Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 260312
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
912 PM CST THU DEC 25 2014

.SHORT TERM...
247 PM CST

REMAINDER OF THIS AFTERNOON AND TONIGHT...

CLOUD COVER FINALLY DISSIPATING THIS AFTERNOON AS WEAK UPPER
RIDGING BUILDS ACROSS THE REGION.  WITH HIGH PRESSURE SETTLING INTO
THE SERN CONUS AND LOW PRESSURE DEVELOPING OVER THE CENTRAL
PLAINS...THE COMBINATION OF RISING HEIGHTS AND LOW LEVEL WARM
ADVECTION HAS ALLOWED TEMPERATURES TO RISE INTO THE LOWER TO MIDDLE
40S ACROSS THE REGION.  HOWEVER...THIS RARE APPEARANCE OF THE SUN
WILL BE RATHER BRIEF WITH SUNSET A LITTLE BEFORE 430 PM CST.
ALSO...MID AND HIGH CLOUD MOVING IN ASSOCIATED WITH A JET STREAM
CORE EXTENDING FROM THE SRN ROCKIES THROUGH THE MIDDLE MISSISSIPPI
VALLEY WILL BRING INCREASING CLOUD COVER FOR THE OVERNIGHT HOURS
TONIGHT.  CONDITIONS SHOULD REMAIN RELATIVELY MILD OVERNIGHT WITH
LOWS ONLY IN THE LOWER TO MIDDLE 30S.  NORMAL MIN TEMPS FOR LATE
DECEMBER ARE IN THE UPPER TEENS.

KREIN

&&

.LONG TERM...
247 PM CST

FRIDAY THROUGH THURSDAY...

THE UNSEASONABLY MILD CONDITIONS WILL CONTINUE INTO THE WEEKEND AS
UPPER RIDGING CONTINUES TO BUILD ACROSS THE CNTRL/ERN CONUS WHILE A
PACIFIC SHORTWAVE CONTINUES TO DROP THROUGH THE GREAT BASIN.  THIS
SYSTEM IS EXPECTED TO CLOSE OFF OVER THE FOUR CORNERS REGION FRIDAY
MORNING...KEEPING DEEP LAYER WARM ADVECTION IN PLACE OVER THE REGION
UNDER SWLY FLOW ALOFT AND AT THE SFC.  MAIN FORECAST CONCERN FOR THE
WEEKEND WILL BE TIMING THE PASSAGE OF A WEAK COLD FRONT DROPPING OUT
OF THE UPPER MISSISSIPPI VALLEY.  THIS FRONT AND ASSOCIATED SFC
TROUGH IS THE SFC REFLECTION OF SOME WEAK PHASING OF A NORTHERN
STREAM SHORT WAVE CROSSING THE NRN PLAINS WITH THE SRN STREAM SYSTEM
LIFTING OUT OF THE FOUR CORNERS REGION.  THE SHORTER TERM GUIDANCE
IS RELATIVELY CONSISTENT THROUGH THE WEEKEND WITH THE EVOLUTION OF
THE PATTERN WITH THE NRN AND SRN STREAM SHORTWAVES PHASING INTO A
HIGH AMPLITUDE LONG WAVE TROUGH EXTENDING FROM THE WESTERN GREAT
LAKES THROUGH THE SONORA DESERT.  THE OVERALL UPPER LEVEL PATTERN
WILL TREND TO A MORE PROGRESSIVE PATTERN AS THE CENTER OF THE UPPER
HIGH DROPS SOUTH TOWARD CUBA WITH RIDGING EXITING NORTH ALONG THE
EAST COAST...AND THE UPPER TROUGH TO THE WEST STARTS TO SHEAR OUT
ACROSS THE GREAT LAKES...WITH SOME ENERGY HANGING BACK OVER WEST
TEXAS.  THE SHEARED OUT UPPER TROUGH WILL HELP DRAG A WEAK COLD
FRONT ACROSS THE AREA LATE SATURDAY NIGHT OR EARLY SUNDAY.  DO NOT
EXPECT A SHARP DROP OFF IN TEMPERATURES IN THE WAKE OF THE INITIAL
COLD FRONT. THE MAIN IMPACT OF THE SYSTEM WILL BE ASSOCIATED WITH
INCREASING LOW LEVEL MOISTURE UNDER THE SWLY FLOW AHEAD OF THE
SYSTEM.  THE SFC PATTERN SETTING UP WILL BE CONDUCIVE TO DRAWING
DEEP LAYER MOISTURE FROM THE GULF OF MEXICO AS SFC HIGH PRESSURE
MOVES OFF THE MID ATLANTIC COAST AND AN ELONGATED FRONTAL TROUGH
EXTENDS FROM CNTRL TX TO THE WRN GREAT LAKES BY FRIDAY EVENING.  THE
LARGER SCALE GUIDANCE IS INDICATING AN INCREASE IN DEEP LAYER
MOISTURE...INDICATING PWATS APPROACHING 1 INCH ALONG THE FRONTAL
ZONE.  WITH TEMPERATURES WELL ABOVE FREEZING EXPECTED...HIGHS IN THE
MIDDLE TO UPPER 40S FOR FRIDAY AND LOWER TO MIDDLE 40S ON
SATURDAY...ANY PCPN SEEN WILL BE LIQUID...LIKELY BEGINNING AS SOME
LIGHT SHOWERS WITH AREAS OF FOG AND DRIZZLE...AND THEN MORE STEADY
RAIN FOR LATE FRIDAY NIGHT THROUGH EARLY SATURDAY MORNING.  THERE IS
A CHANCE FOR SOME COOLER AIR TO FILTER INTO THE REGION AS THE PCPN
IS MOVING AWAY FROM THE FAR ERN PORTIONS OF THE CWA...SO SOME SNOW
COULD MIX IN WITH THE RAIN LATE SATURDAY NIGHT.

