Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 301754
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
154 PM EDT WED JUL 30 2014


.AVIATION...

//DISCUSSION...

A CORRIDOR OF ENHANCED MIDLEVEL MOISTURE OVER THE SOUTHERN HALF OF
THE CWA WILL ALLOW FOR SHOWER AND THUNDERSTORM DEVELOPMENT THIS
AFTERNOON. MODELS REMAIN AGGRESSIVE WITH THE AMOUNT OF SUBSIDENCE
THAT WILL ARRIVE AFTER 21Z. WILL CONTINUE TO HONOR THIS OUTPUT AND
FORECAST THE BEST TIME PERIOD FOR SHOWERS AND POSSIBLE THUNDER
BETWEEN 18-22Z. THE WEATHER PATTERN THIS EVENING WILL BE INFLUENCED
BY LARGE SCALE SUBSIDENCE...WHICH WILL CARRY WELL INTO THE NOCTURNAL
HEATING MINIMUM. FROM THIS VANTAGE POINT...THIS LIMITS THE POTENTIAL
FOR ADDITIONAL SHOWER DEVELOPMENT LATE TONIGHT AND INCREASES THE
UNCERTAINTY IN COVERAGE AND LOCATION. WILL REFRAIN FROM INTRODUCING
PRECIPITATION FOR TONIGHT WITH THIS TAF CYCLE.

FOR DTW... THE MOST FAVORABLE TIME PERIOD FOR SHOWERS TO IMPACT THE
TERMINAL IS 18-22Z.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILING BELOW 5000 FEET THIS AFTERNOON.

&&

.PREV DISCUSSION...ISSUED 1209 PM EDT WED JUL 30 2014

UPDATE...

CLEANED UP THE GRIDS THIS MORNING TO ACCOUNT FOR TRENDS AND GAVE A
SLIGHT NUDGE UPWARD WITH POPS SOUTH OF I 94 TODAY.

A VERY SUBTLE SHORTWAVE FEATURE IS NOW DESCENDING SOUTHWARD ACROSS
THE NORTHERN CWA. LIGHT SHOWER/SPRINKLES THAT PASSED THROUGH THE
NORTHERN CWA WERE SUSTAINED BY SOME LEVEL OF CVA WITHIN A RIBBON
OF RICHER THETA E CONTENT. MODEL DATA SUGGESTS THIS CORRIDOR OF
ENHANCED MIDLEVEL MOISTURE WILL SLOW DOWN FOR THE FIRST HALF OF
THE AFTERNOON ALL THE WHILE THE SHORTWAVE/VORTICITY MAXIMUM
PINCHES DOWN AND CLOSES TO WESTERN LAKE ERIE BY 21-22Z. VISIBLE
SATELLITE IMAGERY SHOWS CUMULUS DEVELOPMENT IS NOW OCCURRING ON THE
NORTHERN EDGE OF THE MOISTURE WITH THE ATMOSPHERE ON ITS WAY TO
MEETING CONVECTIVE TEMPERATURES. SUSPECT IT WILL TAKE AWHILE FOR
THE CONVECTION TO DEEPEN AND MATURE...BUT WILL BE LOOKING AT AN
APPROXIMATE 4 HOUR WINDOW FOR SHOWER (AND POSSIBLY THUNDER) THIS
AFTERNOON PRIMARILY SOUTH OF I 94. CONVECTION ALLOWING
MODELS...INCLUDING IN-HOUSE HI-RES ENSEMBLE OUTPUT ARE SHOWING
SOME POTENTIAL FOR DEVELOPMENT NEAR PORT HURON AND PTS NORTHWARD
ALONG THE LAKE HURON COAST. CONFIDENCE THAT FAR NORTH IS
DEFINITELY LESS. BEST GUESS ON TIMING WILL 17-21Z OR POSSIBLY 18
TO 22Z. WENT AHEAD NUDGED POPS UP TO AROUND 50 PERCENT FOR DETROIT
SOUTHWARD...THE SIGNAL THERE IS THE FORECAST NOW RESIDES ABOVE
THE MOS VALUES. INSTABILITY IS MARGINAL AT A FEW HUNDRED J/KG
POSSIBLE NEAR THE OHIO BORDER. SUBSIDENCE IN THE WAKE OF THE
SHORTWAVE ALONG WITH THE SUBSEQUENT DRYING IS SHOWN TO BE VERY
AGGRESSIVE AFTER 21Z. NO STRONG WEATHER IS ANTICIPATED AS
MATURATION OF CONVECTIVE CELLS AND INCREASED DEPTH SHOULD OCCUR
SOUTH AND EAST OF THE AREA. HOWEVER...A RELATIVELY SURPRISING
AMOUNT OF LIQUID COULD FALL IN SOME ISOLATED LOCATIONS...DUE TO
SLOW NW-SE CELL MOTION OF AROUND 25 MPH.

