Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250004
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
604 PM CST MON NOV 24 2014

.UPDATE...
550 PM CST

HAVE UPDATED FORECAST AND ISSUED SPECIAL WEATHER STATEMENT FOR
PATCHY FREEZING DRIZZLE/LIGHT FREEZING RAIN FOR AREAS SOUTH OF THE
INTERSTATE 80 CORRIDOR EARLY THIS EVENING. REPORTS FROM WFO ILX
INDICATE SEVERAL ACCIDENTS DUE TO LIGHT ICE ACCUMULATION ADJACENT
TO OUR SOUTHWEST COUNTIES.

INDEED...FORECAST SOUNDINGS FROM RAP/NAM RUNS INDICATE MID-LEVEL
DRYING WITH LOW LEVEL MOIST LAYER NOT COLD ENOUGH TO PRODUCE ICE
CRYSTALS AND SNOW FLAKE DEVELOPMENT DESPITE TEMP PROFILES BELOW
FREEZING. THIS MOST APPARENT ACROSS OUR SOUTHERN COUNTIES...THOUGH
SHOWS UP TO A LESSER EXTENT FARTHER NORTH AS WELL. LOOKS LIKE THE
BEST CHANCES FOR FREEZING DRIZZLE/LIGHT FREEZING RAIN WILL BE
ACROSS THE SOUTH...AND THROUGH ABOUT 02Z/8 PM CST BEFORE COLUMN
COOLS A BIT MORE AND DEEPER MOISTURE RETURNS WITH VORT APPROACHING
FROM NORTHEASTERN IA. BY THAT PRECIP SHOULD AGAIN REMAIN LARGELY
SNOW SHOWERS/FLURRIES.

RATZER

&&

.SHORT TERM...
302 PM CST

THROUGH WEDNESDAY...

TONIGHT...

PRIMARY SNOW BAND/DEFORMATION AXIS HAS SHIFTED NORTH BUT
WILL STILL SEE OCCASIONAL SNOW SHOWERS MOVING INTO NORTHERN ILLINOIS
INTO THE EVENING RUSH. SOME OF THESE COULD DROP VISIBILITY DOWN
UNDER 1 MILE AT TIMES...ESPECIALLY AS WINDS CONTINUE TO GUST CLOSE
TO 45 MPH AT TIMES. GOING TO SHIFT END TIME OF ADVISORY FOR
NORTH/NW CWA TO 00Z...WITH THE WIDESPREAD ACCUMULATING SNOW
ALREADY HAVING ENDED. COULD SEE SOME LINGERING LIGHT
SNOW/FLURRIES LATE EVE/OVERNIGHT AS ADDITIONAL UPPER LEVEL ENERGY
TRAVERSES AREA IN CYCLONIC FLOW ALOFT. STRATUS DECK WILL
COUNTERACT LOW LEVEL CAA TO KEEP TEMP FALL IN CHECK TONIGHT...BUT
STILL ANTICIPATING LOW- MID 20S BY DAYBREAK TUESDAY.

TUESDAY/TUES NIGHT...

OTHER THAN POSSIBLY A FEW FLURRIES IN NORTHEAST
IL/NW IN IN THE AM AS SOME LIFT REMAINS OVER THE AREA...LAKE EFFECT
FOCUS WILL LIKELY BE TO OUR EAST WITH WESTERLY FLOW. QUESTION IS
DOES STRATUS ERODE DURING AFTERNOON INTO EVENING. AS IS
TYPICAL...WRF/NAM GUIDANCE IS MOST BULLISH ON LOW LEVEL
MOISTURE...WHILE REMAINDER OF GUIDANCE DOES ENABLE DRY AIR TO MOVE
IN LATE DAY INTO TUESDAY EVENING. CONFIDENCE IS NOT THE
HIGHEST...BUT HAVE TRENDED A BIT MORE OPTIMISTIC WITH CLOUD COVER.
WARM ADVECTION WILL RAMP UP ALOFT IN ADVANCE OF CLIPPER SYSTEM
TOMORROW NIGHT...BUT WITH A TRANSIENT SURFACE RIDGE OVER THE
AREA...CLEAR/PCLOUDY SKIES COULD ENABLE A MORE SIGNIFICANT
TEMPERATURE FALL FROM HIGHS IN THE UPPER 20S-LOW 30S. THIS IS
PARTICULARLY THE CASE FOR AREAS THAT PUT DOWN SNOW COVER TODAY. HAVE
ADJUSTED LOW TEMPS DOWN A FEW DEGREES...TO UPPER TEENS IN NORTHWEST
TO LOW-MID 20S ELSEWHERE.

WEDNESDAY...

