-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KDTX 230410
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1210 AM EDT THU MAY 23 2013


.AVIATION...

-- Changed Discussion --
//DISCUSSION...

PREDOMINANTLY VFR CONDITIONS WILL PREVAIL THROUGH THE VERY EARLY
MORNING HOURS OUTSIDE OF A STRAY SHOWER OR TWO ACROSS THE DETROIT
AREA TAF SITES. THE COMBINATION OF LOW-LEVEL COLD ADVECTION AND
WEAKENING MOISTURE TRANSPORT INTO A SLOW-MOVING FRONTAL ZONE WILL
THEN RESULT IN LIGHT SHOWERS PUSHING SOUTH AND A DETERIORATION OF
CONDITIONS TO AT LEAST MVFR FROM NW TO SE BY APPROX 10Z. LIGHT SW
FLOW WILL GIVE WAY TO GUSTY NORTH WINDS IN THE WAKE OF THE FRONTAL
PASSAGE.

//DTW THRESHOLD THREATS...

* HIGH CONFIDENCE IN CEILINGS BELOW 5KFT BY 12Z
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&&

.PREV DISCUSSION...
-- Changed Discussion --
ISSUED 848 PM EDT WED MAY 22 2013

UPDATE...

TONIGHT`S PORTION OF THE GRIDDED FORECAST HAS BEEN UPDATED TO
INCORPORATE RECENT TRENDS AND LATEST HIGH-RESOLUTION NWP INFORMATION
NOW THAT THE DAY`S CONVECTION HAS ESSENTIALLY RUN ITS COURSE,
INCLUDING CONVECTION ACROSS SE MICHIGAN. WITH THE LOSS OF DIURNAL
HEATING, EXISTING ACTIVITY WILL STEADILY DECREASE IN COVERAGE AND
INTENSITY OVER THE NEXT COUPLE OF HOURS. CONFIDENCE IS HIGH THAT THE
SEVERE THREAT FOR THE EVENING HAS ENDED GIVEN THAT WINDS ASSOCIATED
WITH THE MOST ORGANIZED CONVECTIVE STRUCTURE OF THE EVENING IN
LENAWEE/MONROE COUNTIES WERE UNABLE TO PENETRATE THE DEVELOPING
STABLE LAYER. POPS HAVE THEREFORE BEEN TRIMMED BACK SIGNIFICANTLY WITH
NOTHING MORE THAN A PASSING SHOWER EXPECTED THROUGH LATE TONIGHT.
THE EXCEPTION WILL BE TOWARD THE MI/OH BORDER WHERE DIURNALLY
ENHANCED ACTIVITY OVER EASTERN ILLINOIS/WESTERN INDIANA AT 00Z MAY
BE SUFFICIENTLY MAINTAINED BY FORCING FOR ASCENT AHEAD OF THE
PRIMARY TROUGH AXIS TO GRAZE SOUTHERN AREAS BETWEEN 03-09Z. THE
REMAINDER OF THE FORECAST THEN LOOKS ON TRACK. WEAKENING MOISTURE
TRANSPORT WILL CONTINUE TO FUEL SHOWERS WITHIN A SLOW-MOVING
BAROCLINIC ZONE THAT WILL SAG INTO THE NW CWA BY EARLY MORNING.

PREV DISCUSSION...ISSUED 325 PM EDT WED MAY 22 2013

SHORT TERM...THIS EVENING AND TONIGHT

BROAD UPPER LOW OVER MN/IA THIS AFTERNOON CONTINUES TO PIVOT
SHORTWAVE ENERGY CYCLONICALLY INTO LAKE MICHIGAN AND SW PORTIONS OF
THE STATE. A WEAKER EXTENSION OF THAT ENERGY IS SEEN ON THE IR
SATELLITE AS FAR SE AS DAY/CMH. THAT FORCING WILL MAKE ITS WAY INTO
OUR CWA THIS AFTERNOON. THERE ARE SOME BREAKS IN THE CLOUDS ACROSS
FAR SRN MI PRESENTLY AND THIS WILL HELP DESTABILIZE THE BOUNDARY
LAYER AS WE WAIT FOR MORE FORCING IN THE WAY OF COLD FRONT/TRIPLE
POINT SFC FEATURES TO ADVANCE. WARM FRONT IS NOW DRAPED ACROSS THE
THUMB AREA AS SEEN IN SFC OBS. ALSO OBSERVED ON DTX RADAR ARE
SEVERAL GRAVITY WAVES (SEEN AS FINE LINES A BIT THICKER THAN OUTFLOW
BOUNDARIES) LIKELY PERTURBATING FROM THE PARENT UPPER LOW AND JET
STREAM DYNAMICS.

