Alert archive / Iowa / Mesoscale Discussions in 2015

Mesoscale Discussion 1821

September 22, 201512 counties in Iowa; 5 counties in Nebraska

The Storm Prediction Center issued this mesoscale discussion at Sep 22, 2015, 1:06 PM CDT for 12 counties in Iowa; 5 counties in Nebraska. It was in force for 84 min.

The area

The outline as issued.

The facts

Issued: Sep 22, 2015, 1:06 PM CDT (18:06 UTC)
Expired: Sep 22, 2015, 2:30 PM CDT
In force: 84 min
Issued by: the Storm Prediction Center
Chance of a watch: 5%

Counties Adair County, IA, Audubon County, IA, Carroll County, IA, Cass County, IA, Crawford County, IA, Dallas County, IA, Guthrie County, IA, Harrison County, IA, Ida County, IA, Monona County, IA, Shelby County, IA, Woodbury County, IA, Burt County, NE, Cuming County, NE, Dakota County, NE, Dixon County, NE, Thurston County, NE

The radar

The Omaha radar (KOAX), 93 km from the middle of the area, began a volume at 18:10 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.

The nearest sounding before it

Omaha, NE (KOAX), launched 12:00Z, 6.1 hours before the alert went out and 93 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 529 J/kg
Mixed-layer CIN: -281 J/kg
Mixed-layer LCL: 1052 m above ground
0-6 km bulk shear: 34 kt
0-1 km storm-relative helicity: 150 m²/s²
0-3 km storm-relative helicity: 128 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.2

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

335 
ACUS11 KWNS 221806
SWOMCD
SPC MCD 221806 
IAZ000-NEZ000-SDZ000-221930-

MESOSCALE DISCUSSION 1821
NWS STORM PREDICTION CENTER NORMAN OK
0106 PM CDT TUE SEP 22 2015

AREAS AFFECTED...PARTS OF ERN NEB...WRN IA

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 

VALID 221806Z - 221930Z

PROBABILITY OF WATCH ISSUANCE...5 PERCENT

SUMMARY...AN ISOLATED SEVERE STORM OR TWO MAY LINGER NEAR/EAST OF
THE MID MISSOURI VALLEY ANOTHER COUPLE OF HOURS.  LARGE HAIL APPEARS
THE PRIMARY SEVERE HAZARD...BEFORE ACTIVITY DIMINISHES LATER THIS
AFTERNOON.

DISCUSSION...SUSTAINED...ISOLATED...BUT VIGOROUS THUNDERSTORM
ACTIVITY CONTINUES TO SPREAD SLOWLY EASTWARD ACROSS THE MID MISSOURI
VALLEY...TO THE NORTH OF OMAHA.  THIS CONVECTION APPEARED TO
INITIATE WITHIN A ZONE OF MID-LEVEL CONFLUENCE...PERHAPS MAINTAINED
BY LIFT ASSOCIATED WITH LOWER/MID TROPOSPHERIC WARM ADVECTION. 
HOWEVER...FORCING FOR UPWARD VERTICAL MOTION REMAINS
WEAK/SUBTLE...AND DOES NOT APPEAR WELL HANDLED BY EVEN THE LATEST
HIGH RESOLUTION CONVECTION ALLOWING MODEL GUIDANCE.

IT REMAINS UNCLEAR HOW LONG IT WILL PERSIST...BUT IT STILL SEEMS
MOST PROBABLE THAT IT WILL DIMINISH WITHIN THE NEXT FEW HOURS. 
UNTIL IT DOES...THOUGH...STEEP MID-LEVEL LAPSE RATES...ON THE EDGE
OF WARM ELEVATED MIXED LAYER AIR...MAY CONTINUE TO CONTRIBUTE TO AN
ENVIRONMENT CONDUCIVE TO SEVERE HAIL...IN THE PRESENCE OF AT LEAST
MARGINALLY SUFFICIENT SHEAR FOR SUPERCELLS /MOSTLY DUE TO PRONOUNCED
VEERING WITH HEIGHT OF LOW TO MID-LEVEL WINDS/.

..KERR/HART.. 09/22/2015

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...DMX...FSD...OAX...

LAT...LON   42619722 42709664 42349510 41709393 41279445 41439577
            41989694 42619722

Nearby

Mesoscale Discussions in Iowa, 2015 Mesoscale Discussions in Nebraska, 2015 Every mesoscale discussion in 2015

Sources

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.