Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 050539
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1239 AM CDT TUE MAY 5 2015

.UPDATE...
757 PM CDT

MAIN UPDATES THIS EVENING WERE TO ADJUST TIMING/TRENDS WITH
EXPECTED SHOWERS/THUNDERSTORMS TONIGHT. OUTSIDE OF SOME ISOLATED
THUNDERSTORMS MOVING ACROSS BENTON COUNTY INDIANA EARLIER...DRY
CONDITIONS HAVE BEEN IN PLACE. HAVE ADJUSTED POPS TO REFLECT THIS
AND THE DRY CONDITIONS EXPECTED FOR MOST AREAS DURING THIS
EVENING. WEAK MID LEVEL FEATURE HELPING FOR DEVELOPING CONVECTION
OVER EASTERN IOWA AND WESTERN ILLINOIS WILL CONTINUE EAST AND HELP
FOR THIS DEVELOPMENT TO SLOWLY MOVE INTO NORTH CENTRAL ILLINOIS
OVER THE NEXT COUPLE OF HOURS. HOWEVER...THIS WILL LIKELY REMAIN
OVER THE WEST/SOUTHWEST THIRD OF THE CWA UNTIL STRONGER FORCING
ARRIVES LATER TONIGHT. MORE WIDESPREAD SHOWERS/THUNDERSTORMS WILL
ARRIVE OVER MOST LOCATIONS LATE TONIGHT INTO EARLY TUESDAY
MORNING. HAVE MAINTAINED THUNDER IN THE GRIDS BUT AM NOT OVERLY
EXCITED FOR THIS ACROSS THE ENTIRE CWA TONIGHT...AS ALREADY
WEAKENING INSTABILITY FURTHER WEAKENS. THE BETTER CHANCES FOR
THUNDER WOULD BE SOUTH OF INTERSTATE 80...WHERE THE BEST MOISTURE
AND INSTABILITY WILL RESIDE. HOWEVER...STRONGER FORCING/LLJ COULD
PROVIDE AT LEAST SOME EMBEDDED THUNDER FURTHER NORTH. PERIODS OF
HEAVY RAINFALL WITH SMALL HAIL WILL STILL BE THE MAIN HAZARDS WITH
ANY STRONGER STORMS.

RODRIGUEZ

&&

.SHORT TERM...
249 PM CDT

TONIGHT AND TOMORROW...

APART FROM A COUPLE OF CELLS JUST SOUTH OF THE FORECAST AREA...A
DIFFUSE FRONTAL ZONE STRETCHING ACROSS NORTHERN ILLINOIS THIS
AFTERNOON HAS SO FAR BEEN HAVING TROUBLE SUPPORTING REDEVELOPMENT
OF CONVECTION AS THE PERIOD OF PEAK HEATING APPROACHES...THOUGH
WIDESPREAD CLOUD DEBRIS LIKELY IS INHIBITING INSOLATION AND LOW
LEVEL DESTABILIZATION. THE FRONT HAS BEEN EFFECTIVE AT ALLOWING
WINDS ALONG THE LAKE TO TURN NORTHEAST AND PRODUCE A SHARP THERMAL
GRADIENT...WITH TEMPERATURES DROPPING INTO THE 50S WHERE THE FRONT
HAS PASSED AND STAYING IN THE UPPER 60S TO MIDDLE 70S ELSEWHERE.
THIS COOL AIR IS ANTICIPATED TO SPREAD SOUTH AND WEST ACROSS THE
FORECAST AREA AS EVENING APPROACHES...EVENTUALLY ALLOWING FOR LOWS
IN THE UPPER 40S NORTH BUT AROUND A RELATIVELY MILD 60 DEGREES
SOUTH.

DESPITE THE SLOW START...CONVECTIVE TRENDS LIKELY WILL INCREASE
INTO EARLY EVENING ALONG AND SOUTH OF THE FRONT...SO MAINLY IN THE
SOUTHERN THIRD OF THE FCST AREA. THUNDERSTORM ACTIVITY MAY WANE
WITH LOSS OF HEATING AFTER SUNSET EVEN AS SHOWER TRENDS
PERSIST...AND THEN INCREASE AGAIN TOWARD MORNING ACROSS THE ENTIRE
AREA AS THE FRONTAL ZONE LIFTS BACK NORTH AND IS ENHANCED BY
INCREASED MOISTURE CONVERGENCE INTO THE REGION.

HIGHS TOMORROW NORTH OF THE FRONT WILL BE ABOUT 10 TO 15 DEGREES
COOLER THAN TODAY...THOUGH AREAS SOUTH OF THE FRONT WILL TOP OUT
AT TEMPERATURES VERY SIMILAR TO TODAY...AND MAYBE A DEGREE OR TWO
WARMER.

