000
FXUS63 KDMX 272124
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
310 PM CST MON DEC 27 2010
.UPDATE AVIATION DISCUSSION...
.SHORT TERM /TONIGHT/...
LATEST WATER VAPOR IMAGERY AND SPC MESOANALYSIS IDENTIFY A SHORTWAVE
OVER SE NEBRASKA...REFLECTED IN THE SURFACE BY A WEAK PRESSURE
TROUGH...BUT MORE NOTICEABLY A WIND SHIFT FROM SOUTHERLY OUT AHEAD
OF THE WAVE...TO NORTHWESTERLY BEHIND IT. LOW THIN STRATUS ACROSS
IOWA HAS BROKEN APART ACROSS THE EASTERN HALF OF THE STATE THIS
AFTERNOON...BUT REMAINED INTACT ACROSS THE WESTERN HALF WITH CLOUD
BASES IN THE IFR RANGE. THIS STRATUS DECK IS MOSTLY LIKELY DUE TO
THE MOISTURE TRAPPED BELOW THE LOW LEVEL INVERSION.
HOWEVER...A DIFFERENT STRATUS DECK CURRENTLY OVER SE NEBRASKA IS A
RESULT OF THE AFOREMENTIONED SHORTWAVE. SATELLITE FOG PRODUCT
IMAGERY SHOWS THIS STRATUS DECK AS GRADUALLY INCREASED IN COVERAGE
OVER THE LAST FEW HOURS...AND CURRENT THINKING IS THAT LOW CLOUDS
WILL ONCE AGAIN INCREASE IN COVERAGE OVER THE CWA AS THE SHORTWAVE
MOVES EASTWARD OVERNIGHT. THE LAST FEW RUNS OF THE HRRR SEEM TO
SUPPORT THIS VIA THE CLOUD BASE HEIGHT PRODUCT. THEN EXPECT
SUBSIDENCE FROM WEST TO EAST ON THE BACK SIDE OF THE DEPARTING
SHORTWAVE AFTER 06Z TUESDAY NIGHT...WHICH MAY ATTEMPT TO BREAK UP
THE STRATUS. HOWEVER... THE COMBINATION OF RADIATIONAL COOLING ALONG
WITH THE ADVANCEMENT OF A NARROW THERMAL RIDGE AT 850MB WILL ONLY
ACT TO STRENGTHEN THE INVERSION AND ONCE AGAIN TRAP MOISTURE IN THE
LOW LEVELS. THEREFORE HAVE INCLUDED AREAS OF FOG IN THE GRIDS...WITH
POSSIBLY SOME DENSE FOG AT A FEW LOCATIONS.
.LONG TERM /TUESDAY THROUGH MONDAY/...
UPPER PATTERN TUESDAY AND WEDNESDAY BECOMES NEARLY ZONAL FOR ABOUT
24 HOURS ON TUESDAY AND TUESDAY NIGHT. ZONAL FLOW OVER THE CENTRAL
ROCKIES HELPS INDUCE LEE TROUGHING IN EASTERN COLORADO. DEEPENING
TROUGH SHOULD INCREASE PRESSURE GRADIENT ACROSS THE PLAINS WITH
LARGE SURFACE HIGH OVER THE EASTERN CONUS. RESULTING SOUTHERLY FLOW
INTO CENTRAL DEVELOPS TUESDAY NIGHT AND INCREASES ON WEDNESDAY.
WHATEVER LOW CLOUDS OR PATCHY FOG THAT EXISTS IN THE MORNING SHOULD
DIURNALLY DIMINISH WHICH COULD BE TRICKY THIS TIME OF YEAR.
STRONG LOW-LEVEL MOISTURE TRANSPORT KICKS IN ON WEDNESDAY. MODELS
ALL BRING ABOVE 0C DEWPOINTS RAPIDLY NORTH OVER CENTRAL IA BY
WEDNESDAY. MOISTURE ADVECTION PLUS MELTING SNOW PACK SHOULD RESULT
AN ADVECTION ST/FOG EVENT WEDNESDAY THROUGH THURSDAY NIGHT. LACK OF
DEEP MOISTURE WEDNESDAY NIGHT AND WEAK FORCING SHOULD RESULT IN
AREAS OF DRIZZLE. BASED FREEZING DRIZZLE ON HOURLY TEMPERATURE
FORECAST WHICH SUGGEST FREEZING PRECIPITATION SHOULD BE CONFINED TO
NORTH CENTRAL IOWA.
IN THE LONGER RANGE...THURSDAY THROUGH NEXT MONDAY...PRIMARY WEATHER
PROBLEM WILL BE THE UPPER SYSTEM MOVING THROUGH THE CENTRAL PLAINS
AND UPPER MIDWEST LATE THIS WEEK. 00Z ECMWF AND 12Z GFS SOLUTIONS
ARE DIVERGENT ON THEIR SOLUTIONS AND THESE DIFFERENCES ARE
SIGNIFICANT IN CENTRAL IOWA. THE GFS SOLUTION HAS A STRONGER
SURFACE LOW FROM STRONG SHORTWAVE ENERGY MOVING AROUND THE BASE OF
THE UPPER TROUGH ON FRIDAY. THE ECMWF HAS THE STRONGER FORCING TO
THE NORTHWEST AND ITS SURFACE SOLUTION HAS THE SURFACE LOW TO THE
NORTHWEST. COMPROMISED BETWEEN THESE SOLUTIONS IN THE FORECAST.
THERE ARE A FEW THINGS THAT SHOULD HAPPEN REGARDLESS OF THE VARIOUS
DETAILS DISCUSSED ABOVE...THERE SHOULD BE SIGNIFICANT WARMUP MID TO
LATE WEEK IN THE WARM SECTOR. BROAD SOUTHERLY FLOW AND WARM AIR
ADVECTION PREVAIL. THE WARMER AIR SHOULD MELT MUCH OF CENTRAL IOWA
SNOW COVER. MOISTURE FROM THAT SOURCE AND FROM LOW-LEVEL MOISTURE
FROM ADVECTION FROM THE SOUTH SHOULD RESULT IN AN EXTENDED PERIOD OF
LOW CLOUDS...OCCASIONAL DRIZZLE AND FOG. TIMED THE FRONT THROUGH ON
FRIDAY. AS FOR PCPN TYPE...TRANSITION SHOULD BE QUICK FROM RAIN TO
SNOW AS THE COLD AIR MOVES IN QUICKLY. THEREFORE..KEPT PCPN TYPE
RAIN AND SNOW FOR NOW.
A SHOT OF COLDER AIR FOR NEW YEARS DAY WHICH MODIFIES PRETTY FAST ON
DAYS 6 AND 7. DRY FORECAST AFTER FRIDAY NIGHT.
&&
.AVIATION...27/018Z
THIN STRATUS DECK IS GRADUALLY BREAKING UP ACROSS THE STATE BUT
SOUNDINGS AND SHORT TERM HRRR MODEL SHOW CLOUDS AND/OR FOG
DEVELOPING AGAIN LATE THIS EVENING. EXPECT WIDESPREAD IFR
CONDITIONS...AND WOULD NOT BE SURPRISED TO SEE VISIBILITY
TEMPORARILY REDUCED BELOW 1 MILE AT TIMES. CONDITIONS SHOULD THEN
IMPROVE AROUND MID MORNING.
&&
.DMX WATCHES/WARNINGS/ADVISORIES...NONE
&&
$$
SHORT TERM...BEITLICH
LONG TERM...JOHNSON
AVIATION...BEITLICH
