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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
703 AM EDT TUE JUL 22 2014


.AVIATION...

//DISCUSSION...

THE APPROACH OF A COLD FRONT FROM THE NORTHWEST AND A DEEPENING 
AFTERNOON MIXED LAYER WILL LEAD TO INCREASED S-SW WINDS DURING THE 
AFTERNOON...WITH GUSTS OF 18 TO 22 KNOTS EXPECTED. A COLD FRONT IS 
FORECAST TO TRACK ACROSS SE MI...ENTERING MBS AROUND 04Z AND METRO 
DETROIT AROUND 09Z. THERE WILL BE A CHANCE OF THUNDERSTORMS ALONG AN 
IN ADVANCE OF THIS FRONT. SHALLOW POST FRONTAL COLD AIR ADVECTION 
AND LINGERING LOW LEVEL MOISTURE SUGGEST SOME MVFR STRATUS IS 
POSSIBLE FOLLOWING THE PASSAGE OF THE COLD FRONT.

FOR DTW...DESPITE THE LATE ARRIVAL OF THE COLD FRONT...ENHANCED 
CONVERGENCE AHEAD OF THE FRONT AND BUILDING INSTABILITY TONIGHT WILL 
STILL OFFER A CHANCE OF THUNDERSTORMS /MOST PROBABLE IN THE 06Z TO 
09Z TIME FRAME/. 

//DTW THRESHOLD THREATS...

* LOW CONFIDENCE IN THUNDERSTORM POTENTIAL TONIGHT.

&&

.PREV DISCUSSION...ISSUED 337 AM EDT TUE JUL 22 2014 

SHORT TERM...TODAY AND TONIGHT

A CONVECTIVELY ENHANCED MID LEVEL WAVE OVER NRN MN IS SHOWING UP 
NICELY ON THE SATELLITE/RADAR COMPOSITE. THIS WAVE WILL RACE EAST 
ACROSS LAKE SUPERIOR TODAY...WITH THE ASSOCIATED SFC LOW TRACKING 
EAST AS WELL. THIS SYSTEM WILL DRIVE AN ASSOCIATED COLD FRONT INTO 
NRN LOWER MI THIS EVENING...THEN ACROSS SE MI LATE TONIGHT. 

THE ATMOSPHERE REMAINS STRONGLY CAPPED FROM SRN MN ACROSS IOWA 
COURTESY OF AN EML. AN EXTENSION OF THIS EML WILL SLIDE INTO LOWER 
MI THIS AFTERNOON AND WILL ACT TO FURTHER STRENGTHEN THE CAPPING 
INVERSION ALREADY PRESENT OVER SE MI /PER 00Z DTX RAOB/. THIS CAP 
WILL HOLD THROUGH THE AFTERNOON AND SHOULD SUPPRESS CONVECTION OVER 
SE MI THROUGH AT LEAST 00Z THIS EVENING. STRENGTHENING S-SW FLOW IN 
ADVANCE OF THE FRONT WILL LEAD TO LOW LEVEL WARM AIR ADVECTION INTO 
SE MI...WITH 850MB TEMPS FORECAST TO RISE TO +20C. MODEL SOUNDINGS 
SUGGEST THE BOUNDARY LAYER WILL BECOME WELL MIXED THIS AFTERNOON 
WHICH WILL RESULT IN HIGH TEMPS INTO THE UPPER 80S/LOW 90S. THE 00Z 
MODEL INITIALIZATIONS SHOW AN AXIS OF LOWER DEWPOINTS FROM THE SFC 
TO 850MB EXTENDING FROM ARKANSAS UP THROUGH NE ILLINOIS...AND ARE 
SUPPORTED BY AVAILABLE SFC OBS AND 00Z RAOBS. THESE LOWER DEWPOINTS 
WILL ADVECT INTO SE MI DURING PEAK AFTERNOON HEATING AND WILL RESULT 
IN A DROP IN DEWPOINTS IN THE AFTERNOON. THIS WILL CUT BACK ON THE 
HUMIDITY AND THUS KEEP HEAT INDICES CLOSE TO ACTUAL AIR TEMPS. 

