Mesoscale Discussion 252

Issued

Mar 6, 2017, 1:08 PM CST (19:08 UTC)

Expired

Mar 6, 2017, 3:45 PM CST

In force

3 hr

Issued by

the Storm Prediction Center

Chance of a watch

80%

The Des Moines radar (KDMX), 16 km from the middle of the area, began a volume at 19:11 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

• Tornado Watch 61Mar 6, 1:55 PM CST

369 
ACUS11 KWNS 061908
SWOMCD
SPC MCD 061908 
IAZ000-062145-

Mesoscale Discussion 0252
NWS Storm Prediction Center Norman OK
0108 PM CST Mon Mar 06 2017

Areas affected...Portions of Iowa

Concerning...Severe potential...Tornado Watch likely 

Valid 061908Z - 062145Z

Probability of Watch Issuance...80 percent

SUMMARY...The risk for severe thunderstorms and tornadoes will be
increasing across parts of western IA in the general 2030Z-2200Z
time frame and then subsequently spread eastward. The issuance of a
Tornado Watch will be likely during the next couple of hours.

DISCUSSION...Visible satellite imagery indicates high-based cumulus
development becoming increasingly focused within the deep boundary
layer across parts of central and eastern NE. This high-based
convection is being enhanced by ascent within the left-exit region
of a mid/high-level speed maximum advancing northeastward across the
North-Central States. As large-scale ascent continues to increase, a
pseudo-dryline over the middle Missouri Valley region will continue
to sharpen and serve as a focused zone of thunderstorm development
in the general 2030Z-2200Z time frame. Warm-sector pressure falls of
2.0-2.5 mb per 2 hours have been noted across the region in
association with the strengthening large-scale ascent.

As midlevel flow continues to strengthen, with at least some flow
component orthogonal to the dryline and an approaching baroclinic
zone, initially discrete/semi-discrete supercell modes will be
likely across western IA -- aided by 50-65 kt of effective shear.
With time, upscale growth of this activity should foster storm
clusters and QLCS segments spreading toward the Mississippi River
into the evening hours, especially as the baroclinic zone overtakes
the dryline and accelerates eastward.

While the magnitude of warm-sector moisture return is somewhat
modest (surface dewpoints in the middle 50s), steep midlevel lapse
rates attendant to an overlying elevated mixed layer plume will
support sufficient buoyancy (MLCAPE around 750-1250 J/kg) for robust
convection capable of severe hail, wind, and a few tornadoes. The
tornado risk will be relatively greater for persistent supercell
structures amid effective SRH around 200-300 m2/s2 in the open warm
sector, and perhaps with line-embedded meso-vortices later in the
afternoon/evening.

..Cohen/Grams.. 03/06/2017

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

ATTN...WFO...DVN...ARX...DMX...FSD...OAX...

LAT...LON   40739306 40799508 41819567 42929552 43319441 43189204
            41729132 40809167 40739306

Mesoscale Discussions in Iowa, 2017Every mesoscale discussion in 2017

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.