Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 210829
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
329 AM CDT THU AUG 21 2014

.DISCUSSION...
330 AM CDT

SHORT TERM...THROUGH THURSDAY NIGHT...

FOR THE REMAINDER OF THE OVERNIGHT HOURS...AN AREA OF SHOWERS AND
THUNDERSTORMS HAS DEVELOPED ALONG AN ELEVATED BAROCLINIC
ZONE...NORTH OF A WARM FRONT THAT EXTENDS FROM LOW PRESSURE OVER
SOUTH DAKOTA THROUGH CNTRL IL.  THE THE CELL MOTION OF THESE STORMS
IS TO THE SOUTHEAST AS UPPER RIDGING BUILDS OVER THE UPPER
MISSISSIPPI VALLEY AND AN UPPER LOW SITE OVER GEORGIAN BAY.  THE
RIDGE AXIS SHOULD REMAIN IN PLACE ALONG THE MISSISSIPPI RIVER...SO
FLOW ALOFT SHOULD REMAIN GENERALLY NWLY.  EXPECT THAT PCPN COVERAGE
THROUGH THE REMAINDER OF THE NIGHT SHOULD REMAIN GENERALLY SCATTERED
OR IN SHORT LINES.  WITH DAYTIME WARMING...ANTICIPATE THAT THE TSRA
COVERAGE SHOULD INCREASE...BUT REMAIN FOCUSED ALONG THE ELEVATED
BAROCLINIC ZONE WHICH SHOULD REMAIN OVER NRN IL/NWRN IN.  WHILE THE
SHORT RANGE MODELS HAVE NOT REALLY HANDLED THE CONVECTIVE TRENDS ALL
THAT WELL OVER RECENT RUNS...THE LATEST HIGH RES GUIDANCE SUGGESTS
THAT THERE COULD BE A LULL IN THE ACTIVITY DURG THE AFTERNOON HOURS
WITH SOME WEAK SHORT WAVE RIDGING MOVING OVER THE REGION. HAVE MADE
SOME ADJUSTMENT TO THE MAX TEMPS FOR TODAY AS WELL.  EXPECT THAT
THERE SHOULD BE MORE EXTENSIVE DEBRIS CLOUD OVER MUCH OF THE
AREA...REMNANTS OF THE DECAYING CONVECTIVE ACTIVITY...SO HAVE BACKED
OFF ON MAX TEMPS BY A FEW DEGREES EAST OF A LINE FROM DIXON TO
GIBSON CITY...WHICH COINCIDES WITH THE ELEVATED BAROCLINIC ZONE.
SO...WILL GO WITH HIGHS ARND 83-85F.  THERE IS STILL THE POTENTIAL
FOR THE FAR SOUTHWESTERN PORTIONS OF THE CWA TO SEE TEMPS IN THE
UPPER 80S TO NEAR 90F AS THAT PORTION OF THE CWA LIES SOUTH OF THE
MAIN BAROCLINIC ZONE AND THERE SHOULD BE LESS CLOUD COVER AND MORE
OPPORTUNITY FOR TEMPS TO CLIMB A LITTLE HIGHER.

LATEST THINKING IS THAT THE BEST OPPORTUNITY FOR MORE ORGANIZED
CONVECTION...INCLUDING THE CHANCE FOR LOCALLY HEAVY RAINFALL...WILL
BE DURG THE LATE EVENING THROUGH THE OVERNIGHT HOURS.  THE MODELS
ARE KEYING ON A SHORTWAVE CURRENTLY OVER KANSAS AND NEBRASKA TO RIDE
UP THE NWRN SIDE OF THE RIDGE THURSDAY AFTERNOON...OVERTOP THE RIDGE
EARLY THURSDAY EVENING AND THEN DIVE SOUTHEAST ACROSS NRN IL/IN LATE
THURSDAY EVENING AND OVERNIGHT THURSDAY NIGHT.  FOLLOWING THIS
TREND...HAVE GONE WITH CATEGORICAL POPS OVER THE NRN PORTIONS OF THE
CWA THIS EVENING AND LIKELY POPS FOR MUCH OF THE AREA FOR THE PERIOD
FROM 00Z-12Z FRIDAY.  THE THREAT FOR LOCALLY HEAVY RAINFALL IS STILL
PRESENT AS PWATS WILL BE SLIGHTLY IN EXCESS OF 2 INCHES.
HOWEVER...DO NOT EXPECT A WIDESPREAD FLOOD THREAT AS THUNDERSTORMS
SHOULD BE PROGRESSIVE...BUT LOCALLY HEAVY RAINFALL COULD CAUSE SOME
URBAN AND SMALL STREAM TYPE FLOODING ISSUES.

KREIN

FRIDAY THROUGH SUNDAY...
MAIN FOCUS WILL BE ON CONVECTIVE CHANCES...WHICH WILL ALSO PLAY A
PIVOTAL ROLE ON TEMPS/HEAT INDEX VALUES FOR THE FRI-SUN STRETCH.

