Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 221630 AFDDTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI 1130 AM EST SAT NOV 22 2014 .UPDATE... WARMER AIR SURGING INTO SE MICHIGAN HAS ELIMINATED THE FREEZING RAIN HAZARD BY LIFTING TEMPS INTO THE MID AND UPPER 30S ACROSS THE REGION. UPSTREAM OBSERVATIONS DURING THE LATE MORNING INDICATE EVEN WARMER AIR ON OUR DOORSTEP WITH READINGS IN THE UPPER 40S TO LOWER 50S ENTERING CENTRAL AND SOUTHERN INDIANA. WE WILL LIKELY NOT WARM UP THAT MUCH BASED ON THE PACE OF SURFACE WARM ADVECTION OVER SNOW COVER IN OUR AREA BUT MID TO UPPER 40S LOOK REACHABLE. COVERAGE OF SHOWERS WILL CONTINUE DIMINISHING DURING THE AFTERNOON WITH A DRIZZLE COMPONENT ADDED DUE TO MID LEVEL SUBSIDENCE BETWEEN SHORT WAVES ALOFT. THIS WILL KEEP CONDITIONS DAMP AND CLOUDY WITH FOG ALSO BECOMING MORE NOTICEABLE AS THE INCREASINGLY MILD AIR FLOWS OVER THE SNOW COVER. && .AVIATION...ISSUED 652 AM EST SAT NOV 22 2014 //DISCUSSION... A STRONG SOUTHWEST LOW LEVEL JET (50+ KNOTS) AT THE 2000 TO 3000 FOOT IS PUMPING MOIST AIR INTO LOWER MICHIGAN...AND TAF SITES SHOULD BE LOCKED IN THE MVFR CATEGORY FOR MOST/ALL OF THE PERIOD. A PERIOD OF LIGHT FREEZING RAIN CAN BE EXPECTED THIS MORNING BEFORE CHANGING OVER TO COLD RAIN/DRIZZLE. FURTHER MOISTENING OFF THE LOW LEVELS THROUGH THE DAY WILL LEAD TO FOG CONCERNS TONIGHT AND LIKELY IFR CONDITIONS...AS ADDITIONAL LIGHT RAIN/DRIZZLE CONTINUES. PREVIOUS MENTIONED LOW LEVEL JET WILL LEAD TO WIND SHEAR...AS COLD SURFACE HINDERS BOUNDARY LAYER DEPTHS AND WIND SPEEDS AT THE SURFACE. //DTW THRESHOLD THREATS... * HIGH CONFIDENCE IN CEILINGS BELOW 5000 FEET. * HIGH CONFIDENCE PRECIPITATION WILL CHANGE TO RAIN LATE IN THE MORNING...WITH RAIN OVER THE REMAINDER OF THE TAF PERIOD && .PREV DISCUSSION...ISSUED 330 AM EST SAT NOV 22 2014 SHORT TERM...TODAY AND TONIGHT TEMPERATURES ARE STILL IN THE LOW TO MID 20S EARLY THIS MORNING...WITH SURFACE DEW PTS IN THE LOWER TEENS. WITH THIS AMOUNT OF LOW LEVEL DRY AIR IN PLACE...BOUNDARY LAYER WET BULB TEMPERATURES LOOK TO REMAIN BELOW FREEZING THROUGH THE MORNING HOURS...PER 00Z NAM. MEANWHILE...WATER VAPOR IMAGERY SHOWS MID LEVEL WAVE/CIRCULATION LIFTING INTO THE WESTERN GREAT LAKES...WHICH WILL ALLOW FOR SUBSTANTIAL SURGE OF WARM AND MOIST AIR TO ARRIVE THIS MORNING...WITH 850 MB DEW POINTS RISING TO BETWEEN 4-5 C. THIS WILL LEAD TO SOME INSTABILITY...AS SHOWLATER INDEX FALLS TO AROUND ZERO .WITH GOOD MID LEVEL (700-500 MB) LAPSE RATES IN EXCESS OF 7 C/KM WORKING THROUGH SOUTHERN LOWER MICHIGAN. DUE TO THE CONVECTIVE NATURE OF THE ACTIVITY...TOUGH TO GAUGE QPF AREAL COVERAGE...AS ESPECIALLY THE 00Z GFS IS LIKELY OVERDONE. STILL...BASED ON UPSTREAM RADAR TRENDS...65-80 PERCENT POPS APPEAR APPROPRIATE...AND HAVE ELECTED TO EXTEND FREEZING RAIN ADVISORY SOUTH...AS A QUICK GLAZE OF ICE APPEARS LIKELY WITH THE FROZEN GROUND. SELF-LIMITING PROCESS/LATENT