SPC AC 150548
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1248 AM CDT FRI SEP 15 2006
VALID 151200Z - 161200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTN AND EVE ACROSS PARTS
OF THE CNTRL PLAINS....
...THERE IS A SLGT RISK OF SVR TSTMS TODAY ACROSS PARTS OF THE
EASTERN GREAT BASIN....
AN AMPLIFIED UPPER TROUGH HAS DEVELOPED ACROSS MUCH OF THE WESTERN
UNITED STATES. THE MOST SIGNIFICANT EMBEDDED IMPULSE AND MID-LEVEL
COLD CORE OF THIS SYSTEM IS STILL OVER THE PACIFIC NORTHWEST COAST
STATES...WITH AN ASSOCIATED MID/UPPER JET STREAK DIGGING JUST OFF
NORTHERN PACIFIC COASTAL AREAS. HOWEVER...MODELS SUGGEST THAT THE
EXIT REGION OF THIS STRONG JET STREAK WILL NOSE EASTWARD ACROSS THE
GREAT BASIN BY LATE THIS AFTERNOON...AND THROUGH THE CENTRAL ROCKIES
BY 12Z SATURDAY.
AT LEAST A COUPLE OF WEAKER PERTURBATIONS WILL PRECEDE THE
AFOREMENTIONED FEATURE THROUGH THE BROADER SCALE CYCLONIC FLOW
REGIME ACROSS THE PLAINS. THIS IS WHERE A STRONG SOUTHERLY
LOW-LEVEL JET...ASSOCIATED WITH A DEEP SURFACE CYCLONE IN THE LEE OF
THE NORTHERN ROCKIES...WILL CONTRIBUTE TO A LOW-LEVEL RETURN FLOW.
IN THE WAKE OF A RECENT COOL/DRY INTRUSION ASSOCIATED WITH A
WEAKENING UPPER TROUGH...NOW APPROACHING THE EASTERN SEABOARD...
MOISTURE RETURN WILL NOT BE STRONG...BUT SHOULD STILL BE SUFFICIENT
FOR INCREASING CONVECTIVE POTENTIAL BY THIS AFTERNOON.
THE TROUGH IN THE EAST IS EXPECTED TO REMAIN GENERALLY CUT-OFF FROM
THE STRONGER WESTERLIES...SO EASTWARD PROGRESSION WILL BE SLOW UNTIL
UPSTREAM SYSTEM FORCES AN ACCELERATION OFF ATLANTIC COASTAL AREAS
THIS WEEKEND. THIS WILL MAINTAIN SCATTERED CONVECTIVE POTENTIAL
ACROSS PARTS OF THE MID AND SOUTH ATLANTIC COASTAL STATES.
...PLAINS...
STRONGEST MID/UPPER FORCING ASSOCIATED WITH DEVELOPING WESTERN
TROUGH STILL SEEMS LIKELY TO SUPPORT CONVECTIVE INITIATION ACROSS
PARTS OF EASTERN MONTANA/WESTERN NORTH DAKOTA BY/SHORTLY AFTER THE
BEGINNING OF THE FORECAST PERIOD. ACTIVITY SHOULD BE ELEVATED ABOVE
A DEEP FRONTAL INVERSION LAYER...WITH WEAK CAPE ONLY MARGINALLY
SUPPORTIVE OF LARGE HAIL POTENTIAL. THEREAFTER...AS AN INITIAL
IMPULSE LIFTS THROUGH THE NORTHERN PLAINS...WITH ANOTHER CLOSE ON
ITS HEELS...DESTABILIZATION ACROSS MUCH OF THE NORTHERN PLAINS IS IN
QUESTION DUE TO THE POSSIBILITY OF CONSIDERABLE CLOUD COVER/LIGHT
PRECIPITATION AND DEEP LAYER SOUTHERLY FLOW.
