Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 080902
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
302 AM CST MON FEB 8 2016

.UPDATE...
841 PM CST

NO BIG CHANGES NEEDED TO GOING FORECAST. AREA OF FLURRIES AND
EMBEDDED SNOW SHOWERS CONTINUES TO ADVANCE TO TOWARD THE AREA FROM
THE WEST AND NORTHWEST. LEADING EDGE IS SHOWING UP NICELY ONE KARX
WSR-88D WITH EXTRAPOLATION BRINGING THAT ACTIVITY INTO OUR FAR NW
CWA BY AROUND MIDNIGHT...SPREADING SOUTHEAST ACROSS THE REST OF
THE AREA OVERNIGHT. STEEP LOW-MID LEVEL LAPSE RATES AND
RESPECTABLE CONDITIONAL INSTABILITY SHOULD ALLOW FOR SOME EMBEDDED
HEAVIER SNOW SHOWERS...WHICH HAS ALREADY BEEN THE CASE UPSTREAM
WITH NUMEROUS AWOS/ASOS SITES DROPPING TO 1-2SM IN SHSN WITH ONE
OR TWO BRIEFLY DROPPING BELOW 1SM. TOUCHED UP THE PRECIP WORDING
BUT OTHERWISE MADE NO REAL CHANGES TO GOING FORECAST OTHER THAN
MINOR COSMETIC TIMING TWEAKS.

IZZI

&&

.SHORT TERM...
316 PM CST

TONIGHT THROUGH MONDAY...

MAIN FORECAST CONCERN IS WITH SNOW SHOWER ACTIVITY EXPECTED TO
OCCUR OVERNIGHT THROUGH MONDAY. LARGE VERTICALLY STACKED MID/UPPER
LOW CLEARLY NOTED ON AFTERNOON W/V IMAGERY WILL VERY GRADUALLY
SLIDE SOUTHEAST THROUGH THE PERIOD. SECONDARY COLD FRONT/THERMAL
TROUGH WILL PUSH ACROSS THE AREA OVERNIGHT. COLUMN WILL START OUT
QUITE DRY WHILE FORCING FROM UPPER LOW IS OVERHEAD THIS EVENING.
HOWEVER...WHEN BETTER SATURATION AND LOWERING OF CLOUDS OCCURS
LATE TONIGHT THROUGH THE OVERNIGHT...A SOMEWHAT SHEARED VORT MAX
ROTATING AROUND THE ULL WILL PIVOT ACROSS THE AREA. EXPECTING SNOW
SHOWER ACTIVITY TO BREAK OUT IN RESPONSE FROM NORTHWEST TO
SOUTHEAST. SETUP SHOULD ALSO FAVOR A NARROW MORE CONCENTRATED BAND
OF SNOW SHOWERS THAT WILL DEVELOP OVER THE AREA AND THEN MOVE
SOUTHEAST IN THE PREDAWN THROUGH EARLY MORNING HOURS. EXPERIMENTAL
HRRR HAS BEEN CONSISTENTLY DEPICTING THIS SCENARIO. STEEPENING LOW
LEVEL LAPSE RATES AND SOME 850 MB FGEN COULD POTENTIALLY YIELD
SOME BRIEFLY MODERATE SNOWFALL RATES AND VSBY REDUCTIONS BUT
OVERALL ACCUMS SHOULD BE MINIMAL AT BEST BY THE MONDAY MORNING
RUSH. IF THIS SNOW BAND DOES OCCUR...A FEW TENTHS COULD FALL.
TEMPERATURES ARE EXPECTED TO FALL INTO THE MID 20S TO AROUND 30 BY
DAYBREAK. WARM ROAD TEMPS FROM THE MILD WEEKEND SHOULD ALSO
PRECLUDE SIGNIFICANT TRAVEL IMPACTS.

