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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
710 AM EDT FRI AUG 29 2014

.AVIATION...

//DISCUSSION...

VFR CONDITIONS ARE EXPECTED OVER THE NEXT 24 HOURS.  A WARM FRONT 
WILL LIFT NORTH THROUGH THE WESTERN GREAT LAKES...BUT BULK OF SHOWER 
AND THUNDERSTORM ACTIVITY IS EXPECTED TO REMAIN WEST OF SOUTHEAST 
MICHIGAN. LIGHT SOUTHEAST WINDS (10 KNOTS OR LESS) WILL PERSIST FOR 
MUCH OF THE PERIOD.

//DTW THRESHOLD THREATS...

* LOW CONFIDENCE IN CEILINGS AOB 5000 FEET TOMORROW

&&

.PREV DISCUSSION...ISSUED 337 AM EDT FRI AUG 29 2014 

SHORT TERM...TODAY AND TONIGHT

UPPER LEVEL RIDGE AND DRY AIR (41 C DEW PT DEPRESSION AT 700 MB
PER 00Z DTX SOUNDING) IN PLACE OVER SOUTHEAST MICHIGAN EARLY THIS
MORNING. THE MID LEVELS WILL BEGIN TO MOISTEN UP AS THE DAY WEARS
ON WITH THE RIDGE AXIS DRIFTING EAST...BUT THE SURFACE FLOW WILL
REMAIN OUT OF THE SOUTHEAST...MAINTAINING STABLE CONDITIONS (SFC
LI'S ON THE POSITIVE SIDE) THROUGH TONIGHT (PER 00Z NAM). IT IS
NOT INCONCEIVABLE A FEW SHOWERS MAY SNEAK INTO THE WESTERN PART OF
THE CWA AS THE OVERALL MEAN MOISTURE WILL BE HIGH...PW VALUES
AROUND 1.75 INCHES. IN ADDITION...UPPER LEVEL PV EJECTING FROM
LOWER MISSISSIPPI RIVER VALLEY WILL TRACK THROUGH THE OHIO VALLEY
AND CLIP SOUTHEAST MICHIGAN EARLY THIS EVENING.

925 MB TEMPS OF 19 C THIS AFTERNOON (21Z) SUPPORT MAXES 
PREDOMINATELY IN THE UPPER 70S. FURTHER WARMING OF THE LOW LEVELS 
WILL TAKE PLACE TONIGHT...AS 925 MB TEMPS PROGGED TO RISE TO AROUND 
23 C...WHICH COUPLED WITH CANOPY OF MID/HIGH CLOUDS EXPECTED TO HOLD 
MINS IN THE MID 60S.

LONG TERM...

THE HOLIDAY WEEKEND WEATHER WILL INCLUDE WARM TEMPERATURES TO GO 
ALONG WITH AN ACTIVE RAINFALL PATTERN TO START AND POSSIBLY TO END 
THE PERIOD. THE BEST CHANCE OF RAIN WILL BE DURING SATURDAY AND 
SATURDAY NIGHT WITH A BREAK MOST AREAS SUNDAY AND THEN A RENEWED 
CHANCE OF SHOWERS AND STORMS MONDAY AFTERNOON INTO MONDAY NIGHT.

SURFACE LOW PRESSURE WILL TRACK FROM CENTRAL WISCONSIN TO THE 
EASTERN U.P. DURING SATURDAY WITH THE WARM SECTOR FIRMLY OVER SE 
MICHIGAN. A RICH LOW LEVEL MOISTURE AXIS CONTINUES TO PRESENT HEAVY 
RAINFALL AND THUNDERSTORM POTENTIAL AS THE MAIN CONCERNS IN AN 
ENVIRONMENT OF MODEST INSTABILITY BUT WITH A ROBUST WIND PROFILE. 
MODEL DEPICTIONS OF LOW LEVEL DEWPOINT INDICATE READINGS NEAR 15C 
BEING PUMPED IN BY A 30 KNOT LOW LEVEL JET AND 700 MB DEWPOINT NEAR 
3C WITH SIMILAR WIND SPEED AT THAT LEVEL ON UP TO 500 MB. FORECAST 
SOUNDINGS SHOW SURFACE BASED CAPE BUILDING INTO THE 1000-1500 J/KG 
WITH LOW DENSITY AND MOSTLY IN A NARROW BAND WITHIN A WEAK SURFACE 
PRESSURE TROUGH WHERE DEWPOINT REACHES 70F. THE WEAK SURFACE TROUGH 
IS SYMPTOMATIC OF THE SYNOPTIC SCALE ENVIRONMENT OVERALL AS THE UPPER 
WAVE SHEARS TO OUR WEST AND NORTH. THE WEAK LARGE SCALE FORCING IS A 
CONCERN BUT THE HIGH AMOUNT OF MOISTURE COUPLED WITH DAYTIME HEATING 
SHOULD RESULT IN AT LEAST SCATTERED COVERAGE IN SUPPORT OF THE UPPER 
END CHANCE POPS IN OUR GOING FORECAST. THE STRONG WIND PROFILE 
RAISES A CONCERN FOR SEVERE POTENTIAL BUT NEGATIVE FACTORS APPEAR TO 
WEIGH MORE AGAINST AT THIS POINT MAINLY DUE TO THE WEAKLY FORCED 
SYNOPTIC SCALE AND SUB 6C/KM 700-500 MB LAPSE RATES THROUGH PEAK 
HEATING. THERE WILL BE A TENDENCY FOR MATURE CONVECTION TO ADOPT 
LINEAR MODE BUT WITH LOW POTENTIAL FOR COLD POOL DEVELOPMENT. 
LOCALLY HEAVY RAINFALL WILL BE MORE OF CONCERN GIVEN THE DEEP 
MOISTURE PROFILE...ESPECIALLY IF ACTIVITY LINES UP PARALLEL TO THE 
MEAN FLOW...AND DESPITE INDIVIDUAL CELL MOTION MOVING ALONG AT 30 
KNOTS.

