Area Forecast Discussion
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000
FXUS63 KLOT 260900
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
400 AM CDT WED AUG 26 2015

.SHORT TERM...
255 AM CDT

THROUGH THURSDAY...

THE UPPER LOW INFLUENCING THE WEATHER THE PAST FEW DAYS IS FINALLY
SHIFTING EASTWARD INTO ONTARIO BUT ITS BROAD CIRCULATION HAS
CONTINUED TO STEER CLOUDS INTO THE WESTERN GREAT LAKES. THESE CLOUDS
ARE ASSOCIATED WITH A COOL POCKET OF AIR AROUND 850MB WITHIN THE
CYCLONIC FLOW. AMDAR DATA INDICATES AN INVERSION IN PLACE ABOVE THIS
LEVEL WITH A FAIRLY DEEP NEAR-SATURATED DEPTH OF AROUND 4000 FT. AS
THE LOW-LEVEL FLOW BECOMES MORE NORTHERLY THROUGH THIS
AFTERNOON...AM LESS OPTIMISTIC THAT LONG DURATION CLEARING WILL
OCCUR TODAY. HIGH RESOLUTION GUIDANCE MOISTURE/CONDENSATE FIELDS
SUPPORT THIS INCLUDING THE NAM 850-925MB RELATIVE HUMIDITY...AND
HAVE LEANED HEAVILY ON THAT FOR TODAYS SKY COVER FORECAST. RARE LAKE
GENERATED CLOUDS FOR AUGUST ARE LIKELY TO STEER INTO NORTHWEST
INDIANA AND THEN EVEN NORTHEAST ILLINOIS INTO TONIGHT. THE OVERALL
CLOUD COVER SHOULD ERODE ON ITS EDGES THIS EVENING BUT EXACTLY HOW
WIDESPREAD IT WILL BE IS UNKNOWN...BUT HAVE KEPT AT LEAST PARTLY
CLOUDY SKIES FOR A MAJORITY OF THE CWA THROUGH THE ENTIRE NIGHT.

THE AFOREMENTIONED FACTORS OF CLOUDS AND FLOW TURNING OFF THE LAKE
WILL LIKELY KEEP TEMPERATURES VERY CLOSE TO WHAT THEY WERE
YESTERDAY. EVIDENCE OF RECENT UPWELLING IN THE PAST WEEK IS
INDICATED BY WATER TEMPERATURES OF 48 AT WAUKEGAN BEACH AND 57 AT
SEVERAL CHICAGO BEACHES...AND THIS MAY RESULT IN AIR TEMPERATURES
NEAR THE LAKEFRONT HOLDING OR DROPPING BACK INTO THE MID 60S THIS
AFTERNOON AS ONSHORE FLOW DEVELOPS. FOR TONIGHT...THE SAME
CHALLENGE WITH CLOUDS AND THEIR IMPACT ON TEMPERATURES WILL
REMAIN...WITH ANY CLEAR OUTLYING AREAS POSSIBLY DIPPING TO THE
UPPER 40S...WHILE MORE SO UPPER 50S IF CLOUDS PERSIST THROUGH THE
NIGHT. LIGHT ONSHORE FLOW IS ANTICIPATED AGAIN THURSDAY WHILE
AREAS AWAY FROM THE LAKE SHOULD SEE TEMPERATURES SEVERAL DEGREES
WARMER THAN TODAY.

MTF

&&

.LONG TERM...
255 AM CDT

THURSDAY NIGHT THROUGH TUESDAY...

A SHORT WAVE IMPULSE OVER THE DESERT SOUTHWEST THIS MORNING WILL
ROUND THE SOUTHERN PLAINS UPPER RIDGE AND APPROACH THE AREA FRIDAY
AFTERNOON INTO FRIDAY NIGHT. THE DIFFERENCE IN MODEL GUIDANCE
EVOLUTION OF THIS FEATURE IS QUITE HIGH LIKELY OWING TO THE SLOW
MOVEMENT OF THE SHORT WAVE AND A NORTHERN STREAM COLD FRONT MOVING
THROUGH THE FAR NORTHERN GREAT LAKES AT THE SAME TIME. GLOBAL
26.00 GUIDANCE IS NORTH OF THE MEAN SOLUTION OF ALL AVAILABLE
GUIDANCE...AND NORTH OF THE CWA WITH ITS BETTER RAIN POTENTIAL
AND FOCUS. THE ECMWF ACTUALLY MAINTAINS THE STRENGTH OF THE SHORT
WAVE AND ITS LOW-LEVEL REFLECTION...WHILE OTHER GUIDANCE WEAKENS
IT CONSIDERABLY BY SATURDAY OVER THE REGION. HAVE COLLABORATED AND
FOLLOWED A MEAN AT THIS TIME GIVEN THE SPREAD IN SOLUTIONS.
OVERALL THIS DID LEAD TO AN INCREASE IN POPS FRIDAY NIGHT AND
SATURDAY. IT WOULD SEEM INSTABILITY WOULD TAKE SOME TIME TO
RETURN...POSSIBLY NOT BRINGING THUNDERSTORM CHANCES UNTIL
SATURDAY.