A GENERAL COOLING TREND WILL BEGIN ON SUNDAY FOLLOWING THE PASSAGE
OF THE COLD FRONT.  HOWEVER...THE MORE SIGNIFICANT AIRMASS CHANGE
WILL HOLD OFF UNTIL MONDAY NIGHT AS AN ARCTIC AIRMASS DROPPING OUT
OF WESTERN CANADA SPREADS ACROSS THE NRN PLAINS AND INTO UPPER
MIDWEST AND WESTERN GREAT LAKES REGION. A COLD FRONT ON THE LEADING
EDGE OF THE ARCTIC AIRMASS COULD BRING A CHANCE FOR SOME LIGHT SNOW
IF THERE IS ENOUGH FORCING TO SQUEEZE OUT WHAT LITTLE MOISTURE WILL
BE AVAILABLE WITH THE SYSTEM.  GIVEN THE LIMITED MOISTURE...WILL
ONLY CARRY SLIGHT CHANCE POPS.  LIKELIHOOD OF ANY ACCUMULATION IS
LOW.  AS THE COLDER AIR FILTERS ACROSS THE AREA...TEMPERATURES
SHOULD DROP INTO THE MIDDLE TEENS TO LOWER 20S FOR TUESDAY MORNING.
THE COLDEST DAY SHOULD BE WEDNESDAY WITH LOWS APPROACHING THE SINGLE
DIGITS OVER THE NWRN PORTIONS OF THE CWA AND MIDDLE TEENS ALONG AND
EAST OF THE I-55 CORRIDOR.  FOR THE REMAINDER OF NEXT
WEEK...CONDITIONS SHOULD BE DRY WITH SEASONABLE TEMPERATURES AS HIGH
PRESSURE PREVAILS ACROSS THE REGION.

KREIN

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* MVFR CIGS RETURNING FRIDAY LATE AFTERNOON OR EVENING.
* GUSTY SOUTH WINDS OVERNIGHT AND WINDS INCREASING WITH ALTITUDE
  TO 45KT AT 020 WITH MINIMAL CONCERN FOR LLWS.

BMD

//DISCUSSION...UPDATED 03Z...
BUMPED WINDS BACK UP AS SUSTAINED WINDS HAVE MAINTAINED AROUND 15
KT AT TIMES WITH SPORADIC GUSTS INTO THE LOW 20 KT RANGE.
INCREASING WINDS OFF THE DECK AND WARM ADVECTION OVERNIGHT MAY
CONTINUE TO PROMOTE GUSTS THROUGH THE OVERNIGHT HOURS. WILL HAVE
TO KEEP AN EYE ON LLWS WITH 45 KT AT 020 SHOULD SURFACE WINDS
DECOUPLE AND TAPER OFF.

