Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010547
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1247 AM CDT FRI AUG 1 2014

.DISCUSSION...
845 PM CDT

AS THE SUN HAS SET...CLOUDS HAVE THINNED. TEMPERATURES HAVE BEEN
SLOW TO FALL THIS EVENING...HOWEVER EXPECT A GENERAL TREND TO
AROUND 60 BY DAYBREAK. THE EXCEPTION WILL BE LOCATIONS ADJACENT TO
LAKE MICHIGAN...WHERE TEMPS WILL LIKELY HOLD IN THE UPR 60S TO
PERHAPS ARND 70.

PRECIP THIS EVENING HAS BEEN VERY MINIMAL. IR/WATER VAPOR IMAGERY
INDICATES A WAVE OVER IOWA/SOUTHERN MN SLIDING SOUTHEAST THIS
EVENING. LOCAL ARW8KM INDICATES THIS WAVE WILL ARRIVE
OVERNIGHT...AND COULD HELP TO TOUCH OFF AN ISOLATED SHOWER. AT
THIS TIME IT DOES NOT APPEAR ANY CONVECTION WILL DEVELOP OVER THE
FORECAST AREA...HOWEVER A FEW LIGHTNING STRIKES HAVE BEEN NOTED
WELL UPSTREAM OF THE CWFA.

BEACHLER

//PREV DISCUSSION...
218 PM CDT

VERY LITTLE CHANGE TO THE GOING FORECAST FOR THE NEXT FEW DAYS AS
THE PATTERN...BOTH OBSERVED AND FORECAST REMAINS CONSISTENT.  THE
BIGGEST CHANGES TO THE GOING FORECAST WILL BE IN THE LONGER TERM
WHICH LOOKS LIKE IT COULD BE A LITTLE WETTER.

FOR THE REMAINDER OF THE AFTERNOON AND EVENING...THERE WILL BE A
CONTINUED SMALL CHANCE FOR DIURNAL ISOLATED TSRA AND SCT SHRA AS THE
REGION REMAINS UNDER CYCLONIC FLOW ALOFT...UNDER THE SWRN PERIPHERY
OF THE DEEP UPPER LOW SITTING OVER JAMES BAY.  THE MAIN DIFFERENCE
FOR TODAY OVER THE PAST COUPLE DAYS IS THAT LOW LEVEL WINDS ARE A
BIT STRONGER AND MORE WESTERLY WHICH HAS KEPT ANY LAKE BREEZE
DEVELOPMENT AT BAY.  SO...THE WARMER AIR WILL BE ABLE TO PENETRATE
UP TO THE LAKEFRONT. AS COULD COVER THIS AFTERNOON IS PREDOMINANTLY
DIURNALLY INDUCED...EXPECT THAT SKIES SHOULD BECOME MOSTLY CLEAR
OVERNIGHT...WITH LOW TEMPS AROUND 60F EXPECTED FOR MUCH OF THE
AREA...EXCEPT THE HEAVILY URBANIZED AREAS OF THE CHICAGO METRO
AREA...WHICH SHOULD REMAIN IN THE MIDDLE TO UPPER 60S.

THE REMAINDER OF THE SHORT TERM FORECAST PERIOD WILL ALSO SEE LITTLE
CHANGE FROM THE GOING FORECAST AS THE LATEST MODEL RUNS REMAIN
CONSISTENT WITH PREVIOUS RUNS AND WITH EACH OTHER.  THE VERY SLOWLY
PROGRESSIVE HIGH AMPLITUDE PATTERN WILL PERSIST INTO THE WEEKEND
WITH A SERIES OF WEAK SHORTWAVES DROPPING THROUGH THE NWLY FLOW
ALOFT.  THIS WILL KEEP THE CHANCES FOR ISOLD/SCT SHOWERS OVER THE
AREA...WITH THE CHANCE FOR ISOLD AFTERNOON THUNDERSTORM.  GIVEN THE
SLIGHT TIMING/INTENSITY DIFFERENCES BETWEEN THE MODELS WITH THESE
WEAK SHORTWAVES...AND THE INHERENT UNCERTAINTY IN THE SPECIFIC
DETAILS OF THE SENSIBLE WEATHER FEATURES...IN PARTICULAR PCPN...WILL
KEEP POPS GENERALLY LOW AND FOLLOW DIURNAL TRENDS.  STILL ANTICIPATE
A SLIGHT WARMING TREND INTO THE WEEKEND AS UPPER RIDGING BUILDS OUT
OVER THE WESTERN PLAINS...BRINGING RISING HEIGHTS TO THE
MIDWEST...BUT THE MODELS CONTINUE TO ADVERTISE BROAD TROUGHING
LINGERING INTO THE LOWER MISSISSIPPI VALLEY AS THE SEEMINGLY ENDLESS
SERIES OF WEAK SHORT WAVES DROPPING OUT OF SOUTH CENTRAL CANADA
CONTINUES TO MAINTAIN A LONG WAVE TROUGH OVER THE ERN CONUS.  AT THE
SFC...EXPECT A RETURN TO A WEAKER SFC GRADIENT AS WEAK HIGH PRESSURE
BUILDS OVER THE REGION.  SO...AFTER ONLY ONE DAY OF WARMER AIR
REACHING THE LAKEFRONT...ANTICIPATE A RETURN TO DAILY LAKE BREEZE
DEVELOPMENT FRIDAY THROUGH SUNDAY...MAX INLAND TEMPS TOPPING OUT
AROUND 80F AND LAKEFRONT TEMPS REMAINING SEVERAL DEGREES LOWER.

