FXUS63 KDMX 032340
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
540 PM CST THU DEC 3 2009

.AVIATION DISCUSSION 04/00Z

.SHORT TERM /TONIGHT/...
TONIGHT...MID/UPPER LOW PRESSURE CENTER IS MOVING ACROSS NORTHERN IA 
WITH VERY COLD TEMPS ALOFT...AROUND -40 C AT 500 MB PER 12Z RAOBS. 
AT THE SFC A TROUGH IS MOVING NW TO SE ACROSS OUR AREA WITH WINDS 
TURNING TO NW AND PICKING UP BEHIND THE BOUNDARY. EXTENSIVE CLOUD 
SHIELD COVERING THE REGION HAS SOME BREAKS IN IT ALLOWING BRIEF 
SPELLS OF SUNSHINE. THIS SMALL AMOUNT OF INSOLATION COMBINED WITH 
THE VERY COLD TEMPS ALOFT IS APPARENTLY RESULTING IN A SLIVER OF 
INSTABILITY IN THE PROFILES...AS CONVECTIVE SNOW SHOWERS HAVE 
DEVELOPED ACROSS OUR NORTH CENTRAL AREAS IN THE LAST HOUR OR TWO. 
WEBCAMS INDICATE RAPID CHANGES IN VSBY AS THE SNOW SHOWERS PASS 
OVERHEAD. DESPITE RAPID SNOWFALL RATES WITHIN THE SHOWERS...THEY ARE 
SMALL ENOUGH AND MOVING AT A SUFFICIENT SPEED SO AS TO PRECLUDE 
ANYTHING MORE THAN MINOR ACCUMULATIONS.

EXPECT THE REGION OF CONVECTIVE SHSN TO MOVE EAST SOUTHEAST INTO OUR 
EASTERN COUNTIES LATE THIS AFTERNOON BEFORE EXITING OUR AREA...AND 
THE INTENSITY OF THE SHOWERS SHOULD DIMINISH AS SFC HEATING /SUCH AS 
IT IS/ TAPERS OFF AND THE MID LEVEL COLD AIR LOBE MOVES OFF TO THE 
EAST. BEHIND AND OUTSIDE OF THIS AREA OF SHOWERS STRATIFORM LIGHT TO 
OCCASIONALLY MODERATE SNOW IS SEEN UPSTREAM ACROSS NW IA AND 
SOUTHERN MN...AND THIS WILL LINGER OVER OUR NORTHERN AND 
NORTHEASTERN COUNTIES WELL INTO THE EVENING BEFORE FINALLY PULLING 
AWAY. ASIDE FROM THAT ONLY SPORADIC FLURRIES EXPECTED 
OVERNIGHT...AND EVEN THESE WILL TAPER OFF AS THE NIGHT PROGRESSES. 
IN ANY EVEN EXPECT ANY ADDITIONAL ACCUMULATIONS OF SNOW TO BE LIGHT 
AND PRIMARILY WITHIN THE NEXT FEW HOURS. WILL INCLUDE A PRE FIRST 
PERIOD IN THE FORECAST TO ACCOUNT FOR CONVECTIVE/RADAR TRENDS LATE 
THIS AFTERNOON.

OTHER FORECAST CONCERN FOR TONIGHT IS TEMPERATURE...WHICH IS VERY 
TRICKY. FORECAST THINKING HAS BEEN THAT DESPITE GOOD CAA IN THE LOW 
LEVELS WE WOULD REMAIN FAIRLY WELL MIXED OVERNIGHT WITH OVERCAST 
SKIES HELPING TO KEEP TEMPS UP. HOWEVER...THE CLOUD SHIELD APPEARS 
TO BE VERY THIN WITH SUN BREAKS ALREADY APPEARING...AND AFTER SUNSET 
AS THE CUMULIFORM ELEMENTS DISSIPATE IT IS POSSIBLE THAT EVEN MORE 
BREAKS COULD DEVELOP. IF CLEAR PATCHES TO TRAVERSE THE 
AREA...TEMPERATURES COULD DROP SUBSTANTIALLY DESPITE DECENT 
NORTHWEST BREEZES. HAVE LOWERED OUR MIN TEMPS JUST A DEGREE OR TWO 
TO ACCOUNT FOR THIS POSSIBILITY.

