FXUS63 KDTX 022310
AFDDTX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
610 PM EST WED DEC 2 2009
.AVIATION...
RAIN WILL CONTINUE TO EXPAND ACROSS SOUTHEAST MICHIGAN THROUGH THE
REMAINDER OF THE EVENING AS A DEEPENING LOW PRESSURE SYSTEM TRACKS
ACROSS OHIO TONIGHT...THEN INTO THE EASTERN GREAT LAKES THURSDAY
MORNING. CEILINGS WILL CONTINUE TO LOWER AS BOUNDARY LAYER MOISTURE
INCREASES. EXPECT WIDESPREAD MVFR CEILINGS TO EXPAND INTO THE REST
OF THE TAF SITES BY LATE EVENING...WITH AREAS OF IFR CEILINGS
POSSIBLE BASED ON OBSERVATIONS UPSTREAM. THE LOW LEVEL THERMAL
PROFILES LOOK WARM ENOUGH TO SUPPORT MAINLY RAIN THROUGH THE NIGHT.
WIDESPREAD PRECIPITATION WILL END AROUND 12Z AS THE STORM SYSTEM
EXITS TO THE EAST...LEAVING AN MVFR STRATO CU FIELD ACROSS THE
REGION.
&&
.PREV DISCUSSION...ISSUED 345 PM EST WED DEC 2 2009
SHORT TERM...TONIGHT
FORECAST INTO TONIGHT REMAINS LARGELY IN LINE WITH PRIOR THINKING AS
LIGHT RAIN SPREADS NORTH INTO SOUTHERN PORTIONS OF THE CWA LATE THIS
AFTERNOON...AND ON NORTH THROUGH THE REST OF THE AREA THIS EVENING.
THE ONLY NOTEWORTHY ADJUSTMENT TO THE GOING FORECAST WAS TO DELAY
THE ONSET OF RAIN SHOWERS FROM BETWEEN M 59/I 69 ON NORTH AS DRY AIR
THROUGH 15KFT AS EVIDENT ON DTX 12Z SOUNDING WILL TAKE A NUMBER OF
HOURS TO MOISTEN ENOUGH FOR MEASURABLE PRECIPITATION.
AS THE LOW PRESSURE SYSTEM LIFTS INTO THE OHIO VALLEY LATE TODAY AND
THEN CONTINUES ON INTO THE EASTERN GREAT LAKES TONIGHT...COLDER AIR
WILL FILTER INTO THE AREA AND ALLOW THE RAIN SHOWERS TO MIX WITH AND
CHANGE TO LIGHT SNOW LATE TONIGHT OVER PARTS OF THE CWA...GENERALLY
NORTH AND WEST OF METRO DETROIT. WILL MAINTAIN THE 1/2-1 INCH OF
ACCUMULATIONS OVER THE GENESEE/SAGINAW VALLEY REGION WITH THE IDEA
THAT MOST/ALL OF THIS WILL BE ON GRASSY SURFACES DUE TO THE MILD
BOUNDARY LAYER TEMPERATURES. THE APPROACH/PASSAGE OF THE LOW WILL
ALSO BRING INCREASING NORTHEAST WINDS INTO THE EVENING...WHICH WILL
BACK TO THE NORTH OVERNIGHT AND NORTHWEST ON THURSDAY. TEMPERATURES
WILL FALL INTO THE MID/UPPER 30S OVER SOUTHEAST LOWER MICHIGAN AND
THE LOWER 30S FURTHER NORTH/WEST INTO THE SAGINAW VALLEY AS THE
COLDER AIR MAKES ITS WAY INTO THE CWA.
