Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 291127
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
627 AM CDT FRI MAY 29 2015

.SHORT TERM /TODAY/...
ISSUED AT 348 AM CDT FRI MAY 29 2015

PRIMARY FORECAST CHALLENGE FOR TODAY IS TIMING AND COVERAGE OF
PRECIPITATION ACROSS THE AREA. CURRENTLY STORMS ARE DEVELOPING
QUICKLY ACROSS NORTHWEST IOWA...WITH A SEPARATE BAND OF LIGHT
SHOWERS AND A FEW STORMS LINGERING ACROSS EAST AND SOUTHEAST
IOWA...BUT ELSEWHERE IT HAS BEEN DRY THUS FAR. OBSERVATIONS AND
ANALYSES SHOW SOUTHWESTERLY FLOW ALOFT AND AMPLE LOW LEVEL MOISTURE
BUT ALSO THE LACK OF A FOCUSING MECHANISM OR BOUNDARY AND ONLY LIGHT
SURFACE WINDS. THE IMPLICATION IS THAT WHILE WIDESPREAD BUT MODEST
INSTABILITY AND BROAD WEAK ASCENT WILL SUPPORT ISOLATED TO SCATTERED
SHOWERS AND THUNDERSTORMS AT ANY GIVEN TIME...COVERAGE SHOULD REMAIN
LOW FOR MUCH OF THE DAY AS DEPICTED BY MOST SHORT TERM AND HIGH
RESOLUTION MODELS...WITH TWO EXCEPTIONS. THE FIRST IS THE NEXT
SEVERAL HOURS IN OUR NORTHWEST AND NORTH WHERE THE MORE NUMEROUS
SHOWERS AND STORMS CURRENTLY DEPICTED ON RADAR SHOULD ADVECT ACROSS
THESE AREAS. THERE IS A DISTINCT LACK OF DEVELOPMENT
UPSTREAM...ACROSS NORTHEASTERN NEBRASKA...SO FAR AND IN GENERAL THE
COVERAGE OF STORMS IN THIS AREA SHOULD DIMINISH STEADILY AFTER
SUNRISE. THE SECOND EXCEPTION IS TOWARD THIS EVENING AS A WELL
ADVERTISED COLD FRONT APPROACHES FROM THE NORTHWEST. COMBINED WITH
PEAK HEATING AND PERHAPS A WEAK SHORTWAVE IMPULSE MOVING THROUGH THE
FLOW ALOFT...EXPECT AN INCREASE IN STORM COVERAGE AND INTENSITY
AFTER 21Z OR SO BEGINNING IN THE NORTHWEST AND THEN SPREADING
SOUTHEASTWARD. INSTABILITY AND SHEAR WILL BE LIMITED SO ANY SEVERE
WEATHER THREAT SHOULD BE ISOLATED AND MARGINAL.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
ISSUED AT 348 AM CDT FRI MAY 29 2015

SENSIBLE WEATHER WILL BE RELATIVELY INACTIVE THROUGH THE PERIOD WITH
ONLY APPRECIABLE PRECIP CONCERN EARLY AND AGAIN LATE...AND EVEN
THOSE ARE NOT TOO SIGNIFICANT. WATER VAPOR IMAGERY SHOWS A BROAD
AREA OF LIFT ENTERING THE MO VALLEY WITH SCATTERED WEAK EMBEDDED
CONVECTION. THERE IS NO DISCERNIBLE KINEMATIC OR THERMODYNAMIC
FORCING MECHANISM TO PIN DOWN MUCH IN THE WAY OF LIFT
CONTRIBUTIONS. PRECIP MAINLY APPEARS TO BE UNORGANIZED IN WEAK
INSTABILITY WITH LITTLE CINH. THIS SCATTERED CONVECTION WILL
LIKELY JUST BE EXITING ERN IA AT ONSET WITH ADDITIONAL WEAK
CONVECTION DRIVEN MORE KINEMATICALLY FROM APPROACHING SRN
CANADA/ND SHORT WAVE. THERE MAY ALSO BE SOME WEAK SURFACE BASED
POTENTIAL DUE TO CONVERGENCE ALONG THE FRONT BUT MLCAPE CAPES ARE
MINIMAL SO SPC MARGINAL OUTLOOK SEEMS APPROPRIATE. COULD BE SOME
LOCALLY HEAVY RAINS AS SRN PLAINS ELEVATED PWATS PUSH INTO SERN
IA...BUT RESIDENCE TIME AHEAD OF THE FRONT WILL LIKELY PRECLUDE A
WIDESPREAD OR SIGNIFICANT CONCERN. THE AFOREMENTIONED NRN STREAM
KINEMATIC FORCING WILL LIKELY DRIVE SOME WEAK POST-FRONTAL
CONVECTION OVERNIGHT...LINGERING INTO SAT MORNING SE.

