Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 220540
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1240 AM CDT TUE APR 22 2014

.DISCUSSION...
858 PM CDT

HAVE DONE SOME MAINLY COSMETIC WORK TO POP/WX TO TIME THE SHOWERS
ENDING THIS EVENING AS TROUGH AXIS SHIFTS EAST OF THE AREA. CLOUD
COVER AND WESTERLY WINDS SHOULD ALLOW TEMPS TO HOLD IN THE LOW/MID
60S UNTIL FROPA OVERNIGHT. SPEAKING OF WHICH...COLD FRONT FROM NE
WI SOUTHWEST TO KUIN WILL SWEEP ACROSS THE CWA OVERNIGHT CAUSING
WINDS TO SHIFT TO NNW. THE WIND DIRECTION AND RESEVOUR OF VERY
COLD AIR OVER THE LAKE SHOULD RESULT IN SOME LAKE ENHANCEMENT TO
THE FRONT OVER NE IL AND NW IN WITH A VERY RAPID DROP IN
TEMPERATURES OVERNIGHT AND HAVE NUDGED TEMPS DOWNWARD IN THE GRIDS
AND ATTEMPTED TO ACCOUNT FOR THIS MORE RAPID DROP IN THE HOURLY
TEMP GRIDS. THE NORTHERLY WINDS OFF THE LAKE SHOULD RESULT IN
TEMPS LAKESIDE HOLDING IN THE 40S TOMORROW.

IZZI

//PREV DISCUSSION...
313 PM CDT

FOR THE REST OF THIS AFTERNOON AND TONIGHT...THE MAIN FOCUS WILL
CONTINUE TO BE THE DEVELOPING CONVECTION IN THE WARM SECTOR AHEAD OF
A COLD FRONT.  AT 2PM...THE COLD FRONT EXTENDS FROM CENTRAL
WISCONSIN THROUGH EASTERN IOWA.  A PRE-FRONTAL TROUGH HAS FORMED IN
THE WARM SECTOR AHEAD OF THE MAIN COLD FRONT...WHICH IS MOVING
THROUGH NORTHEASTERN ILLINOIS.  THE BULK OF THE THUNDERSTORM
ACTIVITY HAS FORMED EITHER ALONG THE  OR IN THE
BROAD AREA OF ISENTROPIC LIFT ACROSS NRN IL/NWRN IN IN THE WARM
SECTOR.  SFC DEWPOINTS ARE FINALLY RISING INTO THE LOWER TO MIDDLE
50S BEHIND THE PRE-FRONTAL TROUGH...WHICH WILL SUPPORT INCREASING
SHRA COVERAGE THROUGH THE REMAINDER OF THE AFTERNOON AND INTO THE
EVENING.  MID LEVEL DYNAMICS ARE COMPLICATED WITH  SEVERAL WEAK VORT
MAXIMA INDICATED IN RADAR AND SATELLITE IMAGERY...WHICH WILL
COMPLICATE THE THUNDER FORECAST.  ANTICIPATE THAT CONVECTIVE WILL
REMAIN RELATIVELY SCATTERED WITH SOME SHORT LINES OR SMALL CLUSTERS
POSSIBLY DEVELOPING.  AS DEEP LAYER SHEAR REMAINS RELATIVELY
WEAK...DO NOT EXPECT STORMS TO REACH SEVERE LEVELS...BUT WITH PWATS
OF AROUND 1 INCH PER THE 12Z SOUNDINGS FROM ILX/DVN...OCCASIONAL
HEAVY DOWNPOURS ARE LIKELY AND SOME SMALL HAIL IS NOT OUT OF THE
QUESTION.

