Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Mon Aug 16 2021
Valid 12Z Thu Aug 19 2021 - 12Z Mon Aug 23 2021
***Tropical Depression Grace expected to strengthen back to
tropical storm status over the Gulf of Mexico***
***Emerging North-Central U.S. Heavy Rain Threat ahead of cold
front and upper trough***
...Weather Pattern Overview...
A well defined upper level trough with potential for a cut-off low
should be in place across the Intermountain West and Northern
Rockies to begin the forecast period Thursday, and an upper level
ridge situated over the Southeast U.S. in the wake of Tropical
Storm Fred. The upper trough then reaches the northern Plains and
drives a cold front and surface low eastward, with a building
ridge across the Northeast states. A more robust ridge axis
developing over the Gulf Coast region will tend to steer Tropical
Storm Grace farther westward across the Gulf compared to the more
eastward track of Tropical Storm Fred.
...Model Analysis and preferences...
The 00Z model guidance suite has come into better agreement for
the beginning of the period compared to the 12Z/18Z guidance.
Although still progressive with the northern trough, the GFS
slowed down from its 18Z run and closer to the model consensus.
The past two runs of the CMC have been more amplified with the
western to northern Plains trough, and is slower with the arrival
of the next trough across the Pacific Northwest by the end of the
forecast period. There still remains significant model spread
with the path of Grace, with latest trends suggesting a more
southern track owing to the strength of the ridge to its north.
The WPC fronts/pressures forecast was primarily derived from a 12Z
ECMWF/ECENS/UKMET blend along with a smaller portion of the 18Z
GFS and previous WPC continuity for the end of the week, and then
primarily ECMWF/ECENS/GEFS mean for next weekend.
...Sensible weather and potential hazards...
Residual tropical moisture from what will become the remnants of
Fred will fuel scattered to numerous showers and thunderstorms
across the Northeast U.S. on Thursday prior to exiting the region,
along with some lingering showers on Wednesday. Some instances of
flooding will be possible, particularly across areas that have
received above average rainfall already. Moderate to heavy
rainfall is also becoming increasingly likely across portions of
the central/northern Plains to close out the work week, with the
potential for some scattered 1 to 3 inch totals where mesoscale
convective complexes develop. Another corridor of enhanced
showers and storms may materialize across portions of the
Mid-South this weekend owing to a slow moving front in the
vicinity.
In the temperature department, expect readings to be up to 20
degrees below normal across portions of the northern Rockies and
western portions of the northern Plains for the end of the week,
and some high elevation snow for Wyoming and Montana is within the
realm of possibility. Except for some slightly above average
temperatures across the Great Lakes and New England, no serious
heat waves are currently expected.
Hamrick
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