Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 301744
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1244 PM CDT TUE SEP 30 2014

.SHORT TERM /TODAY/...
ISSUED AT 336 AM CDT TUE SEP 30 2014

.TODAY...FORECAST CONCERNS DEALT WITH TIMING OF PRECIP AND MAX
TEMPERATURES TODAY. LARGE AND STUBBORN 500 MB RIDGE WILL SLOW THE
EASTWARD MOVING OF THE INCOMING 500MB VORT MAX/LOW PRESSURE SYSTEM
INTO THE STATE UNTIL LATE THIS AFTERNOON. THE HRRR/HOPWRF/ARW/NAM12
ALL KEEP PRECIP OUT OF WESTERN IOWA UNTIL AFTER 18-19Z TODAY AND
THEN INTO THE FORECAST AREA BY AROUND 21-22Z TIME FRAME. CUT BACK ON
THE EASTERN EXTENT OF POPS AS WELL AS LOWERED POPS TO CHANCES UNTIL
AFTER 20Z ACROSS THE FAR WEST. NAM/GFS/RAP CRL SOUNDING HAVE RAIN
FALLING B/T 20-23Z DEPENDING THE MODEL. PLUS THE STRONG MOISTURE
TRANSPORT AND THETA-E ADVECTION FINALLY ENTER THE WESTERN PORTION OF
THE FORECAST AREA MID TO LATE AFTERNOON. HIGHER CONFIDENCE WITH THE
DELAYED ONSET OF PRECIP TODAY.

MAX TEMPS WERE TRICKY TODAY WITH THE LESSER EASTWARD EXTENT OF
PRECIP BUT STILL ANTICIPATING AN ABUNDANCE OF MID TO HIGH CLOUD
COVER THROUGH MUCH OF THE DAY. THE PARTIAL STRONG WAA AS SOUTHEAST
FLOW INCREASES THROUGHOUT THE DAY SHOULD HELP WARM TEMPS IN THE
CENTRAL TO SOUTH INTO THE MID-UPPER 70S. LEANED TOWARD THE ECMWF FOR
TEMPERATURES TODAY. NAM SEEMED WAY TOO COLD BUT THE GFS WAS MORE IN
LINE WITH THE ECMWF FOR TEMPERATURES.

