Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211343
AFDLOT

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

.MESOSCALE DISCUSSION...
815 AM CDT

THE FORECAST HAS BEEN UPDATED TO RAISE POPS AND QPF IN EASTERN
AREAS THIS MORNING AS WELL AS ADJUST HOURLY FORECAST TRENDS OF
OTHER ELEMENTS.

A SHORT WAVE CENTER OVER NORTH CENTRAL IL IS MOVING ALONG THE
ELEVATED WARM AIR ADVECTION PATTERN. FORCING AHEAD OF THIS WITHIN
HIGH PWAT AIR /1.82 INCHES ON 12Z DVN RAOB/ HAS LED TO SOME
EFFICIENT RAINFALL RATES.  MUCH OF THE THUNDERSTORM ACTIVITY
HAS BEEN PROGRESSIVE WITH JUST NUISANCE TYPE FLOODING REPORTS.
THIS WAVE SHOULD EXIT THE EASTERN FORECAST AREA BY 10 AM. CONTINUED
WARM AIR ADVECTION MAY SUPPORT SCATTERED STORMS ACROSS MAINLY
NORTHERN IL LATE THIS MORNING ALTHOUGH WITH THE VEERED LOW-LEVEL
JET PER PROFILERS AND THE 12Z DVN RAOB...ORGANIZATION OR MAJOR
INTENSIFICATION IS NOT ANTICIPATED. SUCH SCATTERED ACTIVITY HAS
ALREADY BEEN SEEN ACROSS FAR NORTHWEST IL AND SOUTHWEST WI. THERE
HAS BEEN WARMING OF CLOUD TOPS ON IR WITH THESE STORMS.

THE LOW-LEVEL FRONTAL ZONE IS DRAPED OUT FROM WEST TO EAST ACROSS
THE FORECAST AREA. IT IS TOUGH TO SAY WHERE FOR CERTAIN BUT LOOKS
LIKE IT HAS BEEN CONVECTIVELY DISRUPTED FROM ITS EARLIER NORTH
PLACEMENT. THIS SHOULD BEGIN TO SHIFT SLOWLY BACK NORTH BY EARLY
AFTERNOON BUT WILL BE AN IMPORTANT FEATURE FOR
PLACEMENT/MORPHOLOGY OF ANY AFTERNOON AND EVENING CONVECTION.

MTF

&&

.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 14Z...

* TIMING ADDITIONAL THUNDERSTORM CHANCES TODAY...WITH LOW CHANCES
  LATE THIS MORNING...AND BETTER CHANCES THIS EVENING.

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

KREIN/BMD

//DISCUSSION...UPDATED 12Z..

PRIMARY FORECAST CONCERN WILL REMAIN THE CHANCES FOR THUNDERSTORMS
OVER THE AREA. AT ISSUANCE TIME...WIDESPREAD SHOWERS AND
THUNDERSTORMS COVER NRN IL/NWRN IN. THIS ACTIVITY IS DEVELOPING
ALONG AN ELEVATED WARM FRONT WITH INDIVIDUAL CELLS TRACKING TO THE
SOUTHEAST UNDER NWLY FLOW ARND 30KT ALOFT. THE WHOLE AREA OF PCPN
IS SLOWLY TRANSLATING TO THE EAST AS AN UPPER RIDGE BUILDS OVER
THE MIDDLE/UPPER MISSISSIPPI VALLEY...SO THUNDERSTORMS SHOULD
IMPACT THE TERMINALS THROUGH THE MORNING. HAVE GONE WITH
PREVAILING THUNDER RIGHT OUT OF THE GATE AND ADJUSTED THE INITIAL
TEMPO GROUP MORE TO COVER THE POSSIBILITY FOR PERIODS OF MVFR
CIGS/VIS UNDER THE PCPN SHIELD. TIMING OF BREAKS IN THE ACTIVITY
TODAY IS DIFFICULT AS THE MAIN BAROCLINIC ZONE WILL REMAIN IN
PLACE ALOFT THROUGH THE DAY. LATEST HIGH RES MODEL GUIDANCE
SUGGESTS THAT THE BEST OPPORTUNITY FOR A LULL IN THE ACTIVITY WILL
BE DURG THE AFTERNOON HOURS WHEN WEAK SHORTWAVE RIDGING MOVES OVER
THE REGION. THERE SHOULD BE ANOTHER ROUND OF SHOWERS AND
THUNDERSTORMS ASSOCIATED WITH A SHORTWAVE CURRENTLY MOVING THROUGH
ERN NEBRASKA. ANTICIPATE THAT THIS SHORTWAVE SHOULD TRAVERSE THE
REGION BY LATE THIS EVENING GENERATING MORE SHRA/TSRA OVERNIGHT
TONIGHT. THERE IS ALSO THE POSSIBILITY THAT THIS SHORTWAVE COULD
PHASE WITH THE SHORTWAVE MOVING THROUGH MINNESOTA AND IF THAT
HAPPENS...THE COVERAGE OF SHRA/TSRA COULD BE MORE EXTENSIVE FOR A
LONGER PERIOD OF TIME. THE SHRA/TSRA POTENTIAL SHOULD COME TO AN
END BY EARLY TOMORROW MORNING AS THE UPPER RIDGE BUILDS EAST
ACROSS IL/WI/THE WESTERN GREAT LAKES REGION. AS PCPN ENDS...THERE
COULD BE A PERIOD OF LINGERING MVFR CIGS/VIS INTO LATE FRIDAY
MORNING.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 14Z...

* LOW-MEDIUM CONFIDENCE IN PRECIP CHANCES/GENERAL TIMING REST OF
  TODAY.

KREIN

//OUTLOOK FOR ORD/MDW FOR 00Z SATURDAY-12Z THURSDAY...UPDATED 12Z...

* FRIDAY NIGHT THROUGH WEDNESDAY...PERIODIC CHANCES FOR TSRA/SHRA
  WITH OCNL MVFR CIGS/VIS.

KREIN

&&

.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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