FXUS63 KIWX 230036
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
736 PM EST TUE DEC 22 2009
.UPDATE...
UPDATE ISSUED TO UP POPS THIS EVENING GENERALLY ALONG AND NORTH OF
US 30 IN PERSISTENT AREA OF SNOW. RADAR RETURNS ON THE SOUTHERN
FRINGE OF THE AREA HAVE INCREASED SOMEWHAT WITH VSBYS DROPPING TO
CLOSE TO ONE MILE...INCLUDING HERE AT THE OFFICE. QUICK FEW TENTHS
OF SNOW ACCUMULATED. DRY AIR IS TRYING TO WORK SOUTH BUT STRENGTH
OF LIFT AND AMOUNT OF MSTR SEEMS TO BE WINNING AT THE MOMENT SO
INCREASE WARRANTED. WILL MONITOR TRENDS OVER THE NEXT COUPLE OF
HOURS FOR ADDITIONAL CHANGES.
RADARS TO THE WEST AND SOUTH SHOWING LIGHT RETURNS TRYING TO WORK
TOWARDS SW AREAS BUT NOTHING SHOWING UP ON OBS. SO FOR NOW HAVE
LEFT CHC POPS INTACT IN THESE AREAS.
&&
.PREV DISCUSSION... /ISSUED 642 PM EST TUE DEC 22 2009/
AVIATION /00Z TAFS/...
CHALLENGING TAF PERIOD WITH START OF IT PLAGUED BY STRUGGLE OF
DRIER AIR INVADING FROM THE NE AND LEFT OVER LL MSTR FROM BOTH
DEPARTING WAVE AND ADDITIONAL MSTR STREAMING IN FROM THE SW. AT
KSBN CIGS HAVE BEEN BOUNCING BETWEEN 2800 AND 5000 FT BUT VSBYS
REMAINING SOLID IN THE MVFR RANGE. LOW CLOUDS HOLDING STRONG AT
KFWA BUT OBSERVATIONS JUST TO THE NORTH SHOWING THE DRIER AIR
TRYING TO MOVE TOWARDS THAT SITE. AS LIMITED RIDGING TRIES TO
BUILD SOUTH IT REMAINS POSSIBLE THAT BOTH TAF SITES COULD SEE
IMPROVEMENTS IN CONDITIONS LONGER THAN DEPICTED. HOWEVER GIVEN
RADAR TRENDS OF LGT RETURNS OF WHAT MAY BE EITHER FLURRIES OR
FREEZING DRIZZLE AND MANY SITES IN ILLINOIS SHOWING CIGS AOB 1000
FT...HAVE OPTED TO LET THE DRY AIR HOLD IN GENERAL AT KSBN THROUGH
THE EVENING AND INVADE KFWA OVER THE NEXT FEW HOURS. ONE MORE BAND
OF SOMEWHAT ENHANCED SNOWFALL WAS WORKING NE AND SHOULD DEPART IN
THE NEXT HOUR OR TWO.
BEYOND 6Z FREEZING DRIZZLE OR FLURRIES MAY BECOME MORE OF AN ISSUE
OVER TIME...BUT A LOT OF QUESTIONS REMAIN AS TO AMOUNT OF LIFT AND
MSTR INTO THE FIRST PART OF WEDS FOLLOWED BY GREATER CHALLENGES
BEYOND 18Z WITH POTENTIAL HEAVIER PRECIP. WILL KEEP THINGS
UNCHANGED PAST 7Z FOR NOW.
PREV DISCUSSION... /ISSUED 401 PM EST TUE DEC 22 2009/
SHORT TERM...
