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000
FXUS63 KLOT 080032
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
632 PM CST SAT DEC 7 2013

.DISCUSSION...
306 PM CST

SHORT TERM...TONIGHT THROUGH SUNDAY NIGHT...

PRIMARY FORECAST CONCERN IS WITH LAKE EFFECT SNOW CHANCES LATE
TONIGHT INTO SUNDAY...FOLLOWED BY MINOR SNOW EVENT SUNDAY AFTERNOON
INTO SUNDAY EVENING. ADDITIONAL CONCERNS LIE WITH HOW QUICK
TEMPERATURES WILL DROP THIS EVENING IN ARCTIC AIR MASS AND ALSO
TEMPERATURES MONDAY NIGHT WITH NEXT ARCTIC SHOT.

THIS EVENING AND TONIGHT...ARCTIC HIGH PRESSURE IS CENTERED ACROSS
UPPER MS AND MO RIVER VALLEYS THIS AFTERNOON...WITH VERY COLD AND
DRY AIR MASS IN PLACE ACROSS THE REGION. AS THE CENTER OF THE HIGH
SLIDES SOUTHEAST EARLY THIS EVENING...DEWPOINT READINGS RANGING FROM
0 TO 10 WILL ALLOW TEMPERATURES TO FALL RAPIDLY AFTER DARK. QUESTION
IS HOW FAST AND HOW FAR BEFORE INCREASING CLOUD COVER ARRIVES FROM
THE WEST AND SOUTHWEST. THERE IS A WEAK UPPER WAVE SPREADING SOME
CIRRUS INTO THE NW CWA INTO EARLY EVE...BUT OTHERWISE CLEAR TO
PARTLY CLOUDY SKIES WILL BE THE RULE INITIALLY. BASICALLY LEFT
PREVIOUS FORECAST MINS UNCHANGED AND AM BANKING ON THE GOOD
RADIATIONAL COOLING THIS EVENING TO ATTAIN FORECAST LOWS ROUGHLY
FROM 5 TO 10 DEGREES AWAY FROM CHICAGO...AND LOW TO MID TEENS IN THE
CITY. TEMPERATURES SHOULD SLOWLY RISE OVERNIGHT IN RESPONSE TO
LOWERING AND THICKENING CLOUDS AHEAD OF SUNDAY`S WEAK WAVE.

AS MENTIONED ABOVE...OTHER CONCERN TONIGHT IS WITH LAKE EFFECT SNOW
AS HIGHER RES MODELS SHOW DECENT LOWER LEVEL LIFT AND CONVERGENCE AS
WELL AS DECENT LAKE INDUCED INSTABILITY...ESPECIALLY AFTER 6Z. FLOW
IS EXPECTED TO VEER FROM NNE TO EAST OR EVEN EAST SOUTHEAST BY
DAYBREAK. NEGATING FACTOR WILL BE LOWERING INVERSION HEIGHTS WITH
APPROACH OF SURFACE HIGH. INTRODUCED SLIGHT POPS LATE EVENING FOR
EXTREME NW IN AND PORTION OF IL SHORE THEN BUMPED UP POPS SLIGHTLY
TO HIGHER END CHANCE FROM DOWNTOWN CHICAGO AND NORTH LATER IN THE
OVERNIGHT INTO EARLY SUNDAY. DUE TO DECENT FETCH AND SIGNAL FOR
CONVERGENCE AND LIFT. ASSUMING FAIRLY UNFAVORABLE INVERSION HEIGHTS
DO NOT SUPPRESS LAKE EFFECT SNOW SHOWERS TOO MUCH...SOME PLACES IN
FAR NORTHEAST COOK AND EASTERN LAKE IL COULD SEE UP TO 1" FROM LAKE
EFFECT BY MID MORNING SUNDAY BEFORE FLOW TURNS MORE SOUTHEAST AND
AND ACTIVITY LIFTS NORTH TO THE WISCONSIN SHORE.

