🚀 go-pugleaf

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1230 PM CST MON DEC 17 2012

DISCUSSION...
1215 PM CST

SOME SMALL CHANGES MADE TO THE AFTERNOON FORECAST...MAINLY TO
INHIBIT TEMPS A FEW DEGREES AND TO INCLUDE THE MENTION OF SNOW IN
THE SOUTHEAST FORECAST AREA. WELL-DEFINED UPPER WAVE IS MOVING ENE
OVER THE LOWER OHIO VALLEY EARLY THIS AFTERNOON. A FAIRLY
IMPRESSIVE UPPER LEVEL TEMPERATURE GRADIENT EXISTS ON THE NORTH
AND NORTHWEST SIDE OF THIS SYSTEM...AND WITH THE SYSTEM ITSELF
STILL INTENSIFYING...THIS AREA IS FAVORED FOR FRONTOGENESIS AND
POTENTIALLY ENHANCED PRECIP ENOUGH TO OVERCOME A DRY PART OF THE
COLUMN. SURE ENOUGH...RADAR ECHOES HAVE BEEN INCREASING ACROSS
CENTRAL IL AND THE SOUTHEAST FORECAST AREA. WITH MARGINAL WET BULB
TEMPERATURES...A POTENTIAL FOR WET BIG-FLAKED SNOW EXISTS. THIS
HAS ALREADY BEEN OBSERVED IN LINCOLN /AAA/. BOTH THE RAP AND NAM
INDICATE OMEGA WITHIN THE DENDRITIC GROWTH AREA LIKELY AIDING IN
LARGER FLAKES THAT ARE LESS SUSCEPTIBLE TO MELT/SUBLIMATE ON THEIR
WAY DOWNWARD. THESE MODELS INDICATE THIS FORCING NEAR -15C TO
CONTINUE THROUGH MID-AFTERNOON...WHICH MAKES SENSE ON THE SYNOPTIC
SCALE WITH THE PATH AND EVOLUTION OF THE SYSTEM. A DUSTING COULD
HAPPEN BUT OVERALL NO REAL ACCUMULATION EXPECTED.

EARLY LOOK AT 12Z GUIDANCE FOR THE UPCOMING SNOW EVENT INDICATES
NO DRAMATIC SHIFTS...HOWEVER PLENTY OF INTERESTING FEATURES ARE
EVIDENT. THE NAM/GFS/SREF CONTINUE TO FAVOR A MORE NORTHWARD TRACK
OF THE PANHANDLE HOOK...WITH THE EC AND GEM SOUTH. THIS RESULTS
IN DRASTIC DIFFERENCES IN THERMAL PROFILES AND SNOWFALL DURATION
AND THUS ACCUMULATION ACROSS A LARGE PART OF THE AREA. MOST OF THE
GUIDANCE AGREES ON IMPRESSIVELY STRONG WINDS CORRELATING WITH THE
COLUMN COOLING ON THU AFTERNOON AND EVE. THE GENERAL IDEA REMAINS
STRONG TO HAVE SOME RAIN INDICATED WEDNESDAY NIGHT INTO THURSDAY
MORNING IN THE FORECAST AND THEN TRENDING MORE TOWARD SNOW AND
BLOWING SNOW.

MTF/RC

&&

PREV DISCUSSION...
337 AM CST

FORECAST CONCERNS FOCUS MAINLY ON A COUPLE OF CHANCES FOR SOME LIGHT
PRECIPITATION ACROSS PORTIONS OF THE CWA LATER TODAY/THIS EVENING
AND AGAIN TUESDAY...WITH ATTENTION THEN QUICKLY TURNING TO THE
POTENTIAL FOR A STRONG WINTER STORM FOR THE UPPER MIDWEST WEDNESDAY
NIGHT THROUGH THURSDAY.

