Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 261820
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
120 PM CDT SAT JUL 26 2014

.DISCUSSION...

1150 AM...LATE MORNING UPDATE...TWO FEATURES OF NOTE LATE THIS
MORNING...A WEAK WAVE OVER SOUTHERN MN THAT WILL MOVE ACROSS THE
AREA THIS AFTERNOON AND WHAT IS EXPECTED TO BE THE MAIN WAVE
CURRENTLY OVER NORTHEAST KS MOVING INTO NORTHWEST MO. FORECAST
SOUNDINGS SHOW A FAIRLY STRONG CAP ACROSS MUCH OF THE AREA INTO
THIS AFTERNOON AND WHILE SHORT TERM/HI RES GUIDANCE DOES SUGGEST
SOME ISOLATED ACTIVITY IS POSSIBLE...APPEARS MUCH OF THE AREA WILL
REMAIN DRY INTO THIS AFTERNOON.

THUNDERSTORMS DEVELOPING OVER NORTHWEST MO WILL CONTINUE MOVING
EAST THIS AFTERNOON AND ACROSS A VERY UNSTABLE/MOIST ATMOSPHERE.
THIS ACTIVITY IS EXPECTED TO CONTINUE EAST AND ACROSS THE
SOUTHERN CWA LATE THIS AFTERNOON OR MORE LIKELY DURING THE EVENING
HOURS. NO CHANGES TO PREVIOUS THOUGHTS ON SEVERE POTENTIAL WITH A
LINE OF STORMS WITH THE POTENTIAL FOR WIDESPREAD DAMAGING WINDS
THE MOST LIKELY THREAT. HOW FAR NORTH THIS LINE EXTENDS IS STILL A
BIT UNCERTAIN AND WHILE IT WILL LIKELY REMAIN SOUTH OF I-80...
COULD BE EVEN FURTHER SOUTH...PERHAPS SOUTH OF THE KANKAKEE RIVER.

OTHERWISE...EXTENSIVE CLOUD COVER IS SLOWING THE WARMING TREND.
WITH VERY WARM LOW LEVELS...A FEW HOURS OF SUN...IF THAT WERE TO
OCCUR...COULD PUSH TEMPS TO CURRENT FORECAST HIGHS OF MID 80S...SO
NO LARGE CHANGES TO HIGHS BUT TEMP TRENDS WILL REQUIRE UPDATES.
CMS

//PREV DISCUSSION...
322 AM CDT

SYNOPSIS...UNSETTLED WEATHER IS EXPECTED TODAY WITH SEVERE STORMS
POSSIBLE ALONG AND SOUTH OF I-80.  COOLER CONDITIONS MOVE IN
TOMORROW AND PERSIST THROUGH THE REST OF THE WEEK.

TODAY AND TONIGHT...
SHOWERS HAVE SPROUTED OVER NORTHWEST AND NORTH CENTRAL IL EARLY
THIS MORNING AT THE NOSE OF THE LOW LEVEL JET. EXPECTING SHOWERS
AND A FEW THUNDERSTORMS TO FESTER SOUTH OF I-88 BEFORE PUSHING
EAST OF THE WARNING AREA BY MID MORNING. NOT EXPECTING ANY SEVERE
WEATHER WITH THESE SHOWERS AND STORMS AS A SIGNIFICANT CAP IS IN
PLACE ALONG WITH LIMITED INSTABILITY.

UPPER LEVEL HIGH PRESSURE REMAINS OVER THE SOUTHERN U.S. AND IT IS
STILL EXPECTED TO RETROGRADE OVER THE SW U.S. BY MONDAY NIGHT. THE
UPPER LEVEL LOW OVER SOUTHERN SASKATCHEWAN WILL MOVE OVER WI SUNDAY
MORNING.  AT THE SURFACE...THE LOW OVER THE PLAINS IS SLOWLY
TREKKING EAST AND WILL MOVE OVER CENTRAL IL TONIGHT. THE SECOND LOW
OVER SOUTHERN SASKATCHEWAN WILL SLIDE SOUTHEAST AND MOVE OVER LAKE
MI SUNDAY MORNING.

