Area Forecast Discussion
Issued by NWS Chicago, IL

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116
FXUS63 KLOT 090534
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1134 PM CST MON FEB 8 2016

.UPDATE...

742 PM CST

FOR EVENING UPDATE...

MINOR ADJUSTMENTS TO FORECAST THIS EVENING...TO REFINE TRANSITION
TO HIGHER POPS WITH SNOW SHOWERS SPREADING SOUTH ACROSS THE
NORTHEAST PORTION OF THE FORECAST AREA. ALSO...WITH SOME OF THE
HIGHER REFLECTIVITY RETURNS AND SOME BRIEF 1/2-3/4SM VISIBILITIES
REPORTED HAVE ALSO BUMPED UP QPF/SNOW AMOUNTS A BIT CLOSER TO AN
INCH THROUGH MIDNIGHT ESPECIALLY ACROSS PARTS OF LAKE/EASTERN
MCHENRY AND COOK COUNTIES THIS EVENING.

SNOW SHOWERS HAVE INCREASED IN COVERAGE ACROSS NORTHEAST IL...
SOUTHERN LAKE MICHIGAN AND FAR NORTHWEST INDIANA THIS EVENING IN
RESPONSE TO A SHEARED MID-LEVEL VORT MOVING SOUTH ACROSS LAKE
MICHIGAN. THIS IS EXPECTED TO CONTINUE OVERNIGHT...WITH FORECAST
SOUNDINGS DEPICTING COOLING OF THE SATURATED LAYER ESPECIALLY
BELOW 700 MB AND DEVELOPMENT OF A DEEP DENDRITIC GROWTH ZONE IN
EXCESS OF 200 MB. WHILE FORCING IS NOT TERRIBLY STRONG...THE
THERMODYNAMIC PROFILE SHOULD PRODUCE LIGHT SNOW FAIRLY EASILY.
LIGHT SNOW WAS ALSO SPREADING SOUTHEAST ACROSS THE UPPER
MISSISSIPPI VALLEY IN ASSOCIATION WITH A MID-LEVEL SPEED MAX
DROPPING INTO THE WESTERN PERIPHERY OF THE LARGE SCALE TROUGH OVER
EASTERN NORTH AMERICA...AND LIGHT SNOW SHOULD SPREAD INTO WESTERN
PARTS OF THE FA OVERNIGHT. THUS DESPITE RELATIVELY LOW QPF VALUES
ON THE ORDER OF A FEW HUNDREDTHS OF AN INCH IN ANY GIVEN PERIOD
HAVE CARRIED RELATIVELY HIGH POPS FOR LIGHT SNOW/SNOW SHOWERS INTO
TUESDAY.

OTHERWISE...FORECAST IN GOOD SHAPE WITH ONLY MINOR TWEAKS MADE PER
HOURLY OBS TRENDS EARLY THIS EVENING.

RATZER

&&

.SHORT TERM...
250 PM CST

TONIGHT THROUGH WEDNESDAY...

SNOW/SNOW SHOWER TRENDS AND ACCUMULATIONS...INDIANA LAKE EFFECT
SNOW...AND COLDER TEMPERATURES ARE THE MAIN FORECAST CONCERNS
THROUGH MID-WEEK.

DEEP CLOSED UPPER LOW WAS OVER THE REGION THIS AFTERNOON WITHIN
LARGE EASTERN NORTH AMERICAN UPPER TROUGH. SEVERAL VORTICITY LOBES
WERE ROTATING ABOUT THIS FEATURE...AND ALONG STEEP LOW LEVEL LAPSE
RATES DUE TO STRONG COLD ADVECTION WILL BE THE MAIN MODULATING
FACTORS IN COVERAGE AND INTENSITY OF LIGHT SNOW/SNOW SHOWERS OVER
THE NEXT COUPLE OF DAYS. ONE SUCH WAVE WAS NOTED IN WATER VAPOR
IMAGERY CURRENTLY MOVING SOUTHEAST ACROSS CENTRAL IL...WITH A
CORRESPONDING AREA OF LIGHT SNOW ACROSS SOUTHERN PARTS OF THE
FORECAST AREA THIS AFTERNOON. ANOTHER AXIS OF SHEARED VORTICITY
WAS OVER LAKE MICHIGAN AND EASTERN WISCONSIN WITH ANOTHER BATCH OF
SNOW SHOWERS. FLURRIES WERE LIKELY OCCURRING IN MOST OTHER AREAS
IN SATURATED COLD AIR MASS. LIGHT SNOW WILL LIKELY CONTINUE TO
AFFECT SOUTHERN PARTS OF THE FA FOR THE REMAINDER OF THE AFTERNOON
AS THE CENTRAL IL WAVE CONTINUES TO MOVE OFF TO THE
SOUTHEAST...WITH LIGHT SNOW/SNOW SHOWERS EXPECTED TO INCREASE FROM
THE NORTH AGAIN THIS EVENING/OVERNIGHT AS SHEARED VORT TO OUR
NORTH WRAPS SOUTH-SOUTHEAST AROUND UPPER LOW CENTER WHICH DEVELOPS
ACROSS NORTHEAST INDIANA/NORTHWEST OHIO. THIS UPPER LOW WILL
SLOWLY LIFT OUT TO THE EAST-NORTHEAST ON TUESDAY...WITH ANOTHER
SHORT WAVE DROPPING INTO THE TRAILING PORTION OF THE UPPER TROUGH
AND ACROSS ESPECIALLY EASTERN PARTS OF THE FORECAST AREA TUESDAY
NIGHT. MODEL QPF AMOUNTS REMAIN GENERALLY LIGHT...WITH NON-UNIFORM
COVERAGE DUE TO EXPECTED OFTEN SHOWERY NATURE OF PRECIP.
OVERALL...IT STILL APPEARS THAT WIDESPREAD SNOWFALL OF AN INCH TO
PERHAPS 2-3 INCHES ARE LIKELY BY THE TIME MEASURABLE PRECIP SHIFTS
EAST OF THE AREA LATE TUESDAY NIGHT...GENERALLY LOWEST FAR WEST
AND HIGHER ACROSS THE EAST PARTS OF THE CWA.

