FXUS63 KDMX 090523 AAA
AFDDMX
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE DES MOINES IA
1123 PM CST TUE DEC 8 2009
...UPDATED AVIATION DISCUSSION...
...DANGEROUS BLIZZARD STILL ON TRACK TONIGHT AND WEDNESDAY...
.UPDATE...
SFC LOW DEEPENING OVER NORTHEAST MISSOURI...BUT 00Z MODELS TO FAR
SOUTHEAST WITH ITS POSITION. OTHER THAN THIS SUBTLETY...THE
FORECAST REMAINS ON TRACK. WINDS ARE BACKING TO THE NORTH AND
INCREASING ACROSS WESTERN IOWA...WITH GUSTS NOW OVER 35 MPH.
SNOWFALL RATES OF 1 TO 2 INCHES PER HOUR HAVE RETURNED WITH THE
LATEST BATCH OF SNOW MOVING NORTH THROUGH CENTRAL IOWA. RAPID
REFRESH WRF MODEL SEEMS TO BE ACCURATELY SHOWING THE DEFORMATION
BAND OF SNOW PIVOTING OVER CENTRAL IOWA BETWEEN MIDNIGHT AND 6
AM...WHICH SHOULD KEEP THE HIGHER RATE SNOWFALL FOCUSED THROUGH
THE DSM AND ALO AREAS. BY 6 AM...WINDS WILL BE SOLIDLY INTO
BLIZZARD CRITERIA ALL LOCATIONS...AND EXPECT THE BLIZZARD TO BE
FULL SWING.
&&
.PREV DISCUSSION... /ISSUED 547 PM CST TUE DEC 8 2009/
SHORT TERM /TONIGHT AND WEDNESDAY/...
MAJOR WINTER STORM CONTINUES TO INTENSIFY AS SURFACE LOW EJECTS OUT
OF THE SOUTHERN ROCKIES AND INTO THE SOUTHERN PLAINS/MIDWEST REGION
DURING THE OVERNIGHT HOURS TONIGHT INTO WEDNESDAY. MODELS HAVE BEEN
IN VERY GOOD AGREEMENT WITH TIMING AND PLACEMENT OF THE HEAVIEST
SNOWFALL...SO NO MAJOR CHANGES FROM PREVIOUS FORECAST.
GFS/HRRR HAVE THE HEAVIEST BAND OF SNOW SETTING UP FROM THE
SOUTHWEST TO THE NORTHEAST ACROSS THE CENTRAL PORTION OF THE
CWA...GENERALLY FROM ATLANTIC TO WATERLOO...WHICH INCLUDES THE DES
MOINES AND AMES METRO AREAS. BEST QG FORCING IS BETWEEN 00-06Z WED
ACROSS THE SAME AREA...WITH CROSS SECTIONS OF A DECENT OMEGA
BULLS-EYE WITHIN THE DENDRITIC LAYER. VERY DEEP COLUMN OF MOISTURE
WITH A HUGH AREA OF THETA-E ADVECTION OUT AHEAD OF THE SURFACE
LOW...SO THERE IS PLENTY OF MOISTURE AVAILABLE WITH THIS SYSTEM.
650-600MB FN VECTOR DIVERGENCE AND FRONTOGENESIS MATCHING UP WELL
WITH THE LOCATION OF THE HEAVIEST BAND OF SNOW TONIGHT. CROSS
SECTION OF AGEOSTROPHIC VERTICAL WINDS SHOWING STRONG LIFT ACROSS
THE CWA...THIS COMBINED WITH SOME CONTORTED THETA-ES LINES OVERLAID
WITH EPV. SO MODELS CONTINUE TO SHOW SIGNS OF CSI RELEASE AND THE
POTENTIAL FOR A BRIEF PERIOD OF CONVECTIVE SNOW THROUGH THE EVENING
HOURS. THIS COULD POTENTIALLY LEAD TO AROUND A FOOT AND HALF OF
TOTAL SNOWFALL AMOUNTS IN SOME LOCALIZED AREAS WITHIN THE HEAVIEST
BAND STATED ABOVE. LIFT BEGINS TO DISSIPATE PAST 06Z WED...SO NOT
EXPECTING MUCH DENDRITIC SNOWFALL OVERNIGHT. HOWEVER...THERE WILL BE
THE FINE PARTICULATE SNOWFALL WITH THE STRONGEST WINDS PAST MIDNIGHT
SO THE WINDS WILL NOT HAVE ANY PROBLEMS KEEPING THE SNOW IN
AIRBORNE.
WINDS LOOK TO SHIFT CLOSE TO 03Z WED...WITH THE CAA DEVELOPING AT
850MB. WINDS ATOP THE MIXING LAYER RANGE NEAR 50KTS OVERNIGHT
TONIGHT TOWARDS WEDNESDAY MORNING. 18Z NAM 850MB JET MAX TOPS
60KTS...WHICH IS EVEN HIGHER THAN THE LAST COUPLE OF RUNS. PRESSURE
GRADIENT ACROSS IOWA BY 12Z WED RANGES AROUND 28MB FROM EAST TO
WEST. WITH THIS VERY FINE SNOW WIDESPREAD ACROSS MUCH OF THE
CWA...AS WELL AS SNOW FALLING PAST MIDNIGHT...THERE IS A HIGH
PROBABILITY OF WHITE OUT CONDITIONS OVERNIGHT TONIGHT.
