Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 211703
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
103 PM EDT SUN SEP 21 2014

.AVIATION...

//DISCUSSION...

MVFR SHOWERS WILL BE MOVING THROUGH SOUTHEAST MICHIGAN THIS
AFTERNOON...AND TAPER OFF TOWARD EVENING. THERE IS A SLIGHT CHANCE OF
THUNDER...BUT CHANCES ARE TOO LOW TO INCLUDE IN TAFS. WESTERLY
WINDS GUSTING AROUND 25 KNOTS WILL BE DIMINISHING SOME AS THEY
COME AROUND TO THE NORTHWEST TO NORTH BY EVENING. ONCE THE SHOWERS
END...IT IS EXPECTED THERE WILL BE ENOUGH LOW LEVEL MOISTURE
TRAPPED UNDERNEATH THE DEVELOPING SUBSIDENCE INVERSION TO MAINTAIN
MVFR CEILINGS OVERNIGHT INTO TOMORROW MORNING...BEFORE CIGS SLOWLY
RISE AND MIX OUT DURING THE DAYTIME HOURS OF MONDAY.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CIGS BELOW 5KFT THROUGH THE EVENING...MEDIUM
CONFIDENCE OVERNIGHT INTO MONDAY MORNING.

* LOW CONFIDENCE THAT A THUNDERSTORM WILL IMPACT KDTW THIS AFTERNOON

&&

.PREV DISCUSSION...ISSUED 308 AM EDT SUN SEP 21 2014

SHORT TERM...TODAY AND TONIGHT

CONVECTION IS ONGOING ALONG THE LEAD EDGE OF A STRONG PV ANOMALY
THAT IS PROPAGATING ACROSS THE WESTERN GREAT LAKES EARLY THIS
MORNING. ANOTHER FOCUS OF ACTIVITY IS ONGOING WITHIN THE LOW-LEVEL
THETA-E THAT SUPPORTED LAST NIGHT`S THUNDERSTORM ACTIVITY. RENEWED
COVERAGE AND INTENSITY IS OBSERVED AS HEIGHT FALLS ASSOCIATED WITH
THE UPSTREAM ANOMALY SPREAD INTO THE AREA AND SWLY 300-310K SYSTEM
RELATIVE ISENTROPIC ASCENT RAMPS UP. YET A THIRD FOCUS OF CONVECTION
IS ONGOING ALONG A SURFACE COLD FRONT OVER NORTHERN LOWER MICHIGAN.
THE THERMAL STRUCTURE IN PLACE TODAY IS CORRESPONDINGLY CONVOLUTED
AND DIFFICULT TO DIAGNOSE.

08-12Z IS THE TARGET PERIOD FOR THE FIRST TWO FOCI OF ACTIVITY BASED
ON HIGH RESOLUTION MODEL GUIDANCE AND THE LATEST OBSERVATIONAL TRENDS.
DURING THIS TIME, STEEPER LAPSE RATES WILL BE MAKING INROADS INTO
THE REGION SUPPORTING MARGINAL ELEVATED INSTABILITY OF AROUND 800
J/KG AND SHEAR IN THE EFFECTIVE LAYER IN EXCESS OF 50 KNOTS. THIS
WILL PROVIDE SOME OPPORTUNITY FOR STORM ORGANIZATION AND A LOW END
SEVERE WIND/HAIL THREAT FIRST THING THIS MORNING. THE THREAT SHOULD
REMAIN RATHER LIMITED, HOWEVER, GIVEN MARGINAL INSTABILITY, THE FACT
THAT BETTER LAPSE RATES WILL STILL BE LARGELY LAGGING BEHIND, AND A
MID-LEVEL WIND REGIME THAT MAY SIMPLY BE TOO STRONG FOR UPDRAFTS TO
SURVIVE IN. A BREAK WILL THEN ENSUE DURING MID-MORNING AS THE BETTER
FORCING QUICKLY RACES EAST AND THE BETTER MOISTURE REMAINS TIED TO
THE FRONT OVER NORTHERN LOWER.

MOISTURE CONVERGENCE ALONG THE MID-LEVEL TROUGH AXIS WILL ROTATE
INTO THE AREA FROM THE NORTHWEST DURING THE LATE MORNING AND EARLY
AFTERNOON WHILE THE ASSOCIATED HEIGHT FALLS HELP EASE THE NORTHERN
COLD FRONT SOUTH INTO THE AREA. AT THE SAME TIME, THE TRAILING SHEAR
MAX WILL PIVOT ACROSS THE AREA BRINGING RENEWED LARGE SCALE ASCENT
AND H6-H3 DEFORMATION FORCING. WITH THE THETA-E AXIS HAVING BEEN
SHUNTED EAST FAR EARLIER IN THE DAY, SEVERE PROSPECTS WILL BE
MINIMAL. HOWEVER, H7-H5 LAPSE RATES NEAR 7/C SUPPORTED BY H5 TEMPS
OF -19C WILL SUPPORT TYPICAL COLD INSTABILITY SHOWERS CAPABLE OF
PRODUCING SMALL HAIL AND GUSTY WINDS. MOISTURE AXIS AND DEFORMATION
WILL PIVOT OUT OF THE AREA 00-03Z BRINGING AN END TO SHOWERS.

