323  
FXUS64 KMOB 282345  
AFDMOB  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE MOBILE AL  
645 PM CDT FRI APR 28 2017  
   
DISCUSSION
 
UPDATED FOR LATEST AVIATION DISCUSSION BELOW.  
 

 
   
AVIATION
 
 
29/00Z ISSUANCE...WELL ESTABLISHED SOUTHERLY FLOW PERSISTS IN THE  
NEAR TERM WITH THE MAIN HAZARDS TO APPROACHES AND DEPARTURES BEING  
WIND GUSTS FROM 25 TO 30 KNOTS AT TIMES, ESPECIALLY AFTER AROUND  
29/03Z THIS EVENING AND CONTINUING THROUGH THE DAY ON SATURDAY.  
MVFR CEILINGS THRU AROUND 29/03Z, LOWERING AT TIMES TO IFR  
CATEGORIES OVERNIGHT. CEILINGS CONTINUE INTO THE DAY ON SATURDAY,  
GENERALLY IN MVFR RANGE. 12/DS  
 

 
   
PREV DISCUSSION
 
/ISSUED 308 PM CDT FRI APR 28 2017/  
 
NEAR TERM /NOW THROUGH SATURDAY/...HIGHLY AMPLIFIED PATTERN  
EVOLVES IN THE NEAR TERM WITH DOWNSTREAM UPPER RIDGE FROM THE  
NORTHWEST CARIBBEAN, NORTHWARD ACROSS THE FLORIDA PENINSULA WHILE  
UPPER LEVEL STORM SYSTEM SHARPENS AS IT MOVES EASTWARD OUT OF THE  
FOUR CORNERS OF THE DESERT SOUTHWEST BY SATURDAY. PRECIPITABLE WATER  
LOWERS A QUARTER INCH OR SO TO AROUND 1 INCH BEFORE TRENDING UPWARD  
AGAIN THROUGH THE DAY SATURDAY. AT SAME TIME, A LEAD IMPULSE OVER  
EAST TEXAS, EJECTS NORTHEAST OUT OF THE BASE OF THE LONG WAVE TROF  
POSITION AND APPROACHES THE CENTRAL GULF COAST DURING THE AFTERNOON  
WHEN INSTABILITY WOULD SUPPORT A MENTION OF ISOLATED STORMS.  
PRESSURE GRADIENT FORCE TIGHTENS BETWEEN SURFACE RIDGE OFF THE  
SOUTHEAST US COAST AND LOWERING PRESSURE OVER THE PLAINS RESULTING  
IN INCREASING SOUTHERLY FLOW TO WIND ADVISORY CRITERIA ALONG THE  
COASTAL ZONES ON SATURDAY. THE ADVISORY MAY NEED TO BE EXTENDED  
NORTHWARD OVER THE INTERIOR BUT WOULD LIKE TO GET ANOTHER RUN OF  
GUIDANCE TO ASSESS MIXING AND LOW LEVEL WIND STRENGTH BEFORE DOING  
SO THERE. STRENGTH/DURATION OF SOUTHERLY FLOW ALONG THE COAST, LONG  
PERIOD SWELL AND SPRING TIDAL CYCLES BRING MULTIPLE POTENTIAL  
PROBLEMS ALONG THE COAST, WITH A HIGH RISK OF DANGEROUS TO  
POTENTIALLY DEADLY RIP CURRENTS, HIGH SURF AND RUNUP OVER LOWER  
LYING AREAS NEAR SHORE. LITTLE CHANGE IN TEMPERATURES, LOWS IN THE  
LOWER TO MID 70S. HIGHS ON SATURDAY IN THE MID TO UPPER 80S NORTH OF  
THE BEACHES. /10  
 
SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...THINGS APPEAR ON  
TRACK FROM PREVIOUS THINKING WITH REGARD TO SUNDAY'S POTENTIAL SEVERE  
WEATHER EVENT, BUT FIRST, A BRIEF MENTION OF SATURDAY NIGHT. DUE TO  
A LEAD SHORTWAVE, WE MAY BE DEALING WITH LEFT-OVER LATE AFTERNOON  
AND EVENING CONVECTION. IF SO, DEEP-LAYER VERTICAL WIND SHEAR AND  
INSTABILITY WILL BE GREATEST ENOUGH WEST OF I-65 TO PERHAPS HAVE  
A FEW MULTICELLS THAT MAY NEED TO BE MONITORED CLOSELY. SATURDAY  
NIGHT WILL BE VERY WARM, HUMID AND BREEZY AS APPROACHING SYSTEM'S  
EFFECTS RAMP UP. LOWS IN THE LOW AND MID 70S.  
 
