000 FXUS63 KLOT 100400 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1000 PM CST THU FEB 9 2012 .DISCUSSION... 857 PM CST STRATUS CONTINUES TO SPREAD EAST AND IS NOW ENTERING NORTHWEST INDIANA. HAVE ADJUSTED HOURLY TEMP TRENDS TO REFLECT MORE OF A STEADY CONDITION GIVEN THE CLOUDS AND A STEADY SOUTHWEST BREEZE. DID NUDGE LOWS IN THE NORTHWEST UP SEVERAL DEGREES WITH THEM OCCURRING AROUND OR A BIT AFTER DAYBREAK. THE COLD FRONT ARRIVES DURING THAT TIME WITH COLDER AIR QUICKLY SPILLING IN SO THERE MAY BE A QUICK SEVERAL DEGREE DROP TOWARDS THE END OF THE MIN TEMP PERIOD FOLLOWING GENERALLY STEADY TEMPS OVERNIGHT. WILL CONTINUE WITH LAKE EFFECT SNOW WATCH AS IS. AFTER EVALUATING AFTERNOON AND EARLY EVENING GUIDANCE...INCLUDING A VARIETY OF HIGH RES SOLUTIONS FROM VARIOUS SOURCES...SPECIFICS OF LOCATION AND AMOUNTS NOT COMPLETELY CLEAR...AT LEAST TO THE POINT WHERE CONFIDENCE IS HIGH ENOUGH TO ADJUST TO WARNING OR ADVISORY. SOME GUIDANCE CONTINUES TO SUGGEST THAT AN EASTERLY COMPONENT TO THE FETCH OFF THE LAKE IS LESS LIKELY OR AT LEAST SHORT LIVED SUPPORTING A MORE TRADITIONAL DUE NORTH FETCH BAND INTO NW IN. IT STILL APPEARS THAT THE SHORELINE OF SOUTHERN COOK COUNTY AND WILL COUNTY NEAR THE IL/IN STATE LINE WOULD BE AFFECTED AT LEAST EARLY ON. IT IS POSSIBLE THAT IF WINDS DO NOT HAVE AN INITIAL EASTERLY COMPONENT THEN THE DURATION THAT THEY STAY DUE NORTH WILL BE LONGER BEFORE TURNING NORTHWEST SATURDAY MORNING. THIS COULD RESULT IN A MORE ANCHORED BAND AND HIGHER SNOW TOTALS OVER A MORE FOCUSED AREA...LIKELY SLIGHTLY FURTHER EAST THAN THE CURRENT FORECAST. LIMITING FACTORS OF DRY ADVECTION AND A MORE ANTICYCLONIC CURVATURE TO THE FLOW DEVELOPING FRIDAY NIGHT REMAIN A CONCERN IN TERMS OF MAX SNOWFALL AMOUNTS. OVERALL LAKE COUNTY INDIANA SEEMS TO HAVE THE BEST CHANCE AT WARNING CRITERIA SNOW DESPITE THE RANGE OF SOLUTIONS...WITH POTENTIAL ACROSS FAR EASTERN COOK/WILL COUNTIES AND PORTER COUNTY CONTINUING AS WELL BUT WITH LOWER CONFIDENCE. WILL CONTINUE TO MONITOR NEW GUIDANCE AS IT ARRIVES. TIMING STILL LOOKS ON TRACK WITH LAKE EFFECT SETTING UP LATE AFTERNOON/EARLY EVENING FOLLOWING SYNOPTIC SCALE SNOW. MDB //PREV DISCUSSION... 305 PM CST A LAKE EFFECT SNOW EVENT IS LIKELY FOR THE CITY OF CHICAGO AND THE SOUTH SUBURBS AS WELL AS FAR NORTHWEST INDIANA FROM MIDDAY FRIDAY INTO SATURDAY AFTERNOON. THERE IS THE POTENTIAL FOR UP TO 6 INCHES FOR THE CITY ALONG THE LAKE AND ACROSS THE SOUTH AND SOUTHEAST SIDES AS WELL AS THE SOUTH SUBURBS...WITH UP TO TO 8 INCHES POSSIBLE ACROSS LAKE AND PORTER COUNTY OF NORTHWEST IN. TONIGHT THROUGH SATURDAY... THE COLD FRONT THAT HAS BEEN PUSHING SOUTH-SOUTHEAST ACROSS WESTERN ONTARIO...FAR NORTHWEST MN AND THE DAKOTAS WILL CONTINUE ON THIS PATH THROUGH THE NIGHT...CROSSING INTO FAR NORTH CENTRAL AND FAR