COLD FRONTAL PASSAGE WILL BRING APPRECIABLY COOLER AND ESPECIALLY
DRIER AIR INTO THE WEEKEND WITH BAROCLINIC TROUGH INTO IA SUN AND H85/H7
RETURN FLOW STARTING AN EXTENDED PERIOD OF WARM ADVECTION THROUGH
THE MID/UPPER MO VALLEY EARLY NEXT WEEK. THIS WILL BE ENHANCED BY
SOME KINEMATIC SUPPORT FROM SHORT WAVE EJECTED OUT OF CURRENT BC
TROUGH. INSTABILITY AND MOISTURE RETURN ARE FAIRLY WEAK BUT
300-310K ISENT LIFT SEEMS STRONG ENOUGH TO GENERATE SOME WEAK
CONVECTION AND WITH ESE MEAN WIND FLOW WOULD NOT BE SURPRISED TO
SEE SOMETHING DRIBBLE INTO NRN IA FROM MN...ESPECIALLY
NOCTURNALLY. THESE CONDITIONS GO FROM SUN NIGHT INTO TUE NIGHT AND
DO NOT HAVE THE CONFIDENCE TO ADD THAT EXTENDED OF WORDING YET SO
HAVE OPTED TO KEEP THINGS DRY UNTIL CONFIDENCE INCREASES.
SOMETHING TO MONITOR. SURFACE PATTERN DOES NOT CHANGE MUCH WITH
WEAK SELY LOW LEVEL FLOW...GRADUALLY BECOMING MORE SLY WITH TIME
INTO WED. THERE REALLY IS NOT ANYTHING TO HANG YOUR HAT ON FOR
POPS AGAIN UNTIL LATE IN THE PERIOD...WED NIGHT OR THU AND EVEN
THIS MAY BE OVERDONE. IT WOULD LIKELY BE ELEVATED WITH ANY SURFACE
CONVERGENCE STILL TOO FAR WEST. THIS PATTERN WILL ALSO RETURN MORE
SEASONAL TEMPERATURES AS WELL.

&&

.AVIATION...29/12Z
ISSUED AT 627 AM CDT FRI MAY 29 2015

MAINLY VFR CONDITIONS WILL PREVAIL TODAY...WITH MVFR CONDITIONS
BECOMING WIDESPREAD TONIGHT. SOME LOW MVFR/IFR CIGS...MAINLY UP AT
MCW...SHOULD CLEAR OUT IN THE NEXT SEVERAL HOURS AFTER WHICH
PREVAILING MVFR CIGS ARE EXPECTED. ISOLD SHRA/TSRA WILL BUBBLE UP
DURING THE DAY...BUT PROBABILITY AND DURATION OF ANY IMPACTS ARE
TOO SMALL FOR INCLUSION IN THE TAFS. BY LATE THIS AFTN/EVE WE WILL
SEE AN INCREASE IN SHRA/TSRA COVERAGE AS A COLD FRONT
APPROACHES...THEN PASSES THROUGH THE AREA OVERNIGHT. EVEN OUTSIDE
SHRA/TSRA THIS FRONT WILL BRING PREVAILING MVFR CIGS...PROBABLY
BELOW FL020 AT TIMES...AND HAVE INDICATED THIS IN THE TAFS. ONLY
WENT WITH VCSH FOR NOW AS THERE ARE QUESTIONS AS TO THE FREQUENCY
AND DURATION OF SHRA/TSRA THAT WILL DEVELOP TONIGHT.

&&

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

&&

$$

SHORT TERM...LEE
LONG TERM...SMALL
AVIATION...LEE


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