Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 040904
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
404 AM CDT MON MAY 4 2015

.SHORT TERM...
331 AM CDT

THROUGH TUESDAY...

CONVECTION ASSOCIATED WITH AN APPROACHING COLD FRONT IS MOVING FROM
NORTHWEST TO SOUTHEAST THROUGH NORTHERN IL THIS MORNING. ITS WELL
DEFINED GUST FRONT STRETCHES FROM SOUTHERN LAKE MI THROUGH PONTIAC
IL WITH SCATTERED STORMS FORMING ALONG THE GUST FRONT. A BROADER
AREA OF SHOWERS LAGS BEHIND THE GUST FRONT AND LATEST RADAR IMAGERY
SUGGESTS THE AREA OF SHOWERS IS QUICKLY DISSIPATING.  TWO ADDITIONAL
CONVECTIVE COMPLEXES ARE FURTHER WEST OVER NORTHEAST IOWA AND
NORTHWEST MISSOURI AND THE IOWA COMPLEX MAY PLAY A PART IN WHAT
HAPPENS THIS MORNING.

THINKING WE WILL SEE ADDITIONAL SCATTERED SHOWER AND THUNDERSTORM
DEVELOP ALONG THE GUST FRONT AS IT PLOWS THROUGH THE REST OF THE CWA
EARLY THIS MORNING. MUCH LESS CONFIDENT ON HOW LONG THE SHOWERS WILL
PERSIST OVER NORTH CENTRAL IL AND IF THEY WILL EVEN REACH NORTH EAST
IL AND NW INDIANA. THE ONGOING CONVECTION OVER NORTHEAST IOWA MAY
SHIFT EAST PRODUCING SHOWERS ALONG THE IL/WI STATE LINE...OR IT MAY
DISSIPATE AND WE SEE A DRY MORNING OUTSIDE OF THE CONVECTION ALONG
THE GUST FRONT. SINCE CONFIDENCE IS VERY LIMITED...DECIDED TO KEEP A
LOW END CHANCE TO SLIGHT CHANCE OF RAIN THROUGH MID MORNING.

THE COLD FRONT WHICH CURRENTLY STRETCHES FROM NORTHEAST WI THROUGH
SOUTH CENTRAL IA WILL SLIDE THROUGH THE CWA THIS MORNING AND THEN
STALL OVER LASALLE COUNTY IL THROUGH BENTON COUNTY INDIANA LATE THIS
AFTERNOON.  SHORT TERM GUIDANCE SUGGESTS CAPE WILL INCREASE THIS
AFTERNOON SO HAVE LIKELY POPS SPREADING ACROSS THE CWA THIS AFTN.
HAVE MEDIUM CONFIDENCE IN HOW FAR NORTH THE CONVECTION WILL REACH.
CURRENTLY HAVE LIKELIES OVER THE I-88 CORRIDOR BUT THE NORTHERN
EXTENT MAY REMAIN SOUTH OF I-88.

A MID LEVEL WAVE PUSHES NORTH INTO IOWA THIS EVENING AND EXPECTING
IT TO FORCE THE NEXT ROUND OF STORMS. EXPECTING SHOWERS AND AND A
FEW EMBEDDED THUNDERSTORMS TO COVER THE CWA BY LATE THIS EVENING.
STORMS SHOULD REMAIN TIED TO THE STALLED FRONT TUESDAY AND HAVE
MEDIUM-LOW CONFIDENCE IN HOW CONVECTION WILL BEHAVE. THINKING THE
NOCTURNAL WAVE OF STORMS WILL DISSIPATE AS THEY LIFT NORTH AWAY FROM
THE FRONT...WHILE ADDITIONAL SCATTERED CONVECTION WILL FESTER ALONG
THE FRONT. KEPT AT LEAST A LOW END CHANCE OVER THE WHOLE CWA EVEN
THOUGH AREAS NORTH OF I-80 MAY BE DRY TUESDAY AFTERNOON.

FOR TEMPS...TODAY SHOULD BE A BIT COOLER WITH THE COLD FRONT MOVING
THROUGH SO HAVE HIGHS AROUND 70 ALONG THE IL/WI STATE LINE AND THEN
IN THE UPPER 70S SOUTH OF I-80. WINDS SHOULD TURN ONSHORE ALONG THE
LAKE KEEPING TEMPS IN THE MID 60S TO AROUND 70.  TUESDAY LOOKS EVEN
COOLER IF YOU ARE NORTH OF THE FRONT WITH HIGHS IN THE 60S...AND
ONLY IN THE 50S ALONG THE LAKE. SOUTH OF THE FRONT...LOCATIONS
SHOULD BE IN THE UPPER 70S TO LOW 80S. BREAKS IN CLOUD COVER COULD
LEAD TO A FEW SITES GETTING INTO THE MID 80S.

