667  
FXUS63 KDTX 211421 AAA  
AFDDTX  
 
AREA FORECAST DISCUSSION...UPDATED  
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI  
1021 AM EDT THU OCT 21 2021  
   
UPDATE
 
 
AS OF 1020 AM EDT...CURRENT FORECAST LARGELY REMAINS ON TRACK WITH  
MAIN UPDATES TO ACCOUNT FOR LATEST POP TRENDS IN HI-RES GUIDANCE LATE  
THIS MORNING THROUGH THE AFTERNOON HOURS. SURFACE LOW PRESSURE  
CONTINUES TO LIFT NORTHEAST INTO NORTHERN LOWER LATE THIS MORNING,  
WITH TRAILING COMPACT MIDLEVEL CIRCULATION NEAR SOUTHERN LAKE  
MICHIGAN. SOUTHEAST MICHIGAN CURRENTLY RESIDES IN A RELATIVE DRY SLOT  
BETWEEN THE DEPARTING CONVECTIVE ACTIVITY FROM THIS MORNING  
ACCOMPANYING THE SURGE IN THETA-E ALONG THE INITIAL LEAD FRONTAL  
BOUNDARY, AND A STRONGER COLD FRONT THAT WILL ACCOMPANY THE  
AFOREMENTIONED MIDLEVEL CIRCULATION AND STRONG DCVA APPARENT VIA  
WATER VAPOR IMAGERY.  
 
BREAKS IN CLOUD COVER MOVING THROUGH LOWER MICHIGAN SHOULD ALLOW FOR  
A FEW DEGREE BOOST IN HIGH TEMPERATURES TODAY, WITH MOST AREAS  
SOLIDLY SEEING A SHOT AT MID 60S READINGS. THE CLEARING WILL ALSO  
HELP ENHANCE ALREADY EFFICIENT INBOUND BOUNDARY LAYER MIXING  
POTENTIAL WITH WESTERLY WIND GUSTS INCREASING THIS AFTERNOON TO  
AROUND 25 MPH, WITH 30 MPH GUSTS NOT OUT OF THE QUESTION. AS THE  
MIDLEVEL CIRCULATION SPINS ALONG THE I-96 CORRIDOR LATER THIS  
AFTERNOON INTO THIS EVENING, THE ACCOMPANYING INCREASED DYNAMIC  
FORCING COUPLED WITH A RUSH OF COLDER AIR ALOFT WILL ALLOW FOR  
ANOTHER ROUND OF BLOSSOMING SHOWERS ESPECIALLY ACROSS NORTHERN AREAS,  
BUT WOULD NOT BE SURPRISED TO SEE A FAIRLY WIDESPREAD COVERAGE OF  
NUMEROUS SHOWERS IN THE 3PM-8PM TIMEFRAME. INCREASING 3-6 KM LAPSE  
RATES (APPROACHING 7 C/KM) AND A FEW HUNDRED J/KG CAPE NOTED ON THE  
12Z KAPX UPSTREAM SOUNDING WILL ALSO PROVIDE THE OPPORTUNITY FOR SOME  
ISOLATED THUNDERSTORMS AS WELL. LOW FREEZING LEVELS (DECREASING  
BELOW 8 KFT), MAY SUPPORT SOME SMALL HAIL POTENTIAL AS WELL.  
 

 
   
PREV DISCUSSION
 
 
ISSUED AT 634 AM EDT THU OCT 21 2021  
 
AVIATION...  
 
DEEP MOISTURE AXIS WITH ACCOMPANYING AREA OF SHOWERS AND  
THUNDERSTORMS WILL SHIFT EAST OF THE SOUTHEAST MICHIGAN AIRSPACE AT  
12Z. SOME POTENTIAL REMAINS FOR PERIODIC MVFR CONDITIONS YET THIS  
MORNING, BUT RECENT OBSERVATIONAL TRENDS SUGGEST PRIMARILY LOWER VFR  
GOING FORWARD EARLY TODAY. MODERATELY MIXED WEST-SOUTHWEST FLOW  
WILL SUPPORT GUSTY CONDITIONS INTO THE AFTERNOON - 20 TO 25 KNOTS AT  
TIMES. ADDITIONAL POCKETS OF SHOWERS POSSIBLE, PARTICULARLY AS THE  
TRAILING COLD FRONT SURGES SOUTHEAST THIS EVENING. IMPROVING  
MOISTURE DEPTH ALONG THE FRONTAL ZONE AGAIN BRINGS SOME REDUCTION IN  
CLOUD BASES EARLY TONIGHT, FAVORING MORE WIDESPREAD MVFR CONDITIONS.  
WIND SHIFT TO NORTHWESTERLY EXPECTED WITH THE FRONTAL PASSAGE.  
 
DTW THRESHOLD PROBABILITIES...  
 
* MEDIUM IN CEILINGS AT OR BELOW 5000 FEET THROUGH TONIGHT.  
 
PREV DISCUSSION...  
ISSUED AT 410 AM EDT THU OCT 21 2021  
 
DISCUSSION...  
 
