018  
FXUS63 KDMX 200526  
AFDDMX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE DES MOINES IA  
1126 PM CST SUN NOV 19 2017  
   
SHORT TERM
 
/TONIGHT THROUGH MONDAY/  
ISSUED AT 307 PM CST SUN NOV 19 2017  
 
RIDGE OF HIGH PRESSURE HAS MOVED EAST INTO EASTERN IOWA AND THIS HAS  
LED TO RETURN FLOW AND WARM ADVECTION MOVING INTO THE AREA THIS  
AFTERNOON. THE SOUTH/SOUTHWEST WINDS WILL DIMINISH OVERNIGHT BUT  
SHOULD STILL REMAIN IN THE 5 TO 10 MPH RANGE AND ENOUGH TO KEEP WEAK  
SURFACE WARM ADVECTION CONTINUING WHILE MUCH STRONGER WARM ADVECTION  
OCCURS FROM 900-850 MB. THIS DISPARITY WILL RESULT IN A STRONG  
INVERSION AROUND 900 MB TO START THE DAY ON MONDAY. IN ADDITION, THE  
WARM ADVECTION WILL KEEP OVERNIGHT LOWS MUCH WARMER THAN THIS LOW  
INVERSION WILL LIMIT MIXING POTENTIAL ON MONDAY, THEREFORE, LIMITED  
HIGHS IN THE UPPER 50S DESPITE THE THERMAL RIDGE AND 850 MB  
TEMPERATURES AROUND 10C ARRIVING. SOUTHWEST WINDS WILL BECOME BREEZY  
TO GUSTY BY LATE MORNING AND INTO THE AFTERNOON, HOWEVER, THE  
MOMENTUM TRANSFER OF MIXED LAYER WINDS WILL BE DAMPENED BY THE  
ONGOING WARM ADVECTION. A FEW GUSTS OVER 30 MPH WILL BE POSSIBLE  
THOUGH THE WIND SPEEDS WILL REMAIN BELOW ADVISORY CRITERIA. SOME  
PASSING HIGH LEVEL CIRRUS CLOUDS AT TIMES TONIGHT INTO MONDAY THOUGH  
SHOULD NOT BE DENSE ENOUGH TO PROHIBIT HIGH TEMPERATURES.  
   
LONG TERM
 
/MONDAY NIGHT THROUGH SUNDAY/  
ISSUED AT 307 PM CST SUN NOV 19 2017  
 
TEMPERATURES WILL BE BELOW NORMAL TUESDAY AND WEDNESDAY WITH WARMER  
AIR ARRIVING FOR THANKSGIVING AND FRIDAY BEFORE COLDER AIR ARRIVES  
INTO NEXT WEEKEND. CHANCES FOR WIDESPREAD DRY WEATHER ACROSS CENTRAL  
IOWA REMAINS HIGH THROUGH FRIDAY MORNING.  
 
A SHORTWAVE TROUGH WILL PIVOT TOWARD THE GREAT LAKES AROUND THE  
HUDSON BAY LOW PRESSURE MONDAY NIGHT. AT THE SURFACE, A COLD FRONT  
WILL MOVE FROM THE NORTHWEST TO THE SOUTHEAST AND BE THROUGH MUCH IF  
NOT ALL OF CENTRAL IOWA BY 12Z TUESDAY. WHILE MID AND HIGH LEVEL  
CLOUDS WILL ACCOMPANY THE FRONT, LOW LEVEL MOISTURE WILL BE LACKING  
SO LOOKS TO BE DRY DESPITE A FEW MODELS (E.G. GFS, WRF HIRES WINDOW)  
INDICATED LIGHT PRECIPITATION. BEHIND THIS FRONT, STRONG COLD AIR  
ADVECTION WITH GUSTY WINDS FROM THE NORTHWEST WILL PREVAIL AS SKIES  
BECOME MOSTLY SUNNY ON TUESDAY. 850MB TEMPERATURES WILL DROP TO  
BETWEEN -6 AND -10C WITH A 24 HOUR CHANGE BY 00Z WEDNESDAY OF -15 TO  
-16C. TEMPERATURES WILL ALSO EXHIBIT A NON-DIURNAL TREND OVER THE  
FORECAST AREA. TEMPERATURES MAY FALL THROUGH THE MORNING BEFORE  
BRIEFLY RISING IN THE AFTERNOON OVER NORTHERN IOWA, BE FAIRLY STEADY  
OVER THE CENTRAL PART OF THE STATE, AND FALL THROUGH THE AFTERNOON  
OVER SOUTHERN IOWA. HIGHS WILL RANGE FROM AROUND 30 DEGREES NEAR THE  
MINNESOTA BORDER TO THE LOW 40S OVER SOUTHERN IOWA. BUFKIT NAM  
SOUNDINGS SHOW WINDS AT THE TOP OF THE MIXED LAYER AROUND 40 KNOTS  
NEAR MIDDAY WITH BOTH THE GFS AND NAM SHOWING WINDS AT THE TOP OF  
THE MIXED LAYER OF 30 KNOTS THROUGH AROUND 5PM. THEREFORE, INCREASED  
WINDS FROM INITIAL SUPERBLEND GUIDANCE BY BLENDING IN CONSMOS FROM  
12Z TO 22Z TUESDAY, WHICH WILL HELP TO ACCENTUATE THE CHILL OF THE  
DAY.  
 
