Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 312301 AFDDTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 701 PM EDT THU JUL 31 2014 .AVIATION... //DISCUSSION... THERE REMAINS A LOW CHANCE FOR SHOWERS AND THUNDERSTORMS THROUGH THE FIRST HALF OF THE NIGHT AS WEAK INSTABILITY SLOWLY WANES. COVERAGE REMAINS TOO LOW TO INCLUDE A SPECIFIC MENTION. OTHERWISE CLEARING SKIES WITH THE LOSS OF HEATING. A PERIOD OF THICKER MID CLOUD FOR TONIGHT AND EARLY FRIDAY AS THE NEXT TROUGH ENTERS THE REGION. THIS PROCESS WILL ENCOURAGE SOME DEGREE OF SHOWER AND THUNDERSTORM DEVELOPMENT WITH THE AID OF DAYTIME HEATING FRIDAY AFTERNOON. UNCERTAINTY IN BOTH TIMING AND COVERAGE WILL PRECLUDE A SPECIFIC MENTION AT THIS STAGE. LIGHT SOUTH TO SOUTHWEST WINDS THROUGH THE PERIOD. //DTW THRESHOLD THREATS... * LOW CONFIDENCE IN CEILINGS BELOW 5000 FT ON FRIDAY. && .PREV DISCUSSION...ISSUED 341 PM EDT THU JUL 31 2014 SHORT TERM... THROUGH TONIGHT THE UPPER LOW OVER CANADA STILL RESIDES OVER EASTERN ONTARIO BUT THE PIVOTING SHEARED VORTICITY MAX WILL TIGHTEN ABOUT THE CENTER OF THE LOW WITH THAT MOMENTUM...AND PROXIMITY TO THE JET...HELPING TO LIFT THE SYSTEM NE OVERNIGHT. HEIGHTS WILL STALL WITH SLIGHT TENDENCY TO INCREASE THROUGH THE NIGHT AS THE PIVOTING MOTION PREVENTS GREATER RESPONSE ALOFT. WHILE THIS IS GOING ON...SEVERAL SHORT WAVES SEEN ON WATER VAPOR IMAGERY WILL TRACK THROUGH THE TROUGH AND SE MI. THE FIRST WAVE...CURRENTLY OVER IOWA...SHOULD DIVE JUST TO THE SOUTH OF MI LATER THIS EVENING BUT THE WAVE SURGING SOUTH...JUST NORTH OF LAKE SUPERIOR...WILL SWING THROUGH THE AREA TONIGHT. THIS EVENING WILL FEATURE SOME SCATTERED SHOWERS AND THUNDERSTORMS AS GOOD DIURNAL RESPONSE IN THE CU FIELD WITH THE BUILDING INSTABILITY...CAPE PUSHING 800 J/KG. THERE IS A BIT OF A CAP AROUND 650MB AS SEEN IN THE 12Z DTX SOUNDING WHICH WILL ACT TO KEEP CONVECTION MORE SCATTERED IN NATURE. WITH A BOUNDARY DRAPED ACROSS NORTHERN MI AND SW FLOW RESULTING FROM THE CHANGING ORIENTATION OF THE TROUGH...NOCTURNAL MOISTURE ADVECTION...SEEN IN 850MB DEWPOINTS...FEEDING INTO THE REGION WITH THE APPROACHING WAVE WILL WARRANT THE CONTINUED MENTION OF SLIGHT CHANCE POPS THROUGH MOST OF THE NIGHT. CLOUD COVER WILL BE A BIT CHALLENGING TONIGHT AS THE LOSS OF DIURNAL HEATING WILL TEND TO CLEAR SKIES OUT AS WE HAVE SEEN FOR A COUPLE EVENINGS NOW...BUT THE WAVE WILL BRING CLOUDS BACK INTO THE AREA. THE QUESTION WILL BE IF AND FOR HOW LONG WILL THE POTENTIAL CLEARING LAST? THIS WILL MOSTLY AFFECT MINIMUM TEMPERATURES TONIGHT WHICH SHOULD LARGELY BE IN THE UPPER 50S. OVERALL THE NIGHT SHOULD BE PRETTY UNEVENTFUL FOR MOST LOCATIONS WITH SUCH LOW COVERAGE IN PRECIPITATION. LONG TERM... THE DOMINANCE OF THE UPPER LEVEL RIDGE OVER WESTERN NORTH AMERICA WILL KEEP THE DOWNSTREAM GREAT LAKES REGION RIPE FOR LOWER HEIGHTS AND A LARGE WAVELENGTH UPPER LEVEL TROUGH. THIS REMAINS THE CASE OVER THE NEXT COUPLE OF DAYS EVEN AS THE LARGE PV ANOMALY AND CLOSED LOW PRESSURE CIRCULATION PULLS OUT OF