917  
FXUS63 KDTX 291737  
AFDDTX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI  
137 PM EDT WED JUL 29 2015  
 
   
AVIATION
 
 
A COLD FRONT WILL MOVE ACROSS LOWER MICHIGAN THIS AFTERNOON AND  
EARLY EVENING. CEILINGS WILL DROP TO BETWEEN 2500-4000 FEET OVER THE  
NEXT FEW HOURS AS MOISTURE POOLS AHEAD OF THE FRONT. A LINE OF  
SHOWERS AND THUNDERSTORMS...JUST NOW FORMING AT 17Z...WILL FILL IN  
AND EXPAND SOUTHWARD. THIS WILL AFFECT SOUTHEAST MICHIGAN TERMINALS  
BETWEEN 17-22Z. MOST LIKELY WINDOW REMAINS AS A TEMPO GROUP IN EACH  
TAF. A MODEST WIND SHIFT TO THE WEST WILL OCCUR WITH THE  
FRONT...FOLLOWED BY MUCH DRIER AIR AND CLEARING SKIES TONIGHT. POST-  
FRONTAL AIRMASS WILL NOT BE MUCH COOLER TOMORROW...BUT BREEZY WEST  
WINDS ARE EXPECTED WITH GUSTS TO 25 KTS IN THE AFTERNOON AND EARLY  
EVENING.  
 
FOR DTW...CEILINGS WILL CONTINUE TO FILL IN AND LOWER THROUGH 19Z.  
ALTHOUGH HEIGHT IS A LITTLE UNCERTAIN GIVEN DEVELOPING  
CONVECTION...MOST PROBABLE CEILING LOOKS TO BE AT 3500 FEET.  
GREATEST WINDOW FOR THUNDERSTORMS TO IMPACT THE TERMINAL WILL BE  
BETWEEN 19Z AND 22Z.  
 
 
//DTW THRESHOLD PROBABILITIES...  
 
* HIGH IN CEILINGS BELOW 5000 FT THIS AFTERNOON.  
 
* MEDIUM IN THUNDERSTORMS MID-LATE AFTERNOON.  
 

 
   
PREV DISCUSSION
 
ISSUED 1210 PM EDT WED JUL 29 2015  
 
UPDATE...  
FORECAST STILL ON TRACK AND UPDATE WAS ONLY TO CLEAN UP  
CLOUDS/POPS IN THE VERY NEAR-TERM. COLD FRONT NOW PUSHING INTO  
WESTERN MICHIGAN SHOULD WORK WITH WARM AND HUMID AIRMASS...TEMPS  
IN THE MID/UPPER 80S AND DEWPOINTS PRIMARILY IN THE LOW 70S...TO  
TRIGGER SHOWERS AND THUNDERSTORMS THIS AFTERNOON AND EARLY  
EVENING. COVERAGE STILL EXPECTED TO BE SCATTERED WITH WEAK  
FRONTAL FORCING. BETTER DYNAMICS AND HEIGHT FALLS REMAIN NORTH OF  
THE AREA. HI-RES MODELS STILL SHOWING HIGHER COVERAGE BUT ARE NOT  
HANDLING LACK OF CONVECTION TO OUR WEST WELL. POTENTIAL STILL  
EXISTS FOR A FEW STRONGER STORMS THIS AFTERNOON AS ML CAPE VALUES  
RISE TO 1000-1500 J/KG. COLD FRONT AND STORMS SHOULD EXIT DURING  
THE EVENING.  
 