SATURDAY AFTERNOON CONVECTION WILL LAST WELL INTO SATURDAY EVENING 
BEFORE THE LOW LEVEL THETA-E RIDGE SLIDES EASTWARD AND OUT OF THE 
REGION OVERNIGHT. THE WEAK LOW PRESSURE SYSTEM WILL MANAGE TO PULL 
AN EQUALLY WEAK COLD FRONT INTO THE SOUTH HALF OF LOWER MICHIGAN BY 
SUNDAY MORNING WHICH WILL TAKE MOST OF THE DAY TO SETTLE SOUTHWARD 
AND EVENTUALLY DISSIPATE. DIFFERENCES IN MODEL SOLUTIONS BY THAT 
TIME ARE FOUND MAINLY WITH REGARD TO THE TIMING AND STRENGTH OF MID 
LEVEL SUBSIDENCE TRAILING THE EXITING LOW PRESSURE SYSTEM. ALL 
PRESENT MARKED DRYING OF MEAN RH TOWARD SUNDAY EVENING BUT PROBABLY 
TOO LATE TO RULE OUT A STRAY SHOWER OR THUNDERSTORM ALONG THE FRONT 
BEFORE IT DISSIPATES...ESPECIALLY FROM THE EASTERN THUMB TOWARD THE 
OHIO BORDER WHERE SOME LATE SEASON LAKE BREEZE ENHANCEMENT WILL BE 
POSSIBLE. THAT BEING SAID...EXPECT COVERAGE TO BE CONSIDERABLY LESS 
COMPARED TO SATURDAY WITH LESS CONFIDENCE IN THE CAPABILITY OF THE 
FRONT WITHIN THE SHORT WINDOW OF DAYTIME HEATING AND LINGERING 
WEAK LARGE SCALE SUPPORT. 

THE WEAK SHORT WAVE RIDGE BETWEEN SYSTEMS WILL KEEP SUNDAY NIGHT DRY 
AS THE NEXT LOW PRESSURE SYSTEM MOVES THROUGH THE MIDWEST INTO 
NORTHWEST ONTARIO BY MONDAY. THIS IS A LARGER SYSTEM AND MODEL 
AGREEMENT IS GOOD ON THE POSITION OF THE SURFACE LOW AND TRAILING 
COLD FRONT. RAIN POTENTIAL TO WRAP UP THE HOLIDAY WEEKEND WILL BE 
TIED TO THE RENEWED DEVELOPMENT OF A LOW LEVEL GULF MOISTURE STREAM 
AND THE EASTWARD TIMING OF THE COLD FRONT...BOTH OF WHICH FAVOR 
AFTERNOON AND EVENING TIMING AT THIS POINT. THE MID WEEK PERIOD THEN 
HAS POTENTIAL TO REMAIN CONVECTIVELY ACTIVE IF GLOBAL MODEL 
SOLUTIONS HOLD AND THE FRONT STALLS NEAR THE OHIO BORDER TUESDAY 
INTO WEDNESDAY.

MARINE...

LOW PRESSURE MOVING THROUGH THE PLAINS TODAY WILL REACH CENTRAL 
WISCONSIN SATURDAY MORNING AND THEN THE NORTH SHORE OF LAKE HURON 
SATURDAY NIGHT. THIS SYSTEM WILL PRODUCE MODERATE TO FRESH SOUTHERLY 
FLOW OVER ALL MARINE AREAS. INCREASING POTENTIAL FOR 25 KNOT GUSTS 
SATURDAY WILL BE ACCOMPANIED BY A CHANCE OF THUNDERSTORMS SATURDAY 
AFTERNOON AND EVENING. THIS WILL BE THE MOST ACTIVE PERIOD OF WIND 
AND PRECIPITATION DURING THE HOLIDAY WEEKEND. IMPROVED MARINE 
CONDITIONS ARE EXPECTED SUNDAY AND MOST OF MONDAY BEFORE THE CHANCE 
OF SHOWERS AND STORMS RETURN WITH THE NEXT LOW PRESSURE SYSTEM INTO 
MONDAY NIGHT.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.

&&

$$

AVIATION.....SF
SHORT TERM...SF
LONG TERM....BT
MARINE.......BT


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