Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 272348
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
648 PM CDT THU AUG 27 2015

.SHORT TERM...
246 PM CDT

THROUGH FRIDAY NIGHT...

TONIGHT...
VISIBLE IMAGERY THIS AFTERNOON SHOWS SOME FAIR WEATHER CUMULUS
CLOUDS ACROSS NORTHERN IL/NORTHWEST IN...WITH A CIRRUS SHIELD
ARRIVING OVERHEAD. THIS CIRRUS SHIELD WAS FROM THE AREA OF
SHOWERS/THUNDERSTORMS OVER THE CENTRAL PLAINS. WITH HIGH PRESSURE
CENTERED OVER THE REGION...THE EASTERN PROGRESSION OF PRECIP AND THE
LOW OVER THE CENTRAL PLAINS WILL BE SLOW TO MOVE. THIS WILL ALLOW
QUIET CONDITIONS TO LINGER TONIGHT...SO HAVE MAINTAIN A DRY
FORECAST. SKIES WILL STEADILY THICKEN OVERNIGHT AS THE LOW DRIFTS
EAST. EXPECT OVERNIGHT LOWS TO FALL INTO THE MID/UPR 50S ACROSS THE
BULK OF THE FORECAST AREA...WITH SOME AREAS CLOSER TO LAKE MICHIGAN
SEEING A LIGHT ONSHORE FLOW AND KEEPING TEMPS IN THE LOW 60S.

FRIDAY THROUGH FRIDAY NIGHT... DIFFERENCES AMONGST OPERATIONAL
SOLUTIONS LINGERS INTO FRI MORNING...WITH SOME OF THE HI-RES
SIMULATIONS ANTICIPATING AN AREA OF STORMS/RAIN TO PUSH EAST INTO
EASTERN IOWA THEN QUICKLY DECAY SHORTLY AFTER DAYBREAK FRI. GIVEN A
RATHER STABLE ENVIRONMENT OVER NORTHERN IL/NORTHWEST IN WITH
CONSIDERABLE CLOUD COVER...THIS APPEARS FAVORABLE TO OCCUR.
INSTABILITY DOES NOT LOOK TO EXPAND MUCH DURING THE LATE
MORNING/AFTERNOON HOURS...HOWEVER WITH SOME BETTER FORCING ARRIVING
IN THE MID-LVLS LATE MORNING/AFTERNOON...THIS MAY ALLOW FOR SOME
EMBEDDED THUNDER TO DEVELOP WITH THE SHORTWAVE AS IT APPROACHES THE
REGION. HAVE ONLY MADE MINOR ADJUSTMENTS TO THE PRECIP COVERAGE AND
TIMING.

ONE ITEM THAT MAY NEED FURTHER ADJUSTMENTS WILL BE HIGH TEMPS FOR
FRI...GIVEN THE THICKENING SOLAR SHIELDING. HAVE NUDGED TEMPS DOWN A
DEGREES INTO THE LOW/MID 70S FOR CENTRAL/NORTH AND UPR 70S ACROSS
THE FAR SOUTH. THERE MAY BE ENOUGH CLOUD COVER TO LIMIT
HEATING...AND KEEP TEMPS SEVERAL DEGREES COOLER IN THE UPR 60S TO
LOWER 70S.

PRECIP CHANCES WILL CONTINUE THRU FRI NIGHT...BUT WITH THE SHORTWAVE
LIFTING NORTHEAST ACROSS WISC...THE BEST PROBS FOR PRECIP WILL BE
FOCUSED TO OUR NORTH. HAVE REFLECTED THIS WITHIN THE CURRENT PRECIP
FORECAST WITH LOWER POPS FURTHER SOUTH. TEMPS FRI NIGHT WILL LIKELY
REMAIN MILD GIVEN THE THICK CLOUD COVER...WITH LOWS IN THE MID 60S.

BEACHLER

&&

.LONG TERM...
256 PM CDT

SATURDAY NIGHT THROUGH THURSDAY...

FOCUS ON THE EXTENDED IS FOR WEEKEND PRECIPITATION CHANCES/LINGERING
CLOUD COVER AND IMPACT ON TEMPERATURES...THEN SHIFTING TO AN
EXTENDED PERIOD OF WARM AND DRY CONDITIONS.

