Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 140833
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
233 AM CST SUN FEB 14 2016

.UPDATE...
832 PM CST

A FEW MINOR TWEAKS TO THE FORECAST THIS EVENING.

FIRST...HIGH CLOUDINESS IS SPREADING EAST ACROSS THE AREA AND
NUDGED SKY COVER UP A BIT MORE QUICKLY TONIGHT. AS A RESULT OF THE
CLOUDINESS AND DEVELOPING SOUTHEAST WINDS OVERNIGHT ANTICIPATE
ONLY A MODEST TEMP FALL FROM HERE ON OUT...MOST PRONOUNCED OVER
EASTERN CWA WHERE CLOUDS AND WINDS WILL BE LATEST TO ARRIVE. AS
CLOUDS THICKEN AND WINDS PICK UP TEMPS WILL PROBABLY BEGIN A SLOW
UPWARD REBOUND PRIOR TO DAYBREAK.

NARROW WNW-ESE BAND OF FRONTOGENETICALLY DRIVEN SNOW IS DEVELOPING
EASTWARD THIS EVENING INTO WEST CENTRAL ILLINOIS. SHORT RANGE
GUIDANCE SUGGESTS THAT THE FRONTOGENESIS WILL WEAKEN OVERNIGHT AS
IT LIFTS SLOWLY NORTHWARD. NOT UNCOMMON FOR SHORT RANGE GUIDANCE
TO WEAKEN SUCH FEATURES UNREALISTICALLY AND FOR THEM TO PERSIST
LONGER THAN MODELS SUGGEST. IF THIS WERE TO OCCUR...THEN COULD SEE
SNOW GRADUALLY INCH NORTHEAST INTO OUR SOUTHWESTERN MOST CWA PRIOR
TO DAWN. MIDNIGHT SHIFT WILL NEED TO MONITOR TRENDS CLOSELY.

WATER VAPOR IMAGERY EARLY THIS EVENING SHOWS POWERFUL SHORTWAVE
OVER THE NORTHERN HIGH PLAINS DIGGING SOUTHEASTWARD. NLDN HAS
SHOWN A FAIR AMOUNT OF CONVECTION BREAKING OUT IN ADVANCE OF THIS
FEATURE OVER SOUTH DAKOTA. NUMERICAL GUIDANCE ALL AGREES THAT THIS
SHORTWAVE WILL BEGIN DAMPENING OUT OVERNIGHT AND PARTICULARLY ON
SUNDAY AS IT ENCOUNTERS A MORE CONFLUENT FLOW ON THE BACKSIDE OF
DEPARTING UPPER LOW RESPONSIBLE FOR OUR CURRENT COLD SNAP.

THE MILLION DOLLAR QUESTION TOMORROW IS HOW QUICKLY THE SNOW IN
ADVANCE OF THIS FEATURE WILL WEAKEN IN RESPONSE TO THE WEAKENING
SYSTEM. MADE NO CHANGES TO GOING FORECAST FOR TOMORROW...THOUGH
SOME OF THE LATEST GUIDANCE COMING IN WOULD SUGGEST THAT SNOWFALL
TOTALS COULD END UP BEING SKEWED A BIT MORE TOWARD THE LOWER END
OF THE FORECAST RANGES.

STEEP MID-LEVEL LAPSE COULD ALLOW SOME BANDING FEATURES TO
DEVELOP...EVEN THOUGH FORCING MAY BE WANING WITH TIME TOMORROW.
ANY BANDING FEATURES THAT DO DEVELOP WOULD LOCALLY ENHANCE
SNOWFALL AMOUNTS AND COULD ALLOW SOME AREAS TO SEE SNOWFALL
AMOUNTS A BIT HIGHER THAN LATEST GUIDANCE SUPPORTS AND MORE IN
LINE WITH HIGHER END OF OUR FORECAST SNOWFALL AMOUNT RANGES. GIVEN
THE RANGE OF POSSIBILITIES SEE NO REASONS TO MAKE ANY CHANGES TO
FORECAST AT THIS TIME.

IZZI

&&

.SHORT TERM...

TONIGHT THROUGH MONDAY...