LENNING

&&

.LONG TERM...
249 PM CDT

TUESDAY NIGHT THROUGH MONDAY...

A WESTERN LONGWAVE TROUGH WILL PROGRESS ACROSS THE DESERT SOUTHWEST
TUESDAY NIGHT THEN LIFT NORTHEAST ACROSS THE UPPER MISSISSIPPI RIVER
VALLEY THURSDAY NIGHT. THIS WILL FORCE UPPER RIDGING TO PIVOT ACROSS
THE LOCAL AREA WEDNESDAY AND THURSDAY BEFORE IT SHIFTS TO THE EAST
AND BROADENS THURSDAY. THIS WILL ALLOW THE SURFACE WARM FRONT TO
CONTINUE NORTHWARD ACROSS THE AREA TOMORROW EVENING AND PUSH NORTH
OF THE AREA INTO WEDNESDAY MORNING. THE BULK OF ANY
PRECIP/CONVECTION LOOKS TO SHIFT NORTHWARD THROUGH THE EVENING WITH
ONLY MINIMAL CHANCES CONTINUING ACROSS THE FAR NORTHEASTERN CWA
THROUGH THE OVERNIGHT. THE RIDGE AXIS WILL BE OVERHEAD FOR MUCH OF
WEDNESDAY WHICH WILL MAKE IT DIFFICULT FOR MUCH IF ANY NEW
CONVECTION TO DEVELOP DESPITE A WARM AND HUMID AIRMASS OVER THE
AREA. HOWEVER...WINDS WILL BE TURNING SOUTH TO SOUTHEAST AND BE
LIGHT ENOUGH THAT A MORE PRONOUNCED LAKE BREEZE MAY FORM AND PUSH
INLAND INTO NORTHEAST ILLINOIS. THIS MAY BE ENOUGH TO TRIGGER SOME
ISOLATED CONVECTION BUT OTHERWISE WILL NEED TO RELY ON SOME SORT OF
SUBTLE MID/UPPER WAVE TO HELP KICK ANYTHING OFF. WILL MENTION A
SLIGHT CHANCE FOR NORTHEASTERN AREAS TO MAINTAIN CONTINUITY WITH
PREVIOUS FORECAST FOR NOW. HIGH TEMPS WILL WARM INTO THE LOW TO MID
80S ACROSS MOST OF THE AREA WEDNESDAY WITH MUCH OF THE CHICAGO METRO
BEING HELD INTO THE 70S THANKS TO AN EASTERLY WIND WIND
COMPONENT...WITH ILLINOIS LAKEFRONT AREAS ONLY SEEING LOWER 60S IF
NOT A LITTLE COOLER.

THE UPPER TROUGH WILL LIFT ACROSS THE UPPER MISSISSIPPI RIVER VALLEY
WEDNESDAY NIGHT AND CONVECTION ASSOCIATED WITH IT MAY BE ABLE TO
SKIRT THE WESTERN CWA TOWARD THURSDAY MORNING. THE UPPER RIDGE AXIS
WILL SHIFT EASTWARD BRINGING A STRONGER SOUTHWESTERLY FLOW OVER THE
AREA INTO THE WEEKEND. WESTERN AREAS WILL SEE A CHANCE FOR STORMS
THURSDAY BUT OTHERWISE MOST AREAS WILL REMAIN DRY. FROM THEN ON
PERIODIC CHANCES FOR STORMS WILL PERSIST THROUGH THE WEEKEND AS A
SERIES OF UPPER DISTURBANCES MOVE THROUGH. AT THE SURFACE...A WEAK
FRONTAL BOUNDARY WILL DRIFT SOUTHEAST INTO THE AREA FRIDAY NIGHT AND
BETTER ALIGN WITH THE TRACK OF THE UPPER WAVES YIELDING THE BEST
CHANCE FOR ROUNDS OF CONVECTION FROM LATER FRIDAY THROUGH MONDAY.
HIGH TEMPS THURSDAY AND FRIDAY LOOK TO BE IN THE UPPER 70S AND LOWER
80S WITH LAKESHORE AREAS SEEING THE 70S. DEPENDING ON HOW MUCH
PROGRESS THE EXPECTED SURFACE FRONT MAKES INTO SATURDAY AND THE
AMOUNT OF CONVECTION TEMPS WILL LIKELY BE COOLER...IN THE 70S...WITH
LAKESHORE AREAS PROBABLY BEING HELD INTO THE 60S. A SIMILAR TEMP
REGIME IS EXPECTED SUNDAY.

