Issued by NWS Des Moines, IA
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000 FXUS63 KDMX 221152 AAA AFDDMX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 652 AM CDT MON SEP 22 2008 UPDATED FOR AVIATION DISCUSSION .DISCUSSION... CONVECTION OVER THE DAKOTAS IN RESPONSE TO MOISTURE ADVECTION IN THAT AREA AND A SHORT WAVE LIFTING ACROSS THE DAKOTAS INTO NW MINNESOTA. THE BETTER THETA-E ADVECTION AND MOISTURE WILL REMAIN TO THE NW OF THE FORECAST AREA BASED ON MODEL OUTPUT. HOWEVER...THE STORMS ARE HOLDING TOGETHER BETTER THAN THE MODELS SUGGEST THEREFORE WILL BEEF UP THE POPS A BIT FOR THE MORNING. THE STORMS WILL LOSE THEIR UPPER LEVEL SUPPORT LATER THIS MORNING SO NO PRECIP WAS MENTIONED FOR THE AFTERNOON HOURS. THIS WILL HAVE TO BE WATCHED HOWEVER SHOULD THE STORMS MAINTAIN THEMSELVES AS HEATING TAKES PLACE. FOR TONIGHT...THE MAIN SHORTWAVE BEGINS PUSHING OUT OF CANADA AND THE NORTHERN PLAINS TOWARDS THE UPPER MIDWEST. A SURFACE LOW OVER THE DAKOTAS AND ATTENDANT COLD FRONT WILL BEGIN TO PUSH TOWARDS IOWA IN RESPONSE TO THE UPPER LEVEL SYSTEM SHIFTING EAST. THE FRONT IS ONLY EXPECTED TO MAKE IT TO WESTERN IOWA BY LATE TUESDAY. CAPES AROUND 1000 J/KG COUPLED WITH DECENT CONVERGENCE ALONG THE FRONTAL BOUNDARY AND SOME MOISTURE IN PLACE FROM SOUTHERLY FLOW AHEAD OF THE FRONT SO THUNDERSTORMS APPEAR A FAIR BET TUESDAY. I WOULD EXPECT A FEW ROBUST STORMS AND IF THERE IS SUFFICIENT HEATING BEFORE STORM INITIATION THEN AN ISOLATED STORM OR TWO COULD BE CAPABLE OF PRODUCING LARGE HAIL AND DAMAGING WIND. THE FRONT CONTINUES TO MOVE ACROSS IOWA TUESDAY NIGHT INTO WEDNESDAY AND MODELS DIVERGE HERE ESPECIALLY WITH THE STRENGTH OF THE FRONT AND THE QPF ASSOCIATED WITH ANY SQUALL LINE DEVELOPING. FOR NOW I OPTED TO GO WITH A BLEND OF THE GFS AND EURO...LEANING MORE TOWARDS THE SLOWER AND WETTER EURO MODEL. AS THE UPPER WAVE PUSHES EAST WEDNESDAY THE FRONT SORT OF HANGS ACROSS WISCONSIN AND IOWA BUT BY THIS TIME THE DYNAMICS IS MINIMAL AS IS THE UPPER SUPPORT. STILL THE GFS HAS BEEN A LITTLE MORE CONSISTENT WITH PRODUCING SOME KIND OF LIGHT POPS ACROSS THE AREA WEDNESDAY NIGHT/THURSDAY. I DID LEAVE SOME MENTION OF POPS IN FOR WEDNESDAY NIGHT BUT STILL NOT CONVINCED WE`LL SEE MUCH THURSDAY AS THE BETTER INSTABILITY WILL BE WELL NORTH...IN MINNESOTA. THE GFS DOES HINT AT A MINOR WAVE PUSHING ACROSS NORTHERN IOWA/SOUTHERN MINNESOTA AT THIS TIME BUT FURTHER ANALYSIS SHOWS LITTLE SUPPORT WITH THIS FEATURE SO I LEFT THURSDAY NIGHT AND FRIDAY DRY FOR THE TIME BEING. HOWEVER...THIS WILL NEED TO BE MONITORED FOR POSSIBLE INTRODUCTION OF LIGHT PRECIP OVER THE NORTH LATE FRIDAY. AS FOR TEMPS...GENERALLY STAYED WITH PERSISTENCE THOUGH I DID LOWER HIGHS ON WEDNESDAY A BIT. && .AVIATION...22/12Z LOW CLOUDS AND FOG PRODUCING MVFR TO IFR CIGS AND VISIBILITIES WL DISSIPATE THIS MORNING AS MIXING INCREASES. SOME SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS ARE ALSO EXPECTED ACRS FAR W AND NW PORTIONS OF THE STATE THIS MORNING. HOWEVER...THE PRECIPITATION THREAT SHOULD DIMINISH BY THIS AFTERNOON AS SOUTHEAST SFC WINDS CONTINUE TO INCREASE WITH QUITE BREEZY CONDITIONS BY AFTERNOON. VFR CONDITIONS ARE FORECAST FOR THIS AFTERNOON INTO TONIGHT. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM/AVIATION...COGIL LONG TERM...FAB
