FXUS63 KDMX 212336 AAA
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
535 PM CST MON DEC 21 2009
...UPDATED AVIATION DISCUSSION...
...MAJOR WINTER STORM TO HAVE PROLONGED EFFECT ON IOWA...
.SHORT TERM /TONIGHT/...
STRATUS CONTINUES WIDESPREAD ACRS THE CWA AS WAA INTENSIFIES AHEAD
OF APPROACHING SHORTWAVE. WILL SEE ISENTROPIC LIFT INCREASE FOR THE
REMAINDER OF THE AFTERNOON AND INTO THE EVENING AND WILL SEE DEEPER
SATURATION OCCUR AROUND 00Z WITH SNOW INTENSITIES PICKING UP IN
NORTHERN IA. THIS PERSISTS INTO THE OVERNIGHT ALONG A TIGHT
BAROCLINIC ZONE IN NORTHERN IA. WILL SEE SOME DECENT SNOWFALLS ACRS
THE FAR NORTH AND HAVE GONE WITH ADVISORY FOR 3 TO 5 INCHES IN THOSE
COUNTIES. OTHERWISE...LIGHT SNOW WILL OCCUR FARTHER SOUTH WITH UP TO
AN INCH IN THE HIGHWAY 30 CORRIDOR WITH UNDER AND INCH OR TRACE
AMOUNTS FARTHER SOUTH. HOWEVER...MAY SEE SOME FLURRIES OR FREEZING
DRIZZLE...ESPECIALLY TOWARD MORNING AS SYSTEM PASSES BUT LOW LEVELS
STAY SATURATED WITH NO ICE INTRODUCTION FROM ALOFT.
.LONG TERM /TUESDAY THROUGH MONDAY/...
THE BIG STORY OF THE LONG TERM FORECAST IS THE MAJOR PROLONGED
WINTER STORM SET TO AFFECT THE ENTIRE AREA FROM EARLY WEDNESDAY INTO
THE WEEKEND. HOWEVER...AHEAD OF THAT SYSTEM WE COULD SEE TRAVEL
IMPACTS ON TUESDAY AND TUESDAY NIGHT IN THE FORM OF FREEZING DRIZZLE
OR LIGHT FREEZING RAIN ACROSS MOST OF THE AREA. ONGOING SNOW ACROSS
OUR FAR NORTH/NORTHEAST WILL TAPER OFF BY AROUND 15Z TUESDAY...WITH
MOISTURE INCREASING IN THE NEAR-SURFACE LAYER CWA-WIDE THROUGHOUT
THE DAY. THE MOISTURE WILL BE TRAPPED IN A SHALLOW WARM
LAYER...EXTENDING UP TO AROUND 5000 FT AGL...WITH PROFILES SOLIDLY
BELOW FREEZING NORTH OF I-80 WHERE FREEZING DRIZZLE WILL BE
PREVALENT TUESDAY AFTERNOON AND INTO TUESDAY NIGHT. FURTHER
SOUTH...ESPECIALLY SOUTH OF HIGHWAY 34...THE PRECIP MAY FALL IN THE
FORM OF LIQUID DRIZZLE FOR MUCH OF THIS TIME AS SURFACE TEMPERATURES
WILL BE JUST A BIT WARMER...LIKELY HOVERING A DEGREE OR TWO ABOVE
FREEZING. WHILE NO SIGNIFICANT ICE ACCUMULATIONS ARE EXPECTED...EVEN
LIGHT ACCUMULATIONS COULD RESULT IN ICING OF SOME ROAD SURFACES AND
SLICK TRAVEL CONDITIONS. A FREEZING RAIN ADVISORY MAY BE NEEDED AT
SOME POINT TO ACCOUNT FOR THIS...HOWEVER AT THIS TIME THE MOST
LIKELY AREAS FOR SUCH ACCUMULATIONS ARE DIFFICULT TO IDENTIFY...AND
WITH ANOTHER ADVISORY IN EFFECT ALREADY FOR THE SNOW ACROSS THE
NORTH TONIGHT...AS WELL AS A WINTER STORM WATCH FOR THE LARGER STORM
COMING LATER IN THE WEEK...ADDING IN A THIRD WINTER HAZARD HEADLINE
SEEMS EXCESSIVE.
