Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 260137
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
937 PM EDT MON AUG 25 2014

.UPDATE...

THE NEW 00Z DTX SOUNDING SHOWS THE SHALLOW BUT EFFECTIVE CAP THAT
KEPT STORMS AT BAY DURING THE LATE AFTERNOON AND EARLY EVENING.
ATTENTION THEN TURNS TO CONVECTIVE TRENDS FOR THE REST OF THE
NIGHT THROUGH TUESDAY MORNING...AHEAD OF THE COLD FRONT THAT IS
STILL ON SCHEDULE FOR TUESDAY AFTERNOON. THERE REMAINS POTENTIAL
FOR NOCTURNAL DEVELOPMENT IN THE VICINITY OF THE ILLINOIS-WISCONSIN
BORDER AND THE SOUTH HALF OF LAKE MICHIGAN AS LOW LEVEL FLOW
INCREASES BETWEEN THE FRONT AND THE MESOHIGH LEFT OVER FROM THE
EARLIER MCS. THIS WILL ADVECT A BUBBLE OF HIGHER THETA-E AIR INTO
THE SOUTH HALF OF LOWER MICHIGAN AS SHOWN IN MODEL 870-700 MB
FIELDS. THIS COULD RESULT IN SMALL CLUSTERS OF CONVECTION JUST
ABOUT ANY TIME...BUT THE WIND FIELD IS MODEST ENOUGH TO BRING
COVERAGE INTO QUESTION. THE EARLIER UPDATE BROADENED A LOW END
CHANCE POP IN FORECAST FOR ALL AREAS.

&&

.AVIATION...ISSUED 657 PM EDT MON AUG 25 2014

//DISCUSSION...

THE WEST TO EAST INSTABILITY GRADIENT OVER LOWER MICHIGAN IS
REVEALED BY THE INABILITY OF STORMS TO MAKE IT INTO SE MICHIGAN...
OR DEVELOP OVERHEAD...DURING THE LATE AFTERNOON. THERE IS STILL A
POSSIBILITY OF LATE DEVELOPMENT BUT COVERAGE AND LOCATION ARE TOO
UNCERTAIN TO INCLUDE AT ANY ONE TERMINAL SITE. EXPECT VFR DEBRIS
CLOUDS TO MOVE OVER THE REGION DURING THE EVENING AND GIVE WAY TO
MVFR STRATUS IN THE SOUTHERLY FLOW AHEAD OF THE COLD FRONT
OVERNIGHT. NOCTURNAL THUNDERSTORM DEVELOPMENT IS A POSSIBILITY
BEHIND THE ONGOING STORMS OVER WISCONSIN AND ILLINOIS. THE LATEST
DATA FAVORS A TRACK TO OUR SOUTH AND WEST ALONG THE INSTABILITY AXIS
FOR ANY ORGANIZED ACTIVITY WITH A FEW CLUSTERS POSSIBLE OVERHEAD IN
SE MICHIGAN THROUGH TUESDAY MORNING. THE BEST CHANCE OF STORMS IN
OUR AREA WILL BE WITH THE PASSAGE OF THE COLD FRONT TUESDAY
AFTERNOON.

//DTW THRESHOLD THREATS...

* MEDIUM CONFIDENCE IN CEILING AOB 5KFT OVERNIGHT THROUGH TUESDAY
MORNING.

* LOW CONFIDENCE THAT A TSTORM WILL IMPACT KDTW TONIGHT.

&&

.PREV DISCUSSION...ISSUED 256 PM EDT MON AUG 25 2014

SHORT TERM...THIS AFTERNOON AND TONIGHT

18Z MESOANALYSIS REVEALS WEAKLY SHEARED BUT MODERATELY UNSTABLE
ENVIRONMENT IN PLACE ACROSS SOUTHEAST MICHIGAN. MLCAPE RANGES FROM
AS HIGH AS 1500 J/KG WEST OF US-23 TO JUST OVER 500 J/KG IN THE
THUMB FUELED LARGELY BY DEWPOINTS IN THE UPPER 60S TO LOW 70S.
CONVECTIVE COVERAGE WITHIN THIS ENVIRONMENT HAS, HOWEVER, BEEN QUITE
LIMITED OWING TO A STOUT H8-H7 CAP AND A LACK OF LARGER SCALE
FORCING. AS A RESULT, ACTIVITY HAS BECOME FOCUSED ALONG AN AGGREGATE
OUTFLOW BOUNDARY STRETCHING FROM NORTHERN ILLINOIS INTO LOWER
MICHIGAN AND IS STRUGGLING EASTWARD. ONGOING THETA-E ADVECTION
WITHIN THE MOISTURE AXIS PRECEDING THE UPSTREAM COLD FRONT WILL
CONTINUE HELP TO MAINTAIN THIS ACTIVITY INTO THE EVENING HOURS,
PARTICULARLY WHERE STORM ORIENTATION HAS BECOME ORTHOGONAL TO
WEAKENING LLJ INFLOW, SUCH AS THAT IN THE I-94 TO I-96 CORRIDOR AT
19Z. THERMODYNAMIC PROFILES FEATURING ABUNDANT MID-LEVEL DRY AIR AND
DRY ADIABATIC BOUNDARY LAYER LAPSE RATES SUGGESTS A LIMITED SEVERE
WIND THREAT. HOWEVER, IT HAS BECOME CLEAR THAT CAPPING IS TOO STRONG
TO SUPPORT UPDRAFTS CAPABLE OF REALIZING THIS POTENTIAL. THUS,
NOTHING MORE THAN AN ISOLATED WIND GUST TO 40 MPH IS EXPECTED.

