Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
139 AM EDT Sun Jul 04 2021
Valid 12Z Wed Jul 07 2021 - 12Z Sun Jul 11 2021
...Elsa is forecast to impact Florida and the Southeast through
midweek with potential for heavy rainfall and high winds...
...Repeating heavy rain is may persist over southern Texas into
Friday as low pressure drifts over the region...
...Overview...
The western CONUS ridge/eastern trough persists through the work
week and becomes more amplified next weekend as the upper ridge
over the Great Basin/Desert Southwest strengthens. A leading
shortwave trough pushing east from the northern Great Lakes
Wednesday to eastern Canada Friday will push along a wavy cold
front over the northeastern CONUS through this time. A trailing
Pacific shortwave trough rounds the upper ridge, ejecting from the
WA/OR coast Wednesday morning and is expected to amplify into a
Midwest/Great Lakes by Saturday, supporting another area of low
pressure and associated frontal system for the northern half of
the country east of the Rockies. The primary features of interest
at lower latitudes will be Tropical Storm Elsa which should impact
Florida and parts of the Southeast through the middle of the week,
and an upper trough/low drifting southwest from Texas into
northern Mexico that may produce heavy rainfall over southern
Texas (and northern Mexico).
...Guidance/Predictability Evaluation...
Overall a deterministic 12Z/18Z global model blend (the 12Z CMC
was unavailable for the forecast) transitioning to a GFS/ECMWF
blend with some of their ensemble means produced a decent first
guess forecast. For Elsa, the 18Z and now 00Z GFS continue a trend
toward a weaker and less wet system crossing northeast over
northern Florida Wednesday and the coastal Southeast through
Thursday. The UKMET continues to be slower, but stronger with a
western track like the GFS while the CMC is the strongest with an
eastern Florida Peninsula track and the ECMWF is the slowest,
though it does have more of a system than in prior days with an
eastern track. Given this uncertainty of timing/strength/track the
heavy rain threat is somewhat diminished from previous days, but
there is still potential for this system to be impactful.
Meanwhile there is good agreement in principle for the shortwave
crossing the northeastern quadrant of the lower 48 and a second
shortwave amplifying into the Midwest next weekend after rounding
the western upper ridge. The 18Z and now the 00Z GFS jump ahead to
a faster timing with the latter wave which is a typical bias of
the FV3 GFS.
...Weather Highlights/Hazards...
An upper low/trough drifting southwest over southern Texas into
northern Mexico through next week will support a repeating rain
threat over that region continuing for Wednesday into Friday, with
a somewhat diminished threat for as much repeating heavy rain as
indicated in recent days. However, ample Gulf moisture will be
available to this slow moving disturbance, so there is a flooding
threat. There is still some potential for heavy rain and high
winds for parts of Florida and the coastal Southeast from Elsa
Wednesday into Thursday. Please continue to monitor products from
the National Hurricane Center for the latest Elsa forecast track
information. The wavy front pushing east from the Midwest/Great
Lakes will produce periods of rain that could be locally
moderate/heavy. On Friday tropical moisture from Elsa may converge
on the frontal pattern over the Mid-Atlantic/Northeast and produce
heavy rainfall. The system forecast to reach the Midwest/Great
Lakes this upcoming weekend looks to interact with Gulf-sourced
moisture and result in meaningful rainfall as well.
The heat wave over a significant portion of the West will continue
at least through next weekend as the Great Basin/Southwest upper
ridge restrengthens. The Northwest and northern Great Basin should
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. 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 with
slightly below normal temperatures over the Midwest when rainfall
is forecast.
Jackson/Rausch
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