Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 300800
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
300 AM CDT TUE JUN 30 2015

.UPDATE...
854 PM CDT

FOR EVENING UPDATE...

ONLY A FEW MINOR CHANGES MADE TO HOURLY POP/WX GRIDS INTO THE
OVERNIGHT HOURS TO ADJUST FOR RADAR TRENDS/EXTRAPOLATION OF MID-
LEVEL SHORT WAVE TROUGH AXIS CURRENTLY UPSTREAM OVER NORTHWEST
WI/EAST CENTRAL MN PER VAPOR IMAGERY. ZONE UPDATE SHORTLY.

ISOLATED-SCATTERED SHOWERS AND THUNDERSTORMS HAVE DECREASED IN
COVERAGE AND WERE MAINLY LOCATED PRIMARILY ALONG SOUTHERN PORTIONS
OF THE I-39 CORRIDOR IN LASALLE AND LIVINGSTON COUNTIES AT 845 PM.
THESE CELLS CONTINUE TO PULSE AND EXHIBIT MID-LEVEL ROTATION AT
TIMES ALONG EASTERN GRADIENT OF A REGION OF 25-30 KT 0-6 KM BULK
SHEAR AXIS JUST WEST OF THE CWA. SHOULD CONTINUE TO SEE A GRADUAL
DIURNAL WEAKENING IN THESE CELLS AS THEY CONTINUE TO DRIFT SLOWLY
SOUTH-SOUTHEAST ACROSS LIVINGSTON COUNTY THIS EVENING. FARTHER
UPSTREAM HOWEVER...AREA OF SHOWERS/THUNDERSTORMS ACROSS SOUTH
CENTRAL AND SOUTHWEST WI WERE ADVANCING TO THE SOUTH-SOUTHEAST AS
WELL...IN ASSOCIATION WITH THE AFOREMENTIONED MID-LEVEL WAVE. THIS
ACTIVITY IS EXPECTED TO CONTINUE TO PROPAGATE INTO THE CWA LATE
THIS EVENING...ALTHOUGH THE STRONGEST CONVECTION IS EXPECTED TO
REMAIN ALONG/WEST OF THE MISSISSIPPI RIVER WHERE POOL MID-UPPER
60S SURFACE DEW POINTS AND 2000 J/KG MUCAPE AXIS IS LOCATED PER
SPC/RAP MESOANALYSIS. RECENT RUNS OF THE HRRR DEPICT THIS...WITH
EROSION OF MUCH OF THE DEEPER CONVECTION FARTHER EAST ACROSS OUR
CWA. STILL... THE POTENTIAL FOR SHOWERS/ISOLATED THUNDER WILL
LINGER INTO THE PRE-DAWN HOURS AS THE SHORT WAVE PROGRESSES
SOUTHEAST ACROSS THE AREA. A FEW SHOWERS MAY LINGER PAST SUNRISE
ACROSS THE SOUTHEAST CORNER OF THE CWA.

RATZER

&&

.SHORT TERM...
224 PM CDT

THROUGH TUESDAY...