MDB

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 06Z...

* -SHRA IS MOVING IN AND WILL PERSIST THROUGH THE MORNING. A FEW
  CLAPS OF THUNDER ARE NOT OUT OF THE QUESTION THROUGH DAYBREAK.

* EAST WINDS INCREASE TO 10-11 KT THIS MORNING AND AFTERNOON.

* IFR CIGS ARE EXPECTED BY 12Z AND NOT EXPECTING CIGS TO LIFT TO
  MVFR UNTIL THIS EVENING IF NOT LATER.

* ANOTHER CHANCE OF SHOWERS AND PERHAPS A THUNDERSTORM THIS AFTN.

JEE

//DISCUSSION...UPDATED 06Z...

THE MESSY PATTERN CONTINUES. A SWATH OF SHOWERS AND A FEW
THUNDERSTORMS ARE SLOWLY PUSHING NORTHEAST AND WILL IMPACT ALL OF
THE TERMINALS. LIGHTNING STRIKES ARE FEW AND FAR BTWN SO KEPT
VCTS GOING THROUGH 12Z. IT IS STILL RATHER DRY OUT THERE DESPITE
THE RAIN...SO MVFR CIGS ARE ADVANCING SLOWLY. ALL GUIDANCE MEMBERS
INDICATE THE IFR CIGS TO OUR WEST WILL INFILTRATE THE TERMINALS
EARLY THIS MORNING. THE FRONT CURRENTLY OVER CENTRAL IL WILL
REMAIN SOUTH OF THE TERMINALS TODAY...WHICH SHOULD ME WE ARE
LOCKED INTO IFR CIGS UNTIL THE FRONT LIFTS NORTH WEDNESDAY
MORNING. WINDS WILL BE OUT OF THE EAST AS WELL.

GUIDANCE ALSO HINTS AT ANOTHER ROUND OF SHOWERS AND PERHAPS A
THUNDERSTORM THIS AFTERNOON SO PUT A PROB30 IN AS CONFIDENCE IS
NOT OVERLY HIGH GIVEN HOW MESSY THE PATTERN IS.

JEE


//ORD AND MDW CONFIDENCE...UPDATED 06Z...

* HIGH CONFIDENCE IN SHOWERS THROUGH THIS MORNING AND ONLY A FEW
  LIGHTNING STRIKES THROUGH DAYBREAK. MEDIUM-LOW CONFIDENCE IN
  OCCURRENCE AND COVERAGE OF -SHRA AND TSRA THIS AFTN.

* MEDIUM-HIGH CONFIDENCE IN VSBY...STRONGER DOWNPOURS COULD LOWER
  CIGS TO 2SM.

* MEDIUM-HIGH CONFIDENCE IN CIGS DROPPING TO IFR THIS MORNING.
  MEDIUM CONFIDENCE IN CIGS LIFTING TO MVFR THIS EVENING AS THEY
  COULD REMAIN IFR OVERNIGHT.

* HIGH CONFIDENCE IN ALL OTHER FORECAST ELEMENTS.

JEE

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

WEDNESDAY...SLIGHT CHANCE TSRA. MAINLY VFR. SOUTHEAST WINDS.

THURSDAY...VFR. SOUTH WINDS.

FRIDAY...CHANCE TSRA. MAINLY VFR. SOUTH WINDS.

SATURDAY...CHANCE OF TSRA WITH BRIEF MVFR...THOUGH MAINLY VFR.
SOUTHWEST WINDS BECOMING EAST-NORTHEAST.

SUNDAY...CHANCE OF TSRA. BRIEF MVFR  POSSIBLE. SOUTHEAST WINDS.

RATZER

&&

.MARINE...
257 PM CDT

A COLD FRONT THAT DROPPED SOUTH ACROSS THE LAKE TODAY HAS TURNED
WINDS TO THE NORTH-NORTHEAST AND BROUGHT CONSIDERABLY COOLER AIR
TO THE SOUTHERN END. THIS COOLER AIR COMBINED WITH THE HIGH
MOISTURE ALONG IT HAS PROMPTED SOME REPORTS OF FOG THIS AFTERNOON.
WINDS CONTINUE TO TURN EAST TONIGHT AND TOMORROW THEN BACK TO THE
SOUTHEAST AND SOUTH TOMORROW NIGHT INTO WEDNESDAY AS THE FRONT
LIFTS BACK TO THE NORTH. WINDS GENERALLY STAY LIGHT IN THE WEAK
GRADIENT UNTIL LATE IN THE WEEK WHEN GUSTS MAY CLIMB AGAIN ABOVE
20 KTS.

LENNING

&&

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

$$

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