Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 102333
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
633 PM CDT THU JUL 10 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 405 PM CDT THU JUL 10 2014

MAIN FORECAST CONCERN WAS WITH RAIN CHANCES...OR LACK THEREOF...OVERNIGHT.
LEANED TOWARD A BLEND OF THE NAM12/SREF/ECMWF TONIGHT WHICH RESULTED
IN A MUCH DRIER FORECAST.

WAA CONTINUES TONIGHT AS LARGE SURFACE HIGH SHIFTS EAST TONIGHT.
MODELS HAVE CONTINUALLY SLOWED THE PROGRESSION EAST...AND HENCE
HAVE BACKED OFF ON PRECIPITATION TONIGHT INTO FRIDAY MORNING. THERE
IS A SMALL RIBBON OF THETA-E ADVECTION BETWEEN 700-500MB THAT IS
CURRENTLY PROVIDING SOME LIGHT SHOWER ACTIVITY ACROSS SOUTH DAKOTA
THIS AFTERNOON AND MAY TRICKLE INTO NORTHERN IOWA DURING THE EARLY
EVENING. HOWEVER...BELIEVE MUCH OF THIS ACTIVITY WILL DISSIPATE WITH
THE SUNSET AND KEPT FORECAST DRY THROUGH 03Z.

INCREASING MOISTURE TRANSPORT OVER WESTERN IOWA...ESPECIALLY ALONG
THE 305K TO 315K ISENTROPIC SURFACE. THE FORECAST SOUNDINGS REMAIN
DRY BELOW 700MB OVER THE CENTRAL AND SOUTHERN SECTIONS OF THE
FORECAST AREA AND HAVE LOW CONFIDENCE WITH ANY WIDESPREAD
PRECIPITATION IN THIS AREA...BUT STILL CANNOT COMPLETELY RULE OUT A
THUNDERSTORM OR TWO WITH ANOTHER AREA OF THETA-E ADVECTION MOVING
ACROSS THE STATE COMBINED WITH WEAK ISENTROPIC LIFT PAST 06Z FRIDAY.
THE LATEST HOPWRF...HRRR...SPC 4.0KM WRF KEEP MUCH OF THE FORECAST
AREA DRY THROUGH 12Z FRIDAY WITH A FEW THUNDERSTORMS OVERNIGHT
ACROSS NORTHERN IOWA. TRIMMED BACK POPS CONSIDERABLY AND KEPT LIKELY
POPS GOING FOR ONLY A FEW HOURS OVERNIGHT IN NORTHERN IOWA.

.LONG TERM /FRIDAY THROUGH THURSDAY/...
ISSUED AT 405 PM CDT THU JUL 10 2014

SEVERAL THINGS TO CONSIDER THIS FORECAST WITH CHANGES IN TEMP AND
PRECIP REGIMES. INITIAL CONCERN WILL BE THE FATE OF THE CURRENT ND
SHORT WAVE DRIFTING ACROSS THE TOP OF THE ROCKIES UPPER LEVEL RIDGE.
THIS FORCING..MAINLY KINEMATIC...WILL REACH IA FRIDAY WITH GRADUALLY
INCREASING POPS NW-SE AND THE START OF WHAT MAY BE ACTIVE WEATHER
FOR THE WEEKEND. PERSISTENCE OF SLY LOW LEVEL FLOW WILL ALSO
INTRODUCE BETTER THERMODYNAMIC CONDITIONS INTO THE WEEKEND WITH
SURGING DEWPOINTS AND HIGHER INSTABILITY. CONDITIONS LOOK QUITE
FAVORABLE FOR THUNDERSTORMS FRI NIGHT WITH WEST-EAST FRONTAL
BOUNDARY FORMING ACROSS IA. CONVERGENCE IS NOT TOO STRONG AS NOTED
ALONG THE 305K ISENT SURFACE BUT CONSIDERING ELEVATED CAPES LOOK
TO BE 2000 J/KG OR MORE AND WEAKLY CAPPED ALONG AND NORTH OF THE
BOUNDARY EXPECT MCS TO FORM AND HAVE LIKELY POPS WITH CONTINUED
HEAVY RAIN WORDING. PWS REACH TWO INCHES WITH VERY DEEP WARM CLOUD
DEPTHS AND K INDICES 40 PLUS. HAVE NOT ISSUED WATCH THIS FAR
OUT...AND MCS LOOKS SOMEWHAT PROGRESSIVE...BUT CONDITIONS WILL
CERTAINLY NEED TO BE MONITORED. EFFECTIVE SHEAR SEEMS MORE THAN
ADEQUATE FOR ORGANIZATION AND SEVERE POTENTIAL SO AGREE WITH DAY 2
AND DAY 3 SLIGHT RISK ZONES. PATTERN IS NOT TOO DISSIMILAR SAT AND
SAT NIGHT...ALTHOUGH APPROACHING MO VALLEY SHORT WAVE DOES APPEAR
TO VEER LOW LEVEL WINDS SUFFICIENTLY TO REDUCING CONVERGENCE AND
THERMODYNAMIC SUPPORT.

FORCING STARTS TO WANE INTO EARLY NEXT WEEK WITH SOME LOW POTENTIAL
FOR PRECIP TO FINISH THE WEEKEND...BUT ATTENTION TURNING TOWARD
TEMPERATURES. UPPER LEVEL LOW MATURES OVER CANADA THIS WEEKEND AND
DIVES SHARPLY INTO THE GREAT LAKES BY MON. THIS WILL DRIVE A STRONG
COLD FRONT INTO IA WITH A DRAMATIC DROP IN TEMP AND MOISTURE.
MONDAY NIGHT LOWS AND TUE HIGHS WILL APPROACH OR EXCEED RECORD
LOWS/MIN MAXES RESPECTIVELY. GFS ENSEMBLE LOW LEVEL TEMP ANOMALIES
REACH MINUS 2 TO 3 SDS WITH VERY ANOMALOUS SURFACE HIGH PRESSURE
ACROSS MT/ND. IN FACT GFS SURFACE THETA-E DROPS 65K FROM THE
WEEKEND INTO TUE MORNING AT DES MOINES.

MODERATION THEN LOOKS TO OCCUR INTO THE EXTENDED WITH DIMINISHING
INFLUENCE FROM THE DEPARTING LONG WAVE TROUGH WITH NEUTRAL FORCING
AND LITTLE IF ANY PRECIPITATION IN WEAKER AND RELATIVELY INACTIVE NW
FLOW.  TEMPS WILL STILL REMAIN COOL AND BELOW NORMAL...BUT GRADUALLY
REBOUND AWAY FROM RECORD LOWS.

&&

.AVIATION...11/00Z
ISSUED AT 633 PM CDT THU JUL 10 2014

MAINLY VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. THE
ONLY FLY IN THE OINTMENT IS THE POSSIBILITY OF SHRA/TSRA. WHILE
REGIONAL RADAR IMAGERY CURRENTLY SHOWS LIGHT RETURNS ACROSS NW IA
VERY LITTLE OF IT IS REACHING THE GROUND. IT APPEARS MORE LIKELY
THAT SIGNIFICANT SHRA/TSRA DEVELOPMENT WILL NOT OCCUR UNTIL
SOMETIME FRIDAY MORNING...OR LATER...AND PROBABLY BE CONFINED TO
THE NORTHERN TERMINALS IN THE NEXT 24 HRS...SO HAVE TRENDED TAFS
THAT WAY. OUTSIDE OF SHRA/TSRA ONLY VFR CONDITIONS ARE
ANTICIPATED.

&&

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

&&

$$

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



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