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
A SHORTWAVE TROF WILL DROP INTO THE PAC NW FROM BC FRI NIGHT. MODELS 
NOT IN GOOD AGREEMENT OF ITS PLACEMENT OR INTENSITY AS IT EVOLVES 
INTO AN UPPER LOW. ECMWF PLACES THE UPPER LOW ON MONDAY MORNING OFF 
THE OR COAST...WHILE THE GFS PLACES IT OVER NORTHERN CALIFORNIA. GEM 
APPEARS TO BE THE IN-BETWEEN SOLUTION PLACING IT OVER SOUTHERN 
OREGON. AS PART OF THE EVOLUTION OF THIS SYSTEM...SOME OF THE ENERGY 
FROM THE LOW SPLITS FROM THE MAIN SYSTEM AND PUSHES INTO THE UPPER 
MIDWEST SUNDAY AND MONDAY. TIMING OF THIS SHORTWAVE WILL BE 
DIFFICULT DUE TO THE DIVERGING MODEL SOLUTIONS. GFS IS TRENDING 
TOWARDS A DEEPER LOW FROM PREVIOUS RUNS. GFS/GEM CARRYING A HIGHER 
AMPLITUDE TROUGH INTO THE SOUTHERN ROCKIES WHILE THE ECMWF HAS A 
BROADER LOW-AMPLITUDE TROUGH OVER THE CENTRAL US AT THAT TIME. 

FOCUS OF CONCERN IN THE LONG-TERM WILL BE TIMING OF THE SYSTEM. BEST 
GUESS IS THAT SNOW WILL INITIALLY MOVE INTO WESTERN CWA SUN AFTN IN 
ADVANCE OF A WARM FRONT INDICATED BY GFS/ECMWF/GEM GENERALLY FROM 
CENTRAL OK THROUGH MISSOURI. MODELS ARE CONSISTENT IN LIFTING THAT 
LOW NORTHEAST TO NEAR LAKE MICHIGAN MONDAY MORNING. ANOTHER WAVE IS 
EXPECTED TO DEVELOP ALONG THE FRONT RANGE AND BEGIN IMPACTING IOWA 
WEATHER ON TUES MORNING. MODEL DIFFERENCES VARY IN PLACEMENT OF THE 
SURFACE LOW...BUT RESULTING QPF INDICATES PRECIP CHANCES OVER MOST 
OF IOWA TUES AND WED. BY THURSDAY...HIGH PRESSURE BEGINS TO MOVE IN 
INDICATING AN END TO THE PRECIP.

&&

.AVIATION...
04/00Z...MAIN COLD POOL NOW MOVING OVER CENTRAL SECTIONS OF THE 
FORECAST AREA WITH POCKETS OF VIGOROUS SNOW SNOWS/LIGHT SNOW MOVING 
THROUGH. CONDITIONS VARY QUITE A BIT WITH VFR DROPPING QUICKLY TO 
IFR IN MORE INTENSE BURSTS OF SNOW. ANOTHER TROUGH OF LOW PRESSURE 
NEAR THE IA/MN BORDER IS DROPPING SOUTH AND WILL DRIFT OVER THE 
REGION TNT. THERE SHOULD BE A BRIEF PERIOD...1 TO MAYBE 2 HOURS OF 
CLOUD WITH ADDITIONAL SNOW SHOWERS OR LIGHT SNOW PASSING THROUGH 
PRIOR TO 08Z. AFT THAT WILL SEE SOME BREAK IN THE ACTIVITY WITH CIGS 
LIFTING TO VFR CENTRAL AND SOUTH AND SOME VFR CIGS SHOWING UP NORTH 
SECTIONS AFT 17Z. WINDS TO AGAIN MIX TO 15 TO 18KTS FRI AFT 17Z MOST 
AREAS.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...LEE
AVIATION...REV
LONG TERM...HINSBERGER