SOME SEMBLANCE OF THIS ACTIVITY, OR EVEN A MODEST EXPANSION IN
COVERAGE, MAY HOLD TOGETHER INTO THE OVERNIGHT HOURS AS STEEPER
MID-LEVEL LAPSE RATES STREAM INTO THE AREA IN ASSOCIATION WITH THE
FLATTENING UPPER RIDGE. THUS, MAINTAINED LOW CHANCE POPS FOR ALL
AREA THROUGH THE OVERNIGHT HOURS TO ACCOUNT FOR NOTHING MORE THAN
ISOLATED CONVECTIVE COVERAGE.

HIGH SURFACE MOISTURE CONTENT CHARACTERIZED BY DEWPOINTS AROUND 70
DEGREES AND ONGOING VEERED SWLY FLOW IN ADVANCE OF THE COLD FRONT
WILL KEEP OVERNIGHT LOWS IN THE 70S.

LONG TERM...

UNSTABLE AIRMASS WILL BE PRESENT ON TUESDAY AS PROGRESSIVE MOISTURE
AXIS AHEAD OF THE COLD FRONT WORKS THROUGH THE STATE. MID LEVEL
LAPSE RATES (700-500 MB) AROUND 7 C/KM WITH MLCAPES AROUND 3000
J/KG. LITTLE IF ANY CAP PRESENT...BUT 12Z MODELS (NAM/GEM/GFS) ARE
NOT VERY BULLISH WITH THE PROSPECTS OF CONVECTION...AS THERE IS
LITTLE IN THE WAY OF CONVERGENCE WITH THE FRONT...ALONG WITH 500 MB
HEIGHT FALLS THAT ARE NON-EXISTENT OR MARGINAL AT BEST.
HOWEVER...IF/WHEN A FEW CELLS GET GOING...OUTFLOW BOUNDARIES WILL
PROVIDE ENHANCED CONVERGENCE...AND PREFER TO MAINTAIN HIGHER END
CHANCE POPS. PW VALUES OF 1.75 INCHES WILL SUPPORT A HEAVY RAIN
THREAT. INITIATION TIME OF CONVECTION IS UNKNOWN (DUE TO EVALUATION
OF TONIGHT`S ACTIVITY) AND WILL PLAY A ROLE IN MAX TEMPS...BUT UPPER
80S SEEMS REASONABLE AND A MIDDLE OF THE ROAD FORECAST. THE
SOUTHWARD PROGRESS OF THE SURFACE COLD FRONT AND MOISTURE AXIS WILL
BE KEY AS WE HEAD INTO WEDNESDAY...AS STRONG UPPER WAVE/TROUGH
CURRENTLY OVER ALBERTA SWEEPS THROUGH THE GREAT LAKES REGION
WEDNESDAY MORNING. IT LOOKS TO BE A CLOSE CALL WITH THE SHARP
MOISTURE GRADIENT IN PLACE...BUT WILL COUNT ON THE ENHANCED LOW
LEVEL RIDGING EXPANDING IN...AND CARRY JUST SLIGHT CHANCE POPS OVER
SOUTH HALF OF THE CWA. SEASONABLY STRONG SURFACE HIGH (1021 MB)
PROGGED TO BUILD IN ON THURSDAY UNDERNEATH NORTHWEST CONFLUENT
FLOW...SUPPORTING A DRY DAY WITH SOLIDLY BELOW NORMAL TEMPERATURES
AS 850 MB TEMPS FALL TO 10 C OR SLIGHTLY COOLER.

HIGH PRESSURE WILL SLOWLY SLIDE EASTWARD ON THURSDAY NIGHT BEFORE A
LOW PRESSURE SYSTEM MOVES INTO THE WESTERN GREAT LAKES EARLY FRIDAY
MORNING. RAIN SHOWERS SHOULD BEGIN EARLY FRIDAY MORNING. THE CHANCE
OF RAIN SHOWERS WILL CONTINUE THROUGH THE WEEKEND AS THIS LOW
PRESSURE SYSTEM TRACKS ACROSS THE GREAT LAKES AND IS FOLLOWED BY A
SERIES OF DISTURBANCES IMPACTING THE REGION THROUGH SUNDAY. HIGH
TEMPERATURES WILL BEGIN IN THE MID 70S ON FRIDAY BEFORE RISING INTO
THE UPPER 70S TO LOW 80S THROUGH THE WEEKEND. LOW TEMPERATURES WILL
REMAIN STEADY IN THE LOW TO MID 60S ACROSS THE CWA.

MARINE...

LOW LEVEL WINDS HAVE VEERED AROUND TO A MORE SOUTHERLY DIRECTION
AHEAD OF A SLOW MOVING COLD FRONT...WHICH WILL CLEAR THE CENTRAL
GREAT LAKES LATE TOMORROW. VERY WARM AIRMASS AHEAD OF THE FRONT IS
LEADING TO STRONG STABILITY OVER THE COOL LAKES...KEEPING WIND AND
WAVES AT A MINIMUM. SCATTERED THUNDERSTORMS TRIGGERED BY THE FRONT
WILL BE STRONG TO POSSIBLY SEVERE STORM. THE FRONT WILL STALL OUT
JUST TO THE SOUTH OF THE SOUTHERN GREAT LAKES DURING THE MID WEEK
PERIOD...WITH HIGH PRESSURE ARRIVING OVER LOWER MICHIGAN ON
THURSDAY. NORTHWEST WINDS PRECEDING THE HIGH ON WEDNESDAY SHOULD
REACH AROUND 20 KNOTS OVER NORTHERN LAKE HURON.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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


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