DURING THE DAY ON MONDAY...IT`S CONCEIVABLE THAT THERE WILL BE NO
REAL BREAK IN THE FLURRIES AND SNOW SHOWERS AS BROAD SYNOPTIC LIFT
AND CAA REMAINS OVER THE AREA. HOWEVER...ASSESSING GUIDANCE...BEST
FOCUS FOR CONCENTRATED/POSSIBLY HEAVIER SNOW SHOWERS LOOKS TO BE
ASSOCIATED WITH A STRONG VORT MAX SHIFTING FROM NW IL AROUND 12Z
TO CENTRAL IL BY 18Z. INTRODUCED A SWATH OF CATEGORICAL POPS INTO
THE WESTERN AND SOUTHWEST CWA LATE AM THRU EARLY AFTERNOON. FOR
THE REST OF THE AREA...AS MENTIONED EARLIER...STUCK WITH LIKELY
POPS BUT COULD JUST BE FLURRIES/OCCASIONAL SNOW SHOWERS AND BRIEF
VSBY REDUCTIONS UNDER ANY STEADIER SNOW SHOWERS. TOTAL DAYTIME
SNOWFALL ACCUMULATIONS MAY BE UP TO A HALF INCH OR SO...BUT THERE
WILL LIKELY BE MORE VARIANCE THAN IS SHOWN IN OFFICIAL FORECAST.
ROAD IMPACTS...ESPECIALLY ON HEAVILY TRAVELED ROADS...WILL LIKELY
BE MITIGATED BY LINGERING SURFACE WARMTH FROM THE WEEKEND. GUSTY
WEST-N0RTHWEST WINDS...STRONGEST IN THE WESTERN HALF OF THE
CWA...COULD RESULT IN LOWER VSBY THAN FROM THE FALLING SNOW
ALONE. CLOUDS AND COLD AIR ADVECTION WILL NOT ENABLE TEMPERATURES
TO WARM MUCH FROM OVERNIGHT LOWS. ALL IN ALL...NOT EXPECTING
SIGNIFICANT IMPACTS FROM THE SNOW SHOWERS ON MONDAY...BUT LEAVE
SOME EXTRA TRAVEL TIME JUST IN CASE.

RC

&&

.LONG TERM...
241 PM CST

LONG TERM...

MONDAY NIGHT THROUGH SUNDAY...

FORECAST CONCERNS THROUGH THE EXTENDED PERIOD INCLUDE SNOW
SHOWERS ACROSS THE FORECAST AREA INTO WEDNESDAY...LAKE EFFECT SNOW
ACROSS PARTS OF NORTHWEST INDIANA...AND COLDER TEMPERATURES
THROUGH PERIOD.

DEEP CLOSED UPPER LOW DRIFTS SLOWLY SOUTHEAST FROM THE WESTERN
GREAT LAKES TO THE UPPER OHIO VALLEY THROUGH MID-WEEK...WITHIN
MASSIVE LARGE SCALE UPPER TROUGH ACROSS EASTERN NORTH AMERICA.
DEEP AND PERSISTENT NORTH-NORTHWESTERLY FLOW WEST OF THE TROUGH
AXIS PRODUCES AN INFLUX OF COLD ARCTIC AIR WITH 850 MB TEMPS
DROPPING INTO THE -15 TO -20 C RANGE AND WHICH LINGERS THROUGH
NEXT WEEKEND. STEEP LOW LEVEL LAPSE RATES AND A SERIES OF MID-
LEVEL VORTICITY MAXIMA WILL SET THE STAGE FOR PERIODIC SNOW
SHOWERS...ESPECIALLY FROM MONDAY NIGHT THROUGH EARLY WEDNESDAY
AFTER WHICH THE STRONG VORTICITY CHANNEL SHIFTS OFF TO THE EAST
AND MODELS INDICATE SUBSTANTIAL DRYING OF THE LOW-MID LEVELS BY
WEDNESDAY NIGHT. INTERMITTENT SNOW SHOWERS APPEAR FAIRLY LIKELY
WITHIN THIS REGIME ESPECIALLY EAST...WITH COVERAGE AND INTENSITY
MODULATED BY SMALLER AMPLITUDE SHORT WAVES/VORT MAXIMA ROTATING
THROUGH THE FLOW. WHILE LOW LEVEL LAPSE RATES AND INSTABILITY ARE
STRONGER MONDAY NIGHT...A STRONGER PUSH OF LOW LEVEL COLD AIR
ARRIVES TUESDAY WITH FORECAST SOUNDINGS FROM THE NAM/GFS/ECMWF ALL
INDICATING WEAK FORCING WITHIN A FAIRLY DEEP (200 MB) SATURATED
ISOTHERMAL PROFILE IN THE -10 TO -20 C DENDRITIC GROWTH ZONE FROM
EARLY TUESDAY MORNING INTO TUESDAY EVENING. THUS WHILE HIGHEST
POPS IN THE EXTENDED PERIOD ARE IN THE MONDAY NIGHT-EARLY TUESDAY
PERIOD...LIGHT ACCUMULATIONS WILL LIKELY CONTINUE INTO TUESDAY
AFTERNOON OR EVENING ACROSS THE EASTERN HALF OF THE CWA. MODEL QPF
GENERALLY RANGES FROM 0.05-0.10 INCHES MON NGT/TUESDAY... AND 0.05
INCHES OR LESS TUES NIGHT AND WEDNESDAY WITH LOWEST AMOUNTS
FARTHER WEST. SNOW TO LIQUID RATIOS INCREASING TO NEAR 20 TO 1
WOULD SUGGEST THE POTENTIAL FOR AS MUCH AS 2-3 INCHES OF
ACCUMULATION (OUTSIDE OF LAKE EFFECT AREAS) ACROSS THE EASTERN CWA
OVER ABOUT A 36 HOUR PERIOD...LESS TO THE WEST.

