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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KDTX 202001
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
401 PM EDT MON MAY 20 2013

.SHORT TERM...
-- Changed Discussion --
TODAY AND TONIGHT

WARM SECTOR AIRMASS FIRMLY ENTRENCHED ACROSS SE MICHIGAN THIS
AFTERNOON.  LATEST OBSERVATIONS SHOWING TEMPERATURES MAKING A RUN TO
NEAR 90 DEGREES IN SOME LOCATIONS.  DEEPER PLUME OF LOW LEVEL
MOISTURE BANKED ALONG THE INTERFACE OF THE UPPER RIDGE AXIS HOLDING
JUST TO THE EAST AND THE EXTENSIVE CLOSED LOW CIRCULATING OVER THE
DAKOTAS NOW EXTENDING THROUGHOUT LOWER MICHIGAN.  THIS COMBINATION
TRANSLATING INTO A SOLID INSTABILITY AXIS...RECENT SPC MESOANALYSIS
INDICATING MLCAPE VALUES IN EXCESS 2000 J/KG.  DESPITE THIS...
SOUTHEAST MICHIGAN STILL FEELING THE IMPACT OF THE NEARBY HIGHER
HEIGHT FIELD...AS NOTED BY THE 18Z DTX SOUNDING WHICH SHOWS A FAIRLY
STOUT 900-800 MB CAPPING INVERSION.  THIS EFFECTIVELY CONTAINS THE
CONVECTIVE TEMPERATURE AT 90 DEGREE OR WARMER...LEAVING THE
PROSPECTS FOR SEEING CONVECTION CONDITIONALLY DEPENDENT ON REALIZING
SOME DEGREE OF FORCED ASCENT AND/OR CONSOLIDATING CONVECTIVELY
GENERATED COLD POOL DYNAMICS.

LINEAR EXPANSION OF CONVECTION UNDERWAY TO THE WEST/NORTHWEST
SUSTAINED BY A COMBINATION OF A PASSING MID LEVEL IMPULSE AND SOME
TYPE OF LAKE MICHIGAN MARINE INFLUENCE/BOUNDARY.  THE POTENTIAL
REMAINS FOR SOME FORM OF EAST/NORTHEASTWARD EXPANSION OF THIS
ACTIVITY ALONG A FAVORABLY ALIGNED CORFIDI VECTOR AND INTO THE
INSTABILITY BUBBLE RESIDING OVER SE MICHIGAN.  12Z HI RES MODEL
SOLUTIONS CONTINUE TO POINT IN THIS DIRECTION.  HOWEVER RELATIVELY
MODEST DEEP LAYER SHEAR EAST OF THIS LINE DOES LEAVE SOME QUESTION
AS TO THE ORGANIZATIONAL CAPACITY AND POTENTIAL COVERAGE OF ANY
INBOUND CONVECTION.  NONETHELESS...GIVEN THE DEGREE OF INSTABILITY
IN PLACE...CONDITIONAL SEVERE THUNDERSTORM RISK CERTAINLY
EXISTS...ESPECIALLY ACROSS NORTHERN SECTIONS OF THE FORECAST AREA.
SEVERE THUNDERSTORM WATCH IS IN PLACE THROUGH 8 PM FROM WASHTENAW/
WAYNE COUNTIES NORTHWARD.

DECAYING CONVECTIVE COMPLEX LOCKED ONTO THE NEXT IN A SERIES OF WEAK
IMPULSES EJECTING AROUND THE UPPER MIDWEST LOW CONTINUES TO SHIFT
THROUGH NORTHERN ILLINOIS.  A CONSIDERABLY LESS SUPPORTIVE
THERMODYNAMIC ENVIRONMENT ACROSS THAT CORRIDOR /DEWPOINTS IN THE
50S/ LEADING TO THIS WEAKENING TREND...BUT SOME POTENTIAL DOES EXIST
FOR RENEWED DEVELOPMENT AND UPSCALE GROWTH THROUGH THE LATTER HALF
OF THE EVENING AS THIS WAVE ENCOUNTERS THE BETTER INSTABILITY/
MOISTURE AXIS.  THIS WILL CONTINUE TO SUPPORT A CHANCE POP MENTION.

THE OVERNIGHT/EARLY MORNING PERIOD BECOMES MORE MUDDLED...DEPENDENCE
ON PROPERLY DEFINING THE TRAJECTORY OF ADDITIONAL ILL-DEFINED
SHORTWAVE ENERGY AND/OR CONVECTIVE DEBRIS.   EMERGING NOCTURNAL LOW
LEVEL JET FORCING AND SUSTAINABLE HIGHER LAPSE RATES LEND MORE
CONFIDENCE THAT SOME FORM OF ADDITIONAL CONVECTION WILL TAKE AIM ON
THE REGION LATE TONIGHT...BUT DEFINING THE DETAILS OF
LOCATION/TIMING AND COVERAGE REMAINS A CHALLENGE.  A MILD NIGHT
AHEAD GIVEN THE ELEVATED DEWPOINTS...LOWS MID TO UPPER 60S.
-- End Changed Discussion --

