Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010838
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
238 AM CST SUN MAR 1 2015

.UPDATE...

911 PM CST

MADE A COUPLE TWEAKS TO GOING FORECAST. FIRST LOWERED SNOW AMOUNTS
NORTHERN HALF/TWO-THIRDS OF CWA TONIGHT AND ALSO RAISED FORECAST
LOW TEMPS SEVERAL DEGREES IN SOME INSTANCES.

LARGE SWATH OF WARM AIR ADVECTION DRIVEN SNOW CONTINUES FROM
MISSOURI EAST ACROSS CENTRAL ILLINOIS AND INDIANA...PRIMARILY
SOUTH OF OUR CWA. A LEAD SHEAR VORT MAX MOVING ACROSS LOWER LAKES
THIS EVENING HELPED DRAG THE NORTHERN EXTENT OF THE SNOW NORTH
INTO NORTHERN ILLINOIS BRIEFLY EARLY THIS EVENING...BUT AS THAT
VORT CONTINUES EASTWARD EXPECT THE LIGHT SNOW OVER NORTHERN
ILLINOIS TO TAPER OFF THE NEXT COUPLE HOURS. WHILE SNOW HAS BEEN
KNOCKING VSBY DOWN TO IFR...SNOW FLAKES HAVE BEEN QUITE FINE
ALMOST TO THE POINT TO WHERE IT IS HARD TO EVEN SEE THE FLAKES.
THIS IS RESULTING IN LITTLE OR NO ACCUM. THERE HAVE BEEN SOME
BRIEF TRANSIENT FLARE UPS IN THE REFLECTIVITY WHICH COULD LEAD TO
A QUICK DUSTING/COATING OF SNOW. UPSTREAM SATELLITE TRENDS AND
FORECAST SOUNDINGS SUPPORT CONTINUED DRYING WHICH SHOULD MAKE THE
THREAT OF MEASURABLE SNOWFALL EXTREMELY LOW NORTHERN CWA OVERNIGHT.

FARTHER SOUTH (SOUTH OF I-80) CORRIDOR IT WOULD APPEAR AT LEAST
INTERMITTENT LIGHT SNOW WILL CONTINUE THROUGH THE NIGHT AND COULD
POTENTIALLY RESULT IN A COUPLE INCH OF ACCUMULATION BY
MORNING...WITH THE HEAVIEST SNOW FAR SOUTH NEAR THE ILX/LOT CWA
BORDER. ANOTHER SPEED MAX AND ASSOCIATED VORT WILL ZIP ACROSS THE
REGION SUNDAY AND SUPPORT A RENEWED PERIOD OF BEEFIER SNOW SUNDAY
MORNING/EARLY AFTERNOON SOUTH OF I-80. IN ALL IT STILL LOOKS LIKE
2-5" OF SNOW COULD FALL WITH THE HIGHER AMOUNTS LIKELY ENDING UP
FAR SOUTHERN CWA OR PERHAPS JUST SOUTH OF OUR CWA. GIVEN THE
PERIODIC NATURE AND LONG DURATION OF THE EVENT...THE DAY SHIFT`S
DECISION TO HOLD OFF ON WINTER WEATHER ADVISORY SEEMS LIKE A GOOD
CALL.

CLOUDINESS ALONG WITH WARM/MOIST ADVECTION SHOULD RESULT IN NEARLY
STEADY TEMPS REST OF TONIGHT. THIS SHOULD RESULT IN LOWS A BIT
HIGHER THAN PREVIOUSLY FORECAST AND HAVE ADJUSTED THEM UPWARD
ACCORDINGLY. HAVE ALREADY SENT UPDATED GRIDS AND TEXT PRODUCTS.

IZZI

&&

.SHORT TERM...
318 PM CST

MAIN CONCERNS ARE WITH SNOW POTENTIAL THIS EVENING THROUGH SUNDAY
WITH THE FOCUS FOR MORE ACCUMULATING SNOW SOUTH OF I-80...AND THEN
WITH ANOTHER SYSTEM VARYING PRECIP MONDAY NIGHT INTO TUESDAY.

THROUGH MONDAY...