FROM EARLY SUNDAY THROUGH EARLY MONDAY, REGION WILL BE AFFECTED BY  
SHOWERS AND THUNDERSTORMS, SOME SEVERE, AND ESPECIALLY WEST OF  
I-65 SUNDAY AFTERNOON AND EVENING (NOT SO MUCH EAST OF I-65 AT  
THIS TIME). UP FRONT, THE AMOUNT OF SFC-1 KM MLCAPE AND DEEP LAYER  
VERTICAL WIND SHEAR IS SUFFICIENT TO SUPPORT DEEP MESOCYCLONES,  
MOST OF WHICH WE WILL BE EMBEDDED WITHIN A QUASI-LINEAR CONVECTIVE  
SYSTEM IN THE FORM OF LEWPS\BOWS AS IT MOVES IN FROM THE WEST ON  
SUNDAY AND THEN PROGRESSES TO AND THROUGH OUR EASTERN ZONES LATE  
SUNDAY EVENING AND OVERNIGHT. THE EVENT WILL BE RATHER PROLONGED  
IN NATURE IN THE SENSE THAT IT WILL TAKE A WHILE TO CLEAR OUR  
EASTERN ZONES (4 AM- SUNRISE MONDAY). AREAS WEST OF I-65 WILL HAVE  
TO BE CLOSELY MONITORED FOR RAINFALL VALUES EXCEEDING FLASH FLOOD  
GUIDANCE AS ANOMALOUSLY ABSOLUTE VALUES OF MOISTURE, MOISTURE  
FLUX AND LOW- LEVEL WARMTH PROVIDE THE IMPETUS FOR DEEP AND VERY  
WARM MIXED LAYERS WHICH FAVOR GREAT PCPN EFFICIENCY IN THE WARM  
RAIN PROCESS. THE CURRENT RAINFALL THREAT GRAPHIC DEPICTS THE  
THREAT AREA NICELY. [IT IS IMPORTANT TO NOTE THAT THE MAIN MID AND  
UPPER SYSTEM TRACKS VERY FAR TO OUR WEST AND NORTH (ESPECIALLY  
COMPARED TO PAST FEW EVENTS) AND THAT SOME OF THE MAIN SUPPORT  
WILL BE PULLING AWAY AS THE QLCS ADVANCES EAST OF I-65 LATE  
SUNDAY. WE CURRENTLY THINK THE CURRENT DAY-3 OUTLOOK IS HATCHED  
ABOUT RIGHT GIVEN THE WEAKENING TREND AS THE SYSTEM APPROACHES  
LATE SUNDAY.] ONE ADDITIONAL THING WE DO SEE IS THAT WE HAVE TO BE  
ON THE LOOKOUT FOR A STRONG COLD POOL FORMATION (DUE TO  
EVAPORATIONAL COOLING) WITH THE QLCS GIVEN THAT DRY MID- LEVEL AIR  
IS ADVECTING IN SHARPLY FROM THE WEST RIGHT ON THE EDGE OF THE  
DEEP-LAYER MOISTURE SUNDAY. IF THE COLD POOL BECOMES DOMINANT AS  
THE DEEP-LAYER VWS WEAKENS, IT COULD SPEED THINGS ALONG,  
PARTICULARLY ALONG AND NORTH OF HIGHWAY-84 AND EAST OF I-65. AS  
THE SYSTEM EXITS, DRY WEATHER RETURNS FOR MONDAY AND INTO TUESDAY.  
HIGHS MOSTLY IN THE 70S ON MONDAY WITH OVERNIGHT LOWS IN THE 50S  
BY TUESDAY MORNING. ANY LINGERING COASTAL FLOODING WILL CEASE  
EARLY MONDAY AS WINDS SHIFT OFFSHORE. WIND ADVISORY CONDITIONS  
WILL ALSO BE ON THE WANE AS THE QLCS PASSES. /23 JMM  
 
