Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
301 PM EDT Mon Jul 19 2021
Valid 12Z Thu Jul 22 2021 - 12Z Mon Jul 26 2021
...Heat will continue over the northern Plains and extend into the
Upper Midwest...
...Overview...
Latest guidance continues to show a persistent mean pattern
featuring an upper high dominating the central Rockies/High Plains
while successive shortwaves from northern/eastern Canada will tend
to maintain an upper trough across the northeastern quadrant of
the country. A couple of upper troughs from the northeastern
Pacific are forecast to move onshore across British Columbia
against the strong upper high over the Rockies during the medium
range period, but bringing only a modest cooling trend to the
excessive heat over the northern Plains. Multiple fronts with
attached low pressure waves reaching the East will bring episodes
of showers/storms while keeping temperatures near to slightly
below normal. Farther south, a lingering upper disturbance moving
westward across the southern Plains into the southern Rockies will
promote periods of rain/thunderstorms and below normal high
temperatures over portions of the Desert Southwest and the Four
Corners where the monsoonal regime will remain active.
...Guidance/Predictability Evaluation...
Models and ensembles this morning generally agree quite well for
the U.S. into the medium range period with reasonable increase in
model spread toward Day 7. Guidance has generally abandoned the
idea of bringing an anomalous upper low east of Hudson Bay down
toward the Northeast U.S./eastern Canada by Friday-Saturday. This
has hastened the eastward progression of the low over the Canadian
Maritimes as well as the frontal passages across the Northeast
behind the low. Across the northern Plains, a frontal wave is
forecast to track across the region into the Great Lakes by the
weekend where the ECMWF and GFS indicating possibility of heavy
rain. The Canadian has sped up this frontal wave. Meanwhile, the
ECMWF now agrees with the rest of the guidance to retrograde the
weak upper low over the southern Plains toward the southern
Rockies this weekend. This could bring additional instability
into the southern Rockies/Southwest later this weekend and may
have enhancing effects on the monsoonal rainfall already in place
across the area into early next week.
A general model compromise among the 06Z GFS/GEFS, 00Z ECMWF/EC
mean, and smaller contribution from the 00Z CMC/CMC mean was used
to compose the morning WPC medium range
package. A smaller weight was given to the 06Z GFS beginning on
Day 5 when the predicted flow pattern appears too fast across the
Northeast. A greater weight toward the ensemble means was used
for Days 6-7 to handle the uncertainty. The results only deviated
slightly in details compared with the previous forecast package.
...Weather Highlights/Hazards...
Portions of the northern Plains into Upper Midwest will likely see
highs up to 10-15F above normal on multiple days. Highest
anomalies and best potential for parts of eastern Montana and the
Dakotas to reach at least 100F should be during Thursday-Friday,
when plus 10F anomalies for highs may also extend into the central
Plains. A front extending from Canadian low pressure will likely
bring temperatures down a little from west to east into the
weekend but perhaps by only a few degrees or so. Northern High
Plains temperatures could rebound some next Monday as another
upper trough approaching the Pacific Northwest could give a
northward push to the front stalled over the northern Rockies/High
Plains.
Another period of enhanced monsoonal rainfall appears likely over
the Four Corners states from late this week onward. Some moisture
could reach a little farther northward and westward as well.
Coverage and intensity of rainfall will depend in part on
specifics of low-predictability impulses tracking around the
central Rockies upper high, including possibly the small upper low
initially over the southern Plains. Rain/thunderstorms could be
heavy at times with highest totals during the period currently
expected over central Arizona. The unsettled regime will keep
highs up to 5-10F or so below normal over the southwestern U.S.
Over Texas expect rainfall to continue trending lighter and more
scattered with time while below normal highs trend somewhat closer
to normal.
Lingering moisture along and south of a dissipating southern tier
front may provide localized enhancement to diurnal
showers/thunderstorms into the end of the week. Locations from
the Upper Midwest through the Great Lakes/Ohio Valley into the
Northeast/Mid-Atlantic should see at least a couple episodes of
rain and thunderstorms with fronts that reach the eastern U.S.
Models are providing a general signal that some of this activity
could be locally moderate to heavy but continue to vary
considerably for the details which may take into the short-range
time frame to come into better focus. The upper trough over the
Northeast will keep highs over that region moderately below
normal, especially late this week into the weekend. The remainder
of the East should see readings within a few degrees on either
side of normal.
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