338  
FXUS63 KDTX 210941  
AFDDTX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI  
541 AM EDT FRI JUL 21 2017  
 
   
AVIATION
 
 
MORNING FOG WILL BE SHALLOW AND TRANSIENT IN NATURE, AND SHOULD NOT  
HAVE MUCH OF AN IMPACT ON THE TERMINALS. HIGH PRESSURE OVER THE AREA  
TODAY WILL BRING QUIET WEATHER. WINDS WILL BE LIGHT UNTIL LAKE  
BREEZES WORK INLAND DURING THE AFTERNOON, CAUSING A WIND SHIFT AT  
MOST TERMINALS. WARM AND HUMID AIRMASS OVER THE MIDWEST WILL ALLOW A  
COMPLEX OF THUNDERSTORMS TO DEVELOP TONIGHT, WITH GOOD POTENTIAL FOR  
THIS AREA OF STORMS TO THEN WORK INTO LOWER MICHIGAN VERY LATE  
TONIGHT AND EARLY SATURDAY MORNING. LATEST HI-RES MODELS BRING IT IN  
AFTER 08Z, WHICH IS WHEN BEST FORCING WILL BE ABLE TO PROVIDE  
SUPPORT. TIMING MAY NEED TO BE REFINED, BUT DID INCLUDE A PROB30  
GROUP IN ALL TAFS.  
 
FOR DTW...LATE AFTERNOON LAKE BREEZE PASSAGE WILL SHIFT A  
LIGHT/VARIABLE WIND TO THE SOUTHEAST WITH A SLIGHT INCREASE IN  
SPEED. GOOD CHANCE FOR THE TERMINAL TO BE IMPACTED BY A COMPLEX OF  
THUNDERSTORMS LATE TONIGHT AND EARLY SATURDAY, WITH LATEST IDEA ON  
TIMING IN THE 09-14Z WINDOW.  
 
//DTW THRESHOLD PROBABILITIES...  
 
* MEDIUM FOR THUNDERSTORMS AFTER 09Z SAT  
 
* MEDIUM FOR CIGS AOB 5000 FEET AFTER 09Z SAT.  
 

 
   
HYDROLOGY
 
 
DRY WEATHER IS EXPECTED TODAY THROUGH LATE TONIGHT. THERE IS THEN A  
GOOD CHANCE FOR A ROUND OF SHOWERS AND THUNDERSTORMS TO TRACK  
THROUGH LOWER MICHIGAN LATE TONIGHT THROUGH THE FIRST PART OF  
SATURDAY. TORRENTIAL RAINFALL MAY OCCUR WITH STORMS. THERE IS  
POTENTIAL FOR A SWATH OF 1-2 INCH RAINFALL WITH THIS INITIAL ROUND  
OF STORMS, ESPECIALLY IF A STRONG STORM COMPLEX DEVELOPS. THIS WOULD  
BRING A CONCERN FOR LOCALIZED FLOODING EARLY ON SATURDAY. ADDITIONAL  
STORMS MAY DEVELOP LATE SATURDAY THROUGH SUNDAY, WITH STORMS ON  
SUNDAY NOT CARRYING QUITE AS HIGH OF A POTENTIAL FOR HEAVY RAINFALL.  
 

 
   
PREV DISCUSSION
 
 
ISSUED AT 249 AM EDT FRI JUL 21 2017  
 
DISCUSSION...  
 
VERY WARM AND INCREASINGLY HUMID CONDITIONS ARE EXPECTED OVER THE  
AREA TODAY INTO SATURDAY. HIGHS TODAY WILL RANGE FROM 85 TO 90 TODAY  
AND 80-85 SATURDAY, BUT HEAT INDICES WILL CLIMB INTO THE LOWER 90S  
IN SOME LOCATIONS BOTH DAYS GIVEN THE STEADY INCREASE IN LOW LEVEL  
MOISTURE OVER THE NEXT 30-36 HOURS. THIS BUILDING HEAT/HUMIDITY WILL  
PLAY A PART IN THE MAIN FORECAST CHALLENGE OVER THE NEXT FEW DAYS.  
 
THIS MAIN ISSUE WILL BE BOTH THE TIMING AND LOCATION (AND SEVERITY)  
OF POTENTIAL MCS LATE TONIGHT INTO SATURDAY MORNING. CONVECTION IS  
EXPECTED TO IGNITE OVER THE UPPER MISSISSIPPI VALLEY FROM THIS  
AFTERNOON INTO THIS EVENING WITHIN AREA OF FORCING IN ADVANCE OF A  
SHORTWAVE WHICH WILL TRACK FROM ALONG THE US/CANADIAN BORDER. AS  
PREVIOUSLY ELABORATED, THIS AREA OF CONVECTION WILL ALSO HAVE JET  
SUPPORT WITHIN ENTRANCE REGION OF A RATHER LARGE JET STREAK WHICH  
SHIFTS OFF TO THE EAST/NORTHEAST WITH TIME LATE TODAY/TONIGHT.  
 
