Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 092203
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
503 PM CDT WED JUL 9 2014

.DISCUSSION...
329 PM CDT

OVER THE SEVERAL DAYS THE FORECAST CAN BE SUMMARIZED IN THREE PARTS:
COOLER AND DRIER THROUGH THURSDAY AND MUCH OF FRIDAY...WARM HUMID
AND MORE UNSETTLED LATE FRIDAY THROUGH THE WEEKEND INTO MONDAY...AND
THEN DRY AGAIN WITH ABNORMALLY COOL TEMPERATURES TOWARD THE MIDDLE
OF NEXT WEEK.

IN TERMS OF HIGH TEMPERATURE NORMALS...TODAY BEGINS THE WARMEST
SEVEN DAYS OF THE YEAR AT CHICAGO. THE ANNUAL CURVE AT RFD IS
FLATTER WITH A LONGER PEAK PERIOD OF WARMEST TEMPERATURES...BUT WE
ARE IN THE HEART OF ITS WARMEST TIME OF YEAR AS WELL. THE NEXT TWO
DAYS AND MONDAY WILL SEE HIGHS A FEW DEGREES BELOW THOSE AVERAGE
VALUES...THE WEEKEND WILL PROVIDE TYPICAL JULY WEATHER...THEN
TUESDAY AND WEDNESDAY NEXT WEEK WILL USHER IN A PERIOD THAT WILL
FEEL MORE LIKE A TYPICAL MID TO LATE MAY.

FOR THE SHORT TERM...THROUGH FRIDAY NIGHT...

A BACKDOOR FRONT THAT DROPPED ACROSS THE AREA EARLY TODAY HAS
USHERED IN HIGH PRESSURE AND PUSHED THE BETTER BOUNDARY LAYER
MOISTURE WELL TO OUR SOUTH.  THIS HIGH PROVIDES CONTINUED COOLER AND
DRIER WEATHER TOMORROW AS IT SETTLES OVERHEAD.  ON FRIDAY IT BEGINS
TO ALLOW A BIT OF RETURN FLOW TO WORK INTO NORTHWEST ILLINOIS AS IT
STARTS TO MOVE EAST OF THE AREA.

FRIDAY PROVIDES A BIT OF A CHALLENGING FORECAST.  THE DETERMINISTIC
GFS AND ECMWF BOTH HINT AT THE PRESENCE OF MID TO UPPER LEVEL
SHORTWAVE ENERGY CROSSING THE AREA IN GENERALLY ZONAL OR SLIGHTLY
NORTHWESTERLY FLOW...THOUGH THIS MAY OCCUR A BIT EARLIER THAN THE
BOUNDARY LAYER MOISTURE RETURN WILL HAVE PRIMED THE AIRMASS TO TAKE
ADVANTAGE OF THIS FORCING.  HAVE ELECTED TO GRADUALLY INCREASE
DEWPOINTS AND POPS FROM WEST TO EAST THROUGH THE DAY FRIDAY BUT KEEP
POPS IN THE CHANCE CATEGORY AT MOST.  FORECAST SOUNDINGS SHOW SOME
MID LEVEL SATURATION AND SMALL CAPE VALUES...SO TSRA ALSO IS A
POSSIBILITY BUT SPOTTY SHOWERS MAY BE MORE LIKELY.

BY FRIDAY NIGHT AND INTO THE WEEKEND THE LOW LEVEL MOISTURE IS WELL
ESTABLISHED OVER THE AREA.  MODELS ARE NOT IN ANY BETTER AGREEMENT
AS TO THE TIMING AND LOCATION OF EITHER SYNOPTIC OR MESOSCALE
FORCING FRIDAY NIGHT...BUT ALL CONTINUE TO SUGGEST THERE WOULD BE
SUPPORT FOR SHOWERS AND THUNDERSTORMS OVER AT LEAST SOME OF THE
AREA...PERHAPS WITH THE SOUTHWEST PORTION OF THE CWA IN THE MOST
FAVORABLE LOCATION GIVEN THE EVOLUTION OF THE LOW LEVEL JET RELATIVE
TO THE 850 TO 300 MB THICKNESS GRADIENT.  NEIGHBORS ARE IN GENERAL
AGREEMENT AS TO THESE TRENDS.  STILL AM NOT CONFIDENT ENOUGH TO GO
WITH ANYTHING MORE THAN CHANCE VALUES AT THIS POINT.

LENNING

FOR THE LONGER TERM...SATURDAY THROUGH WEDNESDAY...

THE POSSIBLE STORMS FROM FRIDAY NIGHT MAY LINGER INTO SATURDAY
MORNING. DURING THE DAY...THE GFS AND ECMWF SHOW A MODEST TO STRONG
SHORTWAVE WITH ASSOCIATED CYCLONIC VORTICITY ADVECTION AND WAA.
COMBINED WITH ABUNDANT MOISTURE ALREADY IN PLACE...THERE IS MODERATE
CONFIDENCE OF ADDITIONAL STORM DEVELOPMENT SATURDAY DAY INTO EVE.
THESE STORMS WITH ASSOCIATED CLOUD COVER MAY LIMIT SURFACE HEATING
DURING THE DAY SATURDAY...CONTRIBUTING TO SOME UNCERTAINTY IN HIGH
TEMPS AND AMOUNT OF INSTABILITY. CONFIDENCE IN TIMING OF STORMS ALSO
IS LOW - THE GFS BRINGS PRECIP THROUGH THE CWA SATURDAY DURING THE
DAY AND MOVES IT OUT BY SATURDAY EVE...WHILE THE ECMWF BRINGS THE
BULK OF THE PRECIP LATER SATURDAY NIGHT INTO SUNDAY MORNING. HAVE
HANDLED THIS BY INCREASING POP TRENDS FOR SATURDAY NIGHT...WITH THE
EVENING AROUND 00Z BEING THE BEST OVERLAP PERIOD FOR THE TWO
SOLUTIONS.