SURFACE LOW PRESSURE ACROSS THE NORTHERN GREAT LAKES REGION
CONTINUES TO DRIFT SLOWLY AWAY TO THE NORTHEAST EARLY THIS MORNING.
COLD FRONT WHICH PUSHED ACROSS THE AREA SUNDAY HAS EDGED SOUTHEAST
TO THE OHIO RIVER VALLEY...WHILE WEAK HIGH PRESSURE HAS SPREAD INTO
ILLINOIS FROM THE WEST. LOOKING ALOFT...BROAD TROUGHING LINGERS
ACROSS THE CENTRAL CONUS TO THE SOUTH OF A CLOSED UPPER LOW OVER
WESTERN ONTARIO. SLOW PROGRESSION OF FEATURES AND PRESENCE OF
SEVERAL SMALLER AMPLITUDE MID-LEVEL SHORT WAVES HAVE PREVENTED
STRONG DRYING/SUBSIDENCE AND CLEARING OF LOW CLOUDS ACROSS THE
REGION. GOES WATER VAPOR IMAGERY DEPICTS A COUPLE OF SUCH MID-LEVEL
SHORT WAVES UPSTREAM OF THE REGION TODAY...ONE LIFTING OUT OF THE
RED RIVER VALLEY OF THE SOUTH TOWARD THE LOWER OHIO VALLEY...AND
ANOTHER ACROSS THE CENTRAL PLAINS OF NEBRASKA/KANSAS. NEAR-TERM
GUIDANCE ARE IN AGREEMENT IN BRINGING THESE FEATURES EAST ACROSS THE
MID/UPPER MISSISSIPPI AND OHIO VALLEYS THROUGH THIS EVENING. MAIN
FOCUS FOR MEASURABLE PRECIPITATION SHOULD REMAIN LARGELY WITH WEAK
SURFACE LOW REFLECTION WHICH PROPAGATES ALONG THE AFOREMENTIONED LOW
LEVEL BAROCLINIC ZONE ALONG THE OHIO RIVER...THOUGH VARIOUS GUIDANCE
INCLUDING THE GFS/ECMWF/SREF AND GEM ALL INDICATE A LITTLE LIGHT QPF
STREAK FROM SOUTHEAST KS ACROSS CENTRAL MO AND INTO CENTRAL IL WHICH
INCLUDES OUR FAR SOUTHERN COUNTIES IN ASSOCIATION WITH THE CENTRAL
PLAINS VORT AND POCKET OF MID-LEVEL MOISTURE. ALREADY SEEING SOME
LIGHT RADAR RETURNS ACROSS SOUTHERN KS AT THIS HOUR...WHICH GUIDANCE
SUGGESTS WOULD AFFECT PARTS OF THE WFO LOT CWA ROUGHLY SOUTH OF A
PONTIAC-VALPARAISO LINE THIS AFTERNOON AND EARLY EVENING. LOW LEVEL
THERMAL/MOISTURE PROFILES SUGGEST THIS WOULD MAINLY BE LIGHT
RAIN/SPRINKLES...WITH PRECIP EXPECTED TO END THIS EVENING BEFORE LOW
LEVEL THICKNESS DECREASES ENOUGH FOR MIXED PRECIP.

STRONG UPPER JET STREAK NOTED IN 00Z RAOB ANALYSIS CURRENTLY PUSHING
ACROSS THE OREGON COAST IS PROJECTED TO SPREAD ACROSS THE CENTRAL
CONUS TONIGHT...INDUCING LOW PRESSURE DEVELOPMENT ACROSS THE
NORTHERN PLAINS/UPPER MIDWEST. GUIDANCE DEPICTS WEAK SURFACE LOW
PRESSURE MOVING ALONG THE IL/WI BORDER AREA TUESDAY AFTERNOON...
WITH REGION OF WARM ADVECTION AND RESULTANT ASCENT FOCUSED ACROSS
SOUTHERN WI/FAR NORTHERN IL. ISENTROPIC PROGS IN 290-295 K LAYER
INDICATE A PERIOD OF SATURATION AND FAIRLY GOOD CONVERGENCE ACROSS
THE NORTHERN PARTS OF THE CWA TUESDAY MORNING...WITH SEVERAL
GUIDANCE SETS PRODUCING A LITTLE LIGHT QPF. WHILE NOT EXPECTED TO
AMOUNT TO MUCH...THERMAL PROFILES EARLY TUESDAY DO INDICATE THE
POTENTIAL FOR A LITTLE LIGHT SNOW TO MIX/CHANGE TO LIGHT RAIN DURING
THE MORNING HOURS...BEFORE LARGELY SHIFTING NORTHEAST ACROSS WI/LAKE
MICHIGAN. HAVE ONLY CARRIED SLIGHT CHANCE POPS FOR LIGHT PRECIP OVER
NORTHERN IL...WITH A LITTLE BETTER CHANCE RIGHT ALONG THE WISCONSIN
BORDER.

