Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 172035
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
335 PM CDT THU APR 17 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 303 PM CDT THU APR 17 2014

A SURFACE HIGH PRESSURE RIDGE ALONG THE MISSOURI RIVER WILL TRANSIT
THE STATE TONIGHT WITH THE AXIS POSITIONED JUST EAST OF THE CWA BY
SUNRISE ON FRIDAY. NORTHWESTERLY WINDS WILL LESSEN AS THE RIDGE
APPROACHES AND THEN VEER TO THE EAST AND SOUTHEAST ON THE BACKSIDE
OF THE RIDGE. LOWERED MIN TEMPS A COUPLE OF DEGREES BELOW GUIDANCE
ACROSS THE NORTHEAST WITH THE RIDGE AXIS DIRECTLY OVERHEAD BETWEEN
06 AND 12Z...THOUGH CLOUDS APPROACHING AFTER 09Z MAY KEEP TEMPS
WARMER THAN FORECAST. THESE CLOUDS ARE TIED TO A REGION OF THETA-E
ADVECTION ON THE LEADING EDGE OF THE RETURNING WARM AIR.
HOWEVER...THE BEST THERMODYNAMIC FORCING IS FURTHER TO THE NORTHWEST
OVER SOUTHWEST MN AND EASTERN SD. REMOVED THE SLIGHT CHANCE POPS
THAT WAS ORIGINALLY IN THE FORECAST FOR THE NW CWA WITH THE FORCING
FURTHER NW THAN PREVIOUSLY PROGGED AND SUBSTANTIAL WEDGE OF DRY AIR
BELOW 5-10 KFT AS NOTED ON THE SLB AND FRM NAM BUFR SOUNDINGS.

.LONG TERM /FRIDAY THROUGH THURSDAY/...
ISSUED AT 303 PM CDT THU APR 17 2014

FORECAST CHALLENGES IN EXTENDED REVOLVE AROUND PRECIPITATION
CHANCES NEAR WEEKEND. FOR NEAR TERM...RIDGE WILL BUILD IN FOR
FRIDAY AND SATURDAY AHEAD OF LOW PRESSURE SYSTEM.
HOWEVER...SHORTWAVE WILL PUSH ACROSS NORTHERN IOWA FOR FRIDAY
MORNING/AFTERNOON. HAVE LEFT POPS OUT FOR NOW WITH VERY LIMITED
MOISTURE IN LOW LEVELS. HOWEVER...MAY SEE A FEW SCATTERED SHOWERS
OR SPRINKLES EARLY. WITH WAA AND SOUTHERLY FLOW UNDER RIDGE
TEMPERATURES WILL WARM INTO FRIDAY AND SATURDAY.

FRONTAL BOUNDARY WILL DROP SOUTHEAST ACROSS IOWA SATURDAY NIGHT.
MODELS CONTINUE TO SLOW PROGRESSION OF THIS BOUNDARY...AND HAVE
ALSO CUT BACK ON TIMING OF PRECIPITATION. HOWEVER...THUNDERSTORMS
ARE POSSIBLE SATURDAY AFTERNOON ACROSS THE NORTHWEST AHEAD OF
THE BOUNDARY WITH DECENT INSTABILITY AND FORCING NEAR BOUNDARY.
BOUNDARY WILL SLOWLY PUSH SOUTH...AS SECONDARY LOW IN CENTRAL
KANSAS LIFTS NORTHEAST. MODELS ARE CURRENTLY IN DECENT AGREEMENT
WITH THIS SET UP...THOUGH ARE NOT IN GOOD AGREEMENT WITH TIMING OR
TRACK OF SOUTHERN LOW. HOWEVER...GENERAL THINKING IS THAT LOW
LIFTING NORTHEAST WILL LIMIT SOUTHERN PUSH OF FRONTAL
BOUNDARY...WITH PROLONGED PERIODS OF PRECIPITATION ACROSS NORTHER
CWA. MODELS REMAIN AGGRESSIVE WITH AMOUNT OF MOISTURE PULLED
NORTH...WITH PWAT VALUES NEAR OR ABOVE 1 INCH FOR SATURDAY NIGHT
AND SUNDAY. HOWEVER...DO HAVE SOME CONCERNS ABOUT AMOUNT OF
MOISTURE...AND FEEL MODELS ARE TOO AGGRESSIVE AND WILL NEED TO BE
MONITORED FOR HEAVY RAIN POTENTIAL. IN ADDITION...INSTABILITY IS
SUFFICIENT FOR THUNDERSTORMS...INDICATING SB CAPE VALUES BELOW
1000 J/KG LATE SATURDAY AFTERNOON AND AGAIN SUNDAY AFTERNOON. WILL
NEED TO MONITOR FOR SEVERE POTENTIAL.

BOUNDARY WILL EVENTUALLY PUSH SE OF AREA BY LATE MONDAY...WITH A
RETURN TO RIDGE TUESDAY AND WEDNESDAY...AGAIN ALLOW TEMPERATURES
TO WARM. LARGE AREA OF LOW PRESSURE EXPECTED TO PUSH ACROSS
NORTHERN ROCKIES LATE IN THE WEEK. MODELS ARE NOT IN GOOD
AGREEMENT IN PLACEMENT OF THIS LOW...WITH ECMWF FURTHER
NORTH...AND GFS TRENDING TOWARDS THIS MORE NORTHERN SOLUTION.
HOWEVER...MODELS INDICATE SECONDARY SURFACE LOW AGAIN LIFTING NE
INTO EASTERN NE/IOWA BY LATE THURSDAY...WHICH COULD LEAD TO
PRECIPITATION CHANCES AGAIN. HAVE INCLUDED PRECIPITATION CHANCES
BEGINNING WEDNESDAY AND PERSISTING THROUGH END OF PERIOD.
HOWEVER...GIVEN UNCERTAINTIES...MAY BE TOO AGGRESSIVE WITH PRECIP
ONSET WEDNESDAY.


&&

.AVIATION...17/18Z
ISSUED AT 1240 PM CDT THU APR 17 2014

MVFR STRATUS THAT COVERED MUCH OF CENTRAL IOWA THIS MORNING HAS
QUICKLY BEGUN MIXING OUT FROM WEST TO EAST...AND THIS TREND IS
EXPECTED TO CONTINUE THROUGH THE EARLY AFTERNOON HOURS WILL ALL
SITES CLEARING OUT BEFORE 20Z. NORTHWESTERLY WINDS OF 10 TO 15 KTS
WILL ALSO DECREASE THIS AFTERNOON AS A RIDGE OF HIGH PRESSURE
APPROACHES FROM THE WEST. THE RIDGE PASSES THROUGH OVERNIGHT WITH
SOUTHEASTERLY RETURN FLOW SETTING IN BY DAYBREAK AND INCREASING
THROUGH THE MORNING HOURS.


&&

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

$$

SHORT TERM...SKOW
LONG TERM...AWB
AVIATION...SKOW



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