FXUS63 KIWX 220834
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
334 AM EST TUE DEC 22 2009
.SHORT TERM...
TUESDAY THROUGH WEDNESDAY NIGHT...
LATEST WATER VAPOR LOOP SHOWING UPPER LEVEL SHORT WAVE MAKING ITS
WAY TOWARD THE MID MS VALLEY...KIWX RADAR HAS LIGHT RETURNS
SPREADING ACROSS THE CWA ATTM IN RESPONSE TO THIS DISTURBANCE. BASED
ON LATEST RADAR TRENDS LOCALLY AND UPSTREAM HAVE ADJUSTED WX/POPS TO
LIKELY THROUGH 18Z TODAY. SHORT WAVE EXPECTED TO WEAKEN/SHEAR OUT AS
IT CONTINUES EASTWARD THIS AFTERNOON...DECREASING COVERAGE AND
INTENSITY OF SNOW OVER THE AREA...STILL ANTICIPATE ANY ACCUMULATIONS
TO BE AN INCH OR LESS. BRIEF LULL TONIGHT AND WEDNESDAY BEFORE
SHIFTING FOCUS TO ONSET OF PRECIP ASSOCIATED WITH MUCH ANTICIPATED
STORM SYSTEM GOING INTO THE CHRISTMAS HOLIDAY. SIGNIFICANT WAA
PATTERN TO BEGIN IN EARNEST WEDNESDAY NIGHT AIDED BY 50KT+ LLJ
STREAMING INTO CENTRAL ILLINOIS BY 12Z THU. 295K SFC SHOWING STRONG
ISENTROPIC UPGLIDE TO SUPPORT PRECIP CHANCES IN THE WESTERN CWA. SFC
RIDGING EXTENDING DOWN THROUGH THE EASTERN GREAT LAKES WILL SUPPORT
STRONG EASTERLY FLOW OVER THE AREA EFFECTIVELY DECOUPLING THE
SURFACE FROM THE WAA WED NITE. EXPECT THIS TO STAVE OFF ANY TYPE OF
NON-DIURNAL INCREASE IN TEMPS AND ALLOW FOR A SUB-FREEZING SFC BASED
LAYER OVERNIGHT. RESULTING THERMAL PROFILE LENDING ITSELF MORE
TOWARD A ZR/PL MIX AND HAVE TRENDED WX GRIDS ACCORDINGLY.
&&
.LONG TERM /THURSDAY THROUGH TUESDAY/...
...STRONG STORM TO BRING MIXED BAG CHRISTMAS EVE THROUGH CHRISTMAS
DAY WITH SOME SLICK TRAVEL LIKELY...
MODEL CONSENSUS CONTINUES TO GROW FOR THE UPCOMING LATE WEEK STORM.
EARLY MORNING WATER VAPOR IMAGERY SHOWS ALL OF THE UPCOMING
IMPORTANT PLAYERS. FIRST...SHORTWAVE ENERGY RAPIDLY DIVING DOWN THE
WESTERN COAST OF NOAM IS BEGINNING TO CARVE OUT A WESTERN CONUS
TROUGH. SECONDLY...STRONG DOWNSTREAM BLOCKING CONTINUES WITH THE
REMNANTS OF THE NORTHEASTERN US SNOWSTORM NOW SOUTH OF A REGION OF
POSITIVE HEIGHT ANOMALIES...FORCING BRISK EASTERLY FLOW ALOFT OVER
EASTERN CANADA. AS THE WESTERN TROUGH CROSSES THE ROCKIES...LEE
SIDE CYCLOGENESIS WILL TAKE PLACE...AND TAP AN INCREASINGLY GOM-RICH
AIRMASS TO IT/S EAST ALONG WITH MUCH COLDER AIR PUSHING DOWN THE
EASTERN SLOPES OF THE ROCKIES AS A SECOND SHORTWAVE...SEEN EARLY
THIS MORNING OVER THE YUKON TERRITORY DROPS SOUTH DOWN THE WESTERN
FLANK OF THE ROCKIES TROUGH. THE RESULT WILL BE POTENT CYCLOGENESIS
OVER THE SOUTHERN PLAINS WITH LOW PRESSURE MAKING VERY SLOW PROGRESS
TO THE NORTH AND EAST GIVEN THE DOWNSTREAM BLOCKING AND THE
AFOREMENTIONED ENERGY DROPPING INTO THE REAR OF THE TROUGH. THE LOW
MAY NOT MAKE IT TO THIS LONGITUDE UNTIL THE WEEKEND...BUT THE
PRIMARY CONCERNS WILL BE FOR PRECIPITATION AHEAD OF AND ALONG THE
SURFACE OCCLUSION THURSDAY AND THURSDAY NIGHT.
OVER THE PAST 24 HOURS THERE HAVE ONLY BEEN SLIGHT CHANGES TO THE
ENVELOPE OF MODEL SOLUTIONS. IN GENERAL...THERE HAS BEEN A SLIGHT
SLOWING TO THE OCCLUDED FRONTAL PASSAGE WITH A GOOD CLUSTERING
AROUND THE FRIDAY MORNING PERIOD. THIS WILL SLOW ANY CHANGE BACK TO
SNOW ON CHRISTMAS DAY. OTHERWISE...PREVIOUS FORECAST THINKING HAS
NOT CHANGED SUBSTANTIALLY.