MEDIUM RANGE GUIDANCE REMAINS REMARKABLY SIMILAR IN AMPLIFYING UPPER
FLOW ACROSS THE CONUS TUESDAY NIGHT INTO WEDNESDAY...AS SYSTEM
CURRENTLY SEEN NEAR 150W 40N IN NORTHERN HEMISPHERIC WATER VAPOR
IMAGERY AMPLIFIES ALONG THE WEST COAST AND INDUCES BROAD DOWNSTREAM
RIDGE ACROSS OUR REGION. GUIDANCE MAINTAINS TREND OF UPPER TROUGH
DIGGING ACROSS THE FOUR CORNERS REGION BY 12Z WEDNESDAY...WITH
SURFACE CYCLOGENESIS OCCURRING OVER THE TX/OK PANHANDLE REGION.
UPPER TROUGH THEN ADVANCES ACROSS THE PLAINS...BECOMING NEGATIVELY
TILTED ACROSS THE MIDWEST AND EVENTUALLY CLOSING OFF THE MID-LEVEL
CIRCULATION OVER THE WESTERN GREAT LAKES ON THURSDAY. THE ASSOCIATED
SURFACE LOW MEANWHILE...DEEPENS AS IT TREKS NORTHEAST ACROSS
MISSOURI AND ILLINOIS LATE WEDNESDAY NIGHT/THURSDAY MORNING AND INTO
LOWER MICHIGAN BY THURSDAY EVENING. WHILE MODEL SOLUTIONS HAVE BEEN
FAIRLY STEADY IN THEIR DEPICTION OF THE OVERALL SCENARIO FOR THE
PAST FEW DAYS OF MODEL RUNS...SOME DETAIL DIFFERENCES DO REMAIN WITH
RESPECT TO THE TRACK OF THE SURFACE LOW. 00Z GEFS ENSEMBLE
CLUSTERING HAS TIGHTENED UP FROM ITS 12Z RUN YESTERDAY...WITH A
MEDIAN SOLUTION TRACK JUST SOUTHEAST OF CHICAGO. THE NAM/SREF ARE
THE FARTHEST WEST...ACTUALLY TAKING THE LOW ACROSS FAR NORTHWEST IL.
THE 00Z GEM FOLLOWS A SIMILAR TRACK...THOUGH IS SLOWER BY A GOOD 6+
HOURS. THE 00Z ECMWF IS SIMILAR IN SPEED/TIMING TO THE GEM...THOUGH
TRACKS FARTHER TO THE RIGHT ALONG A DANVILLE-GOSHEN LINE...OR ABOUT
50 MILES WEST OF THE 12Z ECMWF TRACK. OFTEN...DEEPENING NEGATIVELY
TILTED STORM SYSTEMS EMERGING FROM THE SOUTHWEST TEND TO BE
DEEPER...SLOWER AND TRACK FARTHER TO THE WEST/NORTHWEST THAN MODELS
INITIALLY FORECAST...SO PERHAPS IT IS NOT TOO SURPRISING TO SEE THE
ECMWF TREND BACK NORTHWEST JUST A BIT. AT THIS POINT...HAVE BASED
FORECAST ALONG THE LINES OF A 00Z GEM/00Z ECMWF BLEND ALLOWING FOR
THE SLOWER TIMING OF THESE TWO SOLUTIONS...WITH A LOW TRACK THAT AT
LEAST INITIALLY WRAPS WARMER AIR AND RAIN INTO THE EASTERN HALF OF
THE CWA. THUS HAVE MAINTAINED A PROGRESSION OF RAIN/SNOW
NORTHWEST...RAIN EAST...EVENTUALLY CHANGING TO ALL SNOW AS THE LOW
PASSES THURSDAY AND COLD AIR WRAPS INTO THE DEPARTING LOW. WILL
CONTINUE TO HIGHLIGHT THE POTENTIAL FOR A SIGNIFICANT STORM ACROSS
THE REGION IN THE HWO...THOUGH STILL A BIT EARLY TO NAIL DOWN WHERE
SPECIFIC SNOW AMOUNTS WOULD FALL EXCEPT THE CONCEPT OF MORE ACROSS
THE NORTHWEST PART OF THE CWA AND SIGNIFICANTLY LESS IN THE
SOUTHEAST. IN ADDITION TO THE POTENTIAL FOR SNOW...VERY WINDY
CONDITIONS ARE ANTICIPATED BY THURSDAY NIGHT...AS THE DEEPENING LOW
MOVES INTO LOWER MICHIGAN...WITH NORTHWEST WINDS POTENTIALLY GUSTING
INTO THE 45 MPH RANGE.

