FXUS63 KGRR 061229
AFDGRR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
728 AM EST SUN DEC 6 2009

LATEST UPDATE...AVIATION...

.SYNOPSIS...(410 AM EST SUN DEC 6 2009)
SCATTERED LIGHT SNOW SHOWERS AND FLURRIES WILL LINGER TODAY. A WEAK 
LOW PRESSURE SYSTEM WILL BRING SOME STEADIER SNOW FROM LATE TONIGHT 
THROUGH MONDAY. A VIGOROUS LOW PRESSURE SYSTEM WILL MOVE INTO THE 
SOUTHERN PLAINS REGION TUESDAY AND THEN TRACK NORTHEAST INTO OUR 
REGION TUESDAY NIGHT AND WEDNESDAY. THIS INTENSE LOW PRESSURE SYSTEM 
WILL BRING POTENTIAL FOR A MIX OF RAIN AND SNOW CHANGING TO ALL SNOW 
AND HIGH WINDS. THERE IS ALSO POTENTIAL FOR SIGNIFICANT SNOW 
ACCUMULATIONS DEPENDING ON THE EXACT LOW TRACK. IT WILL TURN VERY 
COLD AND VERY WINDY BEHIND THIS SYSTEM WEDNESDAY NIGHT AND THURSDAY 
WITH SNOW AND BLOWING SNOW. 

&&

.SHORT TERM...(410 AM EST SUN DEC 6 2009)
(TODAY THROUGH TUESDAY AFTERNOON)

PRIMARY SHORT TERM FCST CONCERNS INVOLVE ASSESSMENT OF POTENTIAL 
SNOW ACCUMULATIONS WITH THE WEAK LOW PRESSURE SYSTEM LATE TONIGHT 
THROUGH MONDAY. SHORT TERM FCST TWEAKS INCLUDED PUSHING BACK TIMING 
OF MOST OF THE SNOW INTO OUR NORTHERN CWFA UNTIL AFTER 06Z TONIGHT. 

A FEW FLURRIES AND LIGHT SNOW SHOWERS ARE ANTICIPATED TODAY MAINLY 
NORTH TO NORTHWEST OF GRR DUE TO CONTINUED SW FLOW LAKE ENHANCEMENT. 
A BETTER CHANCE FOR STEADIER SNOW WILL COME LATE TONIGHT AS THE WEAK 
LOW PRESSURE SYSTEM MOVES ENE INTO OUR AREA. 00Z MODEL GUIDANCE TIME 
HEIGHT RH PROGS SHOW THE DEEPEST LOW TO MID LEVEL MOISTURE OVER OUR 
NORTHERN CWFA. SNOW WILL ALSO INCREASE IN COVERAGE TOWARD DAYBREAK 
MONDAY IN AN AREA OF PVA AS A SHORTWAVE MOVES IN FROM THE SW. 

LATEST SREF GUIDANCE SUGGESTS AROUND AN INCH TO INCH AND A HALF OF 
SNOW WILL FALL ACROSS OUR NORTHERN CWFA FROM LATE TONIGHT THROUGH 
MONDAY AFTERNOON WITH 00Z GFS IN THE 1 TO 2 INCH RANGE. WE WILL GO 
WITH SLIGHTLY HIGHER FCST ACCUMS NEAR AND NORTH OF A LINE FROM MKG 
TO MOP IN THIS TIME FRAME SINCE MOISTURE WITH THIS SYSTEM WILL BE 
ENHANCED AS IT CROSSES LAKE MI. 

A FEW FLURRIES MAY LINGER INTO MONDAY EVENING BEFORE DRY WX RETURNS 
MONDAY NIGHT THROUGH TUESDAY MORNING. AT THIS TIME WE WILL MAINTAIN 
LOW CHC POPS FOR SNOW TUE AFTN. HOWEVER A CONSENSUS BLEND OF 00Z 
SHORT TO MEDIUM RANGE GUIDANCE SUGGESTS THAT MOST OF THE PCPN 
DEVELOPMENT IN ASSOCIATION WITH THE VIGOROUS MIDWEEK LOW PRESSURE 
SYSTEM SHOULD HOLD OFF UNTIL TUESDAY EVENING.   

