NWS Area Forecast Discussion
FXUS63 KLOT 061605 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 1005 AM CST MON FEB 6 2012 .DISCUSSION... 341 AM CST STRATUS AND FOG ONCE AGAIN THE MAIN FORECAST CHALLENGES. BROAD STRATUS DECK ACROSS THE UPPER MISSISSIPPI VALLEY CONTINUES TO SPREAD EAST AND SHOULD OVERSPREAD THE ENTIRE CWA BY DAYBREAK THIS MORNING. VISIBILITIES UNDER THE STRATUS DECK DROP RAPIDLY BEHIND THE LEADING EDGE OF THE STRATUS TO LESS THAN A HALF MILE OR SO...BUT THEN GRADUALLY IMPROVE GENERALLY INTO THE 1 TO 3 MILE RANGE. OUT AHEAD OF THE STRATUS...PORTIONS OF EAST CENTRAL ILLINOIS AND NORTHWEST INDIANA HAVE ALREADY SEEM AREAS OF DENSE FOG DEVELOP AND EXPECT THIS AREA TO EXPAND UNDER CLEAR SKIES...THUS HAVE HOISTED A DENSE FOG ADVISORY. IF TRENDS HOLD...MAY SEE THESE AREAS IMPROVE SOME AFTER STRATUS OVERSPREADS THE AREA LATER THIS MORNING. SURFACE WINDS THIS MORNING HAVE ALREADY TRANSITIONED TO A WSW FLOW AHEAD OF EXPANSIVE AREA OF HIGH PRESSURE SPANNING THE CENTRAL AND SOUTHERN PLAINS. WEAK WARM ADVECTION ALOFT WILL ACT TO STRENGTHEN LOW LEVEL INVERSION THIS MORNING...TRAPPING ANY MOISTURE UNDERNEATH. MOST GUIDANCE TRIES TO SCATTER/MIX OUT LOW LEVEL MOISTURE THIS MORNING...BUT BELIEVE THAT GUIDANCE IS TOO OPTIMISTIC AND THAT STRATUS WILL HANG AROUND MOST OF THE DAY. CONTINUED COOLER THAN GUIDANCE TEMPERATURES THIS AFTERNOON BASED ON STRATUS LINGERING THROUGH THE DAY. TO THE NORTH...A COLD FRONT DROPPING SOUTH OUT OF CANADA WILL CROSS THE AREA TONIGHT. MOISTURE WITH THIS SYSTEM IS VERY SHALLOW...AND DONT SEE MUCH IN THE WAY OF PRECIP WITH THE FRONT. BEHIND THE FRONT BROAD STRATUS DECK EXPECTED TO PUSH INTO THE CWA. MID LEVEL DRY LAYER PERSISTS...BUT MODELS DO SUGGEST DEEPER MID/UPPER LEVEL SATURATION AS UPPER WAVE PUSHES THROUGH WITH HEIGHT FALLS OVERSPREADING THE AREA. MAY EVEN POTENTIALLY BE BRIEF PERIOD UNDER JET COUPLING TUESDAY AFTERNOON ENHANCING SYNOPTIC SCALE LIFT. SHOULD INGREDIENTS COME TOGETHER...MAY SEE SEEDER/FEEDER SETUP DEVELOP TUESDAY ALLOWING FOR LIGHT SNOW ACROSS THE AREA. OTHERWISE...EXPECT TO SEE DECENT LAKE EFFECT SETUP DEVELOP TUESDAY WITH NORTHEAST BOUNDARY LAYER FLOW STEERING PRECIP INTO NORTHERN ILLINOIS. LONG FETCH WILL BE FAVORABLE...BUT OVERALL THERMODYNAMICS ARE RATHER MARGINAL WITH INVERSION HEIGHTS TOPPING OUT AROUND 5KFT. SO WHILE MINOR ACCUMULATIONS ARE POSSIBLE...NOT LOOKING AT ANYTHING TOO CRAZY AT THIS POINT. COLD AIRMASS BEGINS TO MOVES INTO THE AREA TUESDAY NIGHT BEHIND COLD FRONT WITH H85 TEMPS DROPPING TO NEAR -10C BY WEDNESDAY. MAX TEMPS WEDNESDAY WILL BE MORE SEASONABLE...EVEN SLIGHTLY COOLER THAN NORMAL IN THE LOW 30S...BUT THE BIG COOL DOWN WILL HOLD OFF UNTIL FRIDAY AND SATURDAY AS REINFORCING SHOT OF COLD/ARCTIC AIR MAKES ITS WAY SOUTH. THIS MORNING...ECMWF AND GFS IN DECENT AGREEMENT BRINGING -20C AIR INTO THE AREA...THOUGH RUN-TO-RUN CONSISTENCY HAS NOT BEEN GOOD ON HOW FAR SOUTH TO BRING IT. MODELS BEGIN TO BREAK DOWN WEST COAST BLOCK OVER THE WEEKEND WITH A WARMUP IN STORE FOR EARLY NEXT WEEK...BUT WILL HAVE TO SEE HOW WELL MODELS HANDLE MERIDIONAL PATTERN AND DO SEE SOME POTENTIAL FOR COLD AIR TO LINGER IF THE BLOCKING PATTERN HOLDS. BMD && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 16Z... * IFR CONDITIONS MAY LINGER THROUGH THE PERIOD. * WINDS SHIFT NORTH TONIGHT BEHIND COLD FRONT...BECOMING NORTHEAST TOWARD MORNING AND BECOMING GUSTY NEAR 20 KTS. * POTENTIAL FOR LIGHT SNOW/SNOW SHOWERS TUESDAY MORNING. RATZER/KJB //DISCUSSION...UPDATED 12Z... EXTENSIVE AREA OF LIFR STRATUS/FOG WHICH HAD LINGERED ACROSS MUCH OF IOWA AND MINNESOTA SUNDAY