WITHIN THIS WEATHER REGIME...CONTINUED COLD ADVECTION WILL RESULT
IN LAKE-EFFECT SNOW SHOWERS DEVELOPING ACROSS PARTS OF NORTHWEST
INDIANA TONIGHT...AND MOST LIKELY CONTINUING INTO THURSDAY MORNING
AT LEAST IN LIGHTER FASHION UNTIL LOW LEVEL WINDS BACK TO THE
WEST. THERMODYNAMICS BECOME FAVORABLE FOR LAKE SNOW SHOWERS THIS
EVENING... AS LAKE SURFACE TO 850 MB DELTA-T INCREASES TO AROUND
20 C/KM BY TUESDAY MORNING. COMBINED WITH WITH DEEPER FORCING
PROVIDED BY AFOREMENTIONED VORT LOBE...THE POTENTIAL FOR HEAVIER
SNOW SHOWERS LOOKS TO BE FROM LATER TONIGHT/EARLY TUESDAY...AND
AGAIN TUESDAY NIGHT INTO WEDNESDAY. LOW LEVEL CONVERGENCE AND
FETCH CONTINUE TO FAVOR MAINLY NORTHEAST PORTIONS OF PORTER
COUNTY...WITH THE AXIS OF HEAVIEST SNOW EXPECTED TO BE ACROSS LA
PORTE AND BERRIEN COUNTIES TO THE EAST OF THE CWA. NORTHEAST
PORTER COUNTY DOES HAVE THE POTENTIAL FOR SEVERAL INCHES OF
ACCUMULATION DURING THE PERIOD...DEPENDING UPON LOCATION/INTENSITY
OF BANDING. LONG DURATION OF LIGHT/OCCASIONALLY MODERATE SNOW
SHOWERS TOTALING PERHAPS 4+ INCHES DOES NOT APPEAR TO WARRANT
HEADLINES AT THIS POINT...THOUGH WILL CONTINUE TO MENTION BOTH THE
LAKE EFFECT AND THE POTENTIAL FOR TUESDAY MORNING RUSH HOUR IMPACT
IN AN SPS.

TEMPERATURES VERY SLOWLY FALLING INTO THE LOW-MID 20S THIS
AFTERNOON...AND WILL CONTINUE TO FALL OFF INTO THE TEENS IN MOST
AREAS TONIGHT. LITTLE RECOVERY IS EXPECTED TUESDAY AS COLD
ADVECTION PERSISTS...WITH COLD AIR LINGERING THROUGH MID-WEEK WITH
OVERNIGHT LOWS IN THE SINGLE DIGIT AND HIGHS IN THE TEENS BY
WEDNESDAY.

RATZER

&&

.LONG TERM...
300 PM CST

WEDNESDAY NIGHT THROUGH MONDAY...

BELOW NORMAL TEMPERATURES TO CONTINUE INTO THE WEEKEND AS LARGE
SCALE UPPER TROUGH OVER EASTERN NORTH AMERICA SHIFTS SLOWLY
EAST...BUT IS REINFORCED OVER THE WEEKEND BY ANOTHER CLOSED LOW
WHICH DEVELOPS OVER NEW ENGLAND/QUEBEC. EVENTUALLY THE TROUGH AXIS
SHIFTS FAR ENOUGH TO THE EAST FOR SOME MODERATION SUNDAY NIGHT
INTO MONDAY. STRONG NORTHWESTERLY FLOW ALOFT DOWNSTREAM OF
WESTERN/CENTRAL CONUS RIDGING WILL LIKELY SEE SEVERAL MINOR SHORT
WAVES RIPPLING THROUGH THE FLOW. DIFFERENCES REMAIN IN LONGER
RANGE GUIDANCE WITH LOCATION OF LOW LEVEL BAROCLINIC ZONE TO OUR
SOUTHWEST...THOUGH THE 00Z/12Z ECMWF HAVE MAINTAINED CONTINUITY IN
A MINOR DISTURBANCE TRACKING ACROSS THE MIDWEST FRIDAY WITH THE
POTENTIAL FOR SOME LIGHT SNOW. DEPENDING ON HOW QUICKLY HIGH
PRESSURE EXITS IN THE DAY 6-7 PERIOD THERE IS ALSO A CHANCE OF
LIGHT SNOW AS WARM ADVECTION DEVELOPS MONDAY. UNTIL THEN... COLD
IS THE RULE WITH OVERNIGHT LOWS IN THE SINGLE DIGITS (AND POSSIBLE
AROUND ZERO OR BELOW) AND HIGHS IN THE TEENS TO LOW 20S BEFORE
SLIGHT MODERATION TOWARD THE END OF THE FORECAST.