BY WED MORNING...PRIMARY FOCUS WILL SWITCH FROM SNOW ACCUMULATIONS
TO WINDS WITH BLOWING AND DRIFTING. MAX DGZ OMEGA WILL HAVE EXITED
TO THE EAST...HOWEVER DGZ WILL REMAIN FAIRLY DEEP AS IT LOWERS WITH
COLUMN COOLING. UVM WILL BE WEAK...BUT THIS SHOULD KEEP AT LEAST
LIGHT SNOW GOING...ESPECIALLY THROUGH THE MORNING AND IN NERN
SECTIONS. MAIN CONCERN WILL BE AROUND 30 MPH SUSTAINED WINDS AND
WIND GUSTS OVER 40 MPH WHICH WILL LIKELY PRODUCE BLIZZARD AND
NEAR-WHITE OUT CONDITIONS THROUGH MUCH OF THE DAY. TEMPERATURES
SHOULD BE FALLING THROUGH THE DAY TOO WITH WIND CHILLS IN THE SINGLE
DIGITS AND TEENS BELOW ZERO...POSSIBLY TO AROUND 20 BELOW NORTHWEST.
LONG TERM /WED NIGHT THROUGH TUE/...
LITTLE IN THE WAY OF PRECIPITATION EARLY IN THIS PERIOD. IMPRESSIVE
BROAD LONG WAVE TROUGH SETTLES INTO CONUS WITH MAIN CONCERN TEMP
TRENDS THROUGH THE WORK WEEK. AS IS OFTEN THE CASE...MODELS ARE NOT
CAPTURING THE EXTENT OF COLD AIR UPSTREAM...ESPECIALLY GFS. WITH
THIS IN MIND...AS WELL AS FRESH SNOW COVER...HAVE GONE AT OR BELOW
THE COOLEST GUIDANCE THROUGH AT LEAST FRI.
LOOKING FURTHER AHEAD...SEE NO REASON TO DEVIATE FROM ECMWF
DETERMINISTIC AND ENSEMBLE BLEND TODAY BASED ON CONSISTENCY AND HPC
INPUT WHICH POINTS TOWARD PERSISTENT BUT MODERATING LONG WAVE
TROUGH. GEM/GFS/ECMWF ALL ARE GENERATING LIGHT PRECIP AMOUNTS FRI
NIGHT THROUGH SAT NIGHT. HOWEVER EXAMINATION OF FORECAST SOUNDINGS
SHOW ONLY SHALLOW MOISTURE IS PRESENT IN THE COLUMN SUGGESTING THAT
PRECIP IS EITHER FREEZING DRIZZLE OR NOTHING. MID LEVELS ARE
RELATIVELY DRY WITH NO ICE APPEARING TO BE INTRODUCED SO WILL LEAVE
THAT PERIOD DRY FOR NOW RATHER THAN MENTIONING FREEZING DRIZZLE THAT
FAR OUT.
HAVE ALSO ADDED SLIGHT CHANCE OF SNOW MENTION AROUND SUN WITH WEAK
WAVE PASSING THROUGH MO VALLEY AS MOISTURE DEPENDS SUFFICIENTLY.
AFTER THAT COLD TEMPS ARE THE MAIN STORY AS HIGH PRESSURE DROPS INTO
MO RIVER VALLEY AS TROUGH SHIFTS INTO THE GREAT LAKES. WITH SNOW
COVER AND UNSEASONALLY COLD AIRMASS IN PLACE...WILL UNDERCUT
GUIDANCE.
&&
.AVIATION...
09/06Z...NO REAL CHANGES FROM PREVIOUS FCST. AIRPORTS WILL BE
IMPACTED BY MAJOR BLIZZARD DURNIG THIS TAF PERIOD. SNOW...BLOWING
SNOW AND INCREASING WIND SPEEDS WILL CREATE VLIFR CONDITIONS THROUGH
THE MORNING HOURS. EXPECT SUSTAINED WINDS TO INCREASE TO AT LEAST 30
KTS WITH GUSTS OVER 45 KTS BY 09Z AND CONTINUING THROUGH 16-20Z.
SNOWFALL WILL DIMINISH AFT 12Z...BUT BLOWING SNOW WILL CONTINUE TO
KEEP VSBY 1/2SM OR LESS AND PROBABLY CAUSE A VERTICAL VSBY
ISSUE/CEILING. AT SOME POINT AFT 21Z...WINDS WILL DECREASE ENOUGH TO
LET LOFTED CRYSTALS SETTLE BACK TO THE GROUND AND THUS EXPECT THE
VSBY TO INCREASE AND CEILINGS TO RISE INTO IFR OR MVFR CATEGORY.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
BLIZZARD WARNING UNTIL 6 PM CST WEDNESDAY FOR ADAIR-ADAMS-
APPANOOSE-AUDUBON-BLACK HAWK-BOONE-BREMER-BUTLER-CALHOUN-CARROLL-
CASS-CERRO GORDO-CLARKE-CRAWFORD-DALLAS-DAVIS-DECATUR-EMMET-
FRANKLIN-GREENE-GRUNDY-GUTHRIE-HAMILTON-HANCOCK-HARDIN-HUMBOLDT-
JASPER-KOSSUTH-LUCAS-MADISON-MAHASKA-MARION-MARSHALL-MONROE-PALO
ALTO-POCAHONTAS-POLK-POWESHIEK-RINGGOLD-SAC-STORY-TAMA-TAYLOR-
UNION-WAPELLO-WARREN-WAYNE-WEBSTER-WINNEBAGO-WORTH-WRIGHT.
&&
$$
UPDATE...MOYER
SHORT TERM...PODRAZIK
LONG TERM...SMALL
AVIATION...MOYER