A BRIEF PERIOD OF COLDER BUT QUIET WEATHER MOVES INTO THE AREA
FRIDAY THROUGH THE WEEKEND...WITH SOME LINGERING SNOW SHOWERS
DOWNWIND OF LAKE MICHIGAN INTO INDIANA/MICHIGAN.

RATZER

&&

AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* LENGTH OF TIME MVFR AND LOCALIZED IFR CEILINGS WILL HOLD ON
  INTO THE EVENING/TONIGHT AND IF WILL REMAIN AOA 015 THRESHOLD
  WHICH HAS OPERATIONAL IMPACTS ON ORD AND MDW TRAFFIC.

TRS

//DISCUSSION...UPDATED 18Z...

WIDESPREAD MVFR STRATUS ACROSS THE REGION WITH CEILINGS DOWN TO
IFR WELL OFF TO THE W THROUGH NW ACROSS IA...MN AND SE WI. VERY
WEEK PRESSURE GRADIENT ACROSS THE REGION SO LOW LEVEL FLOW IS IF
NOT NIL THEN VERY LIGHT. AS THE AFTERNOON PROGRESSES AND THROUGH
THE EVENING A WEAK RIDGE OF HIGH PRESSURE WILL BUILD SE FROM MN
AND WI TO OVER THE LOCAL AREA. WINDS SHOULD GENERALLY FAVOR A NW
TO N DIRECTION BUT REMAIN IN THE 3 TO 6 KT RANGE AND THEN BECOME
LIGHT AND VARIABLE OR CALM AGAIN AS THE RIDGE AXIS SETTLES OVER
THE LOCAL TERMINALS TONIGHT.

BY 12Z TUE LOW PRESSURE IS TO HAVE MOVED E FROM THE CENTRAL
PLAINS TO AROUND THE WESTERN IA AREA. IN RESPONSE WINDS BY
DAYBREAK SHOULD HAVE TURNED TO OUT SSE AS THE LOW CONTINUES TO
APPROACH THE AREA FROM THE W.

THERE IS A SLIGHT CHANCE OF SOME VERY LIGHT PRECIPITATION DURING
THU ACROSS NORTHERN IL THOUGH MODELS INDICATING SIGNIFICANT
SATURATION PRESSURE DEFICITS PERSIST WHILE BEST ISENTROPIC
UPGLIDE/OMEGA MOVE OVER THE AREA DURING THE MORNING LEAVING
BETTER CHANCE OF LIGHT SNOWFALL ACROSS SOUTHERN MN AND CENTRAL
WI.

MODEL THERMAL PROFILES AND MOS GUIDANCE ALL SUGGESTING THAT THE
LOW LEVEL INVERSION WILL WEAKEN AND THE STRATUS CLOUDS WILL
DISSIPATE DURING THE NIGHT WHILE MID LEVEL CLOUDS SPREAD OVER THE
AREA AHEAD OF A SHORT WAVE CURRENTLY OVER ID MOVES E TO THE
PLAINS OVERNIGHT.

TRS


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN NIL/LIGHT AND VARIABLE WINDS INTO THE NIGHT.

