Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 251804
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
104 PM CDT MON AUG 25 2014

.DISCUSSION...
327 AM CDT

HOT AND HUMID WEATHER WILL CONTINUE OVER THE NEXT FEW DAYS BEFORE A
BIT OF A REPRIEVE ARRIVES FOR MID WEEK. THUNDERSTORM CHANCES WILL BE
A PERSISTENT CHALLENGE THROUGH THE PERIOD. EARLY MORNING WATER VAPOR
IMAGERY SHOWS A SHARP UPPER RIDGE AXIS STRETCHED FROM THE MID
MISSISSIPPI RIVER VALLEY INTO QUEBEC WHILE A CLOSED UPPER LOW SPINS
OVER NORTH DAKOTA AND SOUTHERN MANITOBA. ANOTHER CLOSED UPPER LOW IS
MOVING SOUTHWARD INTO NORTHERN CALIFORNIA AND NEVADA WITH UPPER
RIDGING TO ITS NORTH OVER THE BRITISH COLUMBIA COAST. AT THE
SURFACE...AN EXPANSIVE AREA OF HIGH PRESSURE IS POSITIONED FROM THE
ST. LAWRENCE RIVER VALLEY TO THE MID-ATLANTIC STATES WITH A RIDGE
AXIS EXTENDING ALL THE WAY TO THE FLORIDA PANHANDLE. LOW PRESSURE IS
CENTERED ACROSS NORTHERN NORTH DAKOTA AND SOUTHERN MANITOBA WITH A
COLD FRONTAL BOUNDARY EXTENDING SOUTH ACROSS MINNESOTA AND IOWA
WHILE A WARM FRONT RUNS JUST NORTHEAST OF THE AREA. THIS HAS ALLOWED
A VERY WARM AND HUMID AIRMASS TO SPREAD ACROSS THE AREA WITH
DEWPOINTS RUNNING IN THE MID AND UPPER 70S.

TODAY...A RATHER DIFFICULT FORECAST THIS MORNING WITH THUNDERSTORMS
BEING THE POTENTIAL WILD CARD IN HOW OPPRESSIVE THE HEAT BECOMES
ACROSS THE AREA. CONVECTION CONTINUES ALONG THE AFOREMENTIONED COLD
FRONTAL BOUNDARY FROM NORTHWEST MISSOURI ACROSS CENTRAL IOWA AND
MUCH OF CENTRAL WISCONSIN. THE UPPER LOW OVER NORTH DAKOTA WILL TAKE
ON A MORE EASTERLY DIRECTION WITH TIME WHICH WILL HELP RE-ORIENT THE
MID LEVEL FLOW FROM ITS CURRENT SOUTHWESTERLY DIRECTION TO A MORE
WESTERLY DIRECTION INTO EARLY AFTERNOON. THIS IS OF CONCERN AS THIS
WOULD LIKELY RESULT IN A MORE EASTWARD MOVEMENT OF ONGOING
CONVECTION THIS MORNING. LATEST GUIDANCE IS MIXED IN TERMS OF
BRINGING CONVECTION ACROSS THE THE NORTHERN PORTION OF THE CWA LATER
THIS MORNING OR EARLY THIS AFTERNOON...WITH HIGH RES GUIDANCE GOING
BACK AND FORTH ON WHAT OCCURS. CONVECTION ACROSS THE AREA COULD
OBVIOUSLY HAVE A BIG IMPACT ON TEMPS...WITH POTENTIAL OUTFLOW FROM
SUCH STORMS POSSIBLY SPREADING SOUTHWARD BEYOND THE STORMS
THEMSELVES. TIMING WOULD ALSO BE KEY AS IT IS POSSIBLE THAT RECOVERY
FROM ANY NEGATIVE TEMPERATURE IMPACTS MAY OCCUR LATE THIS AFTERNOON.
DESPITE THE ABUNDANCE OF WHAT-IFS THIS MORNING FEEL THAT IT IS
PLAUSIBLE THAT SOME OF THE ONGOING CONVECTIVE ACTIVITY WILL MAKE ITS
WAY ACROSS THE NORTHERN CWA FROM MID MORNING POSSIBLY INTO EARLY
AFTERNOON. HOWEVER...WITH SCATTERED CONVECTION ALL THE WAY BACK INTO
NORTHEAST KANSAS A LARGER PORTION OF THE CWA COULD BE AFFECTED LATER
INTO THE AFTERNOON. WILL CARRY LOW CHANCE POPS NORTH ACROSS THE FAR
NORTH LATER THIS MORNING WITH A LARGER AREA OF SLIGHT POPS ACROSS
THE NORTHERN HALF OF THE CWA INTO THE AFTERNOON. IF CONVECTION
REMAINS LOOSELY ORGANIZED WHICH THE FURTHER SOUTH ACTIVITY HAS SO
FAR TONIGHT...THEN THE IMPACT ON TEMPS WILL LIKELY BE SHORT LIVED
AND LATE AFTERNOON RECOVERY A REASONABLE EXPECTATION. WILL MAKE
LITTLE CHANGE TO FORECAST HIGHS OF AROUND 90 NORTH TO THE MID 90S
SOUTHWEST AND LOW 90S ELSEWHERE. WILL CONTINUE WITH THE HEAT
ADVISORY AS-IS WITH THIS AREA LEAST LIKELY TO BE AFFECTED BY
STORMS/CLOUD COVER/OUTFLOW. WILL CONTINUE TO HIGHLIGHT THE
SUB-ADVISORY HEAT/HUMIDITY ACROSS THE REST OF THE AREA WITH A
SPECIAL WEATHER STATEMENT.

