643  
FXUS63 KDMX 201516  
AFDDMX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DES MOINES IA  
916 AM CST WED FEB 20 2019  
   
UPDATE
 
 
ISSUED AT 916 AM CST WED FEB 20 2019  
 
HAVE UPDATED TO EXTEND THE WINTER WEATHER ADVISORY FOR FAR SOUTH  
CENTRAL IOWA AND EXPAND IT INTO CENTRAL IOWA TO REPLACE THE  
EXPIRING WINTER STORM WARNING GIVEN THE LIGHT SNOW AND FREEZING  
DRIZZLE ACROSS THE CENTRAL/SOUTHERN PORTION OF THE STATE. THIS  
SHOULD TAPER OFF THROUGH THE LATE MORNING HOURS AS THE WEAK UPPER  
WAVE TO THE SOUTHWEST MOVES THROUGH THE STATE. LIGHT SNOW TO THEN  
LINGER ACROSS THE NORTH THIS AFTERNOON. LITTLE ADDITIONAL  
ACCUMULATIONS OF SNOW EXPECTED IN THE ADVISORY AREA, MAYBE UP TO  
AN INCH. OTHERWISE MAIN ACCUMULATIONS OF SNOW LIKELY THROUGH THE  
DAY STILL ACROSS NORTHERN IOWA.  
 

 
   
SHORT TERM
 
/TODAY THROUGH THURSDAY/  
ISSUED AT 439 AM CST WED FEB 20 2019  
 
THE PRIMARY CONCERN WILL BE DIFFICULT FORECAST TRENDS AND HEADLINE  
ADJUSTMENTS TODAY WITH CHANGES FROM PREVIOUS EXPECTATIONS. STRONG,  
DEEP QG FORCING CONTINUES TO ADVANCE THROUGH IOWA EARLY THIS MORNING  
BUT ALSO HAS BEEN ACCOMPANIED WITH A PUSH OF MID LEVEL DRY AIR AS  
NOTED IN RECENT WATER VAPOR IMAGERY. THIS HAS RESULTED IN POCKETS OF  
DRY AIR THAT HAS PRODUCED INEFFICIENT SNOW/ICE CRYSTAL PRODUCTION  
AND EVEN PERIODS OF LITTLE TO NO PRECIPITATION ALTOGETHER AT TIMES.  
THIS HAS ENDED UP WITH VARIABLE SNOW ACCUMULATIONS WITH LIMITED  
PREDICTABILITY FROM SPOT TO SPOT, RANGING FROM JUST A DUSTING UP TO  
THREE OR FOUR INCHES DEPENDING ON LOCATION AND WHERE THE DRY POCKET  
OCCURRED.  
 
RECENT IR SATELLITE TRENDS AND RADAR MOSAICS SUGGESTS THE LAST BURST  
OF APPRECIABLE MOISTURE AND LIFT IS PUSHING THROUGH CENTRAL AND  
NORTHERN IOWA RIGHT NOW AND WILL BE FAVORED LOCATIONS FOR ADDITIONAL  
APPRECIABLE ACCUMULATIONS THIS MORNING, ANYWHERE FROM 1 TO 4 INCHES  
DEPENDING ON LOCATIONS. FARTHER SOUTH, WARMER CLOUD TOPS AND SUBTLE  
NEGATIVE DBZ RADAR ECHOES NOTE A TREND MORE TO FLURRIES AND/OR  
FREEZING DRIZZLE SOUTH. THERE MAY BE ANOTHER BURST OF LIGHT TO  
MODERATE SNOW IMMEDIATELY ATTENDANT TO THE UPSTREAM TROUGH AND  
KINEMATIC FORCING AS IT MOVES FROM EASTERN NE INTO IOWA, BUT THIS  
SHOULD NOT LAST TOO LONG FROM POINT TO POINT AS IT MOVES THROUGH  
WITH NO MORE THAN AN INCH OR TWO ADDITIONAL ACCUMULATIONS, AT MOST.  
THIS SEEMS TO BE CAPTURED FAIRLY WELL IN THE HRRR AND ESRL HRRR  
TRENDS, WHICH BRING THE BACK EDGE OF PRECIP THROUGH CENTRAL IA BY  
15Z. THUS HAVE MADE SOME MINOR HEADLINE ADJUSTMENTS TO TRY AND BLEND  
CURRENT EXPECTATIONS VERSUS ADJACENT HEADLINES WITH A DOWNGRADE TO  
AN ADVISORY SOUTH TWO TIERS AND WARNINGS FARTHER NORTH EITHER  
UNCHANGED OR ENDING AT 15Z. REGARDING BLOWING SNOW, THE PRESSURE  
GRADIENT INCREASED A BIT MORE THAN ANTICIPATED AHEAD OF THE  
SOUTHEAST NE SURFACE WAVE, BUT THIS SHOULD DIMINISH LATER THIS  
MORNING AS THE WAVE REACHES NORTHERN IA AND THE GRADIENT PUSHES  
NORTH. SOME MINOR BLOWING SNOW MAY RETURN NORTH LATE AS THE LOW  
DEPARTS THIS AFTERNOON, AND SHOULD NOT BE TOO SIGNIFICANT, BUT FELT  
THE POTENTIAL SUFFICIENT TO KEEP NORTHERN IA HEADLINES GOING UNTIL  
00Z. TEMP WISE, HIGHS ARE EXPECTED TO REBOUND NEAR THE FREEZING  
MARK, SO THAT COMBINED WITH LATE WINTER INSOLATION SHOULD HELP  
REDUCE THE FREEZING DRIZZLE IMPACTS LATER THIS MORNING.  
 
