FXUS63 KGRR 220433
AFDGRR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
1130 PM EST MON DEC 21 2009
LATEST UPDATE...AVIATION...
.SYNOPSIS...(336 PM EST MON DEC 21 2009)
A WEAK COLD FRONT COMES THROUGH SOUTHWEST LOWER MICHIGAN THIS
EVENING. IT WILL BE ACCOMPANIED BY SNOW SHOWERS INTO THE EARLY
EVENING HOURS. BEHIND THAT FRONTAL SYSTEM A CANADIAN POLAR HIGH
PRESSURE SYSTEM BUILDS DOWN FROM CENTRAL CANADA INTO THE CENTRAL
GREAT LAKES. TYPICALLY THAT WOULD BRING AT LEAST PARTLY CLOUDY
SKIES AND COLD TEMPERATURES. HOWEVER THIS TIME AN UPPER AIR
DISTURBANCE PASSING BY TO THE SOUTH OF MICHIGAN WILL BRING SOME
LIGHT SNOW TO AREAS NEAR AND SOUTH OF INTERSTATE 94 TUESDAY.
A LOW PRESSURE SYSTEM MOVING UP THE MISSISSIPPI RIVER VALLEY WILL
BRING OUR AREA POTENTIAL FOR A WINTRY MIX OF SNOW... SLEET...
FREEZING RAIN AND RAIN LATER THURSDAY INTO FRIDAY. BASED ON MEDIUM
RANGE GUIDANCE TRENDS IT APPEARS THAT THE MAJORITY OF SIGNIFICANT
SNOW WITH THIS SYSTEM SHOULD STAY WELL TO OUR WEST NORTHWEST ACROSS
MINNESOTA AND WISCONSIN. WHICH PUTS SOUTHWEST MICHIGAN ON THE WARM
SIDE OF THE STORM FOR CHRISTMAS MORNING. LAKE EFFECT SNOW SHOWERS
WILL MOVE IN BEHIND THE STORM CHRISTMAS NIGHT AND CONTINUE THROUGH
THE WEEKEND.
&&
.SHORT TERM...(336 PM EST MON DEC 21 2009)
(TONIGHT THROUGH WEDNESDAY NIGHT)
THERE REMAIN TWO PERIODS OF SNOW TO WATCH IN THE SHORT TERM PART OF
THE FORECAST. THE SNOW WITH THE COLD FRONT THIS EVENING. THEN THERE
IS THE JET INDUCED MID LEVEL LIFT EVENT THAT WILL PRODUCE A STRIP
OF 1 TO 2 INCH SNOWFALL FROM NORTHERN IOWA ACROSS SOUTHERN WISCONSIN
INTO NORTHERN INDIANA AND SOUTHERN MICHIGAN...SOUTH OF INTERSTATE 94
DURING THE MORNING HOURS OF TUESDAY.
SURFACE CONVERGENCE COMBINED WITH 850 TEMPS NEAR -10C AND 1000 TO
850 MB MEAN RH VALUES OVER 90 PCT ON A LARGELY WEST WIND WILL BRING
SNOW SHOWERS TO THE AREA INTO MID EVENING. THERE IS NOT MUCH LIFT IN
THE DGZ THROUGH SO SNOWFALL AMOUNTS SHOULD BE LIGHT. THE HEAVIEST
SNOWFALLS WILL BE BETWEEN 23Z AND 03Z. THAT IS THE TIME WHEN THE
1000 TO 850 RH IS THE HIGHEST AND SURFACE TO 925 MB WIND CONVERGENCE
IS THE HIGHEST.
ONCE THAT IS THROUGH WINDS BELOW 850 MB BECOME NORTH THEN NORTHEAST
AND THAT WILL TAKE LAKE ENHANCEMENT OUT OF PLAY (BY 06Z). WHICH
WOULD NORMALLY CLEAR THE SKIES BUT NOT TONIGHT. A SHORTWAVE FROM A
STRONG PACIFIC STORM (THE ONE THAT PRODUCES OUR CHRISTMAS STORM)
SENDS A LEAD WAVE EAST ACROSS THE UNITED STATES AROUND 35N DURING
THE NEXT 24 HOURS. THIS SHORTWAVE AND IT'S ASSOCIATED SPEED MAX
INTERACTS WITH THE DEPARTING JET CORE ON THE POLAR JET NEAR LAKE
SUPERIOR. THAT JET COUPLING RESULTS IN FROM FGEN FORCING OVER THE
SOUTHERN CWA TUESDAY MORNING. SO WE WILL SEE SOME LIGHT SNOW OVER
THE SOUTHERN CWA FROM THAT. I INCREASED POPS TO 50 PCT BUT I FEEL WE
WILL LIKELY HAVE TO INCREASE THAT TO CONDITIONAL ON THE NEXT SET OF
ZONES. IT WILL BE A QUICK HITTER WITH AN INCH OR LESS NEAR AND SOUTH
OF I-94.
AFTER THAT WAVES GOES THROUGH SOME CLEARING WILL FOLLOW OVER THE NRN
CWA BY LATE IN THE DAY AND OVERNIGHT TUESDAY. WITH POLAR HIGH
PRESSURE IN PLACE... THE LIGHT WINDS AND NEARLY CLEAR SKIS SHOULD
ALLOW TEMPERATURES TO PLUNGE TO NEAR ZERO. OVER THE CENTRAL AND
SOUTHERN CWA THE HIGH CLOUDS FROM THAT LARGE PACIFIC STORM WILL
LIMIT THE COOLING SO SOME EXTENT. HIGH PRESSURE WILL CONTINUE TO
BRING DRY AIR AND LIMIT CLOUDS TO MID AND HIGH LAYERED CLOUDS
WEDNESDAY INTO WEDNESDAY NIGHT. THE UPPER LEVEL CONFLUENCE ZONE IS
JUST SOUTH OF MICHIGAN DURING THIS TIME FRAME. THAT SHOULD KEEP THE
PRECIPITATION JUST SOUTH OF MICHIGAN THROUGH WEDNESDAY NIGHT.