SPREAD EAST ACROSS THE TERMINALS OVERNIGHT AS WEST WINDS PICKED UP AHEAD OF LOW PRESSURE TROUGH OVER THE FAR UPPER MIDWEST AND NORTHERN GREAT LAKES REGION. TREND THROUGH THE NIGHT HAD BEEN FOR A QUICK DROP TO LIFR WITH VISIBILITY BELOW 1-2SM INITIALLY...THEN IMPROVEMENT IN VIS TO 3-6SM AND BORDERLINE LIFR CIGS AROUND 500 FT AFTER A FEW HOURS. THIS TREND IS EXPECTED TO CONTINUE...WITH VISIBILITY ALREADY SHOWING SOME IMPROVEMENT IN THE LAST COUPLE OF HOURS ACROSS CHICAGO TERMINALS. AREAS NORTH/NORTHWEST OF KORD HAVE ENJOYED VISIBILITIES MORE IN THE 4-7SM RANGE. WOULD EXPECT A CONTINUED GRADUAL IMPROVEMENT IN CIG/VIS CONDITIONS THIS MORNING AFTER SUNRISE. MOST MODEL GUIDANCE CONTINUES TO IMPROVE CIGS TO MVFR AND EVENTUALLY SCATTER OUT DURING THE MIDDAY/AFTERNOON HOURS...THOUGH CURRENT IFR/LIFR DECK EXTENDS ALL THE WAY WEST TO THE MISSOURI RIVER AND SUSPECT GUIDANCE IS MUCH TOO OPTIMISTIC IN SIGNIFICANTLY IMPROVING CONDITIONS ABOVE IFR TODAY. EARLIER PIREP OUT OF KDSM INDICATED STRATUS WAS 2000 FT THICK...AND WITH WARM ADVECTION OCCURRING ALOFT WOULD THINK IT WILL BE DIFFICULT TO SCOUR OUT THE LOW CLOUDS. IF CLOUDS DONT LIFT/SCATTER...IFR CONDITIONS WILL LIKELY CONTINUE RIGHT THROUGH TONIGHT. COLD FRONT DOES SLIP SOUTH ACROSS THE AREA LATE TONIGHT WHICH WILL LEAD TO COLD ADVECTION AND EVENTUALLY DEEPENING OF THE MIXED LAYER AND INCREASING NORTHEAST WINDS TUESDAY MORNING...WHICH WOULD ALLOW FOR RISING CIGS AT THAT TIME. COLDER AIR ALSO RESULTS IN DEEPENING MOISTURE ABOVE THE BOUNDARY LAYER TUESDAY MORNING HOWEVER...WHICH MAY RESULT IN SOME LIGHT SNOW OR SNOW SHOWERS ESPECIALLY ACROSS THE CHICAGO TERMINALS DOWNWIND OF LAKE MICHIGAN. RATZER //ORD AND MDW CONFIDENCE...UPDATED 16Z... * HIGH CONFIDENCE IN IFR CIGS HOLDING ON INTO THE AFTERNOON. * MEDIUM CONFIDENCE IN CIG/VIS TRENDS THIS AFTERNOON AND TONIGHT. * HIGH CONFIDENCE IN OVERALL WIND TRENDS. * LOW CONFIDENCE IN TIMING OF LIGHT SNOW POTENTIAL TUESDAY MORNING. RATZER/KJB //OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 12Z... TUESDAY NIGHT...MVFR CIGS WITH CHANCE OF LIGHT SNOW. WEDNESDAY...MVFR CIGS POSSIBLE EARLY...IMPROVING TO VFR. THURSDAY...VFR. FRIDAY...CHANCE OF MVFR CIGS EARLY...THEN VFR. SATURDAY...MAINLY VFR. SUNDAY...VFR. RATZER && .MARINE... 320 AM CST LOW PRESSURE OVER NORTHERN QUEBEC WILL CONTINUE TO MOVE EAST DRAGGING A COLD FRONT DOWN THE LENGTH OF LAKE MICHIGAN OVERNIGHT TONIGHT AS HIGH PRESSURE OVER THE CENTRAL PLAINS BUILDS SOUTHEAST. THE SURGE OF COLD AIR BEHIND THE FRONT WILL GENERATE NORTH WINDS INCREASING TO AROUND 25 KT...NECESSITATING A SMALL CRAFT ADVISORY FOR THE ILLINOIS NEARSHORE WATERS. ANOTHER AREA OF HIGH PRESSURE WILL THEN BUILD OUT OF CANADA THROUGH THE NORTHERN PLAINS ON TUESDAY AND THROUGH THE CENTRAL AND SOUTHERN PLAINS ON WEDNESDAY. A COLD FRONT IS EXPECTED TO SWEEP THROUGH ONTARIO WEDNESDAY NIGHT INTO THURSDAY MORNING...AND THEN SURGE SOUTH THROUGH THE TROUGH THE GREAT LAKES REGION THURSDAY NIGHT. WEST WINDS AHEAD OF THE FRONT ARE EXPECTED TO INCREASE TO 30KT WITH WINDS QUICKLY SHIFTING TO NORTHERLY BEHIND THE FRONT. THERE IS A CHANCE THAT WINDS MAY REACH GALE FORCE BY FRIDAY. KREIN && .LOT WATCHES/WARNINGS/ADVISORIES... IL...DENSE FOG ADVISORY...ILZ023-ILZ032-ILZ033-ILZ039 UNTIL NOON MONDAY. IN...DENSE FOG ADVISORY...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL NOON MONDAY. LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...4 AM TUESDAY TO 11 AM WEDNESDAY. SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...2 PM TUESDAY TO 11 AM WEDNESDAY. && $$ |
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