EASTERN CANADA. ATYPICAL BECAUSE WHAT SHOULD BE A PERIOD OF BUILDING UPPER LEVEL HEIGHTS AND INCREASED RIDGING INSTEAD...REMAINS DOWNSTREAM OF THE WESTERN UNITED STATES DROUGHT AND FEEDBACK RIDGE STRUCTURE. PERSISTENT NORTHWESTERLY GRADIENT FLOW WILL CHANGE VERY LITTLE IN ORIENTATION OVER THE NEXT FEW DAYS. FORECAST THETA E ALONG WITH MOISTURE FROM AN RH PERSPECTIVE SUGGESTS LITTLE TO TANGIBLE ADVECTION WILL OCCUR. THUS...LOOKING AT A PERSISTENCE FORECAST PLUS SOME INCREMENTAL CHANGE WITH SOME MODEST RECOVERY OF NEAR SURFACE DEWPOINTS DUE TO EVAPOTRANSPIRATION EFFECTS. FCST SOUNDINGS ARE BULLISH IN COMPARISON TO TODAY/YESTERDAY/TUESDAY WITH DECENT 700-600MB LAPSE RATES CONTRIBUTING TO 500-750 J/KG. LITTLE TO NO SHEAR WITH MARGINAL CONVECTIVE INSTABILITY SUPPORTS VERY LITTLE STRONG THUNDERSTORM RISK. LOOKING AT SOME RAINERS. SATURDAY IS IN AN INTERESTING FORECAST WITH SOME COMPETING INTERESTS. SPECIFICALLY...THE STATE WILL BE DIRECTLY UNDERNEATH AN ORGANIZING UPPER LEVEL TROUGH/AXIS. HOWEVER...THE PREEXISTING AIRMASS THAT WILL BE UNDERGOING THE CHANCE/CAUGHT IN BACKSIDE OF DEVELOPING CYCLONIC CIRCULATION WILL BE A CHUNK DIRECTLY OFF OF THE UPPER LEVEL RIDGE FROM PORTIONS OF CENTRAL MANITOBA. THIS AIRMASS WILL NEED TO UNDERGO A SIGNIFICANT AMOUNT OF MODIFICATION TO SUPPORT MUCH PRECIPITATION. WITH THAT SAID...THE AREA WILL BE UNDER CYCLONIC FLOW. THE MODELS ARE LIKELY OVERDOING EVAPOTRANSPIRATION...BUT THERE IS ENOUGH THERE TO GO WITH A CHANCE OF SHOWERS AND THUNDERSTORMS BY SATURDAY AFTERNOON. A DECENT SIGNAL IN THE MODELS THAT TERRAIN WILL PLAY AN IMPORTANT ROLE IN WHERE THE ACTIVITY DEVELOPS. LOW AMPLITUDE UPPER LONGWAVE TROUGH WILL CONTINUE TO BE LOCATED OVER THE GREAT LAKES FROM SUNDAY INTO EARLY NEXT WEEK. HOWEVER...SUNDAY ACTUALLY LOOKS TO FEATURE SOME MID/UPPER LEVEL HEIGHT AND TEMPERATURE RISES AS THE DEEPEST PORTION OF THE TROUGH SHIFTS EAST. NWP STILL SHOWING NEGATIVE LIFTED INDICES AND LONG SKINNY CAPE OVER THE MITTEN...HOWEVER...WITH LACK OF FORCING WILL GO ALONG WITH NEIGHBORING OFFICES IN KEEPING SUNDAY DRY FOR NOW. ILL-DEFINED DISTURBANCES EMBEDDED IN WEAK/MODERATE MIDLEVEL WESTERLIES WILL SPELL DAILY SHOWER/THUNDERSTORM CHANCES INTO EARLY NEXT WEEK...WITH TEMPERATURES REMAINING NEAR NORMAL. MARINE... A WEAK AREA OF SURFACE HIGH PRESSURE WILL BUILD ACROSS THE GREAT LAKES REGION THROUGHOUT THE UPCOMING WEEKEND. A VERY WEAK WIND PROFILE WILL BE IN PLACE LEADING TO LITTLE TO NO SURFACE WINDS OR ORGANIZED WAVE GROWTH. THE SURFACE FEATURE WILL REMAIN VERY WEAK CAUSING AFTERNOON CHANCES FOR SHOWERS AND THUNDERSTORMS TO PERSIST. THE BETTER POTENTIAL FOR SHOWERS AND THUNDERSTORMS FRIDAY AND SATURDAY WILL REMAIN OVER INLAND AREAS. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. LAKE HURON...NONE. LAKE ST CLAIR...NONE. MI WATERS OF LAKE ERIE...NONE. && $$ AVIATION.....MR SHORT TERM...DRK LONG TERM....CB/DT MARINE.......CB YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).

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