Severe Storm Outlook Narrative (AC)
Issued by NWS

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ACUS01 KWNS 260559
SWODY1
SPC AC 260557

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1257 AM CDT SAT JUL 26 2014

VALID 261200Z - 271200Z

...THERE IS A MDT RISK OF SVR TSTMS LATE THIS AFTERNOON INTO
TONIGHT...MAINLY ACROSS MUCH OF CENTRAL ILLINOIS THROUGH CENTRAL AND
SOUTHERN INDIANA...BUT THIS INCLUDES AT LEAST PARTS OF THE GREATER
ST. LOUIS...INDIANAPOLIS...LOUISVILLE AND CINCINNATI METROPOLITAN
AREAS...

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK
AREA ACROSS MUCH OF THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS...

...SUMMARY...
SEVERE STORMS...SOME ACCOMPANIED BY VERY LARGE HAIL...LOCALLY STRONG
WIND GUSTS AND A COUPLE OF TORNADOES ARE EXPECTED TODAY INTO TONIGHT
ACROSS PARTS OF THE MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS.  ONE
OR MORE ORGANIZED CONVECTIVE CLUSTERS CAPABLE OF GENERATING SWATHS
OF POTENTIALLY DAMAGING WINDS MAY EVOLVE LATE THIS AFTERNOON AND
EVENING ACROSS MUCH OF CENTRAL ILLINOIS INTO CENTRAL AND SOUTHERN
INDIANA...PERHAPS REACHING THE CENTRAL APPALACHIANS BY LATE TONIGHT.

...SYNOPSIS...
WHILE BLOCKING APPEARS LIKELY TO REMAIN PROMINENT WITHIN THE
WESTERLIES ACROSS THE HIGHER LATITUDES OF NORTH AMERICA...MODELS
INDICATE UPPER TROUGHING EITHER SIDE OF THE HIGH CENTER OVER
INTERIOR NORTHERN CANADA WILL BEGIN TO UNDERGO AMPLIFICATION DURING
THIS PERIOD.  AS THIS OCCURS...SUBTROPICAL RIDGING IS EXPECTED TO
BUILD NORTHWARD THROUGH MUCH OF THE NORTHERN INTERMOUNTAIN REGION
AND ROCKIES...AND THE CLOSED LOW...NOW UNDERCUTTING THE CANADIAN
HIGH...SHOULD DIG SOUTHEASTWARD ACROSS THE CENTRAL CANADIAN/U.S.
BORDER AREA.

CONSIDERABLE SPREAD LINGERS WITHIN THE VARIOUS MODEL ENSEMBLE OUTPUT
CONCERNING THIS LATTER DEVELOPMENT...BUT THE 12Z ECMWF AND 21Z NCEP
SREF ENSEMBLE MEANS SUGGEST THAT THE REMNANT UPPER LOW/TROUGH
PROBABLY WILL NOT REACH THE UPPER MIDWEST UNTIL LATE THIS EVENING OR
OVERNIGHT.  WHILE THE MAIN TRANSITION TO STRENGTHENING AND
INCREASINGLY CYCLONIC MID-LEVEL FLOW MAY NOT OCCUR UNTIL WELL AFTER
DARK ACROSS THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY...DEEP
LAYER SHEAR WILL BE MORE THAN SUFFICIENT FOR ORGANIZED CONVECTIVE
DEVELOPMENT AS WEAK MID-LEVEL HEIGHT FALLS COMMENCE WITHIN THE
NORTHWEST FLOW REGIME PRECEDING IT.

BENEATH A REMNANT PLUME OF VERY WARM AND CAPPING ELEVATED MIXED
LAYER AIR ALONG THE SOUTHERN EDGE OF THE STRONGER NORTHWEST
FLOW...LOW-LEVEL MOISTENING SHOULD CONTINUE ACROSS THE LOWER
MISSOURI/MIDDLE MISSISSIPPI VALLEYS INTO THE OHIO VALLEY.  THIS IS
EXPECTED TO CONTRIBUTE TO STRONG DESTABILIZATION SUPPORTIVE OF
INTENSE CONVECTIVE DEVELOPMENT AND THE FOCUS FOR THE PRIMARY SEVERE
WEATHER POTENTIAL TODAY.

