Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 201855
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
155 PM CDT WED AUG 20 2014

.DISCUSSION...
320 AM CDT

SHORT TERM...THROUGH WEDNESDAY NIGHT...

FOR THE REMAINDER OF THE OVERNIGHT HOURS...CONDITIONS WILL REMAIN
QUIET WITH HIGH PRESSURE OVERHEAD AND THE PESKY UPPER LOW LIFTING
INTO THE UPPER GREAT LAKES AND ANY ASSOCIATED CONVECTION MOVING WELL
TO THE EAST.  UNDER CLEAR SKIES AND LIGHT WINDS...EXPECT THAT SOME
LOCATIONS COULD SEE SOME FOG DEVELOPMENT...MAINLY LOW LYING AND
SHELTERED AREAS.  DO NOT EXPECT WIDESPREAD DENSE FOG  AS DEWPOINT
DEPRESSIONS ARE STILL 3-5 DEGREES IN MANY PLACES.  CONDITIONS WILL
REMAIN QUIET INTO WEDNESDAY BUT UPPER RIDGING BUILDING OVER THE
CENTRAL PLAINS AND ACROSS THE MIDDLE AND UPPER MISSISSIPPI VALLEY
WHILE THE UPPER LOW CONTINUES TO LIFT TO THE NORTHEAST IS SIGNALING
THE BEGINNING OF A SIGNIFICANT WARMING TREND...MORE LIKE THE TRUE
DOG DAYS OF SUMMER THAT THE UNSEASONABLY COOL WEATHER THAT THE AREA
HAS SEE FOR MUCH OF THIS SUMMER.  ACCOMPANYING THE UPPER RIDGING AND
INCREASING TEMPERATURES ALOFT WILL BE AN INCREASE OF LOW LEVEL
MOISTURE.  THE BERMUDA HIGH IS IN PLACE OVER THE ERN GLFMEX WHILE
A TROUGH OF LOW PRESSURE IS DEVELOPING OVER THE CENTRAL PLAINS.
THIS IS SETTING UP A RETURN FLOW OF MOISTURE FROM A WIDE OPEN GULF
OF MEXICO...WITH 20-30KT 850MB WINDS WITH TRAJECTORIES OFF OF THE
WESTERN GLFMEX POINTING RIGHT INTO THE MIDWEST. UNDER AMPLE SUNSHINE
FOR MOST OF THE DAY TODAY...THERE SHOULD BE LITTLE TROUBLE MIXING
DOWN THE WARMER AIR ALOFT...WHERE 850MB TEMPS SHOULD BE ARND
18-20C.  SO...THE AREA WILL SEE HIGHS TODAY IN THE MIDDLE TO UPPER
80S.  WITH DEWPOINTS IN THE MIDDLE TO UPPER 60S...THE MAX HEAT INDEX
TODAY SHOULD BE IN THE UPPER 80S TO LOWER 90S.  WHILE THESE HEAT
INDEX VALUES ARE NOT ANYWHERE NEAR HEADLINE LEVELS...THEY ARE
CERTAINLY MUCH HIGHER THAN HAS BEEN SEEN FOR THE BULK OF THE
SUMMER.