WILL HOLD ONTO LIKELY POPS FROM 31 WEST ON THURSDAY AS THE EASTERN
FLANK OF THE DEEP GOMEX MOISTURE SURGE AND 290K ISENTROPIC LIFT
BRUSH MY WESTERN ZONES. THERMAL PROFILES SUGGEST A WINTRY MIX TO
START OVER WESTERN AREAS...LIKELY CHANGING TO RAIN SOUTH OF 30 BY
AFTERNOON. CURRENT EXPECTATIONS WOULD BE FOR LESS THAN ONE TENTH OF
AN INCH OF GLAZE...BUT STILL ENOUGH TO CAUSE SOME SLICK TRAVEL...ESP
IN THE MORNING. FURTHER EAST...MAIN QUESTION IS WHETHER ANY
PRECIPITATION ARRIVES HERE. IF IT DOES...A SN/IP/ZR MIX GIVING WAY
TO MORE ZR/IP IS MOST LIKELY /EXCEPT SOUTH OF A WABASH TO BLUFFTON
LINE WHERE ANY PRECIP WOULD LIKELY BE RAIN BY AFTERNOON/.
HOWEVER...DO NOT SEE ANY MORE THAN VERY LIGHT PRECIP THROUGH THE
DAYLIGHT HOURS WITH DRY LLEVEL EASTERLY FLOW AND DEEPER MOISTURE
STILL DISPLACED WEST. SO...EXPECT THE DAYTIME IMPACT IN THIS AREA TO
BE MINIMAL AT THIS TIME.
THURSDAY NIGHT...DEEP MOISTURE PLUME AHEAD OF THE OCCLUSION MOVES
OVERHEAD. FLOW CONFIGURATION AND DISTANCE TO SURFACE LOW SUGGEST
ONLY A VERY SLOW WARMUP. TEMPS WON/T BE ALL THAT COLD TO START THE
EVENT HOWEVER...SO WHILE A PERIOD OF FREEZING RAIN LOOKS LIKELY FROM
33 NORTH AND EAST AS PRECIPITATION GETS GOING IN THIS REGION...DON/T
EXPECT MUCH ICE ACCRUAL...WITH BEST ESTIMATES SUGGESTING A FEW
HUNDREDTHS OF AN INCH OF ICING BEFORE THE CHANGEOVER. FOR A TIME
DURING THE EVENING...HOWEVER...SLICK TRAVEL APPEARS LIKELY ESP ON
NON-TREATED SURFACES. BY MIDNIGHT EXPECT THE CHANGEOVER TO BE
PRETTY MUCH COMPLETE WITH TEMPERATURES CONTINUING TO RISE INTO THE
MID/UPPER 30S THROUGH THE NIGHT.
HAVE UPPED POPS IN THE EAST FRIDAY MORNING GIVEN THE SLIGHTLY SLOWER
TREND AND IT IS POSSIBLE THESE MAY NEED TO BE UPPED FURTHER.
EXITING RAIN IN THE EAST WILL GIVE WAY TO SOME SCATTERED SHRA THAT
WILL MIX WITH AND CHANGE TO SNOW FROM WEST TO WEST AS THE DAY
PROGRESSES AND COLDER AIR FILTERS IN. TEMPS WILL LIKELY FALL
THROUGH THE AFTERNOON BUT A LATE MORNING/EARLY AFTERNOON HIGH IN THE
UPPER 30S TO AROUND 40 LOOKS REASONABLE.
FOR THE SATURDAY THROUGH TUESDAY TIMEFRAME...THERE IS GENERAL
AGREEMENT THAT THE SLOW MOVING CUTOFF LOW IN OUR VICINITY WILL
SLOWLY PULL EAST WITH A LONGWAVE TROUGH MOVING TO A POSITION FROM
HUDSON BAY THROUGH NEW ENGLAND AND INTO THE WESTERN ATLANTIC BY
TUESDAY. WITH THIS EVOLUTION...EXPECT GRADUALLY DECREASING SHSN
POTENTIAL AS CYCLONIC FLOW AND DEEP MOISTURE ARE GRADUALLY LOST.
TEMPS ARE ONLY MARGINALLY SUPPORTIVE FOR A LAKE RESPONSE THROUGH
SATURDAY...AND BY THE TIME THEY COOL DOWN TO THE POINT WHERE
INSTABILITY IS SOMEWHAT MORE FAVORABLE...INVERSION HEIGHTS ARE
FALLING AS HEIGHTS ALOFT BEGIN BUILDING AND CYCLONIC FLOW IS LOST.
THEREFORE...HAVE NOT GONE ANY HIGHER THAN CHC POPS THROUGH THE
PERIOD...AND GRADUALLY TAPER THESE DOWN TO JUST THE CLIMATOLOGICALLY
FAVORED NORTHERN ZONES BY MONDAY...TO A DRY FORECAST BY TUESDAY.
TEMPS SATURDAY WILL STILL REACH NEAR FREEZING...BUT THEN FALL TO
AROUND 5-7F BELOW SEASONAL NORMS TO END THE PERIOD GIVEN COOLING
TEMPS ALOFT ON THE WESTERN FLANK OF THE LONGWAVE TROUGH.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...NONE.
&&
$$
SHORT TERM...LOGSDON
LONG TERM...ARNOTT