NOW ON TO THE BIG STORM SYSTEM...WHICH REMAINS MORE OR LESS ON TRACK
RELATIVE TO THINKING OF PREVIOUS FORECAST SHIFTS. THE OVERALL TIMING
AND LONG DURATION/HIGH QPF NATURE OF THE EVENT HAVE NOT
CHANGED...HOWEVER THE MODELS ARE GENERALLY COMING INTO BETTER
AGREEMENT ON CURVING THE CYCLONE LEFTWARD...MOVING NORTH OR NORTH
NORTHWEST FROM MISSOURI INTO IOWA...AS IT IS BECOMING STACKED AND
OCCLUDED ON WEDNESDAY NIGHT/THURSDAY. IN THIS SITUATION...FORECAST
SOUNDINGS ARE UNANIMOUS IN BRINGING MOST OF THE PRECIP IN OUR
SOUTHEASTERN AREAS FROM WEDNESDAY THROUGH THURSDAY IN THE FORM OF
RAIN...WITH MOSTLY SNOW IN THE NORTHWEST AND A MIX IN BETWEEN.
EVENTUALLY THE PRECIP WILL CHANGE OVER TO SNOW FROM NORTHWEST TO
SOUTHEAST AS THE SYSTEM BEGINS TO MOVE AWAY...HOWEVER THE TIMING OF
THIS CHANGE IS STILL VERY UNCERTAIN AND WILL HAVE SIGNIFICANT
EFFECTS ON FORECAST SNOWFALL TOTALS ACROSS MUCH OF THE
AREA...ESPECIALLY CENTRAL PORTIONS FROM AROUND ATLANTIC...TO DES
MOINES...TO WATERLOO. WE ARE ISSUING A WINTER STORM WATCH FOR AREAS
ROUGHLY ALONG AND NORTHWEST OF THAT LINE WHICH IS WHERE WE EXPECT
SNOW AND/OR FREEZING RAIN/SLEET ON WEDNESDAY. WARNING CRITERIA
ACCUMULATIONS ARE MOST LIKELY FURTHER NORTHWEST WITHIN THE WATCH
AREA...WHILE IN THE DES MOINES METRO AND AREAS TO THE SOUTHWEST TO
NORTHEAST ACCUMULATIONS OF WINTER PRECIP ON WEDNESDAY MAY NOT REACH
WARNING CRITERIA. HOWEVER...THERE IS ENOUGH UNCERTAINTY IN THIS
SCENARIO TO WARRANT A MARGIN OF ERROR...AND EVEN IN THE WARMER
SOLUTIONS OF THE 12Z GFS AND ECMWF WHERE THE PRECIP AT DSM FALLS AS
RAIN FROM AROUND MIDDAY WEDNESDAY THROUGH MOST OR ALL OF
THURSDAY...THERE ARE STILL INDICATIONS OF A BRIEF PERIOD OF
SLEET/FREEZING RAIN ON WEDNESDAY MORNING AS THE PRECIP MOVES IN FROM
THE SOUTH AND BEFORE WAA BECOMES STRONG ENOUGH TO PUSH THE PROFILES
ABOVE FREEZING. WITH THE POSSIBILITY OF THIS FREEZING PRECIP COMING
DURING THE WEDNESDAY MORNING COMMUTE HOURS...AS WELL AS THE
AFOREMENTIONED UNCERTAINTY BUFFER...FEEL IT IS WISE TO INCLUDE THOSE
AREAS IN THE WATCH AT THIS TIME. WE ARE ALSO ISSUING AN ESF TO
ACCOUNT FOR POTENTIAL FLOODING IN OUR SOUTHERN AND SOUTHEASTERN
COUNTIES WHERE SIGNIFICANT RAINFALL TOTALS ARE POSSIBLE ON TOP OF
SNOWPACK. HYDROLOGICAL CONCERNS ARE POTENTIALLY SIGNIFICANT IN SOME
OF THESE AREAS. THE STORM SYSTEM WILL BE MOVING AWAY LATE IN THE
WEEK WITH PRECIP TAPERING OFF FRIDAY NIGHT OR SATURDAY. THEREAFTER A
QUIETER WEATHER PATTERN APPEARS IN STORE FOR THE LATTER HALF OF THE
WEEKEND AND EARLY NEXT WEEK.