SHOWERS AND THUNDERSTORMS WILL BECOME AN INCREASING CONCERN THROUGH
THE AFTERNOON AND INTO WEDNESDAY NIGHT.  EARLY THIS MORNING...THERE
IS ONGOING CONVECTION OVER THE UPPER MISSOURI VALLEY ASSOCIATED WITH
A SHORTWAVE OVER TOPPING THE UPPER RIDGE.  EXPECT THAT THIS ACTIVITY
SHOULD SLIDE SOUTHEAST OF THE CWA THIS MORNING...MOVING THROUGH SRN
IA/NRN MO AND INTO CNTRL/SRN IL.  THE MODELS ARE CONSISTENT IN
DEVELOPING A WARM FRONT EXTENDING EAST FROM A DEEPENING LOW OVER THE
DAKOTAS THROUGH THE MIDDLE MISSISSIPPI VALLEY BY EARLY WEDNESDAY
EVENING.  THIS SHOULD BE THE FOCUS FOR SHRA/TSRA SPREADING INTO THE
REGION FROM SOUTHWEST TO NORTHEAST THROUGH THE EVENING AND OVERNIGHT
HOURS WEDNESDAY NIGHT.  HAVE MAINTAINED A MORE BROADBRUSHED
APPEARANCE TO THE WX/POP/QPF GRIDS FOR WEDNESDAY NIGHT DUE TO SOME
CONCERN WITH TIMING OF THE PCPN SPREADING NWD AS THERE IS STILL SOME
UNCERTAINTY IN THE MODELS AS FAR AS TIMING THE LEADING EDGE OF THE
PCPN...BUT HAVE GONE WITH LIKELY POPS AS CONFIDENCE IS HIGHER IN THE
POTENTIAL FOR PCPN.  CONFIDENCE IS JUST A LITTLE LOWER IN THE ONSET
TIME.  WHILE THE SEVERE POTENTIAL WILL BE LOW...A GREATER CONCERN
WILL BE LOCALLY HEAVY RAINFALL.  AS SWLY FLOW AND WARM/MOIST
ADVECTION INCREASES THROUGH WEDNESDAY NIGHT...THE MODEL GUIDANCE IS
INDICATING PWATS AROUND 2 INCHES OVER THE AREA.  WITH UPPER LEVEL
WINDS FROM JUST ABOVE THE SURFACE THROUGH THE COLUMN EXPECTED TO BE
ARND 25-30KT...THE THUNDERSTORM ACTIVITY SHOULD BE PROGRESSIVE.
SO...WHILE LOCALLY HEAVY RAINFALL IS QUITE POSSIBLE...WIDESPREAD
FLOODING IS NOT ANTICIPATED THOUGH LOCALIZED URBAN AND SMALL STREAM
ADVISORY TYPE FLOODING REMAINS A POSSIBILITY.

KREIN

THURSDAY THROUGH SUNDAY...
MAIN CONCERN IN THE MEDIUM TERM WILL RESIDE ON THE
PLACEMENT/STRENGTH OF THE 500MB RIDGE POISED TO SETUP ACROSS THE
OZARKS. GUIDANCE HAS BEEN CONSISTENTLY SUGGESTING AN ATMOSPHERIC
RIVER DEVELOPING ACROSS THE NORTHERN PLAINS AND BECOMING ORIENTED
ALONG THE NORTHERN PERIPHERY OF THE MID-LVL RIDGE...THEN TURNING
SOUTHEAST ACROSS THE ALLEGHENIES IN PENNSYLVANIA. CONCERN DOES ARISE
WITH THE FACT THAT MEDIUM RANGE GUIDANCE HAS DEMONSTRATED A REDUCED
LEVEL OF CONFIDENCE WITH SPREAD IN HOW AMPLIFIED THE RIDGE WILL
BECOME...WHICH COULD BE STARTING TO HINT THAT THE ACTIVE RING-O-FIRE
MAY END UP CLOSER TO THE FORECAST AREA THUR NGT/FRI. AT THIS TIME
HAVE ONLY MADE MINOR ADJUSTMENTS WITH THE POPS...AND EXPECT A
CHANNEL OF LIKELY POPS TO BE LIFTING NORTH THUR MIDDAY INTO WISC BY
THUR NGT. CURRENT OPER SOLUTIONS STILL FAVOR THIS SCENARIO...PLUS
850MB TEMPS WILL BE STEADILY WARMING AND SHUD LIMIT THE COVERAGE OF
CONVECTION/PRECIP.

THEN INTO FRI SOLUTIONS SUGGEST THE TOP OF THE RIDGE MAY BEGIN TO
FLATTEN. LOCAL ARW8KM ALSO HINTS THAT THE ACTIVE ZONE MAY BE
DRIFTING SOUTH AND COULD BE OVER THE FORECAST AREA FRI AFTN/EVE.
THEN FOR SAT RIDGE REMAINS OVERHEAD...BUT IT STILL LOOKS POSSIBLE
FOR CHC POPS TO START THE WEEKEND. THE ONE CONSISTENCY THROUGH THE
THUR-SAT TIMEFRAME...THE HEAT/HUMIDITY. WHILE PWAT VALUES CONTINUE
TO HOVER IN THE 1.6-1.8" RANGE...THE FORECAST DEW PTS ARE EXPECTED
TO LINGER IN THE LOW TO PERHAPS MID 70S FOR MANY LOCATIONS THUR-SAT.
SFC TEMPS ARE PROGGED TO PUSH WELL INTO THE 80S...AND PERHAPS WARMER
YET DEPENDING ON CLOUD COVER/PRECIP. THE COMBINATION OF ANOMALOUSLY
HIGH DEW PTS AND SFC TEMPS IN THE UPR 80S/NEAR 90 WOULD SIGNAL HEAT
INDEX VALUES IN THE MID/UPR 90S TO PERHAPS THE LOW 100S SOUTH OF
I-80...MAINLY FRI-SUN.

