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000
FXUS63 KLOT 072107
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
307 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...

TO BE ISSUED SHORTLY.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* 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.

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 20Z...

* HIGH CONFIDENCE IN WIND DIR/SPEED FORECAST.
* MEDIUM/HIGH CONFIDENCE IN CIGS.
* MEDIUM CONFIDENCE IN PRECIP TIMING SUN AFTN/EVE.

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...
219 AM CST

A VERY PROGRESSIVE PATTERN IS SETTING UP FOR THE LAKE.  A LARGE
ARCTIC HIGH OVER THE NORTHERN PLAINS THIS MORNING WILL BECOME
CENTERED OVER LAKE MICHIGAN SUNDAY MORNING. WEST TO NORTHWEST FLOW
WILL PERSIST AHEAD OF THE HIGH...THEN BRIEFLY TURN EAST LATE
SUNDAY BEHIND THE DEPARTING HIGH AND AHEAD OF AN APPROACHING LOW.
THE LOW SLIDES OVER MICHIGAN SUNDAY NIGHT AND THEN A SECOND LOW
SWINGS THROUGH THE WESTERN GREAT LAKES TUESDAY. BETWEEN THESE TWO
LOWS THE WINDS MAY RELAX BRIEFLY MONDAY NIGHT...BUT GALES ARE
POSSIBLE BOTH MONDAY AFTERNOON BEHIND THE FIRST LOW AND AGAIN
TUESDAY WITH THE SECOND ONE. FREEZING SPRAY IS LIKELY ACROSS MUCH
OF THE LAKE TUESDAY AND TUESDAY NIGHT BEHIND A COLD FRONT
ASSOCIATED WITH THE SECOND LOW.

LENNING

&&

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

$$

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