Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 051811
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
111 PM CDT TUE MAY 5 2015

.UPDATE...1135 AM CDT

UPDATED TEMPERATURES AND WEATHER FOR THE REST OF TODAY. IT APPEARS
THE BEST CHANCES FOR SHOWERS AND THUNDERSTORM REDEVELOPMENT THIS
AFTERNOON WILL BE PRIMARILY ALONG AND SOUTH OF INTERSTATE 80 IN
CLOSER PROXIMATE TO THE NORTHWARD ADVANCING SURFACE BOUNDARY.
TEMPERATURES ACROSS CENTRAL ILLINOIS..SOUTH OF THIS BOUNDARY ARE
ALREADY WELL INTO THE 70S TO NEAR 80 IN MANY LOCATIONS...WITH DEW
POINTS POOLING INTO THE LOWER TO MID 60S RIGHT ALONG THE BOUNDARY.
MODIFYING THE 12 UTC ILX SOUNDING WITH THESE SURFACE CONDITIONS
YIELDS SCAPE AROUND 1300 J/KG WITH VERY LITTLE CAP. WITH THIS IN
MIND...IN SPITE OF THE FACT THAT THERE IS NO SIGNIFICANT MID LEVEL
DISTURBANCE EXPECTED TO MOVE OVER THE AREA THIS AFTERNOON...IT
SEEMS PROBABLE THAT SCT CONVECTION WILL STILL CONTINUE TO BUBBLE
UP THIS AFTERNOON WITHIN THE VICINTY OF THIS SURFACE
BOUNDARY...WHERE INSTABILITY AND SURFACE CONVERGENCE WILL BE
MAXIMIZED. FARTHER NORTH...VERY COOL AND STABLE CONDITIONS SHOULD
LIMIT SHOWER AND THUNDERSTORM POTENTIAL. THEREFORE...I HAVE
POINTED THE FORECAST IN THIS DIRECTION.

TEMPERATURES ACROSS MUCH OF THE CHICAGO AREA NEAR LAKE MICHGAN
WILL LIKELY REMAIN IN THE LOW TO MID 50S TODAY. HOWEVER...FARTHER
INLAND AND SOUTH...EXPECT TEMPERATURES WELL INTO THE
70S...ESPECIALLY SOUTH OF INTERSTATE 80.

KJB

&&

.SHORT TERM...
329 AM CDT

THROUGH TONIGHT...

A LOOSELY-DEFINED AND NON-INTENSE MCS IS MOVING ACROSS NORTHERN
ILLINOIS DURING THE PREDAWN WITHIN A BROAD ZONE OF WARM ADVECTION.
RADAR MOSAIC AND VAD WIND PROFILER DATA INDICATE THE PRESENCE OF
A MID-LEVEL SHORT WAVE THAT LIKELY MAY BE CONVECTIVELY-ENHANCED
HELPING TO FOCUS THE BULK OF THE RAIN AND EMBEDDED THUNDER. BASED
ON RADAR TRENDS...HAVE GONE A LITTLE FURTHER SOUTH WITH RAIN AND
THUNDER CHANCES THROUGH 9 AM OR SO COMPARED TO THE SUITE OF HIGH-
RESOLUTION GUIDANCE. OVERALL EXPECT THE COVERAGE OF PRECIPITATION
TO CONSIDERABLY DIMINISH DURING THE MID TO LATE MORNING HOURS
WITH THE WAVE PASSING NORTHEAST. TRAILING SHOWERS AND A FEW
STORMS ARE LIKELY TO LINGER ALONG AND NORTH OF I-80 DURING THE
LATTER MORNING.

