Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 201103
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
703 AM EDT SAT SEP 20 2014

.AVIATION...

//DISCUSSION...

THE BROKEN LINE OF SHOWERS AND THUNDERSTORMS BETWEEN GRR AND FNT
AT PRESS TIME IS ASSOCIATED WITH THE LEADING EDGE OF WARMER AND
MORE HUMID AIR ALOFT THAT WILL CONTINUE TO MAKE INROADS OVER LOWER
MICHIGAN DURING THE MORNING. THE MORNING ACTIVITY IS STILL
EXPECTED TO FADE IN FAVOR OF REDEVELOPMENT DURING THE AFTERNOON
AND EVENING. STRENGTHENING SW GRADIENT AND WELL-MIXED CONDITIONS
WILL SUPPORT GUSTS OF 20 TO 25 KNOTS THROUGH THE DAYTIME HOURS AND
CONTINUE TO CARRY LOW LEVEL MOISTURE INTO THE AREA. BOUNDARY
LAYER GROWTH WILL SUPPORT AT LEAST SCT COVERAGE OF DIURNAL CU
AROUND 4KFT DURING THE EARLY AFTERNOON. A LINE OF TSRA IS THEN
FORECAST TO DEVELOP UPSTREAM AHEAD OF AN APPROACHING WAVE AND
MIGRATE INTO THE AREA AFTER 18Z. IT STILL APPEARS THAT INSTABILITY
WILL BE BEST ACROSS THE NW HALF OF THE AREA, SO CONTINUED OMISSION
OF TSRA FOR THE DETROIT AREA SITES. A TEMPO TSRA IS NOW INCLUDED
FOR KMBS/KFNT/KPTK WITH JUST SHRA AND MVFR VSBY FOR THE DETROIT
AREA. TSTORM REDEVELOPMENT WILL BE POSSIBLE LATE TONIGHT AS INFLUX
OF MID-LEVEL COLD AIR MAINTAINS INSTABILITY OVER THE REGION.
CONFIDENCE REMAINS FAR TOO LOW TO INCLUDE IN THE TAFS ATTM...SO
OPTED TO REPRESENT POTENTIAL WITH REINTRODUCTION OF SHRA BY 08Z.
MVFR CEILING EXPECTED TO BE IN PLACE AT ALL LOCATIONS BY LATE
TONIGHT.

//DTW THRESHOLD THREATS...

