Area Forecast Discussion
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000
FXUS63 KLOT 070719
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
219 AM CDT THU MAY 7 2015

.UPDATE...
845 PM CDT

FOG CONTINUES TO BE THE CHALLENGE FOR TONIGHT...MAINLY FOR THE
LAKE MICHIGAN SHORELINE FROM DOWNTOWN CHICAGO NORTH. THE WARM
FRONTAL BOUNDARY CONTINUES TO LIFT NORTH THIS EVENING...HOWEVER
THE PROGRESSION WILL SLOW WITH THE LACK OF WINDS. WINDS WILL
GENERALLY TURN SOUTHERLY POST FROPA...HOWEVER ALONG THE ILLINOIS
LAKE MICHIGAN SHORE WINDS REMAIN ONSHORE OR NORTHEASTERLY. PRIOR
TO SUNSET VISIBLE IMAGERY INDICATED SOME FOG HAD PUSHED BACK
INLAND OR WITHIN A FEW MILES OF THE SHORE. GOES-R SATELLITE
IMAGERY AGAIN TONIGHT DEPICTS THE FOG WELL OVER THE LAKE...AND
INDICATES SOME OF THE FOG BLEEDING INLAND ALONG THE ILLINOIS
SHORE. EXPECT AS THE WINDS TURN SOUTHERLY...THIS WILL SLOWLY LIFT
THE FOG NORTH AND POSSIBLY BACK OVER THE LAKE. HAVE OPTED TO ISSUE
AN SPS TO HANDLE THE PATCHY DENSE FOG OVER LAND...BUT THIS MAY
NEED TO BE MODIFIED LATER IF THE ONSHORE WINDS DO NOT CHANGE
OVERNIGHT AND THE FOG BEGINS TO EXPAND. CURRENTLY HI-RES GUIDANCE
DOES NOT SUGGEST ANY EXPANSION TO THE FOG OVER LAND.

OVERNIGHT TEMPS WILL REMAIN MILD...WITH LOWS STRUGGLING TO COOL
MUCH BEYOND THE LOW 60S SOUTH OF I-80 AND TO THE NORTH MAINLY IN
THE MID TO UPR 50S. THIS WILL ALLOW A HIGHER STARTING POINT FOR
THUR. WITH MINIMAL CLOUD COVER EXPECTED FOR MUCH OF THUR...COUPLED
WITH SOUTHERLY FLOW AND STRONG MIXING...MAX TEMPS MAY MAKE A RUN
AT THE MIDDLE TO UPPER 80S. BASED ON LOCAL ANALYSIS...IT WOULD
APPEAR HIGHS THUR MAY TOUCH 86 DEGREES FOR THE CHICAGO METRO AREA.
ENSEMBLE GUIDANCE CONTINUES TO INDICATE PWAT VALUES JUST OVER
1.00...BUT A FEW HI-RES GUIDANCE MEMBERS SUGGEST ADDTL MOISTURE
ADVECTING NORTH AND COULD PUSH PWAT TO NEARLY 1.50 BY MIDDAY.

BEACHLER

&&

.SHORT TERM...
224 PM CDT

THROUGH THURSDAY...

MUCH OF THE AREA IS GETTING OUT OF THE HAZE/FOG EARLY THIS
AFTERNOON...WITH FOG EVEN LIFTING IN DOWNTOWN CHICAGO. AT 2
PM...THERE WAS A 29 DEGREE DIFFERENCE IN TEMPERATURE BETWEEN
MIDWAY AIRPORT (81) AND NORTHERLY ISLAND (52). THE STATIONARY
FRONT THAT WAS PARKED OVER CENTRAL ILLINOIS YESTERDAY IS MARCHING
NORTHWARD AND ALL BUT FAR NORTHEAST ILLINOIS...INCLUDING NORTHERN
COOK AND LAKE COUNTIES...IS FEELING THE WARMTH BEHIND THIS FRONT
AS TEMPERATURES QUICKLY WARMED TO AROUND 80 ELSEWHERE. AN
EXTENSIVE CUMULUS FIELD IS FILLING IN IN THE WARM SECTOR. IN
ADDITION TO THIS FUN...THE LAKE BREEZE IS NOW MOVING THROUGH COOK
COUNTY WHICH WILL LIMIT THE NORTHEASTWARD MOVEMENT OF THE FRONT.
MEANWHILE ALOFT...SHORTWAVE ENERGY IS ROTATING NORTHWARD AHEAD OF
AN UPPER LOW IN THE PLAINS. SHOWERS AND THUNDERSTORMS REMAIN
CONFINED EAST OF THE MISSISSIPPI.