TONIGHT THROUGH WEDNESDAY...THE UPPER LOW WILL TRACK ACROSS JAMES
BAY TUESDAY MORNING SHUNTING THE RIDGE AXIS EASTWARD AND BRINGING A
FLAT WEST TO EAST FLOW ACROSS MUCH OF THE UPPER MIDWEST AND GREAT
LAKES. THE UPPER LOW NOW OVER CALIFORNIA/NEVADA WILL CUT OFF FROM
THE MAIN FLOW AND MEANDER EASTWARD THROUGH MID WEEK HELPING TO
SUPPLY WEAK SHORTWAVES WHICH WILL TRACK EASTWARD ACROSS THE LOCAL
AREA. THE COOL FRONTAL BOUNDARY CURRENTLY TO THE NORTHWEST WILL MOVE
INTO THE AREA LATER TONIGHT AND SLOWLY MOVE SOUTHWARD ACROSS THE
AREA TUESDAY. THE PRESENCE OF THE FRONT ALONG WITH THE UPPER FLOW
SUPPORTING SHORTWAVE PASSAGES WILL BRING MODESTLY BETTER CHANCES FOR
SHOWERS AND THUNDERSTORMS TONIGHT AND TUESDAY. HOWEVER...THE BEST
CHANCES ARE EXPECTED TUESDAY NIGHT INTO WEDNESDAY MORNING AS A WAVE
TRAVERSES THE NORTHERN STREAM OF FLOW WITH AN ATTENDANT UPPER JET
TRACKING ACROSS THE UPPER GREAT LAKES. THIS PLACES FAVORABLE JET
SUPPORT OVER THE LOCAL AREA. WILL CONTINUE WITH HIGHEST POPS TUESDAY
NIGHT...THOUGH POPS CONTINUE WEDNESDAY WITH THE FRONTAL BOUNDARY
STILL NEARBY. TEMPS LOOK TO BE A LITTLE COOLER TUESDAY THOUGH MANY
AREAS SOUTH OF I-80 WILL PROBABLY SEE LOW 90S ONCE AGAIN. CONVECTION
AND CONVECTIVE DEBRIS WILL ONCE AGAIN BE THE LIMITING FACTOR BUT THE
HEAT ADVISORY MAY NEED TO BE EXTENDED IN THE FAR SOUTHWEST. NORTHERN
AREAS WILL LIKELY BE HELD TO THE UPPER 80S WITH BETTER CHANCES FOR
CLOUDS/PRECIP AND FLOW TURNING NORTHERLY THROUGH THE DAY. WEDNESDAY
WILL SEE A MORE NOTABLE COOLING ALONG THE LAKE WITH 70S
EXPECTED...WHILE INLAND AREAS SEE 80S...WITH WARMEST READINGS SOUTH.

THURSDAY AND BEYOND...THE CUTOFF UPPER LOW WILL STILL BE MEANDERING
EASTWARD OVER THE ROCKIES THURSDAY WITH WEAK RIDGING BUILDING OVER
THE LOCAL AREA AHEAD OF IT. THIS WILL BRING ANOTHER PUSH OF WARM
ADVECTION TO THE AREA LEADING TO MORE SHOWER AND THUNDER
CHANCES...ESPECIALLY FOR NORTHERN AREAS. A SURFACE LOW IS FORECAST
TO PASS TO THE NORTH BRINGING ANOTHER FRONT INTO THE AREA SOME TIME
FRIDAY OR FRIDAY NIGHT WHICH MAY TAKE ITS TIME CROSSING THE AREA AS
WELL. HAVE SEEN CHANGES IN THE MEDIUM RANGE GUIDANCE SOLUTIONS FOR
THIS PERIOD OVER RECENT DAYS SO THE DETAILS ARE STILL FUZZY BUT THE
CHANCES FOR PERIODIC THUNDERSTORMS WILL CONTINUE THROUGH THE
WEEKEND.

MDB

&&

.AVIATION...

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

* VCTS THRU 22-23Z...WITH PERHAPS THE NEED FOR A TEMPO TSRA.

