Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 262000
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
300 PM CDT SUN JUL 26 2015

.SHORT TERM...
224 PM CDT

THROUGH MONDAY NIGHT...

CLOUDS CONTINUED TO THICKEN FROM WEST TO EAST THIS MORNING...AND HAS
PERSISTED THROUGH THIS AFTERNOON FOR MUCH OF THE FORECAST AREA. A
WEAKENING COMPLEX OF THUNDERSTORMS HAD INITIALLY BECOME FOCUSED
ACROSS NORTHERN MISSOURI THIS MORNING...HOWEVER A FEW STRONGER
POCKETS OF CONVECTION REMAINED OVER FAR EASTERN IOWA. THIS ALLOWED
AN AREA OF MODERATE TO AT TIMES HEAVY RAIN AND A FEW EMBEDDED
THUNDERSTORMS TO PERSIST...AND DRIFT EAST INTO LEE/OGLE COUNTIES
EARLY THIS AFTERNOON. DUAL-POL PRECIP ESTIMATES INDICATED A FEW
LOCATIONS IN LEE COUNTY HAVE RECEIVED BETWEEN 2 AND 3 INCHES OF
RAIN. THE LOOP OF REFLECTIVITY SUGGESTS THIS LINE WAS BEGINNING TO
DRIFT NORTHEAST INTO A SLIGHTLY MORE STABLE ENVIRONMENT. WEAK SFC
RIDGE REMAINED CENTERED OVER LOWER MICHIGAN...FEEDING EASTERLY WINDS
BACK FROM NORTHEAST IL/NORTHWEST IN. THIS HAS ALLOWED DEW POINTS FOR
MUCH OF THE FORECAST AREA TO REMAIN IN THE MID/UPR 60S. MEANWHILE
FURTHER WEST/SOUTH AND CLOSER TO THE QUASI-STATIONARY BOUNDARY
MOISTURE CONTINUES TO POOL ALONG THIS FEATURE WITH DEW POINTS IN THE
LOW/MID 70S. VISIBLE IMAGERY INDICATES SOME VERTICAL GROWTH ALONG
THE STALLED FRONTAL BOUNDARY...WHERE GREATER INSTABILITY REMAINS
THIS AFTERNOON. FORTUNATELY A RELATIVELY THICK SOLAR SHIELD REMAINS
AND HAS KEPT TEMPS FROM WARMING TOO MUCH...MAINLY IN THE LOW 80S.

LATE THIS AFTERNOON SOME EROSION TO THE CLOUD COVER SHOULD OCCUR
ACROSS THE WEST/SOUTH CWFA...WHICH SHOULD ALLOW SOME UPTICK IN THE
SHOWERS AND THUNDERSTORMS. ALTHOUGH THE DEEP LAYER SHEAR...AND MU-
CAPE PARAMETERS ARE NOT SUGGESTIVE OF SEVERE WEATHER. THAT BEING
SAID...ANY STRONGER CORES THAT COULD DEVELOP AND QUICKLY DISSIPATE
MAY GENERATE ENOUGH WIND TO PRODUCE A GUST IN EXCESS OF 40MPH.
WHATEVER CONVECTION DOES DEVELOP...THE LARGER CONCERN WOULD BE FOR
BRIEF HEAVY DOWNPOURS...GIVEN SUCH A MOIST LLVL AND LACK OF STEERING
WINDS. THUS CONVECTION WOULD BE SLOW MOVING. EXPECT AS THE EVENING
PROGRESSES...THE MID-LVL VORT MAX WILL PIVOT SOUTHEAST ALONG THE
QUASI-STATIONARY BOUNDARY AND SHOULD PULL THE BEST CHANCE FOR
CONVECTION FURTHER SOUTH/SOUTHEAST. AT THIS TIME FAR NORTHEAST IL
CONTINUES TO REMAIN ON THE DRY SIDE OF GUIDANCE...AND WILL CONTINUE
TO KEEP A DRY FORECAST TONIGHT. CLOUDS WILL LIKELY LINGER
TONIGHT...WHICH SHOULD HELP KEEP TEMPS MILD IN THE UPR 60S TO ARND
70.