&&

.LONG TERM...
-- Changed Discussion --
TUESDAY THROUGH SUNDAY

THE MAIN DRIVER OF THE WEATHER TUESDAY WILL BE THE MOMENTUM TRANSFER
THAT WILL OCCUR OF UPPER LEVEL JET ENERGY OVER THE RIDGE AXIS.
ORIENTATION OF UPPER LEVEL RIDGING OVER NORTHERN GREAT LAKES AND
ONTARIO WILL THEN CHANGE FROM CYCLONIC TO ANTICYCLONIC CURVATURE.
FUNDAMENTALLY...THIS WILL PUT SOUTHEASTERN MICHIGAN IN A DIFFERENT
REGIME AS THE AREA WILL BECOME IN CLOSER PROXIMITY TO THE UPPER
LEVEL JET AXIS. PROXIMITY TO THE UPPER LEVEL JET AXIS IS NEVER A
TRIVIAL QUESTION...BECAUSE A CLOSER PROXIMITY MEANS ONE IS CLOSER TO
THE FIREHOSE OR PATH OF EMBEDDED SHORTWAVE ENERGY. HOWEVER...WHAT IS
A MORE DIFFICULT DETERMINATION IS THE OVERALL CRISPNESS OR EDGE
QUALITY TO THE UPPER LEVEL RIDGE. IN OTHER WORDS...A GREATER
POTENTIAL WILL EXIST FOR PERIODIC SHOWERS AND THUNDERSTORMS
THROUGHOUT TUESDAY AND TUESDAY NIGHT.

THE SEVERE WEATHER THREAT FOR TUESDAY WILL BE PARTIALLY INFLUENCED
BY AREAL COVERAGE AND DURATION OF ONGOING THUNDERSTORMS DURING THE
PRE-NOON TIMEFRAME. HOWEVER...GIVEN LATE MAY NUMBER OF DAYLIGHT
HOURS AND SOLAR INTENSITY...ANY BREAK IN CLOUD COVER WILL ALLOW
SURFACE TEMPERATURES TO CLIMB AND CAPES TO BUILD. THERE IS SURELY
CONTAMINATION OF THE THERMODYNAMIC ENVIRONMENT IN THE MODELS BUT
CONFIDENCE IS HIGH THAT 700-500 MB LAPSE RATES WILL NOT BE ANYWHERE
NEAR AS PRISTINE. LIKELY NOT MOIST ADIABATIC BUT CERTAINLY NOT 7.5-8
DEG C/KM EITHER. SBCAPES ARE EXPECTED TO REACH 1000 J/KG WITH
MLCAPES LIKELY STRUGGLING TO REACH THAT SAME THRESHOLD. CLOSER
PROXIMITY TO JET WILL BRING SOLID 30-40 KNOTS OF 0-6KM BULK
SHEAR....GOOD FOR A POTENTIAL OF STRONG TO POSSIBLY SEVERE
THUNDERSTORMS FOR DAMAGING WIND GUSTS AND LARGE HAIL.

A CONVECTIVELY MODULATED BOUNDARY LAYER AND HIGHER SPEED FLOW
OVERHEAD MAKES THE TIMING OF INDIVIDUAL FORCING ELEMENTS DIFFICULT.
WILL OFFER A THOUGHT OR TWO TO THINK ABOUT DESPITE THE NOISE. SOME
MODEL DATA SUGGESTS THAT DIFFERENTIAL AIRMASS RECOVERY MAY OCCUR
PRIMARILY FROM THE IRISH HILLS TO THE SOUTHEAST. A PERTURBATION TO
THE FLOW OR RIBBON OF ENHANCED CONVERGENCE IS HIGHLIGHTED ON SOME
REMNANT CONVECTIVE SHORTWAVE ENERGY IS HIGHLIGHTED TO TRACK THROUGH
TOMORROW IN SOUTHWEST FLOW.  BEST IDEA ON TIMING FOR STRONG TO
SEVERE STORMS IS 19-23Z. ALL OF SOUTHEASTERN MICHIGAN IS IN THE
SLIGHT RISK ON SWODY2.

CONVECTION THAT DEVELOPS DURING THE DAY ON TUESDAY WILL BEGIN TO
DIMINISH BY EVENING. A LULL IN THE THUNDERSTORMS DEVELOP FOR THE
OVERNIGHT PERIOD AS ONE LOW PRESSURE SYSTEM MOVES OFF INTO CANADA
AND ANOTHER SECONDARY LOW DEEPENS OVER IOWA AND NORTHERN ILLINOIS.
HOWEVER...CURRENT MODEL SOUNDINGS ARE INDICATING MORE LOW LEVEL
MOISTURE IN BETWEEN THESE TWO LOW PRESSURE SYSTEMS THAN PREVIOUS
RUNS SO HAVE INCREASED THE FORECAST TO HIGH CHANCE POPS TO ACCOUNT
FOR ANY SHOWERS THAT WILL CONTINUE THROUGH THE OVERNIGHT PERIOD.

