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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1147 AM CST SAT FEB 16 2013

DISCUSSION...
324 AM CST

THE MAIN FORECAST CONCERNS DURING THE PERIOD WILL BE THE POTENTIAL
FOR A PERIOD OF SNOW SHOWERS AND COLDER TODAY...AND THEN ATTENTION
WILL FOCUS MAINLY ON THE ACTIVE WEATHER EXPECTED NEXT WEEK.

WATER VAPOR IMAGERY EARLY THIS MORNING DEPICTS A DISTURBED CYCLONIC
FLOW PATTERN ACROSS THE UPPER MIDWEST AND GREAT LAKES REGION...WITH
SEVERAL SMALL SCALE DISTURBANCES DROPPING SOUTHEASTWARD ACROSS THE
UPPER MIDWEST. THIS CONTINUES TO LEAD TO AREAS OF CLOUD COVER AND
LIGHT SNOW AND FLURRIES...ESPECIALLY ACROSS MN EARLY THIS MORNING.

FOR TODAY...I EXPECT CLOUDS TO BE ON THE INCREASE AGAIN THROUGH THE
MORNING HOURS AS THE UPSTREAM CLOUD COVER BUILDS SOUTHWARD ACROSS
THE AREA. LOW LEVEL LAPSE RATES SHOULD STEEPEN THROUGH THE MORNING
HOURS...AND THE ADDED WEAK FORCING FOR ASCENT FROM THE DISTURBED
UPPER LEVEL CYCLONIC FLOW SHOULD LEAD TO MORE SCATTERED SNOW SHOWERS
THIS AFTERNOON. ALTHOUGH NO SIGNIFICANT SNOWFALL IS EXPECTED...THE
STEEP LAPSE RATES IN THE LOWER LEVELS COULD BE ENOUGH TO RESULT IN
SOME ISOLATED SNOW SHOWERS CAPABLE OF PRODUCING SOME MINOR
ACCUMULATIONS AND REDUCED VISIBILITIES. THIS WILL ALSO BE THE
COLDEST DAY...WITH TEMPERATURES STRUGGLING TO REACH 20 FAR NORTH AND
THE MID 20S ACROSS CENTRAL ILLINOIS.

SKIES SHOULD CLEAR THIS EVENING AS A SURFACE RIDGE OF HIGH PRESSURE
MOVES OVERHEAD. THIS WILL LIKELY RESULT IN A VERY COLD NIGHT ACROSS
THE AREA...WITH TEMPERATURES LIKELY TO FALL INTO THE LOW TEENS AND
EVEN THE SINGLE DIGITS...ESPECIALLY IN THE CLIMATOLOGICAL FAVORED
AREAS. THINGS SHOULD BEGIN TO WARM ON SUNDAY AS THE FLOW TURNS
SOUTHEAST OUT AHEAD OF LOW PRESSURE TAKING SHAPE ACROSS THE PLAINS
STATES. HOWEVER...IT APPEARS THE REAL WARMTH WILL NOT ARRIVE UNTIL
MONDAY. THE COMBINATION OF THE UNFAVORABLE SOUTHEASTERLY COMPONENT
TO THE WIND AND THE VERY COLD START WILL MAKE IT HARD FOR
TEMPERATURES TO WARM OUT OF THE LOW TO MID 30S SUNDAY.

SUNDAY NIGHT TEMPERATURES WILL LIKELY NOT DROP MUCH...AND MAY
ACTUALLY END UP RISING THROUGH THE NIGHT AS WARM AIR ADVECTION
INCREASES ACROSS THE AREA. THE SURFACE WARM FRONT SHOULD PUSH ACROSS
THE REGION BY EARLY MONDAY...AND VERY WARM CONDITIONS WILL SET UP IN
THE WAKE OF THIS FRONT. IT CURRENTLY APPEARS THAT MANY AREAS ACROSS
CENTRAL ILLINOIS WILL BE NEAR...IF NOT IN THE LOWER 50S...WITH
SLIGHTLY COOLER CONDITIONS ACROSS NORTHERN ILLINOIS. I RAISED
TEMPERATURES A COUPLE OF DEGREES AS THE PUSH OF WARM AIR LOOKS LIKE
IT WILL SUPPORT TEMPERATURES IN THE MID TO UPPER 40S.

