Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 200407
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1207 AM EDT WED AUG 20 2014

.AVIATION...

//DISCUSSION...

UPPER LEVEL LOW PRESSURE SPINNING OVER THE GREAT LAKES WILL SUPPORT
SCATTERED THUNDERSTORM COVERAGE DURING THE EARLY MORNING. THE LATEST
RADAR TREND INDICATES FNT AND THE DTW CORRIDOR MOST LIKELY TO
AFFECTED IN THE 06-09Z TIME WINDOW. IN BETWEEN AND AFTER...VARYING
AMOUNTS OF MID AND HIGH CLOUDS WILL ALLOW ENOUGH BOUNDARY LAYER
COOLING FOR FOG DEVELOPMENT. IFR CONDITIONS WILL INITIALLY CONSIST OF
STRATUS BUT THEN COMBINE WITH FOG THROUGH MID WEDNESDAY MORNING AS
THE UPSTREAM SURFACE LOW DRIFTS INTO NORTHERN LOWER MICHIGAN. A
TYPICAL CYCLE OF CEILING AND VISIBILITY IMPROVEMENT WILL THEN BE
ACCOMPANIED BY INCREASING SHOWER COVERAGE WITH DAYTIME HEATING
DURING THE AFTERNOON INTO WEDNESDAY EVENING.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILING AOB 5000 FT THROUGH MORNING.

* MEDIUM CONFIDENCE IN THUNDERSTORM COVERAGE AND TIMING.

* LOW CONFIDENCE IN CEILING AOB 200 FT/VISIBILITY 1/2SM AROUND
  SUNRISE.

&&

.PREV DISCUSSION...ISSUED 1027 PM EDT TUE AUG 19 2014

UPDATE...

THE EARLIER UPDATE MAINTAINED A CHANCE OF SHOWERS AND STORMS AND
ADDED FOG TO THE FORECAST FOR OVERNIGHT THROUGH MID WEDNESDAY
MORNING. THE COOL AND MOIST BOUNDARY LAYER IS PRIMED FOR FOG
DEVELOPMENT OVER THE TOP OF WET GROUND. LATE EVENING OBSERVATIONS
INDICATE MOSTLY A 1 TO 3 DEGREE SURFACE TEMP/DEWPOINT SPREAD
SUGGESTING FOG WILL DEVELOP QUICKLY UNDER ANY OPEN SKY. THE TIMING
AND EXTENT OF CLEARING IS THE MORE CHALLENGING QUESTION. SO
FAR...WEAK GRADIENT FLOW HAS LED TO STRATUS FORMATION IN THE
SAGINAW VALLEY WHERE MID AND HIGH CLOUDS HAVE PARTIALLY CLEARED
AWAY. AT THE SAME TIME...HOURLY MESOANALYSIS AND MODEL SOUNDINGS
INDICATE ENOUGH ELEVATED INSTABILITY LINGERING UNDER THE UPPER LOW
TO MAINTAIN AREAS OF MID TO HIGH CLOUDS AND SCATTERED COVERAGE OF
SHOWERS...AT LEAST FOR A WHILE. EXPECT THE SAGINAW VALLEY STRATUS TO
BUILD TOWARD THE SURFACE DURING THE NIGHT AND MORE OF AN INITIAL
FOG FORMATION IN THE DTW AREA AS GRADIENT FLOW WEAKENS IN ALL
AREAS AND AS HIGH CLOUDS DIMINISH LATER ON...BUT LOW CONFIDENCE
IN THE CLOUD TRENDS PREVENTS A MENTION OF DENSE FOG FOR NOW.

PREV DISCUSSION...ISSUED 318 PM EDT TUE AUG 19 2014

SHORT TERM...

WITH MLCAPES APPROACHING 2000 J/KG OVER PARTS OF THE CWA NOW THAT
THE BOUNDARY LAYER HAS RECOVERED FROM MORNING CONVECTION...EXPECT
WIDESPREAD SHOWERS AND THUNDERSTORMS TO OVERSPREAD SOUTHEAST
MICHIGAN DURING THE REST OF THE AFTERNOON INTO THIS EVENING. THIS
ACTIVITY WILL SHIFT SOUTHEAST WITH TIME AS AN APPROACHING COLD
FRONT SHIFTS INTO THE REGION.

