FXUS63 KDTX 220455
AFDDTX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
1155 PM EST MON DEC 21 2009
.AVIATION...
LIGHT SNOW WITH CURRENT CLIPPER SYSTEM WILL END DURING THE FIRST FEW
HOURS OF THE FORECAST PERIOD WITH ANY MVFR CONDITIONS BECOMING VFR
DURING THE NIGHT. THIS WILL BE THE CASE THROUGH TUESDAY WITH JUST A
PERIOD OF MVFR POSSIBLE DURING BEST DIURNAL ENHANCEMENT OF STRATOCU
DURING THE MIDDAY. CEILINGS WILL THEN DISSIPATE DURING THE 22Z-02Z
TIME FRAME AS DRIER AIR CONTINUES TO EASE INTO THE AREA ON A LIGHT
NORTHERLY FLOW.
&&
.PREV DISCUSSION...ISSUED 402 PM EST MON DEC 21 2009
SHORT TERM...TONIGHT
SNOW CONTINUES TO INCREASE IN COVERAGE ACROSS LOWER MICHIGAN AS A
WEAK CLIPPER SLIDES FROM NORTHERN MICHIGAN TO THE EASTERN LAKES
REGION. SOUTHWESTERLY FLOW AND SLIGHTLY HIGHER AVAILABLE MOISTURE
OFF OF LAKE MICHIGAN IS HELPING TO MAKE THE MOST OF WEAK ISENTROPIC
LIFT PRECEDING THIS FEATURE. LIGHT SNOW WITH VISIBILITIES DROPPING
DOWN TO 1 TO 2 MILES AT TIMES WILL CONTINUE INTO THIS EVENING
GENERALLY ALONG AND NORTH OF THE M-59 CORRIDOR. REFLECTIVITIES ARE
INCREASING ON THE KGRR RADAR AND LOW LEVEL CONVERGENCE WILL BE
ENHANCED IN THIS AREA AS THE SURFACE WAVE PASSES BY. SCATTERED LIGHT
SNOW SHOWERS WILL ALSO BE POSSIBLE FURTHER SOUTH BUT LITTLE OR NO
ACCUMULATION IS EXPECTED AS THE UPPER LEVEL WAVE BECOMES
INCREASINGLY SHEARED APART WITH A LACK OF HELP FROM LAKE MICHIGAN.
CONVERGENT BOUNDARY THAT HAS BEEN IN PLACE ACROSS LAKE HURON ALL DAY
WILL BE PULLED SOUTH ACROSS THE LAKE LATE TONIGHT AS HIGH PRESSURE
BUILDS OVER ONTARIO. WINDS WILL SHIFT TO THE NORTH WITH HI-RES
MODELS SHOWING THIS FEATURE STARTING TO PUSH SOUTH OVER THE THUMB
BRINGING ADDITIONAL LIGHT SNOW ACCUMULATIONS. LOW TEMPERATURES FOR
TONIGHT ARE JUST ON THE HIGH SIDE OF MAV GUIDANCE AS AN OVERCAST
DECK OF LOW CLOUDS SHOULD SUPPRESS COOLING.
LONG TERM...TUESDAY THROUGH NEXT MONDAY
UPSTREAM RADAR IS SHOWING AN AXIS OF LIGHT TO MODERATE SNOW LOCATED
OVER THE DAKOTAS...LOCATED WITHIN A REGION OF LOWER TO MID
TROPOSPHERIC FRONTOGENESIS. THIS REGION OF FRONTAL FORCING IS
FORECAST TO EXTEND FROM SOUTHERN WI THROUGH CENTRAL OH TUESDAY. THE
NAM WAS NOT WELL INITIALIZED WITH THE 850-700MB THERMAL GRADIENT.