JEE

&&

.LONG TERM...
331 AM CDT

TUESDAY NIGHT THROUGH SUNDAY...

THE STATIONARY FRONT BEGINS TO SHIFT NORTH AS A WARM FRONT TUESDAY
NIGHT AND COULD SEE ISOLATED TO SCATTERED SHOWERS AND PERHAPS
THUNDERSTORM DEVELOPMENT NORTH OF THE FRONT.  CAPE VALUES DROP OFF
QUICKLY OVERNIGHT SO NOT EXPECTING WIDESPREAD THUNDERSTORM
DEVELOPMENT.

WE ARE IN THE WARM SECTOR WEDNESDAY WITH SOUNDINGS SHOWING NO CAP AND
A FEW HUNDREDS OF J/KG OF CAPE. ISOLATED TO SCATTERED THUNDERSTORM
DEVELOPMENT IS POSSIBLE ESPECIALLY WED AFTN NORTH OF A RENSSELAER
TO RFD LINE. TEMPS WILL BE WARMER AWAY FROM THE LAKE WITH HIGHS IN
THE LOW TO MID 80S. IT WILL BE IN THE MID TO UPPER 60S ALONG THE
LAKE.  GUIDANCE FEATURES A FEW LOBES OF VORTICITY MOVING ALONG THE
UPPER LEVEL RIDGE...BUT THEY SHOULD REMAIN WEST OF THE CWA. THIS
WOULD LEAD TO A DRY NIGHT WEDNESDAY NIGHT...BUT IF THE VORTS SHIFT
EAST...COULD SEE SCATTERED SHOWER DEVELOPMENT.

THE NEXT SURFACE LOW FORMS OVER THE UPPER PLAINS/MIDWEST THURSDAY
MORNING AND REACHES SOUTHERN ONTARIO FRIDAY MORNING. ITS COLD FRONT
STALLS OVER NORTHERN IL SUNDAY MORNING.  THEREFORE LOOKING AT
ANOTHER ACTIVE PERIOD WITH MULTIPLE PERIODS OF SHOWERS AND
THUNDERSTORMS LATE THIS WEEK THROUGH THE WEEKEND.  THE FORECAST
LOOKS ODDLY SIMILAR TO WHAT WE WILL GO THROUGH EARLY THIS WEEK.
TEMPERATURES START ABOVE NORMAL IN THE 80S AWAY FROM THE LAKE
THURSDAY AND THEN COOL TO THE 70S SATURDAY AND SUNDAY. A LAKE BREEZE
WILL KEEP TEMPS ALONG THE LAKE IN THE 60S.  FOR WHAT IT IS
WORTH...WPC IS FORECASTING UPWARDS OF 3 INCHES OF RAIN OVER THE NEXT
7 DAYS.

JEE

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* SCATTERED -SHRA WITH A SMALL CHANCE OF -TSRA THROUGH 13Z-15Z.

* WINDS BECOMING VARIABLE THROUGH DAYBREAK AND THEN NORTHWEST
  BEFORE TURNING NORTH-NORTHEAST SOMETIME BETWEEN 15Z-17Z.

* MVFR CIGS LIKELY NEAR AND AFTER THE WIND SHIFT FOR A FEW HOUR
  PERIOD. SMALL CHANCE FOR TEMPORARY IFR CIGS.

* CHANCE OF ADDITIONAL SHRA/TSRA MID AFTERNOON THROUGH EARLY EVE
  WITH CHANCES INCREASING SHARPLY AFTER THAT TIME.


//DISCUSSION...UPDATED 09Z...

COMPLEX OF SHOWERS AND STORMS WEAKENED CONSIDERABLY ACROSS NORTH
CENTRAL ILLINOIS WITH THE OUTFLOW HAVING NOW PUSHED THROUGH. THIS
OUTFLOW TURNED WINDS WESTERLY...BUT NOW THE STRATIFIED AIR WILL
LIKELY LEAD TO VARIABLE CONDITIONS DEVELOPING ACROSS THE TAF
SITES FOR A PERIOD THROUGH 14Z. BECAUSE OF STILL SOME PASSING
SHOWERS WIND SPEEDS MAY BRIEFLY REACH TO 10 KT EVEN WITH VARIABLE
CONDITIONS.

WIND SHIFT TIMING TO NORTH-NORTHEAST DURING THE MID-LATE MORNING
STILL LOOKS ON TRACK.


PREV DISCUSSION...UPDATED 06Z...

