Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 212003 AAA
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
258 PM CDT MON JUL 21 2014

.DISCUSSION...
257 PM CDT

WEATHER HIGHLIGHTS INCLUDE THE HEAT AND HUMIDITY THAT WILL BE ON TAP
THROUGH TOMORROW. A COLD FRONT WILL BRING A CHANCE FOR THUNDERSTORMS
TUESDAY EVENING INTO EARLY WEDNESDAY. COOLER AND DRY CONDITIONS WILL
RETURN FOR THE LATTER HALF OF THE WEEK...FOLLOWED BY A RETURN TO
UNSETTLED THOUGH STILL GENERALLY COOLER WEATHER FOR THE EXTENDED.

IT IS FEELING A BIT MORE SUMMERLIKE THIS AFTERNOON AS DEWPOINTS
ACROSS THE AREA HOVER IN THE MID 60S AND TEMPERATURES INTO THE 80S.
CLOUD COVER OFF THE N IS FADING AS THE STRONG UPPER RIDGE IN THE
NATIONS HEARTLAND EXPANDS NORTH AND EASTWARD INTO NORTHERN
ILLINOIS...AND MUCH LESS IN THE WAY OF DIURNAL CUMULUS CLOUDS.  THE
LAKE BREEZE HAS BEEN HELD LARGELY IN CHECK TODAY DUE TO THE
STRENGTHENING SW WINDS THOUGH IS MAKING AN APPEARANCE ALONG THE
NORTH SHORE.

OUR MAIN FEATURE OF INTEREST IS A SHORTWAVE PUSHING INTO THE DAKOTAS
WORKING ITS WAY ALONG THE PERIPHERY OF THE UPPER RIDGE OUT AHEAD OF
A DIGGING UPPER TROUGH OFF THE PACIFIC NW COAST. THUNDERSTORMS ARE
ONGOING ACROSS THE DAKOTAS AHEAD OF THIS WAVE. THERE IS ADDITIONAL
ACTIVITY SPREADING INTO NEBRASKA WHERE WEAKER FORCING IS IN PLACE.
THIS
DISTURBANCE IS FORECAST TO STRENGTHEN AS IT ENTERS MINNESOTA LATE
TONIGHT WHILE THE UPPER RIDGE FLATTENS SOMEWHAT TO OUR WEST.
THUNDERSTORMS LOOK TO BE ONGOING LATE TONIGHT AND LIKELY IN A
DECAYING PHASE AS THEY PASS BY TO THE NORTH AND POSSIBLY DRIFT
TOWARD OUR FAR WESTERN COUNTIES. SOME HI-RES GUIDANCE SUGGESTS
THAT SOME ACTIVITY MAY SNEAK INTO THESE AREAS ESPECIALLY IF THE
ACTIVITY PUSHING INTO NEBRASKA HOLDS TOGETHER...BUT STILL FEEL THAT
MOST/ALL OF THE AREA WILL REMAIN DRY INTO TUESDAY MORNING WITH ALL
SREF/GEFS MEMBERS KEEPING THINGS OUT OF OUR AREA.

ON TUESDAY THIS SHORTWAVE WILL CONTINUE SLIDING SOUTHEAST AND WILL
DRIVE A COLD FRONT AND SURFACE WAVE THROUGH NE IL/NW IN TUESDAY
NIGHT. BEFORE THAT HAPPENS...THE 850H THERMAL RIDGE BUILDS OVERHEAD.
GUSTY SW WINDS AND MOISTURE POOLING ARE EXPECTED AHEAD OF THE COLD
FRONT WITH THE MOST UNCOMFORTABLE DAY OF THE WEEK. HEAT INDEX VALUES
LOOK TO TOP OUT AROUND 100 OR SO DO TO AN UPTICK IN TEMPERATURES AND
A MORE SIGNIFICANT INCREASE IN DEWPOINTS OVER THAT EXPERIENCED TODAY.

