FXUS63 KDMX 221150
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
550 AM CST TUE DEC 22 2009
.UPDATE...
12Z AVIATION DISCUSSION ADDED. FORECAST ALREADY REFLECTS THIS
THINKING...BUT RADAR IS RUNNING IN MORE SENSITIVE VCP 31 AND WEAK
REFLECTIVITIES ARE VERY GRADUALLY EXPANDING OUTWARD FROM RDA.
FREEZING DRIZZLE MAY BEGIN DEVELOPING SHORTLY.
&&
.SHORT TERM /TODAY AND TONIGHT/...
THE RELATIVE CALM BEFORE THE STORM IN THIS PERIOD...ALTHOUGH THERE
MAY STILL BE ADVERSE SENSIBLE WX IMPACTS. SHORT WAVE WHICH PRODUCED
NRN IA SNOW OVERNIGHT EXTENDS FROM MN ACROSS NERN IA INTO WRN IL.
PRECIPITATION HAS ALL BUT ENDED FOR US AS MORE SIGNIFICANT SNOWS AND
FORCING ARE NOW MOVING INTO IL SO ADVY WILL BE CANCELED EARLY.
AS THIS UVM AND MID LEVEL MOISTURE MOVE AWAY...IT SETS THE STAGE FOR
WHAT MAY BE AN EXTENDED PERIOD OF FREEZING DRIZZLE/DRIZZLE
POTENTIAL...OR POSSIBLY LIGHT MEASURABLE PRECIP DURING TIMES THAT
THE MOISTURE DEPTH GETS DEEP ENOUGH. FOR THE MOST PART ALL MODELS
AGREE THAT THE LOW LEVEL MOISTURE IS 1KM OR LESS...BUT DOES EXPAND
UP TO 1.5KM AT TIMES BY LATER TONIGHT. THIS SHALLOW MOISTURE DEPTH
SHOULD PRECLUDE MUCH IF ANY ICE INTRODUCTION SO HAVE REMOVED SNOW
FROM THE FORECAST THROUGH 12Z WED. BETTER LIKELIHOOD OF MEASURABLE
PRECIP IS TONIGHT...WITH UVM AND MOISTURE TRANSPORT INCREASING ALONG
275/280K ISENT SFCS...ESPECIALLY THE NAM. ANOTHER ADVY MAY BE
NEEDED LATER TODAY IF ICE ACCUMS GET BAD ENOUGH...BUT DO NOT WANT TO
MUDDY THE WATER WITH ADDITIONAL HEADLINES RIGHT NOW AS GREATEST
THREAT AND IMPACTS REMAIN IN WATCH PERIOD.
USING A BLEND OF VARIOUS FORECAST SOUNDINGS YIELDS TEMPS CLOSE TO
THE GFS MOS FOR HIGHS. THIS SAME DATA HAS STEADY OR SLOWLY RISING
READINGS TONIGHT SO MINS ARE SEVERAL DEGREES ABOVE THE WARMEST
GUIDANCE.
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
FORECAST FOCUSED ON MAJOR WINTER STORM TO AFFECT THE STATE WEDNESDAY
THROUGH FRIDAY...LEADING TO POTENTIAL SIGNIFICANT TRAVEL IMPACTS FOR
THE UPCOMING HOLIDAY. MODELS REMAIN IN FAIRLY GOOD AGREEMENT WITH
TIMING OF THE PRECIP AND TRACK OF LOW PRESSURE SYSTEM...BUT THERE
ARE STILL SOME DISCREPANCIES WITH THE THERMAL PROFILES WEDNESDAY
NIGHT INTO THURSDAY. LEANED TOWARD THE SLIGHTLY WARMER NAM WITH THE
STRONGER SIGNATURE OF WAA WEDNESDAY NIGHT/THURSDAY...WHICH LINES UP
FAIRLY WELL WITH PLACING THE SURFACE LOW SLIGHTLY FURTHER NORTHWEST
THAN THE PREVIOUS MODEL RUNS.
WEDNESDAY INTO THURSDAY...BUFR SOUNDINGS DEPICTING A WARM LAYER
ALOFT GENERALLY BETWEEN 900-750MB WEDNESDAY AFTERNOON AND EVENING
OVER MUCH OF THE CWA. ACROSS THE SOUTH...850MB TEMPS WARM TO NEAR
+7C BY WED AFTERNOON AND THE LOWER LEVELS HOVER AROUND FREEZING.
THINKING WITH LATENT HEAT RELEASE AND VERY SHALLOW LAYER OF COLD
AIR NEAR THE SURFACE...EXPECTING MAINLY RAIN ACROSS THE SOUTH WED
INTO THURSDAY. FURTHER NORTH...GENERALLY ALONG AND NORTH OF THE
INTERSTATE 80 CORRIDOR TO THE MN/IA BORDER WILL SEE WINTRY MIX
DURING THE PERIOD. THERE ARE TIMES WHEN SOUNDINGS DEPICT THE LACK
OF ICE INTRODUCTION...BUT REGARDLESS MAY LEAD TO A PROLONG PERIOD
OF FZRA/FZDZ/SNOW. KEPT MENTION OF SLEET AND SNOW WEDNESDAY NIGHT
INTO THURSDAY WITH ALO/MCW DIPPING BELOW THE 0C LINE AT TIMES FOR
THE ENTIRE COLUMN. WENT WITH A TENTH OR TWO OF ICE ACCRUAL WED
INTO THU. CAA FINALLY PUSHES INTO THE STATE THURSDAY NIGHT...SO
HAVE MENTION OF ALL SNOW FOR THE CWA WITH THERMAL PROFILES QUICKLY
COOLING TO BELOW FREEZING. SNOWFALL AMOUNTS LOOK TO BE SIGNIFICANT
ACROSS THE NORTH WHERE IT WILL BE ALL SNOW FOR A LONGER PERIOD.
MAX OMEGA AND ABUNDANT MOISTURE ACROSS THE NORTH LEADS TO THE
POTENTIAL FOR 6+ INCHES TO NEARLY A FOOT OVER THE FAR NORTHWEST.
PRESSURE GRADIENT REMAINS FAIRLY TIGHT ON THE BACKSIDE OF THE
SURFACE...BUT WITH THE 06Z GFS AND 00Z ECMWF PROJECTING THE SURFACE
LOW TO STALL ACROSS THE CENTRAL PORTION OF THE STATE FOR MUCH OF
FRIDAY.
&&
.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...
22/12Z...PRIMARY CONCERN REMAINS LOW CLOUDS...VSBYS AND FREEZING
DRIZZLE POTENTIAL. LIFR/IFR CIGS AND MVFR/IFR VSBYS ARE WIDESPREAD
THIS MORNING AND EXPECTED TO CONTINUE THROUGH ENTIRE VALID TAF
PERIOD. RECENT EXPANSION OF WEAK ECHOES NEAR THE RADAR SUGGESTS
FREEZING DRIZZLE MAY DEVELOP SHORTLY AND PERSIST WELL INTO TONIGHT
WITH MINOR ICING LIKELY. SFC TEMPS MAY JUST WARM ENOUGH TO SWITCH
IT TO DRIZZLE NEAR KOTM/KDSM 21Z/09Z RESPECTIVELY.
&&
.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.
&&
$$
SHORT TERM/AVIATION...SMALL
LONG TERM...PODRAZIK
