FXUS63 KDMX 230537 AAB
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1137 PM CST TUE DEC 22 2009
...UPDATED AVIATION DISCUSSION...
.SHORT TERM /TONIGHT/...
TONIGHT LOOKS TO BE A HIGH POP...VERY LOW QPF EVENT. FREEZING PRECIP
LOOKS TO BE THE DOMINANT PRECIP TYPE...THUS HAVE EMPHASIZED THAT IN
THE GRIDS. FORCING IS NOT IMPRESSIVE OVERNIGHT WHICH SUPPORTS THE
IDEA OF VERY LOW QPF. QUESTION IS HOW FAR SOUTH THE FREEZING PRECIP
WILL GO. THOUGHTS RIGHT NOW ARE THAT IT WOULD EXTEND SOUTH...THROUGH
THE DES MOINES METRO AREA AND POSSIBLY AS FAR SOUTH AS U.S. HWY 34.
MAINLY LIQUID PRECIP IS EXPECTED SOUTH OF THAT HIGHWAY. WILL EXTEND
THE FZRA ADVISORY FARTHER SOUTH THAN THE EXISTING FZRA ADVISORY
AREA...BUT TO START LATER THIS EVENING.
.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
THE STORY OBVIOUSLY IS THE POWERFUL STORM SET TO AFFECT OUR AREA FOR
THE NEXT SEVERAL DAYS. UNFORTUNATELY EVEN AS THE STORM DRAWS CLOSER
THE PROGNOSTIC MODELS CONTINUE TO DIVERGE IN SOME ASPECTS OF THEIR
SOLUTIONS WHICH IS VERY ATYPICAL. THIS IS DUE TO THE PRESENCE OF
ANOTHER STRONG MID/UPPER TROUGH CURRENTLY OVER WESTERN
CANADA...WHICH WILL SWING SOUTHEASTWARD INTO MONTANA BY TOMORROW
NIGHT AND INTERACT WITH THE SOUTHERN STREAM CYCLONE WHICH WILL BE
ROUGHLY OVER THE TEXAS PANHANDLE BY THAT TIME. THE MODELS ARE HAVING
A LOT OF DIFFICULTY RESOLVING THE INTERACTION BETWEEN THESE TWO
SYSTEMS...WHICH IS PRODUCING VERY ERRATIC RESULTS IN THE BEHAVIOR OF
NOT ONLY THE MID-LEVEL LOW CENTERS BUT THE SURFACE LOWS AS
WELL...WHICH OF COURSE IN TURN EFFECTS THERMAL FIELDS...PRECIP
TYPES...TEMPERATURE/MOISTURE ADVECTION...QPFS...ETC. ETC. ETC.
THE FEW THINGS THAT REMAIN RELATIVELY CERTAIN ARE THAT WE WILL HAVE
A VERY LONG DURATION OF PRECIPITATION...THAT STORM TOTAL QPFS WILL
BE QUITE HIGH...AND THAT WINDS WILL ALSO BE QUITE HIGH FOR PERIODS
AS THE SURFACE LOW STALLS NEARBY. IN TERMS OF PRECIP TYPE THE
GENERAL TREND OF THINKING WARMER CONTINUES...WITH GOOD INDICATIONS
THAT THE MAJORITY OF THE PRECIP IN ABOUT OUR SOUTHEASTERN THIRD WILL
BE RAIN FROM WEDNESDAY AFTERNOON THROUGH THURSDAY NIGHT...WITH MIXED
PRECIP NORTHWEST OF THESE AREAS AND MOSTLY SNOW IN OUR FAR
NORTHWESTERN COUNTIES AROUND ESTHERVILLE. COMPLICATING THIS FURTHER
HOWEVER ARE STRENGTHENING INDICATIONS THAT MID-LEVEL DRY AIR POCKETS
WITHIN THE SYSTEM WILL BE AFFECTED BY THE INTERACTION OF THE TWO
GYRES AND RESULT IN PERIODS OF NO ICE CRYSTAL INTRODUCTION ACROSS
THE AREA. THESE POCKETS ARE VIRTUALLY IMPOSSIBLE TO PREDICT AT THIS
TIME...AND COULD THROW A REAL WRENCH INTO P-TYPE/QPF FORECASTS AT
TIMES. HAVE TRIED TO BROAD-BRUSH THE FORECAST AS MUCH AS POSSIBLE
WITH RESPECT TO P-TYPE WITH SUCH A HIGH DEGREE OF UNCERTAINTY.
