Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 182041
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
341 PM CDT THU SEP 18 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 340 PM CDT THU SEP 18 2014

WITH THURSDAY EVENING EXPECTING TO BE QUIET...ONLY FCST ISSUES
ARE POPS TONIGHT AFTER 06Z FRI AND THE POSSIBILITY OF FOG
DEVELOPING IN SOME LOCATIONS AFTER 09Z FRI OR SO. IN TERMS OF POPS
TONIGHT...DROPPED THEM OUT OF SLIGHT WORDING AS NOT FINDING
SUFFICIENT METEOROLOGICAL SUPPORT TO KEEP THEM IN THE FCST. MODELS
IN AGREEMENT WITH SHOWING A WEAK BAND OF THETA-E ADVECTION AND
ASSOCIATED ISENTROPIC LIFT PROPAGATING INTO WESTERN IOWA AFTER 06Z
FRI. LONG STORY SHORT...NOT MUCH ELSE TO SUPPORT POPS OVER DMX
CWA. BEST FORCING APPEARS TO REMAIN WEST OF CWA THROUGH 12Z FRI.
AFTER SUNRISE...STRONG CAPPING INDICATED...WHICH WILL KEEP POPS
DOWN.

OTHER FCST ISSUE IS FOG. MAJOR NEGATIVE HERE WILL BE WIND SPEEDS.
WINDS ARE SLATED TO BOTTOM OUT IN THE 06KT TO 08KT RANGE...WHICH IS
NOT VERY SUPPORTIVE OF RADIATIONAL FOG. SUNSHINE TODAY WILL ALSO NOT
HELP AS IT WILL INCREASE THERMAL CONDUCTIVITY OF THE SFC. IN SUPPORT
OF FOG WILL BE TONIGHT/S EXPECTED CLEAR SKIES AND NARROW DEWPOINT
DEPRESSIONS AT THE SFC. SOILS AND VEGETATION SHOULD STILL BE DAMP
ENOUGH TO HELP WITH SATURATION AS WELL. OVERALL...DEWPOINT
DEPRESSIONS IN SOME LOCATIONS...INCLUDING DES MOINES...SHOULD BE
AROUND 0 TO 2 DEGREES WHICH COULD SUPPORT PATCHY FOG WHEN WINDS
ARE LIGHTEST...ESPECIALLY IN VALLEYS AND OPEN FIELDS. AS OF
NOW...HAVE TIMING OF FOG FROM ROUGHLY 08/09Z TO 12/13Z AND
LOCATIONS FROM AMES TO DES MOINES TO LAMONI AND OTTUMWA IN PATCHY
FOG.

.LONG TERM /FRIDAY THROUGH THURSDAY/...
ISSUED AT 340 PM CDT THU SEP 18 2014

ONLY SENSIBLE WEATHER OF ANY CONCERN WILL BE EARLY IN THE WEEKEND
ASSOCIATED WITH A WEAK COLD FRONTAL PASSAGE.  WRN US/CANADIAN LONG
WAVE TROUGH IS CURRENTLY JUST PHASED...HOWEVER ERN PACIFIC JET WILL
REACH BRITISH COLUMBIA EARLY FRI PUSHING WRN CANADA PORTION INTO THE
NRN PLAINS LEAVING CA CUTOFF BEHIND.  FRI SHOULD BE DRY...HOWEVER
WEAK/MODERATE THERMODYNAMIC AND KINEMATIC FORCING REACHES NRN IA
DURING THE EVENING AND SLOWLY SLIDES NW-SE THROUGH THE STATE INTO
SAT.  SOUNDINGS SHOW MOISTURE AND EFFECTIVE SHEAR WOULD BE MARGINAL
SO ANY CONVECTION WOULD APPEAR TO BE ELEVATED AND MAINLY NON-SEVERE
THROUGH MIDDAY SAT.  WOULD NOT RULE OUT A SPOT WIND OR HAIL REPORT
HOWEVER DUE TO DRY AIR AND FAIRLY STEEP MID LEVEL LAPSE RATES.
SURFACE BASED CONVECTION COMES INTO PLAY SE DURING PEAK HEATING
FRONTAL PASSAGE. MLCAPES REACH 2500-3500 J/KG DEPENDING ON MODEL OF
CHOICE WITH LITTLE MLCINH. SURFACE CONVERGENCE IS SOMEWHAT WEAK
HOWEVER THOUGH SHARP THERMODYNAMIC SUBSIDENCE/LIFT COUPLET ALOFT
AND OVERALL LARGE SCALE FORCING COULD HELP THINGS ALONG. WHATEVER
DOES FORM SHOULD BE SOUTH AND EAST OF THE AREA BY EVENING WITH
FAIR WEATHER THROUGH THE START OF THE NEXT WORK WEEK.

TEMPERATURES WILL REBOUND FRIDAY AND SATURDAY WITH A MORE SUMMER-
LIKE FEEL AS 60S DEWPOINTS RETURN. THE AIRMASS BEHIND THE FRONT
WILL ONLY MODERATE SLIGHTLY TEMPERATURE WISE HOWEVER WITH THE MAIN
DIFFERENCE DRIER NW FLOW. THE NEXT PRECIP WINDOW WILL NOT ARRIVE
UNTIL AROUND THE MIDDLE OF NEXT WEEK. THE REMNANTS OF THE
AFOREMENTIONED CA CUTOFF LOW FINALLY LIFTS THROUGH THE ROCKIES
RIDGE INTO THE MO VALLEY BY WED AND THU. THERMODYNAMIC FORCING
SEEMS BEST AROUND WED ALTHOUGH BROAD SLIGHTLY ACTIVE SW FLOW WILL
STAY IN PLACE INTO THU AS WELL. ANYTHING THAT DOES FORM WOULD STAY
ELEVATED WITH ANY LOW LEVEL CONVERGENCE STILL WELL TO THE WEST.

&&

.AVIATION...18/18Z
ISSUED AT 1217 PM CDT THU SEP 18 2014

SKC TO A FEW CU...ALONG WITH VFR CONDITIONS...EXPECTED THROUGH
09Z FRI. MAJOR FCST CHALLENGE WILL BE FOG FOR FRIDAY MORNING. WITH
MAINLY CLEAR SKIES EXPECTED ACROSS TAF SITES...ESPECIALLY SITES
EAST OF INTERSTATE 35...DWPT DEPRESSIONS WILL BE ZERO TO A FEW
DEGREES. MAJOR INHIBITOR OF FOG WILL BE WINDS...WHICH WILL BE
BLOWING IN THE 6KT TO 8KT RANGE. LOWEST WINDS AT KDSM...SO WENT
3SM VSBY THERE. OTHERWISE DROPPED REST OF TAF SITES TO MVFR
CONDITIONS WITH EXPECTATION OF EVENING/OVERNIGHT SHIFT TO TWEAK
TAFS WHEN FCST BECOMES MORE CLEAR. LOOKS LIKE FOG SHOULD LIFT BY
13Z TO 14Z AND STRONG SOUTHERLY WINDS WITH GUSTS OF 20KTS TO 25KTS
TO MOVE IN FOR THE REST OF FRIDAY.

&&

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

&&

$$

SHORT TERM...KOTENBERG
LONG TERM...SMALL
AVIATION...KOTENBERG


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