PREV DISCUSSION...ISSUED 323 AM EDT WED JUL 30 2014

SHORT TERM... TODAY AND TONIGHT

PATCHY FOG WILL BE THE FIRST CONCERN IN THE FORECAST AS THE RAIN
YESTERDAY AND LAST NIGHT LEFT WET GROUND FOLLOWED BY LIGHT WIND AND
A DECREASE IN CLOUDS DURING THE EARLY MORNING. THE GREATEST
POTENTIAL FOR LOCALLY DENSE FOG WILL BE IN THE TRI CITIES DOWN TO
FLINT WHERE RADIATIONAL COOLING CONDITIONS WILL BE BETTER AND WHERE
MOST OF THE RAIN FELL. THIS WILL BE MONITORED FOR TRENDS IN DURATION
AND AREAL COVERAGE FOR HEADLINE POTENTIAL BEFORE SUNRISE.

THE FOCUS OF THE FORECAST FOR THE REST OF THE DAY WILL BE ON THE
SHORT WAVE ENERGY ROUNDING THE UPPER LOW OVER THE SOUTH END OF LAKE
MICHIGAN. COVERAGE OF SHOWERS SUPPORTED BY THIS FEATURE HAS
DECREASED CONSIDERABLY SINCE MIDNIGHT WITH THE LOSS OF DIURNAL
INSTABILITY. HOWEVER...THE WAVE REMAINS SUPERIMPOSED OVER THE WEAK
NORTHWEST-SOUTHEAST TEMPERATURE AND MOISTURE GRADIENT BEING
MAINTAINED BY THE LONG WAVE PATTERN. LOW TO MID LEVEL THETA-E
ADVECTION AHEAD OF THE SHORT WAVE TRACK HAS A CHANCE TO PRODUCE
SHOWERS THROUGHOUT THE MORNING AS WEAK MOISTURE TRANSPORT GENERATES
SOME ELEVATED INSTABILITY. DAYTIME HEATING WILL THEN ADD SURFACE
BASED INSTABILITY TO THE MIX WITH SOME ENHANCED CONVERGENCE WITHIN
THE SURFACE PRESSURE REFLECTION OF THE WAVE. THUNDERSTORM POTENTIAL
WILL BE SIMILAR TO YESTERDAY BUT WITH SLIGHTLY HIGHER SURFACE
DEWPOINT AND PERHAPS A DEGREE OR TWO INCREASE IN MAX TEMP. A 75/53
PARCEL THEN MODIFIES SURFACE BASED CAPE TO ABOUT 1300 J/KG IN NAM
SOUNDINGS. ALL THAT BEING SAID...EXPECT CHANCE POPS WILL COVER THIS
SCENARIO WHICH WILL AFFECT THE AREA GENERALLY SOUTH OF THE M59
CORRIDOR BUT LIMITED BY AN EARLY EASTWARD EXIT OF THE WAVE. THE 00Z
MODEL PACKAGE HAS A RELIABLE HANDLE ON THE SITUATION BASED ON
ANALYSIS FIELDS...AND FORECASTS SHOW GOOD TEMPORAL EVOLUTION THROUGH
THE OHIO BORDER REGION PLACING THE BULK OF AFTERNOON CONVECTION TO
OUR SOUTH AND EAST BEFORE PEAK SURFACE HEATING.

MID LEVEL SUBSIDENCE TRAILING THE OHIO BORDER WAVE WILL KEEP THINGS
DRY IN THE TRI CITIES DOWN NEAR FLINT DURING THE DAY...BUT THE NEXT
UPPER LEVEL FEATURE WILL ARRIVE SOON INTO TONIGHT. THIS SYSTEM IS
SHOWN CLEARLY IN EARLY MORNING WATER VAPOR IMAGERY OVER NORTHERN
ONTARIO AND LOOKS PLENTY CAPABLE OF SUPPORTING SHOWERS TONIGHT AS IT
MOVES THROUGH LOWER MICHIGAN. MODEL SOLUTIONS INDICATE THE
CIRCULATION WEAKENING FROM A VORTICITY PERSPECTIVE BUT NOT ENOUGH TO
OFFSET THE SHOT OF COLD AIR ALOFT IT WILL BRING DOWN OVER THE
REGION. THE WAVE WILL ALSO BE ABLE TO DRAW SOME LOW TO MID LEVEL
MOISTURE IN FROM THE WEST TO HELP GENERATE MODEST ELEVATED
INSTABILITY TO PROMOTE SCATTERED SHOWER DEVELOPMENT THROUGH SUNRISE
THURSDAY.

