Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 291711
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1211 PM CDT WED JUL 29 2015

.UPDATE:
ISSUED AT 1159 AM CDT WED JUL 29 2015

EXCESSIVE HEAT WARNING WILL BE CANCELLED SHORTLY FOR A GOOD CHUNK
OF THE AREA. CLOUDS AND AND LOWER DEWPOINTS ARE FILTERING IN
BEHIND THE FRONT AND IT LOOKS LIKE 105 HEAT INDEX VALUES WON`T BE
ACHIEVED TODAY.

CARNEY

&&

.SHORT TERM:  (THROUGH LATE THIS AFTERNOON)
ISSUED AT 311 AM CDT WED JUL 29 2015

08Z SURFACE ANALYSIS REVEALS THAT THE EAGERLY-AWAITED COLD FRONT
(AND ITS NOTICEABLY LESS HUMID POST-FRONTAL AIR MASS) WAS
TANTALIZINGLY CLOSE TO THE LSX CWA EARLY THIS MORNING. AT 08Z,
THE FRONT STRETCHED FROM CENTRAL KS THROUGH EXTREME SOUTHEASTERN
NEB AND THROUGH CENTRAL IA INTO WESTERN WI. A NOCTURNAL MCS AHEAD
OF THE FRONT WAS MAKING STEADY SOUTHWARD PROGRESS BUT VERY SLOW
EASTWARD PROGRESS. ADDITIONAL THUNDERSTORMS ARE POSSIBLE WITH THE
FRONT TODAY AS IT MOVES INTO A MOIST AND UNSTABLE AIR MASS DURING
THE AFTERNOON HOURS, AND THE OUTFLOW BOUNDARY MAY ALSO SERVE AS A
TRIGGER FOR AFTN SH/TS.

UNTIL THE COLD FRONT ARRIVES, THERE WILL BE VERY LITTLE CHANGE IN
THE SOUPY AIR MASS STILL IN PLACE ACROSS THE REGION. THE TIMING
OF THE COLD FRONT IS SUCH THAT ROUGHLY THE SOUTHERN TO
SOUTHEASTERN HALF OF THE CWA SHOULD SEE ANOTHER DAY OF HOT/HUMID
CONDITIONS, ALBEIT NOT AS OPPRESSIVE AS YESTERDAY. HEAT INDEX
VALUES SHOULD BE CLOSER TO 105-109 IN THE WORST AREAS TODAY
COMPARED TO THE HEAT INDICES OF 110-117 WHICH WERE OBSERVED
YESTERDAY. DESPITE THE SLIGHTLY LOWER HEAT INDEX VALUES WHICH ARE
EXPECTED TODAY, THE EXCESSIVE HEAT WARNING STILL LOOKS APPROPRIATE
BECAUSE IT IS THE CUMULATIVE EXPOSURE TO EXCESSIVE HEAT WHICH
NEGATIVELY AFFECTS THE HUMAN BODY, AND TODAY WILL BE THE THIRD OR
FOURTH DAY OF SUCH CONDITIONS FOR SOME AREAS.

KANOFSKY

.LONG TERM:  (TONIGHT THROUGH TUESDAY)
ISSUED AT 311 AM CDT WED JUL 29 2015

A TWO-DAY RESPITE FROM THE OPPRESSIVE HEAT/HUMIDITY IS EXPECTED
ON THU/FRI. TEMPERATURES SHOULD BE SEASONABLY WARM (UPPER 80S TO
LOWER 90S), BUT THE DEW POINTS WILL BE MUCH LOWER THAN THEY HAVE
BEEN RECENTLY.

THE UPPER AIR PATTERN WILL HAVE TRANSITIONED TO NORTHWEST FLOW
ALOFT BY FRIDAY DUE TO THE BUILDING RIDGE ACROSS THE WESTERN CONUS
AND A TROUGH OVER THE EASTERN CONUS. MODELS SHOW THAT THIS NW
FLOW PATTERN REMAINS UNCHANGED THROUGH EARLY NEXT WEEK. ALTHOUGH
THE MODELS CONTINUE TO SHOW HINTS OF A SHORTWAVE OR TWO WHICH
COULD SUPPORT CONVECTION (PERHAPS AS EARLY AS SATURDAY NIGHT),
CONFIDENCE IN THE TIMING AND PLACEMENT OF THESE FEATURES IS LOW.
TEMPERATURES LOOK SEASONABLY WARM IN THE UPPER 80S TO LOWER 90S
THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK.