WITH COLD NORTH-NORTHWEST LOW LEVEL FLOW...THE LAKE SNOW BELT
REGION OF EASTERN LAKE AND PORTER COUNTIES WILL SEE LAKE EFFECT
SNOW DURING THE PERIOD...WITH THE FETCH FAVORABLE MAINLY EAST OF
GARY AS WINDS VEER MORE NORTHERLY. LAKE-INDUCED INSTABILITY RAMPS
UP QUICKLY DURING THE EVENING WITH EQUILIBRIUM LEVELS OF 8000-9000
FT...WEAKENING A BIT OVERNIGHT INTO TUESDAY AS LOW LEVELS COOL AND
STRENGTHEN INVERSION NEAR 850 MB. CONTINUED COOLING OF THE COLUMN
LEADS TO INCREASING INSTABILITY AGAIN LATER TUESDAY AFTERNOON INTO
WEDNESDAY...THOUGH GUIDANCE GENERALLY MAINTAINS THE STRONGEST LOW
LEVEL CONVERGENCE JUST EAST OF THE CWA FROM NORTHEAST PORTER INTO
LA PORTE AND BERRIEN COUNTIES. RISING MID-LEVEL HEIGHTS AND
SUBSIDENCE ALONG WITH WEAKENING FLOW/CONVERGENCE AND BACKING OF
LOW LEVEL WINDS TO A MORE WESTERLY DIRECTION SHOULD GRADUALLY
REDUCE ACCUMULATIONS WEDNESDAY AFTERNOON AND NIGHT. SEVERAL INCHES
OF ACCUMULATION DO APPEAR POSSIBLE BY THAT TIME MAINLY ACROSS
NORTHEAST PORTER COUNTY. LOW LEVEL WINDS EVENTUALLY BACK MORE
WESTERLY BY THURSDAY ENDING LIGHTER LAKE SNOW SHOWERS IN PORTER
COUNTY.

UPPER TROUGH SLOWLY DRIFTS EAST THROUGH THE END OF THE WEEK AND
NEXT WEEKEND...WITH SURFACE HIGH PRESSURE EVENTUALLY MOVING ACROSS
THE WESTERN LAKES LATE SATURDAY INTO SUNDAY. ARCTIC AIR WILL KEEP
TEMPERATURES BELOW NORMAL DURING THE TUESDAY-SUNDAY PERIOD HOWEVER
AS WE REMAIN IN NORTHWEST FLOW ALOFT BETWEEN EAST COAST TROUGH AND
DOWNSTREAM OF RIDGING ACROSS THE CENTRAL CONUS. TEMPS THROUGH THE
PERIOD GENERALLY EXPECTED TO SEE LOWS IN THE SINGLE DIGITS AND
HIGHS IN THE TEENS TO LOW 20S. SUB-ZERO MORNING WIND CHILLS ARE
EXPECTED PARTICULARLY WEDNESDAY AND THURSDAY MORNINGS.

