Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 041842
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
142 PM CDT FRI SEP 4 2015

.SHORT TERM /TODAY/...
ISSUED AT 358 AM CDT FRI SEP 4 2015

FORECAST CONCERNS FOR TODAY WILL ONCE AGAIN REVOLVE AROUND
PRECIPITATION CHANCES AND HOT AND HUMID CONDITIONS. EARLY MORNING
PRECIPITATION ACROSS THE EAST WILL CONTINUE TO DIMINISH.
HOWEVER...DEVELOPING CONVECTION IN NE WILL CONTINUE TO EXPAND...AND
LIFT NORTHEAST. THIS MAY PUSH INTO THE WESTERN PORTIONS OF THE
CWA...AND HAVE INCLUDED SLIGHT CHANCE POPS IN THE FAR WEST FOR THE
EARLY MORNING...HOWEVER...STILL UNCERTAINTY IN EXTENT OF
PRECIPITATION AND HOW FAR WEST IT WILL REACH...WITH MODELS
INDICATING A MORE NORTHERLY PUSH TO SYSTEM.

SIMILARLY...A LOT OF UNCERTAINTY IN AFTERNOON PRECIPITATION CHANCES.
EXPECT WAVE TO LIFT NORTH/NORTHEAST AHEAD OF APPROACHING LOW
PRESSURE SYSTEM AND GIVEN THE GENERALLY UNSTABLE AIRMASS IN
PLACE...COULD SEE THUNDERSTORMS DEVELOP IN THE AFTERNOON AS WAVE
ARRIVES. HOWEVER...MIDLEVEL MOISTURE IS SOMEWHAT LACKING AND TRACK
OF WAVE IS STILL UNCERTAIN...WITH SOME MODELS KEEPING SYSTEM NORTH
AND EAST OF CWA. HIGH RES MODELS GENERALLY AGREE WITH AFTERNOON
THUNDERSTORM CHANCES...THOUGH FEEL THEY ARE OVERDONE...HAVE
INTRODUCED A SLIGHT CHANCE FOR THUNDER IN THE NORTHWEST FOR THE
AFTERNOON HOURS.

EXPECT BETTER MIXING TODAY...THOUGH TEMPERATURES ALOFT WILL BE CLOSE
OR SLIGHTLY LOWER THAN YESTERDAY...AND HAVE THEREFORE KEPT
TEMPERATURE NEAR GOING AND CLOSE TO TEMPS THURSDAY. KEPT TEMPS
SLIGHTLY COOLER IN THE NORTHWEST WHERE CLOUDS MAY HELP KEEP
CONDITIONS COOLER. HUMID CONDITIONS WILL PERSIST AGAIN TODAY WITH
HEAT INDEX VALUES GENERALLY IN THE 90S...THOUGH MAY TOP OUT NEAR 100
DEGREES IN THE AFTERNOON...ESPECIALLY IN THE SOUTH.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
ISSUED AT 358 AM CDT FRI SEP 4 2015

THE WEAK UPPER LEVEL LOW CENTERED OVER LOWER MICHIGAN WILL REMAIN
SOMEWHAT PROBLEMATIC THROUGH TONIGHT BEFORE FINALLY DEPARTING TO THE
EAST. THIS SYSTEM HAS SEVERAL PV IMPULSES ROTATING AROUND THE
SYSTEM. STATIC STABILITY HAS BEEN LOW ON THE WESTERN FRINGE OF THIS
AREA HAS AIDED WEAK THUNDERSTORMS OVER PORTIONS OF THE EAST EARLY
THIS MORNING. NOT COMPLETELY CONFIDENT THAT A REPEAT WILL NOT OCCUR
AGAIN TONIGHT THOUGH THE CIRCULATION CERTAINLY IS WEAKENED BY THAT
TIME AND THE UPPER RIDGE WILL BE PUSHING FURTHER INTO THE AREA. THE
INCREASING MONSOONAL FLOW AS THE UPPER LOW DIGS INTO THE PACIFIC
NORTHWEST WILL BE A PRIME TIME PLAYER IN PRECIPITATION CHANCES
THROUGH SUNDAY. IN ADDITION TO GOOD MOISTURE TRANSPORT INTO THE
REGION...NUMEROUS VORTICITY FRAGMENTS SHEARING OFF OF TROPICAL STORM
KEVIN WILL WILL BE INGESTED INTO THE FLOW. THESE INITIAL PIECES OF
ENERGY WILL IMPACT NORTHWEST IOWA BY LATE SATURDAY INTO SATURDAY
NIGHT AND WILL BRING INCREASING THUNDERSTORM CHANCES. THE MOISTURE
TRANSPORT WILL HELP BRING PWATS IN EXCESS OF 2 INCHES BACK TO IOWA
BY SUNDAY AFTERNOON AND NIGHT. A COLD FRONT WILL MOVE INTO THE STATE
DURING THIS TIME AND SHOULD BE MOVING INTO A REGION WILL GOOD
INSTABILITY. WHILE DEEP LAYER SHEAR IS IN THE 30 TO 35 KT
RANGE...THE LOW LEVEL SHEAR BECOMES STRONG AS EVIDENCED BY PROJECTED
EFFECTIVE INFLOW LAYER SRH IN EXCESS OF 250 M2/S2 AND 0-3 KM SHEAR
AT 30 KTS OR GREATER. POTENTIAL FOR THIS TO TURN INTO A QLCS EVENT
SUNDAY EVENING. IN ADDITION TO THE HIGH PWATS...INITIAL STORM
MOTIONS OF 20 KTS OR LESS MAY LEAD TO A HEAVY RAIN THREAT AS WELL.

THE PROGRESSION TO ZONAL FLOW WILL CONTINUE MONDAY. THE SFC BOUNDARY
WILL LIFT BACK NORTH BRIEFLY MONDAY NIGHT BEFORE SHIFTING BACK SOUTH
AS A COLD FRONT AND WILL LEAD TO ANOTHER CHANCE FOR THUNDERSTORMS.
HAVE KEPT POPS TO A MINIMUM TUESDAY NIGHT INTO WEDNESDAY WITH
INCREASING POTENTIAL FOR DRIER AIR ARRIVING WITH SFC HIGH PRESSURE.
ANOTHER PUSH OF COOLER AIR ARRIVES AROUND THURSDAY. AT THIS TIME THE
PRECIPITATION POTENTIAL IS LOW WITH THE AIR MASS REMAINING
RELATIVELY DRY. OTHERWISE...EXPECT WARM AND HUMID CONDITIONS THROUGH
SUNDAY WITH HIGHS IN THE MID 80S TO LOWER 90S AND HEAT INDEX VALUES
OF 90 TO 100. THIS IS LIKELY THE FAREWELL TOUR OF THE TRUE HEAT
FOR THE YEAR.

&&

.AVIATION...04/18Z
ISSUED AT 139 PM CDT FRI SEP 4 2015

MUCH OF THE SAME AT TAF SITES FOR THE UPCOMING TAF PERIOD AS THE
PREVIOUS DAY OR TWO. SOUTHERLY WINDS AROUND 10KTS DURING DAYTIME
HOURS TODAY AND TOMORROW WITH OCCASIONAL GUSTS TO AROUND 15KTS.
WINDS WILL CALM A BIT TO AROUND 5 KTS THIS EVENING AND CLOUD
COVER WILL MOVE IN FROM THE WEST...BUT VFR CIGS WILL DOMINATE
WITH DECKS NOT EXPECTED TO BE MUCH...IF AT ALL...BELOW 10KFT AT
THE MOMENT.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...AWB
LONG TERM...DONAVON
AVIATION...CURTIS


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