Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 280657
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
157 AM CDT FRI AUG 28 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 DISTURBANCES 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. SATURDAY 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 ENVIRONMENT IN THE WARM SECTOR AHEAD OF THE
SURFACE LOW WHERE SOME SHOWERS AND THUNDERSTORMS REMAIN
POSSIBLE...ESPECIALLY 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 AGGRESSIVE 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 06Z...

* WIND SHIFT TO EASTERLY LIKELY LATE FRIDAY AFTERNOON
* CHANCE OF -SHRA/SLIGHT CHANCE OF TS FRIDAY NIGHT

IZZI

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

VFR WITH LIGHT WINDS THROUGH FRIDAY MORNING AS SLOW MOVING STORM
SYSTEM OVER NEBRASKA/WESTERN IOWA SLOWLY MOVES TOWARD REGION.
PRESSURE FALLS IN ADVANCE OF THIS SYSTEM SHOULD CAUSE INITIALLY
SOUTH-SOUTHWEST WINDS FRIDAY TO BACK MORE EASTERLY WITH LAKE
INFLUENCE LIKELY RESULTING IN A MORE DRAMATIC SHIFT TO EASTERLY
FOR CHICAGO TERMINALS. CERTAINLY PLAUSIBLE THAT THERE COULD BE A
PERIOD OF 10-12KT EASTERLY WINDS...PARTICULARLY WITHIN AN HOUR OR
SO OF THE WIND SHIFT...BUT CLOUD COVER AND WEAKER THERMAL GRADIENT
LEAD ME TO BELIEVE THAT IF EASTERLY WINDS REACH PREVAILING 10KT IT
WON`T LAST LONG OR MORE LIKELY JUST A PERIOD OF A COUPLE HOURS OF
OCNL 10KT WINDS. THREAT OF SHOWERS AND PERHAPS ISOLATED TSRA WILL
SLOWLY INCREASE FROM THE WEST/NORTHWEST FRIDAY NIGHT.

IZZI

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

* HIGH CONFIDENCE IN WIND SHIFT TO EAST LATE FRIDAY...LOW-MEDIUM
  CONFIDENCE IN SPEEDS AND PRECISE TIMING
* MEDIUM CONFIDENCE IN PRECIP CHANCES/TIMING
* HIGH CONFIDENCE IN REMAINING FORECAST ELEMENTS

IZZI

//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...
157 AM CDT

FAIRLY TYPICAL SUMMER PATTERN LIKELY TO PERSIST THROUGH THE
UPCOMING WEEK WITH GENERALLY NO SIGNIFICANT WINDS/WAVES
ANTICIPATED. WEAK SURFACE LOW OVER THE CORN BELT WILL TRACK SLOWLY
EAST INTO NORTHERN ILLINOIS/SOUTHERN WISCONSIN THROUGH SATURDAY
BEFORE DISSIPATING. THE MAIN RESULT WILL BE A BACKING OF WINDS TO
EASTERLY OVER SOUTHERN PORTIONS OF THE LAKE LATER TODAY INTO THIS
EVENING BEFORE VEERING SOUTHERLY AGAIN LATE TONIGHT AND FRESHENING
UP JUST A BIT...BUT LIKELY LESS THAN 20KT.

IZZI

&&

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

$$

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