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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
103 AM CDT FRI JUL 11 2014

.DISCUSSION...
313 PM CDT

MAIN FORECAST CONCERNS FOCUS ON TIMING OF THE RETURN OF SHOWERS AND 
THUNDERSTORMS TO THE REGION LATE FRIDAY...WITH THE POTENTIAL FOR 
SOME STRONGER STORMS AND LOCALLY HEAVY RAINFALL DURING THE OVERNIGHT 
HOURS FRIDAY NIGHT. SCATTERED THUNDERSTORMS REMAIN POSSIBLE THROUGH 
THE WEEKEND AS WEAK SURFACE COLD FRONT SLOWLY MOVES THROUGH THE 
REGION WITHIN INCREASINGLY MOIST AIR MASS. FORECAST THEN TRENDS 
TOWARD COOLER WEATHER EARLY NEXT WEEK...AS CLOSED UPPER LOW NOW OVER 
NORTHERN ALBERTA EVOLVES EAST-SOUTHEASTWARD INTO THE GREAT LAKES 
REGION. 

IN THE NEAR TERM...SURFACE HIGH PRESSURE OVER THE GREAT LAKES REGION 
WAS PRODUCING DRY/SUNNY CONDITIONS ACROSS THE AREA THIS AFTERNOON. 
SURFACE WINDS WILL GRADUALLY BECOME MORE SOUTH-SOUTHEAST TONIGHT AS 
THE HIGH DRIFTS TO THE EAST...AND RETURN FLOW DEVELOPS INTO THE 
MISSISSIPPI VALLEY AHEAD OF WEAK TROUGH DEVELOPING ACROSS THE 
PLAINS. A SERIES OF MID-LEVEL SHORT WAVES ARE PROGGED TO TRAVERSE 
WESTERLY FLOW ACROSS THE NORTHERN TIER OF THE CONUS FRIDAY THROUGH 
THE WEEKEND...THOUGH THE INITIAL WAVE CURRENTLY PROPAGATING THROUGH 
THE DAKOTAS WILL BE WORKING AGAINST VERY DRY THERMODYNAMIC PROFILES 
AS NOTED IN 12Z GREAT LAKES/OHIO VALLEY RAOBS. PRECIP CHANCES WILL 
INCREASE BY LATE FRIDAY HOWEVER...AS LOW LEVEL MOISTURE ADVECTS INTO 
THE REGION WITH WINDS BECOMING MORE SOUTH-SOUTHWEST AND AS SOUTHERN 
PERIPHERY OF 100+ KT UPPER JET CORE TRANSLATES ACROSS THE UPPER 
MIDWEST. WITH THE DRY AIR INITIALLY IN PLACE...MODEL GUIDANCE 
CONTINUES TO TREND SLOWER WITH THE ADVANCE OF PRECIP INTO THE CWA 
FRIDAY...WITH MUCH OF THE AREA EXPECTED TO REMAIN DRY THROUGH THE 
DAY. HAVE BACKED POPS OFF TO SLGT CHANCE FAR WEST IN THE AFTERNOON 
BASED ON THIS CONSISTENT TREND...WITH POPS THEN INCREASING FROM WEST 
TO EAST FRIDAY EVENING/NIGHT. FORECAST SOUNDINGS DEPICT MID-LEVEL 
LAPSE RATES ARE FAIRLY UNIMPRESSIVE DURING THE AFTERNOON/EARLY 
EVENING HOURS FOR MUCH IN THE WAY OF THUNDER...THOUGH THUNDERSTORM 
POTENTIAL DOES LOOK TO INCREASE FRIDAY NIGHT AS DEEP MOISTURE 
SPREADS INTO THE REGION ALONG WITH WARM ADVECTION IN THE 900-800 HPA 
WHICH COMBINE TO PRODUCE SOME TALL/SKINNY CAPE PROFILES. DEEP LAYER 
BULK SHEAR (0-6 KM) INCREASES QUICKLY TO 30-40 KT FRIDAY NIGHT WITH 
INCREASE IN BOTH UPPER JET AND LOW LEVEL JET SPEEDS...AND SPC HAS 
PULLED THE 5% SEVERE RISK INTO NORTH CENTRAL ILLINOIS FOR FRIDAY 
NIGHT...THOUGH GIVEN THE RELATIVELY THIN CAPE PROFILES...SUSPECT 
LOCALLY HEAVY RAINFALL MAY BE MORE AF A WEATHER THREAT AS FORECAST 
PRECIPITABLE WATER VALUES CLIMB INTO THE 2.0+ INCH RANGE.

