Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 200605
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
104 AM CDT WED AUG 20 2014

.DISCUSSION...
835 PM CDT

FOR EVENING UPDATE...

HAD ADJUSTED POPS/WX AND SKY COVER EARLIER BASED ON RADAR/SATELLITE
TRENDS. OTHER THAN DETAILS OF ENDING/MOVING ISOLATED SHRA OUT OF
NORTHEAST IL/NORTHWEST IN NEXT COUPLE OF HOURS...NO SIGNIFICANT
CHANGES MADE TO GOING FORECAST.

A FEW ISOLATED SHOWERS CONTINUE TO BUBBLE IN A COUPLE OF SPOTS AT
0115Z...ONE NEAR KANKAKEE...AND THE OTHER ACROSS NORTHERN COOK
COUNTY NEAR OHARE AND JUST OFF THE LAKE SHORE BETWEEN KENILWORTH AND
EVANSTON. THESE SHOWERS WERE OCCURRING ALONG A WEAK SURFACE TROUGH
AXIS WHICH STRETCHED FROM SOUTHERN LAKE MICHIGAN TO NORTH OF ST
LOUIS...AND AHEAD OF A MID-LEVEL SHORT WAVE TROUGH ROTATING EASTWARD
ACROSS NORTHWEST IL/WISCONSIN PER GOES VAPOR IMAGERY. THESE SHOWERS
HAVE DIMINISHED CONSIDERABLY IN COVERAGE WITH SUNSET AND THE
WEAKENING OF DIURNALLY-DRIVEN INSTABILITY...AND LINGERING SHOWERS
ARE EXPECTED TO CONTINUE TO WEAKEN AND DISSIPATE OR MOVE OVER LAKE
MICHIGAN IN THE NEXT HOUR OR TWO. STRONGER STORMS WHICH HAD FORMED
OVER PARTS OF EAST CENTRAL IL AND NORTHWEST INDIANA LATE THIS
AFTERNOON ALSO OCCURRED AHEAD OF A MORE COMPACT VORT MAXIMA SEEN
PUSHING EAST ACROSS CENTRAL IL AT THIS TIME... AND WITHIN A MORE
UNSTABLE ENVIRONMENT ASSOCIATED WITH POOLED UPPER 60S AND LOWER 70S
SURFACE DEW POINTS AHEAD OF THE SURFACE TROUGH. THESE STRONGER
STORMS HAVE MOVED OFF TO THE EAST OF THE CWA INTO CENTRAL AND
NORTHERN INDIANA. GOES 11-3.9 MICRON IR IMAGERY INDICATES LITTLE
LEFT OF DIURNAL CU FIELD ACROSS SOUTHERN WI/NORTHERN IL WEST OF
CHICAGO...SUGGESTING DRY CONDITIONS FOR THE MAJORITY OF THE AREA FOR
THE REMAINDER OF THE EVENING/OVERNIGHT HOURS. ANOTHER MID-LEVEL VORT
ROTATING SOUTH-SOUTHEASTWARD ACROSS CENTRAL MN HAS SOME ISOLATED
CONVECTION NEAR THE MSP AREA...AND HIGH-RES GUIDANCE DOES INDICATE
THE POTENTIAL FOR A FEW ISOLATED SHOWERS TO DEVELOP INTO NORTHEAST
IA/NORTHWEST IL TOWARD MORNING. STILL...APPEARS FAIRLY LOW CHANCE
WITH BETTER PRECIP POTENTIAL BECOMING FOCUSED BACK ACROSS
NEBRASKA/WESTERN IA BY MORNING.

HAD ADJUSTED POPS/WX AND SKY COVER EARLIER BASED ON RADAR/SATELLITE
TRENDS. OTHER THAN DETAILS OF ENDING/MOVING ISOLATED SHRA OUT OF
NORTHEAST IL/NORTHWEST IN NEXT COUPLE OF HOURS...NO SIGNIFICANT
CHANGES MADE TO GOING FORECAST.

