236
FXUS63 KGRR 130520
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
120 AM EDT THU AUG 13 2026
KEY MESSAGES
- CHANCE FOR ISOLATED THUNDERSTORMS THIS AFTERNOON AND EVENING
- FOGGY AND CHANCES FOR LIGHT RAINFALL THROUGH THE SOUTH TO END THE WORK WEEK
- PRECIPITATION CHANCES THIS WEEKEND
DISCUSSION
ISSUED AT 234 PM EDT WED AUG 12 2026
- CHANCE FOR ISOLATED THUNDERSTORMS THIS AFTERNOON AND EVENING
LOW STRATOCU CONTINUES TO REMAIN THROUGH THE SOUTHERN TIER OF
MICHIGAN WITH LIGHT SOUTHERLY WINDS. SEVERE WEATHER THROUGH
INDIANA CONTINUES TO TRACK TO THE SOUTH EAST WITH ANY OUTFLOW
MOVING NORTHWARD RUNNING INTO A WALL OF STABILITY. THAT STABILITY
WILL STIFLE ANY CONVECTION THIS AFTERNOON AND EVENING THROUGH
SOUTHERN MICHIGAN. THE SOLE CAVEAT WILL BE CONVECTION FROM
BOUNDARY INTERACTIONS. CURRENTLY THERE IS THREE DIFFERENT OUTFLOW
BOUNDARIES MOVING IN DIFFERENT DIRECTIONS THAT ARE SET TO
INTERSECT THROUGH SOUTHWEST MI. THERE ARE SOME CAMS THAT SUGGEST
THAT SOME CONVECTION COULD FORM. SFC BASED CAPE IS AROUND 1000
J/KG ALONG THE I 94 CORRIDOR. HOWEVER, THERE IS UPWARDS OF 200
J/KG OF CIN. SO WHILE IT IS POSSIBLE THAT CONVECTION COULD FORM
THROUGH THE SOUTH AND WILL NEED TO BE WATCHED, THE STABLE AIR
COULD PRECLUDE STORM DEVELOPMENT.
THE BETTER CHANCE WILL BE AS A SPEED MAX MOVES FROM THE EAST TO
SOUTHEAST ACROSS CENTRAL LOWER MI. LAPSE RATES IN THE TRI CITY
AREA ARE 6 TO 7 C/KM THROUGH THE MID LEVELS. COUPLE THAT WITH MU
CAPE AROUND 1400 J/KG AND PWATS OF 1.43 INCHES AND YOU HAVE A
DECENT ENVIRONMENT FOR STORM DEVELOPMENT. THERE IS SOME CU
DEVELOPMENT THROUGH CENTRAL AND NORTHER LOWER. HOWEVER, A LARGE
CIRRUS SHIELD SHOULD LIMIT DAYTIME HEATING, THOUGH MAX TEMPS ALONG
THE US 10 CORRIDOR COULD GET INTO THE MID 80S. LATEST CAMS
CONTINUE TO SUGGEST THE POTENTIAL FOR ISOLATED STORMS TO BE
POSSIBLE AFTER 20Z. DCAPES ARE IN THE 800 TO 900 J/KG RANGE SO
DAMAGING WINDS ARE THE MAIN CONCERN. ANY SHOWER AND STORM CHANCES
WILL DWINDLE THROUGH THE EVENING HOURS,
- FOGGY AND CHANCES FOR LIGHT RAINFALL THROUGH THE SOUTH TO END THE WORK WEEK
ANY RAINFALL AND CONVECTION WILL ADD TO THE MOISTURE THROUGHOUT
THE LOWER LEVELS THROUGH SOUTHERN MICHIGAN. CALM WINDS, A STABLE
AIR MASS AND MOIST LOWER LEVELS IS A GREAT RECIPE FOR ANOTHER
ROUND OF FOG THROUGH THURSDAY MORNING. EXPECT AREAS OF FOG TO
DEVELOP OVERNIGHT. FOG COULD BE DENSER THAN THIS MORNING'S BUT
WILL HOLD OFF ON ANY ADVISORIES FOR NOW. ANOTHER UPPER LEVEL
TROUGH WILL DROP DOWN FROM THE WEST TOMORROW. THE TRACK OF THAT
TROUGH IS TRENDING TOWARDS THIS WEEK'S PREVIOUS SYSTEMS. THEY ARE
TRENDING TOWARDS IN SOUTH INTO OHIO. THE ONLY PRECIPITATION COULD
BE ON THE FRINGES ALONG THE I 94 CORRIDOR. ONLY LIGHT RAIN IS
EXPECTED WITH A LOW CHANCE FOR ANY THUNDER.
