Mesoscale Discussion 1279

Issued

Jul 14, 2021, 7:02 AM CDT (12:02 UTC)

Expired

Jul 14, 2021, 9:30 AM CDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

60%

The La Crosse radar (KARX), 152 km from the middle of the area, began a volume at 12:02 UTC, the scan nearest the moment this went out.

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• Severe Thunderstorm Watch 377Jul 14, 8:55 AM CDT

047 
ACUS11 KWNS 141202
SWOMCD
SPC MCD 141202 
WIZ000-ILZ000-IAZ000-MNZ000-141430-

Mesoscale Discussion 1279
NWS Storm Prediction Center Norman OK
0702 AM CDT Wed Jul 14 2021

Areas affected...northern Iowa...southern Minnesota...southwest
Wisconsin...and far northwest Illinois

Concerning...Severe potential...Watch possible 

Valid 141202Z - 141430Z

Probability of Watch Issuance...60 percent

SUMMARY...Ongoing showers and thunderstorms will begin to pose a
sporadic and episodic threat for marginally severe hail and gusty
thunderstorm winds this morning. As further destabilization occurs
later this morning, a more robust severe potential should
materialize. A watch may be needed at some point later this morning
across portions of the area.

DISCUSSION...Widespread showers and thunderstorms associated with a
weak cyclonic perturbation in the mid-level flow continue to move
east across the central Plains this morning. The strongest
convection is organizing on the leading edge of the precipitation
shield, where a seasonably strong 850-mb jet has focused the
strongest low-level theta-e advection this morning. Furthermore,
despite being near the daily minimum in instability, most-unstable
CAPE in excess of 2000 J/kg persists this morning across northwest
Iowa ahead of this organizing convection thanks in part to the
combination of sustained low-level theta-e advection and 700-500 mb
lapse rates between 7 and 8 C/km. This instability is more than
sufficient to sustain the ongoing convection through the morning
hours, especially given the background large-scale ascent associated
with the mid-level cyclonic perturbation. Small hail and gusty winds
are possible with the present convection. 

Thunderstorm intensity should slowly increase through the morning as
diurnal heating commences, slowly eroding the residual nocturnal
boundary layer, and convection organizes into a mesoscale convective
system (MCS). This erosion of the nocturnal boundary layer should
allow for elevated convection to transition to surface-based or
near-surface-based convection through the morning, increasing the
potential for strong thunderstorm downdrafts to reach the surface. 

Two potential limiting factors for a more robust and widespread
severe threat will be the lack of a widespread, higher-end CAPE
reservoir downstream of the ongoing convection and modest deep-layer
shear. With time, deep-layer shear should increase across the
region, aiding the organization of convection into an MCS. Until
then, severe potential should be more sporadic, primarily being
driven by storm-scale processes. Later, despite differences in
intensity and location, high-resolution numerical guidance depicts
development of a belt of enhanced low-level flow as the MCS
organization increases. Development of this feature would act to
enhance corridors of severe wind potential.

It may become necessary for a watch to be needed at some point this
morning across portions of the highlighted area. However,
uncertainties associated with how and when ongoing and developing
convection transitions from elevated to surface based or
near-surface based are too large to pinpoint when and where a watch
would be needed. As such, conditions will continue to be monitored
through the morning.

..Marsh/Hart.. 07/14/2021

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...GRB...MKX...DVN...ARX...MPX...DMX...FSD...OAX...

LAT...LON   43459587 43779531 44649193 44839042 44238938 43238926
            42318967 41889115 41759368 41769548 42699608 43459587

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The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.