RATZER

//PREV DISCUSSION...
304 PM CDT

WARMER AND MORE HUMID CONDITIONS ARE STILL EXPECTED AS WE GO THROUGH
THE REST OF THE WEEK WITH THE BIGGEST CHALLENGE BEING THE
DETERMINATION OF THE EXTENT OF SHOWER AND THUNDERSTORM ACTIVITY.
AFTERNOON WATER VAPOR IMAGERY SHOWS AN UPPER TROUGH OVERHEAD OF THE
LOCAL AREA MAKING GRADUAL PROGRESS TO THE EAST. BROAD LOW-AMPLITUDE
RIDGING IS IN PLACE OVER THE ROCKIES AND HIGH PLAINS WHILE A BROAD
UPPER TROUGH MOVES ACROSS WESTERN CANADA. A CUTOFF UPPER LOW IS
SPINNING SOUTHWARD DOWN THE CALIFORNIA COAST. THIS RIDGE/TROUGH
COMBINATION WILL AMPLIFY OVER THE NEXT FEW DAYS AND MAKE EASTWARD
PROGRESS. AT THE SURFACE...LOW PRESSURE IS OVER NORTHERN MICHIGAN IS
DRAGGING A COOL FRONTAL BOUNDARY/TROUGH SOUTHEAST ACROSS THE LOCAL
AREA.

TONIGHT AND WEDNESDAY...THE UPPER TROUGH OVER THE AREA TODAY WILL BE
SLOW TO MOVE EAST INTO WEDNESDAY KEEPING CYCLONIC FLOW OVERHEAD
THROUGH THE MORNING. THE BULK OF THE CONVECTIVE ACTIVITY IS NOW OFF
TO THE EAST WITH MORE SPOTTY DIURNALLY DRIVEN ACTIVITY WORKING INTO
THE NORTHERN CWA AND MAY SEE SOME ADDITIONAL ACTIVITY INTO CENTRAL
AREAS BEFORE DISSIPATING THIS EVENING. OTHERWISE...WATER VAPOR SHOWS
A WEAK WAVE ROUNDING THE PERIPHERY OF THE UPPER LOW OVER MINNESOTA
THIS AFTERNOON AND THIS SHOULD ROTATE OVER THE LOCAL AREA TONIGHT
AND MAY TRIGGER SOME ADDITIONAL SHOWERS. THE CHANCE DOES NOT LOOK
THAT GREAT ESPECIALLY WITH A MODESTLY DRIER AIRMASS IN PLACE BEHIND
THE AFTERNOON TROUGH. CYCLONIC FLOW REMAINS OVERHEAD WEDNESDAY
MORNING BUT SHIFTS TO THE EAST SO AM NOT EXPECTING PRECIP ACTIVITY
GIVEN THE POOR DIURNAL SUPPORT THEREFORE MUCH OF THE DAY WEDNESDAY
SHOULD BE DRY. HIGHS WILL WARM INTO THE LOW TO MID 80S BUT A LAKE
BREEZE SHOULD COOL LAKESHORE AREAS DURING THE AFTERNOON.