MONDAY THROUGH MONDAY NIGHT... GUIDANCE HAS CONTINUED TO INDICATE
MID-LVL HEIGHTS STEADILY INCREASING FROM WEST TO EAST EARLY MON.
THIS SHOULD HELP PROVIDE SOME DIFFLUENCE IN THE MID-LVLS AND PARTLY
CLOUDY SKIES FOR AT LEAST THE FIRST HALF OF MON. BY LATE IN THE
AFTERNOON 500MB HEIGHTS BEGIN TO FLATTEN...WITH A WEAK LLVL WAVE
OVER THE SOUTHERN PLAINS DRIFTING NORTH AND MAY ALLOW THE QUASI-
STATIONARY BOUNDARY TO LIFT NORTH ACROSS THE WEST/SOUTHWEST CWFA.
GIVEN THE CONTINUED ABUNDANCE OF LLVL MOISTURE MON...IT APPEARS
FAVORABLE THAT THE BEST CHANCE FOR ANY CONVECTION/PRECIP COULD BE
THE WESTERN CWFA.

850MB THERMAL RIDGE AXIS BEGINS TO PIVOT EAST MON...HOWEVER THE
WARMEST AIR CONTINUES TO BE FOCUSED WELL WEST OF THE CWFA IN
IOWA/MISSOURI. HAVE HELD ONTO TEMPS IN THE MID/UPR 80S...ALTHOUGH
COULD SEE A FEW AREAS TOUCHING 90 DEGREES DEPENDING ON CLOUD COVER
AND COVERAGE OF PRECIP. ONSHORE FLOW SHOULD LOCK TEMPS ALONG THE
ILLINOIS NEARSHORE IN THE UPR 70S/ARND 80.

MID-LVL TROUGH WILL BEGIN TO NUDGE EAST FROM THE ROCKIES MON
NGT...PUSHING THE MOIST/WARM CHANNEL ACROSS THE PLAINS FURTHER EAST
MON NGT ACROSS THE FORECAST AREA. THIS SHOULD ALLOW FOR A GREATER
CHANCE FOR SHOWERS/THUNDERSTORMS TO FINALLY ARRIVE OVER NORTHEAST
IL/NORTHWEST IN WITH SOUTHERLY FLOW DEVELOPING AS WELL. OVERNIGHT
LOWS WILL REMAIN MILD ARND 70 TO THE LOW 70S.

BEACHLER

&&

.LONG TERM...

TUESDAY THROUGH SATURDAY...

328 AM...MAIN FORECAST CONCERNS ARE HIGH TEMPS/HEAT INDICES
TUESDAY AND THUNDERSTORM CHANCES WITH A COLD FRONT LATE TUESDAY
NIGHT AND WEDNESDAY.

SIMILAR TO MONDAY...IT IS DIFFICULT TO FIND MUCH TO FOCUS
THUNDERSTORM DEVELOPMENT ON TUESDAY. BUT FROM THIS DISTANCE...
ITS POSSIBLE UPSTREAM CONVECTION OR OUTFLOWS COULD BECOME AN
ISSUE. BUT AT THE MOMENT...WHILE MAINTAINING SLIGHT CHANCE
POPS...TUESDAY COULD BE QUITE WARM/HOT AND CONTINUED VERY HUMID
WITH DEWPOINTS WELL INTO THE 70S. CURRENT HIGHS AROUND 90 WOULD
PUSH PEAK HEAT INDICES TO MID/UPPER 90S NORTH AND INTO THE 100-
105 RANGE SOUTH. THIS TIME PERIOD WILL NEED TO BE MONITORED AS
HEAT INDICES APPROACH ADVISORY CRITERIA ACROSS THE SOUTHERN CWA
TUESDAY AFTERNOON.

TUESDAY NIGHT COULD END UP VERY WARM PERHAPS WITH TEMPS ONLY IN
THE MID 70S...UPPER 70S IN URBAN AREAS. BUT DIDN/T GO THIS WARM
FOR ACTUAL LOW TEMPS JUST YET...AS CONVECTION OR CONVECTIVE
OUTFLOWS MAY BE APPROACHING THE NORTHERN CWA OVERNIGHT OR BY
WEDNESDAY MORNING AS BOTH THE GFS/ECMWF HAVE SPED UP THE FRONTAL
TIMING. THIS INCREASES THE UNCERTAINTY FOR WEDNESDAY BOTH FOR
HIGH TEMPS AND CONVECTIVE TRENDS/TIMING. BUT THIS DOES APPEAR TO
BE THE BEST CHANCE FOR THUNDERSTORMS FOR THE ENTIRE AREA OVER THE
NEXT SEVERAL DAYS. WITH THE FRONT NOW PUSHING THRU BY WEDNESDAY
EVENING...HAVE GONE DRY WEDNESDAY NIGHT.

