SWODY1
SPC AC 200558
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1258 AM CDT SUN SEP 20 2009
VALID 201200Z - 211200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
A STRONG...POLAR JET STREAK DIGGING SEWD FROM THE PACIFIC NW TOWARD
THE CENTRAL ROCKIES WILL SUPPORT INTENSIFICATION OF A LONG WAVE
TROUGH OVER THE CENTRAL CONUS THIS FORECAST PERIOD. A VORTICITY MAX
LOCATED ALONG THE SRN EXTENT OF THE UPPER TROUGH...NOW MOVING EWD
THROUGH THE PACIFIC NW...IS EXPECTED TO REACH THE CENTRAL HIGH
PLAINS BY 21/00Z AND PRECEDE THE ATTENDANT MAIN UPPER TROUGH
AMPLIFYING ESEWD THROUGH THE NRN HIGH PLAINS/DAKOTAS. MEANWHILE...
SEVERAL VORTICITY MAXIMA LOCATED DOWNSTREAM FROM LONG WAVE TROUGH
AXIS WILL LIFT NEWD THROUGH THE MID MS/LOWER OH VALLEYS INTO THE
GREAT LAKES.
...CENTRAL HIGH PLAINS TO MID MO VALLEY...
A COLD FRONT ATTENDANT TO THE AMPLIFYING PACIFIC TROUGH IS EXPECTED
TO EXTEND FROM THE WRN DAKOTAS BORDER INTO CENTRAL WY AT 12Z
TODAY...WITH THIS BOUNDARY PROGRESSING E/SEWD INTO THE CENTRAL
DAKOTAS...WRN NEB AND NERN CO BY 21/00Z. EWD ADVECTION OF EML
PRECEDING MID/UPPER LEVEL TROUGH WILL OCCUR ATOP A MOISTENING
PRE-FRONTAL BOUNDARY LAYER...SUPPORTING A NARROW CORRIDOR OF
MODERATE INSTABILITY /1000-1500 J/KG/ FROM CENTRAL KS/NEB TO SOUTH
CENTRAL SD. CAPPING AT THE BASE OF THE EML IS EXPECTED TO INHIBIT
SURFACE-BASED STORM FORMATION THROUGH THE DAY ACROSS MUCH OF THE
WARM SECTOR. THE EXCEPTIONS WILL BE ALONG THE COLD FRONT OVER WY
AND PRE-FRONTAL TROUGH ALONG THE CENTRAL HIGH PLAINS WHERE
COMBINATION OF LOW-LEVEL CONVERGENCE AND DEEPER FORCING ASSOCIATED
WITH VORT MAX PRECEDING AMPLIFYING UPPER TROUGH OVERCOME INHIBITION.
THIS ACTIVITY WILL OCCUR W OF PRIMARY INSTABILITY AXIS...BUT WITHIN
ENVIRONMENT OF STEEP LAPSE RATES WHICH SHOULD SUPPORT A THREAT FOR
GUSTY WINDS AND SOME HAIL WITH STRONGER STORMS.
ADDITIONAL STORMS...LIKELY ELEVATED...ARE EXPECTED TO DEVELOP ALONG
AND AHEAD OF THE COLD FRONT SUNDAY EVENING OVER THE MID MO VALLEY
WITHIN ZONE OF STRENGTHENING LOW LEVEL WAA. THE PRESENCE OF STEEP
MID LEVEL LAPSE RATES AND MODEST ELEVATED CONVECTIVE INSTABILITY
WILL LIKELY BE SUPPORTIVE OF STRONGER UPDRAFTS CAPABLE OF MAINLY
HAIL.
...OH/TN VALLEYS...
A WEAK SURFACE LOW ASSOCIATED WITH THE NEWD EJECTING VORTICITY
MAXIMA IS EXPECTED TO TRACK NEWD FROM NERN AR TO THE SWRN GREAT
LAKES REGION...IN CONJUCTION WITH WARM FRONT. THIS WILL ALLOW FOR
THE NWD ADVECTION OF A MOIST LOW-LEVEL AIR MASS THROUGH THE TN/OH
VALLEYS...CONTRIBUTING TO WEAK-MODERATE INSTABILITY /MLCAPE OF
500-1000 J PER KG/. GENERALLY WEAK CAPPING AND BROAD WEAK ASCENT
ACROSS WARM SECTOR ARE EXPECTED TO AID IN TSTMS ACROSS THIS REGION.
LATEST MODEL GUIDANCE SUGGESTS THAT DEEP-LAYER SHEAR WILL REMAIN
SOMEWHAT MARGINAL /AOB 30 KT/ WHICH SHOULD LIMIT A GREATER SEVERE
THREAT. HOWEVER...LOCALLY STRONGER LOW-LEVEL SHEAR IS FORECAST
INVOF WARM FRONT AND ALONG LOW TRACK. GIVEN THE MOIST BOUNDARY
LAYER CONDITIONS...SOME POTENTIAL WILL EXIST FOR BRIEF SUPERCELL
STRUCTURES CAPABLE OF LOCALLY GUSTY WINDS AND PERHAPS A BRIEF
TORNADO.
..PETERS/GRAMS.. 09/20/2009
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