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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1207 PM CST TUE FEB 12 2013

DISCUSSION...
252 AM CST

QUITE A VARIETY OF WEATHER CHALLENGES ARE EXPECTED THROUGH THE
FORECAST PERIOD INCLUDING A TEMPERATURE ROLLER COASTER AND SEVERAL
OPPORTUNITIES FOR AT LEAST LOWER END PRECIP CHANCES...THOUGH A MORE
AMPLIFIED AND ACTIVE PATTERN IS LOOKING TO SET UP AS WE GET INTO
NEXT WEEK. A LOOK AT WATER VAPOR IMAGERY SHOWS THE UPPER LOW THAT
AFFECTED THE AREA YESTERDAY NOW OVER QUEBEC. A WEAK LONGWAVE TROUGH
AXIS EXTENDS FROM NORTHERN MINNESOTA BACK INTO THE DESERT SOUTHWEST
WITH FAIRLY FLAT RIDGING EXTENDING NORTHEASTWARD FROM THE PACIFIC
NORTHWEST INTO THE CANADIAN PRAIRIES. FURTHER WEST...AN UPPER LOW IS
SPINNING OFF THE WEST COAST OF ALASKA WITH A PROGRESSIVE ZONAL FLOW
TO ITS SOUTH FEEDING INTO BRITISH COLUMBIA. AT THE SURFACE...LOW
PRESSURE IS OVER THE UPPER ST. LAWRENCE VALLEY WITH WEAK RIDGING
FROM WESTERN IOWA INTO WESTERN/CENTRAL ONTARIO.

TODAY THROUGH WEDNESDAY...FAIRLY QUIET WEATHER IS EXPECTED THROUGH
WEDNESDAY. A SUBTLE UPPER TROUGH IS WORKING ACROSS THE AREA EARLY
THIS MORNING AND BRINGING A PUSH OF WARMER MID LEVEL AIR WITH H85
TEMPS COMING UP TO AROUND -4C BY DAYBREAK. THE AFOREMENTIONED TROUGH
AXIS FROM NORTHERN MN TO THE DESERT SOUTHWEST WILL START TO KICK
EASTWARD WITH THE NORTHERN PORTION OF IT CROSSING THE AREA THIS
AFTERNOON. THIS WILL BRING SLIGHT MID LEVEL COOLING BACK INTO THE
AREA. THE CURRENT BLANKET OF CLOUDS ACROSS THE AREA IS EXPECTED TO
SCATTER MOVE OUT OF THE AREA FROM WEST TO EAST INTO MID AFTERNOON
WITH THE FAR SOUTHERN CWA SEEING A LONGER PERIOD OF SUN. THERE MAY
BE SOME BREAKS/HOLES THAT DEVELOP IN THE CLOUD SHIELD BUT WITH IT
MORE OR LESS MAINTAINING CURRENT COVERAGE IT MAY TAKE UNTIL LATE
AFTERNOON TO CLEAR THE EASTERN CWA. EXPECT HIGHS IN THE LOW/MID 30S
NORTH TO AROUND 40 SOUTH BUT THIS COULD BE OPTIMISTIC BY A FEW
DEGREES IF CLOUDS HOLD ON LONG ENOUGH. IT WILL NOT BE NEARLY AS
WINDY AS MONDAY BUT A STEADY WESTERLY BREEZE IS EXPECTED AS HIGH
PRESSURE WILL BE PASSING TO THE SOUTH WITH SOMEWHAT OF A GRADIENT
REMAINING WITH FAIRLY FLAT RIDGING CROSSING THE LOCAL AREA. THE
SOUTHERN STREAM PORTION OF THE UPPER TROUGH AXIS WILL MOVE EASTWARD
OUT OF OKLAHOMA THIS EVENING AND MOVE UP THE OHIO VALLEY WEDNESDAY.
A NORTHERN STREAM TROUGH WILL BE FOCUSED ACROSS NORTHERN ONTARIO
DURING THIS SAME TIME. A WEAK SURFACE TROUGH AXIS WILL MOVE ACROSS
THE AREA WEDNESDAY BUT THIS WILL RESULT IN THE AREA BEING IN MORE OF
A WEAK FLOW AS A BETTER ORGANIZED TROUGH WILL BE CENTERED TO THE
NORTH AND ANOTHER TO THE SOUTH. OVERALL EXPECT CONTINUED QUIET
WEATHER WITH LIGHT WEST TO SOUTHWEST WINDS AND HIGHS A FEW DEGREES
WARMER THANKS TO SLIGHTLY WARMER MID LEVEL TEMPS AND MORE SUNSHINE.