THE GFS...WHICH APPEARS TO HAVE A MUCH BETTER HANDLE ON THE UPSTREAM
FORCING...BRINGS THE AXIS OF MID LEVEL LIFT INTO SW LOWER MI BY TUES
AFTERNOON. WHILE THE STRONGEST FORCING IS STILL EXPECTED TO PASS TO
THE SOUTH OF THE FORECAST AREA...THE COMBINATION OF DEEPENING
MOISTURE...WEAK MID LEVEL LIFT AND LOW STATIC STABILITY ALOFT WILL
WARRANT A CHANCE OF LIGHT SNOW FROM DETROIT SOUTH ON TUESDAY.
MEANWHILE...THE SURFACE TROUGH AXIS NOW LOCATED OVER CENTRAL LAKE
HURON IS FORECAST TO DROP SOUTH ACROSS THE THUMB TUES MORNING IN
RESPONSE TO STRENGTHENING HIGH PRESSURE TO NORTH OF THE STATE. THIS
WILL ALLOW THE LOW LEVEL WIND FIELD TO TURN MORE NORTHERLY ACROSS
LAKE HURON. THE CONVERGENCE ALONG THE SURFACE TROUGH AND MOISTURE
FLUX OFF LAKE HURON WILL PROMOTE LAKE EFFECT SNOW SHOWERS ACROSS
PORTIONS OF THE THUMB...ESPECIALLY DURING THE MORNING ON TUES. LAKE
MODIFIED MODEL SOUNDINGS NEAR PORT AUSTIN BY 12Z TUES SHOW INVERSION
HEIGHTS UP TO 8K FT WITH LAKE TO 850MB DELTA TS NEAR 16C. INVERSION
HEIGHTS ARE THEN FORECAST TO QUICKLY LOWER DURING THE COURSE OF THE
DAY UNDER PERSISTENT MID LEVEL SUBSIDENCE. THIS SHORT WINDOW OF
OPTIMAL LAKE EFFECT POTENTIAL SHOULD SUPPORT SOME LIGHT
ACCUMULATIONS ACROSS PORTIONS OF HURON COUNTY DURING THE MORNING.
THE SURFACE TROUGH AXIS WILL SLIDE SOUTH ACROSS THE FORECAST AREA
DURING THE COURSE OF THE DAY AND WILL WARRANT AT LEAST A CHANCE OF
SOME LIGHT SNOW SHOWERS INTO THE NORTHERN DETROIT SUBURBS.
THE STRENGTHENING HIGH TO THE NORTH OF THE STATE WILL THEN ADVECT
SOME DRIER AIR INTO SE MI TUES NIGHT INTO WEDNESDAY AND IS
SUPPORTIVE OF THE CURRENT DRY FORECAST. THE NEXT MAJOR CONCERN IN
THE LONG TERM IS THE IMPACTS THAT AN INTENSIFYING STORM SYSTEM OVER
THE CENTRAL US MAY HAVE ON SE MI CHRISTMAS EVE INTO CHRISTMAS DAY.
THE UPPER WAVE NOW OVER BRITISH COLUMBIA IS FORECAST TO DEVELOP INTO
A CLOSED CIRCULATION AS IT SLIDES INTO THE SW US BY WED. THE 12Z
SUITE REMAIN IN GOOD AGREEMENT IN SHOWING THE ASSOCIATED SURFACE LOW
OCCLUDING AS IT LIFTS INTO THE CENTRAL PLAINS THURS/THURS NIGHT.
ALTHOUGH THERE REMAIN SOME MODEL DIFFERENCES IN TERMS OF TIMING AND
DEGREE OF PHASING WITH NORTHERN STREAM ENERGY SLIDING DOWN FROM
WESTERN CANADA...THE 12Z SUITE ARE IN A LITTLE MORE AGREEMENT IN
TERMS OF TIMING AND TRACK OF THE SURFACE LOW /FORECAST TO TRACK FROM
THE CENTRAL PLAINS THURS INTO THE WESTERN GREAT LAKES BY SATURDAY/.
ABUNDANT MOISTURE TRANSPORT AND ISENTROPIC ASCENT IS FORECAST TO
OVERSPREAD SOUTHERN MI THURS NIGHT IN ADVANCE OF THE OCCLUDED FRONT.
