Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 072339
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
539 PM CST SUN FEB 7 2016

.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 00Z TAFS...

COLD FRONT WILL MOVE ACROSS THE TERMINALS DURING THE PRE-DAWN
HOURS MONDAY. BEHIND THE FRONT LOOK FOR MVFR CIGS...FLURRIES...AND
SCTD -SHSN. ATMOSPHERE WILL BE QUITE UNSTABLE AND FORCING VERY
WEAK...SO VERY DIFFICULT TRYING TO PINPOINT TIMING OF SNOW SHOWER
ACTIVITY. WHEN SNOW SHOWERS DO OCCUR THE MORE INTENSE ONES COULD
KNOCK VSBY DOWN TO 1-2SM FOR BRIEF PERIODS OF TIME. BEST ATTEMPT
AT TIMING THE BETTER SNOW SHOWER CHANCES AT THIS DISTANCE WOULD BE
PRE-DAWN THROUGH MID MORNING...THEN AGAIN MONDAY EVENING. IT IS
WORTH NOTING THAT HEALTHY SNOW SHOWERS COULD OCCUR AT JUST ABOUT
ANY POINT IN TIME LATE TONIGHT THROUGH MONDAY NIGHT.

IZZI

&&

.MARINE...
309 PM CST

MAIN MARINE CONCERN IS WITH A PROLONGED PERIOD OF STRONG NORTH-
NORTHWEST WINDS DEVELOPING LATE MONDAY AND CONTINUING INTO MID-
WEEK.

LOW PRESSURE WAS OVER FAR WESTERN LAKE SUPERIOR THIS
AFTERNOON...AND THIS LOW WILL MOVE SOUTHEAST ACROSS LAKE ST CLAIR
BY EARLY MONDAY. A COLD FRONT WILL TRAIL THE LOW ACROSS LAKE
MICHIGAN TONIGHT...WITH A STRONGER SECONDARY COLD FRONT PUSHING
SOUTHEAST ACROSS THE LAKE ON MONDAY. COLD ARCTIC AIR WILL SPILL IN
FROM THE NORTHWEST BEHIND THIS SECOND COLD FRONT...WHICH COMBINED
WITH A STRONG PRESSURE GRADIENT WILL PRODUCE NORTH-NORTWHEST WINDS
AROUND 30 KT FROM MONDAY EVENING INTO WEDNESDAY AS THE LOWS
EASTWARD PROGRESSION IS SLOWED BY ANOTHER STORM OFF THE ATLANTIC
COAST. MARGINAL GALE FORCE GUSTS MAY OCCASIONALLY OCCUR DURING
THIS PERIOD...WITH FREEZING SPRAY LIKELY. HIGH PRESSURE RIDGING
WILL EVENTUALLY SPREAD SOUTHEAST FROM CENTRAL CANADA INTO THE
MISSISSIPPI VALLEY THURSDAY AND THURSDAY NIGHT...ALLOWING WINDS TO
EASE.

RATZER

&&

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

$$

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