Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 160936
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
336 AM CST TUE DEC 16 2014

.SHORT TERM /TODAY/...
ISSUED AT 335 AM CST TUE DEC 16 2014

IMMEDIATE CONCERNS ARE PRECIP TRENDS AND WIND. PARENT SHORT WAVE
CONTINUES TO MOVE ACROSS IL WITH ONLY MINIMAL UPSTREAM INFLUENCE
FROM DAKOTAS/ONTARIO WAVE. LATEST RAP ANALYSIS SHOWS LITTLE IN THE
WAY OF OMEGA OR FORCING ACROSS FORECAST AREA...JUST SOME TOKEN MID
LEVEL LIFT ASSOCIATED WITH DAKOTAS/ONTARIO WAVE BARELY GENERATING
LIGHT SNOW/FLURRIES NW AND LOWER BASED H85/H7 OMEGA ALONG THE
DMX/DVN BORDER. NOT ENTIRELY SURE WHAT IS GENERATING THAT OUTSIDE
OF MINOR SEGMENT OF THETA-E ADVECTION DRIVEN MORE BY WRAP AROUND
MOISTURE THAN TEMPS. REGARDLESS...ANY LINGERING UPWARD VERTICAL
MOTION SHOULD WANE OVER THE NEXT SEVERAL HOURS. ANY PRECIP WILL BE
QUITE LIGHT AND TOKEN...BUT WITH TYPE PROBLEMATIC. IR SATELLITE
CLOUD TOP TEMPS AND SOUNDINGS SUGGEST ICE INTRODUCTION WILL VARY
SO FREEZING AND FROZEN HYDROMETEORS ARE BOTH POSSIBLE...AND EVEN
A BRIEF PERIOD OF LIQUID STILL POSSIBLE FAR EAST WITH WARM LAYER
TEMPS STILL 1-3C THERE. NEARLY ALL MODELS ARE UNDERESTIMATING THE
EXTENT OF STRATUS ACROSS THE GREATER MO VALLEY SO HAVE HELD ONTO
CLOUDS FOR MUCH OF THE DAY. LITTLE CONFIDENCE HOW LONG THEY WILL
HOLD ON...BUT CERTAINLY LONGER THAN MODELS SUGGEST.

NO CHANGES TO CURRENT WIND ADVISORY WITH SEVERAL SITES STILL HITTING
SUSTAINED CRITERIA. THEY SHOULD NOT LAST MUCH LONGER HOWEVER WITH
ISALLOBARIC EFFECTS DIMINISHING WITH PRESSURE RISE MAX INTO SERN
IA...AND MAX MSLP GRADIENT EXITING FORECAST AREA. RAP SOUNDINGS
SUGGEST MIXING WILL SUPPORT GUSTS NOT QUITE TO CRITERIA /39KTS/
BUT STILL WELL INTO THE 30S. TEMPS WILL BE FALLING THROUGH MUCH OF
THE DAY. THERE MAY BE A VERY MINOR REBOUND TOWARD MIDDAY BASED ON
RAW TEMP PLUMES...BUT HIGH TEMPS WILL BE AT BEGINNING OF VALID
PERIOD.

.LONG TERM /TONIGHT THROUGH SUNDAY/...
ISSUED AT 335 AM CST TUE DEC 16 2014

THE SYNOPTIC PATTERN HAS FINALLY CHANGED AND WE WILL SEE COOLER
AND SOMEWHAT MORE ACTIVE WEATHER ACROSS CENTRAL IOWA FOR THE NEXT
SEVEN DAYS AND BEYOND. AT 500 MB...A COMPACT LOW PRESSURE CENTER
MOVING OVER MINNESOTA LATER TODAY WILL SLIDE QUICKLY EASTWARD OVER
THE GREAT LAKES TONIGHT AND WEDNESDAY. AT THE SURFACE...THIS
DEPARTURE WILL BE REFLECTED BY A RIDGE OF HIGH PRESSURE BUILDING
DOWN THE DAKOTAS AND INTO IOWA LATE TONIGHT THROUGH
WEDNESDAY...REINFORCING THE COLDER AIR BUT ALSO PROHIBITING
PRECIPITATION ACROSS THE STATE.

