440  
FXUS63 KDMX 202348  
AFDDMX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DES MOINES IA  
548 PM CST MON FEB 20 2017  
   
SHORT TERM
 
/TONIGHT THROUGH TUESDAY/  
ISSUED AT 328 PM CST MON FEB 20 2017  
 
TREMENDOUS MOISTURE TRANSPORT NORTH FROM THE GULF, ENHANCED BY A  
PRONOUNCED TROUGH EXTENDING FROM SOUTHERN CANADA TO NORTHERN MEXICO,  
HAS BROUGHT RECORD PWAT VALUES TO THE MIDWEST WHICH ARE OVER 400  
PERCENT OF SEASONAL VALUES. WIDESPREAD RAINFALL AMOUNTS TODAY OF ONE  
HALF TO ONE INCH MEASURED. MAINLY LIGHT RAIN STILL ONGOING OVER THE  
FORECAST AREA THIS AFTERNOON. INSTABILITY HAS WEAKENED THIS  
AFTERNOON AS SOME COOLING HAS OCCURRED AROUND 800 MB. A SURFACE  
BOUNDARY MOVING INTO WESTERN IOWA MAY PROVIDE THE FOCUS FOR  
ADDITIONAL INSTABILITY LATER THIS AFTER AND EARLY EVENING AS IT  
MOVES ACROSS CENTRAL IOWA. THEREFORE, A FEW THUNDERSTORMS MAY  
REDEVELOP BEFORE THE SYSTEM EXITS TO THE EAST. MUCH DRIER AIR WILL  
FOLLOW OVERNIGHT AND GOOD SUBSIDENCE WILL DEVELOP AS A RIDGE AXIS  
ARRIVES. FOG POTENTIAL IS A CONCERN CONSIDERING THE RAINFALL TODAY  
AND THEN THE CLEARING SKIES AND LIGHT WINDS ARRIVING OVERNIGHT. HAVE  
ADDED FOG TO THE FORECAST AND EXPECT AN AREA OF DENSE FOG MAY  
DEVELOP LATER TONIGHT INTO TUESDAY MORNING.  
 
TUESDAY WILL BE MILD ONCE AGAIN WITH TEMPERATURES AGAIN NEAR RECORD  
HIGHS. ANY FOG THAT DOES DEVELOP MAY LINGER, ESPECIALLY OVER  
NORTHEAST IOWA FOR MUCH OF THE MORNING AND MAY HAVE AN IMPACT ON  
TEMPERATURES. OTHERWISE, HIGHS WILL BE IN THE 60S TO LOWER 70S.  
   
LONG TERM
 
/TUESDAY NIGHT THROUGH MONDAY/  
ISSUED AT 328 PM CST MON FEB 20 2017  
 
TUE NIGHT THROUGH FRI NIGHT...CONFIDENCE: MEDIUM-HIGH  
 
TWO MAIN STORY LINES WITHIN THIS TIME FRAME. FIRST, WARM, NEAR  
RECORD, TEMPS WILL REMAIN WED. WARM SOUTHERLY FLOW WILL BE AIDED BY  
A PASSING SURFACE LOW TO THE NORTH. 850MB TEMPS IN GOOD AGREEMENT IN  
THE 12-15 DEG C RANGE FROM N TO S, AND WILL PUSH TEMPS BACK INTO THE  
60S TO MID-70S WITH GOOD MIXING EXPECTED. ABNORMALLY WARM TEMPS OF  
LATE THEN BEGIN TO COME TO AN END AS TRAILING COLD FRONT MOVES  
THROUGH, DROPPING TEMPS BACK INTO THE 40S/50S FOR THUR.  
 
SECOND, MAIN SHOW THROUGH THE FORECAST REMAINS A TROUGH DIGGING OUT  
OF THE SW US AND ACCOMPANYING SURFACE LOW TO DEVELOP IN THE TX/OK  
PANHANDLE REGIONS. BY THUR NIGHT, LARGE SURGE OF THETA-E  
ADVECTION/ISENTROPIC LIFT WILL MOVE INTO IOWA AND INITIATE RAIN. AS  
THE LOW DEEPENS THUR NIGHT, TRACK SHOULD TAKE IT ACROSS SE IOWA,  
PROVIDING AN INCREASINGLY LIKELY OPPORTUNITY FOR A SIGNIFICANT  
SNOWFALL EVENT ACROSS PORTIONS OF N/NW IOWA. PERIODS OF BLOWING SNOW  
APPEAR LIKELY AS WELL WITH TIGHTLY WRAPPED LOW AND SUSTAINED 25+ KT  
WINDS. HAVE BUMPED UP WINDS OVER INITIAL GUIDANCE, BUT MAY NEED TO  
BE BUMPED UP A FEW MORE KTS.  
 