RATZER

&&

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

LARGE UPPER LOW CENTERED WILL CONTINUE TO INFLUENCE AVIATION
WEATHER THROUGH TUESDAY EVENING. MAIN FORECAST CONCERN AND FOCUS
CONTINUES TO BE TIMING THE BEST WINDOW OF LOWER VSBY AND POTENTIAL
ACCUMULATIONS WITH OCCASIONAL SNOW SHOWERS. GYY AND MDW LOOKS TO
BE IN FAVORED CORRIDOR THROUGH THE EARLY MORNING HOURS/09-10Z...FOR
TEMPORARY IFR AND EVEN LIFR VSBY AT TIMES. FOR ORD/DPA...GENERALLY
MVFR VSBY IN SNOW SHOWERS DURING THIS TIME...ALTHOUGH CANNOT RULE
OUT IFR VSBY AT TIMES. SHORT RANGE GUIDANCE IS PICKING UP ON A
SUBTLE WAVE THAT COULD RE-ENHANCE SNOWFALL RATES TOWARD DAYBREAK.
AGAIN...GYY SHOULD BE MOST FAVORED...EVEN WITH SOME ADDED LAKE
ENHANCEMENT POTENTIAL. RFD AREA COULD FINALLY GET INTO SOME SNOW
SHOWER ACTIVITY AFTER 07Z...AND POTENTIALLY IFR VSBY AT TIMES.

THE REST OF THE DAY TUESDAY INTO TUESDAY EVENING...STEEP LOW
LEVEL LAPSE RATES AND WEAK LIFT COULD YIELD ADDITIONAL SNOW
SHOWERS AT ANY TIME. CONTINUED TO MAINTAIN THE PROB30 GROUP
DURING THE AFTERNOON FOR IFR VSBY FROM THE PREVIOUS TAF FOR
ORD/MDW/GYY/DPA GIVEN LOWER CONFIDENCE. CONFIDENCE IS ALSO LOWER
IN CIGS...AS THERE ARE POCKETS OF VFR OVER THE AREA. GUIDANCE IS
SHOWING A SIGNAL FOR INCREASED LOW-MID MVFR CIG POTENTIAL DURING
THE AFTERNOON AND POSSIBLY INTO THE EVENING. WINDS WILL BE WEST-
NORTHWEST...WITH GUSTS IN THE 20-25 KT RANGE THIS MORNING THROUGH
TONIGHT.

RC

&&

.MARINE...
223 PM CST

LOW PRESSURE CENTERED NEAR LAKE HURON/LAKE ONTARIO EARLY THIS
AFTERNOON WILL VERY GRADUALLY MEANDER TOWARDS LAKE ONTARIO BY
TOMORROW NIGHT WITH MODERATELY STRONG NORTH TO NORTHWEST WINDS
EXPECTED TO BE IN PLACE ACROSS LAKE MICHIGAN. WINDS ARE EXPECTED
TO REMAIN AROUND 30 KT THIS EVENING THROUGH WEDNESDAY
NIGHT...WITH A FEW GALE FORCE GUSTS POSSIBLE MAINLY TUESDAY INTO
TUESDAY NIGHT. FREEZING SPRAY IS ALSO A CONCERN TUESDAY THROUGH
WEDNESDAY EVENING. HIGH PRESSURE WILL BUILD FROM THE UPPER
MIDWEST WEDNESDAY NIGHT INTO THE OHIO VALLEY THURSDAY NIGHT
RESULTING A MODEST TAPERING OF THE WINDS AS THEY BACK TO THE WEST.
ANOTHER COLD FRONT WILL SWEEP ACROSS LAKE MICHIGAN FRIDAY TURNING
WINDS BACK TO NORTHERLY AND PICKING UP ONCE AGAIN. THIS FRONT WILL
USHER IN A BITTERLY COLD ARCTIC AIRMASS WHICH COULD RESULT IN
ADDITIONAL GALES...PARTICULARLY FRIDAY NIGHT...AND FREEZING SPRAY
ACROSS LAKE MICHIGAN.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 9 AM THURSDAY.

     SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL 3 AM THURSDAY.

&&

$$

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