Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EDT Fri Sep 03 2021
Valid 12Z Mon Sep 06 2021 - 12Z Fri Sep 10 2021
...Overview...
Upper ridging will drift out of the Great Basin early next week
toward the southern Plains as troughing moves into the Pacific
Northwest. Troughing in the Northeast on Monday will lift out by
Tuesday but will be quickly replaced by incoming troughing from
southern Canada. That trough should exit the East Coast around the
end of next week, likely keeping Hurricane Larry over the
Atlantic. The pattern is largely quiet and drier than average
after a very active and wet several weeks.
...Guidance Evaluation and Preferences...
Through the 12Z/18Z cycle, the models and ensembles showed
relatively good clustering through the period, with expected
differences that grew with time. Recent GFS runs were a bit on the
southern side of the spread with low pressure through the Great
Lakes Tue-Wed, but not implausible. Larger differences over the
Pacific translated into the West (particularly the Northwest) due
to system disagreement on existence/timing/track. Ensemble
consensus seemed to favor an area of low pressure approaching the
coast by the end of next week (with its cold front perhaps
onshore) in between the quicker 18Z GFS and 12Z Canadian and the
slower 12Z ECMWF (12Z GFS was also as slow as the ECMWF).
Multi-day ensemble trend has been toward a bit slower and more
amplified trough-ridge-trough pattern over the CONUS, so weighed
the slower ECMWF a bit more than the GFS but with a majority
ensemble mean blend.
Over the Southwest, the movement of the upper high southeastward
may allow for an increase in moisture into the lower CO River
basin late next week. GFS/GEFS were a bit wetter than the
ECMWF/Canadian and their ensembles. Favored a slightly drier
scenario for now. In the Gulf, any limited tropical activity is
forecast to be weak/disorganized, but may spread moisture/rainfall
into the Gulf Coast. Continued to favor the weaker solutions vs
the more robust models.
...Weather/Hazard Highlights...
Rainfall will focus over the Upper Midwest/Great Lakes Monday in
advance of a system out of the northern Plains, which may promote
some heavier rainfall on Tuesday across Michigan. Rainfall along
the cold front may be lighter and continued southward into the
Southeast in a warmer and more humid environment. Temperatures
will be near to above normal over most areas, especially the West
to the Northern Rockies ahead of the incoming Pacific trough.
Temperatures 5-15 degrees above normal are possible as the area of
warmth moves into the southern Plains late next week. Temperatures
in the Northeast will be near to a bit below normal thanks to
generally northwest flow.
Fracasso
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