HEAT RELEASE FROM THE FREEZING RAIN WITH NO COLD AIR FEED WILL ASSURE TRANSITION TO JUST COLD RAIN AS THE DEEP AND STRONG SOUTHWEST FLOW OVERSPREADS THE AREA...AND THE WARMER RAIN DROPS TAKE HOLD. ALTHOUGH ACTIVITY MAY TAPER OFF TO DRIZZLE THIS AFTERNOON...ANOTHER DEEPER SURGE OF MOISTURE TONIGHT...PW VALUES INCREASING TO 1 INCH SHOULD ASSURE FURTHER ADDITIONAL LIGHT RAIN/DRIZZLE...AS MERGING HEIGHT FALLS TAKE PLACE OVER THE CENTRAL CONUS...BACKING THE FLOW OVER THE MISSISSIPPI RIVER VALLEY. WITH FORECASTED (SEE EURO) SURFACE DEW PTS OF 40 F COMING OVER COLD/SNOW COVERED GROUND OVER SOUTHERN LOWER MICHIGAN...ZONES WILL CONTAIN SOME MENTION OF FOG. THE COLD SNOW COVERED GROUND IS ALSO EXPECTED TO INHIBIT THE MIXING DEPTHS TODAY...AND THINK WIND GUSTS WILL BE HARD PRESSED TO EXCEED MUCH PAST 20 KNOTS...DESPITE THE GOOD LOW LEVEL JET (50 KNOTS). LONG TERM... SUNDAY...COMPACT SOUTHERN STREAM MID LEVEL WAVE NOTED ON WATER VAPOR THIS MORNING EJECTING EASTWARD ACROSS NORTHERN MEXICO WILL STEADILY LIFT NORTHEAST AND EVENTUALLY ACROSS THE GREAT LAKES BY SUNDAY NIGHT. MOIST LOW LEVEL SOUTHERLY FLOW ALONG THE EASTERN FLANK OF THE MAIN HEIGHT FALLS WILL MARK CONDITIONS LOCALLY IN ADVANCE OF THIS SYSTEM ON SUNDAY. A RENEWED PERIOD OF STRONGER MOIST ISENTROPIC ASCENT WILL TRANSLATE FROM SOUTH TO NORTH ACROSS THE REGION DURING THE FIRST HALF OF THE DAY. THIS PROCESS WILL IMPROVE THE OVERALL MOISTURE QUALITY /PW AROUND 1 INCH/...IN ADDITION TO SUPPORTING SOME LIGHT SHOWER DEVELOPMENT DURING THIS TIME. A BRIEF LULL IN THE FORCING FOR MIDDAY WILL PROVIDE THE GREATEST WINDOW FOR A BETTER THERMAL RESPONSE. A BACKGROUND OF WEAK WARM AIR ADVECTION WILL COUNTER THE LACK OF MIXING AND INSOLATION POTENTIAL...ALLOWING HIGHS TO MAKE A RUN TOWARD THE 50 DEGREE MARK. SUNDAY NIGHT...STRONG MID LEVEL DYNAMICS ANCHORED BY STOUT LEFT EXIT REGION UPPER JET FORCING...DIVERGENCE AND INCREASING CVA ATTENDANT TO THE INBOUND WAVE WILL SWEEP ACROSS SE MICHIGAN SUNDAY NIGHT. THIS FORCING INTO THE EXISTING VERY MOIST AMBIENT ENVIRONMENT WILL RESULT IN A 6-9 HOUR PERIOD OF WIDESPREAD RAINFALL FOR THE ENTIRE REGION OVERNIGHT. QPF AMOUNTS WITHIN THE .5" TO .75" RANGE LOOK ATTAINABLE GIVEN THE UNDERLYING STRENGTH OF ASCENT...MOISTURE DEPTH AND OVERALL RESIDENCE TIME. A FIRM SOUTHEAST GRADIENT WILL CONTAIN TEMPERATURES IN THE LOWER AND MIDDLE 40S THROUGH THE NIGHT. MONDAY...00Z NAM REMAINS OUTSIDE THE MODEL CONSENSUS WHICH HOLDS WITH A MUCH DEEPER SURFACE LOW AS IT TRACKS FROM THE SOUTH END OF LAKE MICHIGAN THROUGH NORTHERN LAKE HURON DURING THIS TIME. 00Z GFS SEEMINGLY TOO AGGRESSIVE WITH THE RESPONSE /966 MB CENTER AT 18Z MON NORTH OF THE STRAITS/...OWING TO MORE PRONOUNCED PHASING OF THE LEAD PV FEATURE WITH A TRAILING NORTHERN STREAM TROUGH THAT BECOMES NEGATIVELY TILTED. THE NET RESULT IS A MORE SIZABLE DIFFERENCE IN HANDLING THE DETAILS IN THE WAKE OF THE EXITING MAIN