THE INITIAL DRY AIR ASSOCIATED WITH THE STRONG /1030MB/ HIGH PARKED
TO THE NORTHEAST OF THE STATE WILL PROVIDE ENOUGH WET BULB EFFECTS
TO CREATE SOME PRECIP TYPE CONCERNS THURS NIGHT. MODEL SOUNDINGS
INDICATE THAT SNOW/SLEET WILL QUICKLY TRANSITION TO FREEZING
RAIN/RAIN DURING THE NIGHT AS A DEEP ELEVATED WARM LAYER ADVECTS
INTO SE MI. SURFACE WET BULB TEMPS ARE FORECAST TO HOVER IN LOWER
30S THROUGH MUCH OF THE NIGHT BEFORE SURFACE ADVECTIONS WARM THE
BOUNDARY LAYER ENOUGH TO SUPPORT ALL RAIN BY LATE CHRISTMAS MORNING.
THE AMOUNT OF QPF IN THE MODELS THUS NIGHT IS SOMEWHAT ALARMING
/0.5 TO 0.75 INCH/. FROM A PURE PATTERN RECOGNITION STANDPOINT
HOWEVER...THERE ARE CONCERNS THAT THE MODEL QPF MAY BE OVERDONE.
THE MAIN ISSUES ARE THAT IF THIS SYSTEM CUTS OFF SO FAR TO THE
SOUTHWEST OF THE STATE...THE MID LEVEL DRY AIR MAY BE MORE
AGRESSIVE THAN CURRENTLY INDICATED /SHORTENING THE WINDOW OF
OPPORTUNITY FOR PRECIPITATION/. MOISTURE TRANSPORT MAY ALSO BE CUT
SHORT IF A LARGE CONVECTIVE COMPLEX DEVELOPS OVER THE DEEP SOUTH
/WHICH CERTAINLY LOOKS POSSIBLE/. SO AT THIS STAGE IN THE
FORECAST...WILL CONTINUE TO MENTION MIXED PRECIP THURS NIGHT INTO
EARLY CHRISTMAS MORNING AND WILL MONITOR LATER MODEL TRENDS FOR
QPF AMOUNTS. COLD AIR WILL FILTER BACK INTO THE REGION CHRISTMAS
NIGHT...LEADING TO A CHANGE OVER BACK TO SOME SNOW SHOWERS. THE
CHANCES OF SNOW SHOWERS WILL THEN CONTINUE INTO AT LEAST THE FIRST
HALF OF THE HOLIDAY WEEKEND AS THE REMNANT UPPER LOW DRIFTS ACROSS
THE GREAT LAKES.
MARINE...
NORTHERLY WINDS WILL INCREASE ACROSS LAKE HURON INTO THE 10 TO 20
KNOT RANGE FROM LATE TONIGHT THROUGH WEDNESDAY AS HIGH PRESSURE
BUILDS OVER ONTARIO. SMALL CRAFT ADVISORIES MAY BE NEEDED AROUND THE
TIP OF THE THUMB BY TUESDAY NIGHT AS THE LONG FETCH HELPS WAVES
BUILD TO 3 TO 5 FEET.
A STRONG LOW PRESSURE WILL SLOWLY APPROACH FROM THE MID-MISSISSIPPI
VALLEY LATE THURSDAY...FINALLY TRACKING THROUGH THE GREAT LAKES
REGION ON SATURDAY. SOUTHEASTERLY WINDS WILL INCREASE AHEAD OF THIS
SYSTEM WITH THE POTENTIAL FOR GALE FORCE WINDS FROM LATE THURSDAY
NIGHT INTO FRIDAY. STRONG WESTERLY WINDS ARE THEN EXPECTED TO
CONTINUE INTO SUNDAY AS THIS SYSTEM TRACKS NORTHEAST INTO CANADA.
&&
.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.
&&
$$
AVIATION.....BT
SHORT TERM...KEC
LONG TERM....SC
MARINE.......KEC
YOU CAN OBTAIN YOUR LATEST NATIONAL WEATHER SERVICE FORECASTS ONLINE
AT WWW.WEATHER.GOV/DETROIT (ALL LOWER CASE).