FOR THE REST OF THIS AFTERNOON/EARLY EVENING...WEAK SHORTWAVE ENERGY
CONTINUES NORTH WITH THE WARM FRONT. HIGH RES GUIDANCE SUGGESTS THAT
ANY SHOWERS/TSTORMS THAT FORM THIS FAR EAST WOULD BE ISOLATED IN
NATURE AT BEST AS THE UPPER LEVEL HEIGHT FALLS ARE MAINLY OFF INTO
IOWA. MODELS SUGGEST THE WARM FRONT WILL STALL ITS NORTHWARD
MOVEMENT...AND THEREFORE WITH THE MOIST AIRMASS IN PLACE...EXPECT
SOME FOG/HAZE TO REFORM THIS EVENING. NOT QUITE SURE HOW THICK OR
WIDESPREAD IT WILL BE...BUT NOT LIKE IT WAS THIS MORNING. EXPECT
FOG TO REFORM OVER THE LAKE...SO IT COULD BECOME DENSE AGAIN ALONG
THE LAKESHORE AREAS. THROUGH THE NIGHT WE WILL ALSO SEE SOME HIGH
CLOUD OUT AHEAD OF CONVECTION TO OUR WEST.

ON THURSDAY...PRONOUNCED SOUTHERLY FLOW SHOULD FINALLY ALLOW THE
ENTIRE AREA TO HAVE A TASTE OF SUMMER WARMTH. SLIGHT SE FLOW AT THE
SURFACE AT THE LAKE WILL PROVIDE SOME COOLING FROM CHICAGO NORTHWARD
INTO AREAS NEAR THE LAKE IN LAKE COUNTY ILLINOIS AFTER WARMING
THROUGH EARLY AFTERNOON. WIDESPREAD 80S ARE STILL ANTICIPATED WITH
850/925 TEMPS A TOUCH HIGHER THAN TODAY WHERE WE WERE EASILY ABLE TO
QUICKLY REACH THE UPPER 70S TO LOW 80S ALREADY TODAY. THE UPPER LOW
IN THE PLAINS ONLY INCHES EASTWARD...BUT SOME WEAK HEIGHT FALLS IN
THE AFTERNOON MAY ALLOW A FEW SHOWERS/T-STORMS TO PUSH INTO AREAS
ALONG AND EAST OF I-39...BUT MORE ORGANIZED ACTIVITY DURING THE DAY
WILL REMAIN CLOSER TO A COLD FRONT EXTENDING FROM THE UPPER MIDWEST
INTO THE CENTRAL PLAINS.

KMD

&&

.LONG TERM...
320 PM CDT

THURSDAY NIGHT THROUGH WEDNESDAY...

THE EXTENDED PERIOD LOOKS TO BECOME RATHER ACTIVE...ESPECIALLY BY
THE WEEKEND. OVERALL...A DIGGING NORTHERN STREAM JET OVER THE
WESTERN COAST OF THE COUNTRY WILL SET UP A BROAD TROUGH WITH AN
EMBEDDED CLOSED LOW ACROSS THE WESTERN CONUS LATE THIS WEEK. AS
SOUTHWESTERLY FLOW ALOFT CONTINUES TO INCREASE DOWNSTREAM OF THIS
FEATURE IN THE COMING DAYS...NUMEROUS SMALLER SCALE MID-LEVEL SHORT
WAVE DISTURBANCES ARE LIKELY TO EJECT OUT OF THE MAIN SOUTHWEST
TROUGH. MEANWHILE...IN THE LOWER LEVELS...AN INCREASING LOWER LEVEL
WIND FIELD WILL ALLOW A PLUMB OF SUBTROPIC MOISTURE TO BE
TRANSPORTED NORTHWARD ACROSS THE CENTRAL CONUS INTO THE UPCOMING
WEEKEND. THIS ULTIMATELY LOOKS TO RESULT IN SEVERAL PERIODS OF
SHOWERS AND THUNDERSTORMS DURING THE LATER HALF OF THE PERIOD.