EARLY NEXT WEEK...
ENSEMBLES AND OPER SOLUTIONS CONTINUE TO POINT TOWARDS THE HOT/HUMID
AIRMASS BREAKING DOWN AND TRANSITIONING BACK TO A MID CONUS/GREAT
LAKES TROUGH. MON WILL LIKELY STILL BE A HOT/HUMID DAY. GUIDANCE HAS
DEMONSTRATED SOME CONFIDENCE/CONSISTENCY IN A FRONTAL BOUNDARY
APPROACHING THE REGION MON NGT/TUE...WITH MUCH COOLER AIR BEGINNING
TO DRIVE SOUTH ACROSS THE NORTHERN PLAINS TUE...AND ARRIVING ACROSS
THE CWFA TOWARDS THE MIDDLE OF NEXT WEEK.

BEACHLER

&&

.AVIATION...

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

* LAKE BREEZE TURNING WINDS EAST/NORTHEAST THIS EVENING.
* LOW CHANCE FOR TSRA THIS AFTERNOON/EVENING...THEN BETTER CHANCE
  FOR TSRA OVERNIGHT.

BMD

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

A FRONTAL BOUNDARY STRADDLES CENTRAL ILLINOIS THIS AFTERNOON WITH
CONVECTION ONGOING NEAR THE FRONT. SW TO WSW WINDS ARE IN PLACE
ACROSS THE TERMINALS THIS AFTERNOON WITH A FEW SPORADIC GUSTS TO
AROUND 20 KT. WINDS WILL SLOWLY VEER TO THE WNW THROUGH THE
AFTERNOON AND A LATE LAKE BREEZE IS EXPECTED TO PUSH ACROSS THE
TERMINALS THIS EVENING TURNING WINDS TO THE NORTHEAST/EAST. A
CUMULUS FIELD HAS DEVELOPED ACROSS FAR NORTHERN AND NORTHEAST
ILLINOIS...NORTH OF THE CIRRUS SHIELD TO OUR SOUTH AND IS SHOWING
SOME SIGNS OF VERTICAL GROWTH. SEVERAL HIGHER RES MODELS DO HINT AT
ISOLATED TO WIDELY SCATTERED THUNDERSTORMS THIS AFTERNOON WHERE THE
CU FIELD IS CURRENTLY IN PLACE. COVERAGE SHOULD REMAIN LOW ENOUGH TO
PRECLUDE ANY MENTION IN THE TAF AT THIS TIME. BETTER CHANCES FOR
THUNDERSTORMS WILL DEVELOP TONIGHT AS THE FRONT STARTS TO LIFT
NORTH. A LOW LEVEL JET WILL DEVELOP TO OUR WEST WITH RESULTING
CONVECTION TRACKING EAST TOWARDS THE TERMINALS LATE TONIGHT.
THUNDERSTORM PROBABILITIES WILL CONTINUE THURSDAY MORNING INTO EARLY
AFTERNOON WITH THE FRONTAL BOUNDARY OVER NORTHERN ILLINOIS AND
MODELS SUGGESTING THAT THE AREA WILL REMAIN UNSTABLE. COVERAGE IS A
CONCERN SO WILL MAINTAIN A PROB30 FOR NOW.

BMD

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

* HIGH CONFIDENCE IN WINDS THROUGH THIS EVENING...THEN LOW-MEDIUM
  CONFIDENCE IN WIND DIRECTION THIS EVENING AND OVERNIGHT.
* LOW CONFIDENCE IN TSRA THIS AFTERNOON/EVENING...THEN MEDIUM-HIGH
  CONFIDENCE IN TSRA LATE TONIGHT INTO THURSDAY MORNING.

BMD

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

* THURSDAY NIGHT THROUGH TUESDAY...PERIODIC CHANCES FOR TSRA.

KREIN

&&

.MARINE...
155 PM CDT

A STAGNANT PATTERN WILL EXISTS ACROSS THE COUNTRY THROUGH THE
UPCOMING WEEKEND WITH HIGH PRESSURE OF AROUND 30.2 INCHES REMAINING
NEARLY STATIONARY OVER QUEBEC. MEANWHILE...LOW PRESSURE OF AROUND
29.7 INCHES WILL REMAIN NEARLY STATIONARY OVER NORTHERN AND CENTRAL
PLAINS WITH A QUASI-STATIONARY WARM FRONT EXTENDING EAST FROM THIS
LOW ACROSS THE LOWER LAKES REGION.

IZZI

&&

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

$$

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