WHILE THE MID-LEVEL FLOW IS EXPECTED TO BECOME LESS DISTURBED BY
AFTERNOON PER WATER VAPOR ANALYSIS AND RAP AND NAM
FORECASTS...THE CHANCES FOR AFTERNOON CONVECTION LINGER IN THE
HIGH PWAT AIR /1.4 INCHES +/. THESE CHANCES WILL BE ATTACHED TO
OUTFLOWS/CONVERGENCE ZONES TRAILING THE MORNING SHORT WAVE AND
ITS CONVECTION...AS WELL AS THE SYNOPTIC CONVERGENCE ALONG THE
STATIONARY FRONT THAT EARLY THIS MORNING IS ROUGHLY NEAR I-74.
THIS FEATURE IS EXPECTED TO CREEP NORTHWARD THIS MORNING WITH THE
DECAY OF CONVECTION AND HEATING IN THE WARM SECTOR. THIS FRONT
WILL SEPARATE TEMPERATURES LIKELY AROUND 80 IN THE SOUTHERN
FORECAST AREA TO THE COOLER AND CLOUDY 60S IN NORTHEAST
ILLINOIS...WITH EVEN LOWER TO MID 50S ALONG THE LAKE SHORE. DEW
POINTS IN THE LOWER-MID 60S ALONG THE BOUNDARY WILL PROVIDE
MARGINAL BUT SOME MIXED LAYER INSTABILITY NEAR THE BOUNDARY THIS
AFTERNOON. CONVECTIVE ALLOWING MODELS GENERALLY SHOW DEVELOPMENT
THIS AFTERNOON BUT VARY WIDELY IN COVERAGE. WITH WEAK MID-LEVEL
FLOW AND BUILDING HEIGHTS NOT EXPECTING ANYTHING SEVERE AND HAVE
GONE WITH SCATTERED OR CHANCE WORDING DEPENDING ON THE AREA. THE
PRESENT MOST FAVORED FOCUS THIS AFTERNOON LOOKS TO BE ALONG I-80
AND JUST NORTH OF...BUT THAT COULD CERTAINLY SHIFT OR BE MORE
DIFFUSE...AND THE DAY SHIFT WILL BE ABLE TO BETTER ASSESS POST-
MORNING CONVECTIVE TRENDS.

MODELS AGREE ANY SHOWERS/STORMS THIS AFTERNOON WILL WANE IN THE
EVENING BEING MORE DIURNALLY DRIVEN. THE FRONTAL BOUNDARY WILL
LIKELY NOT MAKE TOO MUCH PROGRESS NORTH DURING THE
NIGHT...HANGING AROUND I-80 OR SO. THE HIGH ABSOLUTE MOISTURE
ALONG THE BOUNDARY COULD RESULT IN FOG ACROSS THE AREA...SOME OF
WHICH MAY BE DENSE AND LINGER INTO EARLY WEDNESDAY MORNING. HAVE
ADDED SOME GENERAL MENTION INTO THE FORECAST FOR THE TIME BEING.

MTF

&&

.LONG TERM...
329 AM CDT

WEDNESDAY NIGHT THROUGH MONDAY...

ATTENTION IN THE LONG RANGE IS FIRST ON THE IMPRESSIVE EARLY MAY
WARMTH THAT WILL MAKE IT FEEL LIKE SUMMER IN SOME PLACES WEDNESDAY
AND AREAWIDE THURSDAY AND POTENTIALLY FRIDAY TOO. THE UPPER JET
PATTERN COMING IN FROM THE PACIFIC ON WEDNESDAY INTO THURSDAY
CARVES A MORE AMPLIFIED PATTERN...FAVORING BUILDING HEIGHTS AND
LOW-LEVEL SOUTHERLY FLOW TO INCREASE. GIVEN ALREADY ABOVE-NORMAL
TEMPERATURES UP TO CENTRAL ILLINOIS IN RECENT DAYS...THIS SYNOPTIC
PATTERN WILL FAVOR REPLENISHING AND ADDING TO THAT WARMTH AS WELL
AS SPREADING IT NORTH.