AN UNSETTLED DAY IS IN STORE AS LOW PRESSURE CROSSING LAKE MICHIGAN  
THIS MORNING CONTINUES ITS TREK EASTWARD ACROSS THE STATE. A PLUME  
OF HIGHER THETA-E IS SPREADING ACROSS SOUTHEAST MICHIGAN AT PRESS  
TIME AS THE SYSTEM'S SOUTHWESTERLY LOW-LEVEL JET BECOMES DIRECTED  
OVERHEAD. COINCIDENT WITH THE INCREASING MOISTURE IS THE ARRIVAL OF  
STRONGER SYNOPTIC SCALE LIFT IN THE FORM OF HEIGHT FALLS WITH THE  
INBOUND DYNAMIC UPPER WAVE AND ASCENT TIED TO THE LEFT EXIT REGION  
OF THE UPPER JET. THIS IS COMING TOGETHER TO RESULT IN THE  
CONVECTIVE SHOWERS AND STORMS NOTED IN REGIONAL RADAR MOSAIC THIS  
MORNING. THIS ACTIVITY WILL TRACK ACROSS THE FORECAST AREA OVER THE  
NEXT SEVERAL HOURS WITH BRIEF DOWNPOURS AND LIGHTNING THE MAIN  
THREATS. THE NEAR-SURFACE LAYER REMAINS STABLE AT THIS TIME SO  
STORMS WILL REMAIN MAINLY ELEVATED, BUT CAN'T RULE OUT SOME GUSTY  
WINDS TO 40 MPH IN ANY STRONGER STORMS. A TRAILING COLD FRONT SWEEPS  
ACROSS THE STATE FROM WEST TO EAST DURING THE LATE MORNING AND  
USHERS ANY LINGERING SHOWERS/STORMS OUT.  
 
AFTER THE MID-MORNING ROUND, MOST AREAS WILL SEE A RELATIVE LULL IN  
PRECIP FOR THE LATE MORNING INTO EARLY AFTERNOON AS WE ENTER A POST-  
FRONTAL PSEUDO WARM SECTOR. LOW-LEVEL LAPSE RATES STEEPEN  
CONSIDERABLY WITHIN THE GROWING MIXED LAYER WITH WEST WIND BECOMING  
GUSTY TO AROUND 25 MPH OR SO. HIGH TEMPERATURES DURING THIS PERIOD  
ARE FORECAST TO REACH THE LOWER TO MID 60S. MEANWHILE, A STRONGER  
COLD FRONT IS DRAGGED SOUTHWARD ACROSS NORTHERN LOWER MICHIGAN THIS  
AFTERNOON AS THE LOW DEPARTS ACROSS LAKE HURON. ANOTHER ROUND OF  
LIGHTER SHOWERS IS LIKELY TO ACCOMPANY THIS SECOND FRONT THIS  
EVENING WITH ANY LINGERING SHOWERS TAPERING OFF TONIGHT AS MUCH  
COOLER AIR SETTLES IN. TEMPERATURES FALL TO THE 30S ACROSS MOST OF  
THE AREA BY FRIDAY MORNING WITH METRO DETROIT AND LOCATIONS ALONG  
THE LAKESHORES CLINGING ON TO THE LOWER 40S.  
 
A DRY BUT MOSTLY CLOUDY DAY IS EXPECTED FRIDAY AS COOLER AIR  
CONTINUES TO SETTLE IN. 850MB TEMPS BOTTOM OUT IN THE -2 TO -5 C  
RANGE WHICH POINTS TO HIGH TEMPS IN THE LOWER TO MID 50S. COLD  
ADVECTION CEASES BY FRIDAY AFTERNOON BUT THE COOL AIR MASS LARGELY  
STAYS PUT THIS WEEKEND AS A BROAD UPPER LOW BECOMES ESTABLISHED OVER  
JAMES BAY. THERE WILL BE SEVERAL SPOKES OF SHORTWAVE ENERGY THAT  
PIVOT AROUND THE CYCLONIC FLOW OVER THE COURSE OF THE WEEKEND WITH  
THE MOST SUBSTANTIAL HEIGHT FALLS CURRENTLY PROGGED TO SWING THROUGH  
DURING THE DAY ON SATURDAY. THIS LOOKS TO TRIGGER SOME LAKE ENHANCED  
SHOWERS THAT MAINLY FOCUS OVER THE THUMB AS THE WAVE ENHANCES  
NORTHWEST FLOW OVER THE LAKES.  
 
