382  
FXUS63 KAPX 201030  
AFDAPX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GAYLORD MI  
630 AM EDT SUN AUG 20 2017  
   
NEAR TERM
 
(TODAY THROUGH TONIGHT)  
ISSUED AT 313 AM EDT SUN AUG 20 2017  
 
HIGH IMPACT WEATHER POTENTIAL: A CHANCE OF THUNDERSTORMS OVERNIGHT  
TONIGHT.  
 
PATTERN SYNOPSIS AND FORECAST:  
 
A RIDGE OF SFC HIGH PRESSURE EXTENDED ACROSS THE WESTERN GREAT LAKES  
EARLY THIS MORNING, AHEAD OF SHALLOW MID LEVEL RIDGING. WEAK WAA  
ALOFT HAS STARTED ALOFT, BUT A RELATIVELY DRIER AIR MASS AND NO  
OTHER FORCING ALOFT HAS RESULTED IN MOSTLY CLEAR SKIES. THE ONLY  
CLOUDS AROUND WERE ASSOCIATED WITH LAST EVENING'S POCKET OF LIGHT  
RAIN THAT CROSSED NRN LOWER, AS WELL AS THE FOG AND BIT OF STRATUS  
ACROSS PORTIONS OF EASTERN UPPER. UPSTREAM, A SHORTWAVE TROUGH AND  
ASSOCIATED COLD FRONT WERE ENCROACHING ON THE MISSISSIPPI VALLEY.  
NOT A WHOLE LOT OF SHOWERS AND STORMS ONGOING THERE RIGHT NOW DUE TO  
A GENERAL LACK OF MOISTURE, HOWEVER THERE WAS A POCKET OF ACTIVITY  
IN NE/WESTERN IA WHERE THE LOW LEVEL JET WAS RELATIVELY STRONG.  
ALSO, SOME SCATTERED SHOWERS WERE ACROSS SD. BOTH AREAS WERE  
UNDERNEATH 7C/KM MID LEVEL LAPSE RATES AND IN THE VICINITY OF SOME  
UPPER DIVERGENCE.  
 
THE AFOREMENTIONED SHORTWAVE TROUGH WILL BROADEN OUT AS IT SHOVES  
THE MID LEVEL RIDGE EAST OF THE REGION. THE AIR MASS THROUGH THE DAY  
WILL REMAIN ON THE DRY SIDE, AND ONLY FEW TO SCATTERED CUMULUS IS  
ANTICIPATED AWAY FROM LAKE MICHIGAN WHERE SW WINDS WILL BE STARTING  
TO INCREASE. THUS, MOSTLY SUNNY SKIES ARE EXPECTED, WITH HIGHS  
REACHING THE UPPER 70S AND LOWER HALF OF THE 80S. MOISTURE IS NEVER  
REALLY ABLE TO EFFICIENTLY ADVANCE NORTHWARD AHEAD OF THE FRONT,  
EVEN THROUGH THE NIGHT AS THE FAST WAVE SHOVES THE NRN END OF THE  
FRONT QUICKLY, LAYING IT OUT ACROSS FAR NRN LAKE MICHIGAN AND  
EASTERN UPPER OVERNIGHT. WE WILL HAVE STEEPENING MID LEVEL LAPSE  
RATES TO 6.5C/KM WITH SOME SEMBLANCE OF BETTER WAA/LLJ AND WEAKER  
UPPER DIVERGENCE, BUT NOT AS GOOD AS WHAT IS CURRENTLY UPSTREAM.  
THIS COMBINED WITH THE FRONT AND MUCAPES OF 500-1000J/KG OUGHT TO  
BRING AT LEAST THE CHANCE FOR SHOWERS AND STORMS. INCREASED WINDS  
JUST ABOVE THE SFC WILL KEEP TEMPS FROM FALLING TOO MUCH. LOWS WILL  
BE IN THE LOW TO MID 60S.  
 

 
   
SHORT TERM
 
(MONDAY THROUGH TUESDAY)  
ISSUED AT 313 AM EDT SUN AUG 20 2017  
   
..SHOWER/STORM CHANCES TO START THE WEEK
 
 
HIGH IMPACT WEATHER POTENTIAL: THUNDERSTORM CHANCES AT VARIOUS TIMES  
MONDAY THROUGH TUESDAY. LOCALLY HEAVY RAIN POSSIBLE.  
 