THE OTHER STORY FOR MONDAY WILL BE RAIN CHANCES AND TIMING AS THE
UPPER TROUGH APPROACHES THE AREA. IT CURRENTLY APPEARS THAT THE RAIN
THREAT WILL HOLD OFF UNTIL THE AFTERNOON...WITH A DRY MORNING
EXPECTED. THE SURFACE LOW LOOKS TO TRACK JUST NORTH OF THE AREA AND
PUSH A COLD FRONT THROUGH DURING THE EVENING HOURS. A STRONG PUSH OF
COLD AIR AND RAPID PRESSURE RISES LOOK TO ACCOMPANY THIS FRONT. THIS
COULD RESULT IN SOME STRONG WESTERLY WINDS BEHIND THE FRONT MONDAY
NIGHT INTO TUESDAY. THE COLD AIR MAY MOVE IN QUICK ENOUGH FOR A
TRANSITION TO SOME SNOW BEFORE THE PRECIPITATION ENDS MONDAY NIGHT.
HOWEVER...IT APPEARS THE TRACK OF THE SYSTEM WILL NOT BE CONDUCIVE
TO MUCH IF ANY ACCUMULATION.

TUESDAY LOOKS TO BE A COLD AND WINDY DAY AS THE SURFACE LOW TRACKS
EASTWARD ACROSS THE UPPER GREAT LAKES REGION. TEMPERATURES LOOK TO
BE IN THE LOWER TO MIDDLE 20S...WITH WIND CHILLS LIKELY IN THE SINGLE
DIGITS TO LOW TEENS.

THE MAIN STORY BEYOND TUESDAY CONTINUES TO BE THE POTENTIAL OF A
SIGNIFICANT STORM SYSTEM ACROSS THE CENTRAL PORTION OF THE COUNTRY BY
THE END OF THE WEEK. THE GUIDANCE CONTINUES TO ADVERTISE THIS...WITH
THE LARGE SCALE WEATHER PATTERN ACROSS NORTH AMERICA LOOKING
FAVORABLE FOR A VERY ACTIVE SOUTHERN STREAM JET. HOWEVER...THE
DETAILS OF THE SYSTEM SUCH AS THE TRACK AND STRENGTH...AND THUS ITS
MAIN AFFECT LOCALLY ON THE AREA IS STILL IN QUESTION. THE GUIDANCE
HAS BEGAN CHANGING SOME OF THE DETAILS OF THIS SYSTEM...POSSIBLY
SUGGESTING THAT UPPER LEVEL AHEAD OF THIS SYSTEM MAY ACT AS A
BLOCK...AND KEEP THE SURFACE SYSTEM TO OUR WEST. OVERALL CONFIDENCE
ON THE SPECIFICS OF REMAIN LOW AT THIS TIME.

KJB

&&

AVIATION...

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

* MAINLY VFR WITH A FEW FLURRIES THIS AFTERNOON AND EARLY
  EVENING...THOUGH POTENTIAL FOR SCATTERED -SHSN AND MVFR CIGS.

* NORTHWEST WINDS GUSTING 15-20 KTS THIS AFTERNOON.

* NORTH-NORTHWEST WINDS DIMINISHING TONIGHT...BECOMING VRBL EARLY
  SUNDAY THEN SOUTH-SOUTHEAST SUNDAY AFTERNOON.

RATZER

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

FORECAST AREA REMAINS BENEATH UPPER TROUGH AXIS THIS AFTERNOON...
WITH A WEAK SHORT WAVE MOVING THROUGH THE REGION. COLD AIR ALOFT
WITH THIS FEATURE IS HELPING TO STEEPEN LOW LEVEL LAPSE RATES
ACROSS WISCONSIN AND NORTHERN IL AT MIDDAY...WITH DEVELOPMENT OF
MAINLY VFR STRATOCU DECK AND SOME FLURRIES. WEAK RADAR RETURNS
NORTH OF THE TERMINALS LIKELY CONTAIN SOME EMBEDDED SNOW
SHOWERS...WITH JANESVILLE OBSERVING MVFR CONDITIONS WITH
4SM/BKN019...THOUGH VAST MAJORITY OF OBS SITES REPORTING VFR
CIGS. THUS HAVE MAINTAINED IDEA OF VFR CIGS/VCSH FOR TAFS THIS
AFTN...WITH THE CAVEAT THAT THERE COULD BE SOME BRIEF SPOTTY MVFR
CONDITIONS. COVERAGE/CONFIDENCE IN THESE LOWER CONDITIONS IS QUITE
LOW AT ANY PARTICULAR LOCATION HOWEVER...SO HAVE NOT INCLUDED IN
TAF AT THIS TIME. CLOUDS/SNOW SHOWERS SHOULD QUICKLY DIMINISH THIS
EVENING AS COLD POOL MOVES EAST AND SUBSIDENCE INCREASES BEHIND
DEPARTING TROUGH. VFR CONDITIONS SHOULD PREVAIL THROUGH SUNDAY.