DECIDED TREND ON GUIDANCE AND ENSEMBLES TODAY HAS BEEN TO TRACK
CLIPPER MAINLY SOUTH OF THE CWA...AND HAVE ADJUSTED POPS DOWNWARD
CHANCE SOUTHWEST CWA AND SLIGHT CHANCE ELSEWHERE. UPPER LEVEL ENERGY
ASSOCIATED WITH THIS SYSTEM WAS STILL OFF OF THE BRITISH COLUMBIA
COAST AT RAOB TIME. GIVEN TENDENCY FOR LARGE SHIFTS IN THE SHORT
TERM GUIDANCE WITH THE TRACK OF SUBTLE FEATURES SUCH AS
CLIPPERS...HAVE NOT GONE WITH A COMPLETELY DRY FORECAST YET.
HOWEVER...IF TRENDS CONTINUE...ONLY THE FAR SOUTHWEST CWA WOULD SEE
ANY SENSIBLE WX IMPACTS. TEMPS LOOK TO RISE INTO THE LOW-MID
30S...WITH A FEW DEGREES ABOVE THIS POSSIBLE IF CLIPPER TRACKS
COMPLETELY SOUTH OF THE CWA. THIS WILL BE IN ADVANCE OF NEXT SURGE
OF MODIFIED ARCTIC AIR FOR THANKSGIVING DAY.

RC

&&

.LONG TERM...

WEDNESDAY NIGHT THROUGH TUESDAY...

A SECONDARY TROUGH AXIS WILL CROSS THE REGION WEDNESDAY NIGHT...BUT
OUTSIDE OF BULLISH CANADIAN MODEL...LITTLE SUPPORT FOR ANYTHING MORE
THAN SLIGHT POPS. BIG STORY IS GLANCING BLOW OF ARCTIC AIR SENDING
H85 TEMPS INTO THE LOW-MID MINUS TEENS CELSIUS ON THANKSGIVING
DAY...ONLY SUPPORTING HIGHS FROM THE MID 20S TO LOWER 30S...COLDEST
WHERE THERE WILL STILL BE SOME LEFTOVER SNOW COVER. LAKE EFFECT
FOCUS LOOKS TO BE EAST OF OUR NW IN COUNTIES...BUT WILL STILL NEED
TO MONITOR DUE TO FAVORABLE PARAMETERS.

UPPER LEVEL PATTERN CONTINUES TO LOOK TO QUICKLY SHIFT TO MORE ZONAL
ON LATEST GUIDANCE...SUPPORTING A RAPID MODERATING TREND. PRIOR TO
THAT...GUIDANCE DOES BREAK OUT SOME LIGHT WAA PRECIP ON FRIDAY AS AN
UPPER LEVEL WAVE MOVES OVER THE REGION. BETTER FORCING LOOKS TO BE
INTO SOUTHERN WI AT THIS TIME...BUT STILL A WAYS OUT. HAVE INDICATED
SLIGHT POPS FOR LIGHT SNOW...WITH A MIX OR TRANSITION TO RAIN BY
AFTERNOON FOR PARTS OF AREA AS BOUNDARY LAYER WARMS ABOVE FREEZING.
LOW PRESSURE OVER NORTHERN PLAINS LOOKS TO ADVECT IN A MUCH WARMER
AIR MASS ON SOUTHWEST FLOW ON SATURDAY. H85 TEMPS WARMING TO
POSSIBLY UPWARDS OF +10C COULD SPELL READINGS WELL INTO THE 50S (!)
FOR AT LEAST PORTIONS OF AREA IF TRENDS HOLD. DESPITE FLOW REMAINING
ZONAL...SIGNS THAT COLD AIR OVER CANADA AND FAR NORTHERN TIER COULD
TRY TO OOZE SOUTH LATER INTO THE WEEKEND INTO EARLY NEXT WEEK. THIS
WOULD SET UP A BAROCLINIC ZONE AND POSSIBLY LOW PRECIP CHANCES
RETURNING.

RC

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* INTERMITTENT SNOW SHOWERS THROUGH THE OVERNIGHT WITH BRIEF IFR
  VSBY AT TIMES.

* LOW CHANCE FOR SOME PATCHY LIGHT FREEZING DRIZZLE INTO MID
  EVENING.

* WEST-SOUTHWEST WINDS GUSTING AROUND 35 KT THROUGH EARLY EVENING
  THEN GRADUALLY DIMINISH OVERNIGHT.

* MVFR CIGS PERSIST THROUGH TONIGHT AND MAY CONTINUE TOMORROW AS
  WELL.

MDB

//DISCUSSION...UPDATED 00Z...