257 PM...MAIN FORECAST CONCERN IS SNOWFALL AMOUNTS SUNDAY THROUGH
SUNDAY NIGHT.

CLIPPER SYSTEM TO AFFECT THE REGION IN TWO PARTS WITH THE FIRST
AREA OF SNOW EXTENDING FROM IA INTO CENTRAL IL LATE TONIGHT AND
THEN SWING EAST THRU THE CWA DURING SUNDAY MORNING AND EARLY
SUNDAY AFTERNOON. THIS WILL LIKELY PRODUCE A PERIOD OF MODERATE
SNOW BEFORE DIMINISHING SOME IN THE AFTERNOON...THOUGH STILL
SNOWING LIGHTLY. AS THE SURFACE LOW/TROUGH APPROACHES SUNDAY
NIGHT...THERE WILL LIKELY BE ANOTHER PROLONGED PERIOD OF LIGHT
SNOW...PERHAPS BRIEFLY MODERATE. MODEL QPF FORECASTS HAVE FOCUSED
THE HIGHER AMOUNTS...TWO TO THREE TENTHS...MORE ON NORTHWEST IL
INTO CENTRAL IL WITH AMOUNTS GENERALLY UNDER TWO TENTHS ACROSS
NORTHEAST IL AND NORTHWEST INDIANA. AS A RESULT...SNOWFALL TOTALS
NOW APPEAR TO BE JUST A BIT LOWER FOR THE IMMEDIATE CHICAGO AREA
AND LIKELY IN THE 2 TO 4 INCH RANGE...PERHAPS CLOSER TO 2 INCHES
ACROSS NORTHWEST INDIANA. THESE AMOUNTS ARE ALSO IN LINE WITH
LOWER AMOUNTS FROM WPC. BUT SNOWFALL AMOUNTS ACROSS THE WESTERN
AND SOUTHWEST CWA...WHILE ALSO JUST A BIT LOWER...ARE STILL IN
THE 3-5 INCH RANGE AND HAVE ISSUED A WINTER WEATHER ADVISORY
FROM BOONE SOUTH TO GRUNDY/IROQUOIS. IF LATER MODEL RUNS END UP
BRINGING HIGHER QPF AMOUNTS FURTHER EAST...ITS POSSIBLE THE
ADVISORY MAY NEED TO BE EXTENDED FURTHER EAST.

AS THE SNOW SPREADS EAST ACROSS THE CWA SUNDAY MORNING...THERE
COULD BE A PERIOD OR BURST OF MODERATE SNOW AND THIS IS WHEN
SNOWFALL RATES MAY APPROACH ONE INCH AN HOUR FOR AT LEAST A FEW
HOURS. THE SNOW WILL ALSO BE FAIRLY DRY/FLUFFY AND INCREASING
SOUTHEAST WINDS...PERHAPS GUSTING TO 25 MPH RANGE...WILL LIKELY
LEAD TO SOME BLOWING AND DRIFTING SNOW.

TEMPS WILL LIKELY DROP INTO THE SINGLE DIGITS THIS EVENING...
LEVEL OFF AND THEN START SLOWING RISING AS CLOUD COVER INCREASES
OVERNIGHT. BUT OVERALL TREND IN TEMPS ESPECIALLY THIS EVENING IS
LOW. AFTER LIGHT WINDS THIS EVENING...SOUTHEASTERLY WINDS WILL
LIKELY KEEP WIND CHILLS NEAR ZERO OR SLIGHTLY BELOW ZERO OVERNIGHT
INTO SUNDAY MORNING. TEMPS RISE INTO THE LOWER 20S SUNDAY AND
THEN END UP STEADY TO SLOWLY RISING SUNDAY NIGHT INTO MONDAY AS A
BIT MORE MILD AIR ADVECTS IN BEHIND THIS CLIPPER. DESPITE A NEW
SNOW PACK AND PROBABLY EXTENSIVE CLOUD COVER...THERMAL FIELDS
SUGGEST LOW/MID 30S FOR HIGHS ON MONDAY. CMS

&&

.LONG TERM...
308 PM CST

MONDAY NIGHT THROUGH SATURDAY...

MAIN FORECAST CONCERN IS WITH ANOTHER CLIPPER SYSTEM ON TUESDAY
AND ASSOCIATED PRECIPITATION CHANCES AND TYPE. IN ADDITION...SIGNIFICANT
WARM-UP STILL APPEARS TO BE ON THE WAY TO CLOSE OUT THE WORK WEEK.

