Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181922
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
322 PM EDT MON AUG 18 2014

.SHORT TERM...TONIGHT

RIDGING AT THE SURFACE AND ALOFT THIS AFTERNOON OVER SOUTHEAST
MICHIGAN WILL BE SLIDING EAST EARLY THIS EVENING...ALLOWING FOR A
MODERATE SURGE OF MOISTURE...AS PW VALUES RAMP UP TO AROUND 1.5
INCHES AROUND OR JUST AFTER MIDNIGHT. ALTHOUGH...THE 850-700 MB
THETA-E RIDGE AXIS IS RATHER DIFFUSE AND NOT TERRIBLY FOCUSED. THROW
IN THE FACT THERE IS NOT MUCH OF A LOW LEVEL JET DEVELOPING TONIGHT
AHEAD OF OUR APPROACHING LOW PRESSURE SYSTEM ENTERING THE WESTERN
GREAT LAKES...AND CAN NOT ARGUE FOR MORE THAN CHANCE POPS
TONIGHT...WITH THE BEST CHANCE ACROSS THE TRI-CITIES REGION.
INSTABILITY IS MARGINAL...AS SHOWALTER INDEX GOES EVER SO SLIGHTLY
NEGATIVE FOR THE MOST PART...THUS JUST A SLIGHT CHANCE OF THUNDERSTORMS
(MAINLY WEST OF U.S 23). SURFACE DEW PTS AOA 60 DEGREES POINT TO
MINS PREDOMINATELY IN THE LOWER 60S.

&&

.LONG TERM...

THIS AFTERNOON`S WATER VAPOR IMAGERY INDICATES A CLOSED CIRCULATION
OVER EXTREME NORTHWEST MINNESOTA. THIS LOW IS FORECAST TO CONTINUE
CUTTING OFF FROM THE PREVAILING WESTERLY FLOW AND SLOWLY DRIFT
ACROSS THE GREAT LAKES TUESDAY THROUGH WEDNESDAY NIGHT.

SOUTHWESTERLY FLOW WILL INCREASE THROUGH THE DAY TUESDAY AS
WARM/MOIST ADVECTION AND ISENTROPIC ASCENT INCREASE IN ASSOCIATION
WITH EASTWARD PROGRESSION OF HEIGHT FALLS AND STRONGER SYNOPTIC SCALE
ASCENT. RATHER ILL-DEFINED FORCING IN THE ABSENCE OF A WELL-DEFINED
MID-LEVEL WAVE IS EXPECTED TO RESULT IN NOTHING MORE THAN AN
ISOLATED TO SCATTERED COVERAGE OF WARM ADVECTION TYPE CONVECTION
THROUGH THE FIRST HALF OF TUESDAY, MAINLY FURTHER WEST. WARM SW
FLOW WILL RESULT IN THE DEVELOPMENT OF A MODERATE CAP BY TUESDAY
AFTERNOON, BUT THE GUIDANCE, LOW AND HIGH-RES ALIKE, UNIVERSALLY
ALLOW CONVECTION TO DEVELOP WITHIN PREFRONTAL TROUGHING DURING THE
PEAK HEATING HOURS... A REASONABLE ENOUGH EXPECTATION. DEPENDING ON
EXACT TIMING, MODERATE DESTABILIZATION ACROSS THE EASTERN THIRD OR
SO OF THE CWA COULD RESULT IN A MARGINAL SEVERE THREAT, ALTHOUGH
UPDRAFTS WILL STRUGGLE TO ORGANIZE GIVEN THE SOUTHWARD DISPLACEMENT
OF THE BETTER SHEAR. THE BIGGER THREAT ATTM WILL BE HEAVY DOWNPOURS
GIVEN THE HIGH MOISTURE CONTENT AND DEEPENING WARM CLOUD LAYER FROM
8KFT IN THE MID-AFTERNOON TO 10KFT BY EVENING. A GENERAL HALF INCH
TO ISOLATED ONE INCH AMOUNT SEEMS REASONABLE. AMOUNTS MUCH HIGHER
THAN THAT ARE UNLIKELY GIVEN THE LACK OF ORGANIZED LARGER SCALE
FORCING AND THE EXPECTED TRANSIENT NATURE OF LOW-LEVEL CONVERGENT
AXIS.

