Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 142318
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
618 PM CDT SUN SEP 14 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 330 PM CDT SUN SEP 14 2014

MAIN CONCERN WILL BE AN AREA OF LIGHT RAIN AND ISO THUNDER EXPECTED
TO DEVELOP IN RESPONSE TO AN UPPER LEVEL SHORT WAVE DROPPING
SOUTH OUT OF SOUTHERN MANITOBA/SASKATCHEWAN AREA NOW...WATER
VAPOR IMAGE SHOWS THIS FEATURE NICELY THIS MORNING AND APPROACHING
SHORT WAVE FROM THE CENTRAL/SOUTHERN PLAINS AND ITS ATTENDANT WARM
FRONT WHICH WILL LIFT NORTH TOWARD SOUTHERN IOWA OVERNIGHT.
FARTHER SOUTH BY LATE MORNING...ALREADY HAD SOME RESPONSE TO
RETURN FLOW ALOFT WITH ENHANCED H850-H700 FRONTOGENESIS OVER
NORTHERN NEBRASKA AS MOISTURE FED INTO THE ISENTROPIC LIFT ZONE
WITH SW FLOW ALOFT. RADAR SHOWED MID LEVEL RETURNS NOT QUITE
STRONG ENOUGH TO PRODUCE ANY RAINFALL AT THE SFC BY 15Z...THAT AND
DRY COLUMN OVER THE REGION SHOULD CONTINUE TO SLOWLY GIVE WAY TO
PRECIP LATE THIS EVENING ACROSS THE FAR NORTH INTO SOUTHERN MN AS
THE COLUMN SATURATES. AS THE WAVE DROPS SOUTH TONIGHT...JET ALOFT
WILL ENHANCE PRECIP DEVELOPMENT OVER NORTHERN IOWA AND SOUTHERN
MN. IN THE FAR NORTH AND SOUTH WHERE FORCING IS GREATER...QPF
AMOUNTS WILL BE SOMEWHAT HIGHER. ACROSS THE NORTH INCREASING
MOISTURE CONVERGENCE ALONG THE BOUNDARY SHOULD RESULT IN A STRIPE
OF RAIN ALONG THE BORDER...UP TO A QUARTER INCH. OVER THE SOUTH
MOISTURE IS ALSO MORE READILY AVAILABLE AND SHOWERS AND PERHAPS
SOME ISO THUNDER...ESPECIALLY PRIOR TO 08Z...WILL MOVE INTO THE
SOUTHERN HALF OF THE AREA. SUBJECTIVE H850 ANALYSIS THIS MORNING
SHOWED AMPLE MOISTURE FROM KTOP SOUTHWEST WITH H850 DEW POINTS OF
+10C NEAR IA/KS BORDER TO +15C OVER TX PANHANDLE. NOT LIKELY TO
TAP INTO THE DEEPER MOISTURE...BUT ENOUGH TO GEN UP .25 TO .35 QPF
AMOUNTS OVER THE SOUTH THROUGH THE OVERNIGHT HOURS. ALL MODELS
SLOW TO ORGANIZE MID LEVEL DEVELOPMENT OVER NEBRASKA...BUT THE 00Z
9/14 EURO MODEL HAS A DECENT HANDLE ON PROGRESSION OF EVENT AND
SHOULD PROGRESS SLOWLY AT FIRST AS COLUMN SATURATES...THEN PICK UP
MORE DRAMATICALLY.

.LONG TERM /MONDAY THROUGH SUNDAY/...
ISSUED AT 330 PM CDT SUN SEP 14 2014

TWO BANDS OF PRECIP WILL LIKELY BE STRADDLING THE NORTHERN AND
SOUTHERN COUNTIES OF THE CWA AT THE BEGINNING OF THE PERIOD WITH
LIGHTER SHOWERS IN BETWEEN. THE OVERALL FORCING IS STRONGER ACROSS
THE NORTH OWING TO A MIDLEVEL FRONTOGENESIS BAND AND UPPER LEVEL
DIVERGENCE OUT AHEAD OF AN APPROACHING 300 MB SHORTWAVE/JET STREAK.
HOWEVER...MOISTURE IS DEEPER IN THE SOUTH AS SOUTHERLY FLOW OVERRUNS
A WEAK SURFACE WARM FRONT THAT WILL ADVANCE INTO NORTHERN MISSOURI
AT THE START OF THE PERIOD. WITH THE FORCING AND MOISTURE NOT IN
PHASE...RAINFALL AMOUNTS IN EITHER BAND SHOULD BE NOTHING
SPECTACULAR WITH ONLY A COUPLE OF TENTHS OF AN INCH EXPECTED. KEPT
ISOLATED THUNDER IN THE FAR SOUTH THROUGH 18Z WITH A SMALL RIBBON OF
ELEVATED INSTABILITY PRESENT. THE RESULTANT CLOUDS WILL KEEP
TEMPERATURES CONFINED IN THE LOW 60S FOR HIGHS ON MONDAY. DRIER AIR
QUICKLY PUNCHES IN FROM THE NORTH AS A HIGH BUILDS IN ON MONDAY
AFTERNOON AND SHOULD BRING THE RAIN TO AN END FROM NORTHWEST TO
SOUTHEAST.

