Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 051540
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1040 AM CDT TUE MAY 5 2015

.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 16Z...

* EAST-NORTHEAST WINDS INCREASING BACK TO 10 KT THIS AFTERNOON.

* IFR CIGS POSSIBLY LIFTING TO MVFR THIS AFTERNOON...RETURNING TO
  LIFR TONIGHT.

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

KMD

//DISCUSSION...UPDATED 12Z...

THE RAIN IS SLOWLY SHIFTING NORTHEAST AND WILL CLEAR THE REGION
THIS MORNING. NO ADDITIONAL LIGHTNING STRIKES ARE ANTICIPATED WITH
THESE SHOWERS. THERE ARE SCATTERED REPORTS OF MVFR CIGS AND THE
CIGS SHOULD LIFT TO VFR BEHIND THE SHOWERS. A STATIONARY FRONT
WILL REMAIN SOUTH OF THE TERMINALS TODAY AND COULD SEE SHOWERS AND
PERHAPS A THUNDERSTORM FIRE ALONG THE FRONT THIS AFTERNOON. THE
SHOWERS AND STORMS COULD IMPACT THE EASTERN TERMINALS...BUT DUE
TO LOW CONFIDENCE ONLY HAVE A VCSH. THINKING RFD WILL NOT BE
IMPACTED.

EAST WINDS AT 10 KT DIMINISH TO BELOW 10 KT THIS EVENING AND THEN
BEGIN TO BACK TO SOUTHEAST TOMORROW AS THE FRONT LIFTS NORTH.

JEE

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* MEDIUM-HIGH THAT EAST-NORTHEAST WINDS PICK BACK UP TO AROUND 10
  KT THIS AFTERNOON.

* MEDIUM-HIGH CONFIDENCE IN ORD REMAINING DRY THE REST OF THE
  DAY. MEDIUM THAT AFTERNOON SHOWERS/ISOLATED T-STORMS REMAIN
  SOUTH OF MDW.

* MEDIUM CONFIDENCE IN IFR CIGS LIFTING TO MVFR BUT LOW ON TIMING
  AND DURATION. HIGH THAT IFR/LIFR CIGS AND VSBYS RETURN TONIGHT
  BUT LOW ON TIMING.

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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