THE COLD FRONT SHOULD PUSH THROUGH THE ROCKFORD METRO AREA BY EARLY
EVENING AND THEN PUSH THROUGH THE CHICAGO METRO AREA  SHORTLY BEFORE
MIDNIGHT WHILE CLEARING THE FAR SERN PORTIONS OF THE CWA DURG THE
EARLY MORNING HOURS...AND ALL PCPN SHOULD MOVE OUT OF THE CWA DURG
THE PRE-DAWN HOURS.  COOLER...DRIER AIR WILL QUICKLY FILTER IN
BEHIND THE FRONT AS HIGH PRESSURE BUILDS ACROSS THE NORTHERN PLAINS
AND INTO THE MIDDLE AND UPPER MISSISSIPPI VALLEYS.  TEMPERATURES
WILL BE NEARLY 20 DEGREES LOWER THAN THOSE SEEN THIS AFTERNOON.
AFTER A FEW DAYS OF VERY PLEASANT WEATHER WITH HIGHS IN THE 70S TO
AROUND 80F...COLD ADVECTION UNDER DEEP LAYER NWLY FLOW WILL DROP
TEMPERATURES INTO THE UPPER 50S TO AROUND 60F FOR MUCH OF THE REGION
TOMORROW.  AS THE CENTER OF THE HIGH PRESSURE MOVES ACROSS WISCONSIN
TOMORROW...SFC WINDS WILL GRADUALLY VEER FROM NWLY...THROUGH NLY TO
ENELY BY TOMORROW AFTERNOON.  THIS WILL SET UP FLOW OFF OF THE
RELATIVELY COLD LAKE MICHIGAN WATERS AND THE ILLINOIS AND INDIANA
LAKE FRONTS...INCLUDING DOWNTOWN CHICAGO/WAUKEGAN/GARY WILL LIKELY
SEE HIGHS ONLY IN THE MIDDLE 40S.

FOR THE REMAINDER OF THE SHORT TERM FORECAST PERIOD...LATE TUESDAY
INTO WEDNESDAY...THE UPPER LEVEL PATTERN IS EXPECTED TO
BECOME MORE HIGHLY AMPLIFIED...WITH SLOWING EASTWARD PROGRESSION TO
THE UPPER LEVEL AND SFC PATTERN.  IN PARTICULAR...THE SFC HIGH IS
EXPECTED TO SLOWLY MOVE ACROSS THE CENTRAL GREAT LAKES REGION WITH
SFC RIDGING EXTENDING SOUTH INTO THE OHIO VALLEY.  THIS WILL KEEP
COOL CONDITIONS OVER THE REGION. WITH GENERALLY NORTHERLY SFC WINDS
IN PLACE...THE LAKEFRONT LOCATIONS WILL...AGAIN...BE SIGNIFICANTLY
COOLER THAN FARTHER INLAND...WITH HIGH WEDNESDAY REMAINING IN THE
MIDDLE 40S ALONG THE LAKE.  BY LATE WEDNESDAY AFTERNOON...THE SFC
RIDGE AXIS SHOULD MOVE EAST OF THE AREA AND A RETURN FLOW OR WARMER
AIR SHOULD BRING TO DEVELOP LATE IN THE DAY AND WITH SELY WINDS
SETTING UP IN THE AFTERNOON...INLAND LOCATIONS COULD REBOUND INTO
THE UPPER 50S TO LOWER 60S.

