Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 301025
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
425 AM CST FRI JAN 30 2015

.SHORT TERM /TODAY/...
ISSUED AT 421 AM CST FRI JAN 30 2015

THERE IS LITTLE SENSIBLE WEATHER OF CONCERN IN THIS PERIOD. A WEAK
SHORT WAVE CURRENTLY OVER MT...WHICH SEEMS TO BE MORE STRONGLY
REFLECTED IN WATER VAPOR IMAGERY THAN THE MODELS...AND IS ALREADY
PRODUCING WEAK BUT MOISTURE STARVED LIFT THROUGH THE MID MO VALLEY.
MOISTURE IS SO LIMITED THAT EVEN CLOUDS ARE STRUGGLING TO DEVELOP
HOWEVER.

TEMP WISE...NAM...GFS...HI RES ARW/NMM RAW MODEL TEMPS AND NAM MOS
ALL SEEM TOO WARM AT THE MOMENT WHILE HRRR...RAP AND GFS MOS ARE
PERFORMING BETTER.  WHILE THE LATTER MODELS ARE CURRENTLY BETTER
CAPTURING THE COOLER TEMPS...THEY ALSO REBOUND JUST AS MUCH AS THE
ONES WITH A WARM BIAS.  THUS HAVE TEMPERED THE DEGREE OF REBOUND
FROM WHAT WAS DEPICTED YESTERDAY JUST SLIGHTLY.

.LONG TERM /TONIGHT THROUGH SUNDAY NIGHT/...
ISSUED AT 421 AM CST FRI JAN 30 2015

MAJOR LONG TERM FOCUS IS ON SNOWFALL EVENT FOR THIS WEEKEND. AFTER
HIGH RUN TO RUN INCONSISTENCY ALL WEEK...THE 12Z THU AND 00Z FRI
RUNS FINALLY SEEM TO BE CONVERGING TO A SOLUTION. LONG STORY
SHORT...IT IS TIME TO BUY ON ACCUMULATING SNOWFALL IN OUR CWA.

NOW HERE`S THE LONG STORY...AN UPPER LEVEL TROUGH CAN BE PICKED UP
VERY EASILY ON WATER VAPOR IMAGERY COMING ON SHORE NEAR THE BAJA
PENINSULA. THIS TROUGH WILL BE MOVING OVER THE SOUTHWESTERN US
FRIDAY...PULLING MOISTURE NORTHWARD. MEANWHILE...A FAST MOVING
TROUGH WILL BE COMING DOWN THOUGH EASTERN CANADA AND RACE THROUGH
THE DAKOTAS FRIDAY INTO SATURDAY. THE MODELS SEEM TO BE HANDLING
THIS TROUGH WELL NOW...AND WE WILL HAVE TO WATCH TO SEE IF ITS
STRENGTH VARIES WHEN IT COMES ONSHORE AND CAN BE MORE READILY
SAMPLED. THESE TWO SYSTEMS WILL GRADUALLY PHASE OVER THE MIDWEST AND
WILL LEAD TO SNOW ACROSS OUR CWA. PRECIP SHOULD BE ARRIVING IN OUR
SOUTHWESTERN CWA BY 12Z-15Z SAT. FCST SOUNDINGS SHOW A WARM NOSE
WHICH PEAKS AT AROUND 2C AT 2500 FT. SFC TEMPS AT THIS TIME WILL BE
NEAR FREEZING...SO PRECIP WILL LIKELY HAVE A SLEET/SN/RN COMPONENT
TO IT. SFC TEMPS THEN WARM TO THE UPPER 30S...SO PRECIP WILL CHANGE
TO RAIN. NEAR SUNSET...TEMPS SHOULD COOL TO WHERE P-TYPE IS ALL SNOW
FOR THE REMAINDER OF THIS EVENT. P-TYPE SATURDAY WILL BE A BIG
PLAYER IN SNOWFALL TOTALS ACROSS OUR SOUTH. WITH SATURDAY MORNING
LOOKING VERY OVERCAST FROM TOP-DOWN CLOUD INCREASES...HAVE BUMPED
TEMPS UP FROM GUIDANCE. THIS WILL OBVIOUSLY YIELD LOWER SNOW AMOUNTS
THAN COOLER MODEL SOLUTIONS.

THERE IS VERY STRONG FRONTOGENETICAL FORCING THROUGHOUT THIS EVENT.
ALL MODELS NOW MOVING BAND OF STRONG SFC MOISTURE TRANSPORT WITH
CONVERGENCE BASICALLY RIGHT ALONG I-80 TO SERVE AS A FOCUSING
MECHANISM FOR PRECIP. COLD AIR WILL BE ADVECTED NORTHWARD OF THIS
BOUNDARY SATURDAY NIGHT WHICH WILL SUSTAIN THIS BOUNDARY INTO
SUNDAY. TO QUANTIFY LOW LEVEL MOISTURE...THERE IS SOME MODEL
VARIATION. SOURCE REGION FOR SFC DWPTS FOR THIS EVENT IS THE LOWER
TRANS PECOS OF WEST TEXAS. THE NAM INITIALIZES A LITTLE HIGH ON
DWPTS AND THE GFS INITIALIZES LOW. THE EURO IS IN THE
MIDDLE...WITH A LEAN CLOSER TO THE NAM. THESE INITIALIZATION
DIFFERENCES ARE CARRIED THROUGHOUT THE DURATION OF THIS SYSTEM AS
THE NAM IS CLEARLY THE MOST BULLISH ON SNOWFALL AND QPF AND THE
GFS THE MOST BEARISH. REGARDLESS OF MODEL...CROSS SECTIONS CLEARLY
SHOW THERE IS MOISTURE TRANSPORT AND ISENTROPIC LIFT INTO THE
DENDRITIC ZONE...WHICH RUNS VERY DEEP. THE PEAK OF THIS FORCING
LOOKS TO RUN FROM AROUND 03Z TO 09Z SUNDAY NEAR THE BOUNDARY.

