Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 211745
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1245 PM CDT MON JUL 21 2014

.DISCUSSION...
205 AM CDT

TODAY THROUGH TONIGHT...
MAIN CONCERNS FOR TODAY WILL BE ON HOW WARM SFC TEMPS BECOME...AS
BROAD SFC RIDGE CONTINUES TO PROVIDE A DRY INFLUENCE. THE STRETCH OF
DRY WEATHER OVER THE PAST SEVERAL DAYS...AND DEW PTS NOT POISED TO
START CLIMBING BACK INTO THE UPR 60S TO LOW 70S UNTIL LATE
TONIGHT...THIS WILL ALLOW PARCELS TO EASILY WARM GIVEN THE DRY
ENVIRONMENT...FORTUNATELY GUIDANCE HAS PICKED UP ON THIS AND TEMPS
ARE GENERALLY IN THE MID/UPR 80S. THE SOUTHERLY FLOW SHUD HELP PUSH
TEMPS TO NEAR 90 FOR DOWNTOWN CHICAGO.

THE SFC RIDGE WILL CONTINUE TO DRIFT EAST...WHICH WILL ALLOW THE
FLOW TO SLOWLY TURN SOUTHWESTERLY LATE TONIGHT. ENOUGH SFC RIDGING
WILL LINGER INTO TONIGHT TO HOLD-OFF PRECIP...HOWEVER THE ATMOS
RIVER THAT HAS BEEN PARKED OVER THE CENTRAL PLAINS WILL ARRIVE LATE
THIS EVENING. THIS WILL PUMP DEW POINTS BACK INTO THE UPR 60S OVER
THE REGION...AND HELP HOLD OVERNIGHT LOWS UP IN THE UPR 60S TO PSBLY
THE LOW 70S CLOSER TO DOWNTOWN CHICAGO. GUIDANCE HAS CONTINUED TO
SUGGEST THAT ANY COMPLEX OF THUNDERSTORMS THAT DEVELOP OVER THE
UPPER MIDWEST WITH A VORT MAXIMA TO REMAIN WELL NORTHWEST OF THE
FORECAST AREA TONIGHT...SO HAVE HELD ONTO DRY FORECAST OVERNIGHT.

TUESDAY...
SHORTWAVE WILL BE SLIDING EAST ACROSS LAKE SUPERIOR SHORTLY AFT
DAYBREAK...WITH EXPECTED ONGOING CONVECTION. HI-RES LCL ARW8KM
SUGGESTS A FRONTAL BOUNDARY WILL EXTEND SOUTH FROM THIS FEATURE INTO
SOUTHERN WISC...WHICH COULD ACT AS A TRIGGER TO PRODUCE AN AREA OF
SHOWERS/THUNDERSTORMS EARLY TUE. FORTUNATELY TEMPS ALOFT WILL BE
RISING...NEARING 20-23 DEG C...WHICH WILL PRODUCE A DECENT CAPPED
ENVIRONMENT THRU THE AFTN HOURS TUE. THIS SHUD LIMIT CONVECTIVE
DEVELOPMENT UNTIL LATE IN THE DAY...SO MOST OF TUE COULD END UP
BEING DRY. UNFORTUNATELY THE ATMOS RIVER WILL BE ARRIVING OVERHEAD
WITH DEW PTS PUSHING INTO THE LOW 70S...WITH PWAT VALUES HOVERING
ARND 2" BY TUE AFTN/EVE. SFC TEMPS SHUD BE ABLE TO REALIZE THE WARM
AIR ALOFT...COUPLED WITH THE STRETCH OF DRY DAYS PRECEDING
TUE...THAT SFC TEMPS WILL PUSH TO ARND 90 PSBLY THE LOW 90S. WITH
THE HIGH VOLUME OF MOISTURE IN THE ATMOS...AND TEMPS NEARING THE LOW
90S...HEAT INDEX VALUES WILL BE HOVERING ARND 100 DEG BY MID-AFTN
TUE.

THEN BY LATE AFTN/EVE THE FOCUS WILL TURN TOWARDS A VORT/WAVE THAT
WILL PUSH INTO WESTCENTRAL WISC. BY EARLY EVE THE CAPPED ENVIRONMENT
SHUD BE STARTING TO WEAKEN...HOWEVER THE LCL ARW8KM IS LEANING
TOWARDS THE CAP LINGERING INTO THE EVENING HOURS. THERE WILL BE AN
APPROACHING WAVE APPROACHING TO THE NORTH/NORTHWEST OF THE
FORECAST AREA...WHICH COULD BECOME A FOCAL POINT FOR ANY
CONVECTIVE GROWTH. FORTUNATELY FOR THE FORECAST AREA THERE
REMAINS MARGINAL SHEAR. THE BEST TIMING APPEARS TO BE LATE IN THE
EVENING...BUT THE LACK OF FORCING OVER THE AREA COULD RESULT IN
JUST SOME ISOLATED STORMS. IT WILL ALL HINGE ON THE LONGEVITY OF
THE CAP.

CONFIDENCE IN TEMPS/HEAT INDEX VALUES...HIGH.
CONFIDENCE IN STORMS...MEDIUM.

WEDNESDAY THROUGH THURSDAY...
MID-LVL RIDGE WILL BE BUILDING OVER THE CENTRAL ROCKIES WED WITH A
DOWNSTREAM TROUGH ARRIVING IN THE GREAT LAKES. A SFC RIDGE WILL ALSO
BE ASSOCIATED WITH THE GREAT LAKES TROUGH...WITH TEMPS WED LIKELY
HOLDING IN THE 70S FOR MOST LOCATIONS AND DRY WEATHER RETURNING FOR
WED/THUR. THE ONSHORE FLOW FOR BOTH DAYS COULD HOLD TEMPS ARND 70
FOR ADJACENT LAKE MICHIGAN LOCATIONS. OVERNIGHT LOWS WILL DIP INTO
THE LOW/MID 50S...TO ARND 60 FOR AREAS ADJACENT TO LAKE MICHIGAN.

CONFIDENCE...HIGH.

FRIDAY THROUGH THE WEEKEND...
ENSEMBLES MEMBERS HAVE CONTINUE TO DEMONSTRATE A HIGH CONFIDENCE IN
THE EXTENDED PERIODS...WITH THE LATE WEEK SFC RIDGE HOLDING THRU
MOST OF THE DAY FRI. THIS WILL MAINTAIN DRY WEATHER THRU FRI...ALONG
WITH TEMPS TRYING TO RETURN TO NEAR 80 DEG. THE BROAD MID-LVL RIDGE
OVER CENTRAL ROCKIES APPEARS TO FLATTEN SLIGHTLY FRI...WHICH COULD
RELAX ENOUGH FRI EVENING TO NUDGE THE GREAT LAKES SFC RIDGE FURTHER
EAST. CURRENT EXTENDED OPER GUIDANCE IS INDICATING A SHORTWAVE
SLIDING ALONG THE NORTHERN PERIPHERY OF THE MID-LVL RIDGE FRI
NGT...WITH POP CHANCES RETURNING LATE FRI THRU THE WEEKEND.

ENSEMBLES AND OPER SOLUTIONS HAVE BEEN CONSISTENT IN RETURNING A
HIGH AMPLITUDE RIDGE TO THE WESTERN CONUS EARLY NEXT WEEK...WITH A
HIGH LATITUDE BLOCK OVER THE NORTH ATLANTIC. THIS COULD BRING
TROUGHING AND UNSEASONABLY COOL WEATHER BACK TO THE GREAT LAKES
REGION.

CONFIDENCE...HIGH.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* 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 18Z...

* 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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