THIS SYSTEM IS EXPECTED TO SLOWLY SHIFT EAST BY LATER SUNDAY
ALTHOUGH CONFIDENCE IS LOW THROUGH THE WEEKEND. BEHIND THIS
SYSTEM...THE OVERALL FORECAST SYNOPTIC SCALE PATTERN FAVORS A
DEVELOPING UPPER THICKNESS RIDGE AND LOW-LEVEL THERMAL RIDGE TO
THE WEST. GLOBAL GUIDANCE AND ENSEMBLES ARE INDICATING THIS AND
WOULD FAVOR A WARM PATTERN NEXT WEEK AS THAT RIDGING SHIFTS OVER
THE GREAT LAKES REGION. THE HEMISPHERIC PATTERN SUPPORTS ONE FOR
SLOW MOVEMENT ACROSS CENTRAL NORTH AMERICA AT THAT TIME...SO THIS
WARMING /AT LEAST MID-UPPER 80S/ COULD POSSIBLY LAST THROUGH ALL
OF NEXT WEEK INTO LABOR DAY WEEKEND...AS FAVORED BY THE CPC WEEK
TWO OUTLOOK.

MTF

&&

.AVIATION...

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

* HIGH END MVFR/LOW END VFR CLOUD DECK THROUGH THIS MORNING.

* LAKE BREEZE TURNS WINDS EAST EARLY THIS AFTN WITH SPEEDS LESS
  THAN 10 KT.

JEE

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

BKN TO OVC CIGS SPAN THE REGION WITH HIGH END MVFR CIGS PUSHING
SOUTH OUT OF WISCONSIN. DECIDED TO LOWER CIG HEIGHTS TO HIGH END
MVFR AND KEEP A SCATTERED GROUP FOR PSBL 025 CIGS. GUIDANCE IS NOT
HANDLING THE CIGS WELL SO MVFR CIGS MAY LIFT TO VFR BEFORE
18Z...BUT LEFT THEM IN THROUGH 18Z DUE TO A LACK OF CONFIDENCE.
LIGHT NORTH WINDS WILL BECOME EAST EARLY THIS AFTN BEHIND A LAKE
BREEZE. SOME GUIDANCE MEMBERS INDICATE THE LAKE BREEZE COULD PUSH
THROUGH AS LATE AS 20Z...BUT DECIDED TO LEAVE WELL ENOUGH ALONE
FOR NOW ESPECIALLY SINCE WIND SPEEDS WILL BE LESS THAN 10 KT
BEHIND THE LAKE BREEZE. WINDS WEAKEN EVEN MORE TONIGHT AND BECOME
VARIABLE.

JEE

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

* MEDIUM-LOW CONFIDENCE IN HOW LONG MVFR CIGS WILL PERSIST.

* MEDIUM CONFIDENCE IN TIMING OF LAKE BREEZE...HIGH CONFIDENCE IN
  WIND SPEEDS LESS THAN 10 KT.

JEE

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

THURSDAY...VFR.

FRIDAY...CHANCE OF TSRA LATE.

SATURDAY...CHANCE OF TSRA.

SUNDAY...VFR.

MONDAY...VFR.

RODRIGUEZ

&&

.MARINE...
255 AM CDT

NORTHWEST TO NORTH WINDS UP TO 25 KT OVER THE NORTH END OF THE LAKE
WILL DIMINISH AND BECOME NORTH ACROSS THE LAKE AS HIGH PRESSURE
SPREADS OVER THE LAKE TODAY.  WINDS BECOME VERY LIGHT AND VARIABLE
ACROSS THE LAKE THURSDAY BEFORE BECOMING SOUTHEAST TO SOUTH TO 20 KT
THURSDAY NIGHT AS THE HIGH SHIFTS OVER THE EAST COAST.  THE NEXT LOW
PRESSURE SYSTEM EJECTS OVER THE PLAINS THURSDAY AND WEAKENS AS IT
REACHES THE CENTRAL MISSISSIPPI VALLEY SATURDAY. GUIDANCE DIFFERS ON
THE LOWS EXACT PATH THIS WEEKEND WITH ONE MODEL TAKING IT OVER THE
CENTRAL PORTION OF THE LAKE BEFORE DISSIPATING OVER MICHIGAN...WHILE
ANOTHER MODEL HAS THE LOW MOVING OVER NORTHERN IL AND DISSIPATING
OVER CENTRAL INDIANA.  HAVE LOW CONFIDENCE IN THE FORECAST FOR THIS
WEEKEND SINCE THE LOCATION OF THE LOW WILL HAVE A BIG IMPACT ON WIND
DIRECTION AND SPEED.  SOUTHERLY WINDS AT 15 KT OR LESS ARE THEN
EXPECTED FOR EARLY NEXT WEEK.

JEE

&&

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

$$

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