WEDNESDAY NIGHT THROUGH FRIDAY...THE UPPER RIDGE WILL MOVE FROM THE
HIGH PLAINS AND SLOW ITS EASTWARD PROGRESS OVER THE LOCAL AREA BY
FRIDAY. IT WILL AMPLIFY AS IT MOVES EASTWARD HELPING TO SHARPEN THE
ASSOCIATED MID LEVEL BAROCLINIC ZONE WHICH WILL MOVE NORTHEAST
ACROSS THE FORECAST AREA STARTING BY WEDNESDAY EVENING. WITH A
STRONG PUSH OF MOISTURE AND A WEST TO SOUTHWEST LOW LEVEL JET
DEVELOPING WEDNESDAY EVENING SHOULD SEE A DECENT COVERAGE OF SHOWERS
AND THUNDERSTORMS DEVELOP AND MOVE ACROSS THE AREA INTO THURSDAY
MORNING. BETTER ORGANIZATION COULD RESULT WITH THE PASSAGE OF ANY
SHORTWAVE EJECTING FROM THE AMPLIFYING BUT STILL BROAD WESTERN
TROUGH. LESS CONFIDENT IN PRECIP CHANCES FROM LATE THURSDAY THROUGH
FRIDAY AS THE RIDGE AXIS WILL BE OVER THE AREA BUT THE RIDGE LOOKS
TO BE BROAD ENOUGH THAT THE MORE ACTIVE FLOW AROUND THE PERIPHERY
WILL BE IN CLOSE PROXIMITY. ULTIMATELY A WARM FRONTAL BOUNDARY WILL
LIKELY SET UP TO THE NORTH EXTENDING FROM LOW PRESSURE ACROSS THE
CENTRAL/NORTHERN PLAINS BUT CONVECTION MAY INFLUENCE HOW FAR
NORTH-SOUTH THIS BOUNDARY MOVES WHICH WILL AFFECT POPS AND POSSIBLY
TEMPS AS WELL. WILL CONTINUE WITH THE IDEA OF THE BOUNDARY REMAINING
NORTH LEADING TO HIGHS IN THE MID AND UPPER 80S THURSDAY AND UPPER
80S TO AROUND 90 FRIDAY. THE FLOW MAY BE WEAK ENOUGH TO ALLOW SOME
LAKE INFLUENCE TO COOL SHORELINE AREAS SOMEWHAT. ANY OUTFLOW FROM
STORMS OVER OR NORTH OF THE AREA COULD IMPACT TEMPS AS WELL.

SATURDAY AND BEYOND...THE WESTERN TROUGH WILL EVENTUALLY TRACK EAST
ACROSS THE NORTHERN PLAINS BREAKING DOWN THE RIDGE BUT THE SPEED
WITH WHICH THIS OCCURS IS STILL IN QUESTION. AT THIS POINT THE RIDGE
LOOKS TO FURTHER AMPLIFY OVER THE WEEKEND WHICH COULD KEEP THINGS
DRY BUT GUIDANCE HAS FLUCTUATED ON THE AMPLITUDE. WILL KEEP LOW POPS
SATURDAY AND SUNDAY...MAINLY NORTH CLOSER TO THE MORE ACTIVE FLOW
AROUND THE TOP OF THE RIDGE. TEMPS IN THE UPPER 80S TO AROUND 90
WILL CONTINUE ALONG WITH THE HUMIDITY. AS THE UPPER TROUGH MOVES
EAST SURFACE LOW PRESSURE WILL LIKELY DEVELOP ACROSS THE NORTHERN
PLAINS AND DRAG A COLD FRONT INTO THE AREA EARLY NEXT WEEK. THE
OVERALL UPPER PATTERN WILL BE DE-AMPLIFYING SO THE FRONT MAY STALL
OUT SOMEWHERE NEAR THE AREA BUT STILL QUITE A WAYS OUT THERE FOR ANY
KIND OF CONFIDENCE IN THIS.

MDB

&&

.AVIATION...

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

* CHANCE MVFR VIS OVERNIGHT INTO EARLY WEDNESDAY MORNING.

* POSSIBLE LAKE BREEZE LATE WEDNESDAY AFTERNOON/EARLY EVENING.

KREIN

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

ALL CONVECTIVE ACTIVITY HAS MOVED OUT OF NRN IL/NWRN IN AND SKIES
HAVE BECOME MOSTLY CLEAR WITH CALM TO LIGHT WEST WINDS. CLEAR
SKIES LIGHT WINDS AND DEWPOINTS IN THE 60S SHOULD ALLOW FOR SOME
FG/BR DEVELOPMENT AS TEMPERATURES RADIATE DOWN CLOSE TO THE IN
SITU DEWPOINTS. WILL CARRY MVFR VIS AT ALL TERMINAL EXCEPT ORD/MDW
WHERE URBAN HEAT ISLAND EFFECTS COULD KEEP TEMPS A LITTLE
HIGHER...AND SATURATION WOULD BE LESS LIKELY. A FEW ISOLD POCKETS
OF IFR VIA ARE POSSIBLE IN LOW LYING...SHELTERED LOCATIONS.