FOR THE LONGER TERM FORECAST PERIOD...FROM MONDAY THROUGH
THURSDAY...THE THE LONGER RANGE MODELS REMAIN CONSISTENT ON LIFTING
OUT THE DEEP UPPER LOW FROM THE JAMES BAY AREA.  IT STILL LOOKS LIKE
THE ONLY REAL OPPORTUNITY FOR TEMPS TO HIT SEASONAL NORMAL
LEVELS...HIGHS IN THE MIDDLE 80S...WILL BE MONDAY AS THE AXIS OF THE
UPPER RIDGE EXTENDS INTO THE UPPER MISSISSIPPI VALLEY AND SFC LOW
DEVELOPMENT OVER THE NRN PLAINS SETS UP A SHORT PERIOD OF MODEST
SOUTHERLY-SOUTHWESTERLY FLOW AT THE SFC.  BY TUESDAY...SHORT WAVE
ENERGY WILL REINFORCE THE BROAD UPPER TROUGH OVER THE ERN CONUS.
THE ECMWF WANTS TO GENERATE A CLOSED LOW OVER THE LOWER MISSISSIPPI
VALLEY BY TUESDAY MORNING...WHICH THEN CUTS OFF FROM THE WESTERLIES
AND SLOWLY RETROGRADES INTO THE TEXARKANA AREA.  THE GFS STARTS OUT
EARLY TUESDAY WITH A SIMILAR UPPER TROUGH...BUT THEN BECOMES MUCH
MORE PROGRESSIVE WITH THE UPPER TROUGH.  THE NET EFFECT ON THE
PATTERN IS THAT THE ECMWF IS TRENDING TOWARD MAINTAINING A MORE
HIGHLY AMPLIFIED PATTERN WHILE THE GFS BECOMES MORE ZONAL ALOFT OVER
THE UPPER MIDWEST AND ALLOWS FOR HIGHER CHANCES FOR PCPN THROUGH THE
EXTENDED FORECAST PERIOD FROM A SERIES OF SHORTWAVES TRAVERSING THE
REGION.  AT THIS POINT...THE IDEA OF A CUT-OFF LOW DEVELOPING OVER
THE LOWER MISSISSIPPI VALLEY LOOKS A BIT SUSPICIOUS...SO HAVE
TRENDED THE LONGER TERM FORECAST CLOSER TO THE GFS...WHICH WOULD
IMPLY BETTER CHANCES FOR PCPN OVER THE LOCAL AREA THAN THE
ECMWF...WHICH FAVORS THE UPPER MISSISSIPPI VALLEY THROUGH THE
EASTERN GREAT LAKES ZONE FOR HIGHER PCPN CHANCES.

KREIN

&&

.AVIATION...

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

* CHANCE OF SHRA/TSRA TODAY.
* LAKE BREEZE MID/LATE AFTERNOON.