- PRECIPITATION CHANCES THIS WEEKEND
THE NEXT CHANCE FOR WIDESPREAD PRECIPITATION WILL BE SATURDAY
NIGHT AND SUNDAY AS THE PATTERN SHIFTS AND THE ZONAL FLOW SHIFTS
NORTHWARD BRINGING A MID LEVEL SHORTWAVE PROPAGATES OVER HIGH
PRESSURE BUILDING INTO THE GREAT PLAINS. THAT SHORT WAVE WILL
BRING DECENT SURGE OF MOISTURE SUNDAY. THERE IS SOME QUESTION ON
TIMING AN POSITION OF THE MID LEVEL TROUGH. HOWEVER THERE SHOULD
BE GOOD THICKNESS ADVECTION WITH MUCAPE VALUES IN EXCESS OF 2000
J/KG. SO THERE IS THE PERSISTENT TREND FOR THE POTENTIAL FOR
STRONGER STORMS. MID TO LONG RANGE MODELS ALSO CONTINUE TO SHOW A
35 TO 40 KT 850MB JET THROUGH LOWER MICHIGAN SUNDAY MORNING. THIS
WILL AID STORM DEVELOPMENT SUNDAY.
AVIATION /06Z TAFS THROUGH 06Z FRIDAY/
ISSUED AT 120 AM EDT THU AUG 13 2026
OUR MAIN FOCUS WITH THIS SET OF FORECASTS IS ON THE FOG AND LOW
CLOUD POTENTIAL FOR ALL OF THE TERMINALS. A WEAK FRONT DROPPING
DOWN ACROSS THE AREA IS INTERACTING WITH A MOIST AIR MASS WITH
LIGHTER WINDS. THIS IS A VERY GOOD SET UP FOG FOG. WE ARE STARTING
TO SEE SOME MVFR CONDITIONS DEVELOP, AND CONDITIONS ARE EXPECTED
TO DROP FURTHER INTO THE IFR TO LIFR CATEGORY OVER THE NEXT COUPLE
OF HOURS.
THE LOWER CONDITIONS WILL REMAIN UNTIL ABOUT 13-15Z, BEFORE THE
FOG MIXES OUT, AND CLOUDS BREAK UP. THIS WILL ALLOW FOR VFR
CONDITIONS TO DEVELOP AND REMAIN FOR THE REST OF THE PERIOD.
MARINE
ISSUED AT 247 PM EDT WED AUG 12 2026
NOT EXPECTING MUCH WINDS AND WAVES OVER THE NEXT FEW DAYS.
THE MAIN CONCERNS WILL BE THE POTENTIAL FOR MARINE FOG AND ANY
THUNDERSTORMS MOVING FROM THE WEST IN THE AFTERNOONS. FOG WILL
REMAIN A THREAT OVER THE WATER UNTIL DRIER DEW POINTS MOVE INTO
THE REGION. THAT DOES NOT REALLY HAPPEN THIS WEEK SO THE THREAT OF
FOG WILL CONTINUE THROUGH THE REST OF THE WEEK.
GRR WATCHES/WARNINGS/ADVISORIES
MI...NONE.
MARINE...NONE.
DISCUSSION...CERU
AVIATION...NJJ
MARINE...CERU
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