THE FIRST OF SEVERAL OF THESE SMALL SCALE DISTURBANCES LOOK TO MOVE
NORTHEASTWARD ACROSS THE UPPER MIDWEST THURSDAY NIGHT. MOISTURE
TRANSPORT AND WARM AIR ADVECTION LOOKS TO BEGIN TO INCREASE ACROSS
IOWA AND NORTHWESTERN ILLINOIS BY THURSDAY EVENING. THIS COULD
RESULT IN SOME SCATTERED SHOWER AND STORM DEVELOPMENT ACROSS
PORTIONS OF THE AREA THURSDAY NIGHT. HOWEVER...IT APPEARS THE BETTER
FOCUS FOR ACTIVITY MAY REMAIN TO THE WEST AND NORTH OF MOST OF MY
AREA...SO I HAVE ONLY LOW END POPS ACROSS MY NORTH CENTRAL COUNTIES.

MODEL GUIDANCE IS IN GOOD AGREEMENT THAT ANOTHER DISTURBANCE WILL
SHIFT ACROSS NORTHERN ILLINOIS SOMETIME DURING THE DAY/EVENING ON
FRIDAY...LEADING TO GOOD CHANCES FOR THUNDERSTORM DEVELOPMENT ACROSS
NORTHERN ILLINOIS OUT AHEAD OF A SLOWLY SOUTHEASTWARD ADVANCING
SURFACE COLD FRONT. THIS LOOKS TO PRODUCE OUR BEST CHANCE OF SHOWERS
AND THUNDERSTORMS OVER THE REGION THROUGH AT LEAST LATER IN THE
WEEKEND. THIS DISTURBANCE IS EXPECTED TO INDUCE AN UPTICK IN THE
MOISTURE TRANSPORT OVER THE AREA LATER FRIDAY AND THIS SHOULD ALLOW
SOME HEALTHY PRECIPITABLE WATER VALUES AROUND 1.5" TO SET UP OVER
THE AREA. ALTHOUGH NO SEVERE THUNDERSTORMS ARE EXPECTED WITH THIS
ROUND OF STORMS...IT APPEARS THAT SOME HEAVY RAINFALL COULD OCCUR
GIVEN THE HIGHER PWATS AND HENCE MORE EFFECT RAINFALL POTENTIAL
SETTING UP.

THE SURFACE COLD FRONTAL BOUNDARY IS EXPECTED SHIFT OVER FAR
NORTHERN ILLINOIS BY SATURDAY AFTERNOON...THEN BECOME QUASI-
STATIONARY NEAR THE INTERSTATE 80 CORRIDOR THROUGH SUNDAY. THIS WILL
AGAIN RESULT IN MUCH COOLER CONDITIONS ACROSS NORTHERN SECTIONS OF
THE AREA...WITH ONSHORE FLOW LIKELY TO RESULT IN VERY COOL
CONDITIONS NEAR THE LAKE AGAIN...SIMILAR TO WHAT WE SAW YESTERDAY.
A SHARP TEMPERATURE GRADIENT IS AGAIN LIKELY OVER THE AREA...WITH
TEMPERATURES REMAINING NEAR 80 ACROSS MY SOUTHERN COUNTIES ON THE
WARM SIDE OF THE FRONT. AREAS OF FOG ARE ALSO POSSIBLE AGAIN OVER
NORTHERN ILLINOIS...BUT I WILL LEAVE OUT OF THE FORECAST UNTIL WE
GET A BETTER IDEA ON TIMING.

SHOWERS AND STORMS WILL AGAIN BE POSSIBLE ON SATURDAY...THOUGH THE
BEST CHANCES LOOK TO REMAIN ON THE WARM SIDE OF THE FRONTAL BOUNDARY.
THIS BOUNDARY MAY THEN SHIFT BACK NORTHWARD AS A WARM FRONT AT SOME
POINT LATER IN THE WEEKEND AS THE MAIN SOUTHWESTERN CONUS SYSTEM
SHIFTS OVER THE PLAINS...AND DRIVES A SURFACE LOW EASTWARD OVER THE
MID-MISSOURI VALLEY BY SUNDAY EVENING. INCREASING MID-LEVEL FLOW AND
FALLING HEIGHTS ALOFT ATOP A WARM AND MOIST WARM SECTOR COULD SET
THE STAGE FOR A PERIOD OF STRONG OR EVEN SEVERE THUNDERSTORMS LATER
SUNDAY/SUNDAY EVENING AHEAD OF AN EASTWARD ADVANCING COLD FRONTAL
BOUNDARY.

