FXUS63 KLOT 090446 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1046 PM CST WED FEB 8 2012 .DISCUSSION... 802 PM CST GOING FORECAST IN GOOD SHAPE THIS EVENING. LAST VESTIGES OF CLOUD COVER ARE SKIRTING THE SOUTHERN CWA BORDER BUT WILL EXIT TO THE SOUTH AND EAST OVER THE NEXT HOUR OR TWO. HIGH PRESSURE CENTERED OVER SOUTHWESTERN IOWA AND NORTHERN MISSOURI WILL SLOWLY SETTLE SOUTHWARD KEEPING A MODEST PRESSURE GRADIENT OVER THE LOCAL AREA TONIGHT ALLOWING FOR LIGHT WESTERLY WINDS. THIS WILL SUPPORT A SLOW BUT STEADY COOLING YET THIS EVENING WITH SOMEWHAT OF A QUICKER DECREASE IN TEMPS ACROSS THE WEST OVERNIGHT. FORECAST LOWS LOOK TO BE IN GOOD SHAPE AND HAVE ONLY MADE ADJUSTMENTS TO HOURLY VALUES BASED ON RECENT TRENDS. MDB //PREV DISCUSSION... 311 PM CST IN THE VERY SHORT TERM...THE COMPACT UPPER LOW THAT HAS MOVED FROM FAR SOUTHEAST IA TO NORTH CENTRAL DURING THE MORNING AND EARLY THIS AFTERNOON...WILL CONTINUE DROPPING SOUTHEASTWARD TO CENTRAL IN BY LATE THIS EVENING. THIS...ALONG WITH LOSS OF SURFACE HEATING WITH LOWERING OF THE SUN...WILL RESULT IN THE CONTINUED CLEARING OUT OF CLOUDS NORTHWEST TO SOUTHEAST DURING THE LATER AFTERNOON AND EARLY THIS EVENING.. FOCUS OF FORECAST OVER THE NEXT FEW DAYS TURNS TO THE ARRIVAL OF ARCTIC AIR DURING FRIDAY...AND THE SETUP FOR A LAKE EFFECT SNOW EVENT FOR THE CHICAGO SHORELINE AND NORTHWEST IN FRIDAY AFTERNOON THROUGH SATURDAY MORNING. THE CORE OF COLDEST AIR IS CURRENTLY OVER NORTHERN HUDSON BAY AND BAFFIN ISLAND WITH 850HPA TEMPERATURES AS COLD AS -33C. THE LEADING EDGE OF THIS COLD AIR WILL BE DROPPING SOUTH OVER THE UPPER MIDWEST AND WESTERN GREAT LAKES DURING THURSDAY AND THURSDAY NIGHT...AND IS EXPECTED TO BE OVERSPREADING NORTHERN IL...SOUTHERN LAKE MI...AND NORTHWEST IN BY FRIDAY MORNING AS THE NEAR CIRCULAR POLAR VORTEX CENTERED OVER FAR NORTHERN QUEBEC DEVELOPS A NORTHEAST TO SOUTHWEST AXIS DURING THURSDAY...WHICH EVENTUALLY BECOMES NORTH TO SOUTH-SOUTHWEST FRIDAY AS A JET MAX DROPS SOUTH FROM SOUTH CENTRAL CANADA TO THE DAKOTAS...AND THEN SOUTHEAST OVER THE MID MO AND MID MS VALLEYS FRIDAY. THIS RESULTS IN THE MID AND UPPER FLOW VEERING TO NORTHERLY AND NORTHWESTERLY RESPECTIVELY ALLOWING THE COLD AIR TO SPREAD SOUTHWARD. MOISTURE IS LIMITED AHEAD OF THE COLD FRONT TO DROP ACROSS THE LOCAL FORECAST AREA DURING THE PRE-DAWN AND MORNING HOURS OF FRIDAY...SO ONLY CARRYING A CHANCE OF LIGHT SNOW PRE AND POST FRONTAL PASSAGE ON FRIDAY. BEHIND THE COLD FRONT WILL BE A LARGE AREA OF HIGH PRESSURE OVER SASKATCHEWAN AND MANITOBA BUILDING SOUTH AND SOUTH-SOUTHEAST ACROSS THE NORTHERN PLAINS AND UPPER MIDWEST TO THE CENTRAL PLAINS AND MID MS VALLEY BY LATER FRIDAY. WITH THE CENTER OF THE STRONG ANTICYCLONE STILL FAR TO OUR