SWODY2
SPC AC 190511
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1211 AM CDT SAT SEP 19 2009
VALID 201200Z - 211200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
STRONG...POLAR JET STREAK DIGGING SEWD FROM THE PACIFIC NW INTO
CNTRL ROCKIES WILL SUPPORT INTENSIFICATION OF LONG WAVE TROUGH OVER
THE CNTRL CONUS THIS PERIOD. THE MOST NOTABLE REGION OF HEIGHT
FALLS/DYNAMIC FORCING FOR ASCENT ASSOCIATED WITH THIS SYSTEM WILL
SPREAD FROM THE NRN INTERMOUNTAIN REGION INTO THE NRN AND CNTRL HIGH
PLAINS. MEANWHILE...SEVERAL VORTICITY MAXIMA LOCATED DOWNSTREAM
FROM LONG WAVE TROUGH AXIS WILL LIFT NEWD THROUGH THE MID MS VALLEY
INTO UPPER GREAT LAKES.
...CNTRL HIGH PLAINS INTO MID MO VALLEY...
EWD ADVECTION OF EML PRECEDING MIDLEVEL TROUGH WILL OCCUR ATOP OF
MOISTENING PRE-FRONTAL BOUNDARY LAYER...SUPPORTING A MODERATELY
UNSTABLE AIR MASS /MLCAPE OF 1000-1500 J PER KG/ BY AFTERNOON OVER
THE CNTRL PLAINS. CAPPING AT THE BASE OF EML WILL LIKELY INHIBIT
SURFACE-BASED STORM FORMATION THROUGH THE DAY ACROSS MUCH OF WARM
SECTOR. THE EXCEPTIONS WILL BE ALONG COLD FRONT OVER WY AND
PRE-FRONTAL TROUGH ALONG THE CNTRL HIGH PLAINS WHERE COMBINATION OF
LOW-LEVEL CONVERGENCE AND DEEPER FORCING ASSOCIATED WITH AMPLIFYING
MIDLEVEL TROUGH OVERCOME CONVECTIVE INHIBITION. THIS STORM
DEVELOPMENT IS EXPECTED TO OCCUR W OF PRIMARY INSTABILITY AXIS
WITHIN ENVIRONMENT OF STEEP LAPSE RATES BUT MARGINAL CONVECTIVE
INSTABILITY. THIS MAY TEND TO LIMIT THE POTENTIAL FOR A MORE
ORGANIZED SEVERE THREAT...THOUGH THE STRONGER STORMS WILL BE CAPABLE
OF GUSTY WINDS AND SOME HAIL.
ADDITIONAL STORMS /LIKELY SLIGHTLY ELEVATED/ ARE EXPECTED TO DEVELOP
AHEAD OF COLD FRONT SUNDAY NIGHT OVER PORTIONS OF THE MID MO VALLEY
WITHIN ZONE OF STRENGTHENING LOW-LEVEL WAA. THE PRESENCE OF THE
STEEP MIDLEVEL LAPSE RATES AND MODEST ELEVATED CONVECTIVE
INSTABILITY WILL BE SUPPORTIVE OF STRONGER UPDRAFTS CAPABLE OF
MAINLY HAIL...WARRANTING LOW SEVERE PROBABILITIES.
...OH/TN VALLEYS...
WEAK SURFACE LOW ASSOCIATED WITH NEWD-EJECTING VORTICITY MAXIMA IS
FORECAST TO DEVELOP NNEWD FROM THE LOWER OH VALLEY INTO LOWER
MI...IN CONJUNCTION WITH WARM FRONT. THIS WILL ALLOW FOR THE NWD
ADVECTION OF A MOIST LOW-LEVEL AIR MASS THROUGH THE OH/TN
VALLEYS...SUPPORTING AN AXIS OF SLIGHT TO MODERATE CONVECTIVE
INSTABILITY /MLCAPE OF 500-1000 J PER KG/. GENERALLY WEAK CAPPING
AND BROAD ASCENT ACROSS WARM SECTOR ARE EXPECTED TO FOSTER CLUSTERS
OF DIURNAL STORMS ACROSS THE REGION...WHICH WILL LIKELY CONTINUE
INTO SUNDAY NIGHT.
LATEST MODEL GUIDANCE SUGGESTS THAT DEEP-LAYER SHEAR WILL REMAIN
SOMEWHAT MARGINAL /I.E. AOB 30 KT THROUGH 6 KM/ WHICH SHOULD LIMIT A
MORE SUBSTANTIAL SEVERE THREAT. HOWEVER...LOCALLY STRONGER
LOW-LEVEL SHEAR IS FORECAST INVOF WARM FRONT AND ALONG LOW TRACK.
AND GIVEN THE MOIST BOUNDARY LAYER CONDITIONS...SOME POTENTIAL WILL
EXIST FOR BRIEF SUPERCELL STRUCTURES CAPABLE OF LOCALLY GUSTY WINDS
AND PERHAPS A BRIEF TORNADO.
..MEAD.. 09/19/2009
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