FXUS63 KDMX 080544
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1141 PM CST MON DEC 7 2009
.SHORT TERM /TONIGHT/...
STRONG SHORTWAVE ROUNDING THE BASE OF THE LARGE POSITIVE TILT
LONGWAVE TROUGH OVER THE WRN CONUS WILL MOVE OVER THE SRN ROCKIES
TONIGHT. PLUME OF MOISTURE ALREADY BEING ADVECTED NORTHWARD AHEAD OF
THIS SYSTEM ACROSS THE CENTRAL PLAINS. THETA-E ADVECTION AND
ISENTROPIC LIFT MOVE NORTH OVER IOWA...BUT OVERALL FORCING IS WEAK
THROUGH 12Z. MODEL SOUNDINGS QUICKLY SATURATE SW TO NE AFTER
MIDNIGHT...SO EXPECT LIGHT SNOW TO BEGIN FALLING DURING THIS TIME
FRAME. ACCUMULATIONS BY MORNING WILL ONLY BE AN INCH OR SO SOUTHWEST
TO A TRACE NORTHEAST.
.LONG TERM /TUESDAY THROUGH MONDAY/...
FORECAST REMAINS FOCUSED ON MAJOR WINTER STORM TO IMPACT THE STATE
BEGINNING LATE TONIGHT AND LASTING THROUGH WEDNESDAY. MODELS HAVE
BEEN IN VERY GOOD AGREEMENT WITH TIMING AND PLACEMENT OF SNOW
DEVELOPMENT THROUGHOUT THE DAY TUESDAY INTO TUESDAY NIGHT.
THEN...WINDS INCREASE LATE TUESDAY NIGHT AND MUCH OF THE DAY
WEDNESDAY.
VERY GOOD ISENTROPIC LIFT AHEAD OF THE LOW WITH ABUNDANT MOISTURE
AVAILABLE. BUFKIT SOUNDINGS/CROSS SECTION DEPICTING A LONG TIME
PERIOD OF WEAK OMEGA WITH RH VALUES ABOVE 90 PERCENT FOR MOST OF
TUESDAY AND TUESDAY EVENING...BUT NOT AN IMPRESSIVE BULLS-EYE.
GFS/NAM DEPICTING SOME WEAK ELEVATED CAPE BY 00Z WEDNESDAY DURING
THE HEIGHT OF THE OMEGA FIELD LATE TUESDAY AFTERNOON. CROSS
SECTIONS OF SATURATED THETA-E AND EPV SHOWING WEAK SIGNAL OF CSI
RELEASE DURING THE SAME TIME FRAME...SO CANNOT RULE OUT THE
POTENTIAL FOR SOME ISOLATED CONVECTIVE ELEMENTS LATE TUESDAY
AFTERNOON/EARLY TUESDAY NIGHT. EVEN WITHOUT THIS CONVECTIVE
ELEMENT...THE 18-24 HOURS OF SNOWFALL WILL LEAD TO 8-12 INCHES BY
LATE TUESDAY NIGHT. SNOWFALL RATIOS RANGE FROM 10-1 TO 15-1
...WITH THE LOWEST RATIO ACROSS SOUTHERN IOWA. HOWEVER THE
GREATEST QPF IS ACROSS SOUTHERN IOWA...SO A LARGE SWATH OF THE
HEAVY SNOW IS ANTICIPATED ACROSS MUCH OF THE CWA. THE LATEST ECMWF
HAS A CLOSED 500MB LOW PRESSURE BY 00Z WED AND CONTINUES TO DEEPEN
TOWARD WEDNESDAY MORNING...WITH A SURFACE LOW PROJECTED DOWN BELOW
980MB. BUFR SOUNDINGS SHOWING MIXING LAYER TO AROUND 900MB...WITH
WINDS INCREASING OVERNIGHT INTO WEDNESDAY. 40-45KTS WINDS AT THE
TOP OF THE MIXING LAYER SO POTENTIALLY MIXING DOWN TO 50MPH WINDS
OR GREATER BY WEDNESDAY MORNING. PRESSURE GRADIENT DIFFERENCE IS
ABOUT 25 TO 28 MB BY WEDNESDAY MORNING FROM EAST TO WEST ACROSS
THE STATE...SO WITH THE AMOUNT AND TYPE OF SNOW ANTICIPATED DURING
THE DAY TUESDAY INTO TUESDAY NIGHT...THE POTENTIAL FOR BLIZZARD
CONDITIONS EXIST LATE TUESDAY NIGHT INTO WEDNESDAY.
HIGH PRESSURE DOMINATES THE EXTENDED PERIOD...WITH VERY COLD AIR
MASS IN PLACE BY THURSDAY AND FRIDAY. SOME BELOW ZERO TEMPERATURES
ARE EXPECTED BY THURSDAY MORNING AND MAYBE EVEN FRIDAY MORNING
WITH THE FRESH SNOW PACK. NO MAJOR CHANGES TO THE EXTENDED.
&&
.AVIATION...
08/06Z...FIRST S/WV WILL LIFT ACRS THE STATE THRU THE PREDAWN HRS
WITH A BROAD AREA OF IFR CONDS. CIGS WILL LWR TO IFR ACROSS THE
AREA TUE WITH A BREAK IN THE SNOW FOR SVRL HRS IN THE MNG. MAIN SYS
WILL PRODUCE LIFR STATEWIDE FOR THE MID AFTN AND EVE HRS TUE. WINDS
WILL INCREASE LATE IN THE TAF PD AND MAINTAIN VSBYS BLO 1/2SM THRU
THE NGT IN -SN AND BLSN.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FROM 6 AM TUESDAY TO NOON 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.
BLIZZARD WATCH FROM TUESDAY EVENING THROUGH WEDNESDAY AFTERNOON
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.
&&
$$
SHORT TERM...MOYER
LONG TERM...PODRAZIK
AVIATION...MS DEC 09