PATTERN FORECAST: GRADUALLY LOWERING MID-UPPER LEVEL HEIGHTS MONDAY  
THROUGH TUESDAY WITH SEVERAL MID LEVEL SHORTWAVES SLIDING ALONG AN  
ELONGATED SURFACE FRONTAL BOUNDARY DRAPED ACROSS THE GREAT LAKES.  
THIS WILL RESULT IN UNSETTLED WEATHER TO START THE WEEK.  
 
PRIMARY FORECAST CONCERNS/CHALLENGES: HOW MUCH CLOUDINESS FOR THE  
ECLIPSE? SHOWER/STORM CHANCES MONDAY THROUGH TUESDAY.  
 
STALLING FRONTAL BOUNDARY WILL REMAIN DRAPED ACROSS NORTHERN LOWER  
MICHIGAN ON MONDAY WITH MORE CLOUDS THAN NOT, AND ADDITIONAL  
SCATTERED SHOWERS/STORMS AT VARIOUS TIMES. VERY SOUPY AIRMASS WITH  
LOW LEVEL MOISTURE POOLING ALONG AND SOUTH OF THE BOUNDARY, PUSHING  
SURFACE DEWPOINTS TOWARD 70 DEGF IN SOME AREAS. THIS HIGH DEWPOINT  
AIR, COMBINED WITH HIGH TEMPERATURES PUSHING INTO THE 80S, WILL  
PROMOTE MLCAPES OF NEARLY 2,000 J/KG ACROSS MUCH OF NORTHERN LOWER  
MONDAY AFTERNOON. FORCING IS RATHER NEBULOUS AND NOT TERRIBLY  
IMPRESSIVE DURING THE DAY, BUT THE DEVELOPMENT OF AFTERNOON LAKE  
BREEZES SHOULD BE ENOUGH TO PRODUCE SCATTERED STORMS. GIVEN THE  
AMOUNT OF INSTABILITY AND WITH BULK SHEAR VALUES NEAR 40KTS, A FEW  
ORGANIZED STRONG-SEVERE STORMS WILL BE POSSIBLE.  
 
ECLIPSE VIEWING: STILL LOOKING LIKE MORE CLOUDS THAN SUN ACROSS MUCH  
OF THE AREA EARLY MONDAY AFTERNOON AS A FRONTAL BOUNDARY BISECTS  
NORTHERN LOWER. MODEL TRENDS ARE POINTING TOWARD AFTERNOON DEEP  
LAYER DRYING OVER UPPER MICHIGAN (ESPECIALLY CENTRAL AND WESTERN  
AREAS), WHERE CLOUDS WILL SCATTER OUT DURING THE AFTERNOON AND ALLOW  
FOR BETTER VIEWING. EVEN SOUTH OF THE BRIDGE, A FEW GLIMPSES OF  
SUNSHINE WILL BE POSSIBLE.  
 
STILL SOME MODEL DIFFERENCES IN THE STRENGTH OF ANY SURFACE  
REFLECTION LATER MONDAY NIGHT INTO TUESDAY AS SHORTWAVE ENERGY  
SLIDES ALONG THE QUASI-STATIONARY BOUNDARY ACROSS CENTRAL MICHIGAN.  
BETTER FORCING DEVELOPING WITH UPPER LEVEL JET ENERGY DROPPING INTO  
THE NORTHERN LAKES COINCIDENT WITH SURFACE LOW DEVELOPMENT ALONG THE  
BOUNDARY. PLENTY OF DEEP LAYER MOISTURE IN PLACE WITH PWATS  
APPROACHING 2 INCHES. EVERYTHING POINTS TO A FAIRLY WIDESPREAD RAIN  
EVENT LATER MONDAY NIGHT INTO THE FIRST HALF OF TUESDAY WITH POCKETS  
OF LOCALLY HEAVY RAIN POSSIBLE (WE ARE IN THE MARGINAL EXCESSIVE  
RAINFALL AREA ACCORDING TO WPC).  
   