PRIMARY FEATURE OF NOTE WITH THE TUESDAY CLIPPER IS A MIDLEVEL
SHORTWAVE NOTED ON W/V IMAGERY THIS AFTERNOON OVER THE GULF OF
ALASKA THAT WILL MOVE ASHORE ONTO THE BRITISH COLUMBIA COAST ON
SUNDAY. THIS WAVE WILL THEN DIVE SOUTHEAST...CROSSING THE NORTHERN
PLAINS MONDAY NIGHT. ANOTHER SUBTLE SHORTWAVE TROUGH WILL MOVE
ACROSS THE AREA ON MONDAY NIGHT...WITH DRY AIR AND BEST FORCING
PASSING SOUTH KEEPING CONDITIONS DRY WITH JUST PARTLY/MOSTLY
CLOUDY SKIES AND TEMPS IN THE 20S.

THERE IS FAIRLY SIGNIFICANT SPREAD ON THE OPERATIONAL AND
ENSEMBLE GUIDANCE REGARDING THE TRACK OF THE CLIPPER LOW
PRESSURE...AS WELL AS ITS STRENGTH. NCEP BASED GUIDANCE HAS BEEN
RUNNING NORTH OF FOREIGN COUNTERPARTS....THOUGH 12Z CANADIAN AND
ECMWF (AND ENSEMBLES OF BOTH) DID COME INTO SLIGHTLY BETTER
AGREEMENT WITH GFS. GEFS TRENDED A BIT FLATTER WITH WAVE AND
WEAKER WITH SLP MEAN. BECAUSE OF AFOREMENTIONED DIFFERENCES IN LOW
TRACK AND STRENGTH...CONFIDENCE IS RELATIVELY LOW IN THE EVOLUTION
OF THE SYSTEM AND LOCAL SENSIBLE WEATHER IMPACTS. IT`S POSSIBLE
THAT DOWNSTREAM TROUGH AND SURFACE LOW TO THE EAST COULD ALSO
FORCE A WEAKER...FARTHER SOUTH TRACK.

WITH THIS BEING SAID...AS A NOD TO 12Z GUIDANCE TRENDS...BUMPED
UP POPS TO CHANCE RANGE AREA WIDE. THERMAL PROFILES ARE
MARGINAL...ESPECIALLY WITH SOUTHWARD EXTENT. OPERATIONAL 12Z
NAM/GFS INDICATED A WARM LAYER AT AROUND 925 MB AND WITH EXPECTED
HIGHS ABOVE FREEZING...INDICATED CHANCE LIGHT RAIN/SNOW SOUTH OF
THE IL AND KANKAKEE RIVERS AND LIGHT SNOW NORTH. STRENGTH OF THE
LOW AS IT PASSES NEAR OR SOUTH OF CENTRAL ILLINOIS WILL ALSO HELP
DICTATE MAGNITUDE OF FORCING...WITH POTENTIAL FOR DECENT FGEN ON
NORTH SIDE OF A STRONGER LOW AND CHANGING ANY MIX TO SNOW. IF THIS
SCENARIO OCCURS...SOME WET SNOW ACCUMULATION WOULD BE POSSIBLE
WHILE ON THE OTHER HAND...WEAKER LOW WOULD RESULT IN LITTLE TO NO
SNOW ACCUMS CWA WIDE. SAMPLING OF THE PARENT MIDLEVEL WAVE ON
SUNDAY BY THE RAOB NETWORK IN CANADA SHOULD HELP SORT OUT THE
DIFFERENCES IN THE GUIDANCE SOME.

IN THE WAKE OF THIS SYSTEM...A BRIEF SHOT OF COLDER AIR WILL
FOLLOW ON WEDNESDAY. MID/UPPER PATTERN WILL THEN QUICKLY SHIFT TO
A STRONG RIDGE IN THE MIDSECTION OF THE COUNTRY IN RESPONSE TO
TROUGH TO WEST AND ALSO ONE DIGGING TO OUR EAST. THIS WILL SET THE
STAGE FOR STRONG WARM ADVECTION ALOFT AND ALSO SURFACE WARM
ADVECTION ON THURSDAY INTO FRIDAY BETWEEN EXPANSIVE HIGH PRESSURE
TO THE EAST AND TROUGHING OVER THE PLAINS. EXTENT OF ANY SNOW
COVER WILL AFFECT HOW QUICK TEMPS CAN WARM ON THURSDAY. BUT THEN
ON FRIDAY...PROGGED THERMAL PROFILES SUPPORT HIGHS WELL INTO THE
40S AND 50S. SOME LIGHT RAIN/SHOWERS ARE POSSIBLE IN VICINITY OF
COLD FRONT TRAILING FROM SURFACE LOW OVER THE NORTHERN LAKES.
AIRMASS WILL BE COOLER BUT LIKELY STILL MILD BEHIND FRONT ON
SATURDAY.

