Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 142229
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
529 PM CDT MON APR 14 2014

.DISCUSSION...
314 PM CDT

THE MAIN PERIOD OF ACTIVE WEATHER WILL BE THIS EVENING WITH A MORE
TRANQUIL PERIOD SETTING UP INTO TUESDAY AND CONTINUING THROUGH MUCH
OF THE WEEK. EARLY AFTERNOON WATER VAPOR IMAGERY SHOWS AN EXPANSIVE
LONGWAVE TROUGH OVER THE MID SECTION OF THE COUNTRY WITH THE TROUGH
AXIS SWINGING EASTWARD TOWARD THE LOCAL AREA. A SHARP BUT NARROW
RIDGE TRAILS THE BIG TROUGH OVER WEST COAST WITH A SERIES OF MORE
COMPACT UPPER LOWS SPINNING EASTWARD TOWARDS THE CANADIAN WEST
COAST. AT THE SURFACE...THE MAIN SURFACE LOW WHICH BROUGHT THE
ACTIVE WEATHER OVER THE PAST FEW DAYS IS NOW OVER EASTERN QUEBEC
WITH HIGH PRESSURE SPREADING SOUTHWARD ACROSS THE CANADIAN PRAIRIES
INTO THE CENTRAL AND NORTHERN PLANS.

TONIGHT...WATER VAPOR SHOWS AN EMBEDDED UPPER WAVE MOVING INTO THE
AREA FROM THE SOUTHWEST LATE THIS AFTERNOON. THIS IS TIGHTENING THE
MID LEVEL BAROCLINIC ZONE AND RESULTING IN INTENSIFYING
FRONTOGENESIS IN A CORRIDOR ACROSS THE NORTHWEST CWA WHICH IS
SHIFTING EASTWARD. COLDER AIR AT THE SURFACE AND ALOFT CONTINUES TO
SPREAD SOUTHEASTWARD AS WELL. PRECIP WILL CONTINUE TO INTENSIFY IN
CONJUNCTION WITH THE INCREASED FORCING WHILE THE ENTIRE BAND PIVOTS
EASTWARD AS THE LONGWAVE TROUGH AXIS PUSHES EASTWARD INTO THE
EVENING. FORECAST SOUNDINGS SHOW THAT THE BEST FORCING LINES UP WELL
WITH THE FAVORED SNOW GROWTH REGION SUPPORTING EFFICIENT SNOW GROWTH
WITH THE COLUMN COLD ENOUGH TO SUPPORT ALL SNOW WITHIN THE BAND.
AREAS TO THE SOUTH AND WEST OF THE HAVE BEEN REPORTING VSBY AS LOW
AS 1/4 TO 3/4 SM VSBY UNDER THE HEAVIER PRECIP ELEMENTS. SURFACE
TEMPS...WHILE STILL ABOVE FREEZING WILL COOL WITH TIME FAVORING
ACCUMULATING SNOW BUT GROUND TEMPS WILL BE THE LIMITING FACTOR
OVERALL. HOWEVER...SNOW INTENSITY WITHIN THE HEAVIEST BANDS WILL
LIKELY OVERCOME THE WARMER GROUND...ESPECIALLY ON GRASSY AND
ELEVATED SURFACES...WITH SLUSHY ACCUMULATION POSSIBLE ON ROADWAYS.
BASED ON LATEST TRENDS...AREAS FROM MENDOTA TO CHICAGO NORTHWEST
WILL BE THE FOCUS GOING THROUGH LATE AFTERNOON WITH AN EXPANSION
EXPECTED JUST TO THE SOUTHEAST OF THIS LINE TOWARDS A GARY TO
PONTIAC LINE BY EARLY EVENING. THIS WILL PUT THE CORRIDOR OF
HEAVIEST SNOW ACROSS THE CHICAGO METRO FOR MUCH OF IF NOT ALL OF THE
EVENING RUSH...WITH DEPARTURE FROM THE ROCKFORD AREA RIGHT AROUND
THE START. THE MAIN VORT RESPONSIBLE FOR THIS WILL ZIP NORTHEAST SO
THERE MAY BE A DECREASE IN INTENSITY OF THE BAND TOWARD MID EVENING
BUT BROADER SCALE FORCING WILL CONTINUE TO BRING PRECIP AT A LOWER
RATE THROUGH THE EVENING FROM THE GARY/CHICAGO AREA TO PONTIAC AND
POINTS EAST. ANOTHER WAVE WILL LIFT NORTHEAST INTO NORTH-CENTRAL
INDIANA THIS EVENING AND MAY BRING AN INCREASE IN SNOW TO NORTHWEST
INDIANA MID/LATE EVENING BUT THIS MAY REMAIN TO THE EAST. WILL
CONTINUE WITH ACCUMULATION AROUND AN INCH FOR MOST AREAS EXCEPT FAR
SOUTH WHERE SNOW WILL LIKELY BE LIGHTER AND TEMPS A LITTLE WARMER.
AM THINKING THAT THE CORRIDOR FROM MENDOTA UP TO THE NORTHERN
CHICAGO SUBURBS AND EVEN WAUKEGAN...THEN SOUTHEAST TO THE GARY AND
PONTIAC AREA MAY SEE THE MOST INTENSE SNOW OVERALL AS WELL AS
OVERLAP WITH COLDEST TEMPS GOING INTO THE EVENING. HOWEVER...JUST
ABOUT ANYONE WEST OF GARY-PONTIAC COULD SEE AN INCH GIVEN SNOW
INTENSITY SEEN SO FAR TODAY. DO NOT REALLY HAVE A GOOD FEEL FOR
ACCUMULATION GIVEN THE WARM GROUND ETC...AND CANNOT RULE OUT SOME 2
INCH AMOUNTS UNDER THE MOST PERSISTENT BANDS. SNOW WILL TAPER OFF
FROM WEST TO EAST WITH THE PASSAGE OF THE UPPER TROUGH AXIS LATE
THIS EVENING AND EARLY TONIGHT.