TONIGHT THROUGH WEDNESDAY NIGHT
WEAK SLOW MOVING UPPER LEVEL TROUGH CONTINUES TO SUPPORT LIGHT SNOW
ACROSS THE FA IN A WAA REGIME ALOFT. WEAK CROSS ISOBARIC ASCENT ON
ON 290-300K SFC CONTINUES TO OFFER LL LIFT...SUPPORTING THE LIGHT
SNOW...BEING ENHANCED BY THE DEPARTING WAVE. SUBSIDENCE WILL TAKE
OVER WITH MID LEVEL DRYING EXPECTED THIS EVENING...AS EVIDENT IN
UPSTREAM SAT OBS. AMPLE LL SATURATED PROFILES IN THE PRESENCE OF A
GROWING INVERSION WILL TEND TO HOLD THE LL CLOUD DECK IN PLACE
ACROSS MOST OF THE SOUTHERN/WESTERN FA...WITH BACKING LL WINDS
SUPPORTING DRYING ACROSS THE NORTH AS CP SFC HIGH PRESSURE ADVECTS
ACROSS THE NORTHERN GREAT LAKES. FURTHER UPSTREAM...A POTENT UPPER
LEVEL TROUGH ENTERING THE HIGH DESERT CONTINUES TO DIG INTO THE THAT
REGION...AND WILL BEGIN TO EJECT INTO THE SOUTHERN PLAINS ON WED. A
MORE IMPORTANT AND KEY INGREDIENT TO THE DEVELOPING WINTER STORM IS
THE ADVANCING UPPER SHORTWAVE OVER THE PAC NW. THIS NEGATIVE HEIGHT
ANOMALY AND ITS ASSOCIATED LL CAA FIELDS WILL HELP SUPPORT PHASING
OF THE SOUTHERN STREAM WAVE...ACTING TO SLOW DOWN EASTERN
PROGRESSION OF THE ENTIRE SYSTEM INCLUDING LL REFLECTED FIELDS...AS
UPPER LEVEL BACKSIDE MOMENTUM TRANSPORT CONTINUES TO SUPPORT FLOW
CUTTING TO HIGHER HEIGHTS ALOFT. THIS PROCESS WILL BE CRITICAL TO
THE WX FORECAST IN THE LATER PERIODS AND BEYOND...EFFECTIVELY
SHUNTING THE BEST LL MOISTURE TRANSPORT AND LARGE SCALE ASCENT WEST
OF THE FA THROUGH MUCH OF THE PERIOD. HAVE UTILIZED VARIOUS FORECAST
MODELS...INCLUDING THE NAM/GFS/AND ECWMF WITH NO PARTICULAR FAVORED
WEIGHT TO ONE MODEL GIVEN OVERALL GENERAL HEIGHT AND FLOW AGREEMENT.
TONIGHT...MID LEVEL SUBSIDENT FLOW WILL DOMINANT WITH LINGERING LL
LIFT/UPGLIDE EXPECTED THROUGH OVER MUCH OF THE WESTERN FA PER A WAA
REGIME IN THE H8-H7 LAYER...SUPPORTING LIGHT SNOW AND A PATCHY ZL
THREAT...GIVEN PROGS OF GENERAL SATURATION REMAINING ABOVE -10C. NE
AREAS MAY SEE PARTIAL CLEARING GIVEN BACKED LOCAL FLOW SUPPORTING
NEGATIVE THETA-ADV WITH LL NE FLOW EMANATING FROM ADVECTING SFC HIGH
PRESSURE OVER THE NORTHERN GREAT LAKES. HAVE ADJUSTED LOWS DOWN SOME
IN THE NE WHERE THE OVERLAP OF DRIER PROFILES/CAA/AND PARTIAL
CLEARING WILL RESIDE.
WEDNESDAY...LEAD SURGE OF THETA-E ADVECTION PER THE EJECTING
SOUTHERN STREAM WAVE WILL SUPPORT THE DEVELOPMENT OF A LIGHT PRECIP
BAND PER REFLECTED ACCENT IN THE H85-H7 LAYER. UTILIZING NAM/GFS
290K PROGS...A BAND WOULD BE POSSIBLE NEAR THE WESTERN FA TOMORROW
AFTERNOON INTO WED NIGHT...COINCIDENT WITH A WEAK SHORTWAVE LEAD
DISTURBANCE. STRONG BAROCLINIC ZONE IN PLACE WILL LIKELY ENHANCE
HYDROMETER PRODUCTION LENDING TO LIGHT TO MODERATE RETURNS CLOSER
THE BEST LARGE SCALE ASCENT...WHICH WILL BE WEST OF THE FA. HAVE
OPTED TO FOR A FLURRY AND ZL MENTION IN THE SW FA THROUGH ALL WED
GIVEN INCREASING LL MOISTURE COINCIDENT WITH INCREASING LL LIFT.
LINGERING DRY SUBSIDENT FLOW WILL CONTINUE TO DOMINATE OVER THE
EASTERN HALF OF THE FA INTO THUR UNTIL MID LEVEL RIDGING SHIFTS EAST
OF THE REGION. HENCE...HAVE RETAINED HIGHER POP MENTION IN THE
WESTERN FA...WITH SHARP SW-NE DEGRADATION. PRECIP TYPE BECOMES AN
ISSUE GIVEN SFC-2KFT EASTERLY FLOW SUPPORTING CONTINUE REINFORCEMENT
OF LL COLD DRY AIR. GIVEN FORECAST THERMAL FIELDS ALOFT SUPPORTING
ROBUST WARM NOSE AROUND H925 OF 5 C IN THE WESTERN FA WED
EVENING...EXPECTING ANY PRECIP TO START AS A SLEET MIX...RAPIDLY
CHANGING TO A ZR/IP MIX WITH A PERIOD OF ZR POSSIBLE IN THE WEST.
