Winter Storm in Iowa, March 11 to 12, 2001

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
340 PM CST SUN MAR 11 2001

...WINTER STORM HEADED FOR NORTHWEST IOWA...

MAJOR CONCERN THIS FORECAST PERIOD IS THE STORM ORGANIZING OVER THE
CENTRAL AND SOUTHERN ROCKIES.  THIS IS SOUTHERN STREAM DOMINATED
SYSTEM AND HOW FAR NORTH AND WEST THE HEAVY SNOW GOES IS PRIMARILY
DEPENDENT UPON HOW NEGATIVELY TILTED THIS SYSTEM BECOMES.  APPEARED
THE AVN/ETA HAD CONVERGED ON A SOLUTION AT 12Z WITH HEAVIEST SNOW
FROM KSUX TO KMJQ.  HOWEVER...NEW 18Z MESOETA HAS COME OUT AND
SHIFTED THE HEAVY SNOW FARTHER EAST AND PLACED A SECONDARY BAND OF
SNOW ACROSS THE JAMES VALLEY.  HAVE OPTED FOR A COMPROMISE BETWEEN
THE 12Z AND 18Z RUNS.  THIS MEANS A WINTER STORM WARNING FROM DIXON
COUNTY NEBRASKA NORTHEAST TO WINDOM MN.  ATTM WL GO WITH AN ADVISORY
FROM BON HOMME SD TO SOUTH SIOUX FALLS TO SLAYTON MN.

HERE IS THE CURRENT THOUGHT OF HOW THIS STORM WL PROGRESS.
SATELLITE DSHOWS STRONG WAVE ROTATING OUT OF COLORADO AT THIS TIME
WITH A LARGE AREA OF PCPN OVER ERN NEBRASKA.  ALL MODELS AGREE THAT
850 MB FRONTOGENESIS WL SET UP THIS EVENING FROM SOUTHWEST MN
THROUGH NORTHWEST IOWA.  DURING THE EVEVNING...THIS AREA IS ALSO
WEAKLY UNSTABLE TO SLANTWISE LIFT AND EVEN EVIDENCE OF MIDLEVEL
UPRIGHT INSTABILITY.  THEREFORE EXPECT THAT A BAND OF CONVECTIVE
SNOW WILL DEVELOP ACRS NORTHWEST IOWA SOMETIME AFTER 9 PM.  BEST BET
RIGHT NOW IS FROM EAST OF SIOUX CITY TO SPENCER IOWA.  THIS IS TO
THE WEST OF THE 18Z MESOETA HEAVY PCPN...WHICH PLACES THE HEAVIEST
SNOW EAST OF THE 850 MB FRONTOGENESIS MAXIMUM AND EAST OF SPW.  SEE
NO REASON WHY THIS WOULD OCCUR BASED UPON UPPER DYNAMICS AND
STABILITY.  BEST LOW LEVEL DYNAMICS...AND HEAVIEST PCPN...WL MOVE TO
THE NORTHEAST OF THE CWA BETWEEN 4 AM AND 8 AM.  THEREFORE MOST OF
THE ACCUMULATIONS WL OCCUR BEFORE MORNING RUSH.  TO THE NORTHWEST OF
THIS BAND...PCPN RATES WL DROP OFF RAPIDLY.  THUS WL ONLY HAVE A
NARROW AREA IN AN ADVISORY AND THE NW HALF OF THE CWA NOT IN ANY
HEADLINE.  WIND NOT EXPECTED TO BE A BIG CONCERN OVERNIGHT AS WRN
CWA IN THE RIDGE AND AND ERN CWA NEAR INVERTED TROUGH AXIS.

ON MON...THE HEAVIEST SNOW WILL CONTINUE INTO THE EARLY MORNING OVER
THE SE THIRD OF THE CWA.  THEN EXPECT A SHORT BREAK AND THEN A
SECOND BAND OF LIGHT SNOW TO MOVE ACROSS THE AREA FROM WEST TO
EAST...MAINLY ACROSS THE NORTHERN CWA.  DO NOT EXPECT HEAVY ACCUMS
WITH THIS BAND...LESS THAN AN INCH.  HOWEVER WITH WINDS COMING UP
DURING THE MORNING AND SOME PLACES GETTING MORE THE 6 INCHES OF
SNOW...THIS MAY CAUSE SOME TRAVEL PROBLEMS THROUGH THE LATE MORNING
IN THE WARNING AND ADVISORY AREAS.  EXPECT SKIES TO GRADUALLY CLEAR
ACRS THE WRN HALF OF THE CWA DURING THE AFTERNOON.  WITH 850 MB
TEMPS STILL ABOVE -5C AND WINDS WITH A WRLY COMPONENT...EXPECT THAT
TEMPS WL RISE INTO THE LOWER 30S ACRS MOST OF THE CWA.