* LOW CONFIDENCE THAT A TSTORM WILL IMPACT KDTW TODAY/TONIGHT

* LOW CONFIDENCE IN CEILINGS BELOW 5KFT TODAY, MEDIUM CONFIDENCE TONIGHT

&&

.PREV DISCUSSION...ISSUED 405 AM EDT SAT SEP 20 2014

SHORT TERM...TODAY AND TONIGHT

TODAY. CONVECTION IS ONGOING WITHIN THE UPSTREAM THETA-E AXIS THAT
HAS BEEN DRAWN INTO THE UPPER MS VALLEY AHEAD OF THE BROAD CORRIDOR
OF HEIGHT FALLS SPREADING INTO THE NORTHERN U.S. CONVECTIVE DEBRIS
OVER IA/WI/IL WILL SPREAD EAST NORTHEAST DURING THE MORNING,
ENGULFING MOST OR ALL OF THE FORECAST AREA BY EARLY AFTN. SUBSEQUENT
LIMITED INSOLATION WILL RESULT IN SUSPECT PROSPECTS FOR INSTABILITY
TODAY AS THE THETA-E AXIS FOLDS EAST AND INTO THE AREA. 00Z NAM/RAP
DO STILL SUGGEST A RIBBON OF 1000 J/KG ADVECTING INTO NORTHWEST HALF
OF THE CWA DURING THE AFTERNOON, A REASONABLE PROSPECT GIVEN THE
HEALTHY SOUTHWEST GRADIENT FLOW. IN ADDITION, THE 00Z SUITE HAVE PICKED
UP ON THE POTENT LOW AMPLITUDE SHORTWAVE SEEN DIVING SOUTHEAST OUT OF
MONTANA AT 06Z THIS MORNING. THIS WAVE WAS EITHER UNDERSIMULATED OR
NON-EXISTENT IN THE 12Z GUIDANCE, AND THE MOST RECENT SIMULATIONS
SUGGEST THAT IT WILL BE SUFFICIENTLY STRONG TO INITIATE CONVECTION
ACROSS WESTERN LOWER MI THIS AFTERNOON. 0-6KM SHEAR ON THE ORDER OF
35 KTS FUELED BY A MODERATE SWLY LOW-LEVEL JET WILL BE SUFFICIENT
FOR ORGANIZATION OF THE STRONGEST UPDRAFTS. BUT GENERALLY SPEAKING,
MARGINAL INSTABILITY/LOW CAPE DENSITY WILL BE RESTRICTIVE OF SEVERE
CONVECTION. EXPECT NOTHING MORE THAN A MARGINAL SEVERE WIND/HAIL
THREAT THIS AFTERNOON, MAINLY FOCUSED ACROSS THE NORTHWESTERN HALF
OF THE AREA WHERE CONVECTIVE POTENTIAL WILL MOST FAVORABLY COINCIDE
WITH PEAK HEATING. AFOREMENTIONED SWLY GRADIENT AND WELL-MIXED
BOUNDARY LAYER TO AROUND 875MB WILL RESULT WARM/GUSTY DAY WITH HIGHS
OF 75 TO 80 AND WIND GUSTS TO 25 MPH.

TONIGHT. CYCLONIC SHEAR MAX ALONG THE COLD SIDE OF THE 120KT DIVING
OUT OF CANADA THIS MORNING WILL UNDERGO CONSIDERABLE AMPLIFICATION
AS IT APPROACHES THE GREAT LAKES TONIGHT. SOUTHWESTERLY ADVECTION OF
WARMER LOW-LEVEL AIR AND RAPIDLY COOLING HEIGHTS ALOFT WILL MAINTAIN
1000 TO 2000 J/KG OF CAPE FOR THE BALANCE OF THE NIGHT. AT THE SAME
TIME, THE RAPIDLY TIGHTENING MASS GRADIENT WILL ALLOW A 70 KNOT H5
JET TO IMPINGE ON THE FORECAST AREA RESULTING IN A 0-6KM BULK SHEAR
SOLIDLY IN 60-70 KNOT RANGE BY AROUND 09Z. MEANWHILE, THE LOW-LEVEL
WIND RESPONSE WILL FACILITATE A SECOND THETA-E SURGE INTO THE AREA
POST-06Z WITHIN A BAND OF INCREASING 300-310K SYSTEM RELATIVE
ISENTROPIC ASCENT. IN THE PRESENCE OF INCREASING LARGE SCALE ASCENT,
THIS WILL BE SUPPORTIVE OF ADDITIONAL CONVECTIVE DEVELOPMENT OVER
THE AREA LATE TONIGHT. GIVEN THE PARAMETERS, THE POTENTIAL FOR
MESOCYCLONE DEVELOPMENT AND A SUBSEQUENT SEVERE RISK IS READILY
EVIDENT. HOWEVER, MODELED SOUNDINGS ONCE AGAIN SUGGEST A RATHER LOW
CAPE DENSITY WHICH, GIVEN THE MAGNITUDE OF MID-LEVEL WINDS, MAY
SIMPLY RESULT IN DECAPITATION OF UPDRAFTS AT 15KFT OR SO. THE NATURE
OF TONIGHT`S INSTABILITY PROFILE WILL BE A KEY HERE, BUT ATTM IT
APPEARS LESS IMPRESSIVE IN REALITY THAN IT DOES IN PARAMETER SPACE.
NONETHELESS, OBVIOUSLY CAN`T COMPLETELY RULE OUT SEVERE RISK IN THIS
TYPE OF ENVIRONMENT. THE BEST CHANCE FOR SEVERE WIND/HAIL THREAT
WILL MOST LIKELY LIE ALONG THE EHI MAX WHICH IS CURRENTLY PROGGED TO
ALIGN ALONG/NORTH OF M59, BUT ALL AREAS WILL CARRY AT LEAST A LOW
SLIGHT RISK THROUGH 12Z.

