Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 150320
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1020 PM CDT MON APR 14 2014

.DISCUSSION...
804 PM CDT

FOR EVENING UPDATE...

MADE SOME TWEAKS TO HOURLY POP/WX GRIDS TO SHARPEN UP THE TRAILING
EDGE AND TIGHTEN TIMING OF ENDING OF CURRENT SNOW. ALSO INCREASED
CHANCE POPS SOMEWHAT AND ADDED MENTION OF SOME ADDITIONAL MINOR
ACCUMULATION ACROSS NORTHWEST INDIANA BEYOND MIDNIGHT FOR LAKE
EFFECT SNOW SHOWERS.

UNWELCOME REMINDER THAT APRIL CAN STILL BE WINTER-LIKE IN THE
MIDWEST THIS AFTERNOON/EVENING...AS BAND OF LIGHT/MODERATE SNOW
CONTINUES TO SHIFT EAST ACROSS THE FORECAST AREA AT 730 PM CDT.
THIS BAND OF SNOW IS BEING DRIVEN BY A COMBINATION OF A MID-LEVEL
SHORT WAVE TROUGH LIFTING NORTHEAST FROM MISSOURI...THE APPROACH OF
A LONG WAVE UPPER TROUGH FROM THE WEST...WITH ENHANCEMENT INTO
STRONGER SOUTHWEST-NORTHEAST BANDS BY MID-LEVEL FRONTOGENESIS.
REGIONAL RADAR MOSAIC HAS CONSISTENTLY INDICATED A FAIRLY SHARP
TRAILING EDGE TO THE MORE INTENSE SNOW...ROUGHLY ALONG A SPRINGFIELD
TO STREATOR TO ARLINGTON HEIGHTS AND HIGHLAND PARK IL LINE AT THIS
TIME. FRONTOGENESIS MAXIMA AT 700 HPA DEPICTED NICELY BY THE RAP...
AND EVOLUTION OF THIS FEATURE OFF TO THE EAST SHOWS NICE CORRELATION
WITH RADAR TRENDS OVER THE PAST FEW HOURS AND INDICATES THAT SNOW
WILL TAPER TO FLURRIES AND LARGELY END FROM WEST TO EAST ACROSS MUCH
OF THE CHICAGO AREA BY 9 PM...AND PRIOR TO 11 PM FOR OUR NORTHWEST
INDIANA AND FAR EAST CENTRAL IL COUNTIES. TOTAL SNOWFALL REPORTS
HAVE RANGED FROM A HALF INCH TO NEAR 2 INCHES IN SPOTS THAT HAVE
BEEN AFFECTED BY THE ENHANCED HEAVIER BANDING.

NORTH WINDS AND COLD AIR DO LOOK TO MAINTAIN THE POTENTIAL FOR SOME
LAKE EFFECT SNOW SHOWERS ALONG THE SOUTH END OF THE LAKE THROUGH THE
OVERNIGHT HOURS...WITH LAKE EFFECT PARAMETERS FROM THE 18Z NAM
SUPPORTIVE OF ORGANIZED CONVECTIVE SNOW SHOWERS (DELTA T AROUND
16C...EQUILIBRIUM HEIGHTS OVER 8 KFT...UNIDIRECTIONAL NORTH FLOW
NEAR 20 KT THROUGH MARINE BOUNDARY LAYER). BASED ON THIS...HAVE
INCREASED CHANCE POPS ACROSS LAKE/PORTER COUNTY AFTER MIDNIGHT
TONIGHT AND INTO TUESDAY BEFORE APPROACHING SURFACE RIDGE MAKES THE
LOW LEVEL WIND FIELD INCREASINGLY LESS FAVORABLE.

OTHER CONCERN THIS EVENING WITH LUNAR ECLIPSE TO OCCUR AFTER
MIDNIGHT IS WITH CLEARING OF CLOUD COVER. THICKER/DEEPER CLOUD COVER
ASSOCIATED WITH MID-LEVEL TROUGH LOOKS TO CLEAR OUR EASTERN COUNTIES
AROUND OR SHORTLY AFTER MIDNIGHT BASED ON EXTRAPOLATION OF SATELLITE
IMAGES. WHILE SOME EROSION/DISSIPATION IS POSSIBLE...AREA OF
CLOSED-CELL STRATO-CU UPSTREAM ACROSS IA/MN WITH BASES 5000-7000 FT
MAY TEND TO PERSIST DUE TO FAIRLY STEEP LOW LEVEL LAPSE RATES IN
STRONG LOW LEVEL COLD ADVECTION REGIME. WILL MAINTAIN A BECOMING
PARTLY CLOUDY FORECAST AFTER MIDNIGHT...AND HOPEFULLY THE ECLIPSE
WILL BE VISIBLE BETWEEN PASSING PATCHES OF STRATOCU.

RATZER

//PREV 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 03Z...

* LIGHT SNOW AND LINGERING MVFR CIGS MOVE OUT BY 06Z

IZZI

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

BAND OF SNOW AND ASSOCIATED IFR/LIFR VSBYS WILL SHIFT EAST OF THE
TERMINALS EARLY THIS EVENING WITH A RAPID IMPROVEMENT IN
CONDITIONS EXPECTED SHORTLY THERE AFTER. SHARP CLEARING LINE
NEARING THE MS RIVER JUST BEFORE 00Z IS MAKING QUICK PROGRESS
EASTWARD...THOUGH THERE COULD BE SOME LOWER CLOUDS THAT LINGER A
BIT LONGER AS THE HIGHER CLOUD DECK MOVES OUT. REGARDLESS...FIELD
OF STRATOCUMULUS UPSTREAM OVER MN/IA SHOULD ROTATE INTO THE AREA
AND MAINTAIN ITSELF INTO THE DAY TUESDAY GIVEN THE VERY COLD AIR
ALOFT AND RESULTANT STEEP LAPSE RATES. GUSTINESS OF WINDS SHOULD
ABATE WITH THE PARTIAL CLEARING WITH NORTHWEST WINDS CONTINUING
THROUGH THE DAY TUESDAY. EXPECT A LAKE BREEZE TO DEVELOP AND MOVE
THROUGH GYY IN THE AFTERNOON TUES AND COULD MAKE A RUN AT MDW
TOWARD EVENING...BUT CONFIDENCE IS LOW ENOUGH TO KEEP ANY WIND
SHIFT OUT OF ORD/MDW FOR NOW.

IZZI

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS

IZZI

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

WEDNESDAY THROUGH THURSDAY...VFR.

FRIDAY...RAIN LIKELY. IFR POSSIBLE.

SATURDAY...MVFR CIGS POSSIBLE.

SUNDAY...CHANCE OF RAIN.

IZZI

&&

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

&&

$$

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