000
FXUS63 KDMX 081140
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
540 AM CST SAT MAR 8 2014

.SHORT TERM /TODAY/...
ISSUED AT 330 AM CST SAT MAR 8 2014

AREA OF SURFACE HIGH PRESSURE TO BUILD INTO THE CWA TODAY...WITH
WINDS DIMINISHING AND THE CHILLY AIRMASS IN PLACE TO MODERATE
THROUGH THE DAY. STRATUS SHIELD IN PLACE WITH MODELS SUGGESTING A
SLOW EROSION OF THE CLOUDS FROM NORTHWEST TO SOUTHEAST THROUGHOUT
THE AFTERNOON HOURS. CURRENTLY A BREAK IN THE CLOUDS ACROSS
NORTHWEST IOWA INTO SE SD/SW MN...ALTHOUGH SATELLITE TRENDS SHOW
THIS AREA FILLING IN AS CLOUD SHIELD EXPANDS. IF HAVE SOME BREAKS IN
THE CLOUDS TOWARD DAYLIGHT...COULD SEE CLOUDS ERODE FASTER AS DRIER
MOVES INTO THE CWA AND STRONGER MARCH SUN BURNS SOME OF THE CLOUDS
OFF. THIS MAKES THE TEMPERATURE FORECAST TRICKY WITH CLOUD COVER
KEEPING TEMPS A BIT WARMER THAN EXPECTED OVERNIGHT...WHICH SHOULD
ALLOW TEMPS TO WARM A BIT COMPARED TO THE PREVIOUS FORECAST. HOWEVER
WITH CLOUDS EXPECTED TO LINGER MUCH OF THE DAY AND NORTH TO
NORTHEAST SFC FLOW...WARMING WILL STILL BE LIMITED. THEREFORE OPTED
TO KEEP MAX TEMPS IN THE MID 20S NORTH TO MID 30S SOUTH. AS FOR
PRECIPITATION...CURRENT BAND OF VERY LIGHT SNOW/FLURRIES WILL
CONTINUE TO PUSH EAST SOUTHEAST THROUGH THE NEXT FEW HOURS. BAND HAS
BEEN DIMINISHING IN INTENSITY OVER THE LAST FEW HOURS...AND EXPECTED
THE TREND TO CONTINUE THUS PULLED MENTION OF POPS AND STUCK WITH
FLURRY WORDING THROUGH 11Z...WITH DRY CONDITIONS ACROSS THE ENTIRE
CWA FOR TODAY.

.LONG TERM /TONIGHT THROUGH FRIDAY/...
ISSUED AT 330 AM CST SAT MAR 8 2014

ALTHOUGH THERE IS A WINDOW FOR PRECIP AROUND TUE...THE PRIMARY
CONCERNS THIS PERIOD WILL BE TEMP TRENDS AND POTENTIALLY ICE JAM
EFFECTS. SURFACE HIGH PRESSURE WILL SETTLE INTO IA TONIGHT WITH
RETURN FLOW BEGINNING OVERNIGHT LEADING INTO SIGNIFICANT WARMING
SUN AND MON. LOOKING BACK AT MODEL VERIFICATION DURING DECENT WARM
UP IN FEB SHOWED FORECAST COOL BIAS AND SUGGESTED THAT ECMWF MOS
PERFORMED THE BEST. ECMWF SUITE POINTS TOWARD EVEN GREATER WARM UP
THAN GOING FORECAST SO HAVE INCREASED SUN AND MON HIGHS SEVERAL
DEGREES...WHICH MAKES SENSE WITH DRY SW FLOW SUN AND PROXIMITY TO
THERMAL RIDGE MON. MORE AGGRESSIVELY MIXED AND SEEMINGLY MORE
REALISTIC GFS SOUNDINGS ALSO SUPPORT WARMER. EXPECT LOW 60S OVER
SWRN HALF OF IA MON AND THIS MAY START PERIOD OF HEIGHTENING ICE
JAM POTENTIAL WITH ADDITIONAL DETAILS ON THAT BELOW.

REMNANTS OF CURRENT ERN PACIFIC/GULF OF AK TROUGH REACH THE CENTRAL
CONUS BY TUE.  THIS WILL DROP TEMPS BACK CLOSER TO NORMAL AND ALSO
INCREASE PRECIP CHANCES.  GFS/ECMWF STILL HAVE SOME TIMING
DIFFERENCES WITH GFS A BIT FASTER...BUT ARE GENERALLY SIMILAR WITH
TRENDS. BOTH MODELS SUGGEST MUCH OF THE FORCING FOR IA WILL BE BASED
IN THE MID LEVELS SO WOULD TEND TO FAVOR LIGHTER ECMWF QPF.  MUCH OF
THE 1-3KM FORCING EITHER BRUSHES SOUTHERN IA /ECMWF/ OR STAYS TO
THE SOUTH /GFS/ AND IS RELATIVELY MOISTURE STARVED EITHER WAY.
SOUNDINGS SUGGEST EITHER RAIN OR SNOW FOR PRECIP TYPE WITH WARM
LAYER AND SURFACE TEMPS ESSENTIALLY DROPPING AT THE SAME TIME.

