Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 290322
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1022 PM CDT THU MAY 28 2015

.UPDATE...
840 PM CDT

FOR EVENING UPDATE...

NO REAL CHANGES TO GOING FORECAST THIS EVENING...OTHER THAN ADD
DETAIL TO WIND GRIDS FOR LAKE BREEZE LOCATION PER METAR/RADAR OBS.
18Z NAM AND RECENT RAP/HRRR HIGH RES GUIDANCE DEVELOP SOME ISOLATED
SHOWERS ACROSS PARTS OF THE EASTERN CWA/CHICAGO METRO SHORTLY AFTER
MIDNIGHT...THOUGH FORCING APPEARS PRETTY WEAK PER 00Z RAOB AND RAP
ANALYSIS OTHER THAN SURFACE WARM FRONT PROPAGATING NORTHWARD LATE
TONIGHT. ISOLATED SFC/NEAR SFC BASED SHRA/TSRA WHICH DEVELOPED ALONG
THE WARM SIDE OF THE SURFACE WARM FRONT THIS AFTERNOON WERE QUICKLY
DISSIPATING WITH LOSS OF DIURNAL INSTABILITY. STILL APPEARS THAT THE
GREATER PROSPECTS FOR PRECIP STILL APPEAR TO HOLD OFF UNTIL MUCH
LATER TONIGHT...WITH STRONGER THETA-E ADVECTION AND MOISTURE
TRANSPORT AXIS OVER MO/IA/MISSISSIPPI VALLEY GRADUALLY SHIFTING
EASTWARD INTO WESTERN IL...INCLUDING NORTHWEST AND PARTS OF NORTH
CENTRAL IL...AHEAD OF THE SHORT WAVE CURRENTLY OVER WESTERN MO/AR
PER GOES VAPOR IMAGERY. GOING FORECAST HAS POPS INCREASING MAINLY
ACROSS WESTERN CWA 09-12Z...AND ACROSS THE EASTERN CWA BY/AFTER
12Z...AND THIS STILL LOOKS REASONABLE BASED ON CURRENT DATA.

RATZER

&&

.SHORT TERM...
225 PM CDT

THROUGH TONIGHT...

THE FORECAST AREA FINDS ITSELF BETWEEN TWO STATIONARY FRONTS
THIS AFTERNOON WITH A WEAK RIDGE OF HIGH PRESSURE SANDWICHED IN
BETWEEN DIRECTLY OVER THE AREA. THIS RIDGE HAS ALLOWED A LAKE
BREEZE WITH LAKESIDE COOLING AT THE CHICAGO SHORE ABOUT 14
DEGREES COOLER THAN INLAND AT ORD/MDW AS OF 200 PM. THE FRONT OF
INTEREST FOR OUR AREA TONIGHT WILL BE THE ONE TO THE SOUTH WHICH
WILL LIFT NORTH AS A WARM FRONT...BRINGING A RETURN TO MUGGY AIR
BY DAYBREAK...AS WELL AS KEEP NIGHTTIME TEMPERATURES MILD...EVEN
INCREASING THEM LATE TONIGHT ALONG THE LAKE SHORE AREAS.

MULTIPLE MID-LEVEL DISTURBANCES...SOME LIKELY CONVECTIVELY
ENHANCED...ARE NOTED ON WATER VAPOR AND RADAR MOSAIC IMAGERY ACROSS
THE CENTRAL PLAINS THIS AFTERNOON. ONE OF THESE JUST NORTH OF KANSAS
CITY AS OF 200 PM IS WELL ANALYZED BY THE LATEST RAP AND THIS IS
EXPECTED TO PROPAGATE INTO WISCONSIN OVERNIGHT...WITH ONE OR TWO
WEAKER IMPULSES TO ITS SOUTH APPROACHING THE AREA. WITH A 25-30 KT
LOW-LEVEL JET AHEAD OF THESE AND INCREASING MOISTURE AT THAT
HEIGHT...THERE WILL LIKELY BE SCATTERED SHOWERS OR STORMS FROM
SOUTHWEST WISCONSIN INTO NORTHWEST AND POSSIBLY NORTH CENTRAL
ILLINOIS OVERNIGHT.

MTF

&&

.LONG TERM...
314 PM CDT

FRIDAY THROUGH THURSDAY...

