Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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000
FXUS63 KDVN 202007
AFDDVN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
307 PM CDT WED AUG 20 2014

.SYNOPSIS...
ISSUED AT 306 PM CDT WED AUG 20 2014

12Z UA ANALYSIS HAD DEEP MOISTURE FROM THE SOUTHWEST MONSOON RUNNING
FROM MEXICO TO BETWEEN KOAX AND KTOP. MOISTURE WAS BEING TRANSPORTED
NORTH ON A LLJ RUNNING FROM THE BIG BEND OF TEXAS INTO WESTERN IOWA.
SATELLITE/RADAR TRENDS CONTINUE TO SHOW MID AND HIGH LEVEL CLOUDS
ACROSS THE MIDWEST WITH A COMBINATION OF LINGERING AND NEW
CONVECTION IN AN ELEVATED BAND FROM NEAR KSUX TO NEAR KCMI.

18Z SFC ANALYSIS HAD THE MAIN FRONT FROM SOUTHERN NEBRASKA INTO
SOUTHERN ILLINOIS. WEAK LOWS WERE IN WESTERN SOUTH DAKOTA. A VERY
PRONOUNCED COLD POOL FROM THE OVERNIGHT CONVECTION ACROSS IOWA INTO
WESTERN ILLINOIS. DEW POINTS WERE IN THE 70S ALONG AND SOUTH OF THE
FRONTAL BOUNDARY WITH 60S ACROSS THE UPPER MIDWEST AND GREAT LAKES.

&&

.SHORT TERM...(THIS EVENING THROUGH THURSDAY)
ISSUED AT 306 PM CDT WED AUG 20 2014

UNLIKE THE PAST TWO DAYS...CONVECTION FROM THE NOCTURNAL STORM
COMPLEXES CONTINUES TO LINGER ACROSS THE AREA AND INTO WESTERN IOWA.
ALTHOUGH THE FORCING HAS WEAKENED FROM THIS MORNING...IT IS STILL
SUFFICIENT ALONG AN ELEVATED THERMAL GRADIENT TO PRODUCE SHOWERS AND
A FEW THUNDERSTORMS. THIS CONVECTION WILL PERSIST INTO THE LATE
AFTERNOON AND EVENING AND MAY BRIEFLY END BEFORE THE NEXT NOCTURNAL
THUNDERSTORM COMPLEX DEVELOPS.

THE QUESTION IS WHERE WILL THE NEW THUNDERSTORM COMPLEX DEVELOP
TONIGHT AND EVENTUALLY TRACK. THE PERSISTENT BAND OF ELEVATED
CONVECTION ACROSS IOWA MAY MARK THE FAVORED ZONE OF DEVELOPMENT AND
EVENTUAL TRACK. IF THIS SCENARIO DOES PLAY OUT...THEN THE NORTHERN
OR NORTHEAST THIRD TO 40 PERCENT OF THE AREA WOULD BE FAVORED WITH
THE BETTER CHANCES OF RAIN. BASED ON THE HIGH MOISTURE CONTENT OF
THE AIR...STORMS WILL BE QUITE EFFICIENT IN PRODUCING VERY LOCALIZED
HEAVY RAINFALL.

GOING ON THE IDEA OF A NOCTURNAL THUNDERSTORM COMPLEX MOVING ACROSS
THE NORTH LATE TONIGHT...SAID COMPLEX WILL SLOWLY DECAY AND
DISSIPATE THURSDAY MORNING. RELATIVELY COLDER AIR LEFT OVER FROM
THIS STORM COMPLEX MAY SUPPRESS TEMPERATURES ACROSS THE NORTH ON
THURSDAY.

BOUNDARIES LEFT OVER FROM THE STORM COMPLEX WILL THEN PROVIDE THE
FOCUS FOR NEW DEVELOPMENT THURSDAY AFTERNOON.

