Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 121958
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
158 PM CST FRI FEB 12 2016

.UPDATE...

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 18Z TAFS...

CONFIDENCE REMAINS HIGH THAT WINDS WILL REMAIN NORTHWESTERLY
BETWEEN 310-320 DEG AND BREEZY...WITH SUSTAINED WINDS BETWEEN 15
AND 20 KT AND GUSTS NEARING 28KT AT TIMES THROUGH 2Z. DRY AIR WILL
CONTINUE TO POUR IN FROM THE NORTHWEST AND HELP TO FURTHER ERODE
THE LOW CLOUDS...TRENDING FROM A BKN CIGS OF 3KFT AGL TO A FEW
CLOUDS BY 3KFT AGL AT 2Z. UPSTREAM VISIBLE IMAGERY AND SURFACE
OBSERVATIONS ACROSS WISC WOULD SUPPORT THIS. GUIDANCE THEN
INDICATES THE GUSTY WINDS WILL BEGIN TO DIMINISH LATER
TONIGHT...HOWEVER HAVE HELD ONTO WINDS LONGER INTO THIS EVENING
GIVEN THE STRENGTH OF THE COLD AIR ADVECTION. EARLY SATURDAY
MORNING IT APPEARS WINDS WILL FURTHER DIMINISH TO AROUND 15KT AND
LINGER THROUGH MUCH OF SATURDAY MORNING. DRY AIR WILL KEEP CIGS
MINIMAL IN THE LOWER LEVELS...THUS A HIGH CONFIDENCE IN VFR CONDS
PREVAILING THROUGH OVERNIGHT INTO SATURDAY.

BEACHLER

&&

.MARINE...
150 PM CST

MAIN CONCERNS FOR TONIGHT CONTINUE TO BE FOCUSED ON GALES AND
HEAVY FREEZING SPRAY. WINDS HAVE BEEN STEADILY INCREASING FROM
NORTH TO SOUTH OVER LAKE MICHIGAN TODAY...WITH THE FREQUENCY IN
GALES STEADILY INCREASING. WITH NORTHWEST WINDS PERSISTING FOR A
LONGER PERIOD...THIS WILL BUILD THE WAVES ALONG THE SOUTHERN AND
SOUTHEASTERN PORTIONS OF LAKE MICHIGAN. THE VERY COLD AIR WILL
CONTINUE TO FLOW SOUTH FROM CANADA TONIGHT...WHICH COUPLED WITH
THE EXPECTED GALES TO 40KT WILL CREATE HEAVY FREEZING SPRAY
POTENTIAL FOR ANY VESSELS ON LAKE MICHIGAN TONIGHT. STRONG
CANADIAN HIGH PRESSURE WILL SLIDE SOUTH FROM THE UPPER MIDWEST
INTO THE CENTRAL PLAINS...WHICH WILL KEEP A VERY TIGHT PRESSURE
GRADIENT ALONG THE WESTERN GREAT LAKES AND LAKE MICHIGAN THROUGH
MIDNIGHT. THEN HIGH PRESSURE IS EXPECTED TO SLOWLY DRIFT EAST BY
DAYBREAK SATURDAY AND STEADILY DIMINISH GALES TO BETWEEN 20-25KT
BY DAYBREAK SATURDAY OR SHORTLY AFTER.

HIGH PRESSURE WILL THEN SLIDE OVERHEAD SATURDAY NIGHT AND BRING
MUCH LIGHTER WINDS TO LAKE MICHIGAN...HOWEVER IT WILL LIKELY TAKE
A LITTLE LONGER FOR WAVES TO FULLY SUBSIDE IN THE SOUTHERN PORTION
OF THE LAKE DUE TO THE PROLONGED PERIOD OF NORTH/NORTHWEST
FLOW...WHICH COULD RESULT IN CONTINUED HAZARDOUS WAVES TO SMALL
CRAFT SATURDAY AFTN/EVE MAINLY FOR THE NORTHWEST INDIANA
NEARSHORE. HIGH PRESSURE QUICKLY SHIFTS EAST SUNDAY WITH FLOW
TURNING SOUTHERLY AND INCREASING AHEAD OF YET ANOTHER LOW PRESSURE
SYSTEM FOR SUNDAY NIGHT/MONDAY. CURRENT INDICATIONS ARE THAT
SOUTHERLY FLOW WILL RANGE FROM 15 TO 25 KT.

BEACHLER

&&

.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 UNTIL
     6 AM SATURDAY.

&&

$$

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