EARLY IN THE STORM THE FREEZING DRIZZLE/LIGHT FREEZING RAIN
SPREADING ACROSS THE AREA LATER TONIGHT WILL CONTINUE INTO TOMORROW
MORNING BEFORE MORE MODERATE PRECIPITATION SPREADS NORTHWARD FROM
MISSOURI IN ASSOCIATION WITH THE LEADING WAVE OF WARM THETA-E
ADVECTION AHEAD OF THE CYCLONE. ONCE THE WARM ADVECTION TAKES HOLD
WE EXPECT THE PRECIP TO CHANGE TO RAIN ACROSS THE SOUTH...AND
POSSIBLY ALONG THE I-80 CORRIDOR BY AROUND MIDDAY WEDNESDAY...BUT
REMAIN FZRA OR WINTRY MIX FURTHER NORTH WITH UNCERTAIN AMOUNTS OF
ANY GIVEN PRECIP TYPE. THIS WILL MAKE FOR HAZARDOUS DRIVING
CONDITIONS IN ANY EVENT...HOWEVER IT IS VERY UNCERTAIN AS TO
WHETHER...WHERE...OR WHEN WINTER STORM WARNING CRITERIA OF 0.25 INCH
FZRA OR 0.5 INCH SLEET MAY BE MET TOMORROW. SNOW WILL NOT BE MUCH OF
A FACTOR EVEN IN THE NORTHWEST UNTIL TOMORROW NIGHT. WITH A VERY
COMPLICATED FORECAST AND HAZARD SCENARIO HAVE OPTED TO FOCUS ON THE
PRIMARY INITIAL THREAT WHICH IS LIGHT ICE ACCUMULATIONS ACROSS ALL
BUT THE FAR SOUTH/SOUTHEAST. WITH THIS IN MIND HAVE EXTENDED THE
FREEZING RAIN ADVISORY ACROSS THE NORTH UNTIL 18Z WEDNESDAY...ISSUED
A NEW ONE FURTHER SOUTH FROM 03Z TONIGHT UNTIL 18Z WEDNESDAY...AND
DELAYED THE WINTER STORM WATCH EFFECTIVE TIME UNTIL 18Z DUE TO A
LATER EXPECTED ONSET OF QPFS/ACCUMULATIONS NEAR WARNING CRITERIA.
ANOTHER THING ON OUR MINDS...AND THERE ARE MANY...IS THAT WITH THE
INCREASING INTERACTIONS BETWEEN THE TWO MID/UPPER LOWS OVER THE
MIDWEST LATER IN THE WEEK THERE IS DEFINITELY A POTENTIAL FOR PRECIP
TO LINGER ACROSS OUR AREA FOR LONGER THAN PREVIOUSLY THOUGHT...WITH
THE 12Z ECMWF HAVING THE CONSOLIDATED CYCLONE OVER NORTHEASTERN IOWA
AT 12Z ON SUNDAY. HAVE INCREASED POPS EARLY IN THE WEEKEND TO
ACCOUNT FOR THIS...BUT HESITATE TO EXTEND THEM INTO SUNDAY WITH THIS
MUCH UNCERTAINTY.
&&
.HYDROLOGY...