.LONG TERM /TONIGHT THROUGH MONDAY/...
ISSUED AT 336 AM CDT TUE SEP 30 2014

ACTIVE WEATHER PATTERN RETURNING TO THE AREA AFTER RATHER QUIESCENT
PERIOD. FOR NOW H500 RIDGE REMAINS DOMINANT ACROSS THE CENTRAL
CONUS WITH LARGE SFC HIGH ANCHORED OVER SOUTHERN CANADA JUST NORTH
OF LAKE SUPERIOR. SUBJECTIVE H500 ANALYSIS AT 00Z SHOWS WEAK LOBE
OF HT FALLS MOVING ACROSS WESTERN IA AND EASTERN NE ASSOCIATED
WITH FIRST LOBE OF VORTICITY MOVING NORTHEAST OUT OF THE CENTRAL
ROCKIES. MAIN UPPER LOW CONTINUES TO DRIFT NNE OUT OF CENTRAL CO
WITH SFC SHORTWAVE OVER NORTHEAST COLORADO WITH ATTENDANT TROUGH
BACK NORTHWEST INTO CENTRAL WY. BY 00Z WED...THE H500 SHORT WAVE
WILL CONTINUE TO MOVE NORTH NORTHEAST WHILE THE H500 VORTICITY MAX
OVER NEBRASKA INTERACTS WITH A RIBBON OF H850 MOISTURE EXTENDING
FROM THE DAKOTAS SOUTH TO TEXAS. THIS SHOULD SLOWLY INCREASE
THUNDER CHANCES ACROSS NEBRASKA TODAY WITH SOME ACTIVITY REACHING
THE WESTERN HALF OF THE FORECAST AREA TOWARD 00Z. THE ADVANCEMENT
OF THE BAND OF SHOWERS AND STORMS WILL BE COMPLICATED SOMEWHAT BY
THE UPPER LEVEL RIDGE AND WEAK OMEGA BLOCK ACROSS THE CENTRAL
CONUS. ALONG WITH THAT...REINFORCING DRY AIR CONTINUES TO ADVECT
SOUTHWEST AT THE SFC FROM THE EASTERN GREAT LAKES TOWARD IOWA
WHERE DEW POINTS OF THE MID 40S FROM EASTERN WISCONSIN GIVE WAY TO
DEW POINTS IN THE MID 50S TO LOWER 60S ACROSS WESTERN IOWA.
CURRENT TRENDS AND MODEL GUIDANCE SUGGEST THAT THE PRECIPITATION
SHOULD SLOWLY WORK EAST OVERNIGHT AS THE DRY AIR AND PERSISTENT
NORTHEAST FLOW TODAY GRADUALLY BECOME MORE SOUTHEAST WITH WED
MORNING. HAVE TRIMMED BACK EARLY PERIOD POP AFT 00Z THIS EVENING
AND INCREASED OVERNIGHT MORE IN LINE WITH 12Z/00Z EURO BASED ON
ANALYSIS AND CURRENT TRENDS. ALL MODELS CONTINUE TO INDICATE
LESSENING INSTABILITY AS THE AREA OF STORMS CROSSES THE REGION
TONIGHT. SO...HAVE TRIMMED BACK WORDING FROM CAT THUNDER TO SCT
THUNDER AND THEN ISO AFT 04Z TONIGHT WEST. SINCE THIS IS A SEVERAL
PART SYSTEM WITH THE SECOND WAVE PROJECTED TO CROSS THE AREA ON
WEDNESDAY...POP CHANCES WILL AGAIN INCREASE OVER THE WEST/CENTRAL
AREAS EARLY AND THEN EAST THROUGH THE DAY WITH HIGHER POPS
EXPECTED. ONCE AGAIN WITH SOMEWHAT LIMITED INSTABILITY THUNDER OF
THE ISO TO SCT VARIETY IS EXPECTED. WITH THE SLOWER EVOLUTION OF
THE ENTIRE SYSTEM AND THE SYNOPTIC SCALE MODELS SUGGESTING THE
WESTERN MOST WAVE...NOW OVER NORTHWEST BRITISH COLUMBIA...WILL
HAVE A LONGER TIME TO DIG SOUTHEAST THROUGH WEDNESDAY...AND SHOULD
TRACK THROUGH IOWA THURSDAY AND THURSDAY EVENING. THIS SHOULD
REDUCE POP CHANCES WED EVENING AND OVERNIGHT EXCEPT FOR THE SOUTH.
THE CURRENT 00Z GEM IS FOLLOWING THE EARLIER 12Z EURO EVOLUTION
WHILE THE NAM IS WOEFULLY TOO FAR NORTH WITH ITS FORCING WED NIGHT
AND THURSDAY. THE GFS LIES INBETWEEN...BUT IS LEANING MORE TOWARD
THE 00Z GEM AND 12Z EURO SOLUTIONS. GIVEN OUR SHIFT TO NORTHWEST
FLOW AND MORE ENERGETIC FALL SYSTEMS...PREFERENCE WILL BE GIVEN TO
