Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 101756
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1256 PM CDT THU JUL 10 2014

.DISCUSSION...
337 AM CDT

NICE TODAY...NOT BAD TOMORROW...PARTLY STORMY OVER THE WEEKEND INTO
EARLY NEXT WEEK...THEN THE UNSEASONABLY COOL HEADING INTO NEXT WEEK
WITH A JULY VERSION OF THE POLAR VORTEX...A "POLAR VORTEX LIGHT" IF
YOU WILL.

TAKE YESTERDAY AND ADD A COUPLE DEGREES IN TEMPS AND SUBTRACT ABOUT
5 MPH IN WINDS AND YOU HAVE TODAY`S FORECAST. PLEASANTLY COOL
CONDITIONS SHOULD CONTINUE THROUGH TONIGHT THOUGH AN INCREASE IN
HIGH CLOUDINESS...ESPECIALLY LATE...ALONG WITH DEVELOPING LIGHT
SOUTHERLY WINDS SHOULD KEEP LOWS A BIT MILDER. FRIDAY LOOKS TO BE A
TRANSITION DAY TOWARD THE STORMIER PATTERN WITH DECAYING MCS
PROBABLY MOVING INTO THE AREA BY THE AFTERNOON...THOUGH SUSPECT THE
GLOBAL MODELS ARE TOO AGGRESSIVE WITH MOISTURE RETURN AND PREFER THE
DRIER LOOKING NAM WITH PRECIP PROBABLY LIGHT AND SCATTERED.

FRIDAY NIGHT MODELS HAVE A PRETTY STRONG SIGNAL FOR AN MCS
DEVELOPING OVER IOWA DURING THE EVENING AND THEN TRACKING EAST INTO
THE CWA OVERNIGHT INTO SATURDAY MORNING. GFS/NAM ARE BOTH ARE THE
MOST ROBUST WITH THIS FEATURE AND SHOW A STRONG CONVECTIVELY
ENHANCED VORT AND APPARENT WELL DEVELOPED MCS MOVING ACROSS THE
REGION. ELEVATED INSTABILITY LOOKS TO BE NOT OVERLY IMPRESSIVE WITH
MUCAPES OF 1000 J/KG OR LESS FEEDING INTO THE SYSTEM AND ONLY
MARGINAL DEEP LAYER SHEAR...SO NOT LOOKING LIKE A BIG SEVERE WEATHER
MAKER EVEN IF GFS/NAM VERIFY.

FORECAST CONFIDENCE HEADING INTO SATURDAY IS LOW AND HINGES LARGELY
ON THE FORECASTABILITY OF CONVECTION. ASSUMING A MORNING MCS ROLLS
THROUGH THE AREA...THE REMNANT COLD POOL AND DEBRIS CLOUDINESS
WOULD LIKELY KEEP TEMPS COOLER THAN GUIDANCE AND HAVE SHAVE A COUPLE
DEGREES OFF GOING FORECAST. IF AFOREMENTIONED MCS DOES NOT
MATERIALIZE THEN IT WILL BE A WARM AND RATHER HUMID DAY. ADDITIONAL
CONVECTION SAT AFTERNOON WOULD LIKELY BE DEPEND ON LEFT OVER OUTFLOW
BOUNDARY FROM NOCTURNAL STORMS.

DETAILS VARY SUBSTANTIALLY IN THE MODELS SAT NIGHT THROUGH SUNDAY
NIGHT DUE TO DIFFERING HANDLING OF CONVECTIVE DEVELOPMENT. BASICALLY
IT WILL NOT BE A TOTAL WASH OUT...BUT STORM APPEAR POSSIBLE THROUGH
THE PERIOD AS WE REMAIN IN THE PERTURBED NORTHWEST FLOW ALOFT. IF A
SIGNIFICANT MCS DRIVES EFFECTIVE SFC FRONT SOUTHWARD AT SOME POINT
IN TIME...THEN DRIER WEATHER WOULD LIKELY PREVAIL IN ITS WAKE.