TONIGHT...SHOWER/THUNDER TRENDS WILL BE THE MAIN FOCUS TONIGHT AS
ANOTHER IN A SERIES OF UPPER WAVES TRAVERSES THE AREA. LATE
AFTERNOON SHOWER/THUNDER ACTIVITY WILL FADE AS THE PRIMARY WAVE
DRIVING THE ACTIVITY SHIFTS TO THE EAST AND DAYLIGHT BEGINS TO WANE.
WATER VAPOR IMAGERY SHOWS ADDITIONAL WAVES MOVING SOUTHEAST ACROSS
MINNESOTA WITH THE MOST PRONOUNCED WAVE APPROACHING NORTHWEST
WISCONSIN. SHOWERS AND STORMS CONTINUE TO MARCH SOUTHEASTWARD WITH
THIS SECOND WAVE WHILE NEW SCATTERED ACTIVITY IS DEVELOPING TO THE
SOUTH ACROSS EXTREME SOUTHEAST MN AND SOUTHWEST WI IN CONJUNCTION
WITH THE SE MN WAVE. GUIDANCE IS MIXED ON HOW LONG ALL OF THIS
ACTIVITY WILL CONTINUE INTO THE NIGHT AND THEREFORE HOW MUCH IMPACT
THE CWA WILL SEE. BASED ON LATEST TRENDS AND NEAR TERM
GUIDANCE...SHOWERS AND STORMS COULD REACH THE NORTHWEST CWA BY MID
EVENING THEN CONTINUE ACROSS THE AREA OVERNIGHT IF THEY HOLD
TOGETHER. SOME OF THE ACTIVITY WITH THE SE MN WAVE MAY BE MORE
DIURNALLY DRIVEN SO IT MAY FADE AS IT MOVES CLOSER TO THE AREA EARLY
THIS EVENING. EXPECT THE MORE ORGANIZED NORTHERN ACTIVITY TO MAKE
BETTER PROGRESS AND HOLD TOGETHER LONGER SO WILL WORK A BAND OF LOW
LIKELY/HIGH CHANCE POPS ACROSS THE AREA FROM MID EVENING THROUGH THE
OVERNIGHT HOURS THEN EXIT THEM TO THE EAST EARLY TOMORROW MORNING.
AM A BIT CONCERNED THAT THE ARRIVAL OF POPS MAY BE JUST A BIT SLOW
SO MAY NEED TO SPEED THE PROGRESSION UP BY AN HOUR OR TWO GOING INTO
EARLY EVENING SO STAY TUNED. WOULD EXPECT ANY WIND THREAT WITH THIS
ACTIVITY TO BE DIMINISHING AS IT APPROACHES LEAVING HEAVY
DOWNPOURS/ISOLATED FLOODING AS THE PRIMARY CONCERN.

TUESDAY...ANY LEFTOVER OVERNIGHT PRECIP WILL EXIT TO THE EAST BY MID
TUESDAY MORNING AS AN UPPER TROUGH AXIS CLEARS TO THE EAST. THE MAIN
SURFACE LOW WILL LAG BEHIND THE UPPER WAVE BUT WILL DRAG A COOL
FRONTAL BOUNDARY ACROSS THE AREA THROUGH THE MORNING. THERE DOES NOT
APPEAR TO BE MUCH OF A COHERENT FOCUS FOR ADDITIONAL ORGANIZED
DEVELOPMENT THROUGH THE MORNING BUT WITH BROADER ASCENT IN PLACE
ISOLATED ACTIVITY IS POSSIBLE. HIGH PRESSURE WILL BE BUILDING ACROSS
WESTERN LAKE SUPERIOR WHILE BRINGING A STRONGER PRESSURE GRADIENT TO
NORTHERN SECTIONS OF LAKE MICHIGAN AHEAD OF IT. A SECONDARY COOL
FRONTAL BOUNDARY LOOKS TO DEVELOP AND DROP DOWN LAKE MICHIGAN
TUESDAY AFTERNOON WHICH MAY SERVE TO TRIGGER ISOLATED/SCATTERED
SHRA/TSRA DEVELOPMENT DURING THE AFTERNOON...ESPECIALLY LATER...
GIVEN THAT THE AIRMASS BEHIND THE INITIAL FRONT WILL STILL BE MOIST
AND UNSTABLE. WILL CARRY CHANCE POPS DURING THE MID/LATE AFTERNOON
INTO THE EARLY EVENING HOURS ACROSS NORTHEAST IL AND FAR NORTHWEST
IN. HIGH TEMPS WILL BE IN THE UPPER 70S TO AROUND 80 WITH LAKEFRONT
AREAS COOLING LATE IN THE DAY AS WINDS SHIFT NORTHEAST WITH THE
SECONDARY FRONTAL PASSAGE.

MDB

&&

.LONG TERM...
319 PM CDT

TUESDAY NIGHT THROUGH MONDAY...