MODELS ARE STILL AT ODDS WITH HOW WIDESPREAD THUNDERSTORM ACTIVITY
WILL BE WITH THE FRONTAL PASSAGE DUE TO A STRONG CAP IN
PLACE...MAKING FOR A CHALLENGING FORECAST. I PREFER NOT TO BACK OFF
ON LIKELY POPS BUT FEEL SLIGHTLY LESS CONFIDENT THAN YESTERDAY ON
MORE THAN SCATTERED COVERAGE. WITH THE GENERAL PATTERN CANNOT
RULE OUT ISOLATED ACTIVITY IN THE AFTERNOON WITH CONVERGENCE AHEAD
OF THE FRONT IN THE MOIST WARM SECTOR. BUT THE MORE ORGANIZED TIME
PERIOD WOULD BE IN THE EVENING HOURS. CONFIDENCE IS MODERATE TO
HIGH THAT THERE BE AT LEAST SCATTERED THUNDERSTORM ACTIVITY.
INSTABILITY AND MOISTURE VALUES ARE VERY HIGH...AND BULK SHEAR HAS
SHOWN SIGNS OF INCREASING IN RECENT MODELS SPC HAS HIGHLIGHTED SE
WISCONSIN AND NE ILLINOIS IN THE SLIGHT RISK CATEGORY FOR SEVERE
STORMS TUESDAY NIGHT IN THE REGION WHERE STRONG INSTABILITY/HIGHER
SHEAR WILL BE THE BEST CO- LOCATED...GENERALLY ALONG AND NORTH OF
INTERSTATE 80. SEVERE WIND GUSTS/LARGE HAIL MAY BE POSSIBLE...BUT
THE GREATEST THREAT WILL BE HEAVY RAINFALL RATES GIVEN PWAT VALUES
ABOVE 2" WHICH MEANS STORMS WILL BE VERY EFFICIENT RAIN MAKERS
WITH 1"/HR TOTALS POSSIBLE. STORMS MAY STILL BE SURFACE BASED EVEN
WITH THE FRONTAL PASSAGE COMING IN THE EVENING AS TEMPERATURES AND
DEWPOINTS REMAIN ON THE HIGH SIDE. NO CLEAR SIGNAL OF ANY TRAINING
GIVEN THE PROGRESSIVE NATURE OF THE FRONT. CAPPING ISSUES DECREASE
CONFIDENCE IN COVERAGE FARTHER SOUTH AND EAST...BUT IRONICALLY AS
THE FRONT AND UPPER FORCING HAVE A LONGER TIME TO ACT ON THE CAP
AREAS SOUTH MAY ACTUALLY HAVE A BETTER CHANCE FOR MORE WIDESPREAD
ELEVATED STORMS...THOUGH CONFIDENCE ON SEVERE IN THESE AREAS IS
LOWER.

850 TEMPERATURES DROP SOME 8-10C ON WEDNESDAY WITH THE LOW LEVEL
FLOW TURNING ONSHORE. HIGH PRESSURE BUILDS SOUTH INTO THE UPPER
MIDWEST. THIS WILL KEEP A MODEST PRESSURE GRADIENT DRAPED OVER THE
AREA WEDNESDAY BEHIND THE DEPARTING SURFACE WAVE. THEREFORE EXPECT
GUSTY N/NE WINDS AND HIGHS IN THE 70S. THE SURFACE HIGH BUILDS
OVERHEAD THURSDAY FOR ANOTHER SEASONABLY COOL AND DRY DAY...THEN THE
HIGH DRIFTS EAST ON FRIDAY BRINGING SOME RECOVERY TO TEMPS CLOSER TO
80 OR SO WITH MORE OF SOUTHERLY COMPONENT TO LOWER LEVEL WINDS.

BEYOND THIS TIME FRAME...AN UPPER TROUGH WILL REMAIN PARKED OVER
EASTERN CANADA WHILE ANOTHER UPPER LOW MIGRATES THROUGH THE CANADIAN
PRAIRIES. WEAK RIPPLES AROUND THE CENTRAL U.S. UPPER RIDGE COMBINED
WITH WARM AIR ADVECTION FRIDAY NIGHT INTO SATURDAY. THE UPPER AIR
PATTERN EVOLUTION GETS A BIT MESSIER BEYOND THAT...BUT GENERALLY THE
REGION WILL REMAIN UNDER THE INFLUENCE OF THE EASTERN U.S. TROUGH TO
SOME DEGREE CREATING SOMEWHAT COOLER AND UNSETTLED CONDITIONS.

KMD

&&

.AVIATION...