NORTHWEST THE LOW LEVEL FLOW OVER THE LOCAL AREA BECOMES NORTHERLY AND NORTH-NORTHEASTERLY FOR LATER FRIDAY AND FRIDAY NIGHT BEFORE STARTING TO BACK TO NORTHWESTERLY DURING SATURDAY MORNING AS THE HIGH PRESSURE RIDGE STRETCHES OVER THE LOWER MO AND MID MS VALLEYS BY THEN. WITH MODELS INDICATING 850HPA TEMPERATURES OF -16 TO -18C MOVING OVER THE SOUTHERN PORTION OF LAKE MI FRIDAY EVENING...FORECAST SOUNDINGS OVER THE LAKE SUGGESTING A CLOUD DEPTH OF AROUND 1.5KM...WINDS BELOW AND THROUGH THE CLOUD BEARING LAYER TAKING ON A SLIGHT EASTERLY COMPONENT BY 00Z SAT AND A BIT MORE OVERNIGHT FRIDAY...A PLUME OF LAKE EFFECT SNOW IS LIKELY TO DEVELOP. INITIALLY THIS PLUME IS EXPECTED TO IMPACT THE AREA FROM THE NORTH SIDE OF CHICAGO SOUTH TO FAR EASTERN WILL CO AND E ACROSS LAKE AND NORTHWESTERN PORTER COUNTIES OF NORTHWEST IN DURING FRIDAY AFTERNOON AND THEN SHIFT MARGINALLY WESTWARD DUE TO THE ABOVE MENTIONED ADDITIONAL VEERING OF THE LOW LEVEL FLOW. THIS WOULD BRING THE THREAT OF LAKE EFFECT SNOWS WESTWARD TO THE LAKESHORE OF LAKE AND NORTHERN COOK COUNTIES...AND SOUTHWARD ALONG/NEAR THE STATE LINE DOWN TO THE KANKAKEE RIVER FOR FRIDAY NIGHT. AS THE LOW LEVEL FLOW BEGINS TO BACK SATURDAY MORNING THE TAIL OF THE LAKE EFFECT PLUME IS EXPECTED TO START SHIFTING BACK EASTWARD ACROSS PORTER CO AND TO FAR SOUTHWEST LOWER MI AND NORTH CENTRAL IN BY EARLY SATURDAY AFTERNOON. DUE TO A SOMEWHAT PROLONGED PERIOD OFF LAKE EFFECT...THE CITY OF CHICAGO COULD SEE SNOWFALL AMOUNTS FROM UP TO 1 INCH NORTH SIDE TO 3 TO 4 INCHES FAR SOUTHEAST SIDE...WITH NORTHERN LAKE AND NORTHWEST PORTER COUNTIES POTENTIALLY SEEING 3 TO 4 INCHES. WITH THE INVASION OF THE MUCH COLDER AIR ARRIVING BY DAYBREAK FRIDAY TEMPERATURES WILL BE FALLING ACROSS THE NORTH AND WEST COUNTIES OF THE FORECAST AREA WHILE THE SOUTHEAST COUNTIES WILL HOLD STEADY OR RECOVER A DEGREE OR TWO UNTIL EARLY AFTERNOON AND THEN BEGIN TO DROP. WHILE THE CORE OF THE ARCTIC AIR DROPS OVER THE CENTRAL AND EASTERN GREAT LAKES...THE LOCAL AREA WILL STILL BE IN COLD AIR SATURDAY. A LOWERING SUBSIDENCE INVERSION WITH A BUILDING SURFACE RIDGE TO THE WEST AND RISING HEIGHTS ALOFT WILL KEEP SURFACE TEMPERATURES CONFINED TO THE LOWER AND MID 20S DURING THE DAYTIME SATURDAY. UPPER RIDGING IS TO CONTINUE PUSH EAST FROM THE HIGH PLAINS SATURDAY NIGHT AND THE CENTRAL PLAINS SUNDAY WITH THE LARGE SURFACE HIGH CENTERED SOMEWHERE OVER/NEAR FAR SOUTHERN IL BY 00Z MON. MODERATION OF TEMPERATURES IS TO GET GOING IN EARNEST SUNDAY NIGHT AS THE UPPER RIDGE CONTINUES E TO THE MS VALLEY AND THE SURFACE HIGH MOVE EAST OVER THE OH VALLEY ALLOWING A RETURN FLOW BECOME ESTABLISHED WITH NEAR NORMAL TEMPERATURES RETURNING SUNDAY NIGHT AND MONDAY. TEMPERATURES RISE BACK TO A BIT