NORTHWEST IL TOWARD DAWN...AND THE REST OF NORTHERN IL AND NORTHWEST IN BY EARLY FRIDAY AFTERNOON AS A SHORT WAVE MOVES FROM THE CENTRAL AND SOUTHERN HIGH PLAINS EAST TO THE MID MS VALLEY...AND AN UPPER JET MAX CONTINUES TO DIG SOUTH-SOUTHEAST FROM SASKATCHEWAN AND EASTERN ALBERTA ACROSS THE DAKOTAS TO THE MID MO VALLEY BY 18Z FRIDAY. WHILE THE COLDEST AIR REMAINS OVER HUDSON BAY...NORTHERN QUEBEC AND EASTERN BAFFIN ISLAND...THE SOUTHERN PERIPHERY OF THE COLD AIR MASS HAS MOVED WELL SOUTHWARD SINCE YESTERDAY...WITH AN 850HPA TEMPERATURE OF -5C AT KINL VERSUS -17C AT CWPL AND -20C AT CYQD PER THE 12Z RAOBS. ONCE THE COLD FRONT PASSES BY TOMORROW LOW LEVEL WINDS OVER LAKE MI VEER TO NORTH TO NORTH-NORTHEAST BY MIDDAY. THE DEPTH OF THE VERY COLD AIR DEEPENS DURING FRIDAY AFTERNOON THROUGH FRIDAY NIGHT WHILE MID LEVEL WINDS ALSO VEER TO NORTH TO NORTH-NORTHEAST. WITH 850HPA TEMPERATURES OF -20 TO -22C PROGGED OVER SOUTHERN LAKE MI OVERNIGHT FRIDAY AND SATURDAY MORNING AND +1 TO +3C LAKE WATER...A LAKE EFFECT SNOW BAND IS EXPECTED TO DEVELOP DURING FRIDAY. WITH NEAR UNIDIRECTIONAL N-NNE FLOW THROUGH THE LOWER 3KM CLOUD BEARING LAYER THE BAND IS EXPECTED TO DEVELOP FROM CENTRAL LAKE MI SSW TO THE CHICAGO SHORELINE AND THE FAR NW CORNER OF IN FRIDAY AFTERNOON. WITH COOLING LOW LEVEL TEMPERATURES AND INCREASING DEPTH OF THE COLD AIR...SNOWFALL INTENSITY SHOULD INCREASE FOR LATE FRIDAY AND INTO SATURDAY MORNING INCREASING ACCUMULATION RATES WHERE THIS LAKE EFFECT BAND WILL CROSS THE SHORE AND A BIT DOWNWIND. WHILE MODELS SHOW UP TO A 15-20MI E-W SPREAD IN WHERE THE AXIS OF HEAVIEST SNOW IS LIKELY TO BE THE CONSENSUS CONTINUES TO POINT TO THE AREA FROM DOWNTOWN CHICAGO SOUTH ACROSS THE SOUTHEAST SIDE AND ALONG THE NORTHERN POTION OF THE IL-IN LINE AS WELL AND THE NORTHWEST CORNER OF IN. HEAVIEST SNOWFALL IS EXPECTED TO BE FROM MID FRIDAY EVENING THROUGH THE PRE-DAWN HOURS OF SATURDAY WHEN ELEMENTS FOR LAKE EFFECT SNOW WILL BE MAXIMIZED. BY DAWN SATURDAY THE LOW LEVEL FLOW IS PROGGED TO HAVE BACKED SLIGHTLY SHIFTING THE SNOW BAND AXIS OFF OF THE ILL SHORE AND A BIT EAST IN NORTHWEST IN. THIS BACKING OF THE FLOW TO OUT OF THE NORTHWEST ALONG WITH THE LOWERING OF A SUBSIDENCE INVERSION WILL CONTINUE TO SHIFT THE LAKE EFFECT EASTWARD AND RESULT IN DECREASING INTENSITY. BY MIDDAY SATURDAY IT IS EXPECTED THAT THE REMAINING LAKE EFFECT SNOW WILL HAVE SHIFTED OFF TO SOUTHWEST LOWER MI AND NORTH CENTRAL IN. SATURDAY NIGHT THROUGH SUNDAY NIGHT... A LARGE AREA OF COLD HIGH PRESSURE WILL DOMINATE CENTRAL NORTH AMERICA DURING THIS PERIOD BRINGING THE LOCAL AREA MOSTLY CLEAR...COLD AND DRY CONDITIONS. MONDAY AND MONDAY NIGHT... THE LARGE ANTICYCLONE WILL BE MOVING EAST-SOUTHEAST FROM THE SOUTHEASTERN U.S. OFF THE CAROLINA COAST MONDAY. THIS ALLOWS FOR A RETURN FLOW