FXUS63 KGRR 221734
AFDGRR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GRAND RAPIDS MI
1235 PM EST TUE DEC 22 2009

LATEST UPDATE...AVIATION SHORT TERM AND SYNOPSIS...

.SYNOPSIS...(1235 PM EST TUE DEC 22 2009)
AN UPPER AIR DISTURBANCE PASSING BY TO THE SOUTH OF MICHIGAN THIS 
AFTERNOON WILL CONTINUE TO BRING LIGHT SNOW SOUTH OF A LINE FROM 
SOUTH HAVEN TO KALAMAZOO. MEANWHILE A LARGE CANADIAN POLAR HIGH 
PRESSURE SYSTEM ANCHORED BETWEEN LAKE SUPERIOR AND JAMES BAY WILL 
BRING DRIER AND COLDER AIR INTO THE AREA FOR TONIGHT THROUGH  
WEDNESDAY NIGHT.    

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 LATE THURSDAY INTO FRIDAY. SIGNIFICANT SNOW 
WITH THIS SYSTEM WILL STAY WELL TO OUR WEST. HOWEVER THERE IS 
POTENTIAL FOR FREEZING RAIN ON CHRISTMAS EVE WHICH COULD POTENTIALLY 
CREATE VERY HAZARDOUS TRAVEL CONDITIONS FROM CHRISTMAS EVE INTO 
CHRISTMAS DAY. IT WILL TURN COLDER WITH INTERMITTENT SNOW SHOWERS 
LATE FRIDAY THROUGH THE WEEKEND.     

&&

.SHORT TERM...(1235 PM EST TUE DEC 22 2009)
(THIS AFTERNOON THROUGH THURSDAY AFTERNOON)

THE AREA OF SNOW ASSOCIATED WITH THE COUPLED JET EVENT WILL WEAKEN 
AS THE NORTHERN STREAM JET CORE WEAKENS CONSIDERABLY THIS AFTERNOON. 
EVEN SO THERE IS STILL A CONSIDERABLE AREA OF LIGHT SNOW OVER WI AT 
NOON. IT WILL TAKE AWHILE FOR ALL THAT MOISTURE TO BE GONE. THUS I 
KEEP LIGHT SNOW OVER THE EXTREME SOUTHERN CWA. THERE IS A 
CONSIDERABLE AMOUNT OF MID LEVEL DRY AIR BEING BROUGHT IN FROM THE 
NORTHEAST BETWEEN 4000 FT AND 7000 FT (BUFKIT NAM). DTX 12Z SOUNDING 
SHOWS VERY DRY AIR IN THE 6000 TO 12000 FT LAYER WITH WEST WINDS. 
BOTTOM LINE IS THERE IS TO MUCH DRY AIR FOR THAT SNOW TO MAKE MUCH 
PROGRESS INTO OUR CWA THIS AFTERNOON. THE SNOW SHOULD FOR THE MOST 
PART STAY SOUTH OF THE STATE.  

OVERALL...THE UPSTREAM SHORTWAVE RIDGING WILL PREVAIL AND SLOWLY DRY 
THE ATMOSPHERE OUT SO THAT MOST OF THE CWA SHOULD SEE CLEARING OF 
MOST OF THE CLOUDS TONIGHT.  SOME LOW CLOUDS MAY PREVAIL NEAR AND 
SOUTH OF I-94 DUE TO EAST WINDS OFF LAKE ERIE...INTO THIS EVENING 
BUT THAT SHOULD CLEAR TOO AS WINDS BECOME MORE NORTHEAST.

IN FACT...THE DRY AIR FROM THE SHORTWAVE RIDGE WILL PREVAIL INTO 
THURSDAY AND KEEP THE PRECIPITATION WEST OF THE STATE THROUGH 
THURSDAY AFTERNOON. WE DO HAVE 50 PCT POP IN CURRENT GRIDS AFTER 18Z 
BUT THIS MAY CHANGE. 

&&

.LONG TERM...(330 AM EST TUE DEC 22 2009)
(THURSDAY NIGHT THROUGH MONDAY)

ONLY MADE A FEW TWEAKS TO PRECIP TYPE AND POPS AS SLOW MOVING LOW 
DOMINATES THE WEATHER ACROSS THE GREAT LAKES THROUGH THE PERIOD. 
WENT WITH A COMPROMISE OF GFS AND NAM GUIDANCE...WITH SNOW AND MIXED 
PRECIP ACROSS THE NORTHERN FORECAST AREA AND FREEZING RAIN TO RAIN 
ACROSS THE SOUTH ON THURSDAY NIGHT AS EAST WINDS CONTINUE TO FEED 
COLD AND DRY AIR IN FROM SFC HIGH RETREATING ACROSS EASTERN CANADA.

AFTER A BURST OF WAA/ISENTROPIC LIFT PRECIP THURSDAY NIGHT...THE DRY 
SLOT SHOULD WORK IN ON FRIDAY REDUCING POPS TO CHANCE WITH TEMPS 
ABOVE FREEZING DURING THE DAY. RAIN CHANGING TO WET SNOW DURING THE 
AFTERNOON AS COOLER AIR MOVES IN ALOFT BY 00Z SATURDAY.

SOME LAKE ENHANCED WRAP AROUND SNOWS FOR THE WEEKEND WITH ARCTIC AIR 
ARRIVING ON SUNDAY HELPING TO PRODUCE WIDESPREAD SNOW SHOWERS AND 
SOME ACCUMS OF A FEW INCHES AS THE UPPER LOW MOVES THROUGH.

&&

.AVIATION...(1235 PM EST TUE DEC 22 2009)
LAKE EFFECT CLOUDS OFF LAKE HURON HAS STREAMED INTO JXN AND LAN TAF 
SITES. WAS THE WINDS IN THE 950 MB TO 850 MB LAYER SLOWLY TURN MORE 
EASTERLY AND BECOME MORE DIVERGANT (BY 03Z) THE LOW CLOUDS (CIGS 
015-025) SHOULD BE SHIFTED FARTHER SOUTH. LAN SHOULD CLEAR FIRST. BY 
TONIGHT THE DIVERGING LOW LEVEL WINDS COMING SOUTHWARD FROM THE 
LARGE POLAR HIGH NORTH OF LAKE SUPERIOR SHOULD CLEAR SKIES BELOW 
12000 FT ACROSS THE ENTIRE CWA. I DO NOT EXPECT CLOUDS BELOW 12000 
TO MOVE BACK IN UNTIL THURSDAY AFTERNOON. 

&&

.MARINE...(330 AM EST TUE DEC 22 2009)
WINDS AND WAVES WILL NOT POSE A HAZARD TO MARINERS UNTIL THURSDAY 
AND FRIDAY AS A LOW PRESSURE PRESSURE SYSTEM MOVES IN RESULTING IN 
SIGNIFICANTLY HIGHER WIND AND WAVE ACTION LATE IN THE WEEK. 

&&

.HYDROLOGY...(330 AM EST TUE DEC 22 2009)
CONCERN FOR ICE DEVELOPMENT THAT WOULD LEAD TO ICE JAMS IS MITIGATED 
BY MAX TEMPS MAINLY IN THE 30S THROUGH FRIDAY... RESULTING IN LOW 
ICE JAM POTENTIAL. 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:    OSTUNO
AVIATION:     WDM 
MARINE:       LAURENS
HYDROLOGY:    LAURENS