* WINDS TURNING NE ARND 10-12KT...WITH A FEW OCCASIONAL GUSTS TO
  20-25KT THRU 19Z.

* WIND SPEEDS DIMINISH TO ARND 6-8KT LATE AFTN/EARLY EVE...THEN
  TURN W/NW.

* POSSIBLE ADDTL ISOLATED TSRA OVERNIGHT/EARLY TUE MORNING.

* PATCHY FOG DEVELOPING AFT 04Z...MAY REDUCE VSBYS TO ARND 2-4SM.
  WEST OF TAF SITES VSBYS COULD BE LOWER.

BEACHLER

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

SPOTTY SHOWERS AND THUNDERSTORMS HAVE CONTINUED TO DEVELOP ALONG
AN ORIGINAL OUTFLOW BOUNDARY ACROSS NORTHEAST IL. ADDTL SHRA/TSRA
HAVE DEVELOPED WEST OF TAF SITES...WITH A FEW STRONGER STORMS
CAPABLE OF PRODUCING WIND GUSTS TO 40KTS THIS AFTN. CONCERN IS
THAT A WEAK BOUNDARY SLIDING SOUTH AND A LACK OF A GRADIENT...WILL
ONLY PROLONG THE NORTHEAST WINDS AT ORD/MDW. MEANWHILE FURTHER
WEST AT RFD/DPA THE WINDS SHUD BE ABLE TO TURN NW/W ARND 20-21Z.
WILL CONTINUE TO MENTION VCTS THRU 23Z...AND IF ADDTL STORMS ARE
ABLE TO DEVELOP WE WILL LIKELY NEED TO ADJUST WITH A TEMPO BUT
COVERAGE IS NOT EXPECTED TO BE THERE. THEN LATE THIS EVENING
GUIDANCE IS SUGGESTING THE WINDS WILL BECOME LGT FROM THE NW ARND
5-8KT. THE CONCERN THEN WILL TURN TOWARDS THE LOW CIGS BUT ALSO
THE POTENTIAL FOR PATCHY FOG TO DEVELOP GIVEN HOW MOIST THE
ENVIRONMENT WILL REMAIN OVERNIGHT. FOG MAY BEGIN TO DISSIPATE AFT
DAYBREAK TUE...THEN GUIDANCE SUGGESTS THE WINDS WILL TRY TO TURN
NW WITH SPEEDS ARND 8-11KT.

BEACHLER


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

* HIGH CONFIDENCE IN TSRA REMAINING ISOLATED THIS AFTN...LOW
  CONFIDENCE IN PRECISE TIMING/INTENSITY AT TAF SITES.

* HIGH CONFIDENCE IN WINDS REMAINING NORTHEAST THRU 22Z...THEN LOW
  CONFIDENCE IN TIMING OF WINDS TURNING NW LATE AFTN/EARLY EVE.

* MEDIUM CONFIDENCE IN PATCHY FOG DEVELOPING.

BEACHLER

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

* TUESDAY NIGHT...TSRA LIKELY.
* WEDNESDAY THROUGH SUNDAY...CHANCE OF TSRA.

&&

.MARINE...

243 AM...LOW PRESSURE OVER SOUTHERN MANITOBA WILL MOVE ACROSS
NORTHERN ONTARIO TODAY AND TONIGHT REACHING JAMES BAY TUESDAY
MORNING. A TRAILING COLD FRONT WILL SLOWLY SAG SOUTHEAST ACROSS
THE WESTERN LAKES AND REACH NORTHERN LAKE MICHIGAN BY DAYBREAK
TUESDAY MORNING. THIS FRONT WILL THEN CONTINUE SOUTH ACROSS LAKE
MICHIGAN TUESDAY AND EVENTUALLY STALL NEAR OR JUST SOUTH OF THE
LAKE WEDNESDAY INTO THURSDAY. AHEAD OF THIS FRONT...SOUTH TO
SOUTHEAST WINDS 10-20KTS WILL GRADUALLY SHIFT MORE SOUTHWESTERLY
LATER TODAY AND THIS EVENING. A FEW HIGHER GUSTS ARE POSSIBLE BUT
THE TIGHTEST PORTION OF THE GRADIENT PASSES NORTH OF THE LAKE.
HIGH PRESSURE WILL BUILD ACROSS THE NORTHERN PLAINS ON TUESDAY
AND THEN MOVE ACROSS THE NORTHERN LAKES WEDNESDAY. THIS MAY
STRENGTHEN THE GRADIENT A BIT ON THE SOUTHERN END OF THE LAKE
WITH A PROLONGED NORTHEAST FLOW IN THE 10-20KT RANGE POSSIBLE.
CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...HEAT ADVISORY...ILZ010-ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039
     UNTIL 10 PM MONDAY.

IN...NONE.
LM...NONE.
&&

$$

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