509  
FXUS63 KDTX 191944  
AFDDTX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI  
344 PM EDT THU JUL 19 2018  
   
DISCUSSION
 
 
ANTICYCLONIC FLOW REGIME OF THE LAST FEW DAYS NOW EXITING EAST AS  
HEIGHT FALLS ASSOCIATED WITH THE UPSTREAM CLOSED LOW BEGIN TO SPREAD  
INTO THE AREA. TEMPERATURES IN THE LOW 80S AND DEWPOINTS IN THE 50S  
THIS AFTERNOON ARE CHARACTERISTIC OF THE RESIDENT DRY AIRMASS THAT  
EXTENDS WESTWARD INTO MUCH OF WISCONSIN, INDIANA, AND FAR EASTERN  
IL. AREAS OF SHOWER AND THUNDERSTORM ACTIVITY ARE ONGOING WITHIN THE  
1.5"+ PWAT PLUME THAT DEMARCATES THE AIRMASS BOUNDARY WHICH EXTENDS  
FROM SOUTHEAST MINNESOTA TO THE LOWER PORTIONS OF THE OHIO RIVER  
VALLEY. ISENTROPIC ASCENT AND INCREASED CONVERGENCE ALONG THE LEAD  
EDGE OF THIS MOISTURE AXIS WILL SUPPORT EITHER CONTINUED OR  
REDEVELOPING CONVECTION OVER ILLINOIS AND IOWA OVERNIGHT,  
PARTICULARLY AS UPPER PV ADVECTION INCREASES. LOSS OF DEEPER FORCING  
FOR ASCENT AS FORWARD PROGRESS OF THE LOW ALMOST STOPS ALTOGETHER BY  
FRIDAY MORNING WARRANTS A REDUCTION TO CHC POPS AS ANY BAND OF  
SHOWERS LIFTING THROUGH THE CWA SHOULD BE BOTH TRANSIENT AND  
WEAKENING. DEEP LAYER CYCLONIC FLOW AND ONGOING MOIST ADVECTION WILL  
FORCE DEWPOINTS WELL INTO THE MID OR EVEN UPPER 60S BY LATE FRIDAY,  
POSING A THREAT FOR THUNDERSTORM DEVELOPMENT WITHIN THE WEAKLY  
SHEARED, BUT VERY MOIST ENVIRONMENT. 12Z MODEL PROGS SUGGEST  
ACTIVITY WILL BE FOCUSED ALONG A SECOND WAVE EMBEDDED WITHIN THE  
PARENT LOW OVER SW LOWER MICHIGAN LATE FRI AFTERNOON BEFORE  
SPREADING EAST THROUGH 00Z. COOLER MID-LEVEL TEMPS OPEN THE DOOR FOR  
MODERATE DESTABILIZATION LATE IN THE DAY, BUT LATE ARRIVAL OF  
FORCING AND THE LIKELIHOOD OF EXTENSIVE CLOUD COVER CASTS DOUBT ON  
POTENTIAL. INSTEAD, THE MORE LIKELY OUTCOME IS A RELATIVELY QUICK-  
MOVING HEAVY RAIN THREAT LATE EVE/FIRST HALF OF THE NIGHT WITH ANY  
STRONGER CONVECTION CAPABLE OF PRODUCING SMALL HAIL AND/OR WIND  
GUSTS TO AROUND 40 MPH. SHOWER/TSTORM CHANCES WILL BE PERVASIVE  
HEADING INTO THE FIRST HALF OF THE WEEKEND. LACK OF STEERING FLOW ON  
SATURDAY WILL FAVOR SOME DEGREE OF A HEAVY RAIN THREAT DEPENDING ON  
DEGREE OF DESTABILIZATION BENEATH THE UPPER LOW.  
 
