Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 131621
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1121 AM CDT SUN APR 13 2014

.DISCUSSION...
417 AM CDT

TODAY...
EARLY THIS MORNING LOW PRESSURE LIES OVER IOWA WITH A WARM FRONT
EXTENDING EAST ACROSS FAR NORTHERN ILLINOIS. THE BOUNDARY WAS DRIVEN
SOUTH OVERNIGHT BY CONVECTION/OUTFLOW. ON THE WARM SIDE OF THE
FRONT...TEMPERATURES REMAINED IN THE 60S OVERNIGHT...BUT NORTH OF
THE FRONT TEMPS QUICKLY FALL OFF INTO THE 40S AND THIS TIGHT THERMAL
GRADIENT WILL HOVER ACROSS NORTHERN COUNTIES OF THE STATE THIS
MORNING. AS FOR CONVECTION THIS MORNING...THUNDERSTORMS HAVE MOVED
EAST OVER LAKE MICHIGAN THIS MORNING WITH STABLE CONDITIONS IN THEIR
WAKE AND EXPECT QUIET CONDITIONS TO PERSIST THROUGH THE REMAINDER OF
THE MORNING AND EARLY AFTERNOON. COUPLED JET STILL PROGGED TO
DEVELOP THIS AFTERNOON WITH MODERATE TO STRONG UPPER LEVEL
DIVERGENCE RESULTING IN CYCLOGENENSIS ALONG THE MID MISSISSIPPI
VALLEY. ISENTROPIC ASCENT WILL INCREASE THROUGH THE LATE MORNING
INTO THE AFTERNOON WITH PRECIP OVERSPREADING NORTHERN ILLINOIS FROM
THE WEST. SOUNDINGS STILL INDICATE DEEP SATURATED LAYER WITH PWATS
PUSHING TO AROUND 1.4-1.5 INCHES. MODELS IN DECENT AGREEMENT ON
PLACEMENT OF AXIS OF HEAVIEST RAIN THIS EVENING AND TONIGHT...
STRETCHING FROM EASTERN IOWA TO EAST-CENTRAL WISCONSIN AND CLIPPING
NORTH CENTRAL ILLINOIS INCLUDING THE ROCKFORD AREA. HEAVY PRECIP
REMAINS A CONCERN THOUGH PRECIP SO FAR HAS UNDERPERFORMED FROM PRIOR
EXPECTATIONS AND A PROLONGED PERIOD OF DRY WEATHER OVER NORTH
CENTRAL ILLINOIS THE PAST SEVERAL WEEKS WILL HELP MITIGATE THE FLOOD
THREAT TO SOME DEGREE.

MONDAY...
LONGWAVE TROUGH AXIS WILL TRANSLATE EAST ACROSS THE PLAINS MONDAY
REACHING THE MISSISSIPPI VALLEY BY THE EVENING. MEANWHILE...COLD
ADVECTION INTO THE AREA WILL DRIVE H85 TEMPS DOWN TO AROUND -11C AND
SEVERAL SMALLER SCALE DISTURBANCES LIFTING UP THE LEE SIDE OF THE
TROUGH WILL RESULT IN RAIN CHANGING OVER TO SNOW ACROSS THE AREA.
MODELS HAVE TRENDED MORE BULLISH WITH QPF MONDAY AND CANNOT RULE OUT
LIGHT ACCUMULATIONS AT THIS TIME...MAINLY DURING THE LATE AFTERNOON
INTO THE EVENING WHEN THE AREA IS BETTER ENTRENCHED IN THE COLD AIR.
HIGHS MAY STRUGGLE TO REACH INTO THE LOW 40S FOR MUCH OF THE CWA AND
A GUSTY NORTH WIND WILL MAKE IT FEEL CLOSER TO THE FREEZING MARK.

TUESDAY THROUGH SATURDAY...
GENERALLY QUIET WEATHER EXPECTED TUESDAY AND WEDNESDAY UNDER THE
INFLUENCE OF HIGH PRESSURE. BY WEDNESDAY...LOW PRESSURE OVER THE
PLAINS WILL HELP SPREAD WARM AIR BACK INTO THE MIDWEST WITH A
WARMING TREND THROUGH THURSDAY WHEN TEMPERATURE SHOULD BE CLOSER TO
NORMAL. MORE ACTIVE WEATHER MAY RETURN LATE IN THE WEEK INTO THE
WEEKEND AS ANOTHER SOUTHWEST LOW LIFTS ACROSS THE MIDWEST BRINGING
CHANCES FOR PRECIP AND FLUCTUATING TEMPERATURES.