ABNORMALLY HIGH AMPLITUDE UPPER LEVEL PATTERN FORECAST TO DEVELOP
NEXT WEEK WITH GFS/GEM NOW JUMPING ONTO THE ECMWF BANDWAGON OF THE
PAST COUPLE OF DAYS DEVELOPING AN ANOMALOUSLY STRONG UPPER LEVEL
LOW...A SUMMER VERSION OF THE POLAR VORTEX...TUESDAY INTO WEDNESDAY
NEXT WEEK. VERY IMPRESSIVE HEIGHT ANOMALIES ARE PROJECTED WITH 500MB
HEIGHTS FORECAST TO FALL TO 3+ STANDARD DEVIATIONS BELOW AVERAGE
WITH 850MB TEMPS PROGGED TO DROP TO AROUND 5C. THIS IS LIKELY
OVERDONE SOMEWHAT...AS NARR DATA GOING BACK TO 1979 SHOWS THE
COLDEST 850MB TEMPS WE HAVE SEEN AROUND HERE IN MID-LATE JULY IS
AROUND 7C...SO THIS GOES TO SHOW JUST HOW IMPRESSIVE OF A "COLD"
SNAP IS BEING PROJECTED. HAVE NUDGED TEMPS A BIT BELOW THE BLENDED
MODEL COCKTAIL...AS SIMILAR 850MB TEMPS IN THE SUMMER OF 2009
RESULTED IN HIGHS IN THE UPPER 60S! PATTERN DOES LOOK TO BE
PROGRESSIVE...SO TEMPS LOOK TO MODERATE BY THE END OF THE WEEK...BUT
IF CURRENT MODEL RUNS ARE REASONABLY CLOSE TO VERIFYING THEN THE
SUBURBAN AND RURAL AREAS CAN LOOK FORWARD TO SOME LOWS IN THE 40S
NEXT WEEK WITH NEAR RECORD LOWS NOT TOTALLY OUT OF THE QUESTION!

IZZI

&&

.AVIATION...

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

* INCREASE IN EASTERLY WIND SPEEDS TO AROUND 10 KT THIS AFTERNOON.

RODRIGUEZ

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

HIGH PRESSURE SPANNING THE REGION PROVIDING QUIET WEATHER ONCE
AGAIN THIS AFTERNOON...WITH QUIET AND VFR CONDITIONS EXPECTED TO
PERSIST. DEVELOPING THUNDERSTORMS WELL WEST OF THE TERMINALS
FRIDAY WILL REMAIN WEST OF THE TERMINALS THROUGH THE END OF THE
FORECAST PERIOD. CURRENT EASTERLY WINDS WILL OBSERVE AN INCREASE
IN SPEEDS THROUGH THE AFTERNOON...WITH SPEEDS AROUND 10 KT
BECOMING LIKELY BY MID AFTERNOON.

RODRIGUEZ


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

* HIGH CONFIDENCE WITH VFR CONDITIONS.

* HIGH CONFIDENCE OF AN INCREASE IN WIND SPEEDS...MEDIUM HIGH
  CONFIDENCE OF TIMING.

RODRIGUEZ

//OUTLOOK FOR ORD/MDW FOR 00Z SATURDAY-12Z THURSDAY...UPDATED 12Z...
FRIDAY NIGHT...CHC TSRA

SATURDAY THROUGH MONDAY...PERIODIC CHANCES FOR TSRA. MAINLY VFR.

TUESDAY THROUGH WEDNESDAY...SLIGHT CHC SHRA.

KMD

&&

.MARINE...
326 AM CDT

1020 MB HIGH PRESSURE OVER THE LAKE EARLY THIS MORNING WILL
STRENGTHEN SOME THIS MORNING BEFORE SLOWLY PULLING OFF TO THE
EASTERN LAKES TONIGHT INTO FRIDAY. WINDS WILL THEREFORE REMAIN
LIGHT/VARIABLE WITH LAKE BREEZES ALONG THE SHORES TODAY. ONCE THE
HIGH SHIFTS EAST...SOUTH-SOUTHEAST WINDS TO DEVELOP AND EVENTUALLY
STRENGTHEN INTO FRIDAY NIGHT AS A TROUGH OF LOW PRESSURE ADVANCES
FROM THE PLAINS AND A DEEPER LOW MOVES THROUGH NORTHERN MANITOBA.
SOUTH WINDS WILL INCREASE INTO THE 20 KT RANGE FRIDAY AFTERNOON INTO
SATURDAY MORNING...ESPECIALLY ACROSS THE NORTHERN PORTION OF THE
LAKE. EXPECT WINDS TO SLACKEN SOME LATER SATURDAY AS BOTH LOW
PRESSURE CENTERS WEAKEN THE GRADIENT AND A TROUGH AXIS APPROACHES.
EXPECT GENERALLY A SOUTH-SOUTHWEST COMPONENT THOUGH WIND DIRECTION
MAY BE A BIT VARIABLE IN THE BAGGY GRADIENT OF THE TROUGH AXIS. THE
TROUGH AXIS SLIDES THROUGH PAST THE LAKE AND WINDS SHIFT
WEST-NORTHWEST LATE SUNDAY NIGHT AND MONDAY. STRONGER
NORTHWESTERLIES ARE EXPECTED MONDAY NIGHT INTO EARLY TUESDAY AS A
COLD FRONT DROPS IN. KMD

&&

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

$$

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