RATZER

&&

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

LIGHT REFLECTIVITIES CAN BE SEEN ON RADAR ALONG THE WESTERN IL AND
WI STATE LINES...WITH ONLY A FEW SITES REPORTING FLURRIES. THE
MORE WIDESPREAD SNOW IS ACROSS SOUTHEAST MN AND CENTRAL IOWA
ASSOCIATED WITH THE LOW/S VORTICITY STREAMER. GUIDANCE SUGGESTS
THAT IT WILL TAKE SOME TIME FOR THE COLUMN TO SATURATE AND SNOW TO
REACH THE GROUND...SO PUSHED BACK THE SNOW START TIME BY AN HOUR.
SNOW COULD TAKE EVEN LONGER TO START FALLING ESPECIALLY AT THE
EASTERN TERMINALS. SOME GUIDANCE MEMBERS SUGGEST SNOW WILL NOT
REACH THE GROUND UNTIL THIS EVENING...HOWEVER HAVE LOW CONFIDENCE
IN THOSE MEMBERS. FORCING FROM THE LOW WILL AT LEAST PRODUCE
FLURRIES THROUGHOUT THE DAY. A PERIOD OF MODERATE SNOW AND IFR
VSBY IS PSBL EARLY THIS MORNING. MVFR CIGS WILL ACCOMPANY THE SNOW
SHOWERS.

SOUTHWEST WINDS BECOME GUSTY NORTHWEST WINDS THIS EVENING.

HAVE HIGH CONFIDENCE IN MVFR CIGS AND SNOW FALLING FROM EARLY THIS
MORNING ONWARD. HAVE MEDIUM CONFIDENCE IN PERIODS OF MODERATE SNOW
AND IFR VSBY...BUT LOW CONFIDENCE IN EXACT TIMING. ALSO HAVE LOW
CONFIDENCE IN THE EXACT START TIME OF THE SNOW.

JEE

&&

.MARINE...
301 AM CST

ISSUED A SMALL CRAFT ADVISORY FOR TONIGHT THROUGH EARLY WEDNESDAY
NIGHT FOR THE IL NEARSHORE WATERS...AND INTO THURSDAY MORNING FOR
THE INDIANA NEARSHORE WATERS DUE TO HAZARDOUS WINDS AND WAVES.

A WEAK LOW OVER THE U.P. OF MI WILL DRIFT OVER THE LAKE TODAY AND
REACH THE EASTERN GREAT LAKES LATE THIS AFTERNOON.  WINDS WILL
BECOME NORTH BEHIND THE LOW AND INCREASE TO 30 KT.  HIGH PRESSURE
SINKS SOUTH OVER THE PLAINS TUESDAY NIGHT AND MOVES OVER THE
MISSISSIPPI VALLEY WEDNESDAY EVENING. THE TIGHTENED GRADIENT BETWEEN
THE HIGH AND LOW WILL RESULT IN AN EXTENDED PERIOD OF 30 KT WINDS
THROUGH WEDNESDAY NIGHT. LOW END GALE FORCE GUSTS ARE POSSIBLE LATE
TUESDAY AFTERNOON OVER THE NORTHERN HALF OF THE LAKE...AND THE GALES
REACH THE SOUTHERN END OF THE LAKE TUESDAY NIGHT.  HOWEVER...DO NOT
HAVE ENOUGH CONFIDENCE IN GALES TO ISSUE A GALE WATCH FOR TUESDAY
NIGHT INTO WEDNESDAY.  WINDS DIMINISH AS THE HIGH MOVES OVER THE
LAKE WEDNESDAY NIGHT INTO THURSDAY.

ANOTHER WEAK LOW PASSES OVER THE EASTERN GREAT LAKES THURSDAY NIGHT
AND FRIDAY WHILE HIGH PRESSURE MOVES OVER MINNESOTA FRIDAY NIGHT AND
SATURDAY.  NORTHWEST TO NORTH WINDS INCREASE TO 30 KT LATE IN THE
WEEK.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...MIDNIGHT TUESDAY
     TO 9 AM THURSDAY.

     SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...MIDNIGHT TUESDAY
     TO 3 AM THURSDAY.

&&

$$

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