MUCH COOLER WEATHER LOOKS ON TAP FOR A DAY OR TWO ON TUESDAY AND
POSSIBLY WEDNESDAY OF NEXT WEEK AS A COOL AREA OF HIGH PRESSURE
SHIFTS OVER THE AREA IN THE WAKE OF THE COLD FRONTAL PASSAGE. THE
COOLER AIRMASS MAY ONLY SUPPORT LOWER 60S FOR DAYTIME
HIGHS...THOUGH THE MAGNITUDE OF THIS COLDER AIRMASS IS STILL A
BIT UNCERTAIN AT THIS RANGE.

KJB

&&

.AVIATION...

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

* SOUTH WINDS GUSTING TO 25 KT THIS AFTERNOON.

JEE

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

MUCH QUIETER CONDITIONS WITH MOST SITES AT VFR. ANY MVFR FOG
SHOULD QUICKLY BURN OFF THIS MORNING. LIGHT WINDS WILL BECOME
SOUTHEAST ARND 5 KT OVERNIGHT AND THEN SOUTH BY MID MORNING.
EXPECTING A WARM AND BREEZY DAY WITH GUSTS UP TO 25 KT AT THE
EASTERN TERMINALS AND UP TO 30 KT AT RFD. COULD SEE GUSTS A BIT
HIGHER IF WE MIX MORE. GUSTS DIMINISH THIS EVENING WITH SOUTH
WINDS ARND 10KT TONIGHT.

JEE

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

* HIGH CONFIDENCE IN ALL FORECAST ELEMENTS.

JEE

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

* FRIDAY...SHRA/TSRA LIKELY. MVFR POSSIBLE. SOUTH WINDS.
* SATURDAY...CHANCE OF SHRA/TSRA. MVFR LIKELY. IFR POSSIBLE. NORTH
  WINDS.
* SUNDAY...CHANCE OF SHRA/TSRA. MVFR POSSIBLE. NORTHEAST WINDS.
* MONDAY...SHRA/TSRA LIKELY. MVFR LIKELY. GUSTY SOUTHWEST WINDS.
* TUESDAY...CHANCE OF SHRA. STRONG/GUSTY WESTERLY WINDS.

&&

.MARINE...
218 AM CDT

WIND GUSTS OVER LAND WILL INCREASE TO 25-30 KT TODAY AND THINKING A
FEW GUSTS TO 25 KT WILL BLEED INTO THE NEARSHORE WATERS.
THEREFORE...ISSUED A SMALL CRAFT ADVISORY FOR LATE THIS MORNING
THROUGH THIS AFTN.  ALSO EXTENDED THE DENSE FOG ADVSY OVER THE OPEN
WATERS UNTIL 18Z.  HAVE LOW CONFIDENCE IN HOW THE FOG WILL BEHAVE
TODAY AND HOW QUICKLY IT WILL DISSIPATE.  FORECAST SOUNDINGS LOOK
VERY STABLE OVER THE LAKE BUT WINDS INCREASE THIS EVENING TO 25 KT
WHICH SHOULD HELP MIX OUT ANY LINGERING FOG.  THE NEXT LOW MOVES
OVER ONTARIO TONIGHT AND TOMORROW AND ITS COLD FRONT SWEEPS DOWN THE
LAKE FRIDAY NIGHT. WINDS TURN NORTH BEHIND THE FRONT AND SLOWLY VEER
TO EAST THROUGH SUNDAY.  ANOTHER LOW FORMS OVER THE PLAINS THIS
WEEKEND AND WILL IMPACT THE LAKE EARLY NEXT WEEK. GUIDANCE STILL HAS
CONSIDERABLE DIFFERENCES IN TIMING...PATH...AND STRENGTH OF THIS
LOW. BOTH MODELS DO AGREE THAT HIGH PRESSURE MOVES IN MID WEEK
BEHIND THE LOW.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-
     LMZ777-LMZ779 UNTIL 11 AM THURSDAY.

&&

$$

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