RC

&&

.AVIATION...
FOR THE 06Z TAFS...

STORM SYSTEM WILL MOVE SOUTHEAST TOWARD THE REGION AND WILL LIKELY
RESULT IN A FAIRLY LONG DURATION SNOW EVENT. SNOW LOOKS TO START
MID MORNING AT RFD AND LATER IN THE MORNING AT THE CHICAGO
TERMINALS. ONCE LOW LEVELS ARE SATURATED SNOW SHOULD FALL AT A
FAIRLY GOOD CLIP FOR A COUPLE HOURS SHORTLY AFTER THE ONSET. LEAD
WAVE SHOULD PASS BY MID AFTERNOON AND COULD RESULT IN A PERIOD
WHERE SNOW INTENSITY WANES A BIT BEFORE A SECONDARY WAVE COULD
RESULT IN AN UPTICK IN SNOW INTENSITY AROUND OR SHORTLY AFTER
SUNSET. STILL THINKING THE VAST MAJORITY OF THE EVENT WILL FEATURE
MVFR AND OCNL IFR VSBY...HOWEVER IT REMAINS A POSSIBILITY THAT
THERE COULD BE A PERIOD OF LIFR (LESS THAN 1SM) VSBY IN THE
HEAVIER SNOW. MOST LIKELY TIMING FOR ANY POSSIBLE LIFR VSBY
APPEARS TO BE LATE MORNING/EARLY AFTERNOON. SOUTHEAST WINDS WILL
INCREASE SUNDAY MORNING WITH GUSTS OF 20KT OR JUST A LITTLE HIGHER
SUNDAY AFTERNOON. SOME LIGHT SNOW/SNIZZLE/FLURRIES COULD LINGER
THROUGH MUCH OF THE NIGHT SUNDAY NIGHT.

IZZI

&&

.MARINE...
233 AM CST

SOUTH-SOUTHEAST WINDS WILL STRENGTHEN QUICKLY TO AROUND 25 KT
TODAY...ON THE BACK SIDE OF A SPRAWLING AREA OF HIGH PRESSURE
WHICH WILL MOVE EAST OF THE LAKES TO SOUTHERN NEW ENGLAND AND THE
MID-ATLANTIC REGION BY TONIGHT. A WEAKENING SURFACE WAVE WILL MOVE
INTO THE MID-MISSISSPPI VALLEY ON MONDAY...BUT WILL DISSIPATE AS A
SOUTH GRADIENT IS MAINTAINED BETWEEN THE EAST COAST RIDGE AND AN
AREA OF DEEPER LOW PRESSURE MOVING ACROSS HUDSON BAY. AS THIS
CANADIAN LOW MOVES EAST IT WILL EVENTUALLY TRAIL A TROUGH ACROSS
LAKE MICHIGAN TUESDAY...SHIFTING WINDS TO THE NORTH-NORTHWEST AND
BRIEFLY INCREASING TO 20-30 KT TUESDAY NIGHT INTO EARLY WEDNESDAY
BEFORE HIGH PRESSURE MOVES ACROSS THE WESTERN LAKES WEDNESDAY
NIGHT. ANOTHER DEEP LOW IS EXPECTED TO MOVE ACROSS THE UPPER
MIDWEST TOWARD THE END OF THE WEEK...WITH SOUTH-SOUTHEAST WINDS
STRENGTHENING AGAIN LATE THURSDAY AND FRIDAY.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-
     ILZ019-ILZ020-ILZ021-ILZ032-ILZ033-ILZ039...6 AM SUNDAY TO
     MIDNIGHT MONDAY.

IN...NONE.
LM...NONE.
&&

$$

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