MAIN FORECAST CONCERNS/CHALLENGES DURING THE PERIOD ARE WITH
SHOWER/THUNDERSTORM CHANCES FRIDAY INTO SATURDAY...AND THEN WITH
MUCH COOLER AIR TO PUSH ACROSS THE AREA SATURDAY.

SOME UNCERTAINTY AS TO HOW THE PERIOD WILL BEGIN...DUE TO HOW
PRECIP DEVELOPMENT WILL EVOLVE LATE TONIGHT INTO FRIDAY MORNING.
THE CURRENT UPSTREAM MID LEVEL TROUGH OVER THE CENTRAL PLAINS WILL
CONTINUE MOVING NORTHEAST THROUGH THE CWA FRIDAY MORNING...WITH
LINGERING PRECIP FROM THE OVERNIGHT STILL LIKELY TO AFFECT
PRIMARILY THE WESTERN HALF OF THE CWA IN NORTH CENTRAL ILLINOIS.
LIKELY SCENARIO WILL BE FOR THIS DEVELOPMENT TO CONTINUE EAST
THROUGH THE MORNING. CONFIDENCE WITH OVERALL COVERAGE DURING
FRIDAY MORNING IS LOW...AND HAVE THUS CAPPED POPS AT LIKELYS.
MORNING PRECIP AND EVEN CLOUD COVER SHOULD EXIT THROUGH LATE
MORNING AND MIDDAY...WITH SKIES BECOMING MORE PARTLY CLOUDY/PARTLY
SUNNY. THIS WILL HELP WITH EARLY DESTABILIZATION WITH LITTLE TO NO
INHIBITION AND WITH MID LEVEL ENERGY RETURNING DURING THIS
TIME...THINK DEVELOPMENT WILL ONCE AGAIN BE POSSIBLE IN THE
AFTERNOON. WITH A LACK OF GOOD FOCUS AT THE SURFACE...THINK BEST
DEVELOPMENT WILL STILL BE ACROSS NORTH CENTRAL ILLINOIS WHERE
BETTER LARGE SCALE ASCENT WILL RESIDE. WEAK MID/UPPER LEVEL FLOW
WILL BE A LIMITING FACTOR WITH HOW ORGANIZED ANY STORM WILL BE AND
ALTHOUGH I DONT THINK THERE WILL BE ANY WIDESPREAD SEVERE
THREAT...AN ISOLATED STRONG TO SEVERE STORM WILL STILL BE POSSIBLE
WITH STRONG WINDS AND HEAVY RAINFALL THE MAIN HAZARDS DUE TO
INCREASING INSTABILITY AND MOISTURE. 80 DEGREE WEATHER WILL BE
LIKELY FRIDAY...BUT WITH TEMPS LIKELY BEING LIMITED IN ANY AREA
THAT OBSERVES MORE PRECIP/CLOUD COVER.

STILL THINK THAT THE BEST CHANCES FOR WIDESPREAD
SHOWER/THUNDERSTORM DEVELOPMENT WILL BE FRIDAY NIGHT INTO SATURDAY
MORNING. HOWEVER...GUIDANCE IS VARYING WITH REGARDS TO HOW THE
NIGHT WILL EVOLVE. THIS IS LIKELY DUE TO LARGER TROUGH OVER
SOUTHERN CANADA AND NORTHERN PLAINS MOVING EAST/SOUTHEAST...WHILE
A SEPARATE TROUGH AND SEVERAL IMPULSES WORKS EAST THROUGH THE
PLAINS. NONETHELESS...BEST LOW/MID LEVEL COVERAGE WILL PUSH
THROUGH THE CWA DURING THIS TIME WHILE LARGE SCALE FORCING ALSO
INCREASES. SO DO THINK PRECIP WILL PUSH THROUGH THE CWA...BUT WITH
COVERAGE AND DURATION POSSIBLY LOWER IN SOME LOCATIONS. THUNDER
SHOULD BE DECREASING IN COVERAGE THROUGHOUT THE NIGHT AND
ESPECIALLY INTO SATURDAY MORNING AS THE INSTABILITY AXIS SHIFTS
SOUTH...WITH HEAVY RAINFALL THE MAIN HAZARD DURING THIS TIME. ANY
LINGERING PRECIP WILL CONTINUE SOUTH OF THE CWA THROUGH MIDDAY
SATURDAY AS SURFACE TROUGH AND FRONT PUSH THROUGH THE REGION.
CLOUD COVER WILL LIKELY LINGER THROUGH MUCH OF SATURDAY WHILE
WINDS/GUSTS INCREASE THROUGHOUT THE DAY. HIGH TEMPS WILL LIKELY
OCCUR EARLY SATURDAY MORNING WITH TEMPS FALLING THROUGH THE
DAY...WITH 40 DEGREE WEATHER LIKELY FOR MOST LOCATIONS IN FAR
NORTHEAST ILLINOIS AND NORTHWEST INDIANA FOR MUCH OF THE DAY.