DRY AND MILD NW FLOW LOOKS TO BE IN PLACE TO END THE WORK WEEK WITH
TEMPS BACK INTO THE 50S FOR THU AND FRI. ALTHOUGH A SHORT WAVE DOES
TRAVERSE SRN CANADA INTO THE GREAT LAKES THU INTO FRI...THERE IS
LITTLE EVIDENCE OF SUFFICIENTLY PHASED MOISTURE AND FORCING FOR IA
SO FORECAST REMAINS DRY.  MODELS DO POTENTIALLY POINT TOWARD LIGHT
PRECIP JUST BEYOND THE VALID PERIOD HOWEVER WITH A TRANSIENT PERIOD
OF LIGHT RAIN OR LIGHT SNOW POSSIBLE FRI NIGHT.

&&

.AVIATION...08/12Z
ISSUED AT 540 AM CST SAT MAR 8 2014

STARTING TO SEE SOME EROSION OF THE LOW STRATUS DECK ACROSS
NW/WC IOWA WHICH SHOULD ALLOW FOR NORTHERN TAF SITES CIGS TO
LIFT/CLEAR TOWARD LATE MORNING...WITH THE SOUTHERN TAF SITES
CLEARING OUT TOWARD THE AFTERNOON HOURS. NORTHERLY WINDS WILL
DIMINISH THROUGHOUT THE DAY AS HIGH PRESSURE BUILDS INTO THE
STATE...WITH MOSTLY CLEAR CONDITIONS EXPECTED TONIGHT.

&&

.HYDROLOGY...
ISSUED AT 301 PM CST FRI MAR 7 2014

ICE JAMS AND RESULTING FLOODING WILL BECOME A CONCERN BY EARLY NEXT
WEEK AS TEMPERATURES WARM UP. AREA OF MOST CONCERN INITIALLY
INCLUDES STREAMS SOUTH OF HIGHWAY 30...BUT WILL SHIFT NORTHWARD
WITH TIME TO ENCOMPASS MUCH OF THE CWA BY THE END OF THE WEEK.
TIMING WOULD INITIALLY BE AS EARLY AS NEXT MONDAY NIGHT OR TUESDAY
FOR THE SOUTHERN CWA...AND THE MIDDLE TO LATER PARTS OF NEXT WEEK
ELSEWHERE.

RUNOFF FROM SNOWMELT WILL MAKE ITS WAY INTO OUR STREAMS AS
TEMPERATURES INCREASE OVER THE NEXT FEW DAYS. THE DEEP GROUND FROST
DEPTH WILL MINIMIZE INFILTRATION OF ANY WATER FROM THE
SNOWMELT...AND WOULD THUS HELP MAXIMIZE RUNOFF INTO THE STREAMS.

IF ICE JAMS DO OCCUR EARLY NEXT WEEK THEY WOULD BE DRIVEN PRIMARILY
BY SNOWMELT ALONE. STUDIES OF SUCH PAST ICE JAM EVENTS IN AND NEAR
THE NWS DES MOINES CWA HAVE YIELDED SOME COMMON PRECURSOR FEATURES.
FOR ICE JAMS DRIVEN TOTALLY OR PRIMARILY BY SNOWMELT ONLY...THE ICE
JAMS WERE PRECEDED BY 3 TO 5 CONSECUTIVE DAYS WITH THE DAILY MAX
TEMPERATURE IN THE 40S OR HIGHER. IN ADDITION...THE ICE JAMS
OCCURRED WITH OVERNIGHT LOWS IN THE PRECEDING 3 TO 5 NIGHTS WHICH
WERE BOTH ABOVE AND BELOW FREEZING. SOME OVERNIGHT LOWS WERE AS LOW
AS THE MIDDLE TO UPPER TEENS.

THE STUDIES ALSO SHOWED THAT WHEN THE DAILY AVERAGE TEMPERATURE
REACHED 42F OR HIGHER...ICE JAMS ALMOST ALWAYS OCCURRED WITHIN A DAY
OR TWO. THEY SOMETIMES OCCURRED DURING THE NIGHT IMMEDIATELY
FOLLOWING THE DAYTIME HIGH WHICH RESULTED IN THE DAILY AVERAGE
TEMPERATURE OF 42F OR HIGHER.

LOCATIONS THAT HAVE HISTORICALLY EXPERIENCED ICE JAMS AND RESULTING
FLOODING WOULD BE OF MOST CONCERN. THOSE LOCATIONS INCLUDE BUT ARE
NOT LIMITED TO THE DES MOINES RIVER IN THE FORT DODGE AREA...THE
RACCOON RIVER /INCLUDING IN THE DES MOINES AREA/ AS WELL AS PORTIONS
OF THE SOUTH SKUNK RIVER. GIVEN THE UNUSUALLY THICK NATURE OF THE
ICE...HOWEVER...ICE JAMS AND RESULTING FLOODING MAY OCCUR IN OTHER
LOCATIONS AS WELL. AS HAS HAPPENED WITH PAST ICE JAMS...RIVER LEVELS
MAY RISE RAPIDLY THUS LEADING TO FLASH FLOODING.

FLASH FLOOD WATCHES AND WARNINGS MAY BE ISSUED IF CONCERN BECOMES
HIGH ENOUGH FOR ICE JAM FLOODING.

&&

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

&&

$$

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