Area Forecast Discussion
Issued by NWS Des Moines, IA

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563
FXUS63 KDMX 052345
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
543 PM CST FRI FEB 5 2016

.SHORT TERM /TONIGHT/...
ISSUED AT 407 PM CST FRI FEB 5 2016

PRIMARY CONCERN TONIGHT WILL BE SNOW TRENDS THIS EVENING...AND
POTENTIALLY FOG LATER TONIGHT. LONG WAVE TROUGH CONTINUES TO MOVE
THROUGH THE CENTRAL CONUS WITH TWO SHORT WAVES AFFECTING IA
WEATHER...ONE THROUGH SD AND THE OTHER NEAR THE KC METRO AREA. THE
LIFT BETWEEN THESE TWO SYSTEMS IS TRYING TO PHASE BUT IS SOMEWHAT
DISJOINTED FOR SNOW PRODUCTION. MUCH OF THE VERTICAL MOTION IN
FAVORABLE ICE GENERATION TEMPS...NOTED AROUND 290K ISENT SURFACE OR
H7/H6 ISOBARICALLY...HAS PRODUCED THE COOLER COLD TOP TEMPS INTO
CENTRAL AND ERN IA BUT IS GOING TOWARD SATURATING THE COLUMN RATHER
THAN SURFACE PRECIP WITH NOTED DRY AIR 500-1000M. ONLY THE BACK EDGE
OF THE LIFT...AND LESS FAVORABLE THERMODYNAMICALLY...IS OVERCOMING
THE DRY AIR. THUS EXPECT STEADY STATE CONDITIONS AS SNOW NEARS THE
DES METRO AREA INTO THE EVENING COMMUTE...BUT EXPECT CURRENT 1-3SM
VISIBILITIES TO WANE SOMEWHAT AS THE LIFT DIMINISHES OVERALL LATER
THIS EVENING AND WHAT THERE IS MAINLY APPEARS TO BE WARMER THAN -
10C. THUS EXPECT ACCUMULATIONS LESS THAN AN INCH...IF THAT IN
SPOTS...AS THIS SYSTEM MOVES ACROSS THE AREA.

LOW CLOUDS HAVE DEVELOPED EITHER SIDE OF NE/SD SURFACE TROUGH PER
20Z ANALYSIS AND HIGH RES MODELS SUGGEST THIS WILL EXPAND TO IA INTO
THE NIGHT KEEPING SKIES CLOUDY. THE QUESTION WILL BE WHETHER FOG CAN
BECOME PREVALENT AS WELL. HI RES MODELS /HRRR/RAP/HI RES WINDOW
CORES/ SHOW THE POTENTIAL FOR DENSE FOG OVER WRN IA AS
AFOREMENTIONED TROUGH WEAKENS...AND HOPWRF PROBABILITY PROGS REALLY
HIGHLIGHT NWRN TWO-THIRDS TOO. HAVE ADDED SOME FOG WORDING
OVERNIGHT...WHICH MAKES SOME SENSE WITH HIGHER WEDGE OF 20S
DEWPOINTS NEAR THE TROUGH.