SUNDAY...WARM ADVECTION AND ISENTROPIC ASCENT WILL GRADUALLY RESULT
IN SATURATION OF DRY COLUMN LATE SUNDAY MORNING INTO THE EARLY
AFTERNOON AHEAD OF DECENT MID LEVEL WAVE AND WEAK SURFACE LOW. THE
BEST LARGE SCALE FORCING DOES ONLY GIVE THE NORTHERN CWA A GLANCING
BLOW...BUT IT APPEARS THERE WILL BE ADEQUATE ASCENT FOR A HIGH
POP/LOW QPF TYPE OF EVENT. AIR MASS WITH HIGHS IN MID TO POSSIBLY
UPPER 20S WILL CERTAINLY BE COLD ENOUGH TO SUPPORT SNOW FOR ALL
AREAS...AND SOUNDINGS SHOW ENOUGH ASCENT AND SATURATION THROUGH
DENDRITIC GROWTH ZONE FOR MOST FAVORED AREAS NORTH OF I-88 TO SEE A
PERIOD OF LIGHT TO MODERATE SNOW MID TO LATE AFTERNOON.
HOWEVER...FARTHER SOUTH...ESPECIALLY DOWN IN OUR CENTRAL IL COUNTIES
AND NW IN SOUTH OF THE KANKAKEE RIVER...THERE IS BARELY ENOUGH
SATURATION TO HAVE CRYSTAL GROWTH...ESPECIALLY BY LATE AFTERNOON.
BUMPED UP ALL AREAS TO AT LEAST LIKELY POPS FOR THE HEART OF THE
EVENT...21Z TO 03Z...BUT BY LATE AFTERNOON INTO THE
EVENING...SOUTHERN AREAS COULD SEE AT LEAST A MIX WITH FREEZING
DRIZZLE PERHAPS TRANSITIONING TO ALL FREEZING DRIZZLE BEFORE THE
PRECIP ENDS. GIVEN SOUNDINGS EVEN FARTHER NORTH SHOWING LIKELY LOSS
OF CRYSTAL GROWTH TOMORROW EVENING...THE POTENTIAL EXISTS FOR ALL
AREAS TO TRANSITION TO A PERIOD OF FREEZING DRIZZLE BEFORE PRECIP
ENDS TOMORROW NIGHT. WHEREVER FREEZING DRIZZLE BECOMES PERSISTENT
ENOUGH...CONCERNED THAT SOME AREAS COULD SEE A LIGHT GLAZE ON TOP OF
WHATEVER FALLS EARLIER.

OVERALL...MAINTAINED SIMILAR SNOWFALL FORECAST TO PREVIOUS
SHIFT...WITH 1 TO 3 INCHES NEAR WISCONSIN BORDER...AROUND AN INCH TO
A BIT OVER IN THE I-88 CORRIDOR...1/2 INCH DOWN TO I-80 AND POSSIBLY
A DUSTING TO A FEW TENTHS AT BEST FOR THE SOUTHERN CWA. INCLUDING
LAKE EFFECT TOTALS AND SYNOPTIC SNOW...ITS POSSIBLE THAT PARTS OF
NORTHEAST LAKE IL COULD SEE A BIT OVER 3 INCHES OF SNOW TOTAL. NOT A
BIG EVENT BY ANY MEANS...BUT ANTECEDENT COLD TEMPERATURES WILL
LIKELY ALLOW SNOW TO STICK ON ROAD SURFACES AND COULD LEAD TO
SLIPPERY TRAVEL CONDITIONS.

LONG TERM...MONDAY THROUGH SATURDAY...

BIG STORY WILL BE CONTINUED UNSEASONABLY COLD CONDITIONS. MEDIUM
RANGE GUIDANCE DOES HINT AT SOME MODERATION TOWARD THE END OF THE
WORK WEEK...BUT REINFORCING SHOT OF COLD AIR COULD ARRIVE QUICK
ENOUGH TO KEEP SOME LOCATIONS FROM EVEN CRACKING THE FREEZING MARK
THROUGH THE PERIOD.

ON MONDAY...BEHIND COLD FRONT TRAILING FROM WEAK SURFACE LOW LIFTING
INTO NORTHERN/CENTRAL LAKES...A LOBE OF THE POLAR VORTEX WILL
SHARPEN UPPER TROUGH...WITH GLANCING SHOT OF BITTERLY COLD ARCTIC
AIR PUSHING IN DURING THE DAY. STRONG CAA AND H85 TEMPS APPROACHING
MINUS UPPER TEENS BY EARLY EVENING WILL LIKELY RESULT IN MAINLY
STEADY TEMPERATURES DURING THE DAY...QUICKLY FALLING AFTER DARK AS
SURFACE RIDGE AXIS EXTENDS EAST. IT APPEARS THAT THERE WILL BE
ENOUGH CLEARING OVERNIGHT THAT TEMPERATURES WILL PLUMMET INTO THE
SINGLE DIGITS MOST AREAS...POSSIBLY COLDER IN AREAS THAT HAVE ENOUGH
OF A SNOWPACK...WHICH WOULD LIKELY BE NORTHERN CWA. LOWERED MIN T A
FEW DEGREES FROM PREVIOUS...BUT HAVE SOME CONCERN WITH CLOUD COVER.