GIVEN THE DEGREE OF INSTABILITY...AND MODEST DEEP LAYER SHEAR IN
THE NEIGHBORHOOD OF 25-30 KNOTS...SOME OF THESE STORMS WILL MOST
LIKELY BECOME SEVERE. THE BOUNDARY LAYER WILL BE WELL MIXED AS
STORMS MOVE INTO THE AREA...SO DAMAGING WIND GUSTS WILL BE
POSSIBLE WITH THE STRONGEST STORMS.

HEAVY RAINFALL BE A BIGGER CONCERN AS PRECIPITABLE WATER
INCREASES TO 1.75-2.00 INCHES WITHIN BEST PLUME OF DEEP MOISTURE
FUNNELING NORTHEAST INTO AREA IN ADVANCE OF APPROACHING COLD
FRONT/UPPER TROUGH AXIS. AS ADDITIONAL CONVECTION RIDES NORTHEAST
INTO AREA WITH APPROACHING DISTURBANCE OVER NORTHWEST
INDIANA...THE COVERAGE OF HEAVY RAINFALL WILL INCREASE AND FOCUS
GENERALLY SOUTH OF I 69...AND ESPECIALLY BETWEEN THE M 59 AND I 94
CORRIDORS.

A SEVERE THUNDERSTORM WATCH WILL CONTINUE FROM THE M 59 COUNTIES
SOUTHWARD WITH A FLASH FLOOD WATCH CENTERED ON THE METRO DETROIT
AREA AS RUNOFF IN THE URBAN AREAS IS ENHANCED. A BULK OF THIS
ACTIVITY WILL SHIFT SOUTH AND EAST OF THE AREA THIS EVENING...BUT
LINGERING SCATTERED SHOWERS OR A FEW STORMS WILL REMAIN POSSIBLE.

LONG TERM...

THE AREA ACTUALLY TRANSITIONS INTO A MORE SUMMERLIKE PATTERN LATE
THIS WEEK INTO THE WEEKEND AS THE AN UPPER RIDGE EXPANDS INTO THE
EASTERN CONUS. THIS RIDGE ACTUALLY PERSISTS INTO EARLY NEXT WEEK
BEFORE SETTLING SOUTHEAST. THIS PATTERN WILL BRING AN EXTENDED
PERIOD OF WARMER WEATHER WITH TEMPERATURES WELL INTO THE 80S FROM
LATE THIS WEEK ON THROUGH THE REMAINDER OF THE LONG TERM FORECAST
PERIOD. IN FACT...90 DEGREE READINGS WILL BE POSSIBLE AT TIMES.

THIS BUILDING HEAT AND HUMIDITY WILL ALSO BRING PERIODIC SHOWERS
AND THUNDERSTORMS...PARTICULARLY DURING THE COURSE OF THIS WEEK AS
THE TRANSITION IN THE UPPER LEVEL PATTERN OCCURS AND SHORTWAVE
DISTURBANCES RIDE ALONG THE NORTH AND NORTHEAST EDGE OF THIS
FEATURE AS IT EXPANDS INTO THE GREAT LAKES.

MARINE...

SHOWERS AND THUNDERSTORMS WILL EXPAND OVER THE AREA INTO THIS
EVENING AS LOW PRESSURE PUSHES A COLD FRONT INTO THE REGION. THIS
LOW WILL SLOWLY WOBBLE THROUGH THE AREA THROUGH THE MIDDLE OF THE
WEEK AND MAINTAIN PERIODS OF SHOWERS AND THUNDERSTORMS.

IN GENERAL...HOWEVER...WIND AND WAVE CONDITIONS...OUTSIDE OF
THUNDERSTORMS...WILL REMAIN IN CHECK THIS WEEK AS UPPER RIDING
BUILDS INTO THE AREA.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
UPDATE.......BT
SHORT TERM...DG
LONG TERM....DG
MARINE.......DG


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






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