LONG TERM /TUESDAY THROUGH FRIDAY/...CONTINUED DRY WITH TEMPERATURES  
IN THE LOWER 80S ON TUESDAY. LOWS IN THE UPPER 50S AND LOWER 60S  
WEDNESDAY. AFTER, PREPARE TO GET WET ONCE AGAIN. THE SYNOPTIC  
SITUATION CHANGES QUICKLY WEDNESDAY AS GUIDANCE HINTING AT ANOTHER NOTEWORTHY  
PRECIPITATION EVENT THROUGH REMAINDER OF THE PERIOD. AT FIRST, OVERRUNNING  
PRECIPITATION DEVELOPS ON WEDNESDAY AND SPREADS NORTHEASTWARD OVER  
THE AREA DURING THE ENSUING 12-18 HOURS. THIS IS QUICKLY FOLLOWED BY  
AN APPROACHING DEVELOPING FRONTAL SYSTEM (WHICH EVENTUALLY BECOMES  
CYCLOGENETIC JUST EAST OF OUR AREA BY PERIOD'S END) THAT WILL BRING  
A SECOND DOSE OF RAIN IN THE WAKE OF THE FIRST. RAIN IS FORECAST  
TO END FROM THE WEST ON FRIDAY AS A BLAST OF SIGNIFICANTLY COOLER  
AND DRIER AIR IS FORECAST TO OVERSPREAD THE REGION EITHER LATE  
THURSDAY OR EARLY FRIDAY. THE TIMING OF FRONTAL PASSAGE IS  
UNCERTAIN WITHIN ABOUT 6-12 HOURS, SO STAY TUNED. /23 JMM  
 
MARINE...ONSHORE FLOW IS FORECAST TO REMAIN STRONG GOING INTO THE  
WEEKEND AS A FRONTAL BOUNDARY EJECTS EASTWARD OUT OF THE PLAINS  
SATURDAY INTO THE LOWER MISSISSIPPI RIVER VALLEY ON SUNDAY. THE  
STRENGTH AND DURATION OF THE ONSHORE FETCH THIS WEEKEND CAUSES A  
BUILDING TREND IN SEAS. SMALL CRAFT ADVISORIES IN EFFECT. SHOWERS  
AND STORMS TO INCREASE IN COVERAGE LATE SUNDAY THRU EARLY MONDAY.  
SOME OF THE STORMS COULD BE STRONG TO SEVERE. /10  
 

 
   
MOB WATCHES/WARNINGS/ADVISORIES
 
 
AL...HIGH RIP CURRENT RISK THROUGH LATE SUNDAY NIGHT FOR ALZ265-266.  
 
WIND ADVISORY FROM 6 AM SATURDAY TO MIDNIGHT CDT SUNDAY NIGHT  
FOR ALZ263>266.  
 
COASTAL FLOOD ADVISORY UNTIL 4 AM CDT MONDAY FOR ALZ265-266.  
 
HIGH SURF ADVISORY FROM NOON SATURDAY TO 6 AM CDT MONDAY FOR  
ALZ265-266.  
 
FL...HIGH RIP CURRENT RISK THROUGH LATE SUNDAY NIGHT FOR FLZ202-204-  
206.  
 
WIND ADVISORY FROM 6 AM SATURDAY TO MIDNIGHT CDT SUNDAY NIGHT  
FOR FLZ201>206.  
 
COASTAL FLOOD ADVISORY UNTIL 4 AM CDT MONDAY FOR FLZ202-204-206.  
 
HIGH SURF ADVISORY FROM NOON SATURDAY TO 6 AM CDT MONDAY FOR  
FLZ202-204-206.  
 
MS...NONE.  
GM...SMALL CRAFT ADVISORY UNTIL 10 PM CDT SUNDAY FOR GMZ630>635.  
 
SMALL CRAFT ADVISORY UNTIL 1 AM CDT MONDAY FOR GMZ650-655-670-  
675.  
 

 
 

 
 
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