GUIDANCE CONTINUES TO FAVOR A SLIGHT FLATTENING IN THE MID-LVL RIDGE
FRI...WHICH COULD SETUP THE ATMOSPHERIC RIVER OVER THE CWFA ORIENTED
FROM IOWA EAST THROUGH SOUTHERN WISC/NORTHERN IL...STRETCHING EAST
INTO NORTHERN IN. THE LOCAL ARW8KM HAS CONTINUED TO BE A LITTLE LESS
EXCITED THAN SREF/NAM...HOWEVER THE THEME DOES APPEAR TO BE
CONSISTENT. CONFIDENCE HAS DIMINISHED A LITTLE FOR FRI...BUT HAVE
ONLY MADE MINOR ADJUSTMENTS TO THE GOING FORECAST. IF LESS
CONVECTION/CLOUDS ARE PRESENT FRI...THEN TEMPS SHUD EASILY WARM TO
THE UPR 80S/ARND 90 WITH HEAT INDEX VALUES ARND 100 FOR MANY
LOCATIONS. PERHAPS EVEN THE LOW 100S SOUTH OF I-80.

THE FIRST WAVE OF ACTIVITY SHUD BE MAINLY IN THE MORNING
HOURS...SLIDING EAST/SOUTHEAST. THEN THE THETA-E RIDGE LIFTS NORTH
INTO IOWA...WITH ANOTHER LOBE OF VORTICITY POISED TO RIDE ALONG THIS
FEATURE AND SLIDE SOUTHEAST ACROSS THE FORECAST AREA LATE FRI/EARLY
SAT.

OVER THE NEXT SEVERAL DAYS...SAT IS LIKELY THE MOST CHALLENGING. AT
THIS TIME GUIDANCE HAS COME INTO BETTER AGREEMENT THAT THE FRONTAL
BOUNDARY WILL SLIDE BACK SOUTH ACROSS THE FORECAST AREA...ALLOWING
LLVL FLOW TO TURN NORTHEAST/EASTERLY WITH CLOUDS LINGERING MUCH OF
THE DAY. WITH THIS SCENARIO...SAT WILL LIKELY SEE TEMPS GENERALLY IN
THE MID/UPR 80S BUT COULD BE SLIGHTLY LOWER DEPENDING ON HOW THICK
CLOUD COVER WILL BE. FORTUNATELY THIS WILL KEEP HEAT INDEX VALUES
BELOW 95 DEG FOR MUCH OF THE AREA. THEN PROGRESSING TOWARDS THE
SECOND HALF OF THE WEEKEND AND SUN STILL IS ON TRACK TO BE THE
WARMEST DAY OF THE STRETCH. THE MID-LVL RIDGE REMAINS POISED TO
BUILD BACK INTO THE REGION...LIFTING THE WARM FRONTAL BOUNDARY NORTH
OF THE FORECAST AREA SUN. THIS SHUD KEEP MOST OF THE CONVECTIVE
CHANCES DISPLACED NORTH. SOUNDINGS SUGGEST THE MID-LVLS WILL BE MUCH
WARMER AS WELL...WHICH COULD BE ENOUGH TO EITHER LIMIT OR CAP THE
ENVIRONMENT FROM SEEING CONVECTION DEVELOP SUN. HAVE HEDGED POPS IN
THIS DIRECTION...WITH A DRY FORECAST ACROSS MUCH OF THE FORECAST
AREA SUN. SFC TEMPS SHUD EASILY WARM...WITH HIGHS ARND 90 TO
POSSIBLY THE LOW 90S. HOWEVER...LLVL MOISTURE WILL REMAIN HIGH AS
WELL WITH DEW PTS IN THE 72-74 DEG RANGE. THE COMBINATION WILL
LIKELY PRODUCE HEAT INDEX VALUES IN THE UPR 90S TO PERHAPS AS HIGH
AS 105 SUN AFTN FOR AREAS SOUTH OF I-80.


MONDAY THROUGH WEDNESDAY...
GUIDANCE HAS CONTINUED TO BE LESS EXCITED ABOUT DEPARTING OR
WEAKENING THE MID-LVL RIDGE VERY EARLY NEXT WEEK. AS A RESULT MON
AND PERHAPS TUE LOOK TO MAINTAIN THE HEAT. FORTUNATELY LLVL MOISTURE
DOES APPEAR TO BE A LITTLE LESS THAN THE PRIOR WEEKEND...HOWEVER
HEAT INDEX VALUES ARE STILL PROGGED MON TO REACH THE MID TO PERHAPS
UPR 90S. LLVL MOISTURE BEGINS TO DECLINE FURTHER FOR TUE...WHICH
COULD KEEP TEMPS GENERALLY IN THE MID/UPR 80S. THROUGHOUT THE FIRST
HALF OF NEXT WEEK PERIODIC CHANCES FOR CONVECTION REMAIN. THE FINAL
FEW PERIODS ENSEMBLES HAVE THE IDEA THAT SOME WEAKENING TO THE
ROBUST RIDGE WILL OCCUR.

BEACHLER

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 09Z...

* PERIODIC CHANCES FOR THUNDERSTORMS THROUGH THE PERIOD.

* SWLY WINDS GUSTING UP TO 20KT IN THE AFTERNOON.

