Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 291726
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1126 AM CST THU JAN 29 2015

...UPDATED AVIATION DISCUSSION...

.SHORT TERM /TODAY/...
ISSUED AT 354 AM CST THU JAN 29 2015

PRIMARY QUESTIONS THIS MORNING WILL BE WINDS AND LIGHT PRECIP
TRENDS. IR SATELLITE IMAGERY NICELY SHOWS COMPACT SHORT WAVE PASSING
THROUGH IA AT THE MOMENT WHICH HAS BEEN PRODUCING SOME VERY LIGHT
SNOW ALOFT AND POSSIBLY CONTRIBUTING TO SEEDER/FEEDER LIGHT DRIZZLE
AT THE SURFACE.  FARTHER UPSTREAM WATER VAPOR IMAGERY INDICATES
ANOTHER WAVE FROM THE DAKOTAS INTO MN WITH MORE LIMITED MOISTURE.
ANY FORCING WITH THAT FEATURE APPEARS TO BE WEAK AND POSSIBLY JUST
PHASED WITH THE IA VORT CENTER WITH MID CLOUDS INTO MN.  ANY ICE
INTRODUCTION AND LIGHT SNOW/FLURRY POTENTIAL WITH THE CLOUD
ENHANCEMENT IS FLEETING AND NOT WORTH OF MENTION THIS MORNING.
PRECIP WOULD NEED TO COME FROM THE AT LEAST 1KM THICK STRATUS LAYER.
MODEL CROSS SECTIONS DO NOT SHOW MUCH IN THE WAY OF LIFT HOWEVER SO
MEASURABLE POTENTIAL IS NIL.  WILL JUST MENTION PATCHY LIGHT DRIZZLE
OR FREEZING DRIZZLE NORTH HALF DEPENDING ON SURFACE TEMPS.  PRECIP
SHOULD BE VERY LIGHT...IF AND WHEN IT OCCURS...SO DO NOT EXPECT ANY
TRAVEL PROBLEMS.

WIND GUSTS REACHED 40KTS OR HIGHER OVER WRN IA JUST AFTER
MIDNIGHT...SUGGESTING SURFACE GUSTS WERE TAPPING WINDS TO TOP OF
MIXED LAYER /1KM/.  GUSTS AND SUSTAINED SPEEDS HAVE DIMINISHED
SOMEWHAT HOWEVER WITH ONLY A HANDFUL OF SITES NOW AT EITHER ADVISORY
CRITERIA. 500M WINDS CONTINUE TO GRADUALLY DIMINISH THIS
MORNING...AS SHOULD SUSTAINED SURFACE WINDS WITH ISALLOBARIC MAX
EXITING THE STATE...SO ADVISORY MAY BE MARGINAL AT THIS POINT BUT
PUBLIC SENSIBLE WEATHER PERCEPTION FAVORS KEEPING IT THROUGH
EXPIRATION TIME.

09Z SURFACE ANALYSIS SHOWS FAIRLY HOMOGENEOUS AIRMASS FROM IA
UPSTREAM SO TEMPS ARE NOT EXPECTED TO DROP QUICKLY.  SHOULD SEE A
VERY GRADUAL DROP INTO THE MORNING WITH ONLY A MINOR REBOUND BY
AFTERNOON.  CONFIDENCE IN CLOUD TRENDS IS NOT GREAT...BUT MODEL
CONSENSUS SUGGESTS CLOUDS MAY AT LEAST PARTIALLY BREAK UP BY
AFTERNOON.

.LONG TERM /TONIGHT THROUGH WEDNESDAY/...
ISSUED AT 354 AM CST THU JAN 29 2015

WITH HIGH PRESSURE ROLLING THROUGH THE REGION TO START THE LONG TERM
FCST PERIOD OFF...FRIDAY WILL BE QUIET. SUNSHINE PLUS 850MB TEMPS
OF -2C TO -4C SHOULD HELP THE CWA STAY ABOVE NORMAL WITH TEMPS IN
THE 40S AND 30S.

THE MAIN FORECAST HIGHLIGHT FOR THE NEXT WEEK IS THE SATURDAY INTO
SUNDAY MORNING ACCUMULATING SNOWFALL POTENTIAL. 06Z THU WATER VAPOR
IMAGERY PICKING UP AN UPPER LOW WHICH IS SLATED TO COME ONSHORE NEAR
THE BAJA PENINSULA THURSDAY INTO FRIDAY. MODELS TRYING TO PHASE
AN INVERTED TROUGH...WHICH WILL BE POOLING MOISTURE FROM THE GULF
OF MEXICO...WITH THIS UPPER LOW. ALL MODELS PRODUCE A BAND OF
MOISTURE CONVERGENCE ALONG A BOUNDARY TO SERVE AS A FOCUSING
MECHANISM FOR PRECIP. THE CHALLENGING PART OF THIS FCST HAS BEEN
PEGGING DOWN THE TRACK OF THIS SYSTEM. THE MODELS HAVE BEEN
BOUNCING ALL OVER THE PLACE AND RUN TO RUN CONSISTENCY HAS BEEN
POOR AT BEST. THE NAM IS THE MOST EXCITED WITH THIS SYSTEM AS IT
IS THE FURTHEST NORTH AND MOST AMPLIFIED. THE GFS HAS BEEN THE
LEAST CONSISTENT MODEL OVER THE PAST FEW DAYS AND IS THE FURTHEST
SOUTH WITH THIS SYSTEM. THE EURO LIES SOMEWHAT IN THE MIDDLE AND
HAS BEEN THE MOST CONSISTENT WITH THIS SYSTEM...SO WILL INITIALLY
LEAN TOWARDS THE EURO AND STEER AWAY FROM THE GFS. WITH MORE
RECENT OUTPUTS...A STRONG 1040MB ARCTIC HIGH HAS BEEN INTRODUCED
OVER THE DAKOTAS. THIS HIGH WILL BE WORTH KEEPING AN EYE ON AS IT
MAY HAMMER PRECIP CHANCES...ESPECIALLY ACROSS OUR NORTH...IF IT
IS PLACED FURTHER SOUTH IN FUTURE MODEL RUNS.