WITH THE ABOVE-MENTIONED FORCING MECHANISMS WE ARE STILL ONLY
DEALING WITH THE PREVIOUSLY ADVERTISED 500 J/KG OF CAPE. ADDL SFC
HEATING WILL SQUEEZE 1 TO 1.5 J/G OUT OF A MODIFIED SOUNDING...BUT
TO THIS POINT THE CELLS THAT POPPED UP IN THIS AIRMASS HAVE NOT HAD
STRONG UPDRAFTS. WE DO HAVE A GOOD AMOUNT OF SHEER AS OBSERVED IN AN
18Z DTX SOUNDING...MOSTLY AS SPEED INCREASE WITH HEIGHT, BUT ONLY
SLIGHT VEERING.

ALSO AMONG MARGINAL INGREDIENTS IS MOISTURE CONTENT THAT HOVERS
AROUND MID 60S DEWPOINT TEMPERATURES AND 1.5 INCHES PW. SLIGHT RISK
FOR SEVERE IS STILL WARRANTED AS THE EVOLUTION OF OUR ENVIRONMENT
WILL CHANGE THESE INGREDIENTS WITH TIME. POP-UP CELLS ACROSS SRN MI
AND NRN IN WILL TELL THE STORY FOR FUTURE DEVELOPMENT. WITH THE HIGH
SHEER SETUP, WOULD NOT BE SURPRISED TO SEE WEAK TORNADIC ACTIVITY,
AND ALSO WIND DAMAGE IN ANY LONGER LIVED SINGLE CELLS OR BOWED LINE
SEGMENT STORMS. LIMITED UPDRAFT STRENGTH WILL PROBABLY KEEP HAIL
SIZE ON A SMALLER SCALE.

SEVERE POTENTIAL DROPS OFF AFTER 00Z AS SRN 7 COUNTIES WILL RECOVER
WITH A DRY PERIOD UNDER OVERCAST SKIES. SAGINAW VALLEY REMAINS IN THE
FRONTAL ZONE AS THE UPPER LOW PIVOTS EASTWARD OVERNIGHT AND WILL
HOLD A CATEGORICAL POP THERE...FADED TO LIKELY SOUTH OF THERE.

LONG TERM...THURSDAY THROUGH TUESDAY

THE HIGH AMPLITUDE AND RELATIVELY STATIC LONGWAVE PATTERN OF RECENT
DAYS MARKED BY A CLOSED MID LEVEL CIRCULATION MEANDERING OVER THE
UPPER MS VALLEY WILL FINALLY SEE SOME FORWARD PROGRESS THROUGH THE
END OF THE WEEK.  THE EXPECTED EASTWARD PROPAGATION OF THE ENTIRE
LONGWAVE TROUGH STRUCTURE THROUGH SOUTHEAST MICHIGAN ON THURSDAY
WILL UNFOLD AS EXTENSIVE UPPER RIDGING TAKES RESIDENCE THROUGH THE
PLAINS.  THE ATTENDANT SURFACE COLD FRONTAL BOUNDARY WILL BE
MIGRATING SOUTHEASTWARD AND THROUGH LOWER MICHIGAN BY EARLY
THURSDAY.  GREATEST FORCING WILL ALIGN ALONG THE 850-925 MB FRONTAL
SLOPE...WITH A PERIOD OF HEIGHTENED MID LEVEL FGEN FORCING AND
DEFORMATION EMERGING LOCALLY AS A TRAILING SHARP MID LEVEL SHEAR
AXIS NOW SEEN ON WATER VAPOR STREAMING THROUGH WESTERN ONTARIO
SWEEPS OVER THE FRONTAL ZONE.  LOW LEVEL LAPSE RATES STEEPEN
MODESTLY BELOW THIS LAYER IN RESPONSE TO DEVELOPING WEAK COLD AIR
ADVECTION.  GIVEN A SUFFICIENTLY DEEP MOISTURE PROFILE...THIS
ENVIRONMENT WILL SUPPORT THE MAINTENANCE/EXPANSION IN
SHOWER ACTIVITY AS THE FRONT WORKS THROUGH ON THURSDAY.  THE
COMBINATION OF THE ADVECTIVE PROCESS AND THE LIMITED HEATING
POTENTIAL UNDER SUBSTANTIAL CLOUD COVER WILL CERTAINLY RESTRICT
THE TEMPERATURE RESPONSE...HIGH TEMPERATURES LIKELY CONFINED TO
WITHIN SEVERAL DEGREE OF MORNING LOWS.  THIS TRANSLATES INTO A
SHARPER NORTH TO SOUTH TEMPERATURES GRADIENT...READINGS RANGING
FROM AROUND 50 IN THE THUMB TO MID 60S NEAR THE OHIO BORDER.