MCS WILL LIKELY BE EXITING THE CWA STAGE RIGHT (INDIANA) SATURDAY 
MORNING...WITH THE FORECAST BECOMING SOMEWHAT MESSY IN ITS WAKE. LOW 
LEVEL THETA-E RIDGE AND WARM/MOIST ADVECTION AXIS REMAINS AIMED INTO 
THE FORECAST AREA ON SOUTHWEST WINDS...WITH OUTFLOW FROM 
OVERNIGHT/MORNING ACTIVITY AND APPROACH OF WEAK SURFACE TROUGH FROM 
THE WEST/NORTHWEST LIKELY BECOMING FOCUS FOR CONTINUED CONVECTIVE 
FESTERING OR NEW INITIATION THROUGH THE AFTERNOON. WHILE CONFIDENCE 
IN DETAILS OF TIMING AND LOCATION OF THUNDERSTORM CLUSTERS BECOMING 
LOWER INTO SATURDAY AFTERNOON/NIGHT...HAVE MAINTAINED LIKELY POPS AS 
CONFIDENCE IS FAIRLY HIGH THAT PERIODS OF SCATTERED STORMS ARE A 
PRETTY GOOD BET. POTENTIAL FOR LOCALLY HEAVY RAINFALL WILL REMAIN 
INTO SUNDAY AS WELL...BEFORE DEEP MOIST AXIS SHIFTS SOUTHEAST.  
THUNDERSTORM CHANCES CONTINUE THROUGH SUNDAY AND INTO MONDAY TOO AS 
THE SURFACE COLD FRONT EVENTUALLY MOVES SLOWLY ACROSS THE FORECAST 
AREA...BEFORE THE FOCUS FOR ORGANIZED SHOWERS AND STORMS MOVES OFF 
TO THE SOUTHEAST. 

TEMPS DURING THE FRIDAY-MONDAY PERIOD WILL BE WARMING...WITH AROUND 
80/LOWER 80S FRIDAY AND LOW-MID 80S FOR THE MOST PART SUNDAY BEFORE 
SLIGHT POST-FRONTAL COOLING MONDAY. COULD EVEN BE A FEW UPPER 80S 
SATURDAY AND SUNDAY ACROSS THE SOUTHERN PARTS OF THE CWA...THOUGH 
POTENTIAL FOR EXTENSIVE CLOUDS/PRECIP/OUTFLOW LENDS LOWER CONFIDENCE 
TO REALIZING THAT POTENTIAL.

AS HINTED AT IN THE FIRST PARAGRAPH...WEATHER THEN TAKES A TURN 
TOWARD THE COOL/SHOWERS SIDE HEADING INTO TUESDAY AS THE 
AFOREMENTIONED UPPER LOW DIGS SOUTHEAST AND DEEPENS ACROSS THE GREAT 
LAKES REGION TUESDAY. AFTER A SERIES OF WARM AND HUMID DAYS THROUGH 
THE WEEKEND...TEMPS MAY NOT RISE ABOVE 70 IN A FEW SPOTS TUESDAY AS 
STRONG COLD ADVECTION TAKES HOLD ON BREEZY NORTHWEST WINDS. GRADUAL 
IMPROVEMENT IS EXPECTED HOWEVER...AS THE UPPER LOW SLOWLY LIFTS OUT 
TO THE NORTHEAST THROUGH THE END OF NEXT WEEK...WITH DRY WEATHER AND 
TEMPERATURES MODERATING BACK THROUGH THE 70S EXPECTED 
WEDNESDAY-THURSDAY.  

RATZER

&&

.AVIATION...

