Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Sun Aug 23 2020
Valid 12Z Wed Aug 26 2020 - 12Z Sun Aug 30 2020
...Two tropical systems (Marco and Laura) are forecast to affect
the southern/southeastern U.S. this week...
...Heat continues for the Desert Southwest while much above normal
temperatures spread across much of the northern U.S. through
Thursday...
...19Z Update...
The main changes with the medium range forecast today were due to
the westward track shift of (what is forecast to become) Hurricane
Laura in model guidance and the official National Hurricane Center
tracks. This necessitated a shift in the rainfall forecast farther
west, with rain totals now forecast to be heaviest along the
Texas/Louisiana border into the Lower Mississippi Valley.
Additional shifts in the rainfall forecasts according to the track
are likely, so the position of the heaviest rainfall totals will
change. However, confidence is reasonably high that several inches
of rain should fall along where Laura ends up tracking, which
could cause flooding and flash flooding issues. And 12Z model
guidance thus far has arrived with a pretty consistent track as
most of the 00Z/06Z model guidance through Thu. The mass fields
early on relied on the 00Z/06Z GFS, 00Z ECMWF, and 00Z UKMET,
while the 00Z CMC was not utilized due to it keeping an eastward
track with Laura. Laura is expected to interact with a frontal
boundary and an upper trough dropping southeastward late in the
week, as described in the previous discussion below. There are
some timing differences with when Laura's energy joins with the
jet or accelerates around it. After yesterday's 12Z ECMWF
accelerated it faster than other guidance by late in the week, the
00Z ECMWF was slower than consensus with the feature. Heavily used
the GEFS and EC ensemble means for the mass fields blend by the
later period due to differences there as well as with the upper
low entering southwestern Canada. Please see the previous
discussion below for more information about medium range period.
Tate
...Previous Discussion...
...Overview and Guidance/Predictability Assessment...
Ensembles show the flow pattern across the northern hemisphere
undergoing a fairly significant amplification during the extended
forecast period. One contributor to the pattern shift appears to
be interaction of across eastern Asia of Tropical Storm Bavi with
amplifying higher latitude flow across northeastern China over the
next few days. This appears to trigger a downstream amplification
of mid/upper-level flow across the North Pacific, eventually
reaching North America next weekend, with development of
relatively strong ridging (+2 standard deviations) across the
northeast Pacific and the Gulf of Alaska, and troughing of similar
magnitude setting up across the western CONUS. The amplification
will likely continue even farther east, with an early fall-like
trough expected to amplify across the Great Lakes/Northeast U.S.
next weekend. This signal is noted across ensembles, and thus
confidence in this large scale aspect of the forecast is at least
a bit above average.
Focusing in on the details, by the time the medium range forecast
period begins on day 3 (Wed), Tropical Storm Marco is forecast to
have made landfall along the central Gulf Coast and dissipated,
and all eyes in that part of the country will then turn to
(forecast to be) Hurricane Laura. Laura is forecast to make
landfall as a hurricane along the central Gulf Coast Wed
afternoon/evening. Model consensus surrounding the track of Laura
has improved somewhat over the past day, but a significant amount
of uncertainty remains, especially later in the forecast and after
the system makes landfall. The GFS and ECMWF were initially both
close to the NHC track on Wed. By Thu, the differences begin to
grow, with the GFS taking the system farther west, while the ECMWF
is farther eats but accelerates the system northeastward much more
quickly than other guidance. Please refer to products issued by
the National Hurricane Center for specific details on Tropical
Storm Laura.
A blend of the 18Z GFS/12Z ECMWF was used heavily in the WPC
forecast during days 3-5 (Wed-Fri). More emphasis was placed on
the GFS relative to the ECMWF, for a few reasons. The ECMWF
solution accelerating Laura northeastward more quickly than other
guidance was not preferred, based on the official NHC forecast.
Additionally, the ECMWF was faster than the overall consensus with
northern stream shortwave energy diving southeastward across the
Upper Midwest and Great Lakes Fri. Later in the forecast period,
emphasis on ECENS/GEFS ensemble means was increased coincident
with increasing spread on the forecast details. The GFS continued
to show some utility and remained the preferred deterministic
solution over the ECMWF. As the flow pattern becomes more
amplified, that should favor somewhat slower progression of
shortwave features, and the ECMWF appeared to be too quick with
progressing height falls, associated with the amplifying western
trough, eastward across the northern Rockies by next weekend,
while the GFS was somewhat slower and more in line with a
consensus of ensemble members.
...Weather/Hazard Highlights...
As Laura makes landfall along the central Gulf Coast on Wed, heavy
rainfall is expected to spread across areas near the track of the
system. Based on the current NHC forecast, several inches of
rainfall along with potential flooding impacts are forecast
Wed/Wed night from portions of the central Gulf Coast northward
into the lower Mississippi Valley. By Thu, the system is forecast
to move northward and begin to make a northeast turn, spreading
potentially heavy rains into portions of the Tennessee and Ohio
Valleys. Based on this track, as the remnants of Laura reach the
central Appalachians and Mid-Atlantic on Fri, there is the
potential for interaction with the amplifying Great
Lakes/Northeast upper-level trough and associated frontal
boundary, potentially leading to heavy rain impacts for portions
of the central Appalachians/Mid-Atlantic/Northeast. This is highly
dependent on the eventual track of Laura, however, and confidence
in the specifics of the forecast by that time frame is below
average.
Above average temperatures are expected during the first half of
the medium range from the Southwest to the north central U.S. Many
locations in the Southwest will continue to see temperatures soar
well past the 100 deg F mark, although not to the extent seen
during the peak of the recent heat wave. Isolated record max temps
will remain possible through Wed. Temperatures from portions of
the north central U.S. to the northern Mid-Atlantic will also see
max temperatures 5 to 15 deg above average Wed-Thu ahead of an
incoming cold front, with many areas reaching well into the 90s.
The pattern change (described above) later in the week will bring
an end to the hot temperatures for many areas by next weekend.
High temperatures next Sun are forecast be 5 to 10 deg below
average across much of the Northwest, as well as the Great Lakes
and Northeast.
Ryan
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards 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