Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 231114
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
614 AM CDT SAT MAY 23 2015

.SHORT TERM /TODAY/...
ISSUED AT 332 AM CDT SAT MAY 23 2015

FIRST PUSH OF WARM AIR ADVECTION PRODUCING SPRINKLES OVER NORTHWEST
AREAS EARLY TODAY...ENDING PRIOR TO SUNRISE.  AS IS THE CASE
CURRENTLY...THE SECOND PUSH OF SHOWERS LATER THIS AFTERNOON BETWEEN
21 AND 00Z WILL HAVE SOME DIFFICULTY OVERCOMING DRY AIR AS IT MOVES
INTO SOUTHWEST/SOUTHERN IOWA INITIALLY...SO HAVE CUT BACK ON POP AND
QPF THROUGH 00Z TODAY. BUFR SOUNDINGS AND MODELS CONTINUE TO SHOW
LITTLE IN WAY OF INSTABILITY AS DRY LAYER AND LACK OF ANY TREMENDOUS
MOISTURE PUSH SHOULD RESULT IN SHOWERS WITH LITTLE THREAT FOR ANY
THUNDER TODAY. HIGHS WILL RECOVER DESPITE THE CLOUD COVER INTO THE
LOWER TO MID 70S...COOLER SOUTHWEST WHERE CLOUDS MAY BE THICKER
DURING THE AFTERNOON. AS LOW APPROACHES SOME INCREASE IN SOUTH
SOUTHEAST WINDS BUT GRADIENT QUITE LIGHT SO OVERALL WINDS SHOULD NOT
BE A FACTOR IN TODAYS FORECAST.

.LONG TERM /TONIGHT THROUGH FRIDAY/...
ISSUED AT 332 AM CDT SAT MAY 23 2015

MAIN FOCUS CONTINUES TO BE ON RAIN CHANCES TONIGHT THROUGH THE
HOLIDAY WEEKEND. A LARGE UPPER LEVEL LOW WILL LIFT FROM THE FOUR
CORNERS REGION TODAY INTO SOUTHERN ONTARIO BY LATE MONDAY...WITH
SOUTHWEST FLOW EXPECTED ALOFT. THE ASSOCIATED SFC LOW WILL DEVELOP
ALONG THE FRONT RANGE OF THE ROCKIES AND LIFT NORTHEASTWARD INTO
SRN MN WITH THE SFC TROUGH AXIS STRETCHING ACROSS WESTERN IOWA
INTO THE PLAINS STATES MONDAY. SOUTHERLY FLOW WILL INCREASE AHEAD
OF THIS LOW...WITH MOISTURE TO SURGE NORTHWARD FROM THE GULF OF
MEXICO THROUGH THE WEEKEND. PIECES OF ENERGY ALOFT TO EJECT
NORTHEASTWARD OUT OF THE MAIN UPPER LEVEL TROUGH AND THROUGH THE
REGION TONIGHT THROUGH MONDAY WHICH COMBINED WITH STRONG THETA-E
ADVECTION WILL LEAD TO A LONG-DURATION PRECIPITATION EVENT THROUGH
THE NEXT FEW DAYS. INITIALLY STILL HAVE BRIEF RIDGING ALOFT ACROSS
THE STATE THIS EVENING...WITH ANTICYCLONIC FLOW ALOFT. LOW LEVEL
MOISTURE TRANSPORT VECTORS MORE ORIENTED INTO NEBRASKA/SW IOWA
THIS EVENING...WITH SOME DRIER AIR TO OVERCOME INITIALLY. MOISTURE
TRANSPORT FINALLY ORIENTS MORE INTO CENTRAL IOWA TOWARD 09-12Z
SUNDAY...THEREFORE HAVE CUT BACK ON THE PROGRESSION OF POPS FOR
THIS EVENING SPREADING ACROSS MUCH OF THE CWA WITH CATEGORICAL
POPS TOWARD SUNRISE. MOISTURE TRANSPORT CONTINUES TO BE POINTED
INTO CENTRAL IA AND MN THROUGH THE DAY SUNDAY. EXPECT A FEW SMALL
BREAKS IN THE RAIN FOR SUNDAY...BUT OVERALL WILL LIKELY REMAIN
QUITE STEADY THROUGH THE DAY. CAPE CONTINUES TO BE MARGINAL
THROUGHOUT TONIGHT INTO EARLY SUNDAY...BUT DOES START TO PICK UP
SOME LATE SUNDAY AFTERNOON INTO SUNDAY NIGHT. THEREFORE WENT WITH
AN ISOLATE THUNDER MENTION THROUGH MUCH OF SUNDAY...WITH STRAIGHT
THUNDER FOR SUNDAY NIGHT. RAIN CHANCES CONTINUE INTO MONDAY/MONDAY
NIGHT AS THE UPPER LOW MOVES THROUGH THE REGION. BETTER CHANCES
FOR THUNDERSTORMS COMES MONDAY WITH THE CWA CLEARLY IN THE WARM
SECTOR AND THE SFC TROUGH AXIS TO SWING THROUGH THE CWA MONDAY
NIGHT.