IN THE WAKE OF THE LATTER IMPULSE...HOWEVER...VEERING MID-LEVEL FLOW
AND ASSOCIATED COOLING ALOFT IS EXPECTED TO CONTRIBUTE TO A MORE
FAVORABLE THERMODYNAMIC AND KINEMATIC ENVIRONMENT FOR SEVERE STORMS
ALONG A DEVELOPING DRY LINE...FROM SOUTH CENTRAL SOUTH DAKOTA
THROUGH WEST CENTRAL KANSAS. WEAK MID-LEVEL SUBSIDENCE PROBABLY
WILL CONTRIBUTE TO AT LEAST SOME INHIBITION...BUT SUBSTANTIAL
SURFACE HEATING SHOULD CONTRIBUTE TO CONVECTIVELY UNSTABLE
THERMODYNAMIC PROFILES WITH MIXED LAYER CAPE AROUND 1500 J/KG BY
PEAK HEATING.
FORCING BEYOND BROAD LOW-LEVEL WARM ADVECTION BENEATH WEAKLY
DIFLUENT/DIVERGENT UPPER FLOW IS UNCERTAIN...BUT AT LEAST SCATTERED
THUNDERSTORMS ARE EXPECTED TO INITIATE LATE THIS AFTERNOON.
MODERATELY STRONG AND VEERING WIND FIELDS WITH HEIGHT FROM LOWER
THROUGH MID-LEVELS WILL CREATE FAVORABLE HODOGRAPHS FOR
SUPERCELLS...WITH POTENTIAL FOR LARGE HAIL/DAMAGING WINDS... PERHAPS
AN ISOLATED TORNADO OR TWO. WITH PRIMARY UPPER IMPULSE STILL WELL
UPSTREAM...AND NOT EXPECTED TO REACH THE NORTH CENTRAL ROCKIES UNTIL
LATE TONIGHT...CONVECTIVE POTENTIAL ALONG THE DRY LINE IS EXPECTED
TO DIMINISH AFTER SUNSET.
...GREAT BASIN...
DAYTIME HEATING WILL PROBABLY BE RESTRICTED BY CONSIDERABLE CLOUD
COVER. BUT...MID-LEVEL COOLING AND FORCING IN EXIT REGION OF STRONG
MID/UPPER JET STREAK IS EXPECTED TO CONTRIBUTE TO FAVORABLE
DESTABILIZATION FOR THUNDERSTORM ACTIVITY WITH RISK OF HAIL. PEAK
CAPE VALUES MAY ONLY REACH 500 J/KG OR SO...BUT STRONG SHEAR WILL
SUPPORT UPDRAFT ROTATION...ENHANCING SEVERE POTENTIAL IN STRONGER
CELLS ACROSS PARTS OF EASTERN UTAH/WESTERN COLORADO/WESTERN
WYOMING...MAINLY LATE THIS AFTERNOON.
...FLORIDA...
BOUNDARY LAYER IS EXPECTED TO REMAIN MOIST WITH SURFACE DEW POINTS
AT OR ABOVE 70F ACROSS PARTS OF THE CENTRAL AND SOUTHERN PENINSULA
TODAY. THIS WILL CONTRIBUTE TO CAPE OF 1000 TO 2000 J/KG...WHICH
WILL LIKELY SUPPORT CONSIDERABLE VIGOROUS CONVECTIVE DEVELOPMENT AS
INHIBITION WEAKENS WITH CONTINUING APPROACH OF UPPER TROUGH AXIS.
LOW/MID-LEVEL WIND FIELDS ARE PROGGED TO REMAIN WEAK...BUT 30 TO 50
KT SOUTHWESTERLY HIGH LEVEL FLOW BY THIS AFTERNOON COULD PROVIDE
SUFFICIENT DEEP LAYER SHEAR FOR MID-LEVEL UPDRAFT ROTATION
...PERHAPS ENHANCING WET DOWNBURST POTENTIAL IN HIGH PRECIPITABLE
WATER ENVIRONMENT.
..KERR/CROSBIE.. 09/15/2006
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
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