Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 261955
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
255 PM CDT WED AUG 26 2015

.SHORT TERM...
209 PM CDT

THIS AFTERNOON THROUGH THURSDAY...

MAIN CONCERN FOR THE REMAINDER OF THE AFTERNOON AND INTO EARLY THIS
EVENING WILL BE SKY COVER AND TEMPERATURE TRENDS.  AS AN
UNSEASONABLY DEEP UPPER LOW OVER WRN QUEBEC SLOWLY LIFTS NEWD...THE
GREAT LAKES REGION THROUGH THE OHIO VALLEY REMAINS UNDER COLD
ADVECTION WITH DENSE LOW LEVEL CLOUD COVER.  THUS...TEMPERATURES
ACROSS MUCH OF THE AREA HAVE BEEN STRUGGLING TO RISE THROUGH THE
60S.  SO...HAVE BACKED OFF ON MAX TEMPS FOR THIS AFTERNOON...WITH
MUCH OF NERN IL/NWRN IN REMAINING UNDER 70F.  THE ONLY AREA THAT
COULD TOP 70F WILL LIKELY BE A LINE FROM ROCKFORD TO PONTIAC TO
REMINGTON.

AS THE UPPER LOW CONTINUES TO SLOWLY TRACK NEWD...SFC HIGH PRESSURE
WILL BUILD ACROSS THE UPPER MISSISSIPPI VALLEY AND ACROSS THE
WESTERN GREAT LAKES OVERNIGHT TONIGHT.  HAVE SOME CONCERNS ABOUT
CLOUD COVER OVERNIGHT TONIGHT WHICH WOULD...IN TURN...HAVE AN IMPACT
ON OVERNIGHT LOW TEMPERATURES.  MUCH OF THE GUIDANCE INDICATES
DIMINISHING CLOUD COVER OVERNIGHT...WHICH WHEN COMBINED WITH A DRY
AIRMASS WITH DEWPOINTS ONLY IN THE LOWER TO MIDDLE 50S...AND
LGT/VRBL WINDS UNDER THE HIGH...TEMPERATURES SHOULD DROP INTO THE
LOWER TO MIDDLE 50S.  AS USUAL...THE URBAN HEAT ISLAND OF THE
CHICAGO METRO AREA SHOULD REMAIN A BIT WARMER WITH LOWS AROUND 60F.
HOWEVER...THERE IS A BIT OF A BUST POTENTIAL WITH SKY COVER...AND
CONSEQUENTLY...MIN TEMPS...IF SKY COVER DOES NOT DISSIPATE.  WHILE
THE CLOUD APPEARANCE IS TRANSITIONING FROM STRATUS TO MORE DIURNAL
STRATOCU...PER LATEST SATELLITE IMAGERY...SOME OF THE HIGHER RES
GUIDANCE SUGGESTS THAT SOME MOISTURE COULD REMAIN TRAPPED UNDER A
SUBSIDENCE INVERSION...WHICH COULD KEEP SKY COVER AND MIN TEMPS
HIGHER OVERNIGHT.  HOWEVER...FEEL IT IS MORE LIKELY GIVEN THE MORE
DIURNAL CU APPEARANCE TO CLOUD COVER AND THE CENTER OF THE SFC HIGH
MOVING ACROSS THE IL/WI BORDER OVERNIGHT THAT SKIES SHOULD BECOME
MOSTLY CLEAR AND THAT TEMPS WILL...ONCE AGAIN...DROP TO UNSEASONABLY
LOW LEVELS.

FOR THURSDAY...EXPECT THE BEGINNING OF A WARMING TREND AS THE CENTER
OF THE HIGH MOVES EAST TO THE ERN GREAT LAKES AND LOW PRESSURE
DEEPENS OVER THE CNTRL PLAINS.  THIS SHOULD ALLOW FOR LOW LEVEL FLOW
TO BECOME MORE SLY AND HEIGHTS BUILD ALOFT.  DO NOT EXPECT MUCH IN
THE WAR OF CLOUD COVER TOMORROW WITH THE BUILDING UPPER RIDGE ACROSS
THE REGION...SO EXPECT TEMPERATURES TO REBOUND INTO THE MIDDLE TO
UPPER 70S.  HOWEVER...EXPECT A NORTHWEST-SOUTHEAST ORIENTED LAKE
BREEZE TO DEVELOP WITH SELY WINDS DEVELOPING OVER THE IL LAKEFRONT
THROUGH THE DAY.  SO...THE IL LAKEFRONT...INCLUDING DOWNTOWN
CHICAGO...SHOULD REMAIN A BIT COOLER...WITH HIGHS IN THE UPPER 60S
TO AROUND 70F.

KREIN

&&

.LONG TERM...
255 PM CDT

THURSDAY NIGHT THROUGH WEDNESDAY...

AFTER UPPER HEIGHTS BUILD THURSDAY...THE WEAK UPPER RIDGE AXIS WILL
PASS OVERHEAD THURSDAY NIGHT AND HOLD IN OUR EASTERN AREAS FRIDAY
MORNING. THE UPPER LEVEL CIRCULTATION THAT WILL SLOWLY MIGRATE
THROUGH THE AREA THIS WEEKEND WILL BE POSSIBLY BECOME A BIT BETTER
DEFINED OVERNIGHT...BUT INITIALLY RIDGING AND DRIER ESE FLOW SHOULD
LIMIT THE EASTWARD EXPANSION OF PRECIPITATION ASSOICATED WITH THIS
SYSTEM AND ALLOW IT TO WEAKEN TO SOME DEGREE. ADDITIONALLY...THE
UPPER LOW WILL BECOME REMOVED FROM THE STRONGER WESTERLY FLOW ALOFT
WHICH SHOULD ALSO ACT TO SLOW ITS PROGRESSION. AND WHILE THE SURFACE
LOW WILL BE FILLING...HIGH PRESSURE TO THE EAST WILL MAINTAIN A
MODEST GRADIENT ACROSS OUR AREA.

