Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
212 AM EDT Mon Jul 05 2021
Valid 12Z Thu Jul 08 2021 - 12Z Mon Jul 12 2021
...Elsa is forecast to cross the eastern Carolinas on Thursday and
may result in heavy rainfall near the Carolina Coast...
...Repeating heavy rain may persist over southern Texas into the
weekend as low pressure lingers over northern Mexico...
...Overview...
The western CONUS ridge/eastern mean trough pattern will amplify
this weekend as a shortwave trough from the Pacific rounds the
ridge before stalling over the Upper Mississippi Valley Saturday.
The upper ridge over the Great Basin/Desert Southwest strengthens
in the wake of the ridge riding trough and only intensifies the
ongoing heat wave across The West. An initial shortwave trough
pushing east from the Great Lakes to eastern Canada/New England by
Friday will drag a weak cold front over the northeastern CONUS
through Friday before it lifts back as a warm front ahead of the
ridge riding shortwave trough over the weekend. Tropical Storm
Elsa is forecast to push off the Carolinas on Thursday while an
upper trough/low stalls over northern Mexico that may produce
heavy rainfall over southern Texas (and northern Mexico) into or
through the weekend.
...Guidance/Predictability Evaluation...
The most recent guidance has come into much better agreement with
the ridge riding shortwave trough that stalls near the Upper
Midwest and with the timing and strength of Elsa. The 18Z and now
00Z GFS are stronger with the resultant low over the Upper Midwest
with a more closed and thus slower system than the ECMWF, but
overall the differences are not great and a dominant deterministic
blend focusing on the GFS and ECMWF was able to be used through
Day 6 when separation of the Upper Midwest low is great enough to
warrant greater inclusion of the ensemble means. Meanwhile, the
Elsa forecast for Thursday (as it moves across the eastern
Carolinas to the sea) is in remarkable agreement among global
guidance given all the variance over the past several days.
...Weather Highlights/Hazards...
An upper low/trough that stalls over northern Mexico through next
week will support a repeating rain threat over that region
continuing for Thursday and perhaps well into the weekend, though
the threat for extreme totals continues to diminish. However a
persistent low level flow of Gulf moisture will feed into this
slow moving disturbance, maintaining a flooding threat. Heavy rain
would accompany Elsa as it tracks through the eastern Carolinas
Thursday. Please continue to monitor products from the National
Hurricane Center for the latest Elsa forecast track information.
The initial front looks to be a rainmaker for the Northeast
Thursday before the front shifts offshore. Confidence is
increasing for Elsa associated rain on Friday to be off the
Northeastern Seaboard. The ridge riding trough forecast to reach
the Upper Midwest by Saturday will have access to Gulf-sourced
moisture and should result in significant rainfall for the far
northern Plains to the northern Great Lakes as the system becomes
blocked/stalls.
The heat wave over a significant portion of the West will continue
at least into next week as the Great Basin/Southwest upper ridge
restrengthens in the wake of the ridge riding trough. The Coast
Ranges across the Great Basin to the northern Rockies should all
see highs 10-15F above normal each day with locally higher
readings possible. Several daily high temperature records are
under threat across primarily the Great Basin each day with a
slight western shift to California this weekend as the ridge axis
drifts west. The only slight break over the Pacific Northwest
looks to be Thursday when a weak frontal passage nudges highs down
to near or slightly below 10F above normal. Meanwhile the
strengthening and westward expansion of the upper ridge should
produce a hotter trend with time over interior locations of
California. The southern High Plains/Texas will likely see highs
5-12F below average for most of the week due to cloudiness and
rain from the persistent upper level disturbance drifting over
that region. The Midwest and Great Lakes will see below normal
highs due to rainfall on Thursday and the over the weekend with
the next system. The latter system's cold front may also bring a
cooling trend into the central Plains by next weekend.
Jackson
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