PREV DISCUSSION...ISSUED 354 AM EDT WED JUL 29 2015  
 
SHORT TERM...TODAY AND TONIGHT  
 
SOUTHERN EDGE OF THE LOWER HEIGHTS EMANATING FROM THE LARGE UPPER  
LOW LIFTING INTO WESTERN ONTARIO STEADILY SHIFTING ACROSS THE UPPER  
MS VALLEY EARLY THIS MORNING. WHILE THE GREATER HEIGHT FALLS AND  
ASSOCIATED UPPER JET DYNAMICS WILL ACCOMPANYING THE PARENT LOW INTO  
SOUTHERN CANADA TODAY...THIS EVOLUTION WILL FORCE A COLD FRONT  
THROUGH SE MICHIGAN DURING THE AFTERNOON PERIOD. A SOLID BUT NARROW  
CORRIDOR OF PRE-FRONTAL MOISTURE/THETA-E TRANSPORT WILL SQUEEZE  
NORTHEAST AND INTO THE REGION THROUGHOUT THE FIRST HALF OF THE DAY.  
CONCURRENT WEAK MID LEVEL DESTABILIZATION AS THIS PROCESS UNFOLDS  
WILL PROVIDE AN INITIAL LIMITED OPPORTUNITY FOR ISOLATED CELLS TO  
EMERGE ALONG THE LEAD EDGE. LATEST RADAR TRENDS AND HI RES GUIDANCE  
WOULD FAVOR THE TRI-CITIES FOR A LOW END POP MENTION FOR POSSIBLE  
MORNING ACTIVITY.  
 
GREATEST WINDOW FOR SHOWER/THUNDERSTORM DEVELOPMENT AND EXPANSION  
FOCUSED WITH A MID-LATE AFTERNOON FRONTAL PASSAGE /18Z-22Z/. THE  
EXTENSIVE MID/HIGH CLOUD DECK LOOMING JUST UPSTREAM CONTINUES TO  
CAST SOME DOUBT AS TO POTENTIAL PRE-FRONTAL DESTABILIZATION...SHOULD  
AN EARLY ARRIVING THICKER CANOPY MORE SIGNIFICANTLY UNCUT THE  
DIURNAL HEATING PROCESS. DESPITE THIS...HIGHS OF MID AND UPPER 80S  
ON TARGET GIVEN THE RESIDENT AIRMASS. GIVEN A DEWPOINT CLOSING IN  
ON 70 DEGREES...THIS TRANSLATES INTO BALLPARK MLCAPE VALUES IN THE  
1500 J/KG RANGE. FRONTAL CONVERGENCE WILL BE WEAKENING WITH TIME AS  
THE BETTER HEIGHT FALLS INITIALLY STREAM TO THE NORTH...BUT APPEARS  
ADEQUATE CONSIDERING THE MOISTURE QUALITY AND FAVORABLE DIURNAL  
TIMING TO GENERATE AT LEAST A SCATTERED COVERAGE OF CONVECTION  
EITHER JUST UPSTREAM OR OVERHEAD WITHIN THE 18Z-22Z PERIOD. THE  
MODEST ASCENT AND LOW BULK SHEAR VALUES /LESS THAN 20 KNOTS/ WILL  
HINDER THE PROSPECTS OF SEEING MORE ORGANIZED CONVECTIVE  
GROWTH...SIMPLY A LOW END POTENTIAL FOR SOME STRONGER WIND GUSTS TO  
DEVELOP WITHIN ANY HEALTHIER CORES.  
 
A SLIGHTLY COOLER BUT SIGNIFICANTLY DRIER POST-FRONTAL ENVIRONMENT  
WILL BEGIN TO FILTER IN BY EARLY TONIGHT. THIS WILL PROVIDE A  
NOTEWORTHY DOWNWARD TREND IN HUMIDITY THAT CARRIES RIGHT INTO  
THURSDAY. LOWS TONIGHT IN THE LOWER 60S.  
 
LONG TERM...THURSDAY THROUGH TUESDAY  
 
PRETTY BIG SHIFT IN THE LONGWAVE PATTERN SETTING UP OVER THE END OF  
THE WEEK. THE MID LEVEL CLOSED LOW TRACKING ACROSS SOUTHERN CANADA  
WILL LIFT NE TO THE HUDSON BAY WHERE IT WILL STALL AND DEEPEN OVER  
THE COURSE OF THE WEEKEND WHILE THE RIDGE OVER THE WEST COAST  
AMPLIFIES. THIS PATTERN LOOKS TO BE QUITE STUBBORN LEADING TO  
TROUGHING OVER THE GREAT LAKES THROUGH THE EXTENDED FORECAST PERIOD.  
 