MOISTURE PLUME HEADING INTO MONDAY. SLIVER OF MILDER PRE-COLD FRONTAL AIR WILL EXIST MONDAY MORNING...THE TRAILING COLD FRONTAL PASSAGE MOST LIKELY COMMENCING DURING THE LATTER HALF OF THE DAY. AT THIS POINT...MAIN ELEMENT OF CONCERN BECOMES THE WIND POTENTIAL MONDAY AFTERNOON AND EVENING...AS THE ENSUING ADVECTIVE PROCESS BUILDS THE CONVECTIVE LAYER DEPTH INTO AN ALREADY FIRM WIND FIELD JUST OFF THE SURFACE. A PERIOD OF GOOD 280-285K ISENTROPIC DESCENT WITH 35 TO 40KT WINDS WITHIN THIS LEVEL WOULD SUGGEST PEAK GUSTS IN EXCESS OF 35 MPH ARE ATTAINABLE...WITH AN UPTICK TO HEADLINE WORTHY GUST POTENTIAL CERTAINLY PLAUSIBLE. WINDOW FOR STRONG CONVERGENCE ALONG THE FRONTAL SLOPE AND WITH THE PASSAGE OF THE TRAILING WAVE WILL MAINTAIN A CHANCE FOR SHOWERS INTO MONDAY...PARTICULARLY DURING THE FIRST HALF OF THE DAY. PRE-FRONTAL TEMPERATURES PEAK IN THE UPPER 40S-LOWER 50S...STRONGER COLD AIR ADVECTION AWAITING THE BETTER HEIGHT FALLS TOWARD EVENING. TUESDAY AND BEYOND...SUBSTANTIAL SOUTHWARD EXPANSION OF THE LOWER HEIGHT FIELD WILL SPELL ANOTHER EXTENDED STRETCH OF BELOW NORMAL TEMPERATURES STARTING TUESDAY. PLACEMENT OF AN ENERGETIC NORTHERN PACIFIC JET CORE SOUTHEAST AND NEAR/OVER THE GREAT LAKES WILL BRING THE POTENTIAL FOR A PERIODICALLY ACTIVE PATTERN THROUGH THE UPCOMING HOLIDAY WEEKEND. POTENTIAL FOR LIGHT SNOW SHOWERS TUESDAY WITHIN A MOIST CYCLONIC FLOW...AUGMENTED BY THE MOISTURE FLUX OFF LAKE MICHIGAN. WEAK SHORTWAVE WORKING THROUGH THE NORTHWEST FLOW IS FORECAST TO DEEPEN WITHIN FAVORABLE UPPER JET SUPPORT ON WEDNESDAY...LONG RANGE MODEL GUIDANCE ALLOWING THIS WAVE TO SHIFT INTO SOUTHEAST MICHIGAN BY THANKSGIVING. WHILE SUBJECT TO CHANGE YET AT THIS TIME SCALE...LATEST TRENDS WOULD BRING THE POTENTIAL FOR A LIGHT ACCUMULATING SNOWFALL WITH THIS SYSTEM FOR THE HOLIDAY. MARINE... SOUTHWEST WINDS WILL REMAIN STRONG THROUGH THE FIRST HALF OF TODAY. GUSTS TO GALES OR NEAR GALES WILL EXIST OVER CENTRAL PORTIONS OF LAKE HURON THROUGH THE MORNING PERIOD. WINDS WILL EASE SLIGHTLY THIS AFTERNOON AS THE OVERALL GRADIENT SLACKENS. A MODEST SOUTHERLY WIND WILL EXIST TONIGHT AND SUNDAY IN ADVANCE OF STRENGTHENING LOW PRESSURE. SOME INCREASE IN SOUTHEAST WINDS WILL OCCUR AS THIS LOW TRACKS FROM SOUTHERN LAKE MICHIGAN TO THE STRAITS SUNDAY NIGHT. THE COMBINATION OF AN INCREASING GRADIENT AND THE RETURN OF COLDER AIR WILL BRING THE POTENTIAL FOR GALES ACROSS ALL MARINE AREAS BY MONDAY AFTERNOON AND EVENING. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. LAKE HURON...SMALL CRAFT ADVISORY UNTIL 7 PM EST THIS EVENING FOR LHZ421-441. SMALL CRAFT ADVISORY UNTIL 4 PM EST THIS AFTERNOON FOR LHZ442. LAKE ST CLAIR...NONE. MICHIGAN WATERS OF LAKE ERIE...NONE. && $$ UPDATE.......BT AVIATION.....SF SHORT TERM...SF LONG TERM....MR MARINE.......MR YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).

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