THE SLOWER SOLUTIONS THAT ARE DIGGING THIS SUBSEQUENT WAVE ACROSS
THE CENTRAL CONUS. RAIN CHANCES SHOULD DIMINISH WEDNESDAY NIGHT
INTO EARLY THURSDAY...ONLY TO INCREASE AGAIN AS THE FINAL WAVE
DEEPENS OVER THE MID MISSISSIPPI RIVER VALLEY ON THURSDAY AND A
DEVELOPING FULL FALL CYCLONE EMERGES. GIVEN THE POTENTIAL FOR A
DEF ZONE RAIN AXIS OVER THE NORTHWEST AND THUNDERSTORMS OVER THE
EAST/SOUTHEAST...WILL INCREASE RAIN AND THUNDER CHANCES ACCORDINGLY.
OUTLOOK FOR SEVERE IS THE NEXT ITEM OF DISCUSSION...WE CONTINUE
OUTLOOKED FOR A SLIGHT RISK ACROSS THE SOUTHWEST ON DAY2...BUT
THIS IS PREDICATED ON A RATHER AMBITIOUS WARMING OF THE NAM
SOLUTION IN THE AFTERNOON HOURS ACROSS MUCH OF THE CENTRAL PLAINS.
GIVEN THE UNCERTAINTY OF HOW MUCH IF ANY SUN WE WILL HAVE DURING
THE DAY WED...CONDITIONS REMAIN RATHER CONDITIONAL DURING THE DAY.
AFTER SUNSET...POTENTIAL FOR SOME NOCTURNAL CONVECTION WHICH MAY
LEAD TO WIND OR HAIL THREATS ACROSS THE FAR SOUTHWEST/SOUTH. WITH
THE PASSAGE OF THE FALL CYCLONE THURSDAY AFTERNOON...TEMPS WILL
BEGIN A SIGNIFICANT DOWNWARD TREND. H850 TEMPS ON FRIDAY WITH A
CLOSED H500 LOW AMPLIFYING SOUTH INTO THE REGION LOWERS OUR
DAYTIME H850 TEMPS TO 0 TO -2C. WITH -30C PROGGED AT H500...COLD
POOL SHOULD CLOUD AREA OVER...PERHAPS BRINGING SHOWERS AND
STRONGER MIXING ACROSS THE REGION AS WELL DURING THE DAYTIME
HOURS. BY FRIDAY NIGHT AND EARLY SAT MORNING...COLD AIR SETTLES
INTO THE AREA. FROST MAY BE POSSIBLE DEPENDING ON HOW QUICKLY THE
WINDS DECOUPLE OVERNIGHT. FOR NOW SFC GRADIENT LOOKS STRONG ENOUGH
TO SUPPORT SOME FLOW THROUGH SATURDAY MORNING. BY LATE SAT
AFTERNOON EURO H850 TEMPS RECOVER TO 2C WEST TO -2C EAST. GFS IS
TOO PROGRESSIVE AND WARMING TOO FAST. HIGHS SATURDAY WILL LIKELY
SLOWLY RECOVER A FEW DEGREES TO THE MID TO UPPER 50S WITH A CRISP
FALL DAY EXPECTED. A DECENT PUSH OF WARM AIR ADVECTION SUNDAY
MORNING SHOULD BRING A FEW CLOUDS...AND A MILDER DAY WITH HIGHS
BACK TOWARD 70. WITH WNW FLOW NOW PERSISTING INTO EARLY NEXT
WEEK...EXPECT SOME FAST MOVING CLIPPERS TO TRACK THROUGH THE
REGION...KEEPING TEMPS NEAR SEASONAL NORMS.

&&

.AVIATION...30/18Z
ISSUED AT 1241 PM CDT TUE SEP 30 2014

LOW CEILINGS AND VISIBILITIES WILL CONTINUE TO LIFT AND BURN OFF
INTO THE AFTERNOON WITH DAYTIME HEATING. WILL BEGIN TO SEE THE
INFLUENCE OF THE STRONG UPPER LOW ACROSS THE PLAINS AS BAND OF
PRECIPITATION ENTERS WESTERN IOWA. THE INFLUENCE WILL SPREAD
SLOWLY EASTWARD OVERNIGHT AS MOISTURE ADVECTION CONTINUES AHEAD OF
FRONTAL BOUNDARY. MVFR TO IFR CONDITIONS ARE EXPECTED IN AND
AROUND THE PRECIPITATION...ESPECIALLY WEST OF A KMCW-KDSM-KLWD
LINE. THE RELATIVELY LOWER CEILINGS AND VISIBILITIES WILL PERSIST
INTO WEDNESDAY AS THE PRECIPITATION CONTINUES.

&&

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

&&

$$

SHORT TERM...PODRAZIK
LONG TERM...REV
AVIATION...COGIL


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