SLIGHTLY COOLER AND LESS HUMID AIR WILL THEN ARRIVE THURSDAY.
THERE NOW APPEARS TO BE ONE OR TWO WEAK WAVES MOVING SOUTHEAST
ACROSS THE MIDWEST/LAKES REGION FRIDAY NIGHT AND AGAIN NEXT
WEEKEND...AT LEAST AS SHOWN BY THE GFS...WHICH MAY BRING THE
CHANCE OF PRECIP BACK INTO THE FORECAST FRIDAY NIGHT INTO NEXT
WEEKEND. CMS

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 20Z...

* SCHC -RA THIS AFTERNOON.

* EAST TO NORTHEAST WINDS POTENTIALLY NEARING 10 KT THIS AFTERNOON
  AT TIMES. BETTER CHANCE TOMORROW.

KMD

//DISCUSSION...UPDATED 18Z...

THE UPPER LEVEL WAVE PRODUCING THE COMPLEX OF THUNDERSTORMS IN
WESTERN ILLINOIS WILL CONTINUE TO WEAKEN AND DIVE SOUTHEAST ALONG
THE BETTER INSTABILITY GRADIENT SOUTH AND EAST OF THE MAIN
CHICAGOLAND TERMINALS. EXPECT CONVECTIVE ACTIVITY TO CONTINUE TO
BUBBLE TO THE SOUTHWEST THROUGH THIS AFTERNOON. EXPECT AS A
PORTION OF THE WAVE MARCHES RIGHT ON THROUGH THE AREA THAT A
PERIOD OF LIGHT RAIN/LARGELY SPRINKLES DUE TO THE HIGH CLOUD
BASES WILL IMPACT THE REMAINDER OF THE AREA IN THE COMING HOURS.

MEANWHILE...EAST WINDS CONTINUE TO LARGELY HOLD IN THE 6-9 KT
RANGE...AND NO CHANGE IN THIS IS EXPECTED OTHER THAN SOME
INFREQUENT 10 KT WINDS OWING TO THE CLOUD COVER...WEAKER THERMAL
CONTRAST BETWEEN LAND/LAKE.

KMD

//ORD AND MDW CONFIDENCE...UPDATED 20Z...

* MEDIUM-HIGH IN ACCUMULATING RAIN DIMINISHING BEFORE REACHING
  TERMINALS...THOUGH POSSIBLY SOME SPRINKLES THIS AFTERNOON.

* HIGH THAT WINDS WILL LARGELY REMAIN IN THE 7-9 KT RANGE AND THAT
  THE 10+ KT FREQUENCY WILL BE LOW TODAY. MEDIUM IN WIND SPEED
  TOMORROW.

KMD

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

MONDAY NIGHT AND TUESDAY...VFR.

WEDNESDAY...CHANCE OF TSRA.

THURSDAY THROUGH SATURDAY...PRIMARILY VFR.

IZZI

&&

.MARINE...
225 PM CDT

WEAK HIGH PRESSURE REMAINS ACROSS THE EASTERN GREAT LAKES...WITH A
MINIMAL GRADIENT STRETCHING OVER LAKE MICHIGAN. THE WINDS CONTINUE
TO BE GENERALLY FROM THE EAST...BUT AT TIMES VARIABLE. WINDS WILL
BEGIN TO TURN SOUTHEASTERLY MONDAY NIGHT INTO TUESDAY...AS A COLD
FRONT WILL APPROACH THE LAKE WEDNESDAY. THIS WILL ALLOW WINDS TO
INCREASE ALONG A TIGHT GRADIENT...BEFORE TURNING NORTHWEST. THEN
ANOTHER WEAK RIDGE OF HIGH PRESSURE ARRIVES LATER IN THE WEEK.

BEACHLER

&&

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

$$

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