Area Forecast Discussion
Issued by NWS Des Moines, IA

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305
FXUS63 KDMX 012328
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
528 PM CST MON FEB 1 2016

.SHORT TERM /TONIGHT/...
ISSUED AT 409 PM CST MON FEB 1 2016

FORECAST FOCUS REMAINS SQUARELY ON TIMING AND IMPACTS ASSOCIATED
WITH THE IMPENDING WINTER STORM. 12Z UPPER AIR ANALYSIS AND WATER
VAPOR IMAGERY SHOW THE LARGE NEGATIVE TILT UPPER TROUGH EMERGING IN
THE LEE OF THE ROCKIES.  IN RESPONSE...SFC CYCLOGENESIS CONTINUES TO
INTENSIFY ACROSS THE CENTRAL PLAINS WITH MAX SFC PRESSURE FALLS OF 6-
MB/3HR NOTED OVER KS/OK.  ALTHOUGH HIGH CLOUDS CONTINUE TO SPREAD
NORTHEASTWARD ACROSS THE STATE...MRMS RADAR MOSAIC AND OBSERVATIONS
CONTINUE TO INDICATE ALL THE MEASURABLE PCPN REMAINING OVER NE/KS AS
OF THIS AFTERNOON.

THIS EVENING THROUGH 10PM...SHORT RANGE MODEL GUIDANCE CONTINUE TO
INDICATE THAT A MAJORITY OF THE EVENING SHOULD REMAIN DRY AS MODEL
TIME SECTIONS AND SOUNDINGS SHOW A FAIR AMOUNT OF DRY AIR IN THE LOW
TO MID LEVELS /E.G. 850MB/. LATEST RUNS OF THE HRRR CONTINUE TO
INDICATE THE POTENTIAL FOR A NARROW E/W BAND OF PRECIPITATION
LIFTING NWD ACROSS AREA ALONG THE LEADING SURGE OF ISENTROPIC
ASCENT. HOWEVER...ANY PRECIP /LIKELY A RAIN/SNOW MIX/ WITHIN THIS
BAND SHOULD REMAIN VERY LIGHT...WITH ONLY MINOR SNOW ACCUMS.

TONIGHT AFTER 10 PM...IMPRESSIVE SURGE OF MOISTURE AND LIFT WILL
BEGIN TO IMPACT THE REGION FROM THE SOUTHWEST. COMBINATION OF VERY
STRONG ISENTROPIC LIFT...FRONTOGENETIC FORCING...AND LOWER STATIC
STABILITY SHOULD YIELD ARE LARGE SWATH OF MODERATE TO HEAVY
PRECIPITATION. WOULD NOT BE SURPRISED TO SEE A FEW LIGHTNING STRIKES
TOWARDS 12Z TUE AS WELL DUE TO A PLUME OF VERY STEEP LAPSE RATES
ALOFT CONTRIBUTING TO 100-200 J/KG OF MUCAPE. PORTIONS OF FAR
SOUTHERN IOWA WILL LIKELY BEGIN AS A PERIOD OF MIXED
PRECIPITATION...HOWEVER THE COMBINATION OF VERY STRONG ADIABATIC
ASCENT...AND WET BULB EFFECTS IN THE PBL SHOULD RESULT IN FAIRLY
QUICK CHANGEOVER TO SNOW EXCEPT ALL BUT THE FAR SOUTHEASTERN
COUNTIES.

