283  
FXUS63 KDMX 230008  
AFDDMX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DES MOINES IA  
608 PM CST WED JAN 22 2020  
   
UPDATE
 
 
ISSUED AT 604 PM CST WED JAN 22 2020  
 
HOLDING ONTO THE ADVISORY MAINLY EAST OF I35 AS STILL SOME  
ENHANCED RETURNS AND POOR TRAVEL CONDITIONS IN THE EASTERN AREAS  
OF THE REGION. TEMPERATURES THERE SLIGHTLY COLDER AND PLOW CAMS  
STILL SHOW SNOW COVERED CONDITIONS ALONG THE INTERSTATE AND  
SECONDARY ROADS IN EAST CENTRAL IOWA. WILL EXPIRE AT 9 PM.  
 

 
   
DISCUSSION
 
/TONIGHT THROUGH WEDNESDAY/  
ISSUED AT 309 PM CST WED JAN 22 2020  
 
THE PRIMARY CONCERNS WILL BE FOG TRENDS INTO THURSDAY MORNING IN  
ADDITION TO CONTINUED WINTRY PRECIPITATION TRENDS THROUGH FRIDAY  
WITH CONFIDENCE REMAINING ON THE LOW SIDE. OUR CURRENT SCENARIO  
HAS FAIRLY WIDESPREAD DEFORMATION ZONE PRECIPITATION DRIVEN BY A  
NOTED SHORT WAVE/PV ANOMALY ESSENTIALLY OVERHEAD, WHICH SHOWS UP  
NICELY IN RADAR AND VARIOUS SATELLITE IMAGERY PRODUCTS. THE ONLY  
PROBLEM TODAY HAS BEEN THAT MODELS HAD POORLY HANDLED THE THERMAL  
STRUCTURE, BEING TOO COOL, RESULTING IN MORE MIX/LIQUID THAN THE  
PREVIOUSLY ANTICIPATED SNOW EARLIER TODAY. WE SHOULD BE BACK TO  
MAINLY SNOW THROUGH SUNSET NOW HOWEVER WITH DECENT BUT BRIEF  
SNOWFALL RATES INTO THE EVENING COMMUTE. THIS APPEARS TO BE DRIVEN  
BY SUPERSATURATION IN THE DENDRITIC GROWTH ZONE /RAP/ PRODUCING  
STELLAR DENDRITES, WITH AN 800M ISOTHERMAL LAYER JUST BELOW 0C PER  
THE GFS HELPING NEAR SURFACE AGGREGATION.  
 
THESE KINEMATIC DEFORMATION ZONE LIFT CONTRIBUTIONS SHOULD  
DIMINISH DURING THE EVENING HOWEVER AND BE REPLACED BY LATE  
ARRIVING WARM/THETA-E ADVECTION THERMAL FORCING SOUTH AND EAST  
INTO THE NIGHT. THIS WILL KEEP LIGHT PRECIP GOING IN THOSE  
LOCATIONS. THE PRECIP TYPE MAINLY LOOKS TO BE LIGHT SNOW, ALTHOUGH  
MUCH LIKE TODAY SOME WINTRY MIX MAY OCCUR AT TIMES DUE TO  
AN INTERMITTENT LACK OF ICE INTRODUCTION DURING THE WEAKER LIFT.  
THE FORCING/OMEGA IS WEAK, AND PARTICULARLY NOT MUCH IN THE  
DENDRITIC GROWTH ZONE. THUS THE CURRENT WINTER WEATHER ADVISORY  
WILL BE ALLOWED TO EXPIRE. SNOW RATES SHOULD NOT REACH CRITERIA,  
AND TRAVEL IMPACTS HAVE BEEN MINIMAL DUE TO ROAD TEMPS AND  
DEWPOINTS AT OR ABOVE FREEZING SO ANY ADDITIONAL LIQUID COMPONENT  
OF A WINTRY MIX SHOULD NOT AMOUNT TO MUCH.  
 