REBOUNDING HEIGHTS IN THE WAKE OF THE WAVE WILL CUT OFF ANY SHOWERS  
AND MAY ALLOW AN AREA OF WEAK HIGH PRESSURE TO DEVELOP OVER THE  
REGION SATURDAY NIGHT. MODELED RH FIELDS DEPICT SOME REDUCTION IN  
THE CLOUD FRACTION AS WELL, WHICH WOULD ALLOW FOR A RADIATIVE  
COMPONENT TO AID IN TEMPERATURES FALLING TO THE LOWER TO MID 30S  
ACROSS MUCH OF THE AREA. NOT EXPECTING A WIDESPREAD FROST AT THIS  
POINT, BUT RURAL AREAS IN THE TRI-CITIES AND THUMB REGIONS WILL  
STAND THE BEST CHANCE FOR SOME SUB-FREEZING READINGS SUNDAY MORNING.  
THE NEXT CHANCE FOR WIDESPREAD RAIN ARRIVES LATE SUNDAY INTO MONDAY  
AS A LOW PRESSURE SYSTEM TRACKS FROM THE MID-MISSISSIPPI VALLEY INTO  
THE SOUTHERN GREAT LAKES.  
 
MARINE...  
 
A DEEPENING LOW PRESSURE SYSTEM CURRENTLY OVER THE MIDWEST WILL  
TRACK EAST TODAY ALONG THE STALLED FRONT DRAPED ACROSS CENTRAL LAKE  
HURON. SHOWERS WILL CONTINUE TO DEVELOP THROUGH THE DAY ALONG THIS  
FRONTAL BOUNDARY WITH INCREASED COVERAGE AND THUNDERSTORM CHANCES  
LATER THIS MORNING AS THE LOW SENDS A COLD FRONT EASTWARD ACROSS THE  
AREA. WEST WIND WILL BECOME GUSTY ACROSS WESTERN LAKE ERIE AND LAKE  
ST. CLAIR THIS AFTERNOON WHICH HAS PROMPTED SMALL CRAFT ADVISORIES  
THROUGH THIS EVENING.  
 
THE ELONGATING LOW WILL THEN EXIT TO THE EAST LATER THIS EVENING,  
FORCING A STRONG COLD FRONT SOUTHWARD ACROSS THE LAKES. THIS WILL  
LEAD TO STRONGER NORTH WIND GUSTING UPWARDS OF 30 KT ACROSS LAKE  
HURON AND RAPIDLY BUILDING WAVE HEIGHTS ACROSS THE CENTRAL AND  
SOUTHERN BASINS. SMALL CRAFT ADVISORIES HAVE BEEN ISSUED ALONG THE  
NEARSHORE WATERS OF THE THUMB THIS AFTERNOON THROUGH FRIDAY MORNING.  
NORTH WIND THEN TAPERS OFF ON FRIDAY AS COOLER AIR SETTLES INTO THE  
REGION.  
 
HYDROLOGY...  
 
A LOW PRESSURE SYSTEM TRACKS ACROSS LOWER MICHIGAN TODAY, OFFERING  
SEVERAL PERIODS OF RAINFALL. THE FIRST AND MAIN ROUND WILL BE THIS  
MORNING AS AN AREA OF SHOWERS AND STORMS TRACK WEST TO EAST ACROSS  
THE STATE AHEAD OF A COLD FRONT. SOME LOCALIZED MODERATE TO HEAVY  
DOWNPOURS WILL BE POSSIBLE WITH THIS ACTIVITY. A RELATIVE LULL IS  
THEN EXPECTED FOR THE LATE MORNING TO EARLY AFTERNOON HOURS BEFORE  
THE NEXT COLD FRONT SWINGS THROUGH FROM NORTHWEST TO SOUTHEAST. THIS  
WILL BRING ADDITIONAL LIGHTER SHOWERS DURING THE LATE AFTERNOON AND  
EVENING. ADDITIONAL STORM TOTAL RAINFALL AMOUNTS WILL GENERALLY  
REMAIN NEAR OR BELOW A HALF INCH, BUT LOCALIZED AREAS - PARTICULARLY  
NORTH OF I-69 - MAY SEE HIGHER AMOUNTS. THE PROGRESSIVE NATURE OF  
THE SYSTEM IS EXPECTED TO MINIMIZE POTENTIAL FLOODING APART FROM  
LOCALIZED PONDING ON ROADS AND IN POOR DRAINAGE AREAS.  
 

 
   
DTX WATCHES/WARNINGS/ADVISORIES
 
 
MI...NONE.  
LAKE HURON...SMALL CRAFT ADVISORY FROM 4 PM THIS AFTERNOON TO 10 AM EDT FRIDAY  
FOR LHZ421-441>443.  
 
SMALL CRAFT ADVISORY FROM 4 PM THIS AFTERNOON TO 10 PM EDT THIS  
EVENING FOR LHZ422.  
 
LAKE ST CLAIR...SMALL CRAFT ADVISORY UNTIL 10 PM EDT THIS EVENING FOR LCZ460.  
 
MICHIGAN WATERS OF LAKE ERIE...SMALL CRAFT ADVISORY UNTIL 10 PM EDT THIS EVENING FOR LEZ444.  
 

 
 

 
 
UPDATE.......IRL  
AVIATION.....MR  
DISCUSSION...TF  
MARINE.......DRK/TF  
HYDROLOGY....TF  
 
YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE  
AT WWW.WEATHER.GOV/DETROIT.  
 
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