OVERALL SEVERE THREAT SOMEWHAT MINIMAL GIVEN THE LOWER AMOUNTS OF
CAPE INITIALLY...MOIST SOUNDINGS AND MORE UNILATERAL SHEAR PROFILES.
SLIGHTLY BETTER CHANCES FOR MONDAY...BUT OVERALL A HYDROLOGY ISSUE
MAY BE MORE LIKELY. PWATS TO INCREASE TO AROUND 1.4 TO 1.5 INCHES
THROUGH THE WEEKEND WHICH IS FAIRLY HIGH FOR CLIMATOLOGY. WILL
HAVE TO MONITOR OVER THE NEXT SHIFT OR SO TO SEE IF ANY HEADLINE
IS WARRANTED GIVEN THE EXTENDED PERIOD OF RAIN EXPECTED. FOR THE
REMAINDER OF THE EXTENDED...AN ACTIVE PATTERN WILL CONTINUE WITH
SOUTHWESTERLY TO WESTERLY FLOW EXPECTED ALOFT. THIS WILL ALLOW
ENERGY TO CONTINUE TO TRACK ACROSS THE AREA FROM THE ROCKIES WITH
CHANCES FOR PRECIPITATION EACH DAY. BEST CHANCES LOOK TO BE
THURSDAY WITH A FRONTAL BOUNDARY SAGGING SOUTHWARD ACROSS THE
STATE. OVERALL TEMPERATURES LOOK GOOD THROUGHOUT AS WELL...WITH
H85 TEMPS IN THE TEENS CELSIUS. THIS SHOULD KEEP US NEAR AVERAGE
FOR THIS TIME OF YEAR.

&&

.AVIATION...23/12Z
ISSUED AT 614 AM CDT SAT MAY 23 2015

MINOR CONCERNS EARLY IN THE PERIOD WITH GENERALLY VFR CIGS AND
VCSH OR ISO --SHRA THROUGH 00Z. AS UPPER LEVEL WAVE AND STRONGER
WARM AIR ADVECTION ARRIVES BETWEEN 03-09Z...WILL SEE GRADUAL
INCREASE IN -SHRA COVERAGE AS WELL AS RETURN TO MVFR CIGS MOST
AREAS TOWARD 12Z. WINDS REMAIN SOUTH SOUTHEAST WITH SOME GUSTS
15KT THROUGH 23Z...THEN 10KT OR LESS AFT 00Z. JUST BEYOND PERIOD
FROM 24/12 ON...CIGS WILL DROP QUICKLY TO MVFR/IFR AND -TSRA/SHRA
WILL BE COMMON. /REV

&&

.HYDROLOGY...
ISSUED AT 332 AM CDT SAT MAY 23 2015

NOT MUCH HAS CHANGED...SO HAVE KEPT THE DISCUSSION SIMILAR TO THE
PREVIOUS VERSION.

A WET NEXT FEW DAYS IS EXPECTED TO PROVIDE AN INITIAL RISK FOR
FLASH FLOODING SATURDAY NIGHT INTO SUNDAY NIGHT...AND THEN A
SUBSEQUENT RISK OF AT LEAST ADDITIONAL RISES ON AREA RIVERS...AND
POSSIBLY RIVER FLOODING...BEGINNING AS EARLY AS SUNDAY NIGHT AND
CONTINUING INTO THE EARLY PARTS OF NEXT WEEK. THE MIDDLE AND LOWER
PORTIONS OF THE DES MOINES AND RACCOON BASINS...AS WELL AS
MISSOURI RIVER DRAINAGE BASINS IN THE SOUTHWEST CWA...ARE OF MOST
CONCERN AT THIS TIME.

IN TERMS OF RIVER FLOODING...THE 12Z HYDRO MODEL 72 HR QPF
ENSEMBLES ARE SHOWING A WIDE SPREAD BETWEEN THE EXPECTED AND 95
PERCENT MAX QPF TRACES...INDICATING RELATIVELY HIGH SENSITIVITY IN
THE HYDROLOGIC MODEL TO QPF. FOR EXAMPLE...THE 72 EXPECTED QPF
TRACES SLOW THE FALLS OF MOST STREAMS...WITH SOME SLIGHT TEMPORARY
INCREASES...WHILE THE 72 HR 95 PERCENT MAX QPF TRACES SHOW
SIGNIFICANT RISES...WITH MULTIPLE RIVER FORECAST POINTS RISING TO
NEAR OR ABOVE FLOOD STAGE.

IT SHOULD ALSO BE NOTED THAT THE 12Z QPF ENSEMBLES USED IN THE 12Z
HYDRO MODEL APPEAR TO BE UNDERDONE...WITH TOTAL EXPECTED 72-HR QPF
POSSIBLY FALLING CLOSER TO THE 95 PERCENT MAX QPF ENSEMBLE MEMBER
THAN THE 50 PERCENT QPF ENSEMBLE MEMBER.

GIVEN THE HYDROLOGIC MODEL SENSITIVITY TO THE QPF...AS WELL AS THE
POSSIBILITY OF ENSEMBLE QPF BEING UNDERDONE...UNCERTAINTY IN THE
RIVER RESPONSES IS HIGHER THAN AVERAGE FOR THIS PARTICULAR EVENT.
THIS HIGHER THAN AVERAGE UNCERTAINTY WILL NECESSITATE CLOSE
MONITORING OF QPF TRENDS...AND SUBSEQUENT IMPACTS ON THE HYDRO MODEL
GOING FORWARD.

&&

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

$$

SHORT TERM...REV
LONG TERM...BEERENDS
AVIATION...REV
HYDROLOGY...ZOGG



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