Tropical Depression THIRTEEN

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TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL132007
500 AM EDT THU SEP 27 2007
 
REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER INVESTIGATING
TROPICAL DEPRESSION THIRTEEN INCLUDED A 1500 FT FLIGHT-LEVEL WIND
OF 36 KT IN THE NORTHEASTERN QUADRANT AND A CENTRAL PRESSURE OF
1008 MB.  SATELLITE IMAGERY AND DATA FROM THE MEXICAN RADAR AT
ALVARADO SHOW INCREASING CONVECTION IN THE SOUTHWESTERN
SEMICIRCLE...ALTHOUGH THE BANDING IS NOT WELL DEFINED.  BASED ON
THE AIRCRAFT DATA...THE INITIAL INTENSITY REMAINS 30 KT.

THE AIRCRAFT FOUND THE DEPRESSION SOUTH OF THE PREVIOUS ADVISORY
POSITION...WHILE THE THREE FIXES FROM THE MISSION SUGGEST A
WESTERLY MOTION.  PUTTING THESE TOGETHER YIELDS AN UNCERTAIN MOTION
ESTIMATE OF 245/3.  WATER VAPOR IMAGERY AND RAWINSONDE DATA SHOW
THAT THE MID/LEVEL RIDGE OVER NORTHERN MEXICO IS BUILDING EASTWARD
OVER THE GULF OF MEXICO...WHICH SHOULD STEER THE DEPRESSION
GENERALLY WESTWARD INTO SOUTHEASTERN MEXICO DURING THE NEXT 2-3
DAYS.  THE SPREAD OF THE TRACK GUIDANCE HAS DECREASED SINCE LAST
NIGHT...WITH THE MODELS NOW SPREAD BETWEEN THE WEST-SOUTHWESTWARD
TRACK GFS AND THE WEST-NORTHWESTWARD TRACKING UKMET.  THE NEW
FORECAST TRACK CALLS FOR A GENERALLY WESTWARD MOTION WITH LANDFALL
ON THE COAST OF MEXICO IN ABOUT 36 HR.  IT LIES A LITTLE SOUTH OF
THE PREVIOUS TRACK DUE TO THE INITIAL POSITION.
 
THE DEPRESSION IS UNDER AN UPPER-LEVEL ANTICYCLONE...ALTHOUGH WATER
VAPOR IMAGERY AND MODEL ANALYSES SUGGEST SOME NORTHEASTERLY
UPPER-LEVEL FLOW AND DRY AIR MAY BE TEMPORARILY UNDERCUTTING THE
ANTICYCLONE.  THE MODELS FORECAST THIS FLOW TO DECREASE...LEAVING
THE DEPRESSION IN CONDITIONS FAVORABLE FOR STRENGTHENING.  THE
INTENSITY FORECAST CALLS FOR THE CYCLONE TO REACH 50 KT AT LANDFALL
IN GOOD AGREEMENT WITH THE SHIPS AND HWRF MODELS.  THE CYCLONE
SHOULD WEAKEN AFTER LANDFALL AND DISSIPATE OVER THE MOUNTAINS OF
MEXICO BY 72 HR.

TROPICAL STORM WARNINGS COULD BE REQUIRED FOR PORTIONS OF THE GULF
COAST OF MEXICO LATER TODAY.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      27/0900Z 20.8N  95.1W    30 KT
 12HR VT     27/1800Z 20.7N  95.4W    35 KT
 24HR VT     28/0600Z 20.6N  96.0W    45 KT
 36HR VT     28/1800Z 20.6N  97.1W    50 KT
 48HR VT     29/0600Z 20.9N  98.7W    30 KT...INLAND
 72HR VT     30/0600Z...DISSIPATED
 
$$
FORECASTER BEVEN
 
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