OF MILDER AIR AS THE FLOW BACKS TO SOUTHWEST AHEAD OF A SHORT WAVE TO BE MOVING EAST TO THE UPPER MS AND MID MO VALLEYS BY MONDAY EVENING. MID AND EVENTUALLY LOW LEVEL MOISTURE INCREASE AS THE SHORT WAVE CONTINUE TO APPROACH. WITH DEEP MOISTURE PROGGED OVER THE LOCAL AREA MONDAY AND DECENT VERTICAL ASCENT UP THROUGH THE MID LEVELS WITH THE WARM AIR ADVECTION OCCURRING HAVE A CHANCE OF SNOW MENTIONED FOR DAYTIME MONDAY. TUESDAY THROUGH THURSDAY THE DRY AIR ADVECTION BEHIND THE MONDAY SYSTEM IS FORECAST TO BE GRADUAL...WITH SOME GUIDANCE INDICATING A SECONDARY UPPER LEVEL WAVE WITHIN THE NORTHERN STREAM MOVING OVER THE REGION TUESDAY. AT LEAST CONTINUING CLOUDS LOOK LIKE A GOOD BET IF THE MONDAY SYSTEM TRACK DOES NOT SHIFT MUCH. FORCING LOOKS MINIMAL FOR PRECIPITATION...WITH ANYTHING OCCURRING FAVORED TO BE LIGHT. MODEL SPREAD FROM WEDNESDAY ONWARD IS LARGE...WITH DISAGREEMENTS ON HOW A SOUTHWEST LOW WILL EVOLVE EAST. THE GFS CONTINUES MUCH FASTER...AS WELL AS FURTHER NORTH...WHILE THE EC AND GFS ENSEMBLES OFFERING A NICE MIDDLE GROUND OF THE TWO AND THIS SPEED HAS BEEN FAVORED. ALL IN ALL THOUGH...A LOW CONFIDENCE FORECAST AT THIS POINT. TRS/MTF && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 02Z... * OVERNIGHT STRATUS * WIND SHIFT WITH FRONTAL PASSAGE EARLY AFTERNOON * ASSOCIATED SNOWFALL / TRENDS SHEA //DISCUSSION...UPDATED 00Z... BANK OF STRATUS WILL CONTINUE TO MOVE ACROSS ALL OF THE TERMINALS OVER THE NEXT FEW HOURS AND REMAIN OVERNIGHT. GENERAL TRENDS WILL BE FOR THE BKN/OVC CIGS TO MOVE IN AT 030 AND REMAIN AT THAT LEVEL THROUGH THE EVENING...SLIGHTLY FALLING TO 025 BY MORNING. THE MAIN FOCUS OF THE FORECAST REMAINS WITH THE STRONG COLD FRONT THAT WILL MOVE THROUGH THE REGION LATE TOMORROW MORNING INTO THE AFTERNOON HOURS...VEERING WINDS TO THE NORTHWEST AND THEN NORTH WITH GUSTS EXPECTED IN THE MID 20 KT RANGE. THE NEXT CONCERN WILL BE SNOWFALL AND THEN LAKE EFFECT SNOW BANDING. FOLLOWING THE FRONTAL PASSAGE...CONDITIONS WILL BECOME FAVORABLE FOR WIDESPREAD SNOW...ESPECIALLY FOR THE EASTERN TERMINALS. SNOW ACCUMULATIONS AND REDUCTIONS TO VISIBILITY ARE EXPECTED. SNOWFALL FOR MOST TERMINALS WILL BEGIN IN THE LATE MORNING TO EARLY AFTERNOON...WITH BETTER CHANCES FOR SNOW INTO THE AFTERNOON TIME FRAME. FOCUS WILL THEN TURN TOWARDS LAKE EFFECT BANDING OF SNOW AS CONDITIONS BECOME VERY FAVORABLE BY THE EVENING HOURS. LARGE AMOUNTS OF UNCERTAINTY ARE STILL PRESENT WITH REGARDS TO THE EXACT AREAS THIS LAKE EFFECT SNOW BAND WILL IMPACT AS BOUNDARY LAYER FLOW IS PROGGED TO BE EITHER JUST EAST OF NORTH...OR NORTH KEEPING THE PRIMARY IMPACTS EAST OF ALL BUT GYY. TYPICALLY THESE BANDS OF SNOW ARE VERY HEAVY...BUT ALSO VERY NARROW...LIMITING THE AREAL COVERAGE OF SNOWFALL. THIS RESULTS IN WIDE VARIATIONS IN SNOW AMOUNTS OVER VERY SMALL