.LONG TERM /TUESDAY THROUGH MONDAY/...
ISSUED AT 409 PM CST MON FEB 1 2016

PLENTY TO DISCUSS WITH ALL EYES STILL FOCUSED ON IMPENDING WINTER
STORM. LONG WAVE TROUGH IN QUESTION IS STILL ONLY MOVING THROUGH THE
FOUR CORNERS AREA BUT MID LEVEL FORCING EFFECTS ARE ALREADY
APPROACHING THE MS RIVER VALLEY WITH MAINLY VIRGA FOR NOW. THIS WILL
LIFT NORTHWARD INTO IA AT PERIOD ONSET AND MATURE COMBINING WITH
IMPRESSIVE BAROCLINIC ZONE NOW FORMING FROM KS INTO MO. THETA-E
ADVECTION...FRONTOGENETIC AND KINEMATIC FORCING ARE ALL VERY
STRONG AND IMPRESSIVELY PHASED EARLY IN THE PERIOD...BUT SOME
CONCERN IS THAT MUCH OF THIS LIFT IS BELOW THE DENDRITIC GROWTH
ZONE /DGZ/...WHICH IS NOT THAT DEEP ANYWAYS...AND MAY LEAD TO
LOWER RATIOS. HOWEVER CONVECTIVE SNOW IS ALSO A CONCERN WITH
UPRIGHT CONVECTION OVERWHELMING ANYTHING THAT MAY BE SLANTWISE/CSI
AS MOST MODELS ARE DEPICTING TOKEN MUCAPES. CONVECTION ALLOWING
MODELS ALL REFLECT THIS AS WELL WITH SHOWERY 40-50 DBZ ECHOES IN
SNOW. THUS 1-2 INCH/HOUR RATES ARE QUITE POSSIBLE FROM SOUTHERN
INTO CENTRAL IA AROUND THE MORNING COMMUTE. EXAMINATION OF NAM
CONVECTIVE PRECIP FIELDS SUPPORTS THIS AS WELL...EVEN BRINGING
SPOTTY AMOUNTS TOWARD THE MN BORDER. HAVE EVEN ADDED FLAT OUR
THUNDERSTORM WORDING SE.

WELL ADVERTISED DRY SLOT HAS BECOME EVEN MORE PROMINENT THIS MORNING
AND LOOKS TO DIMINISH PRECIP CHANCES SOUTH INTO THE AFTERNOON WITH
TEMPERATURES AT OR ABOVE FREEZING. THERE COULD VERY WELL BE A BRIEF
PERIOD OF LIGHT FREEZING RAIN TOO AS THE DRY SLOT...RAIN/SNOW LINE
LIFTS NORTHWARD THROUGH THE DAY. ADEQUATE LIFT MAY STILL PHASE WITH
A DECENT MOISTURE DEPTH...ALTHOUGH NOT DEEP AND COOL ENOUGH TO ADD
ICE INTRODUCTION. HOWEVER FARTHER NORTH THE DGZ DEEPENS SOMEWHAT AND
BETTER PHASES WITH THE MAX OMEGA AND SHOULD RESULT IN VERY HEAVY
SNOW ACROSS THE NW THIRD OF THE FORECAST AREA INTO THE EVENING AS
THE DEFORMATION ZONE LINGERS AND PIVOTS OVER THESE LOCATIONS.
ALTHOUGH DIFFICULT TO INJECT INTO THE FORECAST AT THIS POINT THE
OPERATIONAL NAM AND COMPANION NMM CORES DEVELOP WHAT LOOKS TO BE
CONVECTIVE PRECIP JUST NW OF THE SURFACE LOW TRACK EARLY TUE
EVENING...WHICH COULD RESULT IN A HEAVY BURST OF UNDETERMINED WINTRY
PRECIP BEING RIGHT IN THE VICINITY OF THE PRECIP TYPE TRANSITION
LINE. THIS WILL HAVE TO BE WATCH WITH TIME...ESPECIALLY INTO TUE.

TRUE BLIZZARD CONDITIONS MAY BE SOMEWHAT MARGINAL THROUGH THE EVENT
WITH 1/4SM VISIBILITIES AND 35 MPH GUSTS NOT COUPLED FOR EXTENDED
PERIODS AS ONE WIND SURGE MOVES THROUGH TUE...AND THEN SWINGS BACK
THROUGH LATE TUE NIGHT. MOST MODELS DO NOT SHOW TOO DEEP OF MIXING
SO GENERALLY KEPT MAX WIND GUSTS 35-40 MPH. GFS BUFR SOUNDINGS DO
INDICATE SOME POTENTIAL FOR GUSTS EXCEEDING THAT...AND MAY OCCUR
LOCALLY...BUT CONFIDENCE IS NOT THERE TO INJECT INTO THE FORECAST ON
A WIDESPREAD BASIS AS OF YET.

