Shortwave amplifying --> SPC ups tornado potential - Chicago in bulls-eye
Posted by JAC on 6/19/2009, 10:36 am



DAY 1 CONVECTIVE OUTLOOK 
  NWS STORM PREDICTION CENTER NORMAN OK
  0759 AM CDT FRI JUN 19 2009
 
  VALID 191300Z - 201200Z
 
  ...THERE IS A MDT RISK OF SVR TSTMS FROM THE MID MS VLY THROUGH THE
  OH VLY TO THE CNTRL APPALACHIANS...
 
  ...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK
  AREA...FROM THE CNTRL PLNS TO THE MID ATLANTIC CST...
 
  ...SYNOPSIS...
  UPR RIDGE WILL REMAIN ANCHORED OVER THE GULF CST STATES AS A BROAD
  TROUGH CONTINUES ACROSS THE WEST.  SMALLER SCALE FEATURES WITHIN THE
  WRN TROUGH WILL BE THE MAIN FACTORS GOVERNING CONVECTIVE DEVELOPMENT
  THIS PERIOD.
 
  LEAD IMPULSE NOW IN CO APPEARS TO BE AMPLIFYING ATTM IN WATER VAPOR
  IMAGERY.  THIS DISTURBANCE SHOULD CONTINUE ENE INTO WRN IA BY THIS
  EVE...BEFORE TURNING MORE EWD AND FURTHER AMPLIFYING OVER THE UPR OH
  VLY EARLY SATURDAY.  FARTHER NW...ANOTHER TROUGH...NOW APPROACHING
  THE PACIFIC NW...SHOULD MOVE SE INTO CNTRL CA BY 12Z SAT AS NW
  MEXICO UPR LOW EDGES NE TOWARD THE AZ-NM BORDER.
 
  AT LWR LEVELS...WEAK COLD FRONT NOW EXTENDING FROM MN TO THE CNTRL
  PLNS SHOULD CONTINUE STEADILY E/SE ACROSS THE UPR GRT LKS AND MUCH
  OF THE OH VLY BY THE END OF THE PERIOD...WHILE SRN PART STALLS
  OVER SRN KS/NW OK.  NW-SE WARM FRONT NOW IN THE LWR/MID OH VLY
  SHOULD MOVE/REDEVELOP ADVANCE IRREGULARLY EWD...WITH ADVANCE STYMIED
  AT TIMES BY OUTFLOW FROM ASSOCIATED CONVECTION.
 
  ...MID MS VLY/OH VLY TO CNTRL APPALACHIANS/MID ATLANTIC CST...
  MODERATE WSWLY LLJ WILL MAINTAIN/ADVECT RICH LOW LVL MOISTURE ACROSS
  THE MID MS AND LWR/MID OH VLYS TODAY ON NRN FRINGE OF GULF UPR
  RIDGE.  COUPLED WITH SFC HEATING AND EWD MOVEMENT OF RESIDUAL EML...
  EXPECT STRONG SFC-BASED INSTABILITY TO DEVELOP...WITH SBCAPE LIKELY
  TO BE AOA 3000 J/KG FROM NRN MO/SRN IA EWD INTO OH AND WRN PA/WV BY
  AFTN.
 
  TSTMS NOW OVER IND/OH ESSENTIALLY MARK SFC-850 MB WARM FRONT AND
  SHOULD CONTINUE GENERALLY ESE THROUGH AT LEAST MIDDAY.  SOME
  INTENSIFICATION MAY OCCUR WITH THIS ACTIVITY...PARTICULARLY ON THE
  WRN AND SRN FRINGES...WHERE CONVERGENCE/INSTABILITY SHOULD BE
  GREATEST.  THE COLD POOL WITH THESE STORMS MAY TEMPORARILY RETARD
  EWD ADVANCE OF SFC WARM FRONT.
 