AT THE START OF THE PERIOD...THE LEADING UPPER
DISTURBANCE...EMANATING FROM COLORADO...WILL HAVE LIFTED NORTH AND
EAST OF THE AREA. TRAILING ENERGY FROM WYOMING WILL STILL CREATE
SOME LIFT IN THE MORNING...IN ADDITION TO A NORTHERN STREAM PACIFIC
SHORTWAVE ENERGY MOVING INTO THE BACK SIDE OF THE SLOW MOVING UPPER
LOW WHICH WILL KEEP PRECIPITATION CHANCES IN THE FORECAST FOR
SATURDAY AFTERNOON. CONFIDENCE ON THE DETAILS IS A BIT LIMITED AT
THIS POINT AS THERE IS STILL QUITE A BIT OF MODEL UNCERTAINTY AS TO
WHAT DEGREE THESE DISTURBANCES INTERACT AND THEIR TIMING. THE
INTERACTION OF THESE DISTUBBANCES ARE ALSO PLAYING HAVOC ON HOW
QUICKLY THE ASSOCIATED SURFACE LOW MOVES THROUGH...AND MORE
SPECIFICALLY PRECIPITATION CHANCES DURING THE AFTERNOON
SATURDAY...AS IF THE SURFACE BOUNDARY LINGERS...LOW LEVEL
CONVERGENCE AND MOIST UNSTABLE ENVIRONMENT MAY PROMOTE THE
REDEVELOPMENT OF THUNDERSTORMS SATURDAY AFTERNOON AFTER THE MAIN
UPPER FORCING DEPARTS.

EITHER WAY...MOST GUIDANCE SUGGEST SOME DEGREE OF UPPER LEVEL
TROUGHING SATURDAY NIGHT INTO SUNDAY..THOUGH MUCH WEAKER THEN ON
SATURDAY. STURDAY NIGHT INTO SUNDAY MORNING LOOKS TO BE A QUIET
PERIOD. THERE COULD BE SOME LINGERING CLOUDINESS OR MAY SOME LIGHT
RAIN SHOWERS...ESPECIALLY CLOSER TO LAKE MICHIGAN. WE WILL STILL
HAVE A FAIRLY MOIST ENVIORNMENT IN THE WARM SECTOR AHEAD OF THE
SURFACE LOW WHERE SOME SHOWERS AND THUNDERSTORMS REMAIN
POSSIBLE...ESPEICALLY GIVEN THE WPC PREFERENCE FOR A SOLUTION
PROGRESSION OF THE LOW. LOW CONFIDENCE EXISTS NONETHELESS...WITH
AREAS SOUTH AND EAST FAVORED FOR ANY ADDITIONAL PRECIPITATION.

MODELS ARE IN VERY GOOD AGREEMENT WITH REGARDS TO THE EXPANSION OF
THE UPPER RIDGE IN THE WEST OVER MUCH OF THE EASTERN TWO THIRDS OF
THE COUNTY. THIS WILL BE ESPECIALLY TRUE FOR MUCH OF THE NATIONS
HEARTLAND...WHILE THE SOUTH AND THE EAST COAST MAY FEEL SOME IMPACTS
FROM TROPICAL STORM ERIKA AND AN ADDITIONAL CUT OFF LOW IN THE GULF
OF MEXICO.

THE ECMWF REMAINS THE MORE AGREESIVE MODEL WITH REGARDS TO THE LOWER
LEVEL WARMING...BUT BOTH MODELS IN VERY GOOD AGREEMENT FOR AN
EXTENDED PERIOD OF HIGH TEMPERATURES IN THE UPPER 80S TO LOW 90S
...AND IF THE WARMER ECMWF VERIFIES...BEGINNING TUESDAY...AFTER A
TRANSITION DAY OF MID 80S ON MONDAY. ENSEMBLE TEMPERATURE
ANOMALIES ARE STILL ONLY IN THE 2 STANDARD DEVIATION ABOVE NORMAL
CATEGORY...BASED ON THE MORE TEMPERED GFS...AND FORTUNATELY
DEWPOINTS LOOK TO PEAK IN THE MID 60S...RESULTING IN HEAT INDICES
IN THE UPPER 80S TO LOW 90S. WITH A WEAK LOW LEVEL WIND COMPONENT
AND THE SURFACE HIGH FOCUSED TO OUR EAST...ONSHORE FLOW AT TIMES
WILL KEEP AFTERNOON CONDITIONS COOLER AT THE LAKEFRONT.

KMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* EAST WINDS AROUND 10 KT DIMINISHING THIS HOUR.