A COMPLEX OF STORMS WILL CONTINUE TO SHIFT EAST-SOUTHEAST AND VERY
LIKELY ACROSS TAF SITES PRIOR TO OR NEAR DAYBREAK. THIS SHOULD
CONTINUE ON A GRADUAL WEAKENING TREND...BUT ENOUGH INSTABILITY TO
WARRANT TSRA PREVAILING FOR A COUPLE HOUR WINDOW AT NORTHERN IL
TAF SITES. BEHIND THE INITIAL PUSH OF STORMS...SOME REDEVELOPMENT
COULD OCCUR AS SEEN IN CENTRAL IA AS OF 05Z...BUT EVEN IF THAT
WERE TO OCCUR IT SHOULD BE MORE LIGHT AND MAYBE JUST SHRA.

A WIND SHIFT TO THE WEST-NORTHWEST IS LIKELY WITH THE CONVECTION
INCLUDING A FEW GUSTS OVER 20 KT ESPECIALLY AT RFD WHERE THE GUSTS
MAY LAST CLOSE TO TWO HOURS. THE FORECAST FOR WIND DIRECTION THEN
GETS VERY CHALLENGING AS A PERIOD OF VARIABLE WIND IS LOOKING
LIKELY AT AND JUST AFTER DAYBREAK...BUT FOR EXACTLY HOW LONG AND
WHAT SPEED IS DIFFICULT TO SAY AT THIS POINT. A SURFACE COOL FRONT
WILL THEN SWING SOUTHWARD TURNING WINDS TO THE NORTH-NORTHEAST AT
ALL TAF SITES BY MIDDAY. MVFR CIGS ARE LIKELY TO ACCOMPANY THIS
MID SHIFT WITH THE CHANCE FOR IFR....ESPECIALLY FOR TAF SITES NOT
FAR FROM THE LAKE AS LAKE-COOLED AIR COLLIDES WITH DEW POINTS IN
THE MID-UPPER 50S.

LATER TONIGHT INTO EARLY TUESDAY MORNING WILL LIKELY SEE MORE
SHOWERS AND STORMS AS AN UPPER LEVEL DISTURBANCE ADVANCES ALONG
AND TO THE NORTH OF THE FRONTAL BOUNDARY THAT WILL BE LAYING TO
THE SOUTH OF THE TAF SITES. AT THIS POINT THINK BETTER CHANCES ARE
AFTER 06Z TUESDAY.


//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* LOW-MEDIUM ON -SHRA THROUGH 14Z BUT MEDIUM-HIGH THAT CONDITIONS
  WILL REMAIN MAINLY VFR.

* HIGH ON VARIABLE WINDS AT TIMES THROUGH 14Z WITH SPEEDS
  SPORADICALLY OVER 7 KTS.

* MEDIUM ON NORTH-NORTHEAST WIND SHIFT TIMING AND SPEED AFTER
  SHIFT.

* MEDIUM-HIGH ON MVFR BUT LOW ON DURATION.

* MEDIUM-HIGH IN SHRA OCCURRING THIS EVENING INTO OVERNIGHT BUT
  LOW ON SPECIFIC TIMING AND HOW WIDESPREAD TSRA WILL BE.


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

TUESDAY...SHRA WITH PERIODS OF TSRA LIKELY. MVFR LIKELY AT TIMES. EAST-
NORTHEAST WINDS.

WEDNESDAY...CHANCE OF TSRA...THOUGH PRIMARILY VFR. SOUTHEAST WINDS.

THURSDAY...VFR. SOUTH WINDS.

FRIDAY AND SATURDAY...CHANCE OF TSRA. SOUTH WINDS...BECOMING NORTH LATE
SATURDAY.

MTF/RATZER

&&

.MARINE...
257 AM CDT

A COLD FRONT WILL ADVANCE SOUTHWARD ACROSS THE REGION THIS
MORNING REACHING THE FAR SOUTHERN PART OF THE LAKE BY EARLY
AFTERNOON. THIS WILL TURN WINDS TO THE NORTH-NORTHEAST. WHILE THE
LAKE HAS WARMED CONSIDERABLY IN THE PAST FEW WEEKS...THE COOLER
AIR BEHIND THE FRONT COMBINED WITH THE HIGH MOISTURE ALONG IT MAY
GENERATE FOG NAMELY ACROSS THE SOUTHERN PART OF THE LAKE. WINDS
WILL TURN EASTERLY LATER TONIGHT AND INTO TUESDAY AND THEN BACK TO
THE SOUTHEAST AND SOUTH TUESDAY NIGHT INTO WEDNESDAY AS THE FRONT
RETURNS NORTHWARD AS A WARM FRONT. A WARM SECTOR WILL REMAIN OVER
THE LAKE DURING THURSDAY AND FRIDAY MEANING STABILITY SHOULD KEEP
GUSTS MAINLY IN THE 20-25 KT BALLPARK DURING THAT TIME.

MTF

&&

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

$$

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