A DEEPENING OF THE LONGER WAVELENGTH TROUGH OVER NRN ONTARIO AND 
QUEBEC TONIGHT WILL LOWER MID LEVEL HEIGHTS OVER SE MI AND WILL 
ERODE THE CAPPING INVERSION. THIS WILL OCCUR AS THE COLD FRONT 
ADVANCES THROUGH THE FORECAST AREA BETWEEN 03Z AND 09Z. A NARROW 
AXIS OF HIGH LOW LEVEL MOISTURE WILL ACCOMPANY THE SFC 
FRONT...LEADING TO 0-3KM MU CAPE OF 1K TO 1500 J/KG. CONVECTIVE 
INITIATION WILL BE TIED TOWARD THE INCREASED BOUNDARY LAYER 
CONVERGENCE AHEAD OF THE FRONT OR WITHIN ANY PRE FRONTAL TROUGHING. 
WITH SOME UNCERTAINTY AS TO HOW QUICKLY THE EROSION OF THE MID LEVEL 
CAP WILL OCCUR AND WITH WEAK LARGER SCALE DYNAMIC FORCING...NOTHING 
HIGHER THAN CHANCE POPS TONIGHT SEEMS JUSTIFIED AT THIS TIME. WITH 
0-6KM SHEAR ON THE ORDER OF 30 KNOTS AND GOOD CAPE DENSITY SHOWN ON 
MODEL SOUNDINGS...CAN NOT RULE OUT AN ISOLATED SEVERE STORM TONIGHT 
/WITH DAMAGING WIND AND HAIL BEING THE BIGGEST CONCERN/. THE BETTER 
SEVERE POTENTIAL WILL BE SOUTHWEST OF THE AREA /IN THE REGION 
OUTLINED IN THE SPC DAY 1 SLIGHT RISK/ WHERE INSTABILITY WILL BE 
MUCH HIGHER.

LONG TERM...  

THE WEATHER THROUGH THE END OF THE WEEK WILL BE DOMINATED BY A 
STABLE CANADIAN HIGH PRESSURE SYSTEM. TEMPERATURES WEDNESDAY WILL BE 
APPROXIMATELY 12 TO 15 DEGREES COOLER THAN TUESDAY...READINGS 
RUNNING 4 TO 8 DEGREES COOLER THAN AVERAGE. DAYTIME TEMPERATURES ON 
THURSDAY SHOULD BE EVEN COOLER...WORKING OFF A CHILLY START WITH 
AFTERNOON VALUES STRUGGLING TO REACH THE MIDDLE 70S IN MANY 
LOCATIONS.

WEDNESDAY...LATEST GUIDANCE CONTINUES TO ADVERTISE THE MAIN SURFACE 
COLD FRONT SLIPPING ACROSS THE MICHIGAN/OHIO STATELINE SHORTLY AFTER 
12Z. THE H85 EXTENSION IS SHOWN TO LAG BACK SLIGHTLY NORTH...BUT 
EVEN THE MIDLEVEL FRONT DEPICTED TO EXIT THE CWA BY 15Z. AN 
EFFICIENT NORTHERLY FLOW TRAJECTORY SHOULD SCOUR OUT NEAR SURFACE 
Theta-e FAIRLY QUICKLY. THROW IN DIURNAL MIXING PROCESSES AND 
DEWPOINTS WILL DROP RAPIDLY INTO THE 50S. AS A RESULT OF THE LOWER 
HEAT CAPACITY OF THE DRY AIR...LOWERED TEMPS A COUPLE OF DEGREES 
ACROSS THE BOARD. THE SHORELINE AREAS NEAR LAKE HURON SHOULD BE 
ESPECIALLY CHILLY...FACTORING NORTH WINDS OF 20 TO 25 MPH.

THURSDAY...VERY STEEP LAPSE RATES IN THE H7-H8 LAYER WILL LEAD TO AN 
ACTIVE CUMULUS RESPONSE BY MIDDAY. VERY HIGH MIXING HEIGHTS OF UP TO 
6000 FT AGL...WILL EAT INTO CONVECTIVE DEPTHS AND LEAD TO LEGITIMATE 
POTENTIAL FOR PRECIPITATION.

MARINE...

THE MAIN ISSUE OF THE MARINE FORECAST IS THE STRONG SURGE OF 
NORTHERLY WINDS THAT WILL IMMEDIATELY FOLLOW THE COLD FRONT SET TO 
DROP ACROSS LAKE HURON. THE FRONT IS EXPECTED TO EMERGE OVER THE 
STRAITS REGION VERY EARLY THIS EVENING...EXITING THE FAR SOUTHERN 
LAKE HURON BASIN PRIOR TO DAYBREAK WEDNESDAY. A TIGHT PRESSURE 
GRADIENT WILL THEN PERSIST OUT OVER LAKE HURON THROUGHOUT THE DAY. 
WINDS OF UP TO 30 KNOTS WILL BE POSSIBLE TONIGHT THROUGH WEDNESDAY 
AFTERNOON ALONG WITH WAVE HEIGHTS IN THE 3 TO 5 OR 4 TO 6 FOOT 
RANGE. SMALL CRAFT ADVISORIES WILL EVENTUALLY BE NEEDED FOR THE 
NEARSHORE WATERS OF LAKE HURON FROM PORT AUSTIN TO PORT 
HURON...INCLUDING OUTER SAGINAW BAY. A RETURN TO MORE FAVORABLE 
BOATING CONDITIONS WILL OCCUR THURSDAY AND FRIDAY AS A LARGE SURFACE 
HIGH PRESSURE SYSTEM DOMINATES THE REGION.

&&

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

&&

$$

AVIATION.....SC
SHORT TERM...SC
LONG TERM....CB
MARINE.......CB


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