THE FRONTAL BOUNDARY IS LIKELY TO BREAK DOWN SOMEWHAT ON WEDNESDAY
AS JUST BROAD SOUTHERLY FLOW BEGINS. THIS FLOW WILL NOT BE STRONG
ENOUGH TO PREVENT LAKE COOLED AIR TO LIKELY REMAIN OVER NORTHEAST
ILLINOIS AND POSSIBLY EVEN FAR NORTHWEST INDIANA. APART FROM THE
MID 60S TO MID 70S IN THESE AREAS...LOWER TO POSSIBLY MID 80S ARE
EXPECTED ACROSS THE SOUTHERN AND FAR WESTERN FORECAST AREA.

THURSDAY WILL SEE SOUTH-SOUTHWEST WINDS INCREASE ENOUGH TO PREVENT
ANY LAKE COOLING. A THERMAL RIDGE DEFINED BY 925MB TEMPERATURES
OF 18C-19C AND 850MB TEMPERATURES OF 13-14C. USING REANALYSIS DATA
SINCE 1979...WHEN THESE WARM OF TEMPERATURES HAVE BEEN EXPERIENCED
IN THE FIRST HALF OF MAY IT YIELDS AN AVERAGE HIGH OF 83 DEGREES
AT CHICAGO OHARE. WITH SUNSHINE HIGHLY LIKELY ON THURSDAY GIVEN
THE AMPLIFIED FLOW KEEPING CIRRUS/DEBRIS AWAY ON MODEL
SOLUTIONS...HAVE BOOSTED TEMPERATURES TO THESE VALUES AND EVEN
SLIGHTLY HIGHER. SOME UPPER 80S ARE CERTAINLY POSSIBLE GIVEN THIS
PATTERN. THE RECORD HIGH FOR CHICAGO ON THURSDAY MAY 7TH IS 87
DEGREES SET IN 1964.

THE SECOND FOCUS ON THE LONG RANGE IS CONVECTIVE CHANCES. THE
POTENTIAL BEGINS FRIDAY WITH A NORTHERN STREAM FRONT EASING TOWARD
THE AREA...WHICH WILL COOL TEMPERATURES FOR SATURDAY AND POSSIBLY
SUNDAY TOO...ESPECIALLY FOR LAKE ADJACENT AREAS. AS THE UPPER-
LEVEL SOUTHWESTERLY FLOW INCREASES LATER IN THE WEEKEND...STORM
CHANCES WILL INCREASE AS HIGH PWAT AIR IS AGAIN FORECAST TO
ESTABLISH ITSELF ACROSS THE CORN BELT REGION. TIMING OF SHOWERS
AND STORMS LOOK TO AGAIN BE TIED TO MID-LEVEL IMPULSES WHICH CAN
OFTEN BE SUBTLE...WHICH DIMINISHES PREDICTABILITY AT THIS
POINT...SO CONTINUE WITH CHANCES.

MTF

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* EAST-NORTHEAST WINDS AROUND 10 KT THROUGH THIS AFTERNOON.

* IFR/LIFR CIGS THROUGH THE BEGINNING OF THE MORNING RUSH
  WEDNESDAY.

* IFR/LIFR VSBYS RETURNING TONIGHT.

* SCATTERED SHOWERS AND PERHAPS A THUNDERSTORM THIS AFTN...MAINLY SOUTH
  OF MDW.

KMD

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

THE BATTLE OF THE COLD LAKE AND LOW LEVEL ONSHORE FLOW VERSUS THE
WARM MAY SUN IS UNDERWAY...MAKING FOR A CHALLENGING AFTERNOON
FORECAST ACROSS THE TERMINALS. MONITORING WEBCAMS DOWNTOWN SHOW
CLOUDS BRIEFLY ATTEMPTING TO LIFT BEFORE FILLING RIGHT BACK IN. WE
ALSO CONTINUE TO ADVECT WARMER AND MOIST AIR JUST OFF THE DECK
WHICH IS AIDING IN KEEPING THE LOW LEVEL INVERSION IN PLACE. FEEL
THAT THESE LATTER FACTORS WILL WIN OUT AND KEEP MOST OF THE TAF
SITES LOCKED IN LIFR TO IFR CONDITIONS...WHILE POINTS SOUTH WILL
LIFT TO VFR BEFORE MVFR CUMULUS FILL IN AS WELL.

