Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 121509
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
909 AM CST FRI FEB 12 2016

.UPDATE...
909 AM CST

MADE SOME QUICK UPDATES TO POPS AND SNOW AMOUNTS THIS MORNING.
CURRENT RADAR IMAGERY HAS SHOWN A QUICK UPTAKE IN THE SNOW ACROSS THE
AREA. ALTHOUGH MOST GUIDANCE IS NOT HANDING THIS BAND OF SNOW MUCH
AT ALL...THERE APPEARS TO BE A GOOD BAND OF LOWER-LEVEL ENHANCED
FRONTOGENESIS SETTING UP OVER THE AREA IN ASSOCIATION WITH THE
APPROACH OF THE ARCTIC COLD FRONT. GIVEN THE STRENGTHEN OF THIS
BAND...AND THE DENDRITES CURRENTLY FALLING WITH THE 20-30 DBZ ECHOS
ON THE RADAR...I WOULD NOT BE SURPRISED IF A QUICK HALF INCH TO AN
INCH OF SNOW FALLS OVER SOME OF THE WESTERN AND SOUTHERN SUBURBS OF
CHICAGO. THIS BAND OF SNOW SHOULD SHIFT TO THE SOUTHEAST THROUGH THE
MORNING...AND COME TO AN END BY EARLY AFTERNOON.

KJB

&&

.SHORT TERM...
343 AM CST

TODAY THROUGH SATURDAY...

FOCUS IN THE NEAR TERM OF THE FORECAST IS WITH SOME VERY LIGHT
SNOW THIS MORNING ACROSS PARTS OF THE CWA...AND THE POTENTIAL FOR
LAKE EFFECT SNOW ACROSS NORTHWEST INDIANA THROUGH THE PERIOD. COLD
TEMPS WITH A REINFORCING PUSH OF ARCTIC AIR LATER TODAY ARE ALSO A
CONCERN THROUGH THE SHORT-TERM...WITH WINDS CHILLS REACHING
ADVISORY CRITERIA ACROSS FAR NORTHERN ILLINOIS BY SATURDAY
MORNING...WIND CHILL ADVISORY WILL BE ISSUED FOR MIDNIGHT THROUGH
10 AM CST.

00Z UPPER AIR ANALYSIS CONTINUES TO DEPICT UPPER RIDGING ACROSS
WESTERN NORTH AMERICA...AND A BROAD UPPER TROUGH DOWNSTREAM ACROSS
MOST OF EASTERN NORTH AMERICA. THREE INDIVIDUAL SHORT-WAVES WERE
NOTED WITHIN THE NORTHWESTERLY FLOW ALOFT ACROSS THE REGION...ONE
PROPAGATING ACROSS THE LOWER MISSOURI VALLEY WITH AN AREA OF LIGHT
SNOW OCCURRING ROUGHLY ALONG/SOUTH OF A DES MOINES TO SPRINGFIELD
IL LINE...ANOTHER MORE SUBTLE SHEARED DISTURBANCE OVER IA/MN WITH
AN AREA OF EVEN LIGHTER SNOW...AND ANOTHER DIGGING WAVE OVER THE
LAKE WINNIPEG REGION. STRONGEST FORCING FOR ASCENT WILL REMAIN
BOTH TO THE NORTH AND THE SOUTH OF THE FORECAST AREA PER LAYER
Q-CONVERGENCE FIELDS...AND PROGGED ISENTROPIC UPGLIDE AROUND THE
275 K SURFACE. IN ADDITION...SATURATION APPEARS TO BE
BRIEF/INCOMPLETE THROUGH THE COLUMN PER FORECAST SOUNDINGS FROM
VARIOUS MODELS. ALMOST NO GUIDANCE SQUEEZES OUT MEASURABLE PRECIP
FOR US THIS MORNING...THUS WILL CARRY CHANCE POPS FOR LIGHT SNOW
THROUGH MID/LATE MORNING BUT WITH NO ACCUMULATION.