WHAT THIS ALL AMOUNTS TO FOR THE SENSIBLE WEATHER FORECAST IS LITTLE
CHANGE FOR SNOW AMOUNTS IN THE CURRENT BLIZZARD WARNING WITH AMOUNTS
LOWERED SOMEWHAT IN THE WINTER STORM WARNING STRIPE...INCLUDING THE
DES MOINES METRO AREA. BLIZZARD WARNING 8-12...WINTER STORM WARNING
4-9...AND A TIER OF WINTER WEATHER ADVISORY GENERALLY 2-4.

SOME MINOR BLOWING SNOW MAY LINGER INTO WEDNESDAY BUT THE REMAINDER
OF THE FORECAST WILL BE RELATIVELY DRY AND INACTIVE WITH SEASONAL
TEMPERATURES. DRY NW FLOW WILL PERSIST INTO INTO THE WEEKEND BEFORE
THE NEXT ITERATION OF WRN CONUS LONG WAVE TROUGH DEVELOPS TO END THE
WEEKEND. A SHORT WAVE DROPS THROUGH THU BUT IS MOISTURE
STARVED...AND THERE COULD BE SOME TOKEN LIGHT SNOW WITH THE COLD
ADVECTION SURGE MON BUT CHANCES ARE NOT HIGH ENOUGH TO INCLUDE AT
THIS TIME. THE BIGGEST SENSIBLE WEATHER CHANGE IN THE EXTENDED
WILL LIKELY BE COLDER TEMPERATURES...WHICH ARE MAINLY JUST BEYOND
THE VALID PERIOD.

&&

.AVIATION...02/00Z
ISSUED AT 528 PM CST MON FEB 1 2016

SOME DENSE FOG AROUND MAY TRICKLE INTO KDSM SO HIGHLIGHTED WITH A
MENTION THIS EVENING AS REPORTS INTO THE EAST SIDE OF THE METRO
AREA NOW. ALSO PUT INTO KALO AS IT SHOULD BRING VSBYS DOWN UNTIL
WINDS PICK UP LATER. OTHERWISE MAIN STORY IS PRECIP PUSHING IN
LATE TONIGHT WITH CIGS LOWERING AND CONDITIONS DETERIORATING BY
SUNRISE TUESDAY AND CONTINUING TO BE HAZARDOUS THROUGH THE DAY
TUESDAY WITH STRONG GUSTY WINDS...SNOW AND BLOWING SNOW. SE TAF
SITES AT KDSM AND KOTM EXPECTED TO TRANSITION TO RAIN IN THE
AFTERNOON.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 6 AM CST WEDNESDAY
FOR ADAIR-ADAMS-AUDUBON-BOONE-CASS-DALLAS-GUTHRIE-MADISON-POLK.

WINTER WEATHER ADVISORY FROM 3 AM TUESDAY TO 6 AM CST WEDNESDAY
FOR CLARKE-JASPER-POWESHIEK-RINGGOLD-TAMA-TAYLOR-UNION-WARREN.

BLIZZARD WARNING FROM 3 AM TUESDAY TO 6 AM CST WEDNESDAY FOR
CERRO GORDO-EMMET-FRANKLIN-HAMILTON-HANCOCK-HUMBOLDT-KOSSUTH-
PALO ALTO-WINNEBAGO-WORTH-WRIGHT.

WINTER STORM WARNING FROM 3 AM TUESDAY TO 6 AM CST WEDNESDAY FOR
BLACK HAWK-BREMER-BUTLER-GRUNDY-HARDIN-MARSHALL-STORY.

BLIZZARD WARNING FROM MIDNIGHT TONIGHT TO 6 AM CST WEDNESDAY FOR
CALHOUN-CARROLL-CRAWFORD-GREENE-POCAHONTAS-SAC-WEBSTER.

&&

$$

SHORT TERM...FOWLE
LONG TERM...SMALL
AVIATION...BEERENDS



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