Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 210454
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1154 PM CDT WED AUG 20 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 340 PM CDT WED AUG 20 2014

TONIGHTS FORECAST IS QUITE DIFFICULT AND THE MODELS ARE NOT MUCH
HELP.  A WARM FRONT ACROSS IOWA STRETCHES FROM A LOW OVER WEST
CENTRAL SOUTH DAKOTA.  THETA-E ADVECTION AND SOME MODEST INSTABILITY
KEPT THUNDERSTORMS FESTERING AHEAD OF THE FRONTAL BOUNDARY ALL DAY
LONG AND SHEAR ACTUALLY INCREASED THROUGH THE AFTERNOON HOURS.  THE
MODELS CONTINUE THE SHOWERS/STORMS ALONG AND AHEAD OF THE FRONTAL
BOUNDARY IN A ZONE OF WARM ADVECTION THROUGH THE AFTERNOON HOURS
INTO EARLY EVENING.  THE HRRR AND HOPWRF THEN SQUELCH THE PRECIP IN
IOWA OVERNIGHT WHILE THE NAM CONTINUES TO DEVELOP STORMS ACROSS
NORTH CENTRAL/NORTHEAST IOWA...ESPECIALLY WITH A LOW LEVEL JET THAT
INTENSIFIES LATER THIS EVENING.  IT IS VERY HARD TO SAY WHERE THAT
BOUNDARY WILL LIFT TO THIS EVENING...OR WILL IT REMAIN FAIRLY
STATIONARY.  I AM REASONABLY CERTAIN THAT WE WILL NOT BE DRY ACROSS
THE NORTH BUT I AM SKEPTICAL ABOUT KEEPING 60-70 POPS IN THAT AREA
GIVEN THE TRENDS FOR THE PRECIP TO SLOWLY LIFT NORTH.

THE MAIN ADJUSTMENTS TO THE GOING FORECAST WERE TO LOWER POPS SOME
ACROSS THE NORTH AND MENTION HEAVY RAIN ACROSS A PART OF THE NORTH
CENTRAL AND NORTHEAST.  HOWEVER...IF THE NAM12 IS CORRECT MY
FORECAST WILL NEED TO BE ADJUSTED ONCE THE LLJ COMES INTO PLAY AFTER
03Z OR SO AS HIGHER POPS WILL BE NEEDED FURTHER SOUTH.  HEAVY
RAINFALL WILL ALSO BE MORE OF A CONCERN ACROSS AREAS NORTH OF
HIGHWAY 20.

.LONG TERM /THURSDAY THROUGH WEDNESDAY/...
ISSUED AT 340 PM CDT WED AUG 20 2014

RELATIVELY ACTIVE CONDITIONS TO PERSIST FOR MUCH OF THE LONG TERM
WITH BROAD WSW FLOW ALOFT AND SURFACE BOUNDARY NEARBY. ANY ONGOING
CONVECTION ON THURSDAY MORNING SHOULD DISSIPATE WITH FORCING
WEAKENING. WARM SECTOR WILL BE IN PLACE WITH TEMPERATURES CLIMBING
NICELY THROUGH THE DAY. STILL MAY SEE SOME MID TO HIGH CLOUDS
OVERHEAD AND THIS MAY LIMIT DAYTIME HIGHS...HOWEVER STILL A VERY
WARM AND HUMID DAY EXPECTED FOR MOST LOCATIONS. SURFACE BOUNDARY
WILL EASE INTO NORTHERN IOWA ON THURSDAY NIGHT WITH SOME SCATTERED
CONVECTION EXPECTED ALONG THE FRONT OVERNIGHT. THESE STORMS ARE
EXPECTED TO WEAKEN OVERNIGHT TOWARD DAYBREAK AS INFLOW WEAKENS AND
COL REGION PUSHES INTO THE STATE. FRIDAY WILL SEE THE COL PASS
THROUGH IOWA WITH LIMITED CONVECTION CHANCES FOR MUCH OF THE DAY.
HOWEVER...A MORE VIGOROUS SHORTWAVE WILL EJECT INTO THE PLAINS
LATER FRIDAY WITH INCREASING FLOW INTO THE FRONT LATE FRIDAY INTO
FRIDAY NIGHT. MODELS HAVE ALL BEEN INDICATING RATHER ROBUST
CONVECTION BY LATE FRIDAY ACROSS THE PLAINS PUSHING INTO IOWA
OVERNIGHT. HAVE CONTINUED THE HIGHER POPS ON FRIDAY
NIGHT...ESPECIALLY ACROSS THE NORTHWEST WHERE THE BEST FORCING AND
INFLOW INTO SYSTEM IS ANTICIPATED. THE THREAT FOR CONVECTION WILL
PERSIST INTO THE WEEKEND AND EARLY NEXT WEEK AS THE FRONTAL
BOUNDARY REMAINS NEAR THE STATE. AT THIS POINT...IT APPEARS MUCH
OF THE CONVECTION THIS WEEKEND WILL BE NEAR NORTHERN IOWA WITH HOT
AND HUMID CONDITIONS IN THE SOUTH. THE BULK OF THE SYSTEM IS NOT
EXPECTED TO PASS THROUGH THE REGION UNTIL THE MIDDLE OF NEXT WEEK
WHEN GOOD COVERAGE OF STORMS IS ANTICIPATED TUESDAY NIGHT INTO
WEDNESDAY. TEMPERATURES WILL REMAIN VERY WARM TO HOT SOUTH OF THE
BOUNDARY DURING THIS TIME.