THE BEGINNING OF THE PERIOD TUESDAY EVENING MAY BEGIN WITH
ISOLATED TO SCATTERED SHOWERS/THUNDERSTORMS STILL ACROSS THE
CWA...BUT EXPECT WHATEVER LINGERING DEVELOPMENT TO DIMINISH.
DEPARTING LARGE SCALE ASCENT AND A GOOD PUSH OF COOLER/DRIER AIR
ACROSS THE CWA TUESDAY NIGHT...WILL ALLOW FOR ANY FURTHER PRECIP
DEVELOPMENT ACROSS THE REGION TO REMAIN SOUTH OF THE CWA. WITH
THIS COOLER/DRIER AIRMASS IN PLACE AND WITH DIMINISHING
WINDS/CLEARING SKIES...DID LOWER LOW TEMPS TUESDAY NIGHT WITH
TEMPS DROPPING INTO THE 50S FOR MOST AREAS. ALTHOUGH NORTHWEST
FLOW ALOFT WILL PREVAIL ON WEDNESDAY...IT IS BECOMING MORE LIKELY
THAT THE BEST DEVELOPMENT FOR FURTHER PRECIP WILL BE TO THE
SOUTHWEST/SOUTH OF THE CWA. SO DID REMOVE POPS FOR MOST LOCATIONS
DURING THIS TIME BUT DID LEAVE SLIGHT CHANCE WORDING FOR THE FAR
SOUTHWEST CWA OWING TO SOME SLIGHT VARIABILITY AMONG MODEL
GUIDANCE. HOWEVER...COULD SEE THESE SLIGHT CHANCE POPS BEING
REMOVED WITH DRIER CONDITIONS THE MORE LIKELY TREND. ALSO LOWERED
SKY COVER TUESDAY NIGHT INTO WEDNESDAY WITH CLEARER SKIES BECOMING
MORE LIKELY BUT WITH SEVERAL MID LEVEL IMPULSES TRAVERSING THE
FLOW...COULD SEE SOME PERIODIC MID/HIGH LEVEL CLOUD COVER.

THE DRIER TREND WITH A MORE SOUTHWARD PLACEMENT OF BEST PRECIP
DEVELOPMENT WILL LIKELY CONTINUE WEDNESDAY NIGHT INTO
THURSDAY...WITH HIGH PRESSURE EXPANDING ACROSS MUCH OF THE REGION.
BELOW NORMAL TEMPS LOOK TO CONTINUE DURING THIS PERIOD AS WELL
WITH TEMPS FALLING TO THE 50S AGAIN. LOWERED TEMPS DURING THIS
PERIOD AND MAY NOT HAVE LOWERED ENOUGH ESPECIALLY ACROSS FAR
NORTHERN ILLINOIS WHERE TEMPS COULD FALL INTO THE 40S. WITH
STEERING FLOW KEEPING GENERAL MOVEMENT OF APPROACHING MID LEVEL
IMPULSES TO THE SOUTH AND WITH DOMINATING HIGH PRESSURE IN
PLACE...SUNNY SKIES WILL BE IN PLACE THURSDAY. HOWEVER...COOLER
AIRMASS AND ONSHORE FLOW WILL HELP KEEP TEMPS BELOW NORMAL...IN
THE 70S AND THE UPPER 60S NEAR THE LAKE. DRY CONDITIONS SHOULD
CONTINUE THURSDAY NIGHT BUT WITH CONFIDENCE LOWERING INTO
FRIDAY...AS GUIDANCE VARIES WITH A POSSIBLE SYSTEM PUSHING ACROSS
THE CWA. HAVE MAINTAINED CHANCE POPS FOR SHOWERS/THUNDERSTORMS ON
FRIDAY...BUT WILL NEED TO MONITOR THIS PERIOD FOR ANOTHER POSSIBLE
DRY DAY. GOING INTO THE WEEKEND...GUIDANCE DOES VARY BUT AT THIS
TIME...SATURDAY IS APPEARING TO BE ANOTHER DRY PERIOD FOR MOST
AREAS.

RODRIGUEZ

&&

.AVIATION...

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

* FOG OVERNIGHT.
* SCATTERED SHRA/ISOL TSRA TUESDAY AFTERNOON.