THE LOW LEVEL FRONT IS NOT EXPECTED TO LIFT MUCH FARTHER
NORTH...AND THUS EXPECT THE NNE WINDS TO REMAIN IN PLACE. WITH THE
FRONT HOLDING SOUTH...EXPECT ANY CONVECTION THAT STIRS THIS
AFTERNOON TO BE FOCUSED IN THESE AREAS...SOUTH OF INTERSTATE 80.

TONIGHT WINDS SHOULD WEAKEN SOME AND EXPECT CIGS/VSBYS TO REMAIN
LOCKED IN DOWN LOW MOST OF THE NIGHT. FOG COULD BE DENSE
TONIGHT...ESPECIALLY CLOSER TO THE LAKE. LOW LEVEL FRONT GETS
SOME NORTHWARD MOVEMENT EARLY WEDS WHICH SHOULD ALLOW FOR SOME
LIFTING WEDNESDAY MORNING. PRECIP CHANCES DO EXIST TONIGHT/EARLY
WEDS BUT TOO LOW FOR A MENTION IN THE TAFS JUST YET. AT BEST SOME
ISOLATED SHOWERS AHEAD OF THE FRONT 8Z-12Z TIME FRAME FOR THE TAF
SITES.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH THAT EAST-NORTHEAST WINDS HOLD AROUND 10 KT THIS AFTERNOON.

* HIGH IN LIFR CIGS HOLDING OR LIFTING SLIGHTLY AT ORD... AND
  IFR/LIFR CIGS RETURNING TO MDW. MEDIUM ON CIG HEIGHTS OVERNIGHT
  THOUGH LOW-MEDIUM ON CLEARING TIME WEDS.

* MEDIUM-LOW ON VSBYS THIS AFTERNOON AND TONIGHT. COULD BE WORSE
  VSBY OVERNIGHT.

* HIGH IN ORD REMAINING DRY THE REST OF THE DAY. MEDIUM-HIGH THAT
  AFTERNOON SHOWERS/ISOLATED T-STORMS REMAIN SOUTH OF MDW. MEDIUM
  THAT TAF SITES REMAIN DRY OVERNIGHT AS WELL.

KMD

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

WEDNESDAY NIGHT...VFR. SOUTHEAST WINDS BCMG SOUTH.

THURSDAY...VFR. SOUTH WINDS.

FRIDAY...TSRA. MVFR CIGS PSBL OVERNIGHT. SOUTHWEST WINDS.

SATURDAY...SCHC TSRA DURING THE DAY AND CHC SHRA. NORTHWEST WINDS.

SUNDAY AND MONDAY...CHC TSRA. MVFR CIGS PSBL. NORTHEAST WINDS BCMG
SOUTHWEST MONDAY.

JEE

&&

.MARINE...
212 AM CDT

A STATIONARY FRONT IS OVER CENTRAL IL AND IT WILL REMAIN SOUTH OF
THE LAKE UNTIL IT LIFTS NORTH AS A WARM FRONT WEDNESDAY.
THEREFORE...NORTHEAST TO EAST WINDS WILL BECOME SOUTH WEDNESDAY
NIGHT BEHIND THE WARM FRONT.  HIGH PRESSURE CURRENTLY OVER LAKE
SUPERIOR SHIFTS EAST OF LAKE MICHIGAN WEDNESDAY AND REMAINS OVER THE
EASTERN SEABOARD THROUGH THE WEEK.  SOUTH WINDS UP TO 20 KT AT TIMES
ARE EXPECTED THROUGH FRIDAY.  THE NEXT LOW FORMS OVER THE NORTHERN
ROCKIES WEDNESDAY AND MOVES OVER ONTARIO THURSDAY NIGHT AND FRIDAY.
THE LOWS COLD FRONT SWEEPS OVER THE LAKE SATURDAY AND WINDS BECOME
NORTH BEHIND THE FRONT.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$

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