FARTHER WEST THE PRIMARY CONCERN WILL LIKELY TURN TO FOG  
OVERNIGHT, WHICH MAY BE DENSE. HUMIDITIES ARE ALREADY HIGH WITH  
NIGHTFALL AND STEADY STATE LOW LEVEL MOISTURE NEAR THE SURFACE  
TROUGH. THIS CONCEPTUAL MODEL IS ALSO SUPPORTED BY VARIOUS RAW AND  
MOS GUIDANCE SOURCES, WHICH SUGGEST DENSE FOG WELL INTO THE  
MORNING. HAVE ADDED PLENTY OF FOG WORDING AND WILL INTRODUCE A  
DENSE FOG ADVISORY ALONG AND WEST OF I-35 FOR STARTERS BUT FURTHER  
EXPANSION EASTWARD MAY BE NECESSARY.  
 
LOOKING AHEAD TO THURSDAY NIGHT FOG MAY BE A SIMILAR STORY, ALBEIT  
A BIT FARTHER EAST AS THE INVERTED TROUGH PROGRESSES COINCIDENT  
WITH THE MO SURFACE LOW DEVELOPMENT. PRECIP WILL ALSO BEGIN TO  
BECOME MORE PREVALENT AGAIN AS THE CURRENT NORTHERN ROCKIES PV  
TROUGH INCREASES MAINLY KINEMATIC LIFT FROM SOUTHERN IA THU NIGHT  
INTO EASTERN IA BY FRI. THIS WILL GENERATE ONE LAST ROUND OF LIGHT  
SNOW WITH CUMULATIVE AMOUNTS NOW LESS THAN PREVIOUSLY ADVERTISED,  
LITTLE NW TO AS HIGH AS 3-4 INCHES SOUTHEAST.  
 
THE OUTLOOK FOR THE WEEKEND HAS CHANGED LITTLE WITH DRY  
CONDITIONS AND NORMAL TO JUST ABOVE NORMAL TEMPERATURES. A MINOR  
CONCERN WILL BE ONE LAST SHORT WAVE DROPPING INTO THE NORTHWEST  
FLOW AROUND SUNDAY, BUT FOR NOW BOTH FORCING AND SATURATION ARE  
SPOTTY AND NEVER REALLY PHASED. AFTER MORE FAIR WEATHER AND SHORT  
WAVE RIDGING THROUGH MONDAY THE MODELS ARE IN DECENT AGREEMENT,  
CONSIDERING THE TIME RANGE, BRINGING ANOTHER TROUGH FROM THE  
CENTRAL AND SOUTHERN PLAINS INTO THE MID MS VALLEY TUESDAY INTO  
WEDNESDAY. THIS WILL BRING ANOTHER POTENTIALLY PROLONGED PRECIP  
EVENT WITH FORCING AND MOISTURE IN PLACE ACROSS IA FOR A FEW DAYS.  
ROUGH EXAMINATION OF THE THERMAL STRUCTURE SUGGESTS THIS WOULD  
PRIMARILY BE SNOW.  
 

 
   
AVIATION
 
/FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING/  
ISSUED AT 607 PM CST WED JAN 22 2020  
 
MANY CONCERNS REMAIN WITH LOW CIGS, LINGERING PRECIPITATION AND  
EVENTUAL DENSE FOG THAT MAY PERSIST THROUGH MOST OF THURSDAY  
AFTERNOON. WILL NEED TO EVALUATE THE ENDING TIME OF LOW VSBY FOR  
THE 06Z PACKAGE. OTHERWISE, LIFR/IFR CONDITIONS WITH PERIODS OF  
LIGHT SNOW, FOG AND PATCHY FREEZING DRIZZLE EXPECTED. /REV  
 

 
   
DMX WATCHES/WARNINGS/ADVISORIES
 
 
DENSE FOG ADVISORY UNTIL NOON CST THURSDAY FOR IAZ004>007-  
015>017-023>026-033>037-044>048-057>060-070>073-081>083-092>094.  
 
WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR IAZ027-  
028-038-039-049-050-061-062-074-075-084>086-095>097.  
 

 
 

 
 
UPDATE...REV  
DISCUSSION...SMALL  
AVIATION...REV  
 
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