FOR SNOWFALL TOTALS...THERE IS QUITE A DIFFERENCE IN THE COBB
OUTPUTS. GENERALLY THE 06Z GFS COBB OUTPUT HAS RELATIVELY LITTLE
SNOW...WHEREAS THE NAM IS COMPARATIVELY OFF THE CHARTS.
DIFFERENCES ATTRIBUTED TO GFS COBB PRODUCING LOW SNOWFALL RATIOS
BECAUSE IT IS NOT GENERATING LIFT. THE NAM...ON THE OTHER HAND HAS
20:1 RATIOS SUNDAY...WHICH IS OBVIOUSLY QUITE HIGH...GIVEN SFC
TEMPS STILL IN THE 30S. AS HINTED AT BEFORE...WENT CLOSER TO THE
EURO...WHICH WAS IN THE MIDDLE...WITH A SLIGHT BUMP TO THE NAM. IN
ALL...SNOW RATIOS AND SFC TEMPS SATURDAY WILL BE A BIG FACTOR IN
SNOWFALL TOTALS AND MAY OSCILLATE SLIGHTLY BETWEEN NOW AND
SATURDAY. GENERALLY TAKING A SOMEWHAT CONSERVATIVE STANCE ATTM
WITH 4 TO 6 INCHES NEAR AND SOUTH OF INTERSTATE 80 AND 1 TO 3
INCHES FROM IA/MN BORDER TO HIGHWAY 30.

AS THIS SYSTEM PUSHES OFF TO THE EAST SUNDAY...IT WILL LEAVE BEHIND
FALLING TEMPERATURES DURING THE DAY SUNDAY. RECENT MODEL RUNS HAVE
ALSO BEEN STRENGTHENING THE LOW LEVEL MIXED LAYER...PLACING 25KT
WINDS AT ITS TOP. THEREFORE...AM EXPECTING STRONG WINDS SUNDAY
GUSTING UPWARDS OF 20 KTS TO 25 KTS...WHICH COULD LEAD TO DRIFTING
OF SNOW.

.LONG TERM /MONDAY THROUGH THURSDAY/...
ISSUED AT 421 AM CST FRI JAN 30 2015

OUR RECENT STRETCH OF MILD WEATHER WILL BE A DISTANT MEMORY BY THE
END OF NEXT WEEK WITH A FAIRLY ACTIVE AND OVERALL COLD PATTERN. THE
CENTRAL CONUS WILL BE INFLUENCED BY A BROAD BUT FAIRLY LOW AMPLITUDE
MEAN TROUGH WITH THE PASSAGE OF MULTIPLE WEAK SHORT WAVES. THIS
KEEPS AN EXTENDED PERIOD OF LOW END CHANCES IN THE FORECAST MON
THROUGH WED.  ANY ACCUMULATIONS LOOK MINOR AT THIS POINT WITH THE
FORCING FAIRLY TRANSIENT.

TEMPERATURES WILL BE COLD AND BELOW NORMAL MON...AND MAY REBOUND
BRIEFLY INTO TUE...BUT WILL LIKELY TURN THE OTHER DIRECTION SHARPLY
FOR THE LATTER HALF OF THE NEXT WORK WEEK.  THE PASSAGE OF THE
SYSTEM AROUND WED WILL END OUR PRECIP CHANCES INTO THU BUT ALSO
AMPLIFY THE PATTERN AND BRING A COLD FRONT FOLLOWED BY STRONG ARCTIC
HIGH PRESSURE. 00Z ECMWF AND GFS BOTH DEPICT IA/MN HIGH PRESSURE
AROUND 1040MB SO HAVE LOWERED TEMPS. LOWS WED NIGHT NOW DIP INTO THE
SINGLE DIGITS ABOVE AND BELOW ZERO WITH HIGHS THU NO BETTER THAN THE
TEENS. WITH WIDESPREAD SNOW COVER WOULD NOT BE SURPRISED TO HAVE
FORECAST TEMPS THROUGHOUT THIS ENTIRE PERIOD END UP TOO WARM...BUT
ARE AT LEAST HEADING IN THE RIGHT DIRECTION.

&&

.AVIATION...30/06Z
ISSUED AT 1130 PM CST THU JAN 29 2015

CIGS SCATTERING ACROSS CNTRL IA AND EXPECT VFR CONDITIONS AT ALL
SITES THOUGH MUCH OF THE PERIOD. KOTM WILL EXPERIENCE MVFR CIGS
INITIALLY THEN BECOME VFR BY 08Z. NORTHWEST WINDS WILL SWITCH TO
THE SOUTH TO SOUTHWEST ON FRIDAY.


&&

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

$$

SHORT TERM...SMALL
LONG TERM...KOTENBERG/SMALL
AVIATION...DONAVON


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