A WARM FRONT IS OVER CENTRAL MISSOURI AND IT WILL MOVE NORTH INTO
THE SOUTHERN FORECAST AREA THIS MORNING. DEW POINTS WILL RISE INTO
THE 70S. PWAT VALUES WILL ALSO INCREASE TO AROUND 2 INCHES THIS
EVENING...RESULTING IN TORRENTIAL DOWNPOURS WITH ANY STORMS. WHILE
THE NAMS 4000+ J/KG OF CAPE SEEMED A LITTLE EXCESSIVE...INSTABILITY
WILL STILL BE PLENTIFUL WITH MOST GUIDANCE SUGGESTING VALUES OF
3000-4000 J/KG SOUTH OF I-88. THE 0-6 KM SHEAR IN EXCESS OF 50 KTS
AND STEEP LAPSE RATES ALOFT LOOK VERY FAVORABLE FOR ORGANIZED
STORMS CAPABLE OF SEVERE WEATHER...NAMELY WINDS AND HAIL...IN
CENTRAL ILLINOIS INCLUDING THE SOUTHERN FORECAST AREA. THE 0-1 KM
SHEAR IS MAXIMIZED OVER THE FAR SOUTHERN PORTION OF THE
CWA...SOUTH OF A PONTIAC TO RENSSELAER LINE. SPC MENTIONS A CHANCE
FOR TORNADOES IN THE SAME AREA...BUT THINKING TORNADOES WILL NOT
BE THE MAIN THREAT. ONCE AGAIN HEAVY RAIN WILL BE A PRIMARY THREAT
ALONG WITH DAMAGING WINDS AND LARGE HAIL.

HAVE MEDIUM CONFIDENCE IN TIMING AND CONVECTIVE MODE.  HAVE POPS
INCREASING FROM THE EARLY AFTERNOON THROUGH THIS EVENING PRIMARILY
ALONG AND SOUTH OF I-80. THE BEST CHANCE FOR STORMS WILL BE THIS
EVENING INTO TONIGHT AS FORCING IS MAXIMIZED AHEAD OF THE LOW AND
ITS COLD FRONT. COULD SEE SOME ISOLATED STORMS AT FIRST IN THIS
AREA...MAYBE EVEN SUPERCELLS...BUT THE MAIN SHOW SHOULD BE A LINE
OR STORM COMPLEX SOUTH OF I-80 THIS EVENING.

FINALLY...LOWERED HIGH TEMPS TODAY AS EXPECTING THICK OVERCAST SKIES
TO REMAIN IN PLACE. THE CLOUDS WILL LIMIT HEATING AND INSTABILITY.
BASED ON 850 MB TEMPS ALONE...HOWEVER...STILL THINKING WE WILL
EASILY MAKE IT INTO THE MID TO UPPER 80S TODAY.  IF CLOUDS THIN...WE
COULD SEE SPOTS REACH 90 DEGREES.

SUNDAY...
THE UPPER LEVEL LOW WEAKENS AND BECOMES AN OPEN WAVE OVER LAKE MI
SUNDAY AND WE REMAIN ON THE WEST SIDE OF THE UPPER LEVEL TROUGH
THROUGH THE REST OF THE WEEK.  THE SURFACE PLAINS LOW ALSO WEAKENS
AND MERGES WITH THE CANADIAN LOW...AND THEN SHIFT EAST TO OVER NEW
ENGLAND MONDAY MORNING. A SECOND COLD FRONT MOVES THROUGH THE REGION
SUNDAY AFTERNOON/EVENING USHERING IN ANOTHER ROUND OF WELL BELOW
NORMAL TEMPERATURES.

EXPECTING SHOWERS AND STORMS TO FORM OVER THE LAKE SUNDAY MORNING AS
THE UPPER LEVEL LOW MOVES OVER WI. BANDS OF VORTICITY ASSOCIATED
WITH THE LOW WILL PROVIDE LIFT FOR SCATTERED STORM DEVELOPMENT
ACROSS MOST OF THE FORECAST AREA SUNDAY AFTERNOON. SEVERE STORMS
ARE NOT EXPECTED AS INSTABILITY WILL BE WEAK.

HIGH TEMPS WILL GET INTO THE LOW TO MID 80S...BUT WILL FEEL TEMPS
FALL QUICKLY BEHIND THE COLD FRONT LATE SUNDAY AFTERNOON/EVENING.
OVERNIGHT LOWS WILL BE AROUND 60.

