000
FXUS63 KLOT 072218
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
418 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 22Z...

* WINDS BETWEEN 320-330 DEG WILL SLOWLY TURN 340-360 DEG WITH SPEEDS
  GENERALLY ARND 8KT DECREASING TO 4KT OVERNIGHT.

* POSSIBLE FLURRIES/LIGHT SNOW ARND DAYBREAK SUN...THEN STEADIER
  SNOW REDEVELOPS AFTER 18Z SUN.

* POSSIBLE PATCHY MVFR CIGS ARND DAYBREAK SUN.

MDB/BEACHLER

//DISCUSSION...UPDATED 18Z...

HIGH PRESSURE OVERHEAD WILL CONTINUE TO PROVIDE ABUNDANT SUNSHINE
TODAY...WITH WINDS GENERALLY NORTHWESTERLY SLOWLY TURNING TO THE
NORTHEAST. WITH THE LACK OF ANY GRADIENT OVERHEAD...WINDS WILL
MAINLY BE BETWEEN 3-6KT. THEN HIGH PRESSURE DRIFTS
NORTHEAST...WITH WINDS STEADILY TURNING EASTERLY SUNRISE SUN. WITH
COLD AIR IN PLACE AND OVER LAKE MICHIGAN...AND WINDS TURNING
EASTERLY...THIS MAY AID IN DEVELOPING SOME LIGHT SNOW/FLURRIES
ARND DAYBREAK AND LIFTING ACROSS FAR NORTHEAST ILLINOIS. AT THIS
TIME HAVE A DRY TAF GOING FOR ORD/MDW AT DAYBREAK SUN...BUT JUST
SOME LOWER CIGS. HAVE OPTED TO ONLY GO WITH A SCT DECK AT 2KFT
AGL...BUT IF THE LAKE EFFECT SNOW DEVELOPS STRONGER THIS MAY NEED
TO BE ADJUSTED TO LOWER CIGS AND INTRODUCE LIGHT SNOW.

THEN THE FOCUS REMAINS ON A DEVELOPING WINTER SYSTEM THAT IS
POISED TO LIFT NORTHEAST ACROSS THE WESTERN GREAT LAKES SUNDAY
AFTERNOON/EVENING. GUIDANCE HAS BEEN CONSISTENT WITH TIMING OF
MOISTURE ARRIVING ARND MIDDAY/EARLY AFTERNOON. THIS WILL LIKELY BE
JUST A STEADY LOWERING IN THE CLOUDS...THEN EXPECT FLURRIES TO BE
IN THE AREA BY EARLY AFTERNOON SUN. WINDS WILL BE SLOWLY TURNING
SOUTHEASTERLY WITH ADDTL MOISTURE LIFTING INTO NORTHERN
IL/NORTHWEST IN. EXPECT LIGHT SNOW TO THEN DEVELOP AND INCREASE IN
INTENSITY BY SUNDAY LATE AFTERNOON. VSBYS/CIGS WILL LIKELY LOWER
TO AT LEAST MVFR AND VSBYS COULD DROP TO IFR BY 00Z MON. LATE SUN
NIGHT THERE IS ALSO SOME CONCERN THAT THE PRECIP MAY BEGIN TO
LIGHTEN UP AND WITH THE LACK OF A LIFTING MECHANISM...SNOW MAY
TRANSITION TO FRDZ AFTER MIDNIGHT SUNDAY NIGHT.

BEACHLER

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH CONFIDENCE IN WIND DIR/SPEED FORECAST.

* MEDIUM/HIGH CONFIDENCE IN CIGS.

* LOW CONFIDENCE IN PRECIP POTENTIAL EARLY SUNDAY MORNING AND MEDIUM
  CONFIDENCE IN PRECIP TIMING SUN AFTN/EVE.

MDB/BEACHLER

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

SUNDAY NIGHT...SNOW WITH IFR CONDITIONS DEVELOPING LATE.

MONDAY...MVFR OR LOWER EARLY IMPROVING DURING THE DAY.

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

LENNING

&&

.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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