LONG TERM...THURSDAY THROUGH NEXT WEDNESDAY
RESIDUAL LIGHT PRECIPITATION LINGERING ON THE HEELS OF THE LOW
PRESSURE SYSTEM MAY STILL NOT BE FULLY IN THE FORM OF SNOW BY
SUNRISE THURSDAY MORNING. MODEL FORECAST SOUNDINGS AND 1000-850 MB
THICKNESS FIELDS INDICATE WARM AIR LINGERING IN THE BOUNDARY LAYER
WELL AFTER SUNRISE WHICH WILL CUT FURTHER INTO ANY POTENTIAL
ACCUMULATION DURING THIS TIME. COLD ADVECTION WILL BE UNDERWAY BUT
STILL NOT AT AN AGGRESSIVE ENOUGH PACE TO COMPLETELY CHANGE RAIN TO
SNOW BEFORE THE PRECIPITATION SHIELD EXITS TO OUR NORTH AND EAST. NO
MORE THAN AN ADDITIONAL SLUSHY COATING IS EXPECTED BEFORE ENDING BY
NOON. THAT WILL LEAVE THE AFTERNOON HOURS ON THE DRY SIDE BUT WITH
STEADY OR FALLING TEMPERATURES AS COLD AIR CONTINUES TO ADVANCE INTO
THE REGION.
WINTER WILL THEN MAKE A MORE CONVINCING APPEARANCE THURSDAY NIGHT
AND FRIDAY. WITH COLD AIR FIRMLY IN PLACE, THE LAKE EFFECT SNOW
MACHINE WILL BECOME ACTIVE FROM LAKE MICHIGAN AND BECOME CAPABLE OF
REACHING OUR AREA. ALONG WITH SCATTERED SNOW SHOWERS OVER ABOUT THE
ENTIRE REGION, THE SYNOPTIC SCALE PATTERN HAS THE POTENTIAL TO
SUPPORT A STRONG CONVERGENT BAND OFF THE SOUTH END OF LAKE MICHIGAN.
THE NAM12 AND GFS MODELS ARE IN GOOD AGREEMENT CALLING FOR A WIND
FIELD THAT FAVORS THE I-94 TO I-69 CORRIDOR LATER THURSDAY NIGHT AND
THE I-69 CORRIDOR NORTHWARD INTO THE TRI CITIES ON FRIDAY. THE DEPTH
OF CONVECTION WILL BE STRONGLY SUPPORTED BY THE UPPER TROUGH
ADVANCING FROM THE PLAINS INTO THE SOUTHERN LAKES REGION. FORECAST
SOUNDINGS INDICATE EQUILIBRIUM LEVELS MAKING AT RUN AT 700 MB/10000
FEET BY 12Z FRIDAY. COMBINE THIS WITH 850 MB TEMPS DROPPING TO
AROUND -12C, FOR LAKE DELTA-T NEAR 20C, AND WE HAVE THE STRONG CAPE
TO GO ALONG WITH THE HIGH EQUILIBRIUM LEVEL CAPABLE OF SUPPORTING
DEEP LAKE CONVECTION. THE CONVERGENT BAND POTENTIAL WOULD THEN BE
MODULATED BY THIS ENVIRONMENT ALONG WITH GRADUALLY BACKING LOW LEVEL
FLOW DIRECTING THE MOST CONCENTRATED ACTIVITY FROM SOUTH SECTIONS OF
OUR AREA THURSDAY NIGHT INTO NORTH SECTIONS FRIDAY MORNING. WE DO
NOT PRETEND TO BE ABLE TO PINPOINT THIS SCENARIO EXACTLY BUT IT DOES
SEEM REASONABLE TO INDICATE A TIME FRAME MOST FAVORABLE FOR GREATER
SNOW SHOWER ACTIVITY ALONG WITH SOME MODEST ACCUMULATION OF AN INCH
OR TWO. HOWEVER, THIS PATTERN DOES HAVE POTENTIAL FOR HIGHER AMOUNTS
THAT COULD BE INTRODUCED IN THE FORECAST WITH REFINEMENTS IN THE
NEXT FEW FORECAST CYCLES.