LOTS OF UNCERTAINTY IN HOW THINGS WILL EVOLVE HYDRO-WISE WITH THIS
EVENT. NEVERTHELESS...AT LEAST MINOR RIVER FLOODING IS POSSIBLE
ESPECIALLY IN THE SOUTHERN CWA. PRECIP TYPE WILL BE A MAJOR DRIVING
FORCE BEHIND THE HYDROLOGIC RESPONSE. THE WFO MPX SNOW MELT
PREDICTOR MODEL INDICATES THAT THE AMOUNT OF SNOW PACK WE WOULD LOSE
RANGES FROM AROUND 1 1/2 INCHES IN THE CENTRAL CWA...TO AROUND 3 1/3
INCHES IN THE SOUTHEASTERN CWA. THIS LOSS WOULD BE DUE MAINLY TO
TEMPERATURES ONLY. RAINFALL WOULD HELP FACILITATE A LARGER LOSS OF
THE SNOW PACK...THUS PRECIP TYPE WILL BE A LARGE FACTOR. WX MODELS
ARE PROGGING A RELATIVE DRY SLOT INTO OUR CWA...WHICH WILL HELP
REDUCE QPF AMOUNTS. STILL...THOUGH...THERE WOULD BE ENOUGH LIQUID
PRECIP TO RESULT IN POTENTIALLY SIGNIFICANT RUNOFF AND RISES ON
STREAMS...NOT TO MENTION IN AREAS...SUCH AS CITIES...WHOSE NORMAL
DRAINAGE SYSTEMS WOULD HAVE LOWER THAN NORMAL CONVEYANCE DUE TO SNOW
AND ICE OBSTRUCTED INTAKES.
ANOTHER CONCERN IS BREAK UP ICE JAMS ON AREA RIVERS. THE STEFAN ICE
THICKNESS EQUATION INDICATES THAT RIVER ICE DEPTHS RANGE FROM APPROX
2 INCHES IN OUR SOUTHEASTERN CWA...TO APPROX 6 INCHES IN OUR CENTRAL
CWA. SINCE THE RIVERS ONLY NEED TO RISE APPROX 1 1/2 TO 3 TIMES THE
THICKNESS OF THE ICE TO START THE BREAK-UP PROCESS...BREAKING UP THE
ICE WILL BE LIKELY ESPECIALLY AS ONE MOVES SOUTHWARD...DUE TO RUNOFF
FROM ANY RAINFALL AND SNOW MELT. SINCE THE RIVER ICE IS RELATIVELY
THIN...THOUGH...WE WOULD EXPECT ONLY MINOR...LOCALIZED PROBLEMS DUE
TO ICE JAMS.
&&
.AVIATION...
23/06Z...NOT MUCH CHANGE TO PREVIOUS THINKING AS WIDESPREAD IFR/LIFR
CIGS WILL PERSIST FOR ENTIRE FORECAST PERIOD. VSBYS WL ALSO VARY
FROM MVFR TO IFR IN FOG AND LIGHT PRECIPITATION. ICING WILL BE
POSSIBLE AT ALL TERMINALS OVERNIGHT WITH SOME IMPROVEMENT AT KOTM
AND KDSM BY MIDDAY ON WEDNESDAY AS TEMPERATURES CLIMB ABOVE
FREEZING. SFC WINDS WILL REMAIN EASTERLY AND INCREASE THROUGH
WEDNESDAY AS LOW PRESSURE DEEPENS IN THE SOUTHERN PLAINS.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...
FREEZING RAIN ADVISORY FROM 9 PM THIS EVENING TO NOON CST
WEDNESDAY FOR ADAIR-ADAMS-CASS-CLARKE-DALLAS-JASPER-LUCAS-
MADISON-MAHASKA-MARION-POLK-POWESHIEK-UNION-WARREN.
WINTER STORM WATCH FROM WEDNESDAY AFTERNOON 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.
FREEZING RAIN ADVISORY UNTIL NOON CST WEDNESDAY FOR AUDUBON-
BLACK HAWK-BOONE-BREMER-BUTLER-CALHOUN-CARROLL-CERRO GORDO-
CRAWFORD-EMMET-FRANKLIN-GREENE-GRUNDY-GUTHRIE-HAMILTON-HANCOCK-
HARDIN-HUMBOLDT-KOSSUTH-MARSHALL-PALO ALTO-POCAHONTAS-SAC-STORY-
TAMA-WEBSTER-WINNEBAGO-WORTH-WRIGHT.
&&
$$
SHORT TERM...ZOGG
AVIATION...COGIL
LONG TERM...LEE
HYDROLOGY...ZOGG
