Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 252021
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
221 PM CST WED FEB 25 2015

.UPDATE...
ISSUED AT 1228 PM CST WED FEB 25 2015

SURFACE LOW IS NOW IN CASS COUNTY AND WARM AIR HAS REALLY MADE A
PUSH INTO SW IA. THE DRY SLOT OVER CENTRAL IOWA HAS BEEN SHRINKING
OVER THE PAST COUPLE OF HOURS AND IS EVIDENT ON RADAR BUT OUR
FORCING WILL BE SHIFTING TO THE EAST THROUGH THE AFTERNOON. AS A
RESULT...CHANGES NEEDED TO BE MADE TO SNOWFALL AMOUNTS WHICH
RESULTED IN A SHIFT OF HEADLINES AS WELL. FAR SW IA MAY SEE A
PERIOD OF A WINTERY MIX LATE BUT THEY ARE CURRENTLY FROM 45 TO 50
DEGREES AND WILL HAVE A HARD TIME SEEING ANY APPRECIABLE SNOW SO A
FEW COUNTIES IN THE SW WERE REMOVED FROM HEADLINES. THE METRO AREA
WILL STILL SEE 3 TO 5 INCHES LATE AFTERNOON INTO EVENING WITH
HIGHEST AMOUNTS NORTH AND EAST. HEAVIEST SNOW BAND HAS SHIFTED
EAST FROM IA FALLS/MCW SE AND WILL LIKELY EXTEND TOWARDS OTTUMWA
LATER TODAY. NEW HEADLINE PRODUCTS OUT SHORTLY.

UPDATE ISSUED AT 954 AM CST WED FEB 25 2015

THERE WERE A COUPLE OF CHANGES MADE TO THE GOING FORECAST. TEMPS
ACROSS THE SOUTHWEST ARE A BIT WARMER THAN WHAT WE HAD GOING SO
THOSE WERE ADJUSTED AND THE LATEST MODEL DATA WAS LOADED. USING
TOP DOWN FORECASTING TOOLS PUT SOME SLEET INTO THE SOUTHWEST WHICH
SEEMED REASONABLE SINCE UPSTREAM OBS WERE REPORTING UP. I DID NOT
MAKE A BIG CHANGE TO SNOWFALL AMOUNTS AT THIS POINT HOWEVER...
THERE IS AN OBVIOUS DRY SLOT PUNCHING INTO THE I-80 CORRIDOR
AROUND THE DES MOINES AREA UP THROUGH HIGHWAY 30. THE LATEST NAM
HAS THIS HANDLED BUT WRAPS PRECIP/QPF AROUND HITTING THE AREA
LATER IN THE AFTERNOON. CURRENT RADAR TRENDS SHOW HOTTER RETURNS
ALONG THE MO RIVER BASIN IN MONONA COUNTY AND IF THOSE WRAP UP AND
MOVE INTO CENTRAL IOWA WE COULD STILL SEE MORE ROBUST SNOWFALL
LATER THIS AFTERNOON...AS THE NAM WAS SUGGESTING. FOR THAT REASON
SNOWFALL TOTALS WERE LEFT AS IS. HOWEVER...IF THE HEAVIER PRECIP
DOES NOT MATERIALIZE OR FALLS APART AS IT MOVES INTO US THEN
SNOWFALL TOTALS SOUTH OF THE HIGHWAY 30 CORRIDOR FROM GUTHRIE
COUNTY EAST WILL HAVE TO BE KNOCKED DOWN SOME.

&&

.SHORT TERM /TODAY/...
ISSUED AT 354 AM CST WED FEB 25 2015

BIGGEST CONCERN THROUGH TODAY WILL OBVIOUSLY BE TRACK AND AMOUNTS.
TRACK WISE GUIDANCE SUITE HAS CONTINUED TO DRIFT HEAVIEST AXIS NORTH
AND EAST WITH TIME...BUT QUESTION WILL BE HOW FAR. BLEND OF MAIN
OPERATIONAL DETERMINISTIC RUNS NOW PLACES HIGHEST AMOUNTS ALONG
EMMETSBURG TO TAMA OR GRINNELL LINE THROUGH 00Z.  THIS IS ALSO
SUPPORTED BY 00Z SPC SSEO BLEND OF VARIOUS HI RES SOLUTIONS.
CONFIDENCE IS STILL NOT TOO HIGH HOWEVER AS MOST RECENT RAP...HRRR
AND HOPWRF SOLUTIONS...DIFFICULT TO INJECT INTO THE FORECAST AT
POINT...ARE EVEN FARTHER NORTH OF THIS TRACK. THUS FEEL FORECAST IS
HEADING IN THE RIGHT DIRECTION...AT LEAST FOR MAX AMOUNTS AND THEIR
PLACEMENT...BUT POTENTIALLY NOT QUITE THERE YET. THIS THINKING HAS
RESULTED IN A NORTHWARD EXPANSION OF WINTER STORM WARNING...NOW
INCLUDING ALGONA...MASON CITY AND WATERLOO.  CONFIDENCE IN SRN EDGE
OF WARNING IS DECREASING HOWEVER AT THE SAME TIME.