LONG TERM...

THE NEXT SHORT WAVE WILL BE CLOSE ON THE HEELS OF TODAY`S SYSTEM AND
ARRIVE IN TIME TO BRING UNSETTLED WEATHER IN SE MICHIGAN FOR MUCH OF
SUNDAY. THE SYSTEM IS INDICATED IN MORNING SATELLITE IMAGERY MOVING
THROUGH CENTRAL CANADA AND DISPLAYS BOTH ENERGETIC CIRCULATION AND
TEXTURE/CONVECTION IN SUPPORT OF DEPICTIONS IN THE NEW MODEL CYCLE.
UPPER AIR OBSERVATIONS AND MODEL ANALYSIS FIELDS INDICATE
CONSIDERABLY COLDER AIR ALOFT WITH THIS SYSTEM...PUSHING -20C AT 500
MB...THAT WILL BE CARRIED OVER LOWER MICHIGAN DURING DAYTIME
HEATING. IN ADDITION...THE WAVE ITSELF WILL FEED OFF CONTINUED LONG
WAVE AMPLIFICATION AND 100 KT UPPER JET AXIS FOR STRENGTHENING
DYNAMIC SUPPORT OF LARGE SCALE LIFT. IT`S NO SURPRISE THAT MODEL
SOUNDINGS CARRY AN UNSTABLE PROFILE THROUGH THE NIGHT AND SUNDAY
MORNING AS 0-6KM BULK SHEAR RISES TO 50 KNOTS. AN IMPORTANT
DIFFERENCE AMONG THE INDIVIDUAL SOLUTIONS IS IN THE HANDLING OF
BOUNDARY LAYER MOISTURE AND THE RESULTING CAPE PROFILE AS THE COLD
CORE ALOFT MOVES OVERHEAD. THE NAM IS ALONE IN MAINTAINING SURFACE
DEWPOINT IN THE UPPER 60S WITH LITTLE OR NO DRY AIR ADVECTION BEHIND
THE PREFRONTAL TROUGH DRIVING TODAY`S ACTIVITY. THE RESULT IS
SURFACE BASED CAPE NEAR 1500 J/KG BY 15Z ON EARLY DAY MAX TEMPS IN
THE UPPER 60S TO LOWER 70S WHILE THE GFS, ECMWF, AND MOST OF THE
LATEST SREF MEMBERS DROP CAPE BELOW 1000 J/KG WITH SURFACE DEWPOINT
IN THE UPPER 50S TO PERHAPS LOWER 60S OVER THE FAR SOUTHEAST.
HOWEVER...THERE IS SOME MERIT TO THE NAM SOLUTION AS SURFACE
PRESSURE FALLS OVER THE UPPER MIDWEST ELONGATE THE PRIMARY SURFACE
TROUGH ENOUGH TO ALTER THE DOWNSTREAM WIND PATTERN AND POSSIBLY
STALL THE EXIT OF THE LOW LEVEL THETA-E RIDGE AND/OR SIPHON SOME
EXTRA MOISTURE WESTWARD. AT THIS POINT...IT SEEMS PRUDENT TO MAKE
THE SYSTEM PROVE IT WON`T OVERACHIEVE. THEREFORE...LIKELY POPS FOR
SHOWERS WITH A CHANCE OF THUNDERSTORMS WILL BE MAINTAINED IN THE
FORECAST. CONVECTIVE ORGANIZATION WILL STRONGLY DEPEND ON THE
EVENTUAL AMOUNT OF INSTABILITY AVAILABLE TO GO ALONG WITH THE STRONG
WIND FIELD. STORMS HAVE THE POTENTIAL FOR STRONG WIND GUSTS AND
SMALL HAIL GIVEN THE COLD CORE ALOFT AND LOW FREEZING LEVEL.