KANOFSKY

&&

.AVIATION:  (FOR THE 12Z TAFS THROUGH 12Z THURSDAY MORNING)
ISSUED AT 638 AM CDT WED JUL 29 2015

LARGE CONVECTIVE COMPLEX THAT BLANKETED MOST OF S IA AND N MO LATE
YESTERDAY EVENING BECAME OUTFLOW DOMINATED SHORTLY AFTER
05Z...WITH MAIN ELEMENTS DEVELOPING/PROPAGATING SSW WHILE E EDGE
REMAINED Q-STATIONARY BETWEEN KIRK-KUIN.

IN THE SHORT TERM...E EDGE OF THIS DYING PRECIP AREA MAY CLIP
KUIN OVER THE NEXT HOUR OR SO, WHILE DRY WEATHER WILL PREVAIL
ELSEWHERE. VFR CONDITIONS ARE ANTICIPATED...WITH CLOUDS AOA 4KFT.

CURRENT SURFACE ANALYSIS AND LATEST RAP GUIDANCE SUGGESTS TWO
BOUNDARIES THAT COULD PROVIDE FOCUS FOR AFTERNOON CONVECTION...THE
FIRST IS THE OUTFLOW FROM THE EARLY MORNING CONVECTION WHICH
APPEARS TO BE PUSHING THROUGH STL AREA ATTM. THE SECOND BOUNDARY
WILL BE THE ACTUAL SYNOPTIC FRONT. ITS CURRENT LOCATION IS BEING
MASKED BY MESOSCALE IMPACTS OF OVERNIGHT CONVECTION BUT IT APPEARS
TO BE OVER S CENTRAL/SE IA...AND RAP SUGGESTS IT WILL BE WITH
FRONT NEAR THE STL METRO BY MIDDAY. CONVERGENCE WILL BE FAIRLY
WEAK BUT THERE SHOULD CERTAINLY BE ENOUGH INSTABILITY TO CARRY
CURRENT VCTS AT STL METRO TAF SITES IN THE EARLY AFTERNOON.
HOWEVER...HAVE DROPPED MENTION OF VCTS AT KCOU SINCE CONVERGENCE
ALONG SYNOPTIC FRONT SHOULD BE SOUTH OF THAT AREA. PREVAILING
CONDITIONS ACROSS THE AREA FOR THE AFTERNOON HOURS SHOULD BE SCT-
BKN CU 4-5KFT (WITH GREATEST CLOUD COVER NEAR/SOUTH OF COLD
FRONT)...WITH THIS CU FIELD QUICKLY DISSIPATING DURING THE EARLY
EVENING WITH LOSS OF HEATING AND AS THE DRIER POST-FRONTAL AIR
ADVECTS INTO THE AREA.

SPECIFICS FOR KSTL: SCATTERED CLOUDS AOA 4KFT ARE EXPECTED THIS
MORNING...WITH CU FIELD AOA 4KFT BECOMING SCT-BKN BY LATE MORNING
AND INTO THE AFTERNOON. HAVE CONTINUED VCTS IN THE 18-21 TIME
FRAME AS SYNOPTIC FRONT WILL BE PASSING AT THAT TIME. CU FIELD
SHOULD QUICKLY DISSIPATE HEADING INTO THE EARLY EVENING...WITH
A FEW CLOUDS AOA 10KFT OVERNIGHT AND INTO EARLY THURSDAY.

TRUETT

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...EXCESSIVE HEAT WARNING UNTIL 9 PM CDT THIS EVENING FOR IRON MO-
     MADISON MO-REYNOLDS MO-ST. FRANCOIS MO-STE. GENEVIEVE MO.

IL...EXCESSIVE HEAT WARNING UNTIL 9 PM CDT THIS EVENING FOR MARION IL-
     RANDOLPH IL-WASHINGTON IL.

&&

$$

WFO LSX


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