Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 150804
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
304 AM CDT MON SEP 15 2014

.DISCUSSION...
240 PM CDT

MAIN FOCUS IS ON OUR NEXT RAIN CHANCE WHICH COMES VERY LATE TONIGHT
THROUGH MUCH OF MONDAY...AND A CONTINUATION OF COOL/BELOW AVERAGE
TEMPS FOR MID-SEPTEMBER.

IN THE NEAR TERM...A PLEASANT BUT STILL COOL WEATHER WAS IN PLACE
ACROSS THE REGION THIS AFTERNOON. SURFACE HIGH PRESSURE WAS CENTERED
OVER THE EASTERN GREAT LAKES...WITH RIDGING EXTENDING SOUTHWEST
ACROSS INDIANA AND ILLINOIS. THE RESULT IS MOSTLY SUNNY AND SLIGHTLY
MILDER CONDITIONS WITH TEMPS REACHING THE MID 60S IN MOST SPOTS AND
UPPER 60S IN A COUPLE IN THE WEST/SOUTH PARTS OF THE CWA. LIGHT
WINDS AND CLEAR SKIES THIS EVENING WILL ALLOW TEMPS TO FALL OFF
FAIRLY QUICKLY HOWEVER...WITH READINGS DROPPING THROUGH THE 50S THIS
EVENING AND INTO THE MID-UPPER 40S LATER TONIGHT BEFORE CLOUDS BEGIN
TO INCREASE AFTER MIDNIGHT.

THIS INCREASE IN CLOUD COVER LATE TONIGHT WILL BE IN ASSOCIATION
WITH A MID-LEVEL SHORT WAVE TROUGH CURRENTLY DIGGING SOUTHEAST FROM
SASKATCHEWAN INTO MONTANA PER AFTERNOON GOES VAPOR IMAGERY.
INCREASING SOUTHERLY LOW LEVEL FLOW WILL DEVELOP AS THIS WAVE DIGS
SOUTHEAST...WITH 925-850 MB LAYER MOISTURE TRANSPORT INCREASING ON
THE NOSE OF A 30-35 KT LOW LEVEL JET WHICH DEVELOPS FROM THE TX/OK
PANHANDLE REGION. THIS AXIS OF MOISTURE TRANSPORT EVENTUALLY WORKS
INTO NORTHERN MISSOURI AND WEST CENTRAL IL MONDAY MORNING AS THE JET
VEERS MORE WEST-SOUTHWESTERLY. WHILE A FEW SHOWERS MAY DEVELOP
ACROSS FAR NORTH CENTRAL IL PRIOR TO SUNRISE...IT APPEARS THAT THE
BEST FORCING WILL ARRIVE SHORTLY AFTER...AS MID-LEVEL VORT
APPROACHES. A COMBINATION OF 40-50 METER HEIGHT FALLS...UPPER
DIVERGENCE BENEATH THE RIGHT REAR ENTRANCE REGION OF A 100+ KT UPPER
JET STREAK OVER THE GREAT LAKES...AND DEVELOPING FRONTOGENETIC
FORCING WITHIN THE LOW-MID LEVELS SHOULD PROVIDE SUFFICIENT FORCING
FOR PRODUCING RAIN/SHOWERS ACROSS THE AREA DURING THE DAY MONDAY.
WITH VARIOUS MODEL QPF AMOUNTS IN THE 0.25 TO 0.35 RANGE...RAIN IS
EXPECTED TO TAPER OFF FROM THE WEST/NORTHWEST LATE MONDAY AFTERNOON
AND EARLY MONDAY EVENING AS THE RELATIVELY PROGRESSIVE DISTURBANCE
MOVES EAST OF THE FORECAST AREA. STRONG HEIGHT RISES THEN ACCOMPANY
DEVELOPMENT OF SURFACE HIGH PRESSURE ACROSS THE MID-MISSISSIPPI
VALLEY MONDAY NIGHT AND TUESDAY.

SURFACE HIGH PRESSURE IS EXPECTED TO LINGER ACROSS THE ACROSS THE
MIDWEST FOR MUCH OF THE REMAINDER OF THE WEEK...SETTING THE STAGE
FOR DRY WEATHER WITH COOL NIGHTS AND COOLER THAN AVERAGE BUT SLOWLY
MODERATING TEMPS THROUGH NEXT WEEKEND. AFTER CLOUD/PRECIP INFLUENCED
TEMPS ONLY IN THE UPPER 50S/LOWER 60S OVER MUCH OF THE AREA
MONDAY...DAYTIME HIGHS SHOULD MODERATE THROUGH THE 60S THROUGH
WEDNESDAY AND THEN FINALLY INTO THE 70S FOR THE END OF THE
WEEK/WEEKEND. COLDEST OVERNIGHT MINS LOOK TO BE LATE MONDAY NIGHT AS
HIGH PRESSURE BUILDS IN  WITH LOW 40/A FEW UPPER 30S...WITH LOWS
GRADUALLY MODERATING INTO THE 50S BY THE END OF THE WEEK.