BMD

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

SCATTERED SHOWERS SHOULD CONTINUE TO DIMINISH OVER THE NEXT
COUPLE HOURS ACROSS NORTHERN ILLINOIS AS A WEAK SURFACE TROUGH
DROPS SOUTHEAST ACROSS THE REGION. WINDS WILL BE LIGHT WITHIN THE
FRONTAL TROUGH THEN SHOULD TURN NORTHWESTERLY LATER OVERNIGHT OR
EARLY TUESDAY. MAIN CONCERN OVERNIGHT SEEMS TO BE FOG DEVELOPMENT
WITH CONDITIONS ALREADY NEAR SATURATION AND MID LEVEL CLOUD COVER
PROGGED TO CLEAR THE TERMINALS IN THE NEXT 3-4 HOURS. THE
COMBINATION OF LIGHT WINDS AND CLEARING SKIES WILL SET THE STAGE
FOR RADIATION FOG DEVELOPING. AT THIS POINT...NONE OF THE MOS
GUIDANCE IS PICKING UP ON FOG AND THE RAW VSBY GUIDANCE FROM THE
MODELS IS HIT OR MISS. TRENDED VSBY DOWN IN THE LATEST VERSION OF
THE TAFS...BUT CONCERNED AT LEAST PATCHY DENSE FOG OF 1/2 TO 1/4
OF A MILE WILL BE IN PLACE IF NOT MORE WIDESPREAD. WILL WATCH
TRENDS OVER NEXT COUPLE HOURS TO SEE IF FURTHER REDUCTION OF VSBY
IS NEEDED IN THE FORECAST.

TUESDAY SHOULD START OUT WITH NORTHWEST FLOW AND EXPECT TO
GRADUALLY IMPROVE BACK TO VFR BY AROUND MID MORNING. A LAKE BREEZE
IS EXPECTED TO TURN WINDS NE LATE AFTERNOON OR EARLY EVENING FOR
ORD/MDW...AND EARLY AFTERNOON AT GYY. COULD BE SOME SCATTERED
SHOWERS WITH ISOLATED THUNDERSTORMS AGAIN TOMORROW...MAINLY IN THE
AFTERNOON WHEN INSTABILITY PEAKS.

BMD

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

* INCREASING CONFIDENCE IN MVFR/IFR FOG OVERNIGHT. LOW CONFIDENCE
  IN LIFR VSBY.
* LOW-MEDIUM CONFIDENCE IN PRECIP AT TERMINALS TUESDAY AFTERNOON.

BMD

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

* WEDNESDAY AND THURSDAY...DRY/VFR. NORTHEAST WINDS.
* FRIDAY...SLIGHT CHANCE OF SHRA/TSRA. NORTHEAST WINDS.
* SATURDAY...SLIGHT CHANCE OF SHRA. EAST WINDS.
* SUNDAY...SLIGHT SHRA/TSRA. SOUTHEAST WINDS.

&&

.MARINE...
259 AM CDT


A WEAK TROUGH WILL PUSH ACROSS LAKE MICHIGAN TODAY WITH NORTHWEST
TO NORTH WINDS OVERSPREADING THE LAKE THROUGH THE DAY. WINDS
FRESHEN UP TO 15 TO 20 KT ACROSS THE LAKE THIS AFTERNOON AND
EVENING AS THE LOW DEEPENS OVER THE EASTERN LAKES REGION AND HIGH
PRESSURE BUILDS SOUTH FROM MANITOBA. MODEST NORTH TO NORTHEAST
WINDS WILL REMAIN IN PLACE THROUGH WEDNESDAY NIGHT...THEN WILL
BEGIN TO DIMINISH FROM THE NORTH. A GRADIENT WILL HOLD ON ACROSS
THE FAR SOUTH END OF LAKE MICHIGAN THURSDAY AS LOW PRESSURE MOVES
ACROSS THE MID MISSISSIPPI VALLEY...BUT EXPECT WINDS TO DIMINISH
ACROSS THE SOUTH END BY THURSDAY NIGHT AS THE RIDGE CONTINUES TO
BUILD SOUTH.

BMD

&&

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

$$

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