WESTERLY WINDS INCREASE BY MID TO LATE MORNING WEDNESDAY WITH
MODEL FORECAST SOUNDINGS SUGGESTING PREVAILING WINDS AROUND 10
KT...ESPECIALLY DURG THE LATE MORNING HOURS. WINDS ARE EXPECTED TO
WEAKEN DURG BY THE LATE AFTERNOON HOURS AS A WEAK RIDGE AXIS
DEVELOPS FROM WISCONSIN...THROUGH NERN IL/NWRN IN INTO THE LOWER
OHIO VALLEY. A LAKE BREEZE IS EXPECTED TO DEVELOP NEAR THE LAKE
AND EVENTUALLY MOVE INLAND...BUT LIKELY NOT REACHING ORD/MDW UNTIL
LATE AFTERNOON OR EARLY EVENING. WINDS AT ORD/MDW SHOULD TURN
ESELY WITH THE PASSAGE OF THE LAKE BREEZE...BUT THERE WILL ONLY BE
A WEAK PUSH TO THE LAKE BREEZE AND THE SSELY WINDS SHOULD ONLY BE
ARND 5KT OR SO AT ORD/MDW. WINDS WILL THE SETTLE INTO A SSELY TO
SLY DIRECTION THROUGH THE LATE EVENING HOURS. ANOTHER MID LEVEL
IMPULSE IS EXPECTED TO DROP OUT OF THE UPPER MISSISSIPPI VALLEY
LATE WEDNESDAY NIGHT...BRINGING A CHANCE FOR MORE SHOWERS AND
THUNDERSTORMS TO THE REGION.

KREIN

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

*  HIGH CONFIDENCE IN WIND TRENDS THROUGH THE PERIOD...BUT A
   LITTLE LOWER CONFIDENCE IN THE EXACT TIMING OF LAKE BREEZE
   PASSAGE.

*  MEDIUM TO HIGH CONFIDENCE THAT VIS WILL DROP TO MVFR WITH ISOLD
   IFR ACROSS THE REGION...AND MEDIUM CONFIDENCE THAT VIS AT
   ORD/MDW WILL REMAIN 6SM OR HIGHER.

*  LOW CONFIDENCE IN TIMING AND COVERAGE OF SHRA/TSRA WEDNESDAY
   NIGHT.

KREIN

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

* THURSDAY THROUGH SATURDAY...CHANCE OF TSRA.
* SUNDAY...SLIGHT CHANCE OF TSRA.
* MONDAY...DRY/VFR.

&&

.MARINE...
319 PM CDT

GENERALLY QUIET CONDITIONS OVER THE LAKE THROUGH THE WEEKEND WITH
RESPECT TO WINDS AND WAVES...AND PERIODS WHERE DIRECTIONS IN THE
NEARSHORE AREAS WILL BE CONTROLLED BY LAKE BREEZE INFLUENCES BY
DAY. A WARM AND MOIST AIR MASS WILL OVERSPREAD THE REGION AND
UPPER LEVEL DISTURBANCES COULD RESULT IN THUNDERSTORM COMPLEXES
CROSSING THE LAKE AT TIMES...MAINLY FROM WEDNESDAY NIGHT THROUGH
SATURDAY. THE POTENTIAL EXISTS FOR SOME OF THESE COMPLEXES TO
PRODUCE GUSTY WINDS AND LOCALLY HIGHER WAVES...AS WELL AS FREQUENT
CLOUD TO WATER LIGHTNING.

RC

&&

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

$$

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