KREIN

//DISCUSSION...UPDATED 06Z..

PRIMARY FORECAST CONCERN WILL REMAIN THE CHANCES FOR THUNDERSTORMS
OVER THE AREA. AT ISSUANCE TIME...A LINE OF SHOWERS AND A FEW
THUNDERSTORMS WAS DEVELOPING ALONG A NARROW CORRIDOR FROM AROUND
RFD TO JUST SOUTH OF MDW AND INTO NWRN INDIANA. THE PCPN IS
DEVELOPING ALONG AN ELEVATED WARM FRONT...WITH THE SFC WARM FRONT
STILL TO THE SOUTH...EXTENDING FROM LOW PRESSUURE OVER SOUTH
DAKOTA SOUTHEAST TO SRN IL. INDIVIDUAL CELLS ARE TRACKING TO THE
SOUTHEAST...WITH NWLY FLOW ALOFT...BUT THE AREA OF PCPN IS SLOWLY
LIFTING TO THE NORTH. GIVEN THE SOMEWHAT SCATTERED NATURE OF THE
PCPN AREA...HAVE ADDED VCSH MENTION BACK TO THE TAFS THROUGH 12Z.
HAVE RESISTED THE TEMPTATION TO CALL IS VCTS AS THERE IS LITTLE
LIGHTNING ACTIVITY. HOWEVER...TS SHOULD BECOME MORE OF AN ISSUE
FOR THE TERMINALS THROUGH THE DAY TOMORROW AS THE WARM FRONT
SLOWLY LIFTS TO THE NORTH. THE WARM FRONT SHOULD LIFT THROUGH NRN
IL/IN THROUGH THE DAY TOMORROW...PUTTING THE REGION IN THE WARM
SECTOR OF THE SYSTEM...WITH PERSISTENT WARM/MOIST ADVECTION. THE
ENVIRONMENT SHOULD BECOME INCREASINGLY UNSTABLE THROUGH THE DAY
AND ADDITIONAL THUNDERSTORM ACTIVITY IS LIKELY OVER THE AREA...BUT
CONFIDENCE REMAINS LOW IN THE EXACT TIMING AND COVERAGE OF THE
IMPACTS AT THE TERMINALS. SO...HAVE ONLY GONE WITH A VCTS FOR THE
MORNING HOURS...THOUGH CONFIDENCE IS HIGHER THAT THERE SHOULD BE
SOME TS AROUND THE AREA. HAVE KEPT THE TAFS DRY FOR THE AFTERNOON
HOURS...WITH THE SFC WARM FRONT PUSHING INTO WISCONSIN AND AN
ABSENSE OF UPPER LEVEL FORCING...THOUGH THERE STILL COULD BE SOME
ISOLD-SCT SHRA/TSRA. THERE SHOULD BE A BETTER CHANCE FOR MORE
ORGANIZED CONVECTION THUSDAY NIGHT AS THE MODELS ARE INDICATING A
RATHER VIGOROUS SHORTWAVE OVERTOPPING THE UPPER RIDGE OVER THE
MISSISSIPPI VALLEY AND TURNING SOUTHEAST ACROSS NRN IL. THIS
SHORTWAVE...COINCIDENT WITH THE WARM FRONT WHICH SHOULD BE CLOSE
TO THE WISCONSIN BORDER SHOULD PROVIDE BOTH SFC AND UPPER LEVEL
FORCING IN AN ALREADY MOIST AND UNSTABLE ENVIRONMENT. SO...HAVE
INTRODUCED TEMPO TS FOR THE EVENING HOURS AND A PROB30 FOR THE
LATE NIGHT HOURS. AS THE FORECAST PICTURE BECOME A BIT MORE
CLEAR...PREVAILING TS TIMING WILL LIKELY BE ADDED IN THE UPDATES
LATER TODAY.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 09Z...

* MEDIUM TO HIGH CONFIDENCE THAT THERE BE PERIODS OF SHOWERS AND
  THUNDERSTORMS THROUGH THE FORECAST PERIOD...BUT LOW IN THE
  TIMING AND COVERAGE.

KREIN

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

* FRIDAY THROUGH TUESDAY...PERIODIC CHANCES FOR TSRA.

KJB

&&

.MARINE...
310 AM CDT

NOT MUCH IN THE WAY OF A GRADIENT FOR THE NEXT SEVERAL DAYS...WITH
WEAK SYSTEMS EXPECTED. HIGH PRESSURE OVER QUEBEC AND A SERIES OF
WEAK TROUGHS OVER THE NORTHERN PLAINS. THERE WILL BE A QUASI-
STATIONARY FRONTAL BOUNDARY THAT WILL MEANDER NORTH AND SOUTH
ACROSS LAKE MICHIGAN...ALLOWING WINDS TO CHANGE DIRECTION BUT
STILL NOT EXPECTING MUCH OF A WIND GRADIENT TO DEVELOP. THE
HIGHEST WINDS WILL LIKELY BE ASSOCIATED WITH ANY THUNDERSTORMS
THAT CAN PRODUCE BRIEF GUSTS.

BEACHLER

&&

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

$$

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