BMD

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* WIND SHIFT TO NORTHEAST AROUND 10 KT ARRIVING BY 1630-17Z.

* IFR CIGS ARRIVING BY 18Z AND VSBY LOWERING TO AT LEAST MVFR AND
  POSSIBLY IFR BY 19Z.

* PERIODS OF SHRA/TSRA THIS AFTERNOON AND EVENING...HEAVIEST COULD PRODUCE
  IFR VSBY.

* WINDS TURN NORTH AND GUST TO 25-30 KT LATE THIS EVENING/OVERNIGHT.

RC

//DISCUSSION...UPDATED 12Z...

VERY COMPLEX FORECAST. WEAK PRESSURE PATTERN OVER AREA AT THE
MOMENT...WITH WEAK LOW PRESSURE OVER LOWER MICHIGAN AND FRONTAL
BOUNDARY SAGGING SOUTHWARD. MAIN CHANGE IN TAF WAS TO BRING SHIFT
TO EAST OR NORTHEAST WINDS TO ORD SOONER THAN PREVIOUS FCST AND
ALSO INCLUDE MDW IN WIND SHIFT. IT IS POSSIBLE THAT THIS SHIFT
COULD OCCUR AS SOON AS 17 OR 18Z. CONFIDENCE IS LOWER IN GYY
SEEING SHIFT TO NORTHEAST. IFR CIGS BEHIND THE FRONT ACROSS WI SO
THESE SHOULD DEVELOP SOMETIME AFTER THE SHIFT TO EASTERLY WINDS.
ITS POSSIBLE THAT WINDS WILL REMAIN EASTERLY UNTIL EARLY EVENING
NOW AT ORD/MDW. OTHER CHALLENGE IS WITH SHRA/TSRA TRENDS...AS A
COMPLEX LIFTS NORTHEAST FROM IOWA. IT APPEARS THAT THE SHOWERS AND
AT LEAST OCNL EMBEDDED THUNDER WILL ARRIVE A BIT SOONER AT THE
EASTERN TERMINALS...BUT DURATION COULD BE SHORTER THAN INDICATED
IN TEMPO GROUP IN TAF. WILL CONTINUE TO ASSESS OBSERVATIONAL AND
RADAR TRENDS FOR UPDATES TO WIND/CIG/AND SHRA/TSRA FORECAST.

FROM 12Z...

LOW PRESSURE IS OVER THE SOUTHERN PLAINS WITH A FRONT EXTENDING
THROUGH NORTHERN IL/SOUTHERN WI. THE FRONT WILL MEANDER TODAY AND
THINKING IT WILL IMPACT RFD...ORD...AND DPA. WINDS WILL TURN N TO
NE BEHIND THE FRONT...AND BE SOUTH AHEAD OF THE FRONT. RAIN AND
STORMS SPREAD NORTH ACROSS THE TERMINALS EARLY THIS AFTN WITH
CHANCES OF THUNDERSTORMS INCREASING LATE THIS AFTN INTO THIS
EVENING. CIGS WILL LOWER THIS EVENING TO IFR AND THE STRONGEST
SHOWERS AND TSRA WILL LIKELY PRODUCE IFR VSBY.

INSTABILITY WEAKENS AND SHIFTS SE AS THE LOW MOVES THROUGH THE
REGION LATE THIS EVENING. THEREFORE EXPECTING LIMITED TSRA LATE
THIS EVENING AND JUST SHRA OVERNIGHT. WINDS WILL ALSO TURN NORTH
BEHIND THE LOW AND BEGIN GUSTING. THE HIGHEST GUSTS WILL BE SEEN
AT GYY WITH MAX GUSTS OVER 35 KT PSBL. THE OTHER TERMINALS SHOULD
SEE GUSTS BTWN 25 AND 30 KT. THE LOWER ATMOSPHERE BEGINS TO DRY
MID MONDAY MORNING AND CIGS LIFT TO MVFR.

