SPC Jul 11, 2024 Day 4-8 Severe Weather Outlook

[NorthGeorgiaWX]

Day 4-8 Outlook

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0350 AM CDT Thu Jul 11 2024

Valid 141200Z - 191200Z

...DISCUSSION...
An upper-level ridge will remain in place over much of the western
and central CONUS through at least early next week. Multiple
mid-level impulses are expected to traverse the anticyclonic flow
over the northern CONUS through at least Days 4-5 (Sunday-Monday).
These impulses may be accompanied by multiple rounds of strong to
severe thunderstorms given continued low-level WAA/moisture
advection along the eastern periphery of a regenerating EML. By days
6-7 (Tuesday-Wednesday), a mid-level trough will amplify over the
Great Lakes, with a surface cold front expected to sweep southward
across the northern Plains into the Northeast, promoting showers and
thunderstorms (perhaps strong to severe), but also clearing out the
low-level moisture, EML, and associated buoyancy in the process.

...Days 4-5 (Sunday-Monday)...
Medium-range guidance consensus shows a mid-level impulse
approaching the Great Lakes region from the northern Plains on
Sunday (albeit later than earlier model runs). While both the latest
ECMWF and GFS agree on the mid-level impulse overspreading the nose
of the WAA (and associated 3000-4000 J/kg MUCAPE) over the Upper MS
Valley on Sunday, the later arrival of the mid-level impulse past
peak afternoon heating results in a complex forecast. The GFS
depicts potential convection over the northern Great Lakes by late
afternoon/early evening while the ECMWF develops precipitation over
central/southern MN that rapidly progress to the WI/IL border. While
it is unclear which scenario will unfold, both regimes involve
organized convection traversing an axis of strong to extreme
instability amid adequate deep-layer shear to support a severe
threat. This includes the possibility of a severe-wind producing
MCS, warranting a broad 15 percent area. Substantial modifications
may be needed to this highlight in later outlooks as mesoscale
details and associated placement of key baroclinic boundaries
becomes evident.

The latest ECMWF and GFS depicts a more pronounced mid-level trough
traversing the Upper MS Valley by afternoon peak heating on Day 5
(Monday). Continued low-level moisture advection from the south,
beneath a regenerating plume of steep mid-level lapse rates from the
west, will support another day of strong to extreme instability
overspreading the Upper Midwest. This time, stronger deep-layer
shear with the better-timed mid-level impulse will coincide with the
instability axis, fostering an environment favorable for severe
storms. Given impacts from Day 4 (Sunday) convection, it is unclear
how and where mesoscale low-level boundaries will become established
over the Midwest, and these will be key to supporting a severe
threat (including the possibility of severe wind-producing bow-echo
MCSs). 15 percent highlights have been added over the general area
where both the GFS and ECMWF agree on precipitation occurring with
overlapping favorable vertical wind shear and strong/extreme
instability. Like Sunday though, the highlights provided for Monday
will likely need substantial modification based on later-known
placement of important mesoscale meteorological features. 

...Days 6-7 (Tuesday-Wednesday)...
ECMWF and GFS are in general agreement in thunderstorms initiating
along and ahead of a surface cold front, anywhere from the northern
Plains to the Northeast states in the Days 6-7 (Tuesday-Wednesday)
time frame. These storms would traverse a buoyancy axis, possibly
becoming strong or even severe in the process. However, great
uncertainty exists on precisely where and when the greatest severe
threat would materialize, since there is more disagreement among
medium-range guidance members compared to earlier days. Furthermore,
details on severe placement and timing still hinge on the evolution
of earlier thunderstorm events (i.e. where baroclinic boundaries are
left behind). When taking into account both model disagreements and
uncertainties of preceding convective influences, severe
probabilities have been withheld this outlook.

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