Mesoscale Discussion 1771

Issued

Aug 16, 2012, 3:57 AM CDT (08:57 UTC)

Expired

Aug 16, 2012, 6:30 AM CDT

In force

3 hr

Issued by

the Storm Prediction Center

Chance of a watch

20%

The Quad Cities radar (KDVN), 83 km from the middle of the area, began a volume at 08:59 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.

819 
ACUS11 KWNS 160857
SWOMCD
SPC MCD 160857 
WIZ000-ILZ000-IAZ000-MNZ000-161130-

MESOSCALE DISCUSSION 1771
NWS STORM PREDICTION CENTER NORMAN OK
0357 AM CDT THU AUG 16 2012

AREAS AFFECTED...PORTIONS IA...NRN IL...CENTRAL/SRN WI...WRN LM.

CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 

VALID 160857Z - 161130Z

PROBABILITY OF WATCH ISSUANCE...20 PERCENT

SUMMARY...CONVECTION IS INCREASING IN COVERAGE WITHIN BROAD/ELEVATED
WAA REGIME...FROM SRN/WRN IA TO CENTRAL WI.  THIS TREND SHOULD
CONTINUE FOR ANOTHER 2-3 HOURS AS AREA OF TSTMS TRANSLATES EWD
ACROSS REMAINDER DISCUSSION AREA.  SPORADIC LARGE HAIL WILL BE
PRIMARY CONCERN.  ISOLATED GUSTS APCHG SVR LIMITS CANNOT BE RULED
OUT EITHER.

DISCUSSION...AT 08Z...SFC MESOANALYSIS SHOWED COLD FRONT ROUGHLY
FROM APOSTLE ISLANDS WI SWWD NEAR EAU...CCY...DSM...FNB..MHK LINE. 
FRONT SHOULD CONTINUE TO MOVE ESEWD ACROSS THIS REGION 15-20 KT. 
TSTMS HAVE DEVELOPED ON BOTH SIDES OF FRONT IN REGIME OF LOW-LEVEL
WAA ABOVE SFC.  NARROW CORRIDOR OF PRECONVECTIVE MUCAPE NEAR COLD
FRONT WAS ESTIMATED AT AROUND 800 J/KG OVER NERN WI...INCREASING
WITH SWWD EXTENT TO 2000-2500 J/KG RANGE OVER SWRN IA IN ASSOCIATION
WITH GREATER MIDLEVEL LAPSE RATES.  LLJ IS FCST TO
STRENGTHEN...RESULTING IN INCREASED MOISTURE
TRANSPORT/ADVECTION...BUT ALSO TO VEER WITH TIME.  NET RESULT SHOULD
BE NEUTRAL TO SLGT DECREASING TREND IN STORM-RELATIVE INFLOW...BUT
ALSO HIGHER THETAE IN ELEVATED INFLOW LAYERS.  EFFECTIVE SHEAR
MAGNITUDES 30-40 KT ARE EVIDENT OVER PARTS OF THIS AREA...WITH
VALUES STRONGLY DEPENDENT ON CHOSEN LIFTED PARCEL AND RESULTANT
DEPTH OF BUOYANT LAYER. 

SFC AIR MASS ON BOTH SIDES OF FRONT WILL CONTINUE TO STABILIZE
DIABATICALLY FROM COMBINATION OF NOCTURNAL SFC COOLING AND PRECIP
INVOF TSTMS....WITH ADDITIONAL FACTOR OF CAA BEHIND FRONT.  THAT
FACTOR WILL MITIGATE POTENTIAL FOR SVR GUSTS.  HOWEVER...MODIFIED
RAOBS AND FCST SOUNDINGS SUGGEST LAYER OF DRYING BETWEEN SFC AND
BASE OF CAPE-BEARING LAYER AROUND 700 MB...IN SUPPORT OF DOWNDRAFT
MAINTENANCE.

..EDWARDS/WEISS.. 08/16/2012

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

ATTN...WFO...GRB...LOT...ILX...MKX...DVN...ARX...DMX...

LAT...LON   40729118 40709282 41159445 42379475 43519216 44648979
            44928747 44198696 42608711 41798769 40958988 40729118

Mesoscale Discussions in Iowa, 2012Mesoscale Discussions in Wisconsin, 2012Mesoscale Discussions in Illinois, 2012Every mesoscale discussion in 2012

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.