ON MON NIGHT...SKIES WL BE EITHER PARTLY CLOUDY OR MOSTLY CLEAR.
WINDS SHOULD STAY AROUND 10 KTS THROUGH THE NIGHT.  THERE WL BE SOME
CAA THROUGH THE NIGHT BUT LOWS ARE EXPECTED TO STAY IN THE TEENS TO
AROUND 20.  ON TUE...ETA SHOWS ATMOS DRYING OUT.  HOWEVER...BOTH ETA
AND AVN SHOW ANOTHER WAVE MOVING DOWN IN THE NW FLOW.  WITH AN 850
MB BOUNDARY NEARBY...THAT SHOULD BE ENOUGH TO PRODUCE PARTLY TO
MOSTLY CLOUDY SKIES AS SUGGESTED BY AVN.  DO NOT EXPECT PCPN AS MID
LEVELS REMAIN DRY.  WITH NW FLOW...ONCE AGAIN EXPECT TEMPS TO RISE
INTO THE 30S ACRS THE CWA WITH AREAS WITH THE NEWEST SNOWCOVER THE
COLDEST.  TUE NIGHT WL REMAIN WARM AS WRLY FLOW AND WAA OVERSPREAD
THE AREA.  EXPECT MOST AREAS TO HAVE MINS ABOVE 20 DEGREES.

IN THE EXTENDED FORECAST...DID NOT HAVE ENOUGH TIME TO COMPARE
VARIOUS EXTENDED MODELS SO HAVE RELIED ON THE 12Z AVN AND 00Z MRF.
LOOKS LIKE A QUICK WARMUP FOR THE MIDDLE OF THE WEEK WITH WESTERLY
FLOW AT LOW LEVELS AND 850 MB TEMPS 0 TO 5 ABOVE.  HAVE RAISED TEMPS
INTO THE 40S IN SC SD AND EXPECT MOST OF CWA TO RISE ABOVE
FREEZING.  AVN IS ALSO FASTER WITH FIRST WAVE SO ADDED CHC OF
RAIN/SNOW FOR WRN CWA FOR LATE IN DAY.  AFTER THAT...ATTN SHIFTS TO
END OF THE WEEK.  VERY STRONG STORM DEVELOPS AS NRN AND SRN STREAM
MERGE OVER CENTRAL PLAINS.  THIS HAS TWO EFFECTS...FIRST MUCH COLDER
AIR IS BROUGHT INTO THE CWA WITH 850 TEMPS AOB -10C APPEARING ONCE
AGAIN.  SO HAVE DROPPED TEMPS FOR THE END OF THE WEEK WITH ONLY THE
FAR WRN CWA RISING BACK ABOVE FREEZING BY SUN.  OTHER CONCERN IS
PCPN.  EXTENDED MODELS HAVE CONSISTENTLY BEEN TOO FAR SOUTHEAST WITH
EJECTING SW WAVES THIS WINTER.  IN FACT 300 MB PV AND 700 MB TEMPS
WOULD SUGGEST A MORE NW TRACK WITH THIS TRACK.  12Z AVN DOES SHOW A
LITTLE SHIFT TO THE NORTHWEST FROM THE 00Z MRF.  THEREFORE...AM
CONCERNED SNOW WL WRAP BACK INTO CWA...ESP NW IA AND SW MN.  HAVE
KEPT THE WRN THIRD OF CWA BUT HAVE INTRODUCED CHC OF SNOW ALONG AND
E OF I29 FOR THURSDAY AND FRIDAY.  LOOKS DRY BUT COOLER FOR NEXT
WEEKEND.

.FSD...WINTER STORM WARNING TNGT AND MON AM...SE OF LINE FROM
       NEWCASTLE NEB TO WINDOM MN
       SNOW ADVISORY TNGT AND MON AM...SE OF LINE FROM TYNDALL SD
       TO SLAYTON MN.

SCHUMACHER