ANOTHER CONCERN IS LAKE EFFECT POTENTIAL INTO THE OVERNIGHT. COLDER
AIR WILL CONTINUE TO SPREAD SOUTHWARD INTO THE AREA WITH A GOOD
NORTH FETCH DOWN LAKE MICHIGAN. H85 TEMPS FALL TO AROUND -14 C WHICH
IS ENOUGH TO SUPPORT LAKE EFFECT...WITH LAKE WATER TEMPS PROBABLY
/PERHAPS CONSERVATIVELY/ IN THE UPPER 30S ACROSS MUCH OF THE
SOUTHERN PART OF THE LAKE /WITH MID/UPPER 40S AT NEARSHORE OBS
SITES/ DELTA TS ARE AT LEAST IN THE UPPER TEENS. THE PASSAGE OF THE
UPPER TROUGH WILL BRING SOME SUPPRESSION AS WELL AS DEEP LAYER
DRYING INTO THE REGION BUT LAKE INDUCED EQUILIBRIUM LEVELS
MAY PUSH 6-7 KFT OVERNIGHT. WITH A GRADUALLY WEAKENING PRESSURE
GRADIENT TO THE WEST...WINDS WILL EASE UP FROM WEST TO EAST AND THIS
MAY LEAD TO CONVERGENCE NEAR OR OFF THE ILLINOIS SHORE. SO IF PRECIP
CAN DEVELOP...WHICH IS A BIT OF A WILD CARD...THEN SOUTHERN SECTIONS
OF COOK COUNTY AND WESTERN PORTIONS OF LAKE COUNTY INDIANA MAY SEE
SNOW SHOWERS OVERNIGHT. IT IS ALSO NOT OUT OF THE REALM OF
POSSIBILITY THAT A FURTHER INLAND PUSH TO THE WEST COULD OCCUR INTO
THE CHICAGO AND NORTH SHORE AREAS. WILL GO AHEAD AND CARRY LOW POPS
AT THE SHORELINE AND INTO FAR SOUTHEAST COOK AND WESTERN LAKE COUNTY
THROUGH THE OVERNIGHT. SHORELINE AREAS COULD STAY AT OR JUST ABOVE
FREEZING GIVEN THE LAKE SO ANY ACCUMULATION COULD BE SLUSHY OR EVEN
GRADUALLY MELT...BUT AREAS JUST INLAND WILL LIKELY BE IN THE UPPER
20S TO AROUND 30 SO ACCUMULATION IS A CONCERN. WILL SHARE CONCERN
WITH THE EVENING SHIFT FOR FURTHER MONITORING. OTHERWISE WINDS WILL
EASE INLAND WITH SKIES CLEARING ACROSS WESTERN AREAS ALLOWING TEMPS
TO FALL INTO THE LOWER TO MID 20S.