WEDNESDAY NIGHT THROUGH SATURDAY...THINGS BECOME A LITTLE MORE
COMPLEX WEDNESDAY NIGHT AND THURSDAY. A CLIPPER SYSTEM WILL BE
TRACKING SOUTHEASTWARD ACROSS THE NORTHERN PLAINS/UPPER MISSISSIPPI
RIVER VALLEY WEDNESDAY AFTERNOON. WHILE THE MAIN SURFACE LOW WITH
THIS SYSTEM WILL MOVE ACROSS CENTRAL WISCONSIN BEFORE TURNING
EASTWARD ACROSS LAKE MICHIGAN...THE UPPER MID/UPPER TROUGH WILL PUSH
A CORRIDOR OF WARM ADVECTION EASTWARD ACROSS THE CWA WEDNESDAY
EVENING AND EARLY WEDNESDAY NIGHT. THE MAGNITUDE IS NOT VERY STRONG
THIS FAR SOUTH BUT WILL OCCUR IN CONJUNCTION WITH THE PASSAGE OF THE
SOUTHERN LOBE OF A VORT MAX ROUNDING THE BASE OF THE SHORTWAVE
TROUGH AXIS. BASED ON THE TRACK THE BULK OF THE PRECIP WITH THE
SYSTEM WILL BE TO THE NORTH BUT AM THINKING THAT THERE IS AT LEAST A
LOW CHANCE FOR A BAND OF LIGHT SNOW TO CROSS THE NORTHERN CWA DURING
THIS TIME. THIS COULD SIMPLY MANIFEST ITSELF AS FLURRIES BUT A
DUSTING OF SNOW IS NOT OUT OF THE QUESTION...PRIMARILY NEAR THE
WISCONSIN BORDER. WILL ADD A FLURRY MENTION NORTH FOR NOW. THE COLD
FRONT WITH THIS SYSTEM CROSSES THE AREA DURING THE MORNING WITH THE
MAIN PUSH OF UPPER FORCING AND COLD ADVECTION SPREADING ACROSS THE
AREA DURING THE MORNING AND EXITING LATER IN THE AFTERNOON. THIS MAY
GENERATE SOME LIGHT SNOW ACROSS THE NORTHERN THIRD TO HALF OF
THE CWA. TEMPS LOOK TO WARM LATE WEDNESDAY NIGHT AND PEAK THURSDAY
MORNING BEFORE THEY LIKELY FALL THROUGH THE REST OF THE
DAY...ESPECIALLY NORTH. ANOTHER TROUGH AXIS WILL BE SWINGING
SOUTHEASTWARD INTO THE AREA FRIDAY. THIS WILL BRING A SECONDARY PUSH
OF COLD AIR BRINGING H85 TEMPS BACK DOWN TO AROUND -13C BY FRIDAY
EVENING OR SO AND NEAR -17C BY SATURDAY MORNING. THIS ALSO LOOKS TO
PROVIDE ANOTHER POTENTIAL PERIOD OF LIGHT SNOW. LITTLE IF ANY
ACCUMULATION IS EXPECTED SO WILL KEEP POPS PRETTY LOW FOR NOW. BY
SATURDAY THE TROUGH AXIS OF A NEWLY FORMED LONGWAVE TROUGH...THANKS
TO THE PASSAGE OF THESE CLIPPERS...WILL BE SWINGING ACROSS THE AREA
KEEPING THE CORE OF THE COLDEST AIR IN PLACE. SOME LINGERING
FLURRIES ARE STILL POSSIBLE. LAKE EFFECT SNOW WILL OCCUR AS THE
COLDER AIR ARRIVES WITH THE PRIMARY FOCUS AREA BEING TO THE EAST AND
NORTHEAST OF THE CWA BUT THERE ARE INDICATIONS OF A MORE NORTHERLY
WIND FRIDAY INTO SATURDAY AS THE SYNOPTIC FORCING REMAINS IN PLACE
SO SOME LAKE ENHANCEMENT IS POSSIBLE INTO NORTHWEST INDIANA BUT
CONFIDENCE IN THIS IS LOW AT THIS TIME.

