Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 260838
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
238 AM CST FRI DEC 26 2014

.UPDATE...914 PM CST

STEADY SOUTH WINDS REMAIN ACROSS THE AREA THIS EVENING THANKS TO A
PRESSURE GRADIENT WHICH HAS BEEN MODESTLY INTENSIFYING. IN
ADDITION...LOW LEVEL WARM ADVECTION WILL BE PERSISTENT OVERNIGHT
WITH A STEADY STREAM OF MID AND HIGH CLOUD COVER KEEPING SKIES
VARIABLE BETWEEN PARTLY AND MOSTLY CLOUDY. WILL BE MAKING SOME
MINOR ADJUSTMENTS TO TEMP TRENDS THROUGH THE OVERNIGHT AND BUMP
OVERNIGHT LOW IN SPOTS BY A FEW DEGREES.

MDB

&&

.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 06Z...

* GUSTY SOUTH WINDS OVERNIGHT AND WINDS INCREASING WITH ALTITUDE
  TO 40-45KT AT 020 WITH LOW END CONCERN FOR LLWS.

* MVFR CIGS RETURNING FRIDAY LATE AFTERNOON OR EVENING.

* IFR CIGS/VSBYS OVERNIGHT WITH -RA/-DZ

KMD

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

SEEING A FEW 1 MINUTE OBS OF WIND GUSTS AT KMDW...BUT OTHERWISE WIND
GUSTS ARE NOT OCCURRING OFTEN ENOUGH TO PRECLUDE A LLWS MENTION IN
THE TAFS THROUGH 15Z. AMDAR SOUNDINGS DEPICT 40 KT AT 200 DEGREES AROUND
020...WITH SOME VEEERING TO 230 DEGREES THROUGH THE MORNING
POSSIBLE. EXPECT MIXING LATER THIS MORNING END THIS CONCERN. SOME
GUIDANCE DEPICTING SOME MVFR VISBYS AT DAYBREAK BUT HAVE NOT
BOUGHT OFF ON THAT.

STILL HAVE HIGH CONFIDENCE THAT MVFR RETURNS SOMETIME FRIDAY LATE
AFTERNOON OR EVENING AHEAD OF LOW PRESSURE IN THE SOUTHERN PLAINS
THAT WILL APPROACH WESTERN ILLINOIS AFTER MIDNIGHT FRIDAY NIGHT. AS
THE LOW PASSES NW THE TERMINALS...EXPECT CONTINUED SOUTHERLY WINDS
THAT WILL WEAKEN SOME LATER TODAY BUT PICK BACK UP LATE TONIGHT.
EXPECT A PERIOD OF RAIN OR DRIZZLE DURING THE OVERNIGHT HOURS AS
WELL THAT COULD BRING IFR.

KMD

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

* MEDIUM ON WHETHER WINDS GUSTS WILL SURFACE INFREQUENTLY ENOUGH
  TO PRECLUDE LLWS THROUGH MID- MORNING

* HIGH IN VFR CONDITIONS THROUGH MIDDAY FRIDAY AND LIKELY THROUGH
  FRIDAY AFTERNOON...MEDIUM CONFIDENCE IN TIMING RETURN TO MVFR
  FRIDAY EVENING.

* LOW ON IF IFR WILL MATERIALIZE OR NOT LATE FRIDAY NIGHT INTO
  EARLY SATURDAY

KMD

//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...
234 AM CST

HIGH PRESSURE OVER THE SOUTHEAST STRETCHING UP THE ATLANTIC SEABOARD
WILL REMAIN STATIONARY TODAY BEFORE SHIFTING OUT OVER THE ATLANTIC
ON SATURDAY. MEANWHILE A LOW PRESSURE TROUGH AXIS EXTENDS FROM THE
SOUTHERN HIGH PLAINS THROUGH WESTERN WISCONSIN. THE TROUGH AXIS WILL
SHIFT OVER THE NORTHERN HALF OF THE LAKE TODAY WHICH WILL EASE THE
CURRENT GRADIENT OVER THE LAKE THAT IS DRIVING SOME STRONGER
SOUTHERLIES...ESPECIALLY OVER THE SOUTHERN HALF OF THE
LAKE...WHERE WINDS TO 30 KT CAN BE EXPECTED THROUGH MID MORNING. A
SECONDARY LOW MOVING THROUGH THE TROUGH AXIS WILL REACH CENTRAL
LAKE MICHIGAN AFTER DAYBREAK SATURDAY THEN NORTHEAST OF LAKE
HURON SATURDAY NIGHT...DEEPENING AS IT DOES SO. MODEST HIGH
PRESSURE EMERGES BEHIND THIS SYSTEM...BUT WILL DRIVE A PERIOD OF
WESTERLY WINDS TO 30 KT WITH A MODIFIED COLD FRONT. STRONGER HIGH
PRESSURE WILL MOVE OUT OF CANADA BEYOND THIS TIME FRAME...BRINGING
COLDER NORTH- NORTHWESTERLY FLOW OVER THE LAKE WHICH MAY BECOME
GUSTY AT TIMES. BEFORE THE HIGH MOVES INTO THE NORTHERN PLAINS...A
WEAK TROUGH AND COLD FRONT WILL PASS OVER THE LAKE BY
MONDAY...FOLLOWED BY ANOTHER WEAK TROUGH MID-WEEK.

KMD

&&

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

&&

$$

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