LATEST RADAR IMAGERY DEPICTING LARGE AREA OF LIGHT TO MODERATE
SNOW PUSHING NORTHEAST THROUGH PORTIONS OF THE PLAINS AND MID
MISSISSIPPI VALLEY...STEADILY APPROACHING CENTRAL ILLINOIS AT THIS
TIME. THIS AREA OF SNOW IS RIDING ALONG A FAIRLY ENERGETIC SHORT
WAVE...AND STRONG/BROAD WAA. AS THIS WAVE LIFTS NORTHEAST THIS
AFTERNOON AND EARLY EVENING THE FOCUS OF THIS WAA WILL LIKELY WISE
SHIFT EAST...WITH THIS LIGHT SNOW TO AT TIMES MODERATE SNOW
SPREADING OVER MOST OF NORTHERN ILLINOIS AND NORTHWEST INDIANA BY
EARLY TO MID EVENING. WHILE THE BEST FORCING WILL REMAIN ACROSS
THE SOUTHEAST HALF OF THE CWA THROUGH THIS EVENING...BROAD ASCENT
SHOULD STILL BRING AT LEAST A PERIOD OF LIGHT SNOW FOR EVEN AREAS
NEAR THE ILLINOIS/WISCONSIN BORDER. SO DO HAVE LIKELY POPS FOR
AREAS UP NORTH AND THEN CATEGORICAL POPS SOUTHERN HALF OF THE CWA
BY THIS EVENING. WHILE A PERIOD OF LIGHT SNOW IS EXPECTED THIS
EVENING DO FEEL THAT BRIEF BURSTS OF MODERATE SNOW COULD FALL AT
TIMES AND WHILE MORE THAN LIKELY A LOW QPF EVENT...A DEEP DGZ WILL
RESULT IN FAIRLY EFFICIENT SNOW WITH A DUSTING UP TO AN INCH OF
SNOW LIKELY ALONG AND NORTH OF INTERSTATE 88...AND 1 TO 2 INCHES
SOUTH BY AROUND MIDNIGHT TONIGHT. WITH REGARDS TO START
TIMES...PONTIAC SHOULD START OBSERVING SNOW BY 23-00Z WITH AREAS
FROM ROCKFORD TO CHICAGO...TO VALPARAISO BY THE 2-3Z TIME FRAME.
DO EXPECT A 3-4 HOUR WINDOW OF THIS SNOW BEFORE THIS SNOW LIKELY
EXITS.

THERE WILL LIKELY BE A WINDOW WHEN THE SNOW STOPS AND THEN MOVES
BACK ACROSS THE SOUTHERN CWA LATE TONIGHT INTO EARLY SUNDAY
MORNING...WITH THE SNOW THIS EVENING AND THEN ADDITIONAL SNOW
TOMORROW ALMOST TWO SEPARATE EVENTS. INCREASING FLOW ALOFT AND
ADDITIONAL SHORTWAVE ENERGY SHOULD ALLOW THE SNOW TO RETURN BUT
WITH THE FOCUS FROM THIS POINT FORWARD TO REMAIN SOUTH OF
INTERSTATE 80...AND WITH AREAS TO THE NORTH LIKELY ONLY OBSERVING
ADDITIONAL FLURRIES...IF THAT. ONCE AGAIN EXPECT A PERIOD OF LIGHT
SNOW WITH THE POSSIBILITY FOR BRIEF HEAVIER SNOWFALL WITH ANY
POTENTIAL FGEN BANDING THROUGH AT LEAST MIDDAY SUNDAY WITH ANY
REMAINING SNOW EXITING INTO THE AFTERNOON. THE HIGHEST SNOWFALL
AMOUNTS DO REMAIN FAIRLY SIMILAR TO THE PREVIOUS FORECAST AS WELL
AS EVEN THE LOCATION. THROUGH SUNDAY AFTERNOON...TOTAL SNOWFALL IS
EXPECTED TO RANGE FROM A DUSTING UP TO 2 INCHES NORTH OF THE I-80
CORRIDOR WITH 2 TO 5 INCHES SOUTH. HOWEVER...SOUTHERN PORTIONS OF
FORD... IROQUOIS...AND BENTON COUNTIES MAY STILL SEE ISOLATED
AMOUNTS NEAR 6 INCHES IF THE BETTER BANDING WERE MATERIALIZE. THE
LONG DURATION OF THIS LIGHT SNOW AS WELL AS THE ANTICIPATED BREAK
IN SNOWFALL TONIGHT BEFORE STARTING UP ONCE AGAIN SUNDAY MORNING
SHOULD HELP TO LIMIT IMPACTS...AND SO HAVE NOT ISSUED ANY
HEADLINES AT THIS TIME BUT WILL CONTINUE THE SPS.

RODRIGUEZ

&&

.LONG TERM...
318 PM CST

MONDAY NIGHT THROUGH SATURDAY...