FOR THE LONGER TERM FORECAST PERIOD...THURSDAY THROUGH SUNDAY...THE
THE MODEL GUIDANCE IS SETTLING INTO MUCH BETTER AGREEMENT ON THE
LONG-WAVE PATTERN...AT LEAST INTO SATURDAY.  THE MODELS ARE
ADVERTISING THAT THE UPPER LEVEL PATTERN SHOULD REMAIN HIGHLY
AMPLIFIED...BUT STEADILY PROGRESSIVE.  BY THURSDAY...A NRN STREAM
PACIFIC SYSTEM WILL TOP AN UPPER RIDGE OVER THE ROCKIES AND MOVE OUT
INTO THE NRN PLAINS.  A MIDDLE STREAM SHORTWAVE IS EXPECTED TO MOVE
THROUGH THE FOUR CORNERS REGION AND INTO THE CENTRAL PLAINS.  THE
12Z RUNS OF THE GFS AND ECMWF BOTH SHOW SOME DEGREE OF PHASING OF THE
TWO SHORTWAVES...INTO A FULL LATITUDE OPEN TROUGH ALOFT AND A
COMPLEX SYSTEM AT THE SFC WITH A STRENGTHENING TRIPLE POINT LOW OVER
THE CENTRAL PLAINS WITH AN OCCLUSION TO A WEAKENING SFC LOW OVER THE
NRN PLAINS AND A STRONG COLD FRONT EXTENDING SWWD INTO TEXAS.  BY
THURSDAY AFTERNOON/EVENING...THE TRIPLE POINT LOW WILL BECOME THE
DOMINANT LOW...AND THE OCCLUSION EVOLVING INTO MORE OF AN INVERTED
TROUGH.  THE AIRMASS OUT AHEAD OF THE SYSTEM SHOULD BECOME
INCREASINGLY WARM/MOISTER/MORE UNSTABLE WITH LOW LEVEL FLOW OFF OF
AN OPEN GULF OF MEXICO.  GIVEN THE BETTER MODEL CONSENSUS FOR
THURSDAY INTO FRIDAY...WILL CARRY CATEGORICAL POPS WITH THE PASSAGE
OF THE SYSTEM.  THE MODEL GUIDANCE IS EVEN INDICATING SOME SURFACE
BASED INSTABILITY IN THE WARM SECTOR...LEADING HIGHER CONFIDENCE IN
THUNDERSTORM ACTIVITY AS WELL...WITH THE BEST CHANCE FOR
THUNDERSTORMS THURSDAY INTO THURSDAY EVENING.  THE 12Z GFS IS
INDICATING PWATS OF 1.0 TO 1.5 INCHES IN THE AIRMASS AHEAD OF THE
FRONT...SO HEAVY RAINFALL WILL ALSO BE A POSSIBILITY WITH THIS
SYSTEM.

WHILE THE MODELS ARE CONSISTENT IN PUSHING THE SYSTEM EAST  OF THE
AREA BY FRIDAY MORNING...THE MODEL SOLUTIONS BEGIN TO DIVERGE
SIGNIFICANTLY BY FRIDAY NIGHT AND SATURDAY...WITH THE GFS BECOMING
MUCH MORE AMPLIFIED WITH DEEPER TROUGHING OVER THE WEST AND EAST
COAST AND STRONGER RIDGING OVER THE CENTRAL CONUS.  THE GFS
CONTINUES THE TREND OF AMPLIFICATION THROUGH THE WEEKEND WITH CLOSED
UPPER LOWS DEVELOPING OVER THE SRN ROCKIES AND NEW ENGLAND AND A
VERY HIGH AMPLITUDE RIDGE OVER THE CNTRL CONUS.  THE ECMWF DEVELOPS
A SIMILAR...BUT WEAKER UPPER LOW OVER NEW ENGLAND AND ONLY AN OPEN
WAVE OVER THE WRN CONUS.  GIVEN THE LARGE UNCERTAINTY IN THE LATTER
PORTIONS OF THE LONG TERM FORECAST PERIOD...HAVE OPTED TO MAKE
LITTLE CHANGE TO THE GOING FORECAST.

KREIN

&&

.AVIATION...

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

* FROPA WITHIN THE NEXT 1-2 HOURS.

* PATCHY FOG BEFORE FROPA.

* NORTHWEST WINDS TURNING TO THE NORTHEAST BY MID DAY.