SUNDAY THROUGH EARLY NEXT WEEK...THE NEWLY DEVELOPED LONGWAVE WILL
STEADILY SHIFT EASTWARD INTO THE LOWER GREAT LAKES SATURDAY NIGHT
AND SUNDAY MORNING WHILE HIGH AMPLITUDE UPPER RIDGING MOVES INTO THE
MIDDLE OF THE COUNTRY. THIS WILL SEND WARMER MID LEVEL AIR IN FROM
THE WEST ON SUNDAY WITH A BETTER PUSH SUNDAY NIGHT. THIS SHOULD
RESULT IN WARMER HIGHS SUNDAY WITH DRY CONDITIONS. AT THE SAME
TIME...A TROUGH WILL BE DIGGING INTO THE FOUR CORNERS WITH
INDICATIONS THAT IT MAY BE A BIT SPLIT BETWEEN A MORE FOCUSED
NORTHERN STREAM ACROSS THE U.S./CANADIAN BORDER AND THE SOUTHERN
STREAM OVER THE FOUR CORNERS. REGARDLESS...BY MONDAY H85 TEMPS LOOK
TO BE ABOVE 0C WITH A SOUTHERLY LOW LEVEL FLOW LEADING TO A MUCH
MILDER DAY. THINGS BECOME MORE UNCERTAIN IN TERMS OF DETAILS BY LATE
MONDAY AS THE TROUGH TO THE WEST SHIFTS EASTWARD. THIS HAS BEEN
SHOWN FOR THE PAST FEW NIGHTS AND THE RESULT IS LOW PRESSURE MOVING
NORTHEASTWARD ACROSS THE LOCAL AREA MONDAY NIGHT AND TUESDAY. THE
CONCERN IS THE HANDLING OF THE UPPER TROUGH AND DETERMINING THE
DEGREE OF PHASING BETWEEN THE NORTHERN AND SOUTHERN STREAMS. GIVEN
THE TIME RANGE WOULD EXPECT TO SEE SOME VARIABILITY IN SOLUTIONS DUE
TO THIS WITH UPCOMING RUNS. HOWEVER...THERE HAS BEEN SOME
CONSISTENCY THUS FAR WHICH SUPPORT DECENT POPS IN THIS TIME FRAME.

MDB

&&

AVIATION...

AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* MVFR CIGS CONTINUING TIL MID TO LATE AFTERNOON.

TRS

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

LARGE AREA OF STRATUS/STRATOCU EXTENDS UPSTREAM BACK TO FAR
EASTERN IA. MODEL PROGS GENERALLY SHOW A WNW TO W FLOW TODAY AT
THE CLOUD BEARING LEVEL SO SHOULD CONTINUE TO SEE THE MVFR CIGS
ACROSS THE LOCAL AREA INTO THE LATER AFTERNOON.

LATEST EXTRAPOLATION OF BACK EDGE OF CLOUD DECK DOES NOT BRING IT
TO OVER THE CHI AREA TIL EARLY EVENING. IN ADDITION...ON THE SW-W
EDGE OF THE SOLID DECK DIURNAL SCT TO BKN STRATOCU FORMED WHICH
WAS DELAYING THE W TO E CLEARING.

HOWEVER...A MID LEVEL SHORT WAVE TROUGH FROM WI SSW TO FAR W
CENTRAL IL IS PROGGED TO HAVE MOVED ACROSS THE LOCAL AREA LATER
AFTERNOON AN ITS WAY ACROSS LAKE MI AND NW IN BY 00Z. LATEST
MODEL RUNS INDICATE THAT THE TRAILING EDGE OF THE HIGHER LOW
LEVEL RH VALUES WILL MOVE OVER ORD AND MDW 22-22Z.

THE CLOUD DECK IS < = 1K FT THICK PER PIREPS AND AMDAR TEMP AND
RH PROFILES. AIRCRAFT TEMPERATURE MEASUREMENTS DO INDICATE A
STRONG INVERSION BASED BETWEEN 025 AND 030 BUT MODEL FORECAST
SOUNDINGS SHOW A SIGNIFICANT DECREASE IS STRENGTH AS THE MID
LEVEL SHORT WAVE TROF MOVES ACROSS THE LOCAL AREA DURING MID TO
LATER AFTERNOON.

THEREFORE...EXPECT MVFR CIGS TO PERSIST TIL MID OR LATER
AFTERNOON BEFORE SCATTERING OUT WITH MID LEVEL TROUGH PASSAGE.
N EDGE OF CLOUD SHIELD ASSOCIATED WITH THE SYSTEM MOVING FROM THE
SOUTHERN PLAINS TO THE LOWER OH VALLEY DURING THE NIGHT IS
EXPECTED TO REMAIN ON THE SOUTHERN HORIZON AND THEN SOME MID
CLOUDS AND INCREASING HI CLOUDS AS A MINOR SHORT WAVE DROPS
ACROSS THE UPPER MS VALLEY WED MORNING.