THURSDAY WILL FEATURE ELEVATED WESTERLY WINDS DUE TO A WELL MIXED  
AND DRY POST FRONTAL AIRMASS. CONVERGENT WNW FLOW ALOFT BETWEEN THE  
DEVELOPING UPPER LEVEL RIDGE/TROUGH PATTERN...HIGH PRESSURE BUILDING  
IN AT THE SFC...COOLING MID LEVELS... AND PWATS DROPPING UNDER 0.75  
INCHES...WILL PROMOTE STEEP LL LAPSE RATES. IN ADDITION THE WIND  
PROFILE IS DEEP LAYER WESTERLY FLOW WITH SPEEDS UP TO 25 KNOTS DOWN  
TO ALMOST 950MB. NOT A HUGE GRADIENT THROUGH THE COLUMN HENCE  
SUSTAINED WINDS WILL BE 15-20 KNOTS WITH GUSTS CAPPED OFF A LITTLE  
HIGHER AT 25 KNOTS. THESE WINDS ALSO LOOK TO KEEP SOME WARM AIR  
STREAMING INTO THE REGION WITH THE RIDGE REMAINING LOW AMPLITUDE.  
THIS NEARLY ZONAL FLOW WILL HELP KEEP SOME WARM AIR ADVECTING INTO  
THE REGION BEFORE THE TROUGH CAN FURTHER DEEPEN AND DIG INTO THE  
AREA.  
 
FRIDAY LOOKS TO REMAIN DRY BUT LOW END POPS WILL BE IN THE FORECAST  
THROUGH THE WEEKEND AND INTO NEXT WEEK. THIS IS A LOW CONFIDENCE  
FORECAST AS FALL ON THE SOUTHERN PERIPHERY OF THE DEEPENING TROUGH  
WITH A SERIES OF SHORTWAVES LIKELY DROPPING THROUGH THE NW FLOW  
THROUGH THE REGION. THE AIRMASS LOOKS TO REMAIN DRY AND THE BULK OF  
THE ENERGY IN THE PATTERN BRUSHING THE NORTHERN LAKES REGION SO HARD  
TO GET TOO EXCITED ABOUT ANY POP CHANCES AT THIS TIME. THE TROUGH  
WILL YIELD SOME COOLER TEMPS BUT STILL NEAR NORMAL AROUND 80  
DEGREES.  
 
MARINE...  
 
WARM SOUTHERLY FLOW WILL INCREASE AHEAD OF AN APPROACHING COLD FRONT  
TODAY. THE STABLE AIRMASS WILL LIMIT THE GUST POTENTIAL WHICH SHOULD  
TOP OUT NEAR 20 KNOTS. FLOW WILL REMAIN ELEVATED THROUGH THE NIGHT  
AND INTO THURSDAY AS WINDS TURN MORE SOUTHWESTERLY BEHIND THE FRONT.  
SMALL CRAFT ADVISORIES FOR WIND MAY BE NEEDED ON THURSDAY AS WINDS  
OVER LAND MAY REACH 25 KNOTS WHICH MAY EXTEND INTO NEARSHORE ZONES.  
LIGHT GRADIENT WILL SETTLE IN FOR THE EXTENDED PERIOD FAVORING WINDS  
10 KNOTS OR LESS FRIDAY AFTERNOON INTO SUNDAY.  
 

 
   
DTX WATCHES/WARNINGS/ADVISORIES
 
 
MI...NONE.  
LAKE HURON...NONE.  
LAKE ST CLAIR...NONE.  
MICHIGAN WATERS OF LAKE ERIE...NONE.  

 
 

 
 
AVIATION.....HLO  
UPDATE.......HLO  
SHORT TERM...MR  
LONG TERM....DRK  
MARINE.......DRK  
 
YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE  
AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).  
 
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