&&

.LONG TERM...(410 AM EST SUN DEC 6 2009)
(TUESDAY NIGHT THROUGH SATURDAY)

THE ITEM OF INTEREST IS THE DEEP STORM ALL OF THE MODELS CONTINUE TO 
FORECAST MOVING THROUGH THE GREAT LAKES IN THE WED/THU TIME FRAME. 
THERE HAS BEEN GOOD MODEL AGREEMENT FOR DAYS NOW ON THIS SYSTEM 
WHILE THE TRACK HAS SHIFTED SLIGHTLY TOWARD THE NW OVER TIME IT HAS 
NOT SHIFTED MUCH. ALSO GIVEN THE 3 TO 4 STANDARD DEVIATIONS FROM 
NORMAL THE STORMS CENTRAL PRESSURE FALLS TO BY WED MORNING...MY 
CONFIDENCE IN THIS STORM IS HIGH. THE MAIN ISSUE IS ONE OF TRACK.

GIVEN THERE IS COUPLED JET WITH A 170 KNOTS IN THE EXIT REGION OF 
THE SOUTHERN BRANCH WED 00Z AND THERE IS ANOTHER 170 KNOT JET CORE 
DIVING DOWN THE FRONT SIDE THE LARGE UPPER RIDGE CENTERED OVER 
ALASKA (WHICH WILL ONLY SERVE TO DEEPEN THE WESTERN TROUGH... SEEM 
WE WILL HAVE A MERGING OF THE SOUTHERN AND NORTHERN BRANCH OF THE 
JET STREAM. ADD TO THIS THE 1058 MB ARCTIC HIGH OVER BRITISH 
COLUMBIA EARLY THIS SUNDAY MORNING...THERE IS MORE THAN ENOUGH 
THERMAL ENERGY FOR THIS SYSTEM TO PLAY WITH. THE 1.3 INCHES OF 
PRECIPTABLE WATER STREAMING NORTH SOUTH OF THE WARM FRONT WILL 
SURELY HELP THE CAUSE OF DEEPENING THIS SYSTEM TOO. AS WILL THE 
NEGATIVE TILT UPPER TROUGH WEDNESDAY. THE ECWMF HAS A 966 MB LOW 
OVER NORTHERN LAKE HURON BY 00Z THURSDAY WHICH IS NOT TO MUCH LOWER 
THEN THE 972 MB CENTER FORECAST BY THE CANADIAN AND 979 FROM THE 
GFS. SO THIS WILL BE AN UNUSUALLY STRONG WINTER STORM FOR THE GREAT 
LAKES WED INTO THURSDAY. 

THE EXACT TRACK WILL OF COURSE BE CRUCIAL TO THE AMOUNT OF SNOW WE 
GET FROM THIS SYSTEM. THE LATEST THINKING IS FOR THE RAIN SNOW LINE 
TO BE NEAR I-96 WEDNESDAY MORNING. TYPICALLY STORMS THIS DEEP TEND 
TO TRACK WEST OF THE FORECAST TRACK SO WE WILL HAVE TO MONITOR THIS. 
EVEN SO...JUST THE LAKE EFFECT COMPONENT OF THIS STORM WILL BE 
IMPRESSIVE WITH 50 KNOT WINDS AT 850 MB THU AT 12Z FROM THE WEST 
BLOWING IN 850 MB TEMPS NEAR -18C. WHILE THE WIND WILL FORCE THE 
AXIS OF THE HEAVIEST SNOWFALL INLAND OF US-131... THERE WILL STILL A 
SIGNIFICANT AMOUNT OF SNOW ON THE BACK SIDE OF THIS SYSTEM.  SURFACE 
WINDS OF 25 TO 35 MPH WITH GUSTS NEAR 50 NEAR THE LAKE SHORE COULD 
RESULT IN NEAR BLIZZARD CONDITIONS AS SURFACE TEMPS FALL TO NEAR 20F 
BY MID MORNING THURSDAY. ANY PLACE IN THE CWA THAT STAYS ALL SNOW 
COULD SEE NEAR A FOOT OF SNOW FROM THIS SYSTEM. CLEARLY THERE WILL 
BE NUMEROUS TRAVEL ISSUES WED AND THURSDAY ACROSS THE CWA. 