COLD ADVECTION FROM THE SOUTHWEST IN THE WAKE OF THIS MORNING`S
INITIAL WAVE WILL GIVE WAY TO A STRONGER SHOT OF NWLY COLD ADVECTION
LATER THIS AFTERNOON INTO TONIGHT. A STRONG SIGNAL FOR SYSTEM
RELATIVE ISENTROPIC DESCENT IS OBSERVED ON THE 290K SURFACE, WHICH
SUGGESTS HEALTHY DYNAMIC SUPPORT FOR TRANSPORT OF WINDS UP TO 25KT
WINDS TO THE SURFACE. IT WILL BE NEARLY A WASH AGAINST RELATIVELY
STRONG SEPTEMBER INSOLATION, BUT THE SHOT OF CAA MAY BE SUFFICIENT
TO RESULT IN FALLING TEMPS ACROSS THE NORTHERN HALF OF THE CWA
DURING THE AFTERNOON, ESPECIALLY IN THE THUMB. COLD ADVECTION WILL
WANE A BIT DURING THE OVERNIGHT HOURS, BUT THE BOUNDARY LAYER WILL
REMAIN SUFFICIENTLY WELL-MIXED TO SUPPORT WINDS OF 5-10KTS. HEALTHY
MOISTURE FLUX OFF OF THE LAKES WILL SUPPORT WIDESPREAD STRATOCU.
LOWS WILL BOTTOM OUT IN THE UPPER 30S TO MID 40S.

LONG TERM...MONDAY THROUGH SATURDAY.

A SLUG OF COLD AIR WILL MARCH SOUTHWARD ON MONDAY...KEEPING HIGH
TEMPERATURES AROUND 60 DEGREES DURING THE DAYTIME HOURS. THE COLD
AIR ADVECTING SOUTH OVER THE COOL WATER WILL LIKELY LEAD TO
CLOUDS...ESPECIALLY OVER THE THUMB WHERE NORTH NORTHWESTERLY FLOW
CAN PICK UP MOISTURE OFF LAKE HURON. THESE CLOUDS WILL SLOWLY ERODE
FROM WEST TO EAST MONDAY AFTERNOON AS DRIER AIR WORKS ITS WAY IN
FROM THE WEST AND WINDS TURN WESTERLY...STOPPING THE FEED OF
MOISTURE OFF LAKE HURON. THE CLEARING OUT WILL ALLOW MONDAY NIGHT
MINIMUMS TO PLUMMET INTO THE 40S.

HIGH PRESSURE WILL BUILD IN OVER THE SOUTHERN OHIO VALLEY ON
TUESDAY...THEN BUILD EAST OVER THE EASTERN GREAT LAKES AND THE
EASTERN SEABOARD BY THE MIDDLE OF THE WEEK. THIS HIGH WILL STICK
AROUND THROUGH THE REST OF THE WEEK AS AN OMEGA BLOCKING HIGH SETS
UP ACROSS THE EASTERN HALF OF THE CONUS KEEPING THE WEATHER DRY
THROUGH THE REST OF THE EXTENDED. ALTHOUGH WE WILL BE IN A BLOCKING
PATTERN...TEMPERATURES WILL MODERATE BY THE MIDDLE PART OF THE
WEEK...WITH HIGHS GETTING BACK INTO THE 70S BY THE END OF THE WEEK.

MARINE...

A SECONDARY UPPER WAVE AND COLD FRONT WILL LEAD TO ADDITIONAL
SHOWERS AND THUNDERSTORMS TODAY. BRISK NORTHERLY WINDS OF 20 TO 25
KNOTS WILL FOLLOW THIS FRONT THIS AFTERNOON...ALLOWING LARGE WAVE TO
BUILD OVER SOUTHERN LAKE HURON TONIGHT. THIS WILL LEAD TO SMALL
CRAFT ADVISORY CONDITIONS ACROSS ALL OF THE LAKE HURON NEARSHORE
WATERS AS WELL AS OUTER SAGINAW BAY THIS AFTERNOON AND TONIGHT.
THEREFORE WILL ISSUE SMALL CRAFT ADVISORIES FOR THIS TIME FRAME.
HIGH PRESSURE WILL EXERT FAVORABLE INFLUENCE ON THE WATERS STARTING
TUESDAY...AND CONTINUING THROUGH THE MIDDLE OF NEXT WEEK.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.

LAKE HURON...
     SMALL CRAFT ADVISORY...OUTER SAGINAW BAY AND THE NEARSHORE WATERS
     FROM PORT AUSTIN TO HARBOR BEACH...UNTIL 10 AM MONDAY.

     SMALL CRAFT ADVISORY...INNER SAGINAW BAY...UNTIL 8 AM MONDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS FROM HARBOR BEACH TO PORT
     HURON...UNTIL 4 PM MONDAY.

LAKE ST CLAIR...
     SMALL CRAFT ADVISORY...UNTIL 8 PM SUNDAY.

MI WATERS OF LAKE ERIE...
     SMALL CRAFT ADVISORY...UNTIL 8 PM SUNDAY.

&&

$$

AVIATION.....SF
SHORT TERM...JVC
LONG TERM....KURIMSKI
MARINE.......KURIMSKI


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