000
FXUS63 KLOT 080603
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1203 AM CST SUN DEC 8 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 06Z...

* LAKE EFFECT CLOUDINESS AND POSSIBLE -SHSN EARLY THIS MORNING
* SNOW DEVELOPING SUNDAY AFTERNOON AND CONTINUING INTO SUNDAY
  EVENING
* HIGHEST PROBS OF IFR SNOW APPEAR TO BE 22/23Z SUN THROUGH 02/03Z
  MONDAY

IZZI

//DISCUSSION...UPDATED 06Z...

HIGH END MVFR CIGS COULD SPREAD WEST OFF OF LAKE MICHIGAN AND INTO
ORD/MDW PRIOR TO SUNRISE AS GRADIENT FLOW GRADUALLY BECOMES MORE
EASTERLY. SOME POTENTIAL DOES EXIST FOR -SHSN...BUT BETTER THREAT
LOOKS TO REMAIN EAST OF ORD/MDW SO HAVE MAINTAINED THE DRY TAF AND
WILL CLOSELY MONITOR TRENDS THE NEXT FEW HOURS. VIRGA SHOULD BEGIN
OVER THE REGION LATER THIS MORNING AS SNOW ATTEMPTS TO DEVELOP BUT
SUBLIMATES BEFORE REACHING THE GROUND. HAVE OPTED TO BEGIN
FLURRIES IN THE TAFS MIDDAY...BUT STEADIER/HEAVIER SNOW EXPECTED
TO HOLD OFF UNTIL LATE AFTERNOON INTO EARLY EVENING. STILL APPEARS
AS THOUGH CHICAGO TERMINALS WILL BE ON THE SOUTH EDGE OF THE
HEAVIER SNOWFALL...BUT A FEW HOUR LONG PERIOD WITH AT LEAST
PERIODIC IFR VSBY IN -SN SEEMS LIKE A DECENT BET EARLY SUNDAY
EVENING. SNOW SHOULD TAPER OFF TO FLURRIES OR PERHAPS A VERY
FREEZING SNIZZLE FOR BRIEF TIME BEFORE ENDING MID-LATE EVENING
SUNDAY. WINDS THROUGH SUNDAY MORNING WILL BE RATHER SQUIRRELLY AT
THE CHICAGO METRO TERMINALS AS MESOLOW OVER SOUTHERN LAKE MICHIGAN
DEVELOPS AND TRACKS NORTHWEST ACROSS FAR SOUTHERN PORTIONS OF THE
LAKE. MESOLOW SHOULD DISSIPATE BY MID-LATE MORNING SUNDAY ALLOWING
FOR SYNOPTIC E TO SE WINDS TO LOCK IN AROUND 10-15KT THROUGH
SUNDAY AFTERNOON.

IZZI


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* LOW CONFIDENCE REGARDING POTENTIAL MVFR CIGS EARLY THIS MORNING
* MEDIUM CONFIDENCE ANY LAKE EFFECT -SHSN WILL REMAIN EAST OF
  TERMINALS THIS MORNING
* LOW CONFIDENCE IN FLURRY/VERY LIGHT SNOW POTENTIAL MIDDAY TODAY
  THRU MID AFTERNOON
* MEDIUM-HIGH CONFIDENCE IN MOST SIGNIFICANT SNOW OCCURRING VERY
  LATE THIS AFTERNOON INTO EARLY SUNDAY EVENING.
* MEDIUM-HIGH CONFIDENCE IN AT LEAST PERIODIC IFR CIGS DURING
  HEIGHT SNOW
* LOW CONFIDENCE IN WIND DIRECTION THROUGH MID MORNING...THEN HIGH
  CONFIDENCE IN WIND FORECAST THIS AFTERNOON INTO TONIGHT

IZZI

//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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