Severe Thunderstorm Warning 73, NWS Detroit, MI

SOUTHEAST MICHIGAN AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
330 PM EDT FRI SEP 7 2001

SATELLITE AND 88D LOOPS SHOW SHOWERS AND THUNDERSTORMS BREAKING OUT
IN THE OHIO VALLEY AND MOVING NORTHEAST TOWARD SOUTHEAST LOWER. BEST
ORGANIZATION IS OVER EASTERN ILLINOIS CLOSER TO UPPER JET SUPPORT.
MODELS INDICATE JET SUPPORT TO STAY WILL WEST OF MICHIGAN AND WITH
LOSS OF DIURNAL HEATING...ANTICIPATE JUST ORDINARY MULTICELL
CONVECTION INTO THE WESTERN PORTION OF THE CWA BEFORE IT DIES OUT BY
MIDNIGHT. THUNDERSTORMS HAVE ALSO BROKEN OUT IN RICHER AND DEEPER
THETA-E AIR ACROSS THE OHIO VALLEY...WITH MODELS SHOWING RICHER
THETA-E AIR CIRCUMNAVIGATING SOUTHEAST LOWER THIS EVENING...ONCE
AGAIN SUPPORTING THE IDEA THAT BEST CONVECTION WILL STAY OUT OF
SOUTHEAST LOWER. UNIDIRECTIONAL WIND PROFILES OVER SOUTHEAST LOWER
IN ADDITION TO MODEST 45 KNOT MID LEVEL WIND FIELD SHOULD PRECLUDE
ANY SEVERE CONVECTION AS WELL. CAPES HAVE CLIMBED UP TO AROUND 3000
J/KG THOUGH AS SKIES REMAINED MOSTLY SUNNY THROUGH A GOOD PART OF
THE AFTERNOON...LIS RUNNING AROUND -8...BUT AXIS OF GREATEST
INSTABILITY REMAINS OVER THE NORTHERN OHIO VALLEY. JUICY AIR
ESTABLISHED ACROSS ALL OF LOWER MICHIGAN WITH 70 DEGREE DEW POINTS
ALL THE WAY TO THE BRIDGE. THUS OVERNIGHT LOWS WILL BE HARD PRESSED
TO DIP BELOW 70.

MODELS IN GENERAL AGREEMENT THROUGH 60 HOURS...HIGH AMPLITUDE
SOUTHWEST FLOW IS MAKING EASTWARD PROGRESSION OF COLD FRONT VERY
SLOW. FRONT IS PARALLEL TO THE MID LEVEL FLOW AT THIS TIME AND
MODELS SHOW SEVERAL WAVES DEVELOPING ALONG THE FRONT THROUGH MONDAY.
AS UPPER TROF DEEPENS SEVERAL UPPER WAVES MOVE NORTHEAST AND RUN
OVER THE TOP OF THE FRONT INDUCING SURFACE WAVES. THUS FRONTAL
PASSAGE MAY BE SLOWED DOWN BY AS MUCH AS 24 HOURS...STRUGGLING TO
CLEAR THE CWA BY SUNDAY AFTERNOON...ONLY TO BE BOWED BACK TO THE
NORTH AS WAVE COMES UP THROUGH THE OHIO VALLEY.

HOMOGENEOUS AIR MASS OVER SOUTHEAST LOWER NOW AND LITTLE IN THE WAY
OF WARM AIR ADVECTION. +17C 850 MB AIR DID A GOOD JOB IN CAPPING OFF
CONVECTION TODAY AND THERMAL AXIS WILL REMAIN NEARLY STATIONARY OVER
THE CWA THROUGH SUNDAY MORNING...850 MB THETA-E RIDGE AXIS AS WELL.
THUS WILL HAVE TO CONTINUE WITH SCATTERED POPS THROUGH SUNDAY
MORNING...DUE TO LACK OF UPPER SUPPORT...WITH HIGHER DAYTIME/DIURNAL
POPS. ETA SHOWS A WEAK UPPER WAVE MOVING THROUGH THE CWA AROUND 06Z
TONIGHT...BUT FEATURE IS SO WEAK THAT IT CANNOT BE DISCERNED ON
SATELLITE. WILL HAVE TO WAIT FOR BETTER CONVERGENCE ALONG AND AHEAD
OF THE FRONT FOR ANY ORGANIZED CONVECTION. HOWEVER MODELS SHOW LOW
LEVEL FRONTAL CONVERGENCE WEAKENING AS IT MOVES INTO SOUTHEAST
LOWER...AND INTO RIDGE AXIS. THUS WILL CUT GUIDANCE POPS BACK TO
CHANCE CATEGORY FOR SATURDAY NIGHT AND SUNDAY MORNING. GUIDANCE
TEMPERATURES LOOK A LITTLE TO COOL IN THE LATER PERIODS...BASED ON
SLOWER FRONTAL MOVEMENT AND WAVE BOWING FRONT BACK TO THE NORTH
SUNDAY NIGHT. MODELS ALSO WEAK IN REGARD TO CAPES...ANTICIPATE SAME
LEAN MODEL BIAS ON SATURDAY...WITH CAPES LIKELY AROUND 2000-2500J/KG
RATHER THAN 1000J/KG SUGGESTED BY THE ETA DTW SOUNDING.

.DTX...NONE.

GSS