TUESDAY THROUGH FRIDAY...THE UPPER FLOW WILL FLATTEN BEHIND THE
DEPARTING UPPER TROUGH TUESDAY AS SURFACE HIGH PRESSURE CROSSES THE
AREA. MID LEVEL WARM ADVECTION WILL COMMENCE IN THE AFTERNOON. HIGH
TEMPS WILL BE TOUGH GIVEN THE HIGH SUN ANGLE BUT VERY COLD START AND
SLOW START TO MID LEVEL WAA. LOW LEVEL LAPSE RATES STEEPEN QUITE A
BIT BY AFTERNOON AND WITH THE CONTINUED CYCLONIC FLOW ALOFT WILL
LIKELY SEE SCT-BKN CU DEVELOP INTO THE MIDDAY THEN DISSIPATE FROM
WEST TO EAST DURING THE LATE AFTERNOON. THAT SAID...WE HAVE SEEN
SEVERAL DAYS IN RECENT WEEKS WHERE TEMPS WARMED NICELY IN SIMILAR
SITUATIONS. WILL STAY ON THE WARMER SIDE OF THINGS WITH LOWER TO MID
40S WEST AND UPPER 30S TO AROUND 40 EAST. MOISTURE LOOKS PRETTY
SHALLOW BUT CANNOT RULE OUT A FEW SHOWERS DURING THE AFTERNOON BUT
WILL KEEP THE FORECAST DRY FOR NOW. THE TROUGH OFF THE BRITISH
COLUMBIA COAST TODAY WILL WEAKEN AND MOVE SOUTHEAST ACROSS THE UPPER
MIDWEST WEDNESDAY. THIS WILL BRING A PUSH OF WARMER MID LEVEL AREA
WHICH WILL COMBINE WITH INCREASING SOUTHERLY WINDS BEHIND THE
DEPARTING SURFACE HIGH TO PROPEL TEMPS INTO THE 50S WEDNESDAY. A
SURFACE LOW WILL ACCOMPANY THE UPPER TROUGH WITH A WARM FRONT
DEVELOPING NORTH OF THE AREA. THIS SHOULD KEEP THE LOCAL AREA DRY
WEDNESDAY BUT THE ATTENDANT COLD FRONT WILL MOVE IN THURSDAY
POTENTIALLY BRINGING SOME PRECIP. THE FRONT STALLS OUT OVER OR NEAR
THE AREA THURSDAY EVENING AND A SECOND UPPER TROUGH DIGS INTO THE
CENTRAL AND SOUTHERN PLAINS. A NEW SURFACE LOW WILL TRACK NORTHEAST
AND ACROSS THE LOCAL AREA FRIDAY. PRECIP CHANCES LOOK TO RAMP UP
THURSDAY EVENING NORTHWEST OF THE FRONT...THEN EXPAND ACROSS THE
AREA SOME TIME FRIDAY AS THE SURFACE LOW PASSES. PROVIDED THE
BOUNDARY ARRIVES LATE ENOUGH THURSDAY HIGHS WILL LIKELY BE IN THE
60S...WITH FRIDAY BEING COOLER BUT ALSO SEEING A LARGER RANGE OF
TEMPS THANKS TO THE BOUNDARY. EXPECT LOWER 50S NORTH AND UPPER 50S
SOUTH.

SATURDAY AND BEYOND...THE LOW WILL CLEAR TO THE NORTHEAST SOME TIME
EARLY SATURDAY WITH HIGH PRESSURE BUILDING IN THROUGH THE DAY. WITH
THE UPPER FLOW NOT ALL THAT AMPLIFIED THERE WILL BE LITTLE IN THE
WAY OF COOLING ALOFT AND ENOUGH SUNSHINE SHOULD HELP PROPEL TEMPS
INTO THE 50S. ANOTHER TROUGH LOOKS TO CROSS THE AREA SUNDAY BRINGING
MORE CHANCES FOR RAIN BUT OVERALL MILD TEMPS IN THE 50S.

MDB

&&

.CLIMATE...
THE FOLLOWING ARE THE MOST RECENT APRIL DATES AND AMOUNTS OF
ACCUMULATING SNOWFALL:

CHICAGO: APRIL 18 2011 (0.6 IN.)
ROCKFORD: APRIL 5 2009 (0.2 IN.)

THE FOLLOWING ARE RECORD LOWS FOR THE MORNING OF APRIL 15TH. THE
CHICAGO ONE IS THE MOST LIKELY IN JEOPARDY.

CHICAGO:  25 (1943)
ROCKFORD: 16 (1928)

MTF

&&

.AVIATION...