BY SUNDAY, THE LOW PRESSURE SYSTEM THAT HAD BEEN SITUATED OVER THE  
ARE WILL WEAKEN AND SLIDE SOUTHWARD AWAY FROM THE REGION. SCATTERED  
SHOWERS/STORMS WILL COME TO AN END BY LATE SUNDAY AS HIGH PRESSURE  
BUILDS IN. FOR THE MOST PART, THE AREA WILL THEN REMAIN DRY GOING  
INTO NEXT WEEK WITH ONLY SLIGHT CHANCES FOR SOME SHOWER ACTIVITY. AS  
UPPER LEVEL RIDING BUILDS IN NEXT WEEK, TEMPS WILL REBOUND BACK INTO  
THE MID 80S.  
 

 
   
MARINE
 
 
SOUTHEAST WINDS GRADUALLY INCREASE WITH A MINOR WAVE RESPONSE  
TONIGHT AND FRIDAY AS LOW PRESSURE ORGANIZES AND MOVES TOWARD THE  
SOUTH END OF LAKE MICHIGAN. SOUTHEAST WINDS LOOK TO PEAK AROUND 20  
KNOTS ON SATURDAY, WITH LOW PRESSURE THEN SINKING SOUTHEAST INTO THE  
OHIO VALLEY LATE IN THE WEEKEND. THIS WILL ALLOW FOR WINDS TO BECOME  
NORTHEAST, BUT WILL BE LIGHT INTO EARLY NEXT WEEK.  
 

 
   
HYDROLOGY
 
 
INCREASINGLY MOIST AND MODESTLY UNSTABLE CONDITIONS AS LOW PRESSURE  
SLOWLY TRACKS THROUGH THE REGION WILL LEAD TO PERIODIC BOUTS OF  
SHOWERS AND THUNDERSTORMS FRIDAY INTO THE UPCOMING WEEKEND. HIGHLY  
VARIABLE RAINFALL AMOUNTS ARE LIKELY DEPENDING ON THE EVENTUAL TRACK  
AND PACE OF THIS SYSTEM. LATEST FORECAST CALLS FOR RAINFALL AMOUNTS  
RANGING FROM A HALF AN INCH TO AN INCH AND A QUARTER DURING THIS  
PERIOD. SOME LOCALIZED AMOUNTS IN EXCESS OF THIS ARE CERTAINLY  
POSSIBLE IN THUNDERSTORMS, LEADING TO POOR DRAINAGE FLOODING.  
 

 
   
PREV DISCUSSION
 
 
ISSUED AT 104 PM EDT THU JUL 19 2018  
 
AVIATION...  
 
AN INCREASE IN SURFACE MOISTURE HAS LEAD TO DEVELOPMENT OF DIURNAL  
CU OVER SOUTHEAST MICHIGAN, BUT PREDOMIANTLY SCT COVERAGE EXPECTED.  
CU FIELD EXPECTED TO DISSIPATE NEAR SUNSET, WITH HIGH CLOUDS ON THE  
INCREASE TONIGHT AS LARGE LOW PRESSURE SYSTEM MOVES INTO WESTERN  
GREAT LAKES. EXPECTING CLOUD BASES TO HOLD ABOVE 5000 FEET THROUGH  
MORNING HOURS, BUT THEN AN INCREASING CHANCE OF SHOWERS AND  
THUNDERSTORMS AND LOWER CIGS.  
 
FOR DTW...SOUTHEAST WINDS (GENERALLY 10 KNOTS OR LESS) MORE OR LESS  
HOLDING THROUGH THE TAF PERIOD, WITH A CHANCE OF THUNDERSTORMS  
DEVELOPING TOMORROW AFTERNOON, BUT CONFIDENCE IS LOW IN TIMING, AS  
STORMS COULD HOLD OFF UNTIL EVENING HOURS. A SHOWER OR TWO IS  
POSSIBLE IN LATE MORNING HOURS INTO EARLY AFTERNOON WITH INITIAL  
SURGE OF MOISTURE.  
 
DTW THRESHOLD PROBABILITIES...  
 
* LOW CONFIDENCE IN CIGS AOB 5000 FEET TOMORROW AFTERNOON.  
 
* LOW CONFIDENCE IN T-STORMS LATE TOMORROW AFTERNOON.  
 

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

 
 

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