FAIR AND MORE TRANQUIL WEATHER SHOULD RETURN MOMENTARILY INTO  
THURSDAY AS A WEAK MID LEVEL RIDGE APPROACHES. THERE SHOULD BE SOME  
SUN, BUT WEAK SURFACE FLOW AND RIDGING WILL KEEP ANY APPRECIABLE  
REBOUND AT BAY.  
   
LONG TERM
 
/FRIDAY THROUGH TUESDAY/  
ISSUED AT 439 AM CST WED FEB 20 2019  
 
LONG TERM...  
 
SATURDAY-SUNDAY WINTER SYSTEM...  
CONFIDENCE IN NOW HIGH IN THE OCCURRENCE OF THIS SYSTEM DURING  
THE SATURDAY-SUNDAY TIME PERIOD. GOES-R WATER VAPOR IMAGERY  
PICKING UP ON THE PARENT UPPER LOW COMING ONSHORE TO THE PACIFIC  
NW. WITH THE PROXIMITY TO THE US MAINLAND NOW DECENTLY HIGH, THIS  
SYSTEM HAS BEEN INCREASINGLY BETTER SAMPLED THE PAST FEW MODEL  
RUNS. WITH THIS INCREASE SAMPLING A TREND IS STICKING OUT TO BOTH  
PUSH THIS SYSTEM EVER FASTER (ABOUT A 12 HOUR INCREASE SINCE THE  
00Z MON OUTPUT), AND TO INTENSIFY THIS SYSTEM. THE 00Z MON GFS HAD  
THE SFC LOW HAD AN INTENSITY OF AROUND 992 MB. FOR THE 00Z WED  
GFS, THE SFC LOW IS DOWN TO 988MB. THE FORECAST INTENSITY OF THE  
SFC LOW 24 HOURS PRIOR TO ARRIVING IN IOWA IS 1003MB... WHICH IS  
NEARING, BUT JUST SHORT OF THE FORMAL DEFINITION OF "BOMBOGENESIS"  
FOR NOW. WILL BE INTERESTING TO SEE IF THE INTENSIFICATION TREND  
CONTINUES. THIS SYSTEM WILL HAVE ACCESS TO GULF OF MEXICO  
MOISTURE... HOWEVER, DEPENDING UPON THE ULTIMATE TRACK OF THE  
SYSTEM, IT MAY NOT FULLY ACCESS THIS MOISTURE IN IOWA.  
 
THE TIMING OF THIS SETUP IS SUCH THAT A BAND OF PRECIPITATION MAY  
FORM DURING FRIDAY AFTERNOON INTO THE EVENING. AS OF NOW, THIS BAND,  
IF THIS CURRENT SETUP HOLDS, STRETCHES FROM NW IOWA TO SE IOWA,  
COVERING THE DES MOINES METRO. WITH SFC TEMPS IN THE LOW 30S, THIS  
COULD BE MIXED PRECIP... WITH SNOW IN THE NW AND RAIN IN THE SE. BY  
SATURDAY AM, THE SFC LOW TRACKS NEAR TO OVER IOWA THROUGH SUNDAY...  
PUSHING EAST OF IOWA POSSIBLY BY NOON SUNDAY.  
 
ULTIMATELY, THE CHALLENGES THAT WILL MAY OR MAY NOT LEAD TO  
SIGNIFICANT IMPACTS ARE 1.) TRACK OF THE LOW; 2) INTENSITY OF  
SYSTEM...  
 