SURFACE HIGH PRESSURE MOVES INTO THE AREA SUNDAY MORNING ALLOWING
DECREASING NORTHWEST WINDS TONIGHT...AND A PERIOD OF
LIGHT/VARIABLE WINDS SUNDAY MORNING AS THE RIDGE MOVES ACROSS THE
AREA. WINDS WILL THEN BECOME SOUTH-SOUTHEAST SUNDAY AFTERNOON IN
RESPONSE TO PRESSURE FALLS ACROSS THE UPPER MIDWEST WITH LOW
MOVING ALONG CANADIAN BORDER.

RATZER


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

* HIGH CONFIDENCE MOST FORECAST ELEMENTS...THOUGH MEDIUM-LOW
  CONFIDENCE IN COVERAGE OF SCATTERED -SHSN/MVFR CIGS THIS AFTN.

RATZER

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

SUNDAY NIGHT...VFR. WX NIL.

MONDAY...STRONG SOUTH WINDS. RAIN DEVELOPING IN THE AFTERNOON.
CONDITIONS SHOULD DETERIORATE TO MVFR THEN IFR DURING LATE
AFTERNOON/EVENING. WIND SHIFT FROM STRONG SOUTH TO STRONG WEST
OVERNIGHT...THEN PCPN SHOULD END AS -SN BY MORNING.

TUESDAY...MVFR CIGS POSSIBLE. STRONG WEST WINDS.

WEDNESDAY...VFR. WX NIL.

THURSDAY...SNOW DEVELOPING IN THE AFTERNOON AND INCREASING
OVERNIGHT. IFR CONDITIONS LIKELY. STRONG EAST WINDS LIKELY
OVERNIGHT.

FRIDAY... LINGERING -SN AND MVFR CIGS. WIND SHIFT TO WLY LATE.

KREIN

&&

MARINE...
315 AM CST

MAIN CONCERNS WITH THE MARINE FORECAST INCLUDE A PERIOD OF GUSTY
NORTHWEST WINDS ACROSS THE SOUTHERN PORTIONS OF THE LAKE TODAY...
THEN A PERIOD OF STRENGTHENING SOUTHEAST TO SOUTH WINDS SUNDAY
NIGHT AND THEN REACHING GALE FORCE BY MONDAY AS THE GRADIENT
ACROSS THE LAKE STRENGTHENS BETWEEN AN APPROACHING TROUGH OF LOW
PRESSURE AND AN ASSOCIATED STRONG COLD FRONT AND STRONG HIGH
PRESSURE OVER THE EASTERN SEABOARD. THE STRONG COLD FRONT WILL
PUSH ACROSS THE LAKE MONDAY EVENING...TURNING THE GALES
NORTHWESTERLY. STRONG COLD ADVECTION BEHIND THE FRONT WILL
MAINTAIN THE GALES THROUGH TUESDAY. THE COLD AIR...STRONG WINDS
AND HIGHER WAVES ALL COMBINE TO BRING A PERIOD OF MODERATE
FREEZING SPRAY TO THE LAKE FOR TUESDAY AND TUESDAY NIGHT. WINDS
WILL FINALLY SUBSIDE BY WEDNESDAY AS A LARGE AREA OF HIGH
PRESSURE BUILDS ACROSS THE NORTHERN PLAINS AND UPPER MISSISSIPPI
VALLEY. WINDS SHOULD THEN WEAKEN CONSIDERABLY WEDNESDAY NIGHT AND
THURSDAY AND THE HIGH PRESSURE SPREADS ACROSS THE LAKE.

KREIN

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 4 AM SUNDAY