THE SUNDAY SYSTEM WILL EXIT THE GREAT LAKES QUICKLY BY EVENING AND
OPEN THE DOOR FOR COOLER AIR TO SPREAD OVER THE REGION THROUGH
SUNDAY NIGHT. STRONG LOW LEVEL COLD ADVECTION SHOWN IN ALL MODEL
SOLUTIONS WILL CARRY 850 MB TEMPS DOWN INTO THE SINGLE DIGITS BY
MONDAY MORNING ON ROBUST NORTHERLY FLOW. THIS WILL CONFINE MAX TEMPS
TO THE LOWER HALF OF THE 60S...AND LIKELY LIMIT TO UPPER 50S NEAR
THE LAKE HURON SHORELINE.

GLOBAL MODEL SOLUTIONS ARE STABLE ON THE LONG WAVE UPPER LEVEL
PATTERN DURING MID WEEK. A FULL LATITUDE TROUGH OFF THE PACIFIC
COAST WILL ANCHOR A BROAD/DEAMPLIFIED RIDGE OVER MUCH OF NORTH
AMERICA AS THE EASTERN CANADIAN UPPER TROUGH MOVES TOWARD THE NORTH
ATLANTIC. THE BROAD NATURE OF THE RIDGE WILL ALLOW A SLOW ABSORPTION
OF THE CALIFORNIA CLOSED LOW BACK INTO THE WESTERLIES AND AN EQUALLY
SLOW MIGRATION INTO THE PLAINS. THE PRIMARY IMPACT THIS HAS ON THE
FORECAST FOR SE MICHIGAN IS RELATED TO AN EASTWARD DISPLACEMENT OF
NORTHWEST FLOW ALOFT AND A LESS AGGRESSIVE BACKDOOR SURFACE FRONT
INTO THE GREAT LAKES TUESDAY. THIS WAS A BENIGN FEATURE IN PREVIOUS
MODEL RUNS AND IS RENDERED EVEN MORE SO ON THE BACKGROUND OF THE
LATEST DEPICTIONS OF LARGE SCALE PATTERN EVOLUTION. THE END RESULT
WILL BE A RESURGENCE OF SURFACE HIGH PRESSURE THROUGH THE MID WEEK
PERIOD THAT WILL SUPPORT DRY WEATHER WITH NEAR NORMAL TEMPERATURES
UNTIL POSSIBLY FRIDAY WHEN REMNANTS OF THE PLAINS SHORT WAVE COULD
BRING A LOW CHANCE OF RAIN.

MARINE...

SMALL CRAFT ADVISORIES REMAIN IN EFFECT THROUGH TODAY FOR SAGINAW
BAY AROUND THE TIP OF THE THUMB. THIS IS WHERE SOUTHWEST FLOW WILL
BE STRONGEST AS IT CHANNELS DOWN THE BAY AND ALONG THE THUMB
SHORELINE. CONDITIONS WILL BECOME INCREASINGLY UNSETTLED ACROSS ALL
MARINE AREAS AS A PATTERN OF SHOWERS AND THUNDERSTORMS DEVELOPS AND
MOVES ACROSS THE REGION DURING THE AFTERNOON AND TONIGHT. ADDITIONAL
SHOWERS AND STORMS ARE THEN POSSIBLE WITH THE PRIMARY COLD FRONT
THAT WILL SHIFT THE WIND TOWARD THE NORTH SUNDAY AND SUNDAY NIGHT
THROUGH MONDAY. SMALL CRAFT ADVISORIES MAY BE NEEDED AGAIN FOR THE
LARGE WAVE RESPONSE ON THE SOUTH HALF OF LAKE HURON. HIGH PRESSURE
WILL EXERT FAVORABLE INFLUENCE ON THE WATERS DURING THE MIDDLE OF
NEXT WEEK.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.

LAKE HURON...
     SMALL CRAFT ADVISORY...SAGINAW BAY AND THE NEARSHORE WATERS FROM
     PORT AUSTIN TO HARBOR BEACH...UNTIL 8 PM SATURDAY.

LAKE ST CLAIR...NONE.

MI WATERS OF LAKE ERIE...NONE.

&&

$$

AVIATION.....JVC
SHORT TERM...JVC
LONG TERM....BT
MARINE.......BT


YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE
AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).








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