BOTH MODELS AGREE THAT YET ANOTHER SHORTWAVE WILL MOVE THROUGH OUR
CWA IN THE LATE SUNDAY TO MONDAY TIMEFRAME...BRINGING WITH IT
ANOTHER CHANCE OF STORMS. AGAIN...CONFIDENCE IN TIMING AND
PROBABILITY IS LOW TO MODERATE AS THE GFS IS MORE PROGRESSIVE WITH
THESE SHORTWAVES THAN THE ECMWF.

MONDAY AFTERNOON/EVENING INTO TUESDAY REPRESENTS A TRANSITION
PERIOD. DRIER AIR BEGINS FILTERING IN ON MONDAY...ACCOMPANIED BY
CLEARING SKIES DURING THE AFTERNOON/EVENING. MONDAY NIGHT MARKS THE
BEGINNING OF A STRONG CAA PATTERN THANKS TO SIGNIFICANT TROUGHING
OVER THE GREAT LAKES. THIS ABNORMALLY STRONG TROUGHING FOR MID-JULY
WILL RESULT IN TEMPS MUCH BELOW AVERAGE...HIGHS MAY STRUGGLE TO
REACH THE MID-70S ON TUESDAY AND WEDNESDAY. ADDITIONALLY...GUIDANCE
IS KEEPING THE CWA DRY FOR THE REST OF THE FORECAST PERIOD.

CBS/CCH/MDF/RGD/LENNING

&&

.AVIATION...

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

* LAKE BREEZE TIMING...NORTHWEST WINDS SHIFTING TO THE EAST
  NORTHEAST WITH SPEEDS INTIALLY NEAR 10KT THEN SUBSIDING.

RODRIGUEZ/DEUBELBEISS

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

QUIET CONDITIONS ACROSS THE TERMINALS AT THIS TIME WITH THESE
QUIET AND VFR CONDITIONS EXPECTED TO REMAIN THROUGH THE REST OF
THE FORECAST PERIOD. THE ONLY CONCERN IS WITH THE LAKE BREEZE AND
TIMING THIS AFTERNOON. MOVEMENT OF THE LAKE BREEZE HAS BEEN
VARIABLE OVER THE LAST HOUR...WITH LOWER LEVEL SPEEDS STILL ON THE
HIGHER SIDE. EXPECT THESE STRONGER WESTERLY WINDS TO DIMINISH
EARLY THIS AFTERNOON WHILE THE CURRENT SLOW WESTWARD MOVEMENT
INCREASES. CONFIDENCE IS HIGHER WITH LAKE BREEZE TIMING FOR MDW
WITH IT RAPIDLY APPROACHING AT THIS HOUR. WILL CONTINUE TO MONITOR
TIMING FOR ORD...ESPECIALLY WITH SOME INCREASE IN THIS MOVEMENT
ANTICIPATED.

RODRIGUEZ

//ORD AND MDW CONFIDENCE...UPDATED 22Z...

* HIGH CONFIDENCE IN VFR CONDITIONS.

* HIGH CONFIDENCE OF LAKE BREEZE WIND SHIFT...MEDIUM HIGH
  CONFIDENCE WITH SPEEDS...LOW MEDIUM CONFIDENCE WITH EXACT
  TIMING.

RODRIGUEZ/DEUBELBEISS

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

THURSDAY NIGHT...VFR

FRIDAY THROUGH SUNDAY...GENERALLY VFR WITH A CHANCE OF SHOWERS
AND THUNDERSTORMS EACH DAY.

MONDAY AND TUESDAY...VFR.

KJB

&&

.MARINE...
218 PM CDT

LOW PRESSURE CONTINUES TO PULL AWAY ACROSS QUEBEC THIS
AFTERNOON...WITH A WEAK SURFACE TROUGH SAGGING SOUTH ACROSS THE
NORTHERN GREAT LAKES. THIS TROUGH WILL MOVE SOUTHEAST ACROSS THE
LAKE THROUGH TONIGHT...THOUGH THE GRADIENT WILL WEAKEN SIGNIFICANTLY
WITH THE APPROACH OF SURFACE HIGH PRESSURE FROM THE WEST. THIS HIGH
WILL MOVE ACROSS LAKE MICHIGAN TONIGHT INTO THURSDAY...WITH WINDS
BECOMING GENERALLY LIGHT/VARIABLE WITH LAKE BREEZES ALONG THE
SHORES. THE SURFACE RIDGE WILL DRIFT EAST OF THE LAKE FRIDAY...
ALLOWING SYNOPTIC SOUTH-SOUTHEAST FLOW TO DEVELOP AND EVENTUALLY
STRENGTHEN INTO FRIDAY NIGHT AS A TROUGH OF LOW PRESSURE ADVANCES
FROM THE PLAINS. SOUTH WINDS WILL INCREASE INTO THE 20 KT RANGE
FRIDAY AFTERNOON INTO SATURDAY MORNING...ESPECIALLY ACROSS THE
NORTHERN PORTION OF THE LAKE...BEFORE DECREASING AGAIN LATER
SATURDAY AS THE GRADIENT BECOMES WEAKER IN THE VICINITY OF THE
APPROACHING TROUGH AXIS. WIND DIRECTION MAY BE A BIT VARIABLE IN THE
BAGGY GRADIENT OF THE TROUGH AXIS...BEFORE THE TROUGH ACTUALLY MOVES
PAST THE LAKE AND WINDS SHIFT WEST-NORTHWEST LATE SUNDAY NIGHT AND
MONDAY.

RATZER

&&

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

$$

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