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

* GUSTY SOUTH/SOUTHWEST WINDS TUESDAY. CMS

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

SOUTH/SOUTHWEST WINDS WILL CONTINUE THROUGH THE PERIOD AS THE
GRADIENT SLOWLY TIGHTENS. A LAKE BREEZE IS POSSIBLE RIGHT ALONG
THE IL SHORE THIS AFTERNOON...BUT IS NOT EXPECTED TO REACH ANY OF
THE TERMINALS INCLUDING GYY. PREVAILING SPEEDS WILL REMAIN IN THE
8-12 KT RANGE THROUGH SUNSET WITH SOME HIGHER GUSTS INTO THE
MID/UPPER TEEN KT RANGE ACROSS NORTHWEST IL AND RFD. SPEEDS WILL
DIMINISH UNDER 10 KTS THIS EVENING AND THEN INCREASE AGAIN AFTER
SUNRISE TUESDAY MORNING. HOW STRONG WINDS WILL BECOME TUESDAY
MORNING INTO TUESDAY AFTERNOON IS STILL A BIT UNCERTAIN. FORECAST
SOUNDINGS SHOW LOW LEVELS MIXING SHORTLY AFTER SUNRISE...THUS
EXPECT SPEEDS/GUSTS TO BE STEADILY INCREASE THRU 12Z-14Z. IN
ADDITION...QUITE A BIT OF WIND ALOFT DURING THE MORNING AND
CURRENT SPEEDS/GUSTS MAY END UP BEING TOO LOW. GUSTS INTO THE
LOWER/MID 20KT RANGE WILL BE POSSIBLE FROM MID MORNING THROUGH
EARLY TUESDAY EVENING.

THERE IS A CHANCE OF TSRA ACROSS PORTIONS OF NORTHEAST IA AND
SOUTHWEST WI LATE TONIGHT INTO TUESDAY MORNING. ITS POSSIBLE THAT
SOME OF THIS ACTIVITY COULD DRIFT SOUTHEAST TOWARD RFD BEFORE
DISSIPATING TUESDAY MORNING. WITH MUCH OF THIS ACTIVITY EXPECTED
TO STAY NORTHWEST OF RFD...MAINTAINED DRY CONDITIONS WITH THIS
FORECAST. A COLD FRONT WILL MOVE SOUTH ACROSS THE TERMINALS
TUESDAY NIGHT...SHIFTING WINDS NORTHERLY. THERE IS A CHANCE OF
TSRA ALONG AND AHEAD OF THIS FRONT TUESDAY EVENING INTO TUESDAY
NIGHT AND APPEARS THAT ANY TSRA TUESDAY EVENING WOULD BE AFTER
THE END OF THIS 30 HOUR ORD TAF.

PATCHY/SHALLOW FOG FORMED AGAIN THIS MORNING AND THIS MAY
REDEVELOP OVERNIGHT INTO EARLY TUESDAY MORNING. CMS

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

* MEDIUM FOR WINDS THROUGH THE PERIOD. CMS

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

TUESDAY NIGHT...THUNDERSTORMS LIKELY.
WEDNESDAY THROUGH FRIDAY...VFR.
SATURDAY THROUGH SUNDAY...CHANCES FOR THUNDERSTORMS.

BMD

&&

.MARINE...
247 AM CDT

HIGH PRESSURE SPANS FROM NEW ENGLAND INTO THE MID MISSISSIPPI VALLEY
WHILE A COUPLE AREAS OF LOW PRESSURE LIE OVER HUDSON BAY AND THE
NORTHERN PLAINS. A MODEST GRADIENT HAS DEVELOPED OVER NORTHERN LAKE
MICHIGAN THIS MORNING AND WILL SPREAD ACROSS THE REMAINDER OF THE
LAKE TODAY WITH SOUTH WINDS FRESHENING UP INTO THE 15 TO 25 KT RANGE
ACROSS PARTICULARLY THE NORTHERN PORTIONS OF LAKE MICHIGAN. THE AREA
OF LOW PRESSURE OVER THE NORTHERN PLAINS WILL TRACK ACROSS THE
NORTHERN GREAT LAKES REGION TUESDAY NIGHT INTO WEDNESDAY DRAGGING A
COLD FRONT ACROSS LAKE MICHIGAN. WINDS TURN NORTHERLY BEHIND THE
FRONT AND A QUICKLY TIGHTENING GRADIENT AND MODERATE PRESSURE RISES
OVERSPREADING THE LAKE WILL RESULT IN NORTH/NORTHWEST WINDS TO
AROUND 30 KT AT TIMES. HIGH PRESSURE WILL BUILD BACK OVER THE LAKE
DURING THE LATTER HALF OF THE WEEK RESULTING IN TAPERING WINDS AND
WAVES.

BMD

&&

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

$$

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