FXUS63 KIWX 121124
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
624 AM EST SAT DEC 12 2009

.AVIATION...
12Z TAFS...

VFR CONDITIONS TO PERSIST THROUGH 13/03Z. AHEAD OF APPROACHING
TROF/SURFACE LOW...WINDS WILL BECOME MORE SOUTHERLY AND MOISTURE
WILL INCREASE. CLOUDS WILL THICKEN AND LOWER AFTER 22Z...LEADING
TO PRECIPITATION SPREADING FROM SOUTHWEST TO NORTHEAST AFTER
13/00Z. WITH TEMPERATURES AT OR NEAR FREEZING...INITIAL ROUND OF
PRECIPITATION WILL BE A MIXTURE OF -RA/-FZRA BEFORE CHANGING TO
ALL RAIN AFTER 13/09Z. TOWARD THE END OF THE TAF PERIOD WILL NEED
TO MONITOR THE TROF AXIS FOR POTENTIAL LLWS AS THE SYSTEM SHIFTS
NORTH AND EAST AND THE BOUNDARY LAYER WINDS SHIFT AND INCREASE.

&&

.PREV DISCUSSION... /ISSUED 436 AM EST SAT DEC 12 2009/ 

SHORT TERM...
TODAY THROUGH MONDAY MORNING...

CHALLENGE THIS MORNING IS THE TIMING AND TYPE OF PRECIPITATION FOR
THIS EVENING INTO THE MORNING SUNDAY. WARMER AIR IS POISED TO MOVE
INTO THE REGION TODAY AS THE LOW DEEPENS TO OUR WEST. AS OF 08Z
TEMPERATURES ARE ALREADY RUNNING AHEAD OF THE SAME TIME YESTERDAY.
HIGHS ON FRIDAY WERE IN THE MIDDLE 30S TO OUR SOUTH AND SOUTHWEST
AND THESE WILL BE OUR TEMPERATURES TODAY. THEN AS CLOUDS
INCREASE...DO NOT EXPECT TO SEE THE TEMPERATURES DROP AS QUICKLY
TONIGHT. 

ISSENTROPIC ASSENT WILL LEAD TO RAPID SATURATION AND PRECIPITATION
WILL SPREAD FROM SOUTHWEST TO NORTHEAST BEGINNING THIS EARLY
EVENING. EXPECT TO SEE AN INITIAL SHOT OF ZR- AT THE OUTSET...BUT
TEMPERATURES WILL RISE THROUGH THE NIGHT AND BY MIDNIGHT SHOULD BE
AN ALL RAIN EVENT. THEN AS QUICKLY AS IT STARTED THE RAIN WILL END
FROM WEST TO EAST AROUND MID-DAY SUNDAY. HAVE OPTED TO KEEP SNOW
OUT OF THE FORECAST LATE IN THE DAY SUNDAY.

THE NEXT ROUND OF PRECIPITATION WILL MOVE INTO THE AREA DURING THE
OVERNIGHT SUNDAY INTO MONDAY MORNING. 

LONG TERM...
MONDAY THROUGH FRIDAY...THE MAIN CHALLENGES THIS PERIOD ARE THE
TIMING OF THE FRONT LATE MONDAY...THE CHANCE FOR LAKE EFFECT SNOW 
AFTER THE PASSAGE OF THE FRONT AND TEMPERATURES BEHIND THE FRONT 
TUESDAY AND WEDNESDAY. FOR THIS PACKAGE...PREFER THE GFS. THE ECMWF 
HAS BEEN LESS CONSISTENT WITH THE LOW LEVEL THERMAL FIELDS FROM RUN 
TO RUN. UPSTREAM TEMPERATURES OVER MUCH OF THE CANADIAN INTERIOR 
WERE FROM -25F TO -50F. GIVEN UPSTREAM SNOWPACK AND THESE COLD 
TEMPERATURES...THE STRONG AND FAST MOVING COLD FRONT SHOULD BE ABLE 
TO TAP SOME OF THIS COLD AIR GIVEN ADEQUATE UPPER AIR SUPPORT.  THE 
GFS BUFKIT FCST TEMPS HAVE BEEN WORKING EXTREMELY WELL IN THE RECENT 
PAST...SO GUT FEELING OF THE BUFKIT HIGH OF 22F AT SBN WEDNESDAY IS 
IN THE BALLPARK. IT IS POSSIBLE THAT THESE TEMPS MAY STILL BE TOO 
WARM. THERE IS THE POTENTIAL FOR TEMPERATURES TO TURN BITTERLY COLD 
AGAIN BEHIND THIS SYSTEM TUESDAY...SO SUBSEQUENT SHIFTS WILL NEED TO 
MONITOR IF ARCTIC AIR IS ABLE TO BECOME JUST A LITTLE MORE 
ENTRENCHED. CPC OUTLOOK HIGHLIGHTS THESE POTENTIAL HAZARDS OF LAKE 
EFFECT SNOW AND COLD TEMPERATURES VERY WELL. OTHERWISE...KEPT A 
CHANCE FOR LAKE EFFECT SNOW AS DELTA T VALUES SHOULD RISE TO BETWEEN 
15 AND 20 BEHIND THE FRONT. THE FETCH SHOULD BE A LITTLE BETTER WITH 
THIS EVENT THAN WITH THE LAST EVENT AND THE DENDRITIC GROWTH ZONE 
POSITION FAVORABLE FOR THIS EVENT SO HEAVY LAKE EFFECT SNOW IS 
POSSIBLE TUESDAY INTO WEDNESDAY.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...SMALL CRAFT ADVISORY FROM 1 PM THIS AFTERNOON TO 5 AM EST SUNDAY 
     FOR LMZ043-046.

&&

$$

SHORT TERM...LEWIS
LONG TERM...SKIPPER
AVIATION...LEWIS