A PERIOD OF DOWNTIME WILL ENSUE DURING THE LATE EVENING/OVERNIGHT
TUESDAY AS THE INITIAL FORCING DEPARTS PROGRESSES EAST AND NOCTURNAL
STABILIZATION GETS UNDERWAY. THE CLOSED UPPER LOW IS THEN FORECAST TO
SETTLE ALMOST DIRECTLY OVERHEAD ON WEDNESDAY. MOISTURE CONTENT WILL
REMAIN RATHER ROBUST, AROUND 1.5" PWAT WITH DEWPOINTS APPROACHING
70F. THE PRIMARY PV ANOMALY IS FORECAST TO ROTATE ACROSS THE AREA
DURING THE DAY, THE EXACT TIMING OF WHICH WILL PLAY A KEY ROLE IN
INTENSITY/COVERAGE OF THE CONVECTIVE RESPONSE. ONCE AGAIN, THE
PRIMARY CONCERN WILL BE HEAVY DOWNPOURS AS OPPOSED TO LEGITIMATE
SEVERE WEATHER. IT IS NOT OUT OF THE QUESTION THAT THE HEAVIEST
RAINFALL ACTUALLY OCCURS WEDNESDAY GIVEN THE LACK OF DEEP LAYER FLOW
WITH THE LOW OVERHEAD. SUBSIDENCE IN THE WAKE OF THE PV ANOMALY AND
NOCTURNAL COOLING WILL BRING AN END TO CONVECTION WEDNESDAY EVENING.
HIGHS BOTH TUES AND WED WILL BE SEASONAL, AROUND 80, THOUGH
INCREASING MOISTURE WILL LEAD TO A MUGGY FEEL AS WELL AS WARMER
OVERNIGHT LOWS IN THE MID 60S.

RAIN CHANCES WILL STICK AROUND THROUGH MUCH OF THE EXTENDED PERIOD
AS A SERIES OF SHORTWAVES AND LOW PRESSURE SYSTEMS IMPACT SOUTHEAST
MICHIGAN. HAVE LEFT CHANCE POPS IN THE FORECAST THROUGH SUNDAY
NIGHT. TIMING AND COVERAGE OF THE RAIN WILL HAVE TO BE FINE TUNED AS
THE WEEK PROGRESSES AND MODELS GET A BETTER HANDLE ON THESE
DISTURBANCES. TEMPERATURES WILL REMAIN FAIRLY CONSISTENT WITH HIGHS
IN THE UPPER 70S AND LOW 80S AND LOW TEMPERATURES IN THE MID 60S
THROUGH SUNDAY.

&&

.MARINE...

LIGHT WINDS WILL GIVE WAY TO A MODERATE SOUTHEAST BREEZE TUESDAY
INTO TUESDAY EVENING AS A WEAK LOW DRIFTS TROUGH THE AREA. WINDS
WILL BE THE STRONGEST ACROSS NORTHERN LAKE HURON BUT GUSTS THERE
WILL STILL STRUGGLE TO BREAK 20 KNOTS. THIS LOW WILL ALSO BRING A
CHANCE FOR THUNDERSTORMS BOTH TUESDAY AND WEDNESDAY...PARTICULARLY
FOR THE SOUTHERN LAKE HURON BASIN AND POINTS SOUTH.

&&

.AVIATION...ISSUED 114 PM EDT MON AUG 18 2014

//DISCUSSION...

A RIDGE OF HIGH PRESSURE WILL HOLD FOR THE FIRST HALF OF THE TAF
PERIOD...BUT ENOUGH LOW LEVEL MOISTURE AROUND THIS AFTERNOON TO
SUPPORT SCT TO BKN LOW VFR CEILINGS. A LOW PRESSURE SYSTEM ENTERING
THE WESTERN GREAT LAKES TONIGHT WILL THEN LEAD TO AN INCREASING
CHANCE OF SHOWERS AND THUNDERSTORMS LATE TONIGHT INTO
TOMORROW...WITH SHOWERS AND EMBEDDED THUNDERSTORMS LIKELY BY
TOMORROW AFTERNOON. A FURTHER INCREASE IN MOISTURE TONIGHT SHOULD
SUPPORT CEILINGS AT OR BELOW 5000 FEET BY SUNRISE TUESDAY.

//DTW THRESHOLD THREATS...

* LOW CONFIDENCE CEILINGS WILL BE AOB 5000 FEET THROUGH
  TONIGHT...MEDIUM CONFIDENCE TOMORROW MORNING.

* LOW CONFIDENCE IN THUNDERSTORMS TOMORROW.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.

&&

$$

SHORT TERM...SF
LONG TERM....JVC/RK
MARINE.......JVC
AVIATION.....SF


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