STEADY COLD/DRY AIR ADVECTION WILL PROVIDE A DEFINITIVE BACK EDGE TO
THIS MOISTURE AXIS WITH A PROGRESSIVE CLEARING TREND COMMENCING FROM
MID AFTERNOON INTO THE EVENING HOURS FROM NORTHWEST TO SOUTHEAST.
CRISP CP AIRMASS WILL FIRMLY SETTLE IN OVERNIGHT WITHIN DEEPENING
NORTHERLY FLOW ALONG THE EASTERN EDGE OF EXPANDING HIGH PRESSURE
OVER THE UPPER MIDWEST.  850 MB TEMPERATURES STILL LOOK TO BOTTOM
OUT AROUND THE FREEZING MARK BY 12Z FRIDAY MORNING.  BOUNDARY
LAYER WILL LIKELY REMAIN MIXED ENOUGH TO PREVENT A MORE PRONOUNCED
RADIATIONAL COOLING ENVIRONMENT...BUT THE GENERAL STRENGTH OF THIS
AIRMASS WILL SUPPORT LOWS OF UPPER 30S FOR ALL LOCALES OUTSIDE OF
THE DETROIT HEAT ISLAND INFLUENCE.  POCKETS OF PATCHY FROST NOT OUT
OF THE QUESTION...ALTHOUGH A DRYING LOW LEVEL COLUMN AND THE
QUESTION OF LINGERING MIXING REMAIN MITIGATING FACTORS TO POTENTIAL
FORMATION.

EXTENDED STRETCH OF RELATIVELY BENIGN WEATHER CONDITIONS WILL EXIST
FROM FRIDAY RIGHT THROUGH NEXT WEEKEND AS THE REGION SETTLES INTO
DRY CONFLUENT NORTHWEST FLOW ANCHORED OVER GENERAL SURFACE RIDGING.
SHORTWAVE ENERGY AND ANY ASSOCIATED WARM/MOIST AIR ADVECTION
COMPONENT ROUNDING THE APEX OF THE PREVAILING UPPER RIDGE DRAPED
OVER THE PLAINS WILL TEND TO ARC DOWN THE PRIMARY INSTABILITY
GRADIENT CONFINED WELL TO THE SOUTHWEST.  SIMPLE AIRMASS
MODIFICATION DURING THIS TIME UNDER GENERAL MID LEVEL HEIGHT RISES
AND PLENTY OF INSOLATION IN THE WAKE OF THE DEPARTING LONGWAVE
TROUGH.  HIGHS BACK INTO THE MIDDLE 60S FOR INLAND LOCATIONS BY
THE WEEKEND...STILL HOLDING A CATEGORY COOLER ALONG THE THUMB AND
OTHER IMMEDIATE NEARSHORE LOCALES.

MARINE...

AN UNSTABLE ENVIRONMENT THIS EVENING WILL BRING A CHANCE OF STRONG
TO SEVERE THUNDERSTORMS THROUGH 8PM. AFTER THAT TIME LINGERING
THUNDERSHOWERS ARE POSSIBLE. FOG WILL AGAIN BE POSSIBLE OUT AHEAD OF
THE FRONTAL ZONE...MAINLY SOUTHERN HALF OF LAKE HURON OVERNIGHT.
WINDS WILL SHIFT TO THE NORTH ACROSS ALL WATERS BY THURSDAY
MORNING...AS THE MAIN SURFACE LOW AND TRAILING COLD FRONT PUSH OFF
TO THE EAST. WINDS AND WAVES WILL INCREASE GREATLY WITH THE
ADVECTION OF COLD AND DRY AIR...AND THIS LEAD TO ISSUANCE OF A SMALL
CRAFT ADVISORY FOR MUCH OF NEARSHORE LAKE HURON. HIGH PRESSURE WILL
BUILD IN ACROSS THE GREAT LAKES ON FRIDAY...BRINGING QUIET WEATHER
AND COOLER TEMPERATURES TO THE REGION.
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&&

.DTX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
MI...NONE.

LAKE HURON...
     SMALL CRAFT ADVISORY...INNER SAGINAW BAY...FROM 6 AM THURSDAY TO 2
     PM THURSDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS FROM PORT AUSTIN TO PORT
     HURON INCLUDING OUTER SAGINAW BAY...FROM 10 AM THURSDAY TO 10
     PM THURSDAY.

LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.
-- End Changed Discussion --

&&

$$

AVIATION.....JVC
UPDATE.......JVC
SHORT TERM...MM
LONG TERM....MR
MARINE.......MM


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