GUIDANCE IS IN BEST AGREEMENT IN SPREADING PRECIPITATION ACROSS THE
AREA FRIDAY NIGHT INTO SATURDAY. NORTHERN ILLINOIS AND NORTH INTO
WISCONSIN IS THE FAVORED AREA IN THE CO-LOCATION BETWEEN THE UPPER
FORCING AND A WARM FRONT WHICH WILL BE FOCUSED ALONG AND NORTH OF
THE SURFACE LOW TRACK. THAT SAID...A BROAD UPPER TROUGH WILL MOVE
THROUGH MOST OF NORTHEAST ILLINOIS ON SATURDAY...AND WITH A SLIGHT
SOUTHWARD SHIFT OF THE WARM FRONT COULD BRING A BIT MORE
PRECIPITATION...ESPECIALLY SATURDAY MORNING. NAM/GFS DISAGREE AS TO
THE DEGREE OF DESTABILIZATION...BUT SHOWER AND STORM CHAN

THE UPPER LOW LIFTS NORTHEAST LATER SATURDAY...WITH A SECONDARY
SHORTWAVE POTENTIALLY DROPPING SOUTHEAST ON THE BACK SIDE OF THE LOW
SUPPORTING LOWER CHANCES FOR SHOWERS/ISOLATED STORMS SATURDAY NIGHT
INTO SUNDAY AS CAPE DIMINISHES.

ON THE HEELS OF THIS SYSTEM WILL BE AN ANOMALOUSLY STRONG RIDGE OF
HIGH PRESSURE WHICH HAS BEEN DOMINATING THE WEATHER IN THE WESTERN
U.S FOR SEVERAL DAYS AND BRINGING UNSEASONABLY WARM CONDITIONS.
THIS RIDGE WILL BUILD INTO THE MIDWEST SUNDAY AND LINGER THROUGH
MUCH OF THE WEEK. LARGELY DRY CONDITIONS RULE..WITH THE CONCERN THEN
FOCUSED ON THE HEAT EXPECTED TO RETURN FOR MUCH OF THE WEEK.
ENSEMBLE ANOMALIES ARE MODEST...AND 850/925 TEMPERATURE CLIMATOLOGY
FOR EARLY SEPTEMBER LOOKS TO SUPPORT TEMPERATURES AT THIS POINT IN
THE 87-91 DEGREE RANGE OR SO. HEAT INDICES LOOK TO REACH THE 90S FOR
A SEVERAL DAY PERIOD. ONSHORE FLOW AT TIMES WILL KEEP AFTERNOON
CONDITIONS COOLER AT THE LAKEFRONT.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* NORTH WINDS TURNING EAST/NORTHEAST MID/LATE AFTERNOON.
* HIGH END MVFR CIGS THIS AFTERNOON. CMS

//DISCUSSION...UPDATED 18Z...

MAIN CONCERN THIS AFTERNOON ARE WIND DIRECTIONS AND ASSOCIATED
SPEEDS. WINDS HAVE REMAINED MAINLY NORTHERLY SO FAR WITH SPEEDS
UNDER 10KTS. WINDS HAVE TURNED A BIT MORE NORTHEASTERLY ALONG THE
IMMEDIATE LAKE MICHIGAN SHORE AND A LAKE BREEZE IS STILL EXPECTED
THOUGH CONFIDENCE ON TIMING IS DECREASING. SHORT TERM GUIDANCE
CONTINUES TO SUPPORT A SHIFT TO MORE NORTHEAST OR EAST/NORTHEAST
IN THE 20Z-21Z TIME PERIOD. HAVE MAINTAINED THAT TIMING WITH THIS
FORECAST BUT FURTHER REFINEMENT MAY BE NEEDED. SPEEDS WILL LIKELY
REMAIN UNDER 10KTS...THOUGH A PERIOD OF SPEEDS AROUND 10KTS IS
POSSIBLE WITH THE LAKE BREEZE ITSELF. SPEEDS WILL DIMINISH THIS
EVENING AND BECOME LIGHT AND VARIABLE OR CALM TONIGHT. WINDS
SHOULD REMAIN LIGHT THURSDAY MORNING THEN TURN MORE SOUTHEASTERLY
THURSDAY AFTERNOON.

OVC HIGH MVFR DECK IS LOCKED IN PLACE. SOME BREAKS OVER THE LAKE
MAY ARRIVE LATER THIS AFTERNOON WITH SOME SCATTERING POSSIBLE
THIS EVENING AND OVERNIGHT. CMS

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* HIGH FOR MVFR CIGS THIS AFTERNOON...LOW FOR END TIME.
* MEDIUM FOR WINDS SHIFTING NORTHEAST EARLY/MID AFTERNOON... LOW
  FOR TIMING. CMS

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

THURSDAY NIGHT...VFR. SOUTH WINDS.

FRIDAY...CHC SHRA & SCHC TSRA OVERNIGHT. SOUTH WINDS BECOMING
SOUTHEAST OVERNIGHT.

SATURDAY...CHC TSRA. SOUTHEAST WINDS BECOMING NORTHEAST OVERNIGHT.

SUNDAY...VFR. NORTHEAST WINDS BCMG EAST.

MONDAY...VFR. SOUTH WINDS.

TUESDAY...VFR. SOUTHWEST WINDS.

JEE

&&

.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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