CURRENT MODEL RUNS ARE FAIRLY WELL IN AGREEMENT WITH ONE ANOTHER
BRINGING THE CENTER OF THE SURFACE LOW WILL ACROSS CENTRAL MICHIGAN
ON WEDNESDAY AS THE UPPER LOW MERGES INTO THE FLOW OVER CANADA. THIS
WILL BRING A CONTINUED CHANCE OF SHOWERS AND THUNDERSTORMS THROUGH
THE DAY. AS THE COLD FRONT FINALLY MOVES OVER SOUTHERN MICHIGAN ON
WEDNESDAY NIGHT...RAIN WILL CONTINUE THROUGH WEDNESDAY NIGHT. THE
BACK END OF THIS LOW WILL LACK ANY SUPPORT FOR ORGANIZED CONVECTION
ON THURSDAY WITH ONLY RAIN CONTINUING FOR MOST OF THE DAY ON
THURSDAY. COLDER AIR BEGINS TO FILTER IN BEHIND THE PASSING LOW ON
THURSDAY RESULTING IN HIGH TEMPERATURES TO ONLY BE IN THE UPPER 50S TO
LOW 60S ACROSS THE CWA.

A FEW LINGERING RAIN SHOWERS WILL PERSIST INTO THE THURSDAY NIGHT
TIME FRAME BEFORE AN UPPER LEVEL RIDGE BUILDS INTO THE CENTRAL
PLAINS. THIS WILL KEEP SURFACE HIGH PRESSURE OVER THE REGION THROUGH
THE WEEKEND RESULTING IN DRY CONDITIONS. THIS HIGH WILL ALSO
CONTINUE TO USHER IN MUCH COOLER TEMPERATURES WITH HIGHS ON FRIDAY
ONLY IN THE LOW TO MID 60S AND SLOWLY INCREASING TO THE UPPER 60S BY
SUNDAY. LOW TEMPERATURES WILL DIP DOWN INTO THE UPPER 30S AND LOW
40S OVER THE WEEKEND.
-- End Changed Discussion --

&&

.MARINE...

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MODEST SOUTHERLY WINDS WILL REMAIN IN PLACE THROUGH MIDWEEK AS A LOW
PRESSURE SYSTEM SLOWLY LIFTS EAST OUT OF THE UPPER MIDWEST.  A WARM
AND UNSTABLE ENVIRONMENT WILL BRING A CHANCE OF THUNDERSTORMS
THROUGH THIS TIME.  WINDS WILL SHIFT TO THE NORTH ACROSS ALL WATERS
BY THURSDAY MORNING...AS THE LOW AND TRAILING COLD FRONT PUSH OFF TO
THE EAST.
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
ISSUED 110 PM EDT MON MAY 20 2013

//DISCUSSION...

SOUTHEAST MICHIGAN WELL WITHIN A DESTABILIZING WARM SECTOR
ENVIRONMENT THIS AFTERNOON.  DIURNAL CU GROWTH/EXPANSION WILL
CONTINUE THROUGH THE AFTERNOON...CIGS SETTLING INTO THE 4-5K FT
RANGE.  A DEEPENING MIXED LAYER WILL LEAD TO SOME SOUTHERLY WIND
GUSTS IN THE 20 KNOTS RANGE THROUGH PEAK HEATING.  MAIN QUESTION
WITHIN THIS ENVIRONMENT REMAINS DEFINING THE PROSPECTS FOR
THUNDERSTORM DEVELOPMENT...BOTH LATE AFTERNOON INTO THE EVENING AND
AGAIN LATE TONIGHT INTO TUESDAY.  GREATEST POTENTIAL IN THE NEARER
TERM WILL EXIST ROUGHLY 20Z-23Z NORTH AND 23Z-02Z ACROSS METRO
DETROIT...SUPPORTED BY A MID LEVEL WAVE WORKING INTO THE UNSTABLE
CONDITIONS.  WHILE OVERALL CONFIDENCE REMAINS MODEST AS TO THE
EXPECTED COVERAGE...WILL GO AHEAD AND OUTLOOK THE MOST LIKELY
WINDOW WITH A TEMPO MENTION.

FOR DTW...EXPANDING DIURNAL CU FIELD WILL WORK INTO THE TERMINAL
OVER THE NEXT 1-2 HOURS...LEAVING A PREVAILING SUB-5000 FT CEILING
FOR SEVERAL HOURS.  SOME CONCERN THAT DEVELOPING THUNDERSTORM
ACTIVITY LATE THIS AFTERNOON/EVENING WILL TEND TO TRACK NORTH/WEST
OF THE TERMINAL.  HOWEVER THE POTENTIAL FOR A THUNDERSTORM TO IMPACT
THE TERMINAL IS HIGH ENOUGH TO HIGHLIGHT A PERIOD...GREATEST WINDOW
23Z-02Z.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILINGS BELOW 5KFT THIS AFTERNOON AND EVENING.

* MEDIUM CONFIDENCE IN THUNDERSTORM DEVELOPMENT LATE THIS AFTERNOON
AND EVENING.
-- End Changed Discussion --

&&

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

&&

$$

SHORT TERM...MR
LONG TERM....RK/CB
MARINE.......MR
AVIATION.....MR


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