LONG TERM...THURSDAY THROUGH TUESDAY

UPPER LOW CONTINUES TO CHURN NEAR THE ONTARIO/QUEBEC BORDER...WITH
SEVERAL NOTABLE SHORTWAVES VISIBLE ON WATER VAPOR IMAGERY PIVOTING
AROUND THE MAIN SYSTEM. THE LARGER OF THE WAVES (ONE OVER NORTHERN
MICHIGAN AND THE OTHER OVER HUDSON BAY) WILL ROTATE ACROSS THE AREA
TODAY AND TONIGHT...CLEARING THE AREA TO THE EAST BY 12Z THUR. THE
CENTER OF THE UPPER LOW APPEARS TO BE LOCATED NEAR THE SOUTHERN END
OF ST JAMES BAY AT THE MOMENT. MODELS ARE FORECASTING THIS WILL DROP
THROUGH EASTERN ONTARIO THURSDAY AND THURSDAY NIGHT...PUSHING A
SURFACE TROUGH ACROSS NORTHERN MICHIGAN. THE PASSAGE OF THE TROUGH
SHOULD HELP FOCUS SHOWERS AND MAYBE SOME THUNDERSTORMS ONCE AGAIN
ACROSS SOUTHEAST MICHIGAN...WITH THE BEST COVERAGE DURING THE
AFTERNOON AND EVENING AS DIURNAL HEATING PEAKS. THERE IS SOME
UNCERTAINTY WITH DEVELOPMENT AND COVERAGE OF THUNDERSTORMS THROUGH
AT LEAST THE FIRST HALF OF THE DAY. GFS/NAM FORECAST SOUNDINGS HAVE
VARIED OVER THE PAST FEW RUNS BETWEEN A WEAK MID-LEVEL CAP HOLDING
AND NO CAP WITH A SKINNY CAPE PROFILE. THEY DO ADVERTISE A TREND FOR
THE ATMOSPHERE TO BECOME UNCAPPED BY LATE EVENING HOWEVER...WITH
COOLER AIR ALOFT SLIDING DOWN AS THE UPPER LOW DRAWS CLOSER. WILL
CONTINUE TO MENTION THUNDERSTORMS AFTER 15Z AND THROUGH THE
OVERNIGHT PERIOD.

SHORTWAVE ENERGY DIVING DOWN FROM SASKATCHEWAN AND MANITOBA WILL
THEN ALLOW A WEAK UPPER TROUGH TO FORM OVER THE MIDWEST AND WESTERN
GREAT LAKES EARLY FRIDAY. THIS TROUGH WILL THEN SLOWLY TRACK ACROSS
THE GREAT LAKES FRIDAY AND SATURDAY. SURFACE LOW ASSOCIATED WITH
THIS FEATURE IS EXPECTED TO WORK ACROSS THE SOUTHERN OHIO
VALLEY...HOWEVER SOME MID-LEVEL MOISTURE IS EXPECTED TO INCREASE
OVER MICHIGAN FRIDAY INTO FRIDAY NIGHT AS THE TROUGH APPROACHES AND
SOUTHWEST FLOW INCREASES SLIGHTLY...WITH SOME MOISTURE THEN
LINGERING INTO SATURDAY. THIS WILL ONCE AGAIN ALLOW SHOWERS AND
THUNDERSTORMS TO FORM AGAIN...ESPECIALLY EACH AFTERNOON/EVENING AS
WE REMAIN UNDER THE UPPER COLD POOL AND DIURNAL HEATING HELPS TO
INCREASE LAPSE RATES.

SHORTWAVE RIDGE IS EXPECTED TO BRIEFLY BUILD INTO THE AREA FOR
SUNDAY BRINGING A RESPITE FROM DAILY SHOWER/THUNDERSTORM CHANCES.
ANOTHER UPPER TROUGH IS THEN ADVERTISED BY MEDIUM RANGE MODELS TO
DROP ACROSS ONTARIO ON MONDAY...PUSHING ANOTHER SURFACE TROUGH
ACROSS THE WESTERN AND CENTRAL GREAT LAKES AND BRINGING SHOWERS AND
THUNDERSTORMS BACK.

TEMPERATURES THROUGH THE EXTENDED PERIOD ARE EXPECTED TO MODERATE A
BIT AS UPPER TROUGHING OVER THE AREA EXHIBITS A GENERAL WEAKENING
TREND. MAX TEMPS THURSDAY THROUGH EARLY NEXT WEEK SHOULD RUN IN THE
UPPER 70S TO NEAR 80...STILL BELOW NORMAL FOR THIS TIME OF YEAR.

MARINE...

WINDS WILL REMAIN FAIRLY LIGHT THROUGH THE END OF THE WEEK...WITH
THE MOST NOTABLE PERIOD OF WINDS OCCURRING FROM THE SOUTHWEST
THURSDAY AND THURSDAY NIGHT AS A WEAK TROUGH MOVES ACROSS THE
WESTERN AND CENTRAL GREAT LAKES. WINDS ARE STILL EXPECTED TO REMAIN
NEAR OR BELOW 10 KNOTS HOWEVER. OTHERWISE...EXPECT THE AREA TO
REMAIN UNDER A COOL PATTERN WITH SHOWERS AND THUNDERSTORMS
DEVELOPING EACH DAY...MOSTLY DURING THE AFTERNOON AND EVENING.

&&

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

&&

$$

AVIATION.....CB
UPDATE.......CB
SHORT TERM...BT
LONG TERM....HLO
MARINE.......HLO


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





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