&&
.HYDROLOGY...
SOME REAL HYDRO CONCERNS EMERGING WITH THIS SYSTEM AS MODELS
POINTING TOWARDS A WARMER SOLUTION ACROSS THE FORECAST AREA.
APPEARS THAT RAINFALL WILL BE WIDESPREAD ALONG AND SOUTH OF THE
INTERSTATE 80 CORRIDOR...POSSIBLY UP TO THE HIGHWAY 30 CORRIDOR
DEPENDING ON THE EXTENT OF THE WARM AIR. WITH QPF'S FROM 1 TO 2
INCHES AND SNOW WATERS OF 0.75 TO 1.50 INCHES ON THE
GROUND...COULD SEE SOME HIGH RUNOFFS AND RAPID RISES ON STREAMS
AND RIVERS. TEMPERATURES WILL PLAY BIG IN THIS AND IF READINGS
HOLD JUST ABOVE FREEZING...RUNOFF MAY NOT BE AS BAD. HOWEVER...IF
WE HEAD TOWARD 40 DEGREES...ESPECIALLY DURING THE NIGHT...WE COULD
HAVE QUITE A FLUSHING DOWN THE RIVERS. GOOD THING IS THAT RIVERS
ARE RELATIVELY LOW...HOWEVER THE RAPID RISES AND ICE ON THE RIVERS
COULD CAUSE PROBLEMS. WILL HIGHLIGHT THESE ISSUES IN AN ESF ISSUED
LATE THIS AFTERNOON.
&&
.AVIATION...
22/00Z...SNOW IS EXPECTED OVER NORTHERN IOWA TONIGHT THEN CHANCES OF
FREEZING DRIZZLE THE ENTIRE AREA ON TUESDAY. FOR TONIGHT...A BRIEF
BREAK WITH MVFR CEILINGS AND MVFR VISIBILITIES WILL MOVE THROUGH
KMCW AND NEAR KALO. IFR TO LIFR CEILINGS WITH FOG AND SNOW WILL THEN
BE IN PLACE THE REMAINDER OF THE NIGHT. FURTHER SOUTH...PERIODS OF
VFR CONDITIONS WILL BE IN PLACE BEFORE DEVELOPING IFR AND LIFR
CEILINGS LATE. BY TUESDAY...WIDESPREAD LIFR CEILINGS WILL BE OVER
IOWA WITH FREEZING DRIZZLE DEVELOPING. SFC TEMPS SHOULD RISE ABOVE
FREEZING AT OTM IN THE AFTERNOON AND PRECIP WILL CHANGE TO DZ.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM WEDNESDAY MORNING THROUGH FRIDAY AFTERNOON
FOR ADAIR-AUDUBON-BLACK HAWK-BOONE-BREMER-BUTLER-CALHOUN-CARROLL-
CASS-CERRO GORDO-CRAWFORD-DALLAS-EMMET-FRANKLIN-GREENE-GRUNDY-
GUTHRIE-HAMILTON-HANCOCK-HARDIN-HUMBOLDT-KOSSUTH-MARSHALL-PALO
ALTO-POCAHONTAS-POLK-SAC-STORY-TAMA-WEBSTER-WINNEBAGO-WORTH-WRIGHT.
WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 6 AM CST
TUESDAY FOR BLACK HAWK-BREMER-BUTLER-CERRO GORDO-EMMET-FRANKLIN-
HANCOCK-KOSSUTH-PALO ALTO-WINNEBAGO-WORTH.
$$
SHORT TERM/AVIATION...COGIL/DONAVON
LONG TERM...LEE