WEST SOUTHWESTWARD ALONG AN INITIAL SURFACE FRONT ADVANCING
SOUTHWARD INTO THE CENTRAL PLAINS...SEVERE WEATHER POTENTIAL APPEARS
LOWER DUE TO STRONGER LINGERING MID-LEVEL INHIBITION AND WEAKER
SHEAR.  HOWEVER...ISOLATED STRONG/SEVERE STORM DEVELOPMENT MAY NOT
BE OUT OF THE QUESTION NEAR OR SHORTLY AFTER PEAK HEATING.
OTHERWISE...CONDITIONS MAY ALSO BECOME AT LEAST MARGINALLY
SUFFICIENT FOR STRONG/SEVERE STORM DEVELOPMENT IN RESPONSE TO THE
EVOLVING UPPER PATTERN...ACROSS PARTS OF THE UPPER GREAT LAKES
REGION THIS AFTERNOON THROUGH TONIGHT...AND PERHAPS AS FAR EAST AS
THE LOWER GREAT LAKES/UPPER OHIO VALLEY AND CENTRAL APPALACHIANS BY
LATE TONIGHT.

MEANWHILE...MONSOONAL MOISTURE ACROSS THE HIGHER TERRAIN OF THE
SOUTHERN PLATEAU INTO CENTRAL AND SOUTHERN ROCKIES WILL AGAIN
SUPPORT CONSIDERABLE THUNDERSTORM ACTIVITY TODAY AND TONIGHT.

...MIDDLE MISSISSIPPI INTO LOWER OHIO VALLEYS...
CONVECTION ALLOWING MODEL GUIDANCE DOES NOT PROVIDE CLARITY FOR THIS
FORECAST...OFFERING A RATHER BROAD RANGE OF POSSIBLE SOLUTIONS.
HOWEVER...A FAIRLY CONSISTENT SIGNAL HAS BEEN EVIDENT IN THE NAM AND
ECMWF...AMONG OTHER MODEL OUTPUT...SUGGESTING THAT POTENTIAL EXISTS
FOR THE EVOLUTION OF ONE OR MORE ORGANIZED MESOSCALE CONVECTIVE
SYSTEMS ACROSS PORTIONS OF THE MIDDLE MISSISSIPPI THROUGH LOWER OHIO
VALLEYS DURING THIS PERIOD.  GIVEN THE AFOREMENTIONED ENVIRONMENT
CONDITIONS ANTICIPATED ACROSS THIS REGION BY LATE
AFTERNOON...CONFIDENCE IS FAIRLY HIGH THAT A SIGNIFICANT SEVERE
WEATHER EVENT WILL BE REALIZED.

ALONG/NORTH AND NORTHEAST OF AN AXIS OF STRONG SURFACE HEATING
ACROSS CENTRAL MISSOURI INTO CENTRAL ILLINOIS...MOISTENING BENEATH
THE AXIS OF STEEPEST MID-LEVEL LAPSE RATES WILL CONTRIBUTE TO LARGE
CAPE...BEFORE INHIBITION GRADUALLY ERODES.  PROBABLY AIDED BY
FORCING ASSOCIATED WITH LOWER/MID TROPOSPHERIC WARM
ADVECTION...INITIATION OF STORMS MAY OCCUR PRIOR TO 21Z ACROSS PARTS
OF SOUTHERN IOWA AND NORTHERN MISSOURI.  INITIALLY DISCRETE...A FEW
SUPERCELLS APPEAR POSSIBLE WITH THE POTENTIAL FOR VERY LARGE
HAIL...PERHAPS TORNADOES...BEFORE ACTIVITY GRADUALLY CONSOLIDATES
INTO AN ORGANIZING MESOSCALE CONVECTIVE SYSTEM AND DEVELOPS
SOUTHEASTWARD ACROSS CENTRAL ILLINOIS AND CENTRAL/SOUTHERN INDIANA
THROUGH THE EVENING.

ALTHOUGH MORE UNCERTAIN...AS INITIAL ACTIVITY CONTINUES TO SPREAD
TOWARD THE CENTRAL APPALACHIANS OVERNIGHT...FORCING ASSOCIATED WITH
THE DIGGING UPSTREAM IMPULSE MAY SUPPORT RENEWED CONVECTIVE
DEVELOPMENT ALONG THE TRAILING OUTFLOW BOUNDARY ACROSS THE MIDDLE
MISSISSIPPI VALLEY.  AT LEAST SOME GUIDANCE SUGGESTS THAT THIS COULD
EVOLVE INTO ANOTHER SOUTHEASTWARD PROPAGATING MESOSCALE CONVECTIVE
SYSTEM...IN AN ENVIRONMENT THAT SHOULD REMAIN CONDUCIVE TO THE
PRODUCTION OF STRONG...POTENTIALLY DAMAGING WIND GUSTS.

..KERR/SMITH.. 07/26/2014




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