  FARTHER W...LOOSELY-ORGANIZED STORM CLUSTER OVER IA ALSO MAY
  INCREASE WITH SFC HEATING LATER THIS MORNING.  WIND FIELD AND
  THERMODYNAMIC SETUP APPEARS FAVORABLE FOR N-S ORIENTED BOWING
  SEGMENTS WITH HIGH WIND.  DOWNSTREAM DEVELOPMENT OF STORMS LIKELY
  WILL BE COMPLICATED/AFFECTED BY OUTFLOW BOUNDARY IN NRN
  IL/IND...LEFT BY OVERNIGHT MCS NOW IN OH.  A NON-ZERO THREAT FOR
  TORNADOES COULD EVOLVE LATE THIS MORNING/EARLY THE AFTN IF
  SEMI-DISCRETE STORMS ARE ABLE TO FAVORABLY INTERACT WITH RECOVERING
  BOUNDARY LAYER AIR IMMEDIATELY N/E OF STALLING GUST FRONT.
 
  OTHER STORMS/STORM CLUSTERS LIKELY WILL FORM THIS AFTN AND EVE
  FARTHER W ACROSS PARTS OF IA/MO...IN AREA OF INCREASING UVV AHEAD OF
  APPROACHING CO/NEB SHORTWAVE TROUGH.  REGION WILL BE IN BELT OF
  ENHANCED /40-50 KT/ MID LVL WSW FLOW WITH THE TROUGH...YIELDING
  AMPLE DEEP SHEAR FOR SUPERCELLS AND BOWS.  THESE STORMS LIKELY WILL
  EVOLVE INTO ONE OR TWO SVR FORWARD-PROPAGATING MCSS.  THESE BOWING
  SYSTEMS WILL POSE A THREAT FOR WIDESPREAD DAMAGING WIND AS THEY MOVE
  RAPIDLY EWD.  LARGE HAIL COULD ALSO ACCOMPANY THE STORMS....AS WELL
  AS A FEW SUPERCELL TORNADOES.  ISOLD TORNADOES MAY ALSO OCCUR INVOF
  WARM FRONTAL SEGMENTS OVER THE MID/UPR OH VLY/CNTRL APPALACHIANS.
  OVERALL...HOWEVER...TORNADO THREAT SHOULD BE SOMEWHAT LIMITED
  RELATIVE TO STRAIGHT-LINE WINDS BY VEERED LOW LVL FLOW AND
  RELATIVELY SMALL LOW LVL HODOGRAPHS.
 
  STORMS/STORM CLUSTERS LIKELY WILL PERSIST INTO EARLY SAT FROM IND/OH
  EWD ACROSS THE CNTRL APPALACHIANS TO THE MID ATLANTIC CST GIVEN
  CONTINUED AMPLIFICATION OF UPR TROUGH AND ASSOCIATED EWD
  DESTABILIZATION DESPITE TIME OF DAY.
 
  ...MN/CNTRL-NRN WI/NRN MI...
  CLOUDS WILL LIMIT LOW LVL DESTABILIZATION HEATING FOR AT LEAST PART
  OF THE DAY OVER THE UPR MS VLY/UPR GRT LKS...AND REGION WILL LIE N
  OF STRONGEST MID/UPR LVL FLOW.  BUT SUFFICIENT FORCING FOR ASCENT
  LIKELY WILL DEVELOP TO SUPPORT A SEPARATE AREA OF STORMS LATER TODAY
  IN FAR ERN MN OR...MORE LIKELY...CNTRL/NRN WI INTO NRN MI.  THIS
  ACTIVITY COULD YIELD SVR HAIL/LOCALLY DMGG WIND THROUGH TONIGHT.
 
  ...GRT BASIN...
  A COLD FRONT ASSOCIATED WITH PACIFIC NW SHORTWAVE TROUGH WILL BE THE
  FOCUS FOR SCTD HI-BASED DIURNALLY-ENHANCED STORMS.  OTHER STORMS MAY
  FORM AHEAD OF FRONT BY EVE IN ZONE OF INCREASING UVV. INVERTED-V
  THERMODYNAMIC ENVIRONMENT WILL PROMOTE A THREAT OF ISOLD DMGG WIND
  AND MARGINALLY SVR HAIL WITH STRONGER STORMS.
 