.LONG TERM /SATURDAY THROUGH FRIDAY/...
ISSUED AT 407 PM CST FRI FEB 5 2016

PRIMARY FORECAST FOCUS IS ON THE POTENTIAL FOR BLOWING SNOW AND
BLIZZARD CONDITIONS LATE SUNDAY AFTERNOON THROUGH AT LEAST MONDAY
MORNING. SEVERAL FACTORS TO CONSIDER. TEMPERATURES SATURDAY AND
SUNDAY ARE FORECAST TO BE ABOVE FREEZING THEREFORE THE QUESTION IS
HOW MUCH MELTING AND HOW MUCH OF A CRUST CAN FORM ON THE EXISTING
SNOW PACK. DEWPOINTS ARE EXPECTED TO REMAIN IN THE MID TO UPPER
20S OVER MUCH OF THE AREA WHICH WOULD KEEP SFC WETBULB
TEMPERATURES BELOW 32. THIS WILL MITIGATE SOME OF THE MELTING
ALONG WITH SOME EVAPORATIVE COOLING SO NOT EXPECTING HIGHS IN THE
32 TO 34 DEGREE RANGE TO HAVE MUCH IMPACT. AN STRONG PV ANOMALY
WILL MOVE INTO CNTRL MN LATE SATURDAY THEN DROP SOUTHEAST SUNDAY
INTO SUNDAY NIGHT AND DRAG A COLD FRONT THROUGH THE STATE. THIS
WILL LEAD TO STRONG MIXED LAYER WINDS AND WITH THE COLD
ADVECTION...WILL LEAD TO AN ENHANCED MOMENTUM TRANSFER EFFICIENCY
OF THE MIXED LAYER WINDS TO THE SFC. BOTH THE NAM/GFS SUGGESTING
MAX WINDS WITHIN THE MIXED LAYER EXCEED 50 KTS AT TIMES AND WITH
GOOD MOMENTUM TRANSFER AND A STRENGTHENING SFC PRESSURE
GRADIENT...SHOULD REALIZE GUSTS OVER 50 MPH AT TIMES. THIS ALONE
WILL ERODE ANY POTENTIAL CRUST ON THE EXISTING SNOW...OPENING IT
UP FOR WIDESPREAD BLOWING AND BECOME AIRBORNE. LIGHT SNOW IS ALSO
EXPECTED...POSSIBLY IN THE FORM OF SNOW SHOWERS OR HORIZONTAL
CONVECTIVE ROLLS WHICH APPEAR FAVORABLE CONSIDERING THE DEEP
DENDRITIC LAYER...AMPLE MOISTURE AND GOOD LOW LEVEL SHEAR OF 25
KTS OR MORE. THIS WOULD DROP VISIBILITIES DUE TO FALLING SNOW BUT
ALSO ADD MORE SNOW FOR BLOWING. THEREFORE...HAVE ISSUED A BLIZZARD
WATCH FOR PORTIONS OF NORTH CENTRAL IOWA. LIKELY WILL NEED TO ADD
SOME ADJACENT AREAS WITH UPCOMING FORECASTS AND MAY NEED SOME
TIMING ADJUSTMENTS...PARTICULARLY FURTHER INTO MONDAY.

THE DEEP AND COLD TROUGH WILL PASS THROUGH BY TUESDAY. FORECAST
WIND CHILLS TUESDAY AND WEDNESDAY MORNING REMAIN 10 TO 20 BELOW
ZERO. MAY CLIP WIND CHILL ADVISORY OVER THE FAR NORTH. GRADUAL
WARMING THOUGH TEMPERATURES WILL REMAIN NEAR TO BELOW SEASONAL
VALUES THROUGH MUCH OF THE PERIOD ALONG WITH GENERALLY DRY
CONDITIONS.

&&

.AVIATION...06/00Z
ISSUED AT 543 PM CST FRI FEB 5 2016

A COUPLE OF DISTURBANCES WILL PASS ACROSS IOWA THIS EVENING...ONE
RIGHT ON THE HEELS OF THE OTHER.  THE FIRST WAVE WILL BRING MVFR
CONDITIONS WITH FLURRIES TO A DUSTING OF SNOW.  EXPECT VSBYS TO DROP
TO A MILE AT TIMES AND CIGS TO 2K-3KFT.  THE SECOND WAVE WILL BRING
LIGHT SNOW WITH A DUSTING POSSIBLE AND VSBYS AROUND 1SM WITH CIGS
AROUND 700FT AGL.  FOG WILL OCCUR LATER TONIGHT AS SLIGHTLY WARMER
AIR MOVE IN ALOFT. VFR CONDITIONS WILL RESUME ACROSS TAF LOCATIONS
AFTER 15Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
BLIZZARD WATCH FROM SUNDAY AFTERNOON THROUGH MONDAY MORNING FOR
CALHOUN-CERRO GORDO-EMMET-FRANKLIN-HAMILTON-HANCOCK-HARDIN-
HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-SAC-WEBSTER-WINNEBAGO-
WORTH-WRIGHT.

&&

$$

SHORT TERM...SMALL
LONG TERM...DONAVON
AVIATION...FAB



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