ABOVE NORMAL FOR TUESDAY AND WELL ABOVE FOR TUESDAY NIGHT AND WEDNESDAY AS STRONG WARM AIR ADVECTION PUSHES ACROSS THE MID MS VALLEY AND THE OH VALLEY. MODELS SHOW MID COLUMN FROM 2 KM ON UP BECOMING SATURATED LEVELS BY MON EVENING. WITH INCREASING DEPTH OF MOISTURE AND THE ISENTROPIC LIFT WITH THE WARM AIR ADVECTION AS WELL AS A SHORT WAVE TROUGH MOVING EAST ACROSS THE MS VALLEY HAVE A CHANCE MENTION FOR SNOW MONDAY NIGHT. ANOTHER SHORT WAVE IS TO APPROACH TUESDAY NIGHT AND CROSS THE MS RIVER WEDNESDAY. SIGNIFICANT DIFFERENCES IN STRENGTH...SURFACE DEVELOPMENT AND THERMAL STRUCTURE IN MODELS OUT THIS FAR SO HAVE LIMITED IT TO A CHANCE OF SNOW/ SNOW AND RAIN. TRS && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 06Z... * NO CONCERNS OVERNIGHT SHEA //DISCUSSION...UPDATED 06Z... HIGH PRESSURE CONTINUES TO SLOWLY MOVE IN AND RELAX THE GRADIENT. FLOW WILL EVENTUALLY VEER AROUND TO 270-280 OVERNIGHT BETWEEN 6-8 KTS. THE FEW PASSING CIRRUS WILL ALSO SHIFT EAST...LEAVING GENERALLY CLEAR SKIES OVERNIGHT AND ON THURSDAY. WEST TO SOUTHWESTERLY SURFACE FLOW ACROSS THE AREA WILL CONTINUE THROUGH THAT TIME...PICKING UP SLIGHTLY TOMORROW AFTERNOON. NO WEATHER CONCERNS ARE EXPECTED IN THE NEXT 24 HOURS. CHANCES FOR LIGHT SNOW SHOWERS WILL INCREASE BEYOND 12Z TOMORROW NIGHT. SHEA //ORD AND MDW CONFIDENCE...UPDATED 06Z... * HIGH CONFIDENCE IN ALL FORECAST ELEMENTS SHEA //OUTLOOK FOR ORD/MDW FOR 12Z FRIDAY-12Z WEDNESDAY...UPDATED 00Z... * FRIDAY/SATURDAY...CHC LAKE EFFECT SNOW WITH MVFR/IFR POSSIBLE * SUNDAY/MONDAY...VFR * TUESDAY...CHC RAIN/SNOW * SHEA && .MARINE... 225 PM CST THE CONCERNS WITH THE MARINE FORECAST MAINLY CENTER ON THE POTENTIAL GALE FORCE WINDS ON FRIDAY WITH THE PASSAGE OF AN ARCTIC FRONT. IN THE MEANTIME...A RIDGE OF HIGH PRESSURE OVER THE SOUTHERN LAKES WILL GRADUALLY SLIDE SOUTH THROUGH TONIGHT AND INTO THURSDAY. WINDS AND WAVES SHOULD STAY FAIRLY STEADY IN THAT TIME. LOW PRESSURE WILL MOVE SOUTHEAST ACROSS ONTARIO ON THURSDAY NIGHT AND SWING A STRONG COLD FRONT SOUTH ACROSS LAKE MICHIGAN ON FRIDAY. IMMEDIATELY AFTER THIS PASSAGE...A PERIOD OF NORTHERLY GALE FORCE GUSTS LOOK LIKELY AT THIS TIME...INCLUDING POTENTIALLY ALONG THE INDIANA NEAR SHORE. SOME SCATTERED FREEZING SPRAY APPEARS POSSIBLE OVER THE LAKE GIVEN THE DEGREE OF COLD AIR FORECAST...PRIMARILY OVER THE NORTHERN PORTION. IN ADDITION...SOME PROLONGED LAKE EFFECT SNOW IS LIKELY ACROSS THE FAR SOUTHERN PART OF THE LAKE. WINDS WILL BE VERY SLOW TO DIMINISH AND WAVES JUST AS STUBBORN TO SUBSIDE THROUGH FRIDAY NIGHT AND SATURDAY. MTF && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...6 AM FRIDAY TO 5 PM SATURDAY. SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...NOON FRIDAY TO 5 PM SATURDAY. && $$