DUE TO THE SLIGHTLY MORE WESTERLY TRACK OF THE SYSTEM FROM THE 
LATEST MODELS... I PUT FREEZING RAIN AND SLEET INTO THE ZONES AS FAR 
NORTH AS I-96 WED MORNING. I ALSO INCREASED POP TO CATEGORICAL FOR 
TUESDAY NIGHT AND WEDNESDAY.  WILL QUESTION OF SNOW SHOWERS THURSDAY 
SO I INCREASED POP TO LIKELY THERE AND ADDED BLOWING SNOW TO THE 
ZONES FOR WEDNESDAY NIGHT AND THURSDAY. 

WITH THE POLAR VORTEX FORECAST TO BE DISPLACED FARTHER SOUTH INTO 
THIS COMING WEEKEND (ARCTIC OSCILLATION INDEX GOES OFF SO NEGATIVE 
IT IS OFF THE CPC SCALE THEY USE TO DISPLAY THE FORECAST FOR IT). 
THIS WOULD IMPLY COLD WEATHER IS LIKELY SO I LOWERED TEMPS ONE 
CATEGORY IN MOST FORECAST PERIODS AFTER WEDNESDAY DAY TIME. ALSO 
WITH THAT POLAR JET FARTHER SOUTH WE WILL BE SEEING LAKE EFFECT SNOW 
OR CLIPPER SYSTEM SNOWS EACH DAY. THUS WE HAVE SNOW SHOWERS IN THE 
FORECAST THROUGH SAT. 
&&

.AVIATION...(728 AM EST SUN DEC 6 2009)
FOR THE MOST PART VFR CIGS/VSBY SHOULD PREVAIL AT MOST OF GRR'S TAF 
SITES THROUGH SUNSET. THE NEXT SYSTEM UPSTREAM COULD BRING SNOW TO 
THE MKG TAF SITE TOWARD SUNRISE.

AS FOR TODAY...THE 850 MB TEMP IS FORECAST TO RISE TO NEAR 8C BY 
MIDDAY. GIVEN THE LAKE TEMP IS NEAR 5C...THAT WOULD BE MARGINAL FOR 
SNOW SHOWERS. SO THE SERIES OF WEAK SHORTWAVES WILL KICK OF SNOW 
SHOWERS OVER MICHIGAN TODAY BUT MOST THEY WILL BE OVER THE NORTH 1/3 
OF LAKE MICHIGAN.  

&&

.MARINE...(410 AM EST SUN DEC 6 2009)
SOUTHWESTERLY WINDS OF AROUND 5 TO 15 KTS WILL RESULT IN 2 TO 4 FOOT 
WAVE HEIGHTS THIS MORNING AND AFTERNOON THAT WILL SUBSIDE TO 1 TO 2 
FEET LATE THIS AFTERNOON THROUGH TONIGHT AS WIND SPEEDS DIMINISH AND 
FLOW BECOMES OFFSHORE. 

&&

.HYDROLOGY...(410 AM EST SUN DEC 6 2009)
THE ONLY FLOODING POTENTIAL WOULD COME ON WEDNESDAY IF THE ROBUST 
LOW PRESSURE SYSTEM TAKES A TRACK OVER OUR WEST OF OUR CWFA WHICH 
COULD RESULT IN SIGNIFICANT RAINFALL. HOWEVER OVERALL FLOODING 
POTENTIAL MID TO LATE WEEK SEEMS FAIRLY LOW SINCE TEMPERATURES WILL 
PLUMMET INTO THE TEENS TO LOWER 20S FROM WEDNESDAY NIGHT THROUGH 
LATE WEEK. 

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...NONE. 
LM...NONE.

&&

$$

SYNOPSIS:     LAURENS
SHORT TERM:   LAURENS
LONG TERM:    WDM
AVIATION:     WDM
MARINE:       LAURENS
HYDROLOGY:    LAURENS