DEGREE OF FORCING REMAINS UNCHANGED HOWEVER AND EXPLODES INTO IA
THROUGH THE DAY...INITIAL KINEMATICALLY DOMINATED BUT EVENTUALLY AIDED
BY MORE THERMODYNAMIC FORCING AS FRONTOGENETICAL RESPONSE INCREASES
INTO THE AFTERNOON. THIS CAN BE SEEN QUITE NICELY IN THE HI RES
SIMULATED REFLECTIVITY WITH MATURING SW-NE LOBE OF SNOW PASSING
THROUGH LATE. SNOWFALL RATES ACROSS NRN IA SHOULD BE 1 IN/HR PLUS
WITH NOTED MODEL OMEGA AND QG FORCING IN 290-300K OR H7/H6 DENDRITIC
GROWTH ZONE. 1-2KM DRY AIR MAY INITIALLY LIMIT PRECIP...BUT SNOWFALL
RATES ARE EXPECTED TO OVERCOME THAT.  SURFACE OBS ARE ALREADY
SHOWING VISIBILITIES 3/4 TO 2SM JUST UPSTREAM AND THE SYSTEM REALLY
HAS NOT MATURED YET.

HAVE NOT CANCELLED ANY HEADLINES...BUT MORE NRN TRACK WILL ALSO
LEAVE SNOW AMOUNTS IN DOUBT SW DUE TO WARMER AIR WITH A PERIOD OF
RAIN EVEN POSSIBLE. SNOWFALL RATES MAY STILL BE INTENSE FOR A SMALL
WINDOW HOWEVER SUPPORTING A HEADLINE EVEN THOUGH TOTAL SNOW CRITERIA
MAY NOT BE MET.  FARTHER SOUTH SNOW RATIOS LOOK LOWER WITH MAX LIFT
WARMER THAN -10C.  WARMER COLUMN WILL LIKELY RESULT IN WETTER SNOW
SOUTH VS MORE FLUFFY NORTH. HAVE ALSO PUSHED BACK SOME HEADLINES IN
TIME TO BETTER ACCOUNT FOR SLOWER TIMING AND HITTING THOSE MAIN
IMPACTS.

.LONG TERM /TONIGHT THROUGH TUESDAY/...
ISSUED AT 354 AM CST WED FEB 25 2015

TODAYS SNOWFALL EVENT...DISCUSSED IN THE SHORT TERM SECTION
ABOVE...WILL CONTINUE INTO TONIGHT AS THE SYSTEM EXITS TO THE
SOUTHEAST. FORECAST SOUNDINGS INDICATE A DEEP DENDRITIC GROWTH
ZONE FOR SEVERAL HOURS AS THE COLUMN COOLS...AND DESPITE A STEADY
DECREASE IN FORCING HAVE HELD POPS LONGER THROUGH THE EVENING AS
SNOW SHOULD PERSIST UNTIL THE DRY AIR CAN REALLY WORK IN FROM THE
NORTH. DUE TO THE SNOW LINGERING LONGER THROUGH THE EVENING...PART
OF THE WINTER STORM WARNING IN OUR SOUTHEAST HAS BEEN EXTENDED TO
06Z.

FROM THURSDAY INTO SATURDAY MUCH COLDER WEATHER WILL PREVAIL AS A
LARGE HIGH PRESSURE AREA BUILDS DOWN FROM CANADA ACROSS THE
DAKOTAS AND IOWA. HIGH TEMPERATURES ON THURSDAY WILL NOT BE VERY
FAR ABOVE MORNING LOWS...THEN ON THURSDAY NIGHT AS THE CENTER OF
THE HIGH MOVES NEAR SOUTHWEST IOWA TEMPERATURES SHOULD PLUMMET
ESPECIALLY WITH THE FRESH SNOW PACK FROM WEDNESDAYS SYSTEM
COVERING MUCH OF THE AREA. ON FRIDAY THE HIGH WILL BE SLOW TO PULL
AWAY...LIMITING ANY OPPORTUNITY FOR MODERATION...THEN ON FRIDAY
NIGHT ITS INFLUENCE WILL KEEP WINDS LIGHT OR CALM EARLY IN THE
NIGHT ESPECIALLY ACROSS OUR NORTHEAST AND EAST WHERE THE NEW SNOW
WILL LIKELY BE DEEPEST. IN LIGHT OF THESE FACTORS...HAVE LOWERED
THE TEMPERATURE FORECAST BY SEVERAL DEGREES FOR EACH PERIOD FROM
THURSDAY THROUGH FRIDAY NIGHT. WE ARE NOW PREDICTING THURSDAY
HIGHS AND THURSDAY NIGHT LOWS BOTH NEAR RECORD COLD LEVELS FOR THE
DATE. WIND CHILL HEADLINES ARE A NEAR CERTAINTY FOR MUCH OF THIS
PERIOD AS WELL.