* HIGH CONFIDENCE CEILINGS WILL REMAIN AOA 015 THRESHOLD REST OF
  DAY.

* MODERATE CONFIDENCE STRATUS DECK WILL DISSIPATE BUT CONFIDENCE
  IN TIMING DURING THE OVERNIGHT IS LOW.

* HIGH CONFIDENCE THAT NO SIGNIFICANT PRECIPITATION WILL FALL OVER
  THE LOCAL AREA.

TRS

//OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 12Z...

* TUESDAY NIGHT AND WEDNESDAY...VFR.
* WEDNESDAY NIGHT...RAIN DEVELOPING. IFR POSSIBLE.
* THURSDAY...RAIN CHANGING TO SNOW. IFR/LIFR LIKELY. BECOMING
  VERY WINDY WITH GUSTS 35+ KTS BY EVENING.
* THURSDAY NIGHT...VERY WINDY WITH GUSTS 35+ KT. SNOW ENDING.
  BLOWING SNOW. LIFR IMPROVING TO MVFR.
* FRIDAY...GUSTY NORTHWEST WINDS. VFR.
* SATURDAY AND SUNDAY...VFR.

&&

MARINE...

312 AM...NORTHWEST STORM FORCE WINDS TO 50 KT NOW EXPECTED BY
LATE THURSDAY AFTERNOON AND CONTINUING INTO THURSDAY NIGHT.

MAIN FORECAST CONCERN/CHALLENGE REMAINS THE POWERFUL WINTER STORM
EXPECTED TO MOVE ACROSS THE GREAT LAKES REGION THURSDAY/THURSDAY
NIGHT. THERE REMAINS UNCERTAINTY REGARDING THE EXACT TRACK OF THE
LOW FROM THE VARIOUS FORECAST MODELS THOUGH THERE IS SOME
CONSENSUS THAT THIS STORM WILL TRACK VERY CLOSE TO OR PERHAPS OVER
SOUTHERN LAKE MICHIGAN. THE STORM WILL BE IN THE PROCESS OF
RAPIDLY DEEPENING DURING THIS TIME. IF THIS STORM DOES INDEED
FOLLOW THIS TRACK...WINDS WILL RAPIDLY INCREASE THURSDAY
AFTERNOON AND CONTINUE INTO THURSDAY NIGHT WITH FORECAST SOUNDINGS
SHOWING GUSTS INTO THE 45-50KT RANGE WHICH COULD BE TOO LOW THOUGH
STRONGEST WINDS PROBABLY WOULD ONLY LAST 4-8 HOURS THOUGH GALES
SHOULD PERSIST WELL INTO FRIDAY AS THEY SLOWLY RELAX INTO FRIDAY
EVENING.

IN THE SHORTER TERM...LOW PRESSURE OVER THE NORTHERN LAKES WILL
LIFT NORTH TODAY AND WEAKEN...THIS WILL MAINTAIN A MODEST WESTERLY
GRADIENT OVER THE NORTHERN HALF OF THE LAKE THIS MORNING. WINDS
WILL TURN NORTHERLY OVER THE SOUTH END OF THE LAKE AHEAD OF A WEAK
RIDGE OF HIGH PRESSURE WHICH WILL MOVE ACROSS THE WESTERN LAKES
OVERNIGHT. ANOTHER LOW WILL BE MOVING NORTHEAST ACROSS THE OHIO
VALLEY THIS AFTERNOON AND DEEPENING AS IT MOVES TOWARD NEW ENGLAND
ON TUESDAY. SO HOW STRONG THE NORTHERLY GRADIENT BECOMES ON THE
SOUTH END OF THE LAKE LATER TODAY WILL NEED TO BE MONITORED.

ANOTHER FAST MOVING LOW WILL MOVE ACROSS THE IL/WI REGION ON
TUESDAY AND THEN ACROSS SOUTHERN LAKE MICHIGAN TUESDAY EVENING.
THE GRADIENT MAY INCREASE ENOUGH TO SUPPORT 15-25 KT ON THE FAR
SOUTH END OF THE LAKE AHEAD OF THE LOW AND AGAIN FROM THE WEST
TUESDAY EVENING AS THE LOW PASSES. CMS

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$