GEOGRAPHIC AREAS. THEREFORE IT IS QUITE POSSIBLE THAT SOME OF THE TAF SITES ACROSS THE AREA WILL BE SPARED BY THE HEAVY LAKE EFFECT SNOW. SHEA //ORD AND MDW CONFIDENCE...UPDATED 02Z... * MEDIUM CONFIDENCE IN FROPA TIMING * MEDIUM-HIGH CONFIDENCE ON A PERIOD OF SNOW FRIDAY AFTERNOON * LOW CONFIDENCE ON VIS TRENDS IN THE SNOW * HIGH CONFIDENCE IN WIND FORECAST ELEMENTS SHEA //OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z... * SATURDAY NIGHT/SUNDAY...VFR * MONDAY/TUESDAY...SLIGHT CHANCE SNOW * WEDNESDAY...VFR * THURSDAY...CHANCE OF RAIN OR SNOW SHEA && .MARINE... 240 PM CST AREA OF LOW PRESSURE CONTINUES TO SLIDE EAST ACROSS SOUTHERN ONTARIO...JUST NORTH OF LAKE SUPERIOR. GUIDANCE CONTINUES TO SUGGEST THIS SYSTEM WILL PUSH EAST TOWARDS WESTERN QUEBEC...AS A TRAILING COLD FRONT ARRIVES ACROSS THE UPPER MIDWEST/NORTHWEST GREAT LAKES REGION EARLY FRIDAY. BEHIND THIS BOUNDARY AN ARCTIC AIRMASS IS POISED TO SLIDE SOUTHEAST ACROSS LAKE MICHIGAN FRIDAY AFTERNOON/EVENING. THIS EVENING THE PRESSURE GRADIENT WILL TIGHTEN UP ACROSS NORTHERN LAKE MICHIGAN AND REMAIN TIGHT THROUGH LATE TONIGHT...WHICH SHOULD ALLOW GALES TO 35 KT FOR FAR NORTHERN LAKE MICHIGAN. FOR FRIDAY THE CHALLENGE HAS BEEN HOW STRONG WINDS WILL BE WITH COLD AIR ADVECTION EXPECTED THROUGH SATURDAY. GUIDANCE HAS BACKED OFF MARGINALLY WITH POTENCY OF WINDS...AND IT APPEARS TO FAVOR MORE OF A SUB-GALE CONDITIONS. HOWEVER...COMING UP WITH OCCASIONAL GALES TO 35 KT. AT THIS TIME HAVE WORDED THINGS TO BE OCCASIONAL GALE FORCE GUSTS. THE FREQUENCY MAY NOT BE PRESENT TO WARRANT A GALE WARNING AT THIS TIME...ALTHOUGH IT WILL BE CLOSE. FOR NOW NO HEADLINES AFTER THE CURRENT GALE WARNING FOR NORTHERN LAKE MICHIGAN. IN ADDITION CONDITIONS APPEAR TO BE FAVORABLE FOR FREEZING SPRAY CONCERNS. BASED ON WINDS/TEMPERATURES HAVE DERIVED AT TIMES MODERATE FREEZING SPRAY FOR THE OPEN WATERS THROUGH SATURDAY. THEN WINDS BEGIN TO SLACKEN AS A LARGE DOME OF HIGH PRESSURE SAGS SOUTH INTO THE CENTRAL PLAINS FOR THE SECOND HALF OF THE WEEKEND. ON THE EASTERN END OF THIS RIDGE...WINDS WILL CONTINUE TO BE OCCASIONALLY GUSTY FROM THE NORTHWEST ALTHOUGH SUB-GALES EXPECTED. FOR THE NEARSHORE WATERS...THIS MAY RESULT IN A PROLONGED SETUP FOR SMALL CRAFT CONDITIONS. THUS WILL MAINTAIN THE SMALL CRAFT ADVISORY GIVEN THE LONG FETCH...ESPECIALLY FOR THE WATERS OF LAKE MICHIGAN IN NORTHWEST INDIANA. BEACHLER && .LOT WATCHES/WARNINGS/ADVISORIES... IL...LAKE EFFECT SNOW WATCH...ILZ014-ILZ022...NOON FRIDAY TO 3 PM SATURDAY. IN...LAKE EFFECT SNOW WATCH...INZ001-INZ002...NOON FRIDAY TO 3 PM SATURDAY. LM...GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ563 UNTIL 3 AM FRIDAY. SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 5 PM SATURDAY. SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 2 PM SUNDAY. && $$