A SERIES OF UPPER DISTURBANCES WILL CONTINUE TO DRIVE BANDS OF
SNOW SHOWERS INTO THE OVERNIGHT HOURS. MAY SEE A BIT OF A BREAK IN
THE COVERAGE LATE THIS EVENING BUT ANOTHER WAVE COMING ACROSS
NORTHEAST IOWA WILL LIKELY BRING AN INCREASE IN ACTIVITY AROUND
MIDNIGHT INTO THE OVERNIGHT. WILL INDICATE TEMPO IFR VSBY BASED ON
UPSTREAM TRENDS. ACCUMULATION WILL BE LIGHT WITH LESS THAN A HALF
INCH ADDITIONAL EXPECTED.

AM A LITTLE CONCERNED ABOUT SOME LIGHT FREEZING DRIZZLE THIS
EVENING AS WELL AS MOISTURE ALOFT DECREASES FOR A TIME. AREAS
SOUTH OF THE TERMINALS ARE STARTING TO REPORT THIS AND IT IS MORE
FAVORED TO THE SOUTH BUT IT IS POSSIBLE SOME COULD OCCUR AT THE
TERMINALS. POTENTIAL OF OCCURRENCE IS TOO LOW TO INCLUDE IN THE
TAF FOR NOW BUT WILL MONITOR.

WEST-SOUTHWEST WINDS WILL CONTINUE TO GUST TO AROUND 35 KT THROUGH
EARLY EVENING WITH SNOW SHOWERS HELPING THEM TO OCCUR. EXPECT
GUSTS TO STEADILY DECREASE LATER THIS EVENING INTO THE OVERNIGHT.
MVFR CIGS PERSIST AND SHOULD DO SO OVERNIGHT BUT MAY LIFT A FEW
HUNDRED FEET AT TIMES. LIGHTER WEST WINDS ARE EXPECTED TUESDAY
WITH MVFR STRATUS TO START THE MORNING. HAVE LOW CONFIDENCE IN HOW
LONG MVFR MAY PERSIST OR IF IT LIFTS TO VFR OR SCATTERS AT SOME
POINT DURING THE DAY. WILL LIKELY ENED TO ADJUST THIS WITH LATER
UPDATES.

MDB


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH CONFIDENCE IN INTERMITTENT SNOW SHOWERS THROUGH AT LEAST EARLY
  TONIGHT. MEDIUM CONFIDENCE THAT BRIEF IFR VSBY WILL OCCUR AT
  TIMES.

* MEDIUM-HIGH CONIFDENCE THAT THREAT FOR -FZDZ IS LOW AND WOULD BE
  VERY LIGHT INTO MID EVENING IF IT DID OCCUR.

* HIGH CONFIDENCE IN WIND TRENDS.

* HIGH CONFIDENCE IN MVFR CIGS PERSISTING TONIGHT WITH LOW
  CONFIDENCE IN HOW LONG THEY MAY LAST TUESDAY.

MDB

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

WEDNESDAY...SCHC SNOW. OTHERWISE VFR. SOUTHEAST TO EAST WINDS.

THURSDAY...A FEW FLURRIES POSSIBLE...OTHERWISE VFR. BREEZY NORTHWEST WINDS.

FRIDAY...SCHC SNOW. SOUTH WINDS.

SATURDAY...VFR. WEST-SOUTHWEST WINDS.

SUNDAY...VFR. NORTH WINDS.

MDB

&&

.MARINE...
235 PM CST

LOW PRESSURE THAT MOVED OVER THE STRAITS THIS MORNING AND DRAGGED A
COLD FRONT ACROSS THE LAKE WILL HAVE LIFTED INTO THE JAMES BAY AREA
LATE THIS EVENING.  STRONG WEST GALES ACROSS THE SOUTH HALF OF THE
LAKE WILL PERSIST INTO THE OVERNIGHT HOURS...WITH GUSTY BUT WEAKER
NORTHWEST WINDS ACROSS THE NORTHERN HALF OF THE LAKE.  SNOW MOVES
OUT TONIGHT EXCEPT FOR LINGERING LAKE EFFECT ALONG THE MICHIGAN
SHORE.

WINDS REMAIN WESTERLY BUT RELAX DURING THE DAY TUESDAY AND THEN
RELAX FURTHER BY WEDNESDAY IN A WEAK AND DISORGANIZED PRESSURE
GRADIENT.  BY WEDNESDAY NIGHT INTO THURSDAY THE PATTERN BECOMES
BETTER ORGANIZED AGAIN AND WINDS PICK UP FROM THE WEST TO NORTH.
WEATHER REMAINS MOSTLY QUIET THIS WEEK AFTER TONIGHT ASIDE FROM A
BRIEF SHOT OF SNOW AGAIN LATE WEDNESDAY AND THE MICHIGAN SIDE LAKE
EFFECT.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012 UNTIL 6 PM MONDAY.

IN...NONE.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
     LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
     LMZ874-LMZ876-LMZ878 UNTIL 3 AM TUESDAY.

&&

$$

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