WINDS THIS AFTERNOON GENERALLY WESTERLY AOB 10KT THEN LIGHT SW
AFTER SUNDOWN.

TRS

PREVIOUS DISCUSSION...UPDATED 12Z...

BACK EDGE OF MVFR CLOUDS CONTINUES TO MAKE SLOW BUT STEADY
PROGRESS EAST ACROSS IOWA THIS MORNING. CIGS ALSO CONTINUE TO
SLOWLY LOWER TO 1600-2000FT. WITH THE INVERSION ALOFT CONTINUING
TO STRENGTHEN AND LOWER...NOT EXPECTING MUCH CHANGE THROUGH THE
MORNING HRS AND EXPECT MVFR CIGS TO CONTINUE INTO THIS AFTERNOON.
EXACT TIMING OF THIS BACK EDGE OR ANY POTENTIAL SCATTERING WILL
LIKELY NEED TO BE TWEAKED BUT HAVE TRENDED MORE PESSIMISTIC WITH
THIS FORECAST. ONCE LOW CLOUDS DO FINALLY CLEAR...JUST CIRRUS
EXPECTED INTO WEDNESDAY MORNING. WESTERLY WINDS HAVE REMAINED A
BIT GUSTY AT TIMES AND GUSTS INTO THE MID/UPPER TEENS WILL STILL
BE POSSIBLE THIS MORNING WITH PREVAILING SPEEDS 10-14KTS FOR MUCH
OF THE DAY. WINDS WILL DIMINISH WITH SUNSET AND TURN MORE
SOUTHERLY THIS EVENING. CMS

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* HIGH CONFIDENCE IN MVFR CIGS PERSISTING INTO EARLY AFTERNOON
  THEN LOSS OF CIGS DURING LATER AFTERNOON.

* MEDIUM CONFIDENCE IN TIMING OF MVFR CIGS BECOMING VFR WITHIN
  +/- 2HRS OF FCST TIMING.

* HIGH CONFIDENCE IS WIND FORECASTS.


TRS

//OUTLOOK FOR ORD/MDW FOR 00Z THURSDAY-12Z TUESDAY...UPDATED 12Z...

* WEDNESDAY NIGHT...DRY/VFR.
* THURSDAY...GUSTY WESTERLY WINDS. SLIGHT CHANCE OF SNOW. MVFR
  POSSIBLE.
* FRIDAY...CHANCE OF SNOW SHOWERS. MVFR POSSIBLE.
* SATURDAY...SLIGHT CHANCE OF SNOW SHOWERS.
* SUNDAY...DRY/VFR.
* MONDAY...CHANCE OF RAIN/SNOW. MVFR POSSIBLE.

&&

MARINE...

240 AM...THE GRADIENT CONTINUES TO SLOWLY RELAX AS LOW PRESSURE
NOW OVER SOUTHERN QUEBEC WEAKENS AND MOVES EAST. MODEST WESTERLY
FLOW...15-25KTS...WILL CONTINUE INTO WEDNESDAY AS A RIDGE OF HIGH
PRESSURE ACROSS THE CENTRAL PLAINS SHIFTS EAST ACROSS THE OHIO
VALLEY TODAY AND MID ATLANTIC TONIGHT. AS NEW LOW PRESSURE MOVES
SOUTHEAST ACROSS CENTRAL WI WEDNESDAY THEN PIVOTS EAST ACROSS THE
NORTHERN LAKES THURSDAY...WINDS WILL TURN MORE SOUTHWESTERLY. THE
EXACT TRACK OF THIS LOW IS STILL A BIT UNCERTAIN BUT APPEARS IT
WILL TRACK ACROSS NORTHERN LAKE MICHIGAN DURING THE LATE MORNING
OF THURSDAY. WINDS WILL TURN WEST THEN NORTHWEST AS THE LOW PASSES
AND EXPECT SPEEDS TO INCREASE TO 30KTS ON THE SOUTH END OF THE
LAKE. THESE STRONG WINDS WILL THEN WORK THEIR WAY NORTH AS THE LOW
DEPARTS. SHOULD THE LOW PASS FURTHER NORTH...THEN STRONGER WINDS
CAN BE EXPECTED SOONER ACROSS NORTHERN PARTS OF THE LAKE. A TIGHT
GRADIENT COMBINED WITH COLDER AIR SPREADING ACROSS THE REGION
SHOULD ALLOW FOR 30 KTS TO CONTINUE INTO FRIDAY MORNING. COULD BE
SOME GUSTS TO GALE FORCE AND WILL NEED TO MONITOR WITH LATER
FORECASTS. CMS

&&

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

$$