SOUNDINGS EASILY SATURATE ALOFT BY 18Z SATURDAY AND CONTINUE TO
SATURATE FROM TOP DOWN AS SYSTEM APPROACHES. BY 00Z SUN...SOUTHERN
IOWA BEGINS TO SATURATE NEAR THE SFC AS DWPT DEPRESSIONS TIGHTEN.
MAX TEMPS SATURDAY TRENDING WARMER THE PAST FEW DAYS AND HAVE BEEN
BUMPED TO THE UPPER 30S/LOW 40S ALONG AND SOUTH OF INTERSTATE 80.
THIS WILL OBVIOUSLY KICK P-TYPE TO RAIN DURING PEAK HEATING HOURS.
SOUNDING PROFILE ALOFT DROPS TO BELOW FREEZING BY 00Z SUN. THUS
WHEN THE SUN GOES DOWN...PTYPE SHOULD CHANGE TO SNOW FAIRLY
QUICKLY GIVEN THE EXPECTED CAA. IN ALL HAVE ROUGHLY 1 TO 2 INCHES
OF SNOW GOING ACROSS OUR SOUTH. GIVEN THE RECENT MODEL
CONSISTENCY...CONFIDENCE IS NOT EXTREMELY HIGH AT THIS POINT SO
THERE MAY BE SOME VARIATION IN THIS SNOWFALL FORECAST OVER THE
NEXT FEW DAYS.

AT THIS POINT...THE BIGGEST DRIVER OF ADVERSE IMPACTS WILL BE THE
WINDS. TODAY`S MODEL RUNS HAVE SHOWN CONSISTENCY WITH DEVELOPING A
MIXED LAYER WHICH FEATURES 15KT TO 20KT WINDS AT ITS TOP. IT
APPEARS THESE WINDS SHOULD EASILY MIX DOWN TO THE SFC...SO HAVE
BUMPED UP WINDS FROM PVS FCST AND WILL NEED TO CONTINUE TO MONITOR
THIS TREND FOR STRONGER WINDS AS THEY WOULD LEAD TO GREATER
REDUCED VISIBILITIES DURING PERIODS OF SNOW.

BEHIND THIS SYSTEM FOR SUNDAY INTO THE MIDDLE OF NEXT WEEK...MODELS
HAVE BEEN ALL OVER THE PLACE FOR TEMPERATURES. THE ONLY CONSISTENT
TREND HAS BEEN A LOWERING OF TEMPERATURES FOR SUNDAY. WITH CLOUDS
EXPECTED AT LEAST THROUGHOUT THE MORNING...THIS SEEMS
REASONABLE...AND HAVE THUS LOWERED TEMPS FROM GUIDANCE A FEW
DEGREES. FOR TUESDAY...THE GFS RUN FROM 24 HOURS AGO HAD STRONG CAA
PRODUCING TEMPS OF 19/-1 AT KDSM AND NOW HAS 35/14. MAIN REASON FOR
DIFF IS MOST RECENT RUNS PROPAGATING LOW PRESSURE THROUGH CENTRAL
MN...PLACING OUR CWA IN SOUTHERLY FLOW. BEYOND THIS...CONFIDENCE
INCREASING THAT THE CWA WILL REMAIN IN NORTHERLY/NORTHWESTERLY FLOW
INTO NEXT WEEKEND...WHICH WOULD SUGGEST NEAR TO BELOW NORMAL
TEMPERATURES TO PERSIST.

&&

.AVIATION...29/18Z
ISSUED AT 1126 AM CST THU JAN 29 2015

MVFR CIGS LOOK TO PERSIST THROUGH THIS EVENING AND POTENTIALLY
INTO FRIDAY MORNING. CONFIDENCE IS HIGHER AT MCW/ALO AND EVEN OTM
FOR THE STRATUS TO LINGER A BIT LONGER...BUT CERTAINLY FOD AND DSM
EXTENDED THE MVFR CIGS TIME FRAME THROUGH LATE THIS EVENING.

&&

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

&&

$$

SHORT TERM...SMALL
LONG TERM...KOTENBERG
AVIATION...PODRAZIK



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