1.) TRACK OF THE LOW: THIS WILL BE THE BIGGEST DETERMINER OF TYPE  
OF IMPACT ACROSS IOWA. IN THE SE/WARM SECTOR OF THIS SYSTEM,  
PRECIP WILL FAVOR RAIN AS MAX WETBULB TEMPS MAY EXCEED +5C. IN THE  
NW/COLD SECTOR OF THIS SYSTEM, SNOW WILL BE FAVORED. THERE WILL  
BE A TIGHT TEMPERATURE GRADIENT WITH THIS SYSTEM... TO PUT IT IN  
PERSPECTIVE, EASTERN IOWA MAY BE AROUND +5C AT 850MB, AND WESTERN  
IOWA MAY BE AROUND -5C AT 850MB. THE OVERALL PROGRESSION OF THIS  
SYSTEM APPEARS TO BE A PUSH OF MIXED/ICED PRECIP FROM SOUTH TO  
NORTH EARLY SATURDAY MORNING... BY SATURDAY AFTERNOON, THIS  
FREEZING PRECIP "LINE" PIVOTS TO BECOME MORE ORIENTED SW-NE. THEN  
AS THE SYSTEM PULLS AWAYS, COLD AIR WILL SURGE IN, QUICKLY  
TRANSITIONING ALL PRECIP TO SNOW. AT THIS TIME, SWATH OF FREEZING  
RAIN/ICE PELLETS/SLEET LIKELY TOO WIDE. AS CONFIDENCE IN TRACK  
INCREASES, CAN NARROW DOWN THIS BAND OF ICED PRECIP TYPES.  
 
2.) INTENSITY OF THIS SYSTEM: AS THE LOW HAS TRENDED TO DEEPEN, ONE  
OF THE RESULTS IS STRENGTHENING OF THE FORECAST WINDS. ON THE COLD  
SIDE OF THIS SYSTEM, A 50KT LLJ IS SLATED TO PERSIST THROUGH IOWA  
SATURDAY NIGHT INTO SUNDAY. WITH AN INCREDIBLY TIGHT PRESSURE  
GRADIENT, STRONG WINDS WILL BE SUPPORTED. FORECAST SOUNDINGS HAVE  
OVER 50 KTS THROUGH THE TRANSPORT LAYER. THIS WILL LIKELY LEAD TO  
BLOWING SNOW... ESPECIALLY ON THE COLD SIDE- LIKELY TO SET UP IN NW  
IOWA. WILL BE WATCHING THIS CLOSELY. AT THE VERY LEAST, WINDS COULD  
BE HEADLINE CRITERIA DURING THIS TIMEFRAME.  
 
MONDAY-TUESDAY SYSTEM...  
FOR THE PAST FEW RUNS, LONG-RANGE MODELS HAVE BEEN HINTING AT A  
SYSTEM TO IMPACT IOWA SOMETIME MONDAY INTO TUESDAY VIA SOME SORT  
OF AN INVERTED TROUGH. DUE TO LACK OF CONFIDENCE, WILL NOT GET  
INTO MESOSCALE DETAILS AT THIS TIME. BUT THIS COULD BRING A FEW  
MORE INCHES OF SNOW, SHOULD IT COME TO FRUITION. WINDS MAY BE  
STRONG ON THE BACKSIDE OF THIS POTENTIAL SYSTEM AS WELL.  
 

 
   
AVIATION
 
/FOR THE 12Z TAFS THROUGH 12Z THURSDAY MORNING/  
ISSUED AT 553 AM CST WED FEB 20 2019  
 
VARIABLE CONDITIONS FROM MVFR TO LIFR WERE IN PLACE ACROSS CENTRAL  
IA AT 12Z WITH SNOW OR FREEZING DRIZZLE IN PROGRESS AT MANY  
LOCATIONS. MOST OF THE IFR AND LIFR LOCATIONS WERE CENTRAL AND  
NORTH, AS WAS MUCH OF THE SNOW. THE PRECIPITATION SHOULD DIMINISH  
THROUGH MIDDAY, ENDING DURING THE AFTERNOON, WITH A LIKELY RETURN  
TO VFR CONDITIONS BY TONIGHT.  
 

 
   
DMX WATCHES/WARNINGS/ADVISORIES
 
 
WINTER WEATHER ADVISORY UNTIL NOON CST TODAY FOR IAZ057>062-  
070>075-081>086-092>097.  
 
WINTER STORM WARNING UNTIL 6 PM CST THIS EVENING FOR IAZ004>007-  
015>017-023>028-037>039-050.  
 
WINTER STORM WARNING UNTIL NOON CST TODAY FOR IAZ044>049.  
 
WINTER STORM WARNING UNTIL 3 PM CST THIS AFTERNOON FOR  
IAZ033>036.  
 

 
 

 
 
UPDATE...BEERENDS  
SHORT TERM...SMALL  
LONG TERM...KOTENBERG  
AVIATION...SMALL  
 
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