DEEPENING SURFACE LOW PRESSURE CENTER OVER THE NORTHERN LAKES
THIS MORNING WILL TRAIL A REINFORCING ARCTIC COLD FRONT ACROSS THE
FORECAST AREA LATE THIS MORNING...RESULTING IN SURFACE WINDS
VEERING TO NORTHWEST AND GUSTING AROUND 30 MPH THIS AFTERNOON.
AFTER HIGHS IN THE 20-25 DEGREE RANGE THIS AM/MID-DAY...THIS WILL
LIKELY PRODUCE FALLING TEMPS FROM THE NORTHWEST BY MID-LATE
AFTERNOON ALONG WITH SUB-ZERO WIND CHILLS. COLD ADVECTION AND A
NORTHWEST FETCH WILL LIKELY RESULT IN LAKE EFFECT SNOW SHOWERS
DEVELOPING WESTWARD INTO PORTER COUNTY AS WELL. THERMODYNAMIC
PARAMETERS ARE VERY FAVORABLE FOR HEAVY LAKE EFFECT WITH LAKE
SURFACE TO 850 MB DELTA-T INCREASING TO AROUND 25 C/KM TONIGHT AND
EQUILIBRIUM LEVELS ABOVE 10 KFT... ALTHOUGH THE FOCUS FOR LOW
LEVEL CONVERGENCE REMAINS GENERALLY EAST OF PORTER COUNTY INTO
SOUTHWEST LOWER MICHIGAN. HAVE INDICATED 1-3 INCHES ACROSS FAR
NORTHEAST PORTER COUNTY AT THIS TIME FOR LATE TODAY THROUGH
SATURDAY IN THIS SETUP.

WITH THE SURGE OF ARCTIC AIR MOVING INTO THE
REGION...TEMPERATURES ARE EXPECTED TO FALL OFF INTO THE SINGLE
DIGITS BELOW ZERO ACROSS NORTH CENTRAL AND NORTHEAST IL...AND
AROUND ZERO OR A COUPLE OF DEGREES ABOVE IN THE CORE OF CHICAGO
AND SOUTH OF I-80. COMBINED WITH WINDS AROUND 15 MPH...THIS WILL
PRODUCE WIND CHILL READINGS AROUND -20 GENERALLY ALONG/NORTH OF
THE I-88 CORRIDOR ACROSS NORTHERN IL AND AROUND -15 FARTHER SOUTH.
THUS HAVE ELECTED TO ISSUE A WIND CHILL ADVISORY FOR THE NORTHERN
TWO TIERS OF IL COUNTIES IN COORDINATION WITH WFOS DVN AND MKX.
THE ADVISORY WILL RUN FROM MIDNIGHT THROUGH 9 AM CST SATURDAY
MORNING. IT WILL REMAIN COLD DURING THE DAY SATURDAY WITH
TEMPERATURES STRUGGLING INTO THE 10-15 ABOVE RANGE AND WIND CHILLS
NEAR ZERO. WINDS WILL DIMINISH DURING THE AFTERNOON HOWEVER AS
SURFACE HIGH PRESSURE SPREADS ACROSS THE AREA.

RATZER

&&

.LONG TERM...
343 AM CST

SATURDAY NIGHT THROUGH THURSDAY...

THE MAIN FORECAST CONCERN IN THE EXTENDED PERIOD IS WITH THE
POTENTIAL FOR ACCUMULATING SNOW SUNDAY WITH A CLIPPER-LIKE SYSTEM
WHICH WILL MOVE ACROSS THE AREA.

GUIDANCE IS IN FAIRLY DECENT AGREEMENT IN THE TRACK OF THE MID-
LEVEL SHORT WAVE FEATURE MOVING ACROSS THE FORECAST AREA
SUNDAY...WITH A STRONG WARM ADVECTION RESPONSE AND HEIGHT FALLS
PROVIDING FAIRLY STRONG ASCENT WITHIN A THERMODYNAMIC COLUMN
FEATURING A DEEP DENDRITIC GROWTH ZONE FROM JUST OFF THE SURFACE
THROUGH ABOUT 600 MB. ISENTROPIC FORECASTS ON THE 285 K SURFACE
DEPICT GOOD UPGLIDE AND CONDENSATION PRESSURE DEFICITS LESS THAN 5
MB...WITH MIXING RATIOS AROUND 3 G/KG. THIS ALONG WITH A BLENDED
GUIDANCE QPF OF NEARLY 0.25 INCHES ACROSS NORTH CENTRAL IL
COMBINED WITH FORECAST SNOW TO LIQUID RATIOS IN THE 15-18:1 RANGE
THIS WOULD PRODUCE ON THE ORDER OF 3-5 INCHES ACROSS NORTH CENTRAL
IL...AND 2-4 INCHES FARTHER TO THE SOUTHEAST. BLUSTERY SOUTH-
SOUTHEAST WINDS 20-25 MPH MAY INCREASE THE IMPACT OF BLOWING AND
DRIFTING SNOW AS WELL. WILL HAVE TO CONTINUE TO WATCH THIS
SYSTEM...AND A WINTER WEATHER ADVISORY MAY ULTIMATELY BE NEEDED IT
MODELS REMAIN CONSISTENT IN THE ABOVE SCENARIO.