"WARM" ADVECTION SOUTHWEST FLOW IN RESPONSE TO CLIPPER PASSING
THROUGH NORTHERN LAKES ON TUESDAY SHOULD ENABLE HIGHS TO REACH
LOW-MID 20S. THEN ANOTHER REINFORCING SHOT OF COLD AIR WILL SEND
TEMPS BACK DOWN TO THE TEENS AND LOW 20S ON WEDNESDAY. GFS AND ECMWF
DO SHOW SOME POTENTIAL FOR LIGHT SNOW OR FLURRIES ALONG SOUTHWARD
ADVANCING ARCTIC FRONT TUES NIGHT...MAINLY NEAR WI BORDER. FOR
NOW...KEPT SLIGHT POPS FROM GUIDANCE BLEND. GUIDANCE VARIES ON HOW
MUCH MODERATION THERE IS BY FRIDAY...WITH GFS SHOWING 850 MB TEMPS
APPROACHING 0C ON FRIDAY AND ECMWF SEVERAL DEGREES COOLER. FRIDAY
OFFERS THE BEST CHANCE FOR SOME AREAS TO REACH OR EVEN EXCEED THE
FREEZING MARK FOR HIGHS...BUT THEN IT APPEARS YET ANOTHER
REINFORCING SHOT OF COLD AIR WILL SEND TEMPERATURES DOWNWARD TO
START NEXT WEEKEND. THIS COULD SET THE STAGE FOR A DECENT LAKE
EFFECT SET-UP OVER LAKE MICHIGAN. SLIGHT AND LOW CHANCE POPS FRI
NIGHT INTO SATURDAY COULD BE A BIT TOO HIGH BASED ON 12Z GUIDANCE
TRENDS WITH EXPANSIVE HIGH PRESSURE SINKING SOUTHWARD.
HOWEVER...DUE TO UNCERTAINTY...MAINTAINED GUIDANCE BLEND POPS FOR
NOW.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* LOW CHANCE FOR LIGHT LAKE EFFECT SNOW SHOWERS OR FLURRIES FROM
  APPROX 09-13Z.

* EAST-SOUTHEAST WINDS INCREASING TO AROUND 12 KT SUNDAY MORNING
  WITH GUSTS TO AROUND 18 KT POSSIBLE.

* SNOW/MVFR EXPECTED TO ARRIVE TOWARD 21Z...WITH MOST INTENSE SNOW
  EXPECTED AROUND 00Z WITH IFR CIGS/VSBY POSSIBLE.

* PERIOD OF FZDZ POSSIBLE AS SNOW WINDS DOWN LATE SUNDAY EVENING.

MDB

//DISCUSSION...UPDATED 00Z...

HIGH PRESSURE IS SPREAD FROM THE NORTHERN PLAINS TO THE OHIO
VALLEY AND WILL BE DRIFTING EAST THROUGH THE PERIOD. WINDS WILL
TURN NORTH THEN EAST THEN EAST-SOUTHEAST INTO SUNDAY MORNING WITH
SPEEDS REMAINING LIGHT OVERNIGHT THEN INCREASING SUNDAY MORNING.
VFR IS EXPECTED THROUGH THE OVERNIGHT BUT LAKE EFFECT PROCESSES
MAY TAKE AIM ON THE CHICAGO/GARY AREA BRINGING IN SCATTERED MVFR
CLOUDS LATE TONIGHT. IN ADDITION...THERE IS AT LEAST A LOW CHANCE
FOR SOME LIGHT LAKE EFFECT SNOW SHOWERS TO AFFECT ORD/MDW LATE
TONIGHT THROUGH DAYBREAK SUNDAY AS WINDS TURN EASTERLY. THIS WOULD
LIKELY SHIFT OFF TO THE NORTH BY MID SUNDAY MORNING. WILL NOT
MENTION SNOW SHOWER POTENTIAL IN THE TAF AS THE CHANCE IS AROUND
30 PERCENT. IN ADDITION...INTENSITY WOULD LIKELY BE LIGHT TO WHERE
ANY VSBY RESTRICTION WOULD BE MINIMAL. HOWEVER...IF SNOW CAN OCCUR
AND CONTINUES LONG ENOUGH THEN A DUSTING IS NOT OUT OF THE
QUESTION.