DEEPER SW FLOW AND WARMING LOW LEVEL TEMPERATURES WILL GRADUALLY
DIRECT LAKE EFFECT TO OUR WEST AND NORTH BY SATURDAY. THE PRESENCE
OF THE COLD UPPER LEVEL TROUGH WILL KEEP SNOW SHOWERS POSSIBLE BUT
THEY WILL BE LIGHT AND SCATTERED AT BEST BEFORE THE UPPER TROUGH
SHIFTS TOWARD THE EAST BY SUNDAY. CONFIDENCE REMAINS REASONABLE IN
MODEL DATA SHOWING A PROGRESSION OF THE SMALLER SCALE UPPER LEVEL
FEATURES IN ORDER TO FOSTER CONTINUED EVOLUTION OF A BLOCKING LONG
WAVE PATTERN. A SHORT WAVE THEN IN PLACE OVER THE GREAT LAKES ON
SUNDAY, ALONG WITH CONTINUED DEEPER SW FLOW, WILL HELP WITH A SMALL
TEMPERATURE MODERATION BEFORE A RETURN TO A MORE ACTIVE PATTERN NEXT
WEEK.
THE SLOWLY EVOLVING BLOCKED LONG WAVE PATTERN LOOKS LIKE A SOLID
SOLUTION IN THE GLOBAL MODELS THROUGH THE FIRST HALF OF NEXT WEEK.
THE CHALLENGING PART OF THE FORECAST WILL BE HANDLING OF THE SHORT
WAVE FEATURES EMBEDDED WITHIN THIS FLOW. THE LONG WAVE TROUGH AXIS
WILL BE TO OUR NORTH AND WEST THROUGH THE PERIOD WITH A WESTERLY
UPPER FLOW FROM THE PACIFIC INTO THE GREAT LAKES. THIS FLOW WILL
SENT A SHORT WAVE OUR WAY PERIODICALLY AND SUBJECT TO DAILY
ADJUSTMENTS TO TIMING AND INTENSITY DUE TO THE LOW PREDICTABILITY OF
THE SMALL SCALE FEATURES. ONE THING WE CAN SAY WITH SOME CERTAINTY
IS THAT THE TEMPERATURE PROFILE WILL SUPPORT SNOW FROM THESE
SYSTEMS, ESPECIALLY IF THE SURFACE LOW TRACK REMAINS TO OUR SOUTH AS
OFFERED IN THE LATEST MODEL RUNS.
MARINE...
WINDS WILL INCREASE FROM THE NORTHEAST TONIGHT IN RESPONSE TO A LOW
PRESSURE SYSTEM LIFTING THROUGH THE OHIO VALLEY WITH GUSTS REACHING
30 KNOTS AT TIMES OVERNIGHT INTO THURSDAY. THIS ALONG WITH THE
ONSHORE WIND DIRECTION WILL CREATE WAVE CONDITIONS BUILDING UP TO 10
FEET ON THE NEARSHORE WATERS OF LAKE HURON. THE POTENTIAL EXISTS FOR
GUSTS TO BRIEFLY REACH GALE FORCE ON SOUTHERN LAKE HURON BUT THE
LIMITED DURATION WILL PRECLUDE THE NEED FOR A GALE WARNING.
SLACKENING PRESSURE GRADIENT WILL ALLOW GUSTS TO BRIEFLY DIMINISH
THURSDAY NIGHT, WITH A CORRESPONDING DROP IN WAVE HEIGHT. SOUTHWEST
WINDS WILL THEN STRENGTHEN AGAIN ON FRIDAY AND COMBINE WITH COLD AIR
MOVING OVER THE WARMER WATERS AND LEAD TO UNSTABLE CONDITIONS ONCE
AGAIN.
&&
.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...
SMALL CRAFT ADVISORY...NEARSHORE WATERS FROM PORT AUSTIN TO PORT
HURON INCLUDING OUTER SAGINAW BAY...FROM 10 PM WEDNESDAY TO 10
PM THURSDAY.
SMALL CRAFT ADVISORY...INNER SAGINAW BAY...FROM 10 PM WEDNESDAY TO
4 PM THURSDAY.
LAKE ST CLAIR...
SMALL CRAFT ADVISORY...FROM 10 PM WEDNESDAY TO 4 PM THURSDAY.
MI WATERS OF LAKE ERIE...
SMALL CRAFT ADVISORY...FROM 10 PM WEDNESDAY TO 4 PM THURSDAY.
&&
$$
AVIATION.....SC
SHORT TERM...DG
LONG TERM....BT
MARINE.......BT
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AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).