FOG IS A POSSIBILITY IN LOW LYING AREAS AND RIVER VALLEYS ON MONDAY
NIGHT AND TUESDAY MORNING WITH THE GROUND LIKELY STILL DAMP FROM THE
RAIN AND SURFACE HIGH PRESSURE TRANSITING THE CWA DURING THIS
TIME. BY TUESDAY EVENING RETURN SOUTHWESTERLY FLOW AND WAA SPREAD
ACROSS THE CWA. A FEW SHOWERS AND THUNDERSTORMS ARE POSSIBLE
ALONG THE NOSE OF THE THETA-E ADVECTION LATE TUESDAY INTO
WEDNESDAY MORNING. THE 12Z GFS IS FAR MORE BULLISH ON THE
DEVELOPMENT OF THE PRECIP THAN THE EC...SO WENT WITH A BLEND OF
THE TWO MODELS AND INTRODUCED SLIGHT CHANCE POPS IN THE WEST
DURING THIS TIMEFRAME.

TEMPERATURES AFTER THE MONDAY SYSTEM WILL ONLY GRADUALLY WARM WITH
NO STRONG OR PERSISTENT PUSH OF WAA UNTIL FRIDAY...WITH 850MB TEMPS
BETWEEN TUESDAY AND EARLY FRIDAY ONLY CLIMBING FROM +7C TO +14C. THE
LONGWAVE RIDGE RESPONSIBLE FOR OUR RELATIVELY DRY STRETCH OF WEATHER
BREAKS DOWN LATE ON THURSDAY. A LEE SURFACE TROUGH MOVES TOWARDS
IOWA ON FRIDAY AS THE PATTERN BECOMES MORE PROGRESSIVE AND AN UPPER
LEVEL IMPULSE MOVES EAST ALONG THE US/CANADIAN BORDER. THIS WILL SET
UP AN ACTIVE WEATHER PATTERN FOR IOWA ON FRIDAY AND SATURDAY. BOTH
THE GFS AND EC HINT AT AN MCS TRACKING ACROSS IOWA ON FRIDAY MORNING
ON THE LEADING EDGE OF THE LOW-MID LEVEL WAA. WHILE SOLUTIONS DIFFER
ON THE SYNOPTIC SETUP...BOTH MODELS ARE INDICATING THUNDERSTORM
REDEVELOPMENT FRIDAY AFTERNOON AND EVENING. SOME OF THE
THUNDERSTORMS IN NORTHERN AND NORTHWEST IOWA ON FRIDAY AFTERNOON MAY
APPROACH SEVERE LEVELS WITH SBCAPE VALUES OVER 2500 J/KG AND MODEST
DEEP SHEAR.

&&

.AVIATION...15/00Z
ISSUED AT 614 PM CDT SUN SEP 14 2014

WEAK COLD FRONT EXTENDING FROM THE EASTERN UPPER PENINSULA OF MI
THROUGH IA INTO THE NEBRASKA PANHANDLE WILL MOVE SOUTHEAST ACROSS
THE STATE TONIGHT. MOISTURE WILL INCREASE IVOF OF THE FRONT WITH
SHOWERS BECOMING NUMEROUS AND A BAND OF MVFR CIGS DEVELOPING.
CLEARING WILL SPREAD SOUTH AND EAST MONDAY AS HIGH PRESSURE BUILDS
IN FROM THE NORTHWEST.

OUTLOOK...HIGH PRESSURE WILL BRING GOOD VFR CONDITIONS MONDAY
NIGHT THROUGH MIDWEEK. PATCHY FOG MAY DEVELOP MONDAY NIGHT IN
VALLEYS HOWEVER. WEATHER WILL BECOME INCREASINGLY UNSETTLED LATER
IN THE WEEK.

&&

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

$$

SHORT TERM...REV
LONG TERM...SKOW
AVIATION...MS SEP 14



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