BY SATURDAY MORNING THE 500 MB PATTERN WILL BE TRANSITIONING TO A
NEW CONFIGURATION...WITH A LARGE AND BROAD TROUGH GRADUALLY
DIGGING DOWN THE PACIFIC U.S. COAST AND GENERALLY ZONAL FLOW
EXTENDING DOWNSTREAM OVER IOWA. WITHIN THIS REGIME IT IS EXPECTED
THAT SEVERAL MODEST IMPULSES WILL EJECT OUT OF THE TROUGH AND MOVE
QUICKLY OVER OUR REGION...WITH THE FLOW EVENTUALLY TURNING TO
SOUTHWEST BEFORE THE LARGE TROUGH FINALLY SWEEPS THROUGH SOMETIME
AROUND TUESDAY. AT THE SURFACE...THE INITIAL STAGES OF THE PATTERN
WILL MANIFEST IN THE DEVELOPMENT OF A BROAD QUASI-STATIONARY
FRONTAL ZONE DEVELOPING AROUND SOUTHERN IOWA OR MISSOURI OVER THE
WEEKEND. AS THE AFOREMENTIONED IMPULSES MOVE OVER THIS REGION THEY
WILL GENERATE PERIODS OF PRECIPITATION WITH SNOW THE MOST LIKELY
MODE GIVEN FORECAST TEMPERATURES AND SOUNDINGS. WHILE AMOUNTS AND
LOCATION ARE STILL UNCERTAIN...PRECIPITATION APPEARS LIKELY ACROSS
OUR SOUTHERN COUNTIES AND POPS HAVE THUS BEEN RAISED TO THE
LIKELY CATEGORY IN THIS AREA ON SATURDAY NIGHT AND SUNDAY. LOOKING
INTO EARLY NEXT WEEK...PRECIPITATION WILL AGAIN BE LIKELY SOMETIME
AROUND MONDAY NIGHT AND TUESDAY AS THE BULK OF THE ENERGY FROM THE
FORMER WESTERN U.S. TROUGH FINALLY MOVES THROUGH...BUT PREDICTIONS
AS TO TYPE AND A STORM PATH ARE HIGHLY VARIABLE AT THIS POINT TO
POPS HAVE BEEN HELD IN THE CHANCE RANGE FOR NOW. THERE IS SOME
POTENTIAL FOR A MIXED PRECIPITATION IN OUR AREA DURING THIS TIME
FRAME...BUT WE WILL NEED TO WAIT FOR A MORE STABLE SOLUTION BEFORE
ADDING MORE DETAIL TO THE FORECAST.

&&

.AVIATION...25/18Z
ISSUED AT 221 PM CST WED FEB 25 2015

IFR/LIFR CONDITIONS ASSOCIATED LOW PRESSURE SYSTEM SHIFTING ACROSS
IA.  CIGS/VSBYS WILL BE BOUNCING AROUND BETWEEN LOCATIONS WITH
MODERATE SNOW AND THOSE WITHOUT.  CONDITIONS WILL IMPROVE AFT 06Z.
SFC WND WILL GO FROM ERLY TO NRLY.  SPEEDS WILL BE FM 10-20MPH
INCREASING TO 15 TO 25 MPH.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR APPANOOSE-
CLARKE-DALLAS-DECATUR-LUCAS-MADISON-MARION-MONROE-POLK-WARREN-
WAYNE.

WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR ADAIR-
AUDUBON-CALHOUN-CARROLL-CASS-CRAWFORD-GREENE-GUTHRIE-SAC.

WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR BLACK HAWK-
GRUNDY-JASPER-MARSHALL-POWESHIEK-STORY-TAMA.

WINTER STORM WARNING UNTIL 9 PM CST THIS EVENING FOR BOONE-
BREMER-BUTLER-CERRO GORDO-EMMET-FRANKLIN-HAMILTON-HANCOCK-HARDIN-
HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-WEBSTER-WINNEBAGO-WORTH-
WRIGHT.

WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR DAVIS-
MAHASKA-WAPELLO.

&&

$$

UPDATE...FAB
SHORT TERM...SMALL
LONG TERM...LEE
AVIATION...FAB



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