WHILE MODELS DO OFFER DIFFERING SOLUTIONS, ESPECIALLY IN TERMS OF  
TIMING THIS SYSTEM ON ITS TREK FROM THE UPPER MIDWEST INTO THE  
SOUTHERN/CENTRAL GREAT LAKES, ALL DO HAVE A RATHER WELL ORGANIZED  
CONVECTIVE SYSTEM TRACKING INTO THE VICINITY SOMEWHERE IN BETWEEN  
THE 06Z-08Z AND 12Z-14Z TIME FRAME. HIRES MODELS OF LATE HAVE,  
HOWEVER, HAD A GREAT DEAL OF DIFFICULTY HANDLING MCS DEVELOPMENT AND  
SUBSEQUENT EVOLUTION. CASE IN POINT WOULD BE A GENERAL UNDERPLAYING  
OF ONGOING CONVECTION FROM IOWA INTO NORTHERN ILLINOIS. WHILE THIS  
DEFICIENCY MAY IMPACT MODEL PERFORMANCE SOMEWHAT ON LATE DAY STORMS,  
RECENT HRRR RUNS, WHICH DO SIMULATE THE CURRENT MIDWEST CONVECTION  
PRETTY WELL, SUGGEST THE DEVELOPMENT AND EXPANSION OF CONVECTION  
FORM MINNESOTA INTO WISCONSIN THIS AFTERNOON/EVENING (WELL WITHIN  
CONSENSUS OF 00Z HIRES MODEL SOLUTIONS).  
 
WITH THIS IN MIND, STILL FEEL PRETTY GOOD ABOUT THE GOING LIKELY  
PRECIPITATION CHANCES LATE TONIGHT INTO SATURDAY MORNING. WILL  
REMOVE POPS FROM THE 00Z-06Z WINDOW THIS EVENING AS TIMING DOES NOT  
SEEM TO SUPPORT THIS, BUT LEAVE THE 06Z-18Z TIME FRAME UNCHANGED AT  
THIS POINT. AREA REMAINS IN A MARGINAL/SLIGHT RISK OF SEVERE WEATHER  
DURING THIS PERIOD, WHICH WILL DEPEND A GREAT DEAL ON THE ABILITY OF  
AN MCS TO REMAIN WELL ORGANIZED DURING A DIURNAL CONVECTIVE MINIMUM.  
THINK THE MORE LIKELY ISSUE MAY BE LOCALLY TORRENTIAL RAINS/FLOODING  
AS PWATS INCREASE TO 2.00 INCHES. THIS WILL DEPEND LARGELY ON HOW  
WELL CONVECTIVE SCALE FORCING IS ABLE TO FIRE ADDITIONAL STORMS ALONG  
THE FRONTAL BOUNDARY DRAPED IN THE VICINITY. IN THEORY, THIS IDEA  
MAKES SOME SENSE GIVEN THE LOCATION OF BOUNDARY AND UPPER JET  
SUPPORT WHICH SHOULD ALLOW FOR CONVECTION TO REMAIN FAIRLY FAR NORTH  
ON A E/ESE TRAJECTORY AS OPPOSED TO PROPAGATING INTO HIGHER CAPE  
ENVIRONMENT TO THE SOUTH.  
 
THE REMAINDER OF THE FORECAST SHOWS A RETURN TO A MORE MODERATE  
ENVIRONMENT AS COLD FRONT ASSOCIATED WITH THE AFOREMENTIONED WAVE  
ALONG THE US/CANADIAN BORDER USHERS IN COOL (AND ESPECIALLY LESS  
HUMID AIR FROM LATE SUNDAY ON INTO NEXT WEEK. WHILE SCATTERED  
SHOWERS OR STORMS WILL BE POSSIBLE ALONG THIS FRONT SUNDAY, THE  
ACTIVITY SHOULD BE MINOR COMPARED TO ANY ACTIVITY IN THE VICINITY  
TONIGHT/SATURDAY. WITH THE PASSAGE OF THIS FRONT THE HUMID AIRMASS  
WITH HIGH TEMPERATURES INTO THE 85-90 DEGREE RANGE WILL EDGE BACK  
INTO THE UPPER 70S/LOWER 80S ON AVERAGE.  
 
MARINE...  
 
QUIET WEATHER WITH LIGHT WINDS IS EXPECTED TODAY AS BROAD HIGH  
PRESSURE ENCOMPASSES THE AREA. LOW PRESSURE TRACKING INTO THE GREAT  
LAKES WILL PROVIDE A VERY GOOD CHANCE FOR SHOWERS AND THUNDERSTORMS  
LATE TONIGHT INTO SATURDAY, ESPECIALLY ACROSS WESTERN LAKE ERIE,  
LAKE ST CLAIR AND SOUTHERN LAKE HURON. SOME OF THESE STORMS COULD BE  
STRONG TO SEVERE. ADDITIONAL ROUNDS OF THUNDERSTORMS ARE POSSIBLE  
LATE SATURDAY THROUGH SUNDAY AS THE LOW TRACKS OVERHEAD. WINDS  
OUTSIDE OF THUNDERSTORMS WILL BE FAIRLY LIGHT THROUGH TONIGHT,  
BEFORE EASTERLY WINDS OVER LAKE HURON INCREASE 10 TO 20 KNOTS  
SATURDAY AS THE LOW TRACKS ACROSS MICHIGAN. WINDS WILL DECREASE FOR  
SATURDAY NIGHT AND SUNDAY, BEFORE A RESURGENCE OF GUSTY WINDS FROM  
THE NORTHEAST OCCURS OVER LAKE HURON SUNDAY NIGHT INTO MONDAY ON THE  
BACKSIDE OF THE LOW. WAVE HEIGHTS COULD POTENTIALLY GROW TO MORE  
THAN 4 FEET OVER SOUTHERN LAKE HURON ON MONDAY, AND MAY NECESSITATE  
SMALL CRAFT ADVISORIES.  
 

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

 
 

 
 
AVIATION.....HLO  
DISCUSSION...DG  
MARINE.......HLO  
HYDROLOGY....HLO  
 
YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE  
AT WWW.WEATHER.GOV/DETROIT.  
 
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