RODRIGUEZ

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

COLD FRONT CONTINUES TO SLOWLY DROP SOUTHEAST ACROSS THE AREA WITH
RFD ALREADY OBSERVING FROPA...WITH THE REMAINING TERMINALS LIKELY
OBSERVING FROPA WITHIN THE NEXT 1-2 HOURS AND THE WESTERLY WINDS
BECOMING MORE NORTHWEST/NORTH WHILE INCREASING. ALTHOUGH OUT AHEAD
OF THIS FRONT...RATHER MOIST CONDITIONS HAVE ALLOWED FOR FOG AND
LOW CEILINGS TO DEVELOP WITH VIS GENERALLY STAYING WITHIN THE
3-5SM RANGE. DO EXPECT FOG TO BE MORE OF A PREVAILING FEATURE
BEFORE FROPA AND AM NOT OVERLY CONCERNED WITH THE LOWER CEILINGS
BECOMING AN ISSUE. WITH FROPA...DRIER AIR ON THE BACKSIDE WILL
HELP SCOUR ANY FOG/LOW CLOUD COVER WITH VFR CONDITIONS THEN
EXPECTED FOR THE TERMINALS THROUGH THE REMAINING FORECAST PERIOD.
NORTHWEST WINDS WILL LIKELY INCREASE BY MID MORNING BUT WITH A
NORTHWEST/NORTH DIRECTION...WINDS SHOULD EASILY SHIFT TO THE
NORTHEAST FOR THE TERMINALS BY THE LAKE BY MID DAY AND REMAIN THIS
DIRECTION THROUGH THE END OF THE FORECAST PERIOD.

RODRIGUEZ


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

* MEDIUM HIGH CONFIDENCE WITH FROPA...AND WIND SHIFT LATER TODAY.

* HIGH CONFIDENCE WITH FOG EARLY THIS MORNING.

RODRIGUEZ

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

WEDNESDAY...CHC OF SHOWERS. OTHERWISE VFR.

THURSDAY...SHRA/TSRA LIKELY WITH PERIODS OF MVFR CIGS/VSBYS IN
THUNDERSTORMS.

FRIDAY...SLIGHT CHC OF SHOWERS EARLY. MAINLY VFR.

SATURDAY THROUGH SUNDAY...SLIGHT CHANCE OF SHOWERS. MAINLY VFR.

BEACHLER

&&

.MARINE...
253 PM CDT

A BROAD LOW PRESSURE SYSTEM WILL MOVE ACROSS THE GREAT LAKES THIS
EVENING AND TONIGHT DRAGGING A COLD FRONT DOWN LAKE MICHIGAN FROM
MID TO LATE THIS EVENING. NNW WINDS WILL PICK UP BEHIND THE FRONT
WITH A WINDOW OF 30 KT GUSTS POSSIBLE. A SMALL CRAFT ADVISORY
REMAINS IN EFFECT FROM MID EVENING THROUGH TOMORROW MORNING THOUGH
WILL BE PARTICULARLY MARGINAL FOR THE ILLINOIS SHORES. EXTENDED
THE ADVISORY INTO THE AFTERNOON FOR INDIANA THOUGH WHICH WILL HAVE
HIGHER WAVES FOR A FEW MORE HOURS INTO THE AFTERNOON. HIGH
PRESSURE WILL ALLOW WINDS TO DIMINISH SOME TUESDAY ALLOWING LAKE
BREEZES TO DEVELOP...ESPECIALLY ON THE WESTERN SIDE OF THE LAKE
WHERE THE GRADIENT IS MORE RELAXED. THE HIGH WILL MOVE OVERHEAD
WEDNESDAY WITH WINDS BECOMING LIGHT AND VARIABLE FOR A
PERIOD...AND AGAIN ALLOWING LAKE BREEZES TO DEVELOP IN THE
AFTERNOON. SOUTHEASTERLY FLOW BEGINS TO RAMP UP WEDNESDAY NIGHT
INTO THURSDAY AS LOW PRESSURE TAKES SHAPE OVER THE CENTRAL PLAINS
AND LIFTS INTO THE UPPER MIDWEST. SOUTHEAST SUSTAINED WINDS TO
AROUND 25 KT ARE EXPECTED...THOUGH GUSTS SHOULD STAY IN CHECK AS A
STABLE MARINE LAYER DEVELOPS. BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 3 PM TUESDAY.

&&

$$

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