&&
.LONG TERM...(336 PM EST MON DEC 21 2009)
(THURSDAY THROUGH MONDAY)
A COMPLEX SYSTEM WILL BE IMPACTING THE REGION AROUND THE HOLIDAY
TRAVEL TIME FRAME. THE MODELS ARE IN DECENT AGREEMENT THAT LOW
PRESSURE OVER WESTERN MO ON THURSDAY WILL REMAIN TO OUR WEST. IT
APPEARS THE LOW WILL HEAD FOR THE EASTERN U.P. BY EARLY SATURDAY.
THIS WILL INITIALLY ALLOW A WARM SURGE TO OCCUR ALOFT WHILE AN
ARCTIC HIGH OVER THE HUDSON BAY REGION CONTINUES TO SUPPLY COLD
DRAINAGE AIR OVER THE BOUNDARY LAYER. THEREFORE THE MAIN THREAT
WILL BE MIXED PCPN...PROBABLY LARGELY FREEZING RAIN. THE PCPN MAY
BE SLOW TO BEGIN ON THURSDAY AS WE WILL HAVE SOME DRY LOW LEVELS TO
OVERCOME. SO HAVE LEFT CHANCE POPS...BUT INSTEAD OF SNOW...HAVE
GONE WITH MIXED PCPN. THE MAIN SURGE OF PCPN SHOULD COME CHRISTMAS
EVE WHEN A LLJ PENETRATES THE REGION. MIXED PCPN SHOULD SWITCH TO
ALL RAIN ACROSS THE SOUTHERN CWA THROUGH THE NIGHT...WHILE THE MIX
MAY CONTINUE ACROSS THE NORTH. AS THE LOW CENTER DRIFTS NORTH
ACROSS WISCONSIN ON CHRISTMAS DAY...COOLER AIR WILL WRAP IN BY LATE
IN THE DAY. ANY RAIN SHOULD SWITCH BACK OVER TO SNOW TOWARD
NIGHTFALL.
FRIDAY NIGHT THROUGH MONDAY WE WILL SEE CYCLONIC FLOW PERSIST.
SHORT WAVES IN THE MEAN FLOW WILL BRING SNOW SHOWERS AND GRADUALLY
COLDER AIR TO THE REGION. BY MONDAY H8 TEMPS ARE EXPECTED TO BE
AROUND -12C LEADING TO PERSISTENT SNOW SHOWERS.
&&
.AVIATION...(1130 PM EST MON DEC 21 2009)
A COUPLE OF CHALLENGES WITH THE AVIATION FORECAST. SOME FREEZING
DRIZZLE ONGOING AT KLAN. THIS WAS ASSOCIATED WITH A WIND SHIFT
MOVING SOUTHWARD THROUGH THE REGION. AS DEEPER MOISTURE ARRIVES
OVERNIGHT...ANY PRECIPITATION SHOULD CHANGE BACK OVER TO SNOW.
HOWEVER WITH THIS WIND SHIFT MOVING SOUTH...KJXN...KBTL AND KAZO
WILL FEATURE A POTENTIAL FOR FREEZING DRIZZLE FOR A FEW HOURS AT THE
BEGINNING OF THE TAF. STILL IFR CONDITIONS HAVE BEEN AND WILL
CONTINUE TO IMPACT TAF SITES WITH THIS WIND SHIFT.
THE NEXT WAVE OF LOW PRESSURE TO IMPACT THE REGION ARRIVES LATER
TONIGHT. THIS FEATURE IS EXPECTED TO IMPACT MAINLY SOUTHERN TAF
SITES WITH POTENTIALLY IFR SNOW AND CEILINGS. THIS SYSTEM HAS THE
ABILITY TO CAUSE CONDITIONS TO GO LOWER...WHICH WILL NEED TO BE
MONITORED THROUGH THE NIGHT. CURRENTLY THIS SYSTEM IS IMPACTING
PARTS OF NRN IL AND IA.
THE DRIER AIR WILL BE DELAYED SEVERAL HOURS ON TUE AS LOW LEVEL
MOISTURE WAS STILL DROPPING SOUTH THROUGH NRN MI.
&&
.MARINE...(336 PM EST MON DEC 21 2009)
UNTIL THE STORM ON THURSDAY ARRIVES WINDS WILL BE LIGHT AND THAT
MEANS NO NEED FOR A SCA UNTIL THE CHRISTMAS TIME FRAME.
&&
.HYDROLOGY...(336 PM EST MON DEC 21 2009)
THE ONLY HYDRO ISSUE THAT CONTINUES IS THE CONCERN FOR ICE
DEVELOPMENT THAT WOULD LEAD TO ICE JAMS. HOWEVER WITH MAX TEMPS
MAINLY IN THE UPPER 20S TO MID 30S THROUGH THE UPCOMING WEEK... THE
POTENTIAL FOR THIS TO OCCUR IS LOW. PCPN THURSDAY THROUGH FRIDAY
WILL NOT BE SIGNIFICANT ENOUGH TO CAUSE HYDRO ISSUES.
&&
.GRR WATCHES/WARNINGS/ADVISORIES...
MI...NONE
LM...NONE.
&&
$$
SYNOPSIS: WDM
SHORT TERM: WDM
LONG TERM: JK
AVIATION: MJS
MARINE: WDM
HYDROLOGY: WDM
