ACUS48 KWNS 240855
SWOD48
SPC AC 240854

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0354 AM CDT Sat Aug 24 2024

Valid 271200Z - 011200Z

...DISCUSSION...
Within the faster belt of upper flow across the northern U.S.,
substantial differences in timing of the disturbances in the flow
are manifest from the very start of the medium-range period.  On
Tuesday/Day 4, an energetic short-wave trough should be crossing the
north-central U.S., headed toward the Upper Mississippi
Valley/southern Ontario, while an upstream trough crosses western
Canada and the Pacific Northwest, and a downstream one crosses
northern Quebec.  However, noted differences in both timing and
orientation of these features is immediately apparent.  The
north-central U.S. feature -- the one associated with Day-4 severe
risk over the Great Lakes region -- is roughly 12 hours slower, and
evolves to a more negatively tilted configuration in the ECMWF, as
compared to the GFS.  This results in fairly substantial differences
in the surface pattern as well, prohibiting any forecast of the
specifics regarding the severe-weather potential on Tuesday.  Still,
severe-weather risk is apparent despite east-west geographical
uncertainties.  Thus, will introduce an elongated 15% area across
the Upper Midwest and Great Lakes region, given likelihood for
existence of a favorably unstable/amply sheared environment in the
vicinity of a surface front likely to stretch east-northeastward to
west-southwestward across this region.  Damaging winds and hail
would likely be the primary risks.

By Day 5, differences continue to amplify.  Risk for severe weather
may evolve across portions of the Northeast (favored by the GFS),
and/or parts of the Midwest/Ohio Valley (favored by the ECMWF).
However, the differences are substantial enough amongst the models
to preclude an areal highlight at this time.  Meanwhile farther
west, an upper low/trough is forecast to be moving across the
northwestern U.S. and Canadian Prairie early in the day, but
location/timing, and intensity differences with this feature are
pronounced.  While some severe risk may evolve across parts of the
north-central U.S. ahead of this feature and an associated cold
front, the differences preclude any areal outlines at this time.

Predictability issues persist through the remainder of the period,
with model solutions diverging steadily through the upcoming Labor
Day weekend.

..Goss.. 08/24/2024

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