ATTENTION TURNS TO SUNDAY AFTERNOON AS WEAK LOW PRESSURE MOVES
NORTHEAST TOWARD THE AREA. PRECIPITATION WILL SPREAD NORTH AND
NORTHEAST INTO THE AREA DURING THE AFTERNOON. SNOW WILL SPREAD
NORTHEASTWARD THROUGH THE AFTERNOON WITH THE DRY LOW LEVEL AIR
INITIALLY IN PLACE WORKING TO KEEP MUCH IF ANYTHING FROM REACHING
THE GROUND DURING THE EARLY AFTERNOON. HOWEVER...A BAND OF MORE
INTENSE PRECIP MAY ARRIVE WITH WARMER AIR ALOFT SO THERE IS A
CHANCE FOR SOME LIGHT SNOW TO MAKE IT TO THE GROUND PRIOR TO THE
START TIME IN THE CURRENT TAFS. HAVE BETTER CONFIDENCE THAT
STEADIER SNOW WILL ARRIVE MID/LATE AFTERNOON ACROSS THE ILLINOIS
TERMINALS AND A BIT LATER AT GYY BRINGING REDUCED VSBY AND
LOWERING CIGS. OVERALL SNOW INTENSITY LOOKS TO BE LIGHT SO EXPECT
VSBY GENERALLY IN THE 2-4SM RANGE FOR THE MOST PART WITH A FEW
HOUR PERIOD WHERE 1-2SM VSBY MAY OCCUR...LIKELY AROUND 00Z. SNOW
WILL WIND DOWN DURING THE LATE EVENING BUT FREEZING DRIZZLE MAY
DEVELOP AS THIS OCCURS FROM LATE EVENING THROUGH THE EARLY
OVERNIGHT.

MDB


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* LOW CONFIDENCE IN LAKE EFFECT SNOW POTENTIAL OVERNIGHT BUT
  MEDIUM CONFIDENCE IN TIMING.

* HIGH CONFIDENCE IN WIND FORECAST.

* HIGH CONFIDENCE IN SNOW OCCURRING SUNDAY
  AFTERNOON/EVENING...WITH MEDIUM-HIGH CONFIDENCE IN
  TIMING/CIG/VSBY DETAILS.

* MEDIUM CONFIDENCE IN FZDZ POTENTIAL LATER SUNDAY EVENING/SUNDAY
  NIGHT.

MDB

//OUTLOOK FOR ORD/MDW FOR 12Z MONDAY-12Z SATURDAY...UPDATED 00Z...

MONDAY...IFR/MVFR CIGS IMPROVING TO VFR.

TUESDAY THROUGH FRIDAY...MAINLY VFR WITH MVFR POSSIBLE WEDNESDAY.

MDB

&&

.MARINE...
415 PM CST

WINDS ON LAKE MI WILL CONTINUE TO GRADUALLY SUBSIDE AS A RIDGE OF
HIGH PRESSURE FROM SD TO IA MOVES E ACROSS THE MIDWEST AND INTO
THE GREAT LAKES REGION. DURING SUN WINDS WILL BECOME SE AND WILL
BE INCREASING AS THE LOW CONTINUES TO MOVE E TO THE ST. LAWRENCE
VALLEY AND LOW PRESSURE MOVES E FROM THE SOUTHERN HIGH PLAINS.
THE LOW DEEPENS AS IT TURNS NE SUNDAY...TRAVELING FROM THE LOWER
MO VALLEY TO LAKE SUPERIOR...AND THEN NNE INTO ONTARIO SUN NIGHT.

A COLD FRONT TRAILING FROM THIS LOW WILL PUSH E ACROSS LAKE MI
DURING OVERNIGHT SUN AND MON MORNING. FOLLOWING THE COLD FRONTAL
PASSAGE WINDS ON THE N PORTIONS OF THE LAKE ARE LIKELY TO
INCREASE TO GALE FORCE. THE LOW CONTINUES OFF TO NEAR JAMES BAY
BY MON WHILE ANOTHER HIGH MOVES E ACROSS THE PLAINS. THE HIGH
SETTLES OVER THE SOUTHERN PLAINS MON NIGHT WHILE LOW PRESSURE
MOVES SE TO WESTERN LAKE SUPERIOR.

WITH THE LOW CONTINUING TO THE E ON TUE ANOTHER COLD FRONT WILL
MOVE ACROSS THE LAKE WITH GALES LIKELY ACROSS MUCH OR ALL OF THE
LAKE MON INTO MON NIGHT. THERE IS ALSO LIKELY TO BE FREEZING SPRAY
ON MUCH OF LAKE DURING THE GALES.

A SMALL CRAFT ADVISORY MAY BE NEEDED ON MON WITH BRISK WINDS
BEHIND THE FIRS COLD FRONT.

TRS

&&

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

$$

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