SWODY1
SPC AC 101959
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0259 PM CDT THU SEP 10 2009
VALID 102000Z - 111200Z
...THERE IS A SLGT RISK OF SVR TSTMS OVER THE NRN PLAINS LAST THIS
AFTERNOON AND TONIGHT...
...NRN PLAINS...
NO CHANGES HAVE BEEN MADE TO THE SEVERE WEATHER THREAT AREAS FOR
THIS OUTLOOK.
EARLY AFTERNOON SURFACE ANALYSES SHOWED A COLD FRONT...ATTENDANT TO
THE AMPLIFYING SHORTWAVE TROUGH...EXTENDED FROM NERN MT TO NWRN WY.
19Z BIS SOUNDING SHOWED VERY LITTLE CAPPING LEFT AROUND 750 MB...
WITH OBJECTIVE ANALYSES ALSO INDICATING SURFACE BASED INHIBITION HAS
BEEN ELIMINATED GIVEN THE COMBINATION OF LOW LEVEL MOISTURE RETURN
/SURFACE DEWPOINTS IN THE LOWER-MID 60S...SURFACE HEATING...AND
COOLING MID LEVEL TEMPERATURES. DESPITE THESE THERMODYNAMIC
PROCESSES RESULTING IN MODERATE INSTABILITY...SHORTWAVE RIDGING HAS
THUS FAR LIMITED MORE ROBUST CONVECTIVE DEVELOPMENT. INCREASING WLY
MID LEVEL FLOW WITH APPROACH OF UPPER TROUGH IS SUPPORTING
STRENGTHENING EFFECTIVE BULK SHEAR /35-45 KT/ FROM NWRN SD TO ERN
ND/NWRN MN. TIMING OF STORM INITIATION SHOULD OCCUR AROUND 21-23Z
ACROSS CENTRAL/ERN ND...WITH SIMILAR STORM MODE EVOLUTION EXPECTED
AS INDICATED BY PREVIOUS FORECAST.
VIS IMAGERY/LIGHTNING DATA INDICATED AN ISOLATED STORM OVER THE
BLACK HILLS. CONVECTION WILL REMAIN POSSIBLE ACROSS THE HIGHER
TERRAIN...BUT WEAKER INSTABILITY ACROSS FAR WRN SD MAY TEND TO LIMIT
SEVERE WEATHER THREAT ACROSS THIS PART OF THE SLIGHT RISK AREA.
..PETERS.. 09/10/2009
.PREV DISCUSSION... /ISSUED 1100 AM CDT THU SEP 10 2009/
...NRN PLAINS...
MID LEVEL TROUGH...LOCATED OVER SRN ALBERTA THIS MORNING...IS
FORECAST TO DIG SEWD INTO NERN MT LATE AFTERNOON AND INTO CENTRAL ND
BY EARLY FRIDAY MORNING. ACCOMPANYING COLD FRONT...CURRENTLY MOVING
SEWD THROUGH CENTRAL MT...IS FORECAST TO EXTEND FROM FAR NWRN MN
SWWD INTO SERN WY BY 11/12Z. SLY FLOW AHEAD OF THE FRONT HAS
ADVECTED RICHER BOUNDARY LAYER MOISTURE INTO CENTRAL SD...AND SHOULD
SPREAD NNEWD ACROSS CENTRAL/SRN ND THIS AFTERNOON. THE COMBINATION
OF STRONG INSOLATION...SURFACE DEWPOINTS IN THE LOW 60S AND STEEP
MID LEVEL LAPSE RATES WILL SUPPORT MLCAPE VALUES OF 1500-2500 J/KG
LATE THIS AFTERNOON. THUNDERSTORMS MAY FORM BY LATE
AFTERNOON...AFTER 22Z...IN ERN ND IN REGION OF WEAK WARM ADVECTION.
HOWEVER...MORE WIDESPREAD THUNDERSTORM COVERAGE SHOULD OCCUR AFTER
DARK AS LARGE SCALE FORCING STRENGTHENS. DEEP LAYER SHEAR AT 35-40
KT FAVORS A MIX OF MULTICELL CLUSTERS AND SUPERCELL STRUCTURES. THE
THERMODYNAMIC STRUCTURE APPEARS MOST FAVORABLE FOR HAIL WITH THE
EARLY STORMS...BUT THE STORMS ARE EXPECTED TO CONSOLIDATE INTO A
LINEAR SYSTEM OVERNIGHT ALONG THE COLD FRONT...WITH WIND DAMAGE
BECOMING THE GREATER THREAT AT THAT TIME.
...FAR ERN DELMARVA AND SRN NJ...
BROAD UPPER LOW WAS SITUATED IN THE VA/WV AREA THIS MORNING...WITH
SEVERAL VORTICITY CENTERS ROTATING AROUND THIS SYSTEM. PAST AND
LATEST MODEL FORECASTS HAVE BEEN CONSISTENT IN SUGGESTING THAT AS
ONE OF THESE VORT MAXES ROTATES NWWD INTO THE MID ATLANTIC
STATES...THE OFFSHORE LOW...LOCATED ABOUT 150 NM EAST OF THE NC
OUTER BANKS AT 15Z...WOULD MOVE NWWD AND INTO THE NRN DELMARVA AREA
LATE TONIGHT. THIS WOULD ADVECT RICHER ATLANTIC MOISTURE WWD AND
STRONG AMBIENT LOW/DEEP LAYER SHEAR MIGHT SUPPORT A SUPERCELL OR TWO
WITH A VERY LOW TORNADO/SEVERE WIND POTENTIAL. HOWEVER...THE GREATER
INSTABILITY AND ASSOCIATED WARM SECTOR IS MOST LIKELY TO REMAIN
OFFSHORE...SO ONLY LOW TORNADO PROBABILITIES ARE FORECAST ALONG THE
COAST FROM FAR ERN MD NEWD INTO SRN NJ.
...NWRN GULF/TX COASTAL REGION...
WIDESPREAD CONVECTION WAS LOCATED ACROSS SRN TX AND THE NWRN
GULF...ALONG AND NORTH OF A SURFACE TROUGH IN THE GULF...AND AHEAD
OF AN UPPER WAVE LOCATED NEAR LRD. WIDESPREAD CLOUDS/PCPN ALONG WITH
WEAK ENVIRONMENTAL WINDS SHOULD LIMIT THE SEVERE THREAT...THOUGH PW
VALUES FROM 2.25-2.50 INCHES MAY SUPPORT LOCALLY HEAVY RAINFALL
RATES.
...AZ/NM THIS AFTERNOON/EVENING...
WEAK MID/HIGH LEVEL CIRCULATION IN THE GRAND CANYON REGION COMBINED
WITH STRONG HEATING WILL SUPPORT SCATTERED AFTERNOON/EVENING
THUNDERSTORMS. THE SEVERE THREAT APPEARS REMOTE AS THE UPDRAFTS WILL
BE DISORGANIZED AND SHORT-LIVED DUE TO WEAK WINDS/SHEAR...ESPECIALLY
FROM THE SURFACE-500 MB.
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