.LONG TERM...(THURSDAY NIGHT THROUGH WEDNESDAY)
ISSUED AT 306 PM CDT WED AUG 20 2014

THE LONG TERM WILL FOCUS ON RATHER PERSISTENT LOW CONFIDENCE
THUNDERSTORM POTENTIAL IN A BROAD RING OF FIRE PATTERN...EVENTUALLY
REPLACED BY A MORE HIGH AMPLITUDE PATTERN...FEATURING A STALLED
FRONT FROM KANSAS THROUGH WISCONSIN. THUS...I VIEW THIS FORECAST AS
LOW CONFIDENCE...HIGH IMPACT. OUR FORECAST COULD EASILY GO TWO WAYS
ON SEVERAL DAYS...EITHER THE MCS STAYS NORTH RESULTING IN HOT AND
HUMID CONDITIONS AND POTENTIAL HEAT ADVISORY LEVELS OF HEAT
INDEX...OR LIKE TODAY...MCS REMNANTS KEEP THE CWA COOLER.
UNFORTUNATELY...IT WILL BE DIFFICULT TO IMPOSSIBLE TO HAVE A DAY TO
DAY HIGH CONFIDENCE GOING THIS WEEK...AS MESOSCALE PROCESSES WITH
NIGHTLY MCS ACTIVITY WILL DETERMINE THE POSITION OF THE WEST TO EAST
FRONT.  AGAIN...SHOULD THE BOUNDARY BE SOUTH...A COOL HUMID DAY LIKE
TODAY SHOULD RESULT...WHILE ANY DAY WITH THE MCS GOING NORTH WILL BE
IN THE UPPER 80S TO MID 90S...WITH HIGH HUMIDITY. I WILL CARRY A
FORECAST THAT SLIGHTLY FAVORS HEAT OVER RAIN...BUT THIS IS BASED
SOMEWHAT ON OUR RECENT TREND TO AVOID MCS IN OUR CENTRAL AND
NORTHERN CWA...BUT I HAVE ONLY TWEAKED POPS SLIGHTLY DUE TO THIS.

SEVERE STORMS CANNOT BE RULED OUT GIVEN AMPLE CAPE...BUT WITH POOR
CONFIDENCE ON WAVES EJECTING FROM THE MONSOONAL FLOW TO THE
SOUTHWEST...THE MESOSCALE EVOLUTION IS JUST TOO POORLY DEFINED TO
HIGHLIGHT POTENTIAL.

HEAVY RAIN APPEARS LIKELY IN ANY GIVEN MCS EVENT. LIKE THE PAST TWO
NIGHTS...AS WELL AS IS TYPICAL FOR LATE SUMMER RING OF FIRE
PATTERNS...WE WILL SEE SOME TRAINING OF STORMS...AS WELL AS ROBUST
PWAT VALUES. HOWEVER...SINCE THE NORTHERN 3/4 OF THE CWA HAS BEEN
DRY ABOUT 1 MONTH...WE CAN TAKE QUITE A BIT OF MOISTURE WITHOUT
PROBLEMS. ONCE AN MCS OR TWO HITS OUR CENTRAL AND NORTH...THERE
COULD BE POTENTIAL RUN OFF PROBLEMS OUTSIDE OF URBAN
LOCATIONS...WHICH DO NOT REQUITE SATURATION FOR RUN OFF PROBLEMS.

THE HIGH AMPLITUDE PATTERN EXPECTED LATE THIS WEEK INTO THE WEEKEND
IS FORECAST BY BOTH THE GFS AND ECMWF 12Z RUNS...AND IS MODERATELY
CONFIDENT.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1236 PM CDT WED AUG 20 2014

CONVECTION ACROSS IOWA AND WESTERN ILLINOIS WILL DISSIPATE PRIOR
TO 00Z/21. OTHERWISE EXPECT VFR CONDITIONS THROUGH 06Z/21. AFT
06Z/21 ANOTHER THUNDERSTORM COMPLEX WILL DEVELOP AND MOVE
SOUTHEAST TOWARD THE OHIO VALLEY. ALL TAF SITES HAVE VCTS BUT
KCID/KBRL MAY BE THE LEAST AFFECTED. KDBQ/KMLI HAVE A SOMEWHAT
BETTER CHANCE OF SEEING A TSRA. MVFR VSBYS ARE ALSO POSSIBLE AFT
06Z/21. IF A TSRA AFFECTS A TAF SITE THEN A BRIEF PERIOD OF IFR
CONDITIONS MAY BE POSSIBLE.


&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...08
SHORT TERM...08
LONG TERM...ERVIN
AVIATION...08





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