FOCUS NEXT WEEK CONTINUES TO BE WITH LARGER SYSTEM TO AFFECT MUCH
OF TH REGION EARLY NEXT WEEK THROUGH MID WEEK...AND LOCALLY MONDAY
NIGHT INTO TUESDAY. HIGHLY AMPLIFIED PATTERN STILL ANTICIPATED
WITH APPROACHING LONGWAVE THROUGH EXPECTED TO STEER A SURFACE LOW
ACROSS THE AREA MONDAY NIGHT INTO TUESDAY. WARM/MOIST ADVECTION
OUT AHEAD OF THIS SYSTEM WILL HELP SPREAD PRECIP ACROSS THE CWA
LATE MONDAY NIGHT AND EARLY TUESDAY MORNING. LATEST GUIDANCE TODAY
INDICATING THAT INITIAL PROFILES WILL BE SUPPORTIVE OF SNOW AT THE
ONSET...AND THEN WITH A WINTRY MIX LIKELY ACROSS THE ENTIRE CWA
THROUGH TUESDAY MORNING. A TRANSITION OVER TO ALL RAIN IS STILL
EXPECTED THROUGHOUT THE DAY ON TUESDAY BUT SOME GUIDANCE HINTING
AT A POSSIBLE COLDER SOLUTION FOR THE CWA. IF THIS WERE TO
MATERIALIZE...THEN LESS OF THE CWA...LIKELY FAR NORTHERN
ILLINOIS...WOULD OBSERVE LESS RAIN AND MORE FROZEN TYPE PRECIP.
THIS WOULD ALSO RESULT IN MORE A TRANSITION OVER TO ALL SNOW BY
TUESDAY AFTERNOON. HOWEVER THIS PANS OUT...TUESDAY MORNING IS
STILL A PERIOD TO MONITOR CLOSELY WITH THE VARYING PRECIP TYPE AND
THEN STILL POSSIBLE HYDRO ISSUES WITH RAIN AND SNOW MELT.

RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* LIGHT SNOW AND BRIEF MVFR VIS ENDING BY OR SHORTLY AFTER 06Z.

* SOME POTENTIAL FOR MVFR VIS IN FOG OVERNIGHT.

* MVFR CIGS PROBABLE LATER TONIGHT INTO SUNDAY MORNING.

RATZER

//DISCUSSION...UPDATED 06Z...

AREA OF LIGHT SNOW WHICH OVERSPREAD THE TERMINALS DURING THE
EVENING IS JUST ABOUT OVER AT 06Z...WITH CONDITIONS TRENDING BACK
TO VFR IN MOST AREAS. IR SATELLITE IMAGERY HAS DEPICTED AN AREA OF
CLEARING OVER EASTERN IOWA AND NORTHWEST IL OVER THE PAST FEW
HOURS...THOUGH DOES APPEAR TO BE FILLING IN WITH PATCHY MVFR DECK
WHICH IS EXPECTED TO EXPAND EAST ACROSS THE TERMINALS DURING THE
PREDAWN HOURS. RELATIVELY MOIST SURFACE CONDITIONS WITH SMALL
TEMP/DEW POINT SPREADS...SO IF SKIES DO CLEAR FOR A TIME EARLY
THIS MORNING THERE COULD BE SOME MVFR FOG DEVELOPMENT WITH LIGHT
WINDS. THIS WOULD BE MOST LIKELY WEST OF CHICAGO...PRIOR TO
AFOREMENTIONED MVFR DECK EXPANDS EAST BEHIND DEPARTING HIGHER
CLOUDS.

ON SUNDAY...IT APPEARS ADDITIONAL LIGHT SNOW WILL REMAIN JUST OFF
TO THE SOUTH OF THE TERMINALS ACROSS CENTRAL IL/IN. LIGHT
SOUTHEAST WINDS TO BECOME LIGHT SOUTHWEST DURING THE MORNING...
THEN EXPECTED TO SHIFT WEST-NORTHWEST AS COLD FRONT PASSES SUNDAY
EVENING. SOME GUIDANCE INDICATES A BRIEF PERIOD OF GUSTS AS LAPSE
RATES STEEPEN BEHIND THE FRONT SUNDAY EVENING...THOUGH GRADIENT
WEAKENS OVERNIGHT AS HIGH PRESSURE RIDGE QUICKLY BUILDS EAST BY
MONDAY MORNING. ANY LINGERING MVFR STRATUS EXPECTED TO CLEAR OUT
DURING THE EVENING AS WELL.

RATZER

//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* LOW IN MVFR VIS IN FOG OVERNIGHT.

* MEDIUM IN MVFR CIGS DEVELOPING OVERNIGHT/SUNDAY.

* HIGH IN ALL OTHER ELEMENTS.

RATZER

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

MONDAY...VFR.

