Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
228 AM EDT Tue Sep 28 2021
Valid 12Z Fri Oct 01 2021 - 12Z Tue Oct 05 2021
... Heavy rain threat over portions of Texas late this week...
...Overview...
The medium range period will begin Friday with a closed upper low
centered over the Northeast, while ridging in the east-central
U.S. separates it from mean troughing in the West. Moisture and a
stalled frontal boundary ahead of a southern stream trough should
produce heavy rainfall in the Southern Plains for the latter part
of the week. By early next week, model guidance is coming into
better agreement with shortwave energy over the north-central U.S.
(underneath an upper high moving across western and central
Canada), which should maintain a surface low pressure/frontal
system across the Ohio Valley.
...Guidance Evaluation/Preferences...
12/18Z model guidance continues to show good agreement with the
location of the closed upper low centered over southeastern Canada
and spreading troughing to the Northeast through the end of the
week, with perhaps some differences in the energy distribution
potentially narrowing the low or not early on, and somewhat minor
differences in the speed it lifts out by Monday/Tuesday. A
multi-model deterministic blend worked well for this feature.
In the West, troughing is expected for the end of the week in both
the northern and southern streams. With the northern stream
shortwave, model guidance is generally agreeable in its track
passing through the Northwest Friday/Saturday, while splitting
some energy off into the eastern Pacific offshore of California.
The 12Z CMC is the exception, which once again spins most of its
energy southward to create a closed low over Oregon for the
weekend. Meanwhile with the southern stream trough, while there
are some differences in its strength and whether or not a closed
low develops briefly, no guidance stood out particularly as being
an outlier, so a deterministic blend of models excluding the CMC
was able to be used in the West through the end of the week.
As the northern stream shortwave moves eastward early next week,
and the southern stream shortwave lifts northeastward, they should
generally produce lower heights/troughing in the north-central
U.S. while an upper high slides atop them into Canada. Detail
differences certainly remain, but the GFS runs have trended
somewhat weaker with energy and a closed low there while ECMWF
runs have trended somewhat stronger with energy in this area,
meaning that for the 12/18Z cycle, their patterns matched pretty
well. The latter part of the forecast period used a blend of the
deterministic GFS and ECMWF and the GEFS and EC ensemble means to
temper individual model differences.
...Weather and Potential Hazards...
Portions of the Southern Plains can expect multiple days of
widespread and potentially heavy rainfall for the latter half of
the week, as the upper low in the Southwest exits into the High
Plains and interacts with tropical moisture coming from the Gulf
of Mexico. The specifics of rainfall duration and intensity remain
somewhat uncertain given model differences in the timing and track
of the upper low, but the current forecast calls for 2 of 4 inches
of rain with locally higher amounts extending through a good
portion of Texas. Periods of rain can be expected farther north
across the rest of the Plains and the Midwest in the vicinity of
the stalled front, with a tendency for the rain to slowly shift
eastward into the Ohio Valley as the surface low pressure system
stalls by early next week. Meanwhile, a round of showers is
possible for the Northern Rockies/Plains on Friday as a front
passes through, scattered showers and thunderstorms are possible
for southern Florida through the period, and some moisture may
return to the Four Corners states by Monday/Tuesday.
Warmer than normal temperatures are forecast across the
north-central tier of the CONUS through the end of the week, with
highs in the 70s through the Midwest underneath upper ridging. On
the other hand, cooler than average temperatures (especially
highs) are expected for the Rockies into southern/central portions
of the High Plains. Friday should be particularly cool over New
Mexico/Colorado/Texas given the clouds and precipitation over the
region. The West Coast can expect temperatures a few degrees above
normal by the weekend, while slightly below average temperatures
are expected for the Northeast with the lingering upper low.
Tate
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