* SOUTH WINDS SHIFTING TO EAST-SOUTHEAST FRIDAY AFTERNOON.

* CHANCE OF SHRA LATE FRIDAY AFTERNOON THROUGH FRIDAY EVENING.

RC

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

AREA REMAINS INFLUENCED BY HIGH PRESSURE CENTERED OVER CENTRAL AND
EASTERN GREAT LAKES. THE HIGH PRESSURE WILL SLIDE EAST ON FRIDAY
AS WEAKENING LOW PRESSURE SLOWLY MOVES ACROSS IOWA. EAST WINDS
BEHIND LAKE BREEZE WILL DIMINISH QUICKLY THIS EVENING AND TURN
LIGHT SOUTH OVERNIGHT. SOUTH WINDS WILL TURN SOUTHEAST TO EAST-
SOUTHEAST DURING FRIDAY AFTERNOON...POSSIBLY WITH SOME LAKE
INFLUENCE AND SPEEDS ARE EXPECTED TO BE AROUND OR SLIGHTLY OVER 10
KT.

PRIMARILY VFR CIGS ARE EXPECTED THROUGH THE PERIOD...THOUGH SOME
GUIDANCE INDICATES A PERIOD OF HIGH END MVFR CIGS FRIDAY MORNING.
ALSO MVFR BR IS POSSIBLE MAINLY AT DPA LATE TONIGHT INTO EARLY
FRIDAY. AN UPPER LEVEL DISTURBANCE ASSOCIATED WITH THE LOW
PRESSURE WILL FOCUS BEST FORCING FOR PRECIP INTO SOUTHERN
WISCONSIN FRIDAY AFTERNOON. A FEW ISOLATED OR SCATTERED SHRA
CANNOT BE RULED OUT...ESPECIALLY AT RFD...BUT HAVE MAINTAINED A
DRY TAF THERE DUE TO LOW CONFIDENCE. BETTER CHANCE FOR SHRA AT ORD
LOOKS TO ARRIVE MAINLY AFTER 06Z TOMORROW NIGHT INTO SATURDAY AT
ORD...SO ALSO HAVE A DRY FORECAST IN ORD 30-HR TAF. THE CHANCE FOR
ANY THUNDER WITH THIS SYSTEM APPEARS TO BE LOW.

RC


//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* HIGH IN WIND TRENDS THROUGH FRIDAY MORNING...MEDIUM FRIDAY
  AFTERNOON AND EVENING.

* LOW IN SHRA AT TERMINALS FRIDAY AFTERNOON AND EVENING.

RC

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

SATURDAY...A FEW PERIODS OF SHRA PROBABLE AND ASSOCIATED VSBY
IMPACTS. SLIGHT CHANCE OF TSRA. VFR CIGS LIKELY.

SUNDAY...PRIMARILY VFR...SLIGHT CHANCE OF SHRA. NORTH TO NORTHEAST
WINDS.

MONDAY THROUGH WEDNESDAY...VFR. AFTERNOON WIND SHIFTS TO EAST-
SOUTHEAST POSSIBLE.

RC

&&

.MARINE...
256 PM CDT

HIGH PRESSURE EAST OF LAKE MICHIGAN WILL CONTINUE EAST TONIGHT...AS
LOW PRESSURE OVER THE CNETRAL PLAINS DEEPENS AND MOVES INTO IOWA
FRIDAY AFTERNOON. THE LOW WILL SLOW ITS EASTWARD PROGRESS AS THE
RIDGE AXIS ASSOCAITED WITH THE DEPARTING HIGH EXTENDS BACK
NORTHWESTWARD TO THE NORTHERN LAKES.  AS THE LOW APPROACHES FRIDAY
NIGHT...A PERIOD OF GUSTY EAST THEN SOUTHEAST WINDS WILL SETUP
ACROSS THE SOUTHERN HALF OF THE LAKE...POSSIBLY APPROACHING 25 KT.
THE LOW WILL PASS OVER THE SOUTHERN TIP OF LAKE MICHIGAN LATER
SATURDAY INTO EARLY SUNDAY AND WEAKEN. HIGH PRESSURE WILL THEN TAKE
CONTROL OF THE EASTERN THIRD OF THE COUNTRY FOR MOST OF THE UPCOMING
WEEK...WITH LOW PRESSURE REMAINING IN THE CENTRAL HIGH PLAINS.

KMD

&&

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

$$

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