central North America
mid-late week, the 12Z CMC became the most suspect solution by day
5 Friday and beyond--closing off an upper low near Nova Scotia
instead of carrying the core of the upper trough well northward.
The new run is closer to consensus. Meanwhile some guidance has
trended a little more amplified over eastern Canada in a partial
nod to some recent GFS runs. The upstream trough expected to
track from the Northwest into the Plains still exhibits various
trends and differences in character due in part to involvement of
multiple pieces of shortwave energy. Heading into yesterday there
had been somewhat of a faster trend but 12Z/18Z consensus had
become a bit slower and more amplified. The new 00Z GFS and ECMWF
have added a new wrinkle, trending out of sync with other guidance
(including the GEFS mean) for northeastern Pacific flow by the
latter half of the period. This leads to a shortwave dropping
into the Northwest instead of feeding into the Plains trough. It
will be interesting to see if/how much the 00Z ECMWF mean follows
this idea. The 12Z/18Z GFS runs compared much better to the means
(and also somewhat better versus the 12Z ECMWF) late in the period.
In the tropical realm, attention will turn to the northwestern
Caribbean through the western Gulf of Mexico where guidance is
suggesting at least a wave if not a defined tropical system. CMC
runs had been on the strong and northern side of the spread though
the new 00Z run has trended southward, a sensible adjustment given
the prevalence of upper ridging to the north. The manual forecast
for Sunday depicts extrapolation of yesterday's tropical wave for
Saturday that was coordinated with the National Hurricane Center.
A blend of 12Z/18Z operational models represented consensus well
for days 3-4 Wednesday-Thursday. The questionable 12Z CMC
over/near the Northeast required phasing out its solution after
Thursday and increasing detail uncertainties led to increasing 18Z
GEFS/12Z ECens input by next weekend. The 12Z/18Z GEFS and 12Z
ECMWF provided some continued operational model weight through the
weekend.
...Sensible Weather/Hazard Highlights...
Surface waves and associated fronts will cross the northern U.S.
and southern Canada, producing episodes of showers and
thunderstorms that could be locally heavy. Guidance continues to
highlight the best potential for heavy rainfall over the Upper
Mississippi Valley and Midwest from late this week into the
weekend. This region will see an early frontal passage, followed
by the front stalling to the south (Mid-Mississippi Valley) and
then lifting back north as a warm front ahead of another system
whose trailing cold front will move into the area during next
weekend. Parts of this region have been dry recently so some of
the rain may initially be beneficial. However a multi-day period
of significant rainfall could end up being problematic over some
areas. Central and southern parts of the East may see areas of
diurnally-favored convection within a humid and warm to hot
airmass. Upper level energy from a combination of the tail end of
the initial East Coast trough and what tracks underneath the
western Atlantic ridge may enhance convection over parts of the
South and/or southern Mid-Atlantic. Specific details of this
energy have low predictability. Rainfall may increase along the
central/western Gulf Coast next Sunday as a possible wave reaches
the western Gulf of Mexico.
Very warm to hot temperatures from the central-southern Plains
through much of the East as of Wednesday (plus 10F or so anomalies
for highs from the Mid Mississippi Valley into Northeast) should
gradually moderate with time, especially with highs trending to
within a few degrees of normal by next Sunday. The highest heat
index values should be on Wednesday over the Mid/Lower Mississippi
Valley and along the Gulf Coast where max values may locally reach
110F or so. Morning lows will be more persistently above normal
with areas from the Midwest into Great Lakes/Ohio Valley tending
to see readings stay close to or a little above 10F above normal
through the period. Some daily records for warm lows may be
challenged even away from areas with the highest anomalies. A
cold front and trailing high pressure will bring the Northeast a
break from the warmth late week into the weekend. Based on recent
guidance consensus, expansion of the upper ridge across southern
parts of the West would support increasing coverage of highs 5-10F
above normal over southern parts of the West and then across much
of California later in the week, followed by up to plus 10-15F
highs reaching western Oregon next weekend. More western
troughing aloft in the new 00Z GFS/ECMWF would significantly
curtail this warming trend though. The Northern Plains region
will be consistently cool during the period. Some 10-15F below
normal highs are possible from Wednesday into Saturday.
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