UPPER PATTERN EVOLVES INTO A LONG WAVE TROUGH OVER THE EAST COAST
WITH AN UPPER RIDGE ACROSS THE ROCKIES BY FRIDAY. SLIGHT EASTWARD
PROGRESSION AND FLATTENING OF THE PATTERN ALLOWS RETURN OF
WARMER/HIGHER MOISTURE CONTENT AIR INTO THE REGION INTO THE WEEKEND.
THIS YIELDS AN INCREASING CHANCE OF SHOWERS/THUNDERSTORMS AS A
MID-LEVEL DISTURBANCE PROPAGATES THROUGH THE FLOW AND A SURFACE
TROUGH APPROACHES AND MOVES ACROSS THE AREA FRIDAY NIGHT THROUGH
SUNDAY.

RATZER

&&

.AVIATION...

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

* COLD FRONT MIDDAY MONDAY BRINGING A WIND SHIFT TO THE NW OR NNE
  ALONG WITH MVFR POSSIBLY BRIEFLY IFR CONDITIONS AND RAIN.

BMD

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

OVERNIGHT THROUGH AROUND MID MORNING MONDAY EXPECT STEADY STATE
CONDITIONS WITH VERY LIGHT SSW FLOW AND VFR CONDITIONS. AN UPPER
LEVEL WAVE WILL MOVE ACROSS THE REGION MONDAY BRINGING SHOWERS TO
THE AREA BY LATE MORNING OR EARLY AFTERNOON. CIGS ARE EXPECTED TO
LOWER TO AT LEAST MVFR AND SOME MODELS SUGGEST THE POSSIBILITY OF A
COUPLE HOURS OF IFR CIGS AS THE SURFACE TROUGH DROPS ACROSS THE
AREA. CIGS ARE EXPECTED TO QUICKLY SCATTER OUT THOUGH EARLY TO MID
EVENING BEHIND THE FRONT. WINDS WILL BE TRICKY DURING THE DAY
MONDAY. WINDS ARE EXPECTED TO SHIFT TO SOME SORT OF NORTHERLY
COMPONENT BUT NOT QUITE SURE AT THIS POINT IF IT WILL BE NW OR NNE
AT THE CHICAGO TERMINALS WHERE SOME GUIDANCE SHOWS THE FRONT SURGING
AHEAD AND TURNING WINDS NNE OFF OF LAKE MICHIGAN. GIVEN THE RELATIVE
WARMTH OF THE LAKE RELATIVE TO THE AIRMASS...WILL LEAN AWAY FROM
THIS SOLUTION IN THE MEANTIME...BUT IF WINDS DO END UP TURNING MORE
NNE IT WOULD LIKELY BE DURING THE AFTERNOON...THEN WINDS SHOULD
EVENTUALLY SETTLE BACK TOWARDS THE NW LATER IN THE EVENING.

BMD

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

* LOW CONFIDENCE IN WIND DIRECTION POST FRONTAL MONDAY AFTERNOON.
  GUIDANCE RANGES FROM NW TO NNE.
* MEDIUM CONFIDENCE IN MVFR CONDITIONS AND RAIN...LOW CONFIDENCE
  IN IFR.

BMD

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

TUESDAY...VFR. NORTHWEST WINDS BECOMING WEST.

WEDNESDAY...VFR. WEST WINDS BECOMING NORTHEAST.

THURSDAY...VFR. EAST WINDS.

FRIDAY...VFR. SOUTHEAST WINDS.

SATURDAY...VFR. BREEZY SOUTH-SOUTHWEST WINDS. CHANCE OF
THUNDERSTORMS.

MDB

&&

.MARINE...
303 AM CDT

LOW PRESSURE NEAR JAMES BAY THIS MORNING WILL DRAG A COLD FRONT
ACROSS LAKE MICHIGAN. WINDS WILL TURN NORTHWEST TO NORTH BEHIND THE
FRONT...AND MAY BRIEFLY INCREASE INTO THE 20 TO 25 KT RANGE
ESPECIALLY ON THE SOUTH END OF THE LAKE THIS EVENING. HIGH PRESSURE
WILL BUILD IN BEHIND THE FRONT ALLOWING WINDS TO REMAIN MODEST FOR A
PERIOD BEFORE SOUTHWEST FLOW BEGINS TO FRESHEN UP AGAIN OVER THE
NORTH HALF OF LAKE MICHIGAN MIDDAY TUESDAY. ANOTHER COLD FRONT WILL
DIVE SOUTH ACROSS LAKE MICHIGAN ON WEDNESDAY TURNING WINDS NORTH TO
NORTHEAST AND INCREASING FOR A SHORT PERIOD TO AROUND 30 KT ONCE
AGAIN.

BMD

&&

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

$$

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