FURTHER S/SE, INITIAL PRECIP WILL BE RAIN WITH A FEW THUNDERSTORMS  
MIXED IN AS SUGGESTED BY MUCAPE VALUES AROUND 100-400 J/KG. AS  
SURFACE LOW CONTINUES PROGRESSION NE, STRONG COLD FRONT AND WINDS  
WILL PASS THROUGH THE STATE TURNING PRECIP ALL SNOW. WITH FRONTAL  
PASSAGE, SUSTAINED WINDS AROUND 25KTS AND GUSTS IN EXCESS OF 35 KTS  
WILL BE POSSIBLE FRI AFTERNOON AND INTO THE EVENING. TEMPS WILL  
PLUMMET INTO THE TEENS/LOW 20S FOR LOWS SAT MORNING AND 20S/LOW 30S  
HIGHS. CHANGEOVER TO SNOW SHOULD BE QUICK WITH ENTIRE COLUMN RAPIDLY  
COOLING BELOW FREEZING, THOUGH SOME CONCERN FOR A FREEZE ON ROADS OF  
MELTED/MELTING SNOW DEPENDENT ON HOW QUICKLY GROUND TEMPS REACT. A  
COUPLE OTHER THINGS OF NOTE, PWATS VERY HIGH FOR THIS TIME OF THE  
YEAR IN THE WARM SECTOR AROUND 1-1.1 IN AND EVEN TO AN EXTENT IN THE  
COLD SECTOR AROUND 0.60-0.75 IN. AS A RESULT, PERIODS OF HEAVY  
RAINFALL APPEAR INCREASINGLY LIKELY IN SE IOWA AND HEAVY SNOWFALL  
BACK OVER NW/N IOWA. DGZ DEPTH HAS BACKED OFF A BIT OVER PREVIOUS  
RUNS, CURBING SOME CONCERN FOR SUSTAINED HEAVY SNOWFALL IN THE WRAP  
AROUND/DEFORMATION ZONE THOUGH. WHILE MODELS ARE IN FAIRLY GOOD  
AGREEMENT INTO FRIDAY, GFS SOMEWHAT CURIOUSLY HANGS THE SURFACE LOW  
UP IN SE IA/NW IL, CAUSING IT TO BE ABOUT 6-12 HRS SLOWER THAN THE  
EURO IN EXITING IOWA. GIVEN THE LACK OF BLOCKING FEATURES TO THE  
EAST, HAVE TRENDED POPS TOWARD EURO, BUT POSSIBLY NOT ENOUGH.  
 
SATURDAY THROUGH MONDAY...CONFIDENCE: LOW  
 
LARGE SCALE PATTERN REMAINS IN FAIR AGREEMENT BETWEEN EURO/GFS, WITH  
500 MB FLOW RELATIVELY ZONAL, HOWEVER DETAILS IN WAVES MOVING  
THROUGH THE FLOW DIFFER SIGNIFICANTLY. GFS CONSISTENTLY MORE  
AGGRESSIVE WITH PRECIPITATION CHANCES SUN AND MON AND EURO ONLY  
CREEPING CHANCES IN LATE MON. TEMPS SIGNIFICANTLY DIFFERENT AS WELL  
DUE TO HANDLING OF WAVES, WITH GFS SIGNIFICANTLY COOLER THAN THE  
EURO. HAVE KEPT WITH A BLEND OF THE TWO FOR THE TIME BEING.  
 

 
   
AVIATION
 
/FOR THE 00Z TAFS THROUGH 00Z TUESDAY EVENING/  
ISSUED AT 546 PM CST MON FEB 20 2017  
 
EXTREMELY VARIABLE CONDITIONS ANYWHERE FROM LIFR TO VFR EXIST  
ACROSS THE FORECAST AREA AHEAD OF AN APPROACHING SURFACE TROUGH  
JUST WEST OF THE MO RIVER AT 00Z. GENERALLY THE LOWER END OF THAT  
RANGE EXISTS IN THE KMCW/KALO AREAS. THESE CONDITIONS SHOULD  
GRADUALLY IMPROVE WEST TO EAST AS THE TROUGH MOVES THROUGH  
TONIGHT. HIGH PRESSURE, CLEARING SKIES AND LIGHT/VARIABLE WINDS  
WILL FOLLOW QUICKLY ON ITS HEALS HOWEVER WHICH MAY LEAD TO  
RADIATION FOG DEVELOPMENT EARLY TUE MORNING AS LOW LEVEL MOISTURE  
LINGERS, ESPECIALLY NORTH AND EAST. VFR CONDITIONS ARE  
ANTICIPATED TO RETURN AREA WIDE BY VERY LATE MORNING HOWEVER WITH  
SW WINDS AND SOME SUNSHINE.  
 

 
   
DMX WATCHES/WARNINGS/ADVISORIES
 
NONE.  
 

 
 

 
 
SHORT TERM...DONAVON  
LONG TERM...CURTIS  
AVIATION...SMALL  
 
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