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

* POTENTIAL FOR A LAKE BREEZE FRIDAY AFTERNOON/EVENING.

KMD

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

VFR CONDITIONS WILL PREVAIL AT THE TERMINALS THROUGH AT LEAST THE
NEXT 24 HOURS WITH HIGH PRESSURE INCHING EAST AWAY FROM THE
REGION. E TO ESE WINDS WILL CONTINUE TO DROP OFF AND LIKELY GO
LIGHT AND VARIABLE IN MANY LOCATIONS THOUGH DIRECTION SHOULD
GENERALLY VEER TOWARDS THE SOUTH. WINDS WILL PICK UP OUT OF THE
SOUTH AROUND 10 KT OR SO DURING THE DAY FRIDAY. A LAKE BREEZE IS
EXPECTED TO FORM...HOWEVER IT IS UNSURE AT THIS POINT WHETHER OR
NOT IT WILL HAVE A STRONG ENOUGH PUSH TO MAKE IT ACROSS THE
TERMINALS. SOME OF THE HIGHER RESOLUTION GUIDANCE IS HINTING AT A
SHIFT TO SE WINDS AT KMDW BUT GENERALLY KEEPS SPEEDS ON THE
LIGHTER SIDE. WITH A SSW WIND AROUND OR SLIGHTLY ABOVE 10 KT...A
LAKE BREEZE WOULD LIKELY SLOWLY PUSH INTO NORTHERN COOK COUNTY
TOWARDS ORD...BUT MAY BE STOPPED SHY OF THE TERMINAL. WILL
CONTINUE WITH THIS REASONING FOR NOW BUT OBVIOUSLY THIS IS A SITUATION
THAT WILL HAVE TO BE MONITORED...AND THE PICTURE MAY BECOME MORE
CLEAR AS IT STARTS TO COME IN RANGE OF MORE OF THE HIGHER-RES
MODEL SOLUTIONS OVERNIGHT AND EARLY TOMORROW.

CONCERNS WILL SHIFT TO POTENTIAL FOR THUNDERSTORMS AFTER 6Z...BUT
WILL GIVE THIS A CLOSER LOOK WITH THE 12Z TAF PACKAGE.

KMD


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

* LOW CONFIDENCE IN LAKE BREEZE REACHING TERMINALS FRIDAY.
* HIGH CONFIDENCE IN REMAINING FORECAST ELEMENTS.

KMD

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

SATURDAY...TSRA. MVFR/IFR PSBL.
SUNDAY THROUGH TUESDAY...PERIODIC CHANCES FOR TSRA/SHRA. PRIMARILY VFR.
WEDNESDAY...VFR.

BMD

&&

.MARINE...
229 PM CDT

HIGH PRESSURE IS OVER THE EASTERN GREAT LAKES BUT LIGHT AND VARIABLE 
WINDS REMAIN OVER THE LAKE. WINDS WILL BECOME SOUTH AS THE HIGH 
SHIFTS FURTHER EAST TONIGHT.  SOUTH WINDS INCREASE TO 15 TO 25 KT 
OVER THE NORTHERN HALF OF THE LAKE TOMORROW WHILE THE SOUTH HALF 
WILL SEE S TO SE WINDS AT 10-20 KT.  WINDS DIMINISH SATURDAY AS A 
TROUGH OF LOW PRESSURE MOVES OVERHEAD AND BECOME SOUTHWEST SATURDAY 
NIGHT.

A LOW PASSES WELL NORTH OF THE LAKE SATURDAY NIGHT AND SUNDAY AND 
ITS COLD FRONT SWINGS OVER THE LAKE SUNDAY MORNING. WINDS BECOME 
WEST BEHIND THE FRONT AND THEN A SECOND FRONT PUSHES DOWN THE LAKE 
SUNDAY NIGHT INTO MONDAY.  WINDS WILL BECOME NORTH TO NORTHWEST 
10-20 KT BEHIND THE FRONT.  WINDS DIMINISH TUESDAY NIGHT AND THEN 
HIGH PRESSURE MOVES OVER THE LAKE WEDNESDAY NIGHT.

JEE

&&

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

$$

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