JEE

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* HIGH CONFIDENCE IN WIND SHIFT TO NORTHEAST...LOW-MEDIUM CONFIDENCE IN
  TIMING OF SHIFT BACK TO SOUTH-SOUTHWEST THIS EVENING.

* HIGH CONFIDENCE IN IFR CIGS...MEDIUM-HIGH IN ARRIVAL BY
  1730-18Z.

* HIGH IN VSBY LOWERING TO MVFR IN BR...MEDIUM IN TIMING...AND
  MEDIUM IN POTENTIAL FOR IFR VSBY IN BR AND HEAVIEST SHRA/TSRA.

* MEDIUM CONFIDENCE IN OVERALL SHRA/TSRA TRENDS AND TIMING THIS
  AFTERNOON AND EVENING...WITH MEDIUM-HIGH CONFIDENCE THERE WILL
  BE AT LEAST OCNL TSRA AT THE TERMINALS BY MID AFTERNOON.

* HIGH CONFIDENCE IN SHIFT TO GUSTY NORTHERLY WINDS LATE
  TONIGHT...MEDIUM IN TIMING.

RC

//OUTLOOK FOR ORD/MDW FOR 00Z TUESDAY-12Z SUNDAY...UPDATED 12Z...

MONDAY NIGHT...-RASN PSBLY BCMG -SN AFTER MIDNIGHT. MVFR TO IFR
CIGS IMPROVING TO VFR.

TUESDAY...VFR.

WEDNESDAY...VFR.

THURSDAY...CHANCE OF RAIN OVERNIGHT...MVFR/IFR CIGS PSBL OVERNIGHT.

FRIDAY...CHANCE OF RAIN...MVFR TO IFR CIGS PSBL.

SATURDAY...CHANCE OF RAIN.

JEE

&&

.MARINE...
317 AM CDT

HEADLINES...WILL UPGRADE THE GALE WATCHES TO WARNINGS AS GALES ARE
STILL EXPECTED.

LOW PRESSURE IS OVER THE OK PANHANDLE EARLY THIS MORNING WITH A
BROAD TROUGH EXTENDING ALL THE WAY THROUGH THE IL/WI STATE LINE.
THERE IS A FRONT IN THE TROUGH BUT IT HOLDS A VARIETY OF FORMS AKA
COLD/STATIONARY/WARM AS YOU MOVE ALONG IT...SO WILL REFER TO IT AS A
TROUGH FOR SIMPLICITY.  THE FRONT WILL MEANDER OVER THE FAR SOUTHERN
END OF THE LAKE THIS MORNING WHILE THE LOW MOVES UP THE TROUGH AND
PASSES OVER THE SOUTHERN END OF THE LAKE THIS EVENING.  NORTH GALES
WILL DEVELOP BEHIND THE LOW AND EXTEND THROUGH THE NSH WATERS
TONIGHT AND THROUGH MID AFTN MONDAY.  WINDS CONTINUE TO DIMINISH
THROUGH TUESDAY AS A RIDGE OF HIGH PRESSURE MOVES OVER THE LAKE
TUESDAY NIGHT.  WINDS TURN SOUTH BEHIND THE RIDGE AND INCREASE TO 30
KT AS A WEAKENING TROUGH OF LOW PRESSURE APPROACHES WEDNESDAY.  THE
TROUGH PASSES OVER THE LAKE THURSDAY WITH NORTHEAST TO EAST WINDS
BEHIND IT.  GUIDANCE THEN FEATURES EXTREME DIFFERENCES IN THE
PATTERN LATE IN THE WEEK SO HAVE LOW CONFIDENCE IN THE FORECAST PAST
THURSDAY.  FOR REFERENCE THE ECMWF FEATURES A HIGH MOVING OVER THE
LAKE LATE THIS WEEK WHILE THE GFS HAS A LOW MOVING UP THE LAKE AT
THE SAME TIME.

JEE

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...4 PM SUNDAY TO 4
     AM MONDAY.

     GALE WARNING...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...4 AM
     MONDAY TO 4 PM MONDAY.

     GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
     LMZ669-LMZ671-LMZ673-LMZ868-LMZ870-LMZ872-LMZ874...10 PM
     SUNDAY TO 3 PM MONDAY.

     GALE WARNING...LMZ080-LMZ675-LMZ777-LMZ779-LMZ876-LMZ878...3 AM
     MONDAY TO 3 PM MONDAY.

&&

$$

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