FEEL AFOREMENTIONED THERMAL NUMBERS ARE REPRESENTATIVE GIVEN WEAK A
ADIABATIC COOLING REPONSE IN THE PRESENCE OF MINIMAL LL WET BULB
EFFECTS. QPF PROGS/EXPECTED WEAK LSA NUMBERS/AND CONCERNS ON
DURATION PRECLUDE ANY HEADLINES/HIGHLIGHTS ATTM...ESPECIALLY GIVEN
THE POSSIBILITY OF A WESTWARD HEIGHT PATTERN SHIFT GIVEN PRONOUNCED
WAVE PHASING...AND WILL DEFER ANY TO THE NEXT SHIFT GIVEN THE PERIOD
AND THE NEED FOR ADDITIONAL OBSERVATIONAL DATA. REGARDLESS...A SHORT
DURATION OF ZR WOULD CAUSE MAJOR TRAVEL CONCERNS AND THIS THREAT
SHOULD BE MONITORED.
LONG TERM...
THURSDAY THROUGH TUESDAY...THE MAIN CONCERN THIS PERIOD IS THE
POTENTIAL FOR A WINTRY MIX LATE THURSDAY THROUGH THURSDAY NIGHT.
HAVE BLENDED THE NAM 12 KM WRF...GFS AND SREF ENSEMBLE FOR THIS
PACKAGE. GIVEN THERMAL PROFILES...EXPECT TO SEE A MIX OF MAINLY
SLEET AND FREEZING RAIN NORTH THURSDAY AFTERNOON...WITH JUST RAIN
SOUTH. GIVEN RECENT MODEL HANDLING OF THIS TYPE OF SHALLOW COLD AIR
EVENT WITH OVERRUNNING PRECIPITATION...CONCERN THAT TEMPS WILL STILL
BE BELOW FREEZING THROUGH THURSDAY AFTERNOON OVER FAR NORTHERN
INDIANA INTO LOWER MICHIGAN AND FAR NW OHIO. BUFKIT SFC TEMPS OVER
NORTHERN AREAS 0C TO 1C. ALTHOUGH NOT OVERWHELMING SUPPORT FROM
THESE MODELS FOR A WIDESPREAD FREEZING RAIN/ICE EVENT...BELIEVE THE
POTENTIAL FOR ROADS TO BECOME EXTREMELY SLIPPERY BY THURSDAY EVENING
ESPECIALLY NORTH OF ROUTE 6 WITH SFC TEMPS JUST BELOW FREEZING.
GIVEN THE POTENTIAL FOR A HUGE HOLIDAY TRAVEL IMPACT...HAVE DECIDED
AFTER COORDINATING WITH NEIGHBORING OFFICES TO ISSUE A WINTER STORM
WATCH FOR THIS WINTRY FREEZING RAIN AND SLEET MIX OVER FAR NORTHERN
INDIANA INTO LOWER MICHIGAN. A LARGE SFC ANTICYCLONE WILL INITIALLY
KEEP A COOL AND DRY EASTERLY FEED INTO THE REGION BEFORE MOISTURE
FLUX INCREASES RAPIDLY THURSDAY NIGHT. TEMPERATURES OVER THE ENTIRE
AREA SHOULD RISE ABOVE FREEZING AS THE DRY SLOT SURGES NORTH. MUCH
COLDER AIR WILL THEN MOVE INTO THE AREA DURING THE AFTERNOON AND
EVENING WITH A FLASH FREEZE LIKELY. TEMPS SHOULD BE BELOW NORMAL
THIS WEEKEND INTO EARLY NEXT WEEK. AS FOR LAKE EFFECT SNOW...GIVEN
THE CUT OFF POSITION OF THE LOW...THE WIND DIRECTION SHOULD NOT BE
FAVORABLE FOR LAKE EFFECT SNOW...SO MAINLY LIGHTER SYNOPTIC SNOW IS
EXPECTED.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...WINTER STORM WATCH FROM THURSDAY AFTERNOON THROUGH FRIDAY
MORNING FOR INZ003>007.
MI...WINTER STORM WATCH FROM THURSDAY AFTERNOON THROUGH FRIDAY
MORNING FOR MIZ077>081.
OH...NONE.
LM...GALE WATCH FROM THURSDAY MORNING THROUGH FRIDAY AFTERNOON FOR
LMZ043-046.
&&
$$
SHORT TERM...JC
LONG TERM...SKIPPER
AVIATION...FISHER
UPDATE...GREENAWALT/FISHER