MONDAY THROUGH FRIDAY...
WE REMAIN ON THE WEST SIDE OF THE UPPER LEVEL TROUGH NEXT WEEK AND A
SURFACE HIGH MOVES OVER THE REGION TUESDAY.  IN GENERAL LOOKING AT
BELOW NORMAL TEMPS THAT SLOWLY WARM TO AROUND 80 BY THE END OF NEXT
WEEK.  HAVE A FEW PERIODS OF SHOWERS AND STORMS IN THE
EXTENDED...BUT THINKING STORMS WILL BE VERY DIURNAL. ALSO NOT
EXPECTING A TON OF ACTIVITY...AS THE FORCING SOURCE WILL BE WEAK
VORTICITY STREAMERS THAT TRAVEL AROUND THE UPPER LEVEL TROUGH.

JEE

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* LOW CHANCE ISOLATED SHRA/TSRA THIS AFTERNOON. GREATER TSRA
  POTENTIAL REMAINS SOUTH OF AREA INTO CENTRAL IL LATE THIS
  AFTERNOON/EVENING.

* LIGHT AND SOMEWHAT VARIABLE SFC WINDS INTO TONIGHT. SOME CHANCE
  OF LAKE BREEZE AND SHIFT TO AN EASTERLY DIRECTION MID/LATE
  AFTERNOON.

* POTENTIAL FOR MVFR VIS IN FOG DEVELOPMENT OVERNIGHT.

* LOW CHANCE SHRA/TSRA AROUND SUNRISE SUNDAY...AND AGAIN SUNDAY
  AFTERNOON.

* WINDS BECOMING WEST-NORTHWEST SUNDAY MORNING AND INCREASING IN
  STRENGTH. GUSTS 25-30 KT LIKELY BY EARLY/MID AFTERNOON.

RATZER

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

COMPLICATED AND RATHER LOW CONFIDENCE FORECAST THROUGH TONIGHT
ESPECIALLY WITH RESPECT TO WIND DIRECTION AND SHRA/TSRA COVERAGE.

DISSIPATING SURFACE COLD FRONT WAS ACROSS THE FORECAST AREA FROM
FAR NORTHEAST IL TO FAR NORTHERN MISSOURI THIS AFTERNOON. WEAK
PRESSURE GRADIENT IN THE VICINITY OF THIS BOUNDARY WAS PRODUCING
RELATIVELY LIGHT/VARIABLE SURFACE WINDS...THOUGH LIGHT WESTERLY
WINDS WERE AT THE TOP/ABOVE SHALLOW SURFACE MIXED LAYER. LAKE
BREEZE WAS DEVELOPING ACROSS EASTERN LAKE/NORTHEAST COOK COUNTY...
THOUGH TDWR IMAGERY INDICATES THAT IT HAS SLOWED ITS WEST-
SOUTHWESTWARD PUSH NORTH OF DOWNTOWN CHICAGO...AND MOST HIGH-RES
MODEL GUIDANCE DOES NOT BRING IT AS FAR INLAND INTO ORD/MDW.

MVFR CIGS HAD DEVELOPED LATE MORNING WITH DIURNAL MIXING OF
SHALLOW MOIST BOUNDARY LAYER...THOUGH SOME SCATTERING AND LIFTING
OF THIS LOWEST LAYER OCCURRING PER SATELLITE/SFC OBS. FORECAST
SOUNDINGS ALSO DEPICT DRYING AND MIXING ABOVE BASE OF STRONG
INVERSION NEAR 2500 FT THROUGH THE AFTERNOON SUGGESTING MVFR CIGS
WILL NOT PERSIST. OTHER CONCERN IS THE POTENTIAL FOR SHRA/TSRA
DEVELOPMENT AS A MID-LEVEL DISTURBANCE TRACKS SOUTHEAST ACROSS THE
AREA THIS AFTERNOON. FORECAST SOUNDINGS INDICATE AFOREMENTIONED
INVERSION WOULD PROVIDE STRONG CAPPING FOR SURFACE BASED
PARCELS...THOUGH STEEP MID/UPPER LEVEL LAPSE RATES WOULD SUPPORT
SUBSTANTIAL INSTABILITY IF TAPPED. HIGH-RES GUIDANCE DOES DEPICT
SOME LIMITED DEVELOPMENT ACROSS THE AREA DURING THE AFTERNOON WITH
SUPPORT FROM RECENT RADAR TRENDS...THOUGH CONVECTION IS EXPECTED
TO STRUGGLE ACROSS TERMINALS WITH LOW CONFIDENCE IN COVERAGE.
STRONG/SEVERE THUNDERSTORMS ARE MUCH MORE LIKELY TO AFFECT
PORTIONS OF CENTRAL IL LATE THIS AFTERNOON/EVENING...EVOLVING EAST
FROM CURRENT TSRA COMPLEX SEEN OVER NORTHERN MO. MOIST LOW LEVEL
ENVIRONMENT AND LIGHT WINDS MAY SUPPORT FOG OR LOW STRATUS
DEVELOPMENT LATER TONIGHT...THOUGH MID/HIGH CLOUD COVER IS
EXPECTED TO LIMIT DEVELOPMENT SOMEWHAT.