TUESDAY...RAIN...FREEZING RAIN...OR SNOW. IFR PROBABLE.

WEDNESDAY...MVFR CIGS POSSIBLE.

THURSDAY AND FRIDAY...MAINLY VFR.

IZZI

&&

.MARINE...
237 AM CST

MARINE FOCUS REMAINS ON A COUPLE PERIODS OF STRONGER WINDS NEAR 30
KT OVER THE NEXT SEVERAL DAYS. THE FIRST ONE IS TONIGHT AS HIGH
PRESSURE BUILDS IN BEHIND A COLD FRONT PUSHING ACROSS THE LAKE. THE
OTHER WILL BE TUESDAY AHEAD OF AN APPROACHING LOW...AND TUESDAY
NIGHT INTO WEDNESDAY AS COLD AIR SURGES INTO THE AREA ONCE AGAIN AS
THE DEEPENING LOW CONTINUES TO MOVE OFF TO THE NORTHEAST. A PERIOD
OF WEST-NORTHWESTERLY GALES ARE POSSIBLE TUESDAY NIGHT.

IN THE NEAR TERM...RELATIVELY QUIET WEATHER WAS ACROSS THE LAKE THIS
MORNING WITH WEAK HIGH PRESSURE RIDGING JUST SOUTH OF THE LAKES FROM
ILLINOIS TO NEW ENGLAND. A COLD FRONT WAS OVER THE UPPER MIDWEST...
AND WILL MOVE SOUTHEAST AND ACROSS THE LAKE THIS EVENING. THE
COMBINATION OF COLD AIR SPREADING IN BEHIND THE FRONT AND STRONG
HIGH PRESSURE BUILDING EAST FROM THE PLAINS WILL TIGHTEN THE
GRADIENT FOR A PERIOD TONIGHT...ALLOWING WEST-NORTHWEST WINDS TO
INCREASE TO 25-30 KT BEFORE DIMINISHING QUICKLY MONDAY MORNING AS
THE HIGH MOVES INTO THE REGION. THE RIDGE WILL MOVE EAST OF THE LAKE
MONDAY AFTERNOON...WITH A TROUGH OF LOW PRESSURE DEVELOPING FROM
MANITOBA INTO THE HIGH PLAINS. LOW PRESSURE WILL DEVELOP OVER THE
PLAINS AND MOVE NORTHEAST ACROSS KANSAS BY EARLY TUESDAY AND MOVING
NEAR SOUTHERN OR CENTRAL LAKE MICHIGAN BY TUESDAY EVENING. WINDS
WILL BECOME SOUTH-SOUTHEAST AHEAD OF THE LOW EARLY
TUESDAY...INCREASING TO 20-25 KT FOR A TIME BEFORE THE GRADIENT
WEAKENS NEAR THE LOW/TROUGH CENTER LATE IN THE DAY. THE LOW WILL BE
DEEPENING AS IT PASSES AND LIFTS NORTHEAST AWAY FROM LAKE MICHIGAN
TUESDAY NIGHT...WITH WINDS VEERING WEST-NORTHWEST AND INCREASING
INTO THE 30 KT RANGE OVERNIGHT AND INTO WEDNESDAY. SOME GUIDANCE
CONTINUES TO SUGGEST A PERIOD OF NORTHWEST GALES POSSIBLE DURING THE
LATE TUESDAY NIGHT/EARLY WEDNESDAY TIME FRAME...THOUGH THERE REMAINS
SOME DIFFERENCES IN THE DETAILS OF THE STRENGTH OF THE LOW. FOR NOW
HAVE MAINTAINED A MENTION OF GALE FORCE GUSTS TUESDAY NIGHT.
WEST-NORTHWEST WINDS LOOK TO REMAIN FAIRLY STOUT THROUGH WEDNESDAY
AND EVEN INTO WEDNESDAY NIGHT...AS HIGH PRESSURE DEVELOPS SOUTHEAST
FROM THE PLAINS TO THE OHIO VALLEY AND KEEPS THE WESTERN LAKES IN A
TIGHTER GRADIENT NORTH/NORTHEAST OF THE HIGH CENTER.

ILLINOIS/INDIANA NEAR SHORE WATERS REMAIN MOSTLY ICE COVERED. IF NOT
FOR LACK OF OPEN WATER...INCREASING SOUTHEAST WINDS LATE MONDAY
NIGHT COULD OTHERWISE LEAD TO SMALL CRAFT ADVISORY CONDITIONS MAINLY
NORTH OF CHICAGO BY TUESDAY MORNING.

RATZER

&&

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

$$

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