THE THREAT FOR HEAVY RAINFALL WILL BE EVER PRESENT WITH THIS
CONVECTION FOR MUCH OF THE FORECAST. PWAT VALUES WILL IN GENERAL
BE AT OR ABOVE 1.50" WITH CONVECTION EAST OR SOUTHEAST AND
TRAINING A POSSIBILITY FROM TIME TO TIME. WHILE WIDESPREAD
FLOODING IS NOT EXPECTED GIVEN MATURE VEGETATIVE
GROWTH...LOCALIZED FLOODING MAY OCCUR. THE PRIMARY FLASH FLOOD
THREAT WILL REMAIN IN URBAN AREAS. HOWEVER...IF SEVERAL NIGHTS OF
RAIN DO OCCUR IN THE SAME AREA...A MORE GENERALIZED FLASH FLOOD
THREAT MAY MATERIALIZE.

THE OTHER CONCERN WILL BE WITH HOT AND HUMID CONDITIONS SOUTH OF
THE BOUNDARY. HEAT INDEX VALUES ARE EXPECTED TO CLIMB ABOVE 100
DEGREES FROM TIME TO TIME AND MAY BECOME PROBLEMATIC...ESPECIALLY
IN SOUTHERN IOWA SHOULD SEVERAL DAY STRING TOGETHER OF HOT AND
STICKY CONDITIONS. MUCH WILL DEPEND ON THE EXTEND OF CONVECTION
AND WHERE OUTFLOW BOUNDARYS AND SUBSEQUENT CONVECTION END UP. AT
THIS POINT...GIVEN THE UNCERTAINTY...NO HEADLINES FOR HEAT WILL BE
ISSUED.

&&

.AVIATION...21/06Z
ISSUED AT 1150 PM CDT WED AUG 20 2014

BOUNDARY NEAR NORTHERN IOWA WILL ALLOW FOR TSRA CHANCES AT SITES.
HAVE INCLUDED MENTION AT NORTHERN SITES KMCW/KALO...WITH
CONDITIONS DROPPING TO MVFR OR IFR...POSSIBLY LOWER IF HEAVY RAIN
REDUCES VISIBILITIES NEAR 12Z. SYSTEM WILL PUSH EAST...WITH A
RETURN TO VFR CONDITIONS AT SITES. WINDS WILL BE FROM THE SOUTH
AND MAY BE GUSTY IN THE AFTERNOON. ADDITIONAL CHANCES FOR
SHRA/TSRA WILL BE POSSIBLE AT SITES LATE IN THE PERIOD...THOUGH
HAVE LEFT OUT MENTION ATTM DUE TO UNCERTAINTIES IN TIMING AND
PLACEMENT.

&&

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

&&

$$

SHORT TERM...FAB
LONG TERM...COGIL
AVIATION...AWB


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