RATZER

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

MID-LEVEL TROUGH WILL PROPAGATE SLOWLY ACROSS THE REGION TODAY.
FORCING FOR ORGANIZED CONVECTION APPEARS RATHER WEAK...THOUGH
SLIGHTLY COOLER TEMPERATURES ALOFT WILL RESULT IN UNSTABLE
CONDITIONS DEVELOPING BY MID/LATE MORNING AS SURFACE TEMPERATURES
RISE. AREA OF SHOWERS CURRENTLY OVER SOUTHEAST IOWA APPEAR TO BE
ASSOCIATED WITH MID-LEVEL CIRCULATION WITHIN MID-LEVEL TROUGH...
AND CONVECTION-ALLOWING MODEL GUIDANCE INDICATES THESE WILL
CONTINUE TO DRIFT EAST AND PERSIST SOUTH OF THE TERMINALS. IT IS
AHEAD OF THIS FEATURE INTO EAST CENTRAL ILLINOIS AND NORTHWEST
INDIANA WHERE SHRA/TSRA LIKELY TO INITIALLY EXPAND WITH INCREASING
DAYTIME INSTABILITY...WITH GENERALLY LOWER COVERAGE OF SHRA/TSRA
EXPECTED NORTH OF A BDF-SBN LINE. HOWEVER...LIGHT WINDS BOTH
SURFACE AND ALOFT ARE EXPECTED TO ALLOW LAKE BREEZE DEVELOPMENT BY
EARLY AFTERNOON...AND THE LAKE BREEZE BOUNDARY MAY BECOME A FOCUS
FOR ADDITIONAL SHRA/TSRA DEVELOPMENT WITH POTENTIAL IMPACTS FOR
ESPECIALLY THE ORD/MDW/DPA AND GYY TERMINALS. WEAK WIND FIELD WILL
RESULT IN SHRA/TSRA OUTFLOW BOUNDARIES PROVIDING FOCUS FOR NEW
DEVELOPMENT...AS WELL AS FAIRLY SLOW STORM MOTION, THUS WHILE
COVERAGE OF POTENTIAL SHRA/TSRA IS OF SOMEWHAT LOW CONFIDENCE...
ANY SHRA/TSRA WHICH FORM COULD HAVE SUBSTANTIAL IMPACT ON LOCAL
AIRSPACE GIVEN LIKELY SLOW MOVEMENT AND VARIABLE WIND DIRECTION
FROM OUTFLOW.

AS FOR WINDS...GENERALLY LIGHT WEST-SOUTHWEST PREVAILING WIND LESS
THAN 10 KT SHOULD DEVELOP AFTER SUNRISE. LAKE BREEZE WILL LIKELY
DEVELOP BY LATE MORNING...PUSHING INTO ORD/MDW/GYY DURING THE
AFTERNOON. SOME EASTERLY COMPONENT GENERALLY LESS THAN 10 KT
EXPECTED BEHIND LAKE BREEZE...WHICH WOULD EVENTUALLY BECOME
SOUTH-SOUTHEAST DURING THE EVENING. AS INDICATED ABOVE HOWEVER...
ANY SHRA/TSRA OUTFLOW MAY RESULT IN HIGHLY VARIABLE WIND
DIRECTION.

RATZER


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

* LOW CONFIDENCE FOR SHRA/TSRA DIRECTLY OVER TERMINALS.
* MEDIUM CONFIDENCE IN LAKE BREEZE BY MID/LATE AFTERNOON.

RATZER

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

* SATURDAY...CHANCE OF SHRA/TSRA. AFTERNOON LAKE BREEZE.
* SUNDAY...DRY/VFR.
* MONDAY THROUGH WEDNESDAY...CHANCE OF SHRA/TSRA.

&&

.MARINE...
211 PM CDT

HIGH PRESSURE IN THE PLAINS WITH LOW PRESSURE IN EASTERN ONTARIO IS
KEEPING THE LAKE BREEZE AT BAY TODAY WITH WESTERLY WINDS IN THE
NEARSHORE WATERS AND SOUTH WINDS ON THE OPEN LAKE. THERE ARE EVEN
SOME PATCHES OF FOG WITH VISIBILITIES REPORTED AS LOW AS 2
NAUTICAL MILES. THE LOW IN ONTARIO WILL CONTINUE SHIFTING NORTH
AS HIGH PRESSURE EXPANDS OVER THE GREAT LAKES THE NEXT FEW DAYS.
THIS WILL SLACKEN THE GRADIENT OVER LAKE MICHIGAN AND KEEP
VARIABLE WINDS UNDER 10-15 KT...WHILE LAKE BREEZES RETURN FRIDAY.
WITH THE WEAK GRADIENT ALSO EXPECT FOG AT TIMES. A WEAK COLD
FRONT WILL PUSH IN SATURDAY AS THE HIGH BUILDS SHIFTING WINDS BACK
TO NORTHERLY FOR A TIME. EXPECT LAKE BREEZES INTO EARLY NEXT WEEK
WITH A RETURN TO SOUTHERLY FLOW IN THE OPEN WATERS AS STRONGER LOW
PRESSURE ORGANIZES IN THE CENTRAL PLAINS. THE LOW WILL TRACK
EASTWARD THROUGH THE GREAT LAKES REGION MID TO LATE WEEK...BUT
STILL SOME UNCERTAINTY AS TO WHETHER IT WILL PASS OVER THE LAKE OR
DIP TO THE SOUTH. KMD

&&

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

$$

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