MEANWHILE...A LARGE AND BROAD 500 MB TROUGH WILL BUILD OVER THE
ROCKIES AND WESTERN U.S. ON WEDNESDAY...AND EVENTUALLY PROGRESS
EASTWARD AS A SOMEWHAT CONFUSED AREA OF GENERAL TROUGHINESS
PERSISTING FOR THE REST OF THE WORK WEEK. THE SURFACE RIDGE
DISCUSSED EARLIER WILL STUBBORNLY CLING TO MINNESOTA AND MUCH OF
IOWA DURING THIS TIME EVEN AS ATMOSPHERIC MOISTURE ABOVE A VERY
SHALLOW NEAR SURFACE LAYER INCREASES DRAMATICALLY AROUND THURSDAY.
FORECAST SOUNDINGS ARE UNANIMOUS IN ILLUSTRATING THIS MOISTURE
INCREASE COINCIDENT WITH THE ARRIVAL OF THE BROAD 500 MB
TROUGH...HOWEVER...ANY FORCING ASSOCIATED WITH THIS UPPER FEATURE
WILL BE MODEST AT BEST AND THE RESULT WILL LIKELY BE THICK LAYERED
CLOUDS AND SOME FLURRIES OR LIGHT SNOW AT TIMES. HAVE INCLUDED LOW
POPS IN THE 20 TO 30 RANGE FOR SOME OF THIS PERIOD...BUT NO
SIGNIFICANT IMPACTS ARE ANTICIPATED.

A MORE INTERESTING ATMOSPHERIC FEATURE APPEARS IN THE FORM OF A
LONGWAVE BUT INITIALLY SUBTLE 500 MB TROUGH MOVING ASHORE OVER THE
PACIFIC U.S. COAST AROUND SATURDAY. AS THIS TROUGH SUBSEQUENTLY
MOVES EASTWARD OVER THE ROCKIES AND EMERGES OVER THE HIGH PLAINS
AND MIDWEST IT SHOULD INTENSIFY TO SOME EXTENT...THEN BE
REINFORCED BY TRAILING SHORTWAVES AND CARVE OUT A LARGE CENTRAL
U.S. TROUGH EARLY NEXT WEEK. THERE IS OF COURSE A HIGHER DEGREE OF
UNCERTAINTY AT SUCH A LONG RANGE...HOWEVER...IT DOES APPEAR THAT
THERE WILL BE A DECENT CHANCE FOR SOME KIND OF PRECIPITATION
ACROSS IOWA SOMETIME BETWEEN AROUND SUNDAY NIGHT AND TUESDAY OF
NEXT WEEK. EVEN SO...IT REMAINS TO BE SEEN WHETHER TEMPERATURES
WOULD BE MORE SUPPORTIVE OF RAIN OR SNOW FROM THIS SYSTEM. FOR NOW
HAVE ADVERTISED CHANCE POPS EARLY NEXT WEEK AND IT WILL TAKE SOME
TIME BEFORE THERE IS ANY DEGREE OF CONFIDENCE TO RAISE OR
ELIMINATE THEM.

&&

.AVIATION...16/06Z
ISSUED AT 1136 PM CST MON DEC 15 2014

NORTHWEST WINDS WILL REMAIN GUSTY OVERNIGHT WITH A FEW GUSTS OVER
35 KTS POSSIBLE THROUGH 12Z. RAIN HAS MOVED OVER EASTERN IOWA WITH
SOME WINTRY MIX OVER THE WESTERN PORTION OF THE STATE. MAINLY
NORTHERN SITES WILL HAVE SOME SNOW MIX THOUGH ANY VSBY REDUCTIONS
IN SNOW WILL BE BRIEF. LIFR CIGS ARE BEGINNING TO MOVE EAST.
EXPECT IFR THEN A TRANSITION TO MVFR LATER TONIGHT INTO TUESDAY.
NO CONFIDENCE WHEN THE LARGE STRATUS SHIELD WILL MOVE OUT OF THE
AREA THEREFORE KEPT MVFR STRATUS THROUGH THE PERIOD.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY UNTIL 6 AM CST EARLY THIS MORNING FOR AUDUBON-
BOONE-CALHOUN-CARROLL-CERRO GORDO-CRAWFORD-EMMET-FRANKLIN-GREENE-
HAMILTON-HANCOCK-HARDIN-HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-
SAC-STORY-WEBSTER-WINNEBAGO-WORTH-WRIGHT.

&&

$$

SHORT TERM...SMALL
LONG TERM...LEE
AVIATION...DONAVON


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