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

* A PERIOD OF IFR/LIFR SNOW THROUGH MID EVENING

* MVFR CIGS THROUGH THIS EVENING

* GUSTY NORTH/NORTHWEST WINDS

IZZI

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

COMPLEX FORECAST THIS PERIOD AS A WINTRY MIX IS LIKELY THIS
AFTERNOON INTO THE EVENING. LATEST RADAR IMAGERY DEPICTING
CONTINUED LIGHT DEVELOPMENT AT THIS HOUR WITH ALL SNOW BEING
REPORTED ACROSS NORTH/NORTHWEST ILLINOIS...AND THEN A RAIN SNOW
MIX BEING REPORTED OVER AREAS IN NORTHEAST ILLINOIS. THIS LIGHTER
DEVELOPMENT WILL LIKELY CONTINUE THROUGH MID AFTERNOON BEFORE A
HEAVIER/STEADIER BAND OF LIGHT TO MODERATE SNOW MOVES THROUGH LATE
THIS AFTERNOON INTO THE EARL EVENING...WITH ANY AREAS OBSERVING A
RAIN/SNOW MIX OBSERVING A QUICK TRANSITION TO ALL SNOW. VIS AND
CEILINGS WILL LIKELY LOWER UNDER THIS BETTER DEVELOPMENT...WITH
IFR VIS AND LOW END MVFR CEILINGS LIKELY. THIS STEADIER SNOW
SHOULD BEGIN TO SHIFT EAST OF THE TERMINALS LATER THIS
EVENING...BUT WITH LAKE EFFECT DEVELOPMENT POSSIBLY LINGERING
THROUGH THE MIDNIGHT TIME FRAME. CONFIDENCE IS LOW WITH REGARDS TO
LAKE EFFECT SNOW DEVELOPMENT THIS EVENING/TONIGHT...AND WILL NEED
TO MONITOR THIS POSSIBLE DEVELOPMENT. AREAS WILL THEN OBSERVE A
CLEARING TREND OVERNIGHT...WITH VFR CONDITIONS THEN LIKELY
TUESDAY. NORTHERLY WINDS MAY CONTINUE TO VARY BETWEEN NNW AND NNE
OVER THE NEXT SEVERAL HOURS...BUT WITH A TREND TOWARDS NNW. HAVE
CONTINUED TO LOWER SPEEDS/GUSTS THIS AFTERNOON WITH THE HIGHEST
GUSTS OF AROUND 21KT NOW LIKELY EARLY THIS AFTERNOON. NORTHWEST
WINDS WILL PREVAIL TONIGHT INTO TUESDAY...WITH SPEEDS DIMINISHING.

RODRIGUEZ

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

* MEDIUM-HIGH CONFIDENCE WITH CEILING TRENDS

* MEDIUM-HIGH CONFIDENCE IN WIND DIRECTION/SPEED

* HIGH CONFIDENCE IN SNOW TRENDS

* LOW CONFIDENCE WITH LAKE EFFECT SNOW SHOWERS

IZZI

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

TUESDAY NIGHT...VFR.
WEDNESDAY...VFR.
THURSDAY...VFR.
FRIDAY...CHC RAIN. MVFR/IFR PSBL.
SATURDAY...SLT CHC RAIN. MVFR/IFR PSBL.
SUNDAY...CHC RAIN. MVFR/IFR PSBL.

BMD

&&

.MARINE...
313 PM CDT

LOW PRESSURE OVER QUEBEC DRAGGED A COLD FRONT OVER THE LAKE THIS
MORNING. NORTH GALES BEHIND THE FRONT HAVE PERSISTED INTO MID
AFTERNOON AND ARE EXPECTED TO EASE QUICKLY THIS EVENING...BUT EAST
SIDE OF THE ENTIRE LAKE IS STILL SEEING GUSTS INTO THE MID 30S TO
AROUND 40 KT RANGE...SO GALE WARNING HAS BEEN EXTENDED INTO THE
EVENING. TO THE WEST...HIGH PRESSURE WILL BUILD ACROSS THE PLAINS
TONIGHT THEN INTO THE WESTERN GREAT LAKES LATER TOMORROW. ANOTHER
LOW WILL DEVELOP ACROSS THE CENTRAL AND SOUTHERN PLAINS WEDNESDAY
WITH TROUGHING EXTENDING TO THE NORTHEAST WHICH WILL MOVE OVER
LAKE MICHIGAN WEDNESDAY NIGHT INTO THURSDAY. LOW PRESSURE WILL
THEN LIFT NORTHEAST ACROSS THE MIDWEST OVER THE WEEKEND.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
     LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
     LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL 9 PM MONDAY.

     SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 5 PM TUESDAY.

&&

$$

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