SWODY1
SPC AC 090553
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1253 AM CDT THU APR 09 2009
VALID 091200Z - 101200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTN AND TONIGHT ACROSS
PARTS OF THE SRN PLAINS INTO THE OZARK PLATEAU....
...SYNOPSIS...
SIGNIFICANT AMPLIFICATION OF A SHORT WAVE TROUGH WITHIN THE SOUTHERN
BRANCH OF SPLIT POLAR WESTERLIES IS UNDERWAY ACROSS THE EASTERN
PACIFIC...AND MODELS CONTINUE TO SUGGEST THAT THIS SYSTEM WILL DIG
INTO THE VICINITY OF THE CALIFORNIA COAST AND BAJA BY 12Z FRIDAY.
AS THIS OCCURS...THE LARGE COLD DOWNSTREAM TROUGH IS PROGGED TO
FINALLY LIFT AWAY FROM THE ATLANTIC SEABOARD. IN BETWEEN THESE TWO
FEATURES...AND SOUTH OF A WEAKENING MID-LEVEL CIRCULATION WITHIN
BROADER SCALE NORTHERN BRANCH RIDGING ACROSS THE NORTH CENTRAL
STATES...GUIDANCE HAS BEEN CONSISTENT IN INDICATING THAT A
RELATIVELY COMPACT AND VIGOROUS CLOSED LOW WILL FORM WITHIN THE
SOUTHERN STREAM TODAY...TO THE LEE OF THE SOUTHERN ROCKIES. THIS
FEATURE IS EXPECTED TO EVOLVE FROM THE STRONG SHORT WAVE IMPULSE NOW
TURNING EASTWARD ACROSS THE FOUR CORNERS STATES...AND IS PROGGED TO
MIGRATE ACROSS THE SOUTH CENTRAL PLAINS INTO THE OZARK PLATEAU LATER
TODAY/TONIGHT...ACCOMPANIED BY A DEEP SURFACE CYCLONE WHICH HAS
ALREADY FORMED TO THE LEE OF THE CENTRAL/SOUTHERN ROCKIES.
DESPITE A FAVORABLE LARGE-SCALE PATTERN FOR RAPID MOISTURE RETURN
THROUGH THE SOUTHERN PLAINS AND LOWER HALF OF THE MISSISSIPPI
VALLEY...BOUNDARY LAYER MODIFICATION IN THE WAKE OF THE EASTERN
TROUGH IS STILL AN ONGOING PROCESS ACROSS THE GULF OF MEXICO AND
GULF STATES. AND...IT IS NOT YET CLEAR THAT MOISTENING WILL BE AS
SUBSTANTIAL AS LATEST NAM FORECASTS OF LOWER/MID 60S SURFACE DEW
POINTS MIGHT INDICATE. HOWEVER...GOES BLENDED TOTAL PRECIPITABLE
WATER DATA DOES INDICATE AT LEAST A NARROW AXIS OF 1+ INCH VALUES
RETURNING NORTHWARD ALONG THE MEXICAN GULF COAST. NCEP SREF IS
SUGGESTIVE THAT THIS WILL RAPIDLY ADVECT NORTHWARD ACROSS EAST TEXAS
INTO THE OZARK PLATEAU THROUGH THIS PERIOD.
AT THE SAME TIME...WARM ELEVATED MIXED LAYER AIR ADVECTING EAST
NORTHEASTWARD OFF THE MEXICAN/SOUTHERN U.S. PLATEAU REGION WILL
CONTRIBUTE TO AIR MASS DESTABILIZATION...WHICH WILL BECOME
SUPPORTIVE OF INCREASING CONVECTIVE POTENTIAL...INCLUDING THE RISK
OF SEVERE STORMS...ACROSS THE SOUTH CENTRAL STATES.
...SOUTH CENTRAL PLAINS INTO THE OZARK PLATEAU...
A WARM FRONT EXTENDING EAST OF THE SURFACE CYCLONE INTO THE OZARK
PLATEAU SEEMS LIKELY TO BE THE FOCUS FOR THE MOST WIDESPREAD
CONVECTIVE DEVELOPMENT TODAY. MODELS ARE SUGGESTIVE OF STRONG
CONVERGENCE ALONG THIS BOUNDARY...PARTICULARLY JUST EAST OF THE LOW
CENTER EXPECTED TO BE MIGRATING ACROSS NORTH CENTRAL OKLAHOMA DURING
THE LATE AFTERNOON HOURS. THIS FORCING...COUPLED WITH THAT OF THE
APPROACH OF THE UPPER IMPULSE...APPEARS LIKELY TO SUPPORT THE
INITIATION OF STORMS BY OR SHORTLY AFTER 21Z NEAR THE
KANSAS/OKLAHOMA BORDER AREA. IN THE PRESENCE OF STEEP LAPSE RATES
...AND MIXED LAYER CAPE PROBABLY ON THE ORDER OF 1000 J/KG... LARGE
CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS WILL BE SUPPORTIVE OF
SUPERCELLS WITH VERY LARGE HAIL AND A FEW TORNADOES. THIS ACTIVITY
IS EXPECTED TO THEN SPREAD EASTWARD AHEAD OF THE SURFACE
LOW...ACROSS SOUTHEAST KANSAS/NORTHEAST OKLAHOMA INTO SOUTHERN
MISSOURI/NORTHERN ARKANSAS...WITH LARGE-SCALE FORCING SUPPORTING THE
GRADUAL CONSOLIDATION AND UPSCALE GROWTH INTO AT LEAST A SMALL
MESOSCALE CONVECTIVE SYSTEM. DURING THE EVENING HOURS...THE
TRANSITION TO MAINLY A DAMAGING WIND THREAT SEEMS PROBABLE... BEFORE
ACTIVITY SPREADS INTO THE LOWER OHIO/TENNESSEE VALLEYS AND WEAKENS
OVERNIGHT.
ALONG THE DRY LINE TRAILING SOUTH OF THE SURFACE LOW CENTER...
ACROSS PARTS OF EASTERN OKLAHOMA INTO EASTERN TEXAS LATE THIS
AFTERNOON...MODELS CONTINUE TO INDICATE A RATHER STRONG WESTERLY
COMPONENT TO THE LOW-LEVEL FLOW JUST OFF THE SURFACE. AND...
CONCERN EXISTS THAT THE NEAR SURFACE MOIST LAYER COULD BE SOMEWHAT
SHALLOW. EVAPORATIVE COOLING NEAR DOWNDRAFTS COULD TEMPER TORNADIC
POTENTIAL SOMEWHAT...DESPITE THE GREATER LIKELIHOOD FOR LONGER
HODOGRAPHS...MORE FAVORABLE FOR CLASSIC LONG-LIVED SUPERCELL TYPE
STRUCTURES...THAN COMPARED TO THOSE ANTICIPATED ALONG THE WARM
FRONT. BUT...A WINDOW OF OPPORTUNITY APPEARS TO EXIST DURING THE
LATE AFTERNOON/EVENING HOURS FOR SCATTERED DISCRETE STORM
DEVELOPMENT ACROSS EASTERN OKLAHOMA/NORTHEAST TEXAS INTO WESTERN
ARKANSAS AND NORTHWEST LOUISIANA. IF THIS OCCURS... STORMS WILL
LIKELY BE CAPABLE OF AT LEAST PRODUCING VERY LARGE HAIL AND
LOCALIZED DAMAGING WIND GUSTS.
..KERR/GRAMS.. 04/09/2009
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