PREVIOUS DISCUSSION...
QUIET WEATHER EXPECTED OVERNIGHT WITH SOUTH/SOUTHWEST FLOW AND
VFR CONDITIONS IN PLACE BEHIND A DEPARTING HIGH AND AHEAD OF LOW
PRESSURE OVER THE CENTRAL PLAINS. LINGERING WIND GUSTS THIS
EVENING SHOULD CONTINUE TO TAPER OVERNIGHT...BUT ARE EXPECTED TO
RETURN WITH DAYTIME HEATING DURING THE DAY FRIDAY. GULF MOISTURE
IS PROGGED TO ADVECT NORTH INTO THE MIDWEST...WITH CLOUD COVER
INCREASING THROUGH THE DAY. EXPECT MVFR LEVEL CIGS TO BECOME
BROKEN ANYWHERE FROM MID TO LATE AFTERNOON ON...BUT TIMING REMAINS
UNCERTAIN AT THIS TIME.

BMD

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* HIGH CONFIDENCE IN VFR CONDITIONS THROUGH MIDDAY
FRIDAY...LOW-MEDIUM CONFIDENCE IN RETURN TO MVFR.

BMD

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

SATURDAY...CHC SHOWERS. IFR/MVFR PSBL.
SUNDAY...VFR.
MONDAY...SLT CHC SHOWERS. MVFR PSBL.
TUESDAY THROUGH WEDNESDAY...VFR.

BMD

&&

.MARINE...

230 PM...THE GRADIENT REMAINS FAIRLY STRONG ACROSS LAKE MICHIGAN
THIS AFTERNOON AS DEEP LOW PRESSURE OVER QUEBEC CONTINUES TO LIFT
NORTHEAST AND HIGH PRESSURE OVER THE GULF COAST LIFTS NORTHEAST TO
THE CAROLINAS AND MID ATLANTIC TONIGHT AND FRIDAY. THE GRADIENT
WILL SLOWLY BEGIN TO WEAKEN ACROSS THE NORTHERN PORTIONS OF THE
LAKE OVERNIGHT INTO FRIDAY MORNING AS A WEAK TROUGH OF LOW
PRESSURE SAGS INTO THE WESTERN LAKES. THIS TROUGH WILL REMAIN
STATIONARY NEAR THE NORTH CENTRAL PORTIONS OF THE LAKE INTO
SATURDAY. THIS TROUGH EXTENDS BACK INTO THE PLAINS AND NEW LOW
PRESSURE WILL DEVELOP ALONG THIS TROUGH OVER MISSOURI FRIDAY
NIGHT...LIFTING NORTH ACROSS CENTRAL LAKE MICHIGAN SATURDAY...AS
IT BEGINS TO DEEPEN. ITS EXACT TRACK/TIMING/STRENGTH WILL LIKELY
RESULT IN CHANGES TO THE FORECAST...BUT A PERIOD OF INCREASING
SOUTHERLY WINDS ARE POSSIBLE AHEAD OF THE LOW AND THEN STRONG
NORTHWEST WINDS SATURDAY AFTERNOON INTO SATURDAY NIGHT. THESE
WINDS WILL BE SOMEWHAT SHORT LIVED HOWEVER...AS HIGH PRESSURE
BUILDS ACROSS THE SOUTHERN LAKES AND OHIO VALLEY SUNDAY AND WINDS
BEGIN TO DIMINISH FROM SOUTH TO NORTH.

A WEAK TROUGH OF LOW PRESSURE WILL THEN MOVE ACROSS THE LAKE
SUNDAY NIGHT SHIFTING WINDS TO THE NORTH. AS VERY STRONG ARCTIC
HIGH PRESSURE BUILDS INTO THE PLAINS MONDAY AND TUESDAY...THE
GRADIENT WILL TIGHTEN WITH INCREASING WEST/NORTHWEST WINDS WHICH
MAY INCREASE FURTHER MIDWEEK AS LOW PRESSURE MOVES ACROSS HUDSON
BAY...TIGHTENING THE GRADIENT. CMS

&&

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

&&

$$

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