RODRIGUEZ

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 03Z...

* -SHRA AND TSRA TIMING ON FRIDAY.

KJB

//DISCUSSION...UPDATED 00Z...

THE MAIN FORECAST CONCERNS WILL BE THE TIMING OF CONVECTION ON
FRIDAY.

A WARM FRONT OVER CENTRAL ILLINOIS...WILL SHIFT NORTHWARD OVER
NORTHERN ILLINOIS LATER TONIGHT...AND THIS WILL ALLOW THE EASTERLY
WINDS TO BECOME SOUTHERLY BY MORNING...AND EVENTUALLY SOUTH-
SOUTHWEST DURING THE DAY ON FRIDAY. IT APPEARS THAT A PERIOD OR TWO
OF THUNDERSTORMS COULD IMPACT THE TERMINALS ON FRIDAY. ALTHOUGH
TIMING OF THESE STORMS IS A BIT UNCERTAIN...IT APPEARS THAT SOME
ISOLATED TO WIDELY SCATTERED STORMS MAY DEVELOP OVER THE AREA
AROUND...OR SHORTLY BEFORE DAYBREAK FRIDAY. A SECOND WEATHER
DISTURBANCE COULD SPARK OFF SOME ADDITIONAL SHOWERS AND STORMS
AROUND MIDDAY...POSSIBLY CONTINUING INTO THE EARLY AFTERNOON HOURS.
THIS IS THE DIRECTION WE HAVE GONE WITH THE LATEST TAFS...THOUGH
TIMING ADJUSTMENTS MAY STILL BE NEEDED IN THE FUTURE.

KJB

//ORD AND MDW CONFIDENCE...UPDATED 03Z...

* LOW-MEDIUM WITH SHRA AND TSRA TIMING ON FRIDAY.

KJB

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

SATURDAY....CHANCE SHRA EARLY. MVFR POSSIBLE. STRONG NORTH-
NORTHEAST WINDS.

SUNDAY...VFR/WX NIL. NORTHEAST WINDS.

MONDAY...VFR/WX NIL. EAST WINDS.

TUESDAY...VFR/WX NIL. SOUTHEAST WINDS.

WEDNESDAY...SLIGHT CHANCE TSRA. SOUTH WINDS.

KJB

&&

.MARINE...
225 PM CDT

A SURFACE COLD FRONT DRAPED FROM WEST-TO-EAST ACROSS THE NORTHERN
PART OF THE LAKE WILL SHIFT NORTHWARD THIS EVENING. THIS WILL BE A
SOURCE FOR FOG ALONG IT AND THIS HAS BEEN SEEN ON VISIBLE SATELLITE
AT TIMES DURING THURSDAY. EVEN HIGHER MOISTURE WILL MOVE NORTHWARD
ACROSS THE LAKE ON FRIDAY ALLOWING FOR THE POSSIBILITY OF FOG AS
WELL...ESPECIALLY WHERE SHOWERS ARE NOT PREVALENT.

LOW PRESSURE WILL MOVE EAST INTO QUEBEC ON FRIDAY NIGHT AND STEER
A COLD FRONT SOUTHEASTWARD ACROSS THE LAKE. AS HIGH PRESSURE
BUILDS SOUTHWARD FROM ONTARIO ON SATURDAY...IT WILL DRIVE A STRONG
NORTHERLY WINDS FOR LATE MAY ACROSS THE CENTRAL AND SOUTHERN PARTS
OF THE LAKE. THIS WILL RESULT IN SMALL CRAFT ADVISORY CONDITIONS
FOR THE ILLINOIS AND INDIANA NEARSHORE AREAS SATURDAY AFTERNOON
AND NIGHT WITH LINGERING HIGH WAVES ON SUNDAY.

MTF

&&

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

$$

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