STRONGER MID-LEVEL SYSTEM DIGS SOUTHEAST ACROSS THE REGION EARLY
SUNDAY ACCOMPANIED BY A STRONGER COLD FRONTAL PUSH. SOME POTENTIAL
WILL EXIST FOR SHRA/TSRA AROUND 12Z WITH THIS FRONT...THOUGH POOR
DIURNAL TIMING SUGGESTS LOW COVERAGE AT THAT HOUR AND PRECLUDES
MENTION IN TAF AT THIS TIME. SAME WITH POTENTIAL FOR INSTABILITY
SHOWERS LATER SUNDAY AFTERNOON. COLD ADVECTION BEHIND STRONGER
FRONT DOES DEEPEN THE MIXED LAYER SUNDAY...WITH WINDS SHIFTING
WEST-NORTHWEST AND BECOMING FAIRLY GUSTY NEAR 30 KT IN THE
AFTERNOON.

RATZER


//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* LOW CONFIDENCE IN SHRA/TSRA DIRECTLY AFFECTING ORD/MDW THROUGH
  PERIOD.

* LOW CONFIDENCE IN WIND DIRECTION THIS AFTERNOON AND TONIGHT...
  THOUGH HIGH CONFIDENCE IN PREVAILING SPEEDS LESS THAN 10 KT.

* LOW CONFIDENCE IN FOG AND LOW CIGS TONIGHT.

* HIGH CONFIDENCE IN WEST-NORTHWEST WINDS BECOMING GUSTY SUNDAY.

RATZER

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

SUNDAY NIGHT...SLIGHT CHANCE OF SHRA IN THE EVE.
MONDAY...VFR. GUSTY NORTH-NORTHEAST WINDS.
TUESDAY...VFR.
WEDNESDAY-FRIDAY...SLIGHT CHANCE OF TSRA.

MTF

&&

.MARINE...
217 AM CDT

AN ACTIVE PERIOD FOR SUMMERTIME IS UPCOMING OVER THE LAKE BUT FIRST
TODAY AND TONIGHT WILL SEE SOUTHERLY WINDS AHEAD OF AN APPROACHING
SURFACE LOW. WINDS ACTUALLY LOOK TO BE LIGHT ENOUGH IN INDIANA AND
ILLINOIS NEARSHORE ZONES TO ALLOW FOR A LAKE BREEZE TO DEVELOP
FOR TEMPORARY AFTERNOON ONSHORE FLOW. THE SURFACE LOW WILL EVOLVE
EASTWARD OVER THE MIDDLE OF THE LAKE ON SUNDAY MORNING WITH COOL
AIR ADVECTION ENVELOPING THE LAKE BY LATE IN THE DAY AND
ESPECIALLY IN THE EVENING. NORTHERLY WINDS WITHIN UNSTABLE
THERMAL PROFILES OVER THE WATER WILL SUPPORT 20-25 KT SUSTAINED
WINDS WITH POTENTIALLY SOME GUSTS KNOCKING ON THE DOOR TO GALE
STRENGTH SUNDAY NIGHT AND EARLY MONDAY. WAVES ARE LIKELY TO BUILD
WHERE THEY WERE WITH THE LAST COOL ADVECTION EPISODE EARLIER THIS
WEEK /WEDNESDAY/...BOTH OVER THE OPEN WATERS AND NEARSHORE AREAS
OF ILLINOIS AND INDIANA. THE FLOW WILL DIMINISH INTO TUESDAY BUT
CONTINUE FROM THE NORTH BEFORE HIGH PRESSURE SETTLES IN MIDWEEK.

MTF

&&

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

$$

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