  ..CORFIDI/GRAMS.. 06/19/2009



AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
706 AM CDT FRI JUN 19 2009

.DISCUSSION...
324 AM CDT

ONGOING CONVECTION OCCURRING AS NOSE OF LOW LEVEL JET RIDES OVER
WARM FRONT WHICH WAS FROM DBQ TO MDW AS OF 08Z. EXTREMELY MOIST
AND UNSTABLE AIRMASS SOUTH OF THE WARM FRONT WITH DEW POINTS IN
THE MID 70S AND CAPES OVER 4000 EVEN AT THIS HOUR. AREA IS FAIRLY
WELL CAPPED SOUTH OF WARM FRONT. THUS MAIN FOCUS FOR THIS MORNING
WILL BE FAR NE IL INTO NW IN. EXPECT CONVECTION TO MOVE OFF AND
WEAKEN DIURNALLY THIS MORNING. THINNING CLOUDS SHOULD ALLOW GOOD
HEATING THIS AFTERNOON AS SHORT WAVE APPROACHES FROM THE PLAINS.
DEPENDING ON CLOUD COVER FROM THIS MORNINGS MCS...COULD SEE
TEMPERATURES IN THE LOWER 90S SOUTH OF WARM FRONT...WITH THE
EXCEPTION BEING NEAR THE WI STATE LINE WHERE RAIN HAS BEEN
FALLING AND MORE RECOVERY WILL BE NEEDED.

EXPECT STRONG TO SEVERE STORMS AGAIN THIS AFTERNOON INTO THIS
EVENING. SHORT WAVE TO MOVE OUT OF THE PLAINS INTO NORTHERN IL BY
EVENING. DEW POINTS WILL CONTINUE TO POOL IN MID 70S ALONG WARM
FRONT AND CAPE VALUES COULD EXCEED 4000-5000 J/KG AGAIN. 0-6KM BULK
SHEAR WILL ALSO BE SUFFICIENT FOR ORGANIZED SEVERE STORMS. WITH
PRECIPITABLE WATER VALUES OF AROUND 2 INCHES...FLASH FLOODING WILL
ALSO BE A CONCERN.

SHOULD SEE SUBSIDENCE IN WAKE OF SHORT WAVE AND CLEARING BY
SATURDAY MORNING. MODELS THEN BRING WARM FRONT BACK NORTH
SATURDAY NIGHT AND SUNDAY UNDER BUILDING UPPER RIDGE. GFS SEEMS
THE MOST AGGRESSIVE IN BRINGING CONVECTION INTO THE AREA. ECMWF KEEPS
LOW LEVEL JET MORE FOCUSED FROM KS TO IA INTO MN WITH 850 RIDGE
ACROSS IL. WILL INTRODUCE SLIGHT CHANCE POPS BUT NOT MUCH
CONFIDENCE IN CONVECTION SPREADING THIS FAR EAST.

SHARP RIDGE BUILDS MONDAY AND TUESDAY. THIS SHOULD SUFFICIENTLY
CAP THINGS AND THUS HAVE REMOVED SLIGHT CHANCE POPS. NAM AND ECMWF
BRING 850 TEMPERATURES UP TO 20-22C...ALTHOUGH GFS NOT QUITE AS
WARM. COULD SEE SUMMER LIKE TEMPERATURES AROUND 90 INTO MID WEEK. MEDIUM
RANGE MODELS BRING SHORT WAVE ACROSS SOUTHERN CANADA AND FAR
NORTHERN US WED OR THU NEXT WEEK. THIS MAY PUSH A SURFACE FRONT
DOWN TOWARDS NORTHERN IL AND BRING THE NEXT CHANCE OF
THUNDERSTORMS. BUT WILL NOT MAKE ANY CHANGES TO EXTENDED AT THIS
TIME.

ALLSOPP

15
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Friday Afternoon Severe Weather for IA, IL, IN - JAC, 6/19/2009, 8:29 am
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