THE UPPER PATTERN CONTINUES TO FEATURE AN EASTERN TROUGH AND
WESTERN RIDGE THROUGH MID-WEEK...WITH A SERIES OF CLIPPERS
DEPICTED IN GLOBAL GUIDANCE THROUGH ABOUT WEDNESDAY. TIMING AND
STRENGTH OF THESE FEATURES SHOW SOME DIFFERENCES BETWEEN
MODELS...WITH TUESDAY LOOKING MOST LIKELY FOR ANOTHER CHANCE OF
LIGHT SNOW. EVENTUALLY...THE WESTERN RIDGE SPREADS EAST ACROSS THE
PLAINS AND TOWARD THE MIDWEST BY THE END OF THE PERIOD...ALLOWING
A MODERATION IN TEMPERATURES BACK ABOVE NORMAL...AND ABOVE
FREEZING...BY THE END OF THE WEEK.

RATZER

&&

.AVIATION...
FOR THE 12Z TAFS...

MAIN FORECAST CONCERNS/CHALLENGES ARE WITH MVFR CEILINGS AND VIS
IN SNOW THIS MORNING...AND STRONG NORTHWEST WINDS.

RADAR IMAGERY DEPICTING LIGHT SNOW STILL DROPPING SOUTHEAST ACROSS
SOUTHERN WISCONSIN AND NORTHERN ILLINOIS AT THIS TIME. THE
TERMINALS WILL SEE SNOW HERE SHORTLY WITH A WINDOW OF SLIGHTLY
MORE INTENSE SNOW STILL POSSIBLE. VIS OF 3-5SM STILL CURRENTLY
PRESENT IN THIS SNOW AND THINK THAT STILL SEEMS TO BE POSSIBLE
BRIEFLY THIS MORNING. MVFR CEILINGS ALSO STILL EXPECTED TO MOVE
OVERHEAD THIS MORNING WITH A POSSIBILITY FOR THEM TO REMAIN
SLIGHTLY LONGER INTO MIDDAY. ANY MVFR CEILINGS WILL LIKELY BECOME
VFR LATER TODAY BEFORE SCATTERING EARLY THIS EVENING. CURRENT WEST
WINDS WILL TURN TO THE NORTHWEST THIS MORNING WHILE INCREASING
WITH GUSTS AROUND 30KT STILL EXPECTED THIS AFTERNOON.

RODRIGUEZ

&&

.MARINE...
215 AM CST

MAIN FORECAST UPDATE WAS TO BRING UP THE START TIME OF THE GALE
AND FREEZING SPRAY WARNINGS...WITH GALES EXPECTED TO START ACROSS
THE NORTH THIS MORNING AND ACROSS THE SOUTH AND NEARSHORE EARLY
THIS AFTERNOON.

WITH LOW PRESSURE TO THE NORTH AND WITH STRONG HIGH PRESSURE
BUILDING EAST/SOUTHEAST JUST TO THE WEST EXPECT SPEEDS TO REALLY
INCREASE TODAY ACROSS THE ENTIRE LAKE. WINDS ARE CURRENTLY THE
STRONGEST OVER THE NORTH HALF OF THE LAKE WHERE WESTERLY WINDS TO
30KT ARE BEING OBSERVED. THERE HAVE BEEN A FEW GALE FORCE GUSTS IN
THIS LOCATION OVERNIGHT AND DO EXPECT THESE GUSTS TO BE POSSIBLE
THIS MORNING. HOWEVER...MORE SUSTAINED GALES WONT ARRIVE UNTIL MID
TO LATE MORNING AND THEN CONTINUE THROUGH TONIGHT...WHILE THE
SOUTH AND NEARSHORE WATERS SEE GALES THIS AFTERNOON INTO TONIGHT.
GALES SHOULD DIMINISH BY LATE TONIGHT INTO EARLY SATURDAY MORNING
BUT WILL NEED TO MONITOR FOR GALES TO POSSIBLY PERSIST A LITTLE
LONGER OVER THE NORTH HALF. WINDS OVER THE ENTIRE LAKE WILL
DIMINISH SATURDAY INTO SATURDAY NIGHT AS HIGH PRESSURE MOVES
ACROSS THE REGION.

RODRIGUEZ

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WIND CHILL ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014...MIDNIGHT SATURDAY TO 9 AM
     SATURDAY.

IN...NONE.
LM...GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-
     LMZ779 UNTIL 6 AM SATURDAY.

     HEAVY FREEZING SPRAY WARNING...LMZ744-LMZ745-LMZ777-LMZ779...3
     PM FRIDAY TO 6 AM SATURDAY.

&&

$$

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