Tropical Storm Karen - 10am Friday Discussion

By: National Hurricane Center
By: National Hurricane Center

TROPICAL STORM KAREN DISCUSSION NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL AL122013
1000 AM CDT FRI OCT 04 2013

AFTER BEING DISPLACED WELL EAST OF THE LOW-LEVEL CENTER OF KAREN... DEEP CONVECTION HAS RECENTLY DEVELOPED A LITTLE CLOSER TO THE CENTER OVER THE PAST HOUR OR TWO...BUT THE CYCLONE IS STILL BEING AFFECTED BY 20-25 KT OF WESTERLY SHEAR AND DRY AIR BEING ADVECTED INTO THE CYCLONE. THE LAST CENTRAL PRESSURE REPORTED BY THE AIRCRAFT WAS 1003 MB...AND BASED ON THE LATEST ROUND OF AIRCRAFT DATA THE INTENSITY HAS BEEN REDUCED TO 45 KT. THE ENVIRONMENT DOES NOT LOOK FAVORABLE FOR SIGNIFICANT INTENSIFICATION...WITH MODERATE SHEAR EXPECTED TO CONTINUE FOR THE NEXT DAY OR TWO. HOWEVER...IF THE SHEAR DOES LESSEN...EVEN FOR A SHORT PERIOD OF TIME...DEEP CONVECTION COULD RE-DEVELOP CLOSER TO THE CENTER AND ALLOW FOR SOME INTENSIFICATION. IN ADDITION...BY 48 HOURS UPPER-LEVEL DIVERGENCE AHEAD OF AN APPROACHING MID/UPPER-LEVEL TROUGH COULD ALLOW FOR SOME STRENGTHENING. THE NEW NHC INTENSITY FORECAST HAS BEEN ADJUSTED DOWNWARD AND SHOWS LITTLE CHANGE IN THE NEXT 24 HOURS AND A STRENGTHENING TO 55 KT BY 48 HOURS.

THE INITIAL MOTION HAS BEEN WOBBLING BETWEEN 325 AND 330 DEGREES AT ABOUT 9 OR 10 KT OVER THE PAST FEW HOURS. THE FUTURE TRACK WILL BE QUITE SENSITIVE TO THE STRUCTURE OF KAREN OVER THE NEXT COUPLE OF DAYS. IN THE SHORT TERM A WEAKER SHALLOWER SYSTEM WILL BE STEERED MORE TOWARD THE LEFT BY THE LOW-LEVEL FLOW...WHILE A DEEPER MORE VERTICALLY COHERENT CYCLONE WOULD TURN NORTHWARD MORE QUICKLY DUE TO A MID-LEVEL RIDGE TO THE EAST. GIVEN THE CURRENT APPEARANCE OF KAREN AND THE CONTINUED SHEAR...THE NHC TRACK HAS BEEN ADJUSTED A LITTLE TO THE LEFT DURING THE FIRST 24 HOURS TOWARD THE UKMET AND ECMWF MODELS...WHICH SHOW A WEAKER SYSTEM. ALL OF THE GUIDANCE SHOWS A NORTHEASTWARD TURN IN 36 TO 48 HOURS...BUT WITH LARGE DIFFERENCES IN THE LATITUDE AT WHICH THE TURN OCCURS AND SIGNIFICANT SPREAD IN WHERE THE CENTER CROSSES THE COAST. AT 36 HOURS AND BEYOND THE NHC TRACK IS CLOSE TO THE ECMWF AND THE MULTI-MODEL CONSENSUS...AND CLOSE TO THE PREVIOUS OFFICIAL FORECAST. GIVEN THE LARGE SPREAD AND THE DEPENDENCE OF THE TRACK ON THE STRUCTURE AND INTENSITY OF KAREN...CONFIDENCE IN THE DETAILS OF THE TRACK FORECAST...AND THEREFORE THE DISTRIBUTION OF IMPACTS ALONG THE COAST...IS LOW.

THE NOAA GULFSTREAM-IV JET SAMPLED THE ENVIRONMENT NEAR KAREN AND OVER THE NORTHERN GULF OF MEXICO OVERNIGHT. THESE DATA WILL BE INCORPORATED INTO THE 12Z MODEL CYCLE...AND HOPEFULLY IMPROVE THE ANALYSIS OF THE ENVIRONMENT NEAR AND AHEAD OF THE STORM.

GIVEN THE WESTWARD SHIFT IN THE NHC TRACK...THE TROPICAL STORM WARNING HAS BEEN EXTENDED WESTWARD TO MORGAN CITY LOUISIANA. GIVEN THE WEAKENING TREND AND THE REDUCTION IN THE INTENSITY FORECAST...IT HAS BECOME A LITTLE LESS LIKELKY THAT KAREN WILL REACH HURRICANE STRENGTH. IF THESE TRENDS CONTINUE...THE HURRICANE WATCH COULD BE CHANGED TO A TROPICAL STORM WATCH OR WARNING LATER TODAY.


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