LONG TERM
 
(TUESDAY NIGHT THROUGH SATURDAY)  
ISSUED AT 313 AM EDT SUN AUG 20 2017  
 
ANOTHER DEEP UPPER LEVEL TROUGH DIGS INTO THE GREAT LAKES AND  
NORTHEAST STATES FOR THE MIDDLE AND END OF THE WEEK, PUSHING AN  
ABNORMALLY COOL AIRMASS INTO THE AREA. DEEP LAYER DRYING AND  
SUBSIDENCE TUESDAY NIGHT BEHIND THE FRONT, LIKELY RESULTING IN  
MAINLY DRY CONDITIONS BUT BREEZY AND TURNING COOLER. SECONDARY  
TROUGH ON WEDNESDAY BRINGS AN UPTICK IN H8-H7 MOISTURE OVER NORTHERN  
AREAS AND POSSIBLY RESULTS IN A FEW SHOWERS. THE REMAINDER OF THE  
WEEK INTO THE FIRST PART OF THE WEEKEND LOOKS QUIET AS A LARGE AREA  
OF CANADIAN HIGH PRESSURE SLOWLY BUILDS SOUTH/EAST. TEMPERATURES ON  
THE COOL SIDE FOR AUGUST, ALTHOUGH READINGS WILL MODERATE HEADING  
INTO THE WEEKEND. IDEAL RADIATIONAL COOLING CONDITIONS ON SEVERAL  
NIGHTS WILL LIKELY RESULT IN SOME OF THE TYPICALLY COLDER LOCATIONS  
DIPPING INTO THE 30S.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)  
ISSUED AT 630 AM EDT SUN AUG 20 2017  
 
SFC HIGH PRESSURE WILL TRACK EAST OF THE REGION TODAY, WHILE A  
COLD FRONT APPROACHES FROM THE NW AND NEARS PLN LATE IN THE TAF  
PERIOD. S/SW WINDS WILL INCREASE WITH THE APPROACH OF THE FRONT,  
WITH LOW END LLWS EVENTUALLY BEING NEEDED TONIGHT. THE ATMOSPHERE  
HAS QUITE A BIT OF INSTABILITY TODAY, BUT MOISTURE IS RATHER  
SCANT, WITH ONLY FEW TO SCATTERED CUMULUS EXPECTED. NO RAINFALL  
UNTIL MAINLY AFTER MIDNIGHT WHEN THERE'S A CHANCE FOR SHOWERS AND  
THUNDERSTORMS.  
 

 
   
MARINE
 
 
ISSUED AT 313 AM EDT SUN AUG 20 2017  
 
HIGHER PRESSURE SLIDES EAST TODAY WITH A COLD FRONT LAYING OUT  
ACROSS NRN MICHIGAN LATE TONIGHT THROUGH LATER TUESDAY. SW WINDS  
INCREASE TO LOW END ADVISORIES OVER MANY OF THE NEARSHORES LATER  
THIS AFTERNOON AND TONIGHT, BEFORE THE GRADIENT WEAKENS  
CONSIDERABLY. CHANCES FOR SHOWERS AND STORMS WILL BE ASSOCIATED WITH  
THIS FRONT LATE TONIGHT THROUGH TUESDAY. NO SEVERE WEATHER IS  
EXPECTED. THEN, A LARGER UPPER TROUGH PLOWS INTO THE REGION AND  
SHOVES THE COLD FRONT WELL SOUTH. THE COLD ADVECTION AND STRONGER NW  
WINDS WILL BRING LATE TUESDAY AFTERNOON INTO WEDNESDAY.  
 

 
   
APX WATCHES/WARNINGS/ADVISORIES
 
 
MI...NONE.  
LH...SMALL CRAFT ADVISORY FROM 7 PM THIS EVENING TO 5 AM EDT MONDAY  
FOR LHZ347.  
LM...SMALL CRAFT ADVISORY FROM 7 PM THIS EVENING TO 5 AM EDT MONDAY  
FOR LMZ341-342.  
SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 5 AM EDT MONDAY  
FOR LMZ344>346.  
LS...SMALL CRAFT ADVISORY FROM 7 PM THIS EVENING TO 5 AM EDT MONDAY  
FOR LSZ321.  

 
 

 
 
NEAR TERM...SMD  
SHORT TERM...JK  
LONG TERM...JK  
AVIATION...SMD  
MARINE...SMD  
 
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