THROUGHOUT THE DAY TUESDAY INTO TUESDAY NIGHT, A 1030MB HIGH  
PRESSURE WILL BE MOVING THROUGH THE DAKOTAS INTO EASTERN NEBRASKA  
BECOMING CENTERED AROUND KANSAS CITY, MO BY 12Z WEDNESDAY. WITH  
CROSS SECTIONS AND FORECAST SOUNDINGS SHOWING A FAIRLY DRY AIRMASS  
AND WINDS DROPPING OFF IN RESPONSE TO THE HIGH, ESPECIALLY ACROSS  
WESTERN IOWA, DID LOWER TEMPERATURES A DEGREE OR TWO OVER THESE  
AREAS.  
 
AS THE HIGH PRESSURE MOVES AWAY TO THE EAST ON WEDNESDAY, A WEAK PV  
ANOMALY WILL MOVE FROM THE SOUTHERN CANADIAN PRAIRIES INTO THE  
WESTERN GREAT LAKES WEDNESDAY AFTERNOON INTO THURSDAY MORNING. THE  
PRECIPITATION LOOKS TO STAY PREDOMINATELY NORTH OF THE AREA WITH  
GFS/ECMWF FORECAST SOUNDINGS SHOWING PLENTY OF MID-LEVEL SATURATION,  
BUT A LACK OF LOW LEVEL SATURATION. THEREFORE, WILL MAINTAIN A DRY  
FORECAST DURING THIS TIME. AS THE PV ANOMALY MOVES AWAY, WARM AIR  
ADVECTION WILL BEGIN ON THANKSGIVING DAY AS TEMPERATURES REBOUND TO  
NEAR NORMAL.  
 
TEMPERATURES ARE EXPECTED TO CONTINUE TO RISE ON FRIDAY, THOUGH BY  
HOW MUCH REMAINS A BIT IN QUESTION. AFTER THE 18/12Z ECMWF RUN  
SUDDENLY TRENDED LOWER THAN THE 18/00Z RUN, THE ECMWF 19/00Z AND  
19/12Z RUNS ARE BACK TO MUCH HIGHER TEMPERATURES IN THE UPPER 50S TO  
PERHAPS UPPER 60S OVER THE FAR SOUTHWEST FORECAST AREA. THE GFS OVER  
ITS PAST SEVERAL RUNS REMAINS QUITE WARM WITH 60S OVER AT LEAST THE  
SOUTHWEST HALF OF IOWA. LAST THREE 00Z RUNS OF THE NORTH AMERICAN  
ENSEMBLE FORECAST SYSTEM (NAEFS) AT KDSM HAS BEEN TRENDING HIGHER  
WITH AROUND 9C FROM THE 17TH RUN, 10C FROM THE 18TH RUN, AND THE  
LATEST RUN BEING AROUND 13C. WHILE THE RANGE HAS BEEN DECREASING  
OVER THESE RUNS, THE RANGE BETWEEN THE MINIMUM AND MAXIMUM VALUES  
REMAINS AROUND 20C WEIGHTED TOWARD THE MINIMUM SIDE. PART OF THE  
LARGE RANGE IN TEMPERATURES IS UNCERTAINTY ABOUT WHAT IF ANY  
CLOUDS MAY IMPACT THE AREA LATER FRIDAY AS A SHORTWAVE TROUGH  
CROSSES SOUTHERN CANADA/THE NORTHERN US STATES WITH A COLD FRONT  
APPROACHING THE AREA. FORECAST SOUNDINGS FROM THE GFS/ECMWF  
CONTINUE TO SHOW VARYING INCREASES OF MOISTURE IN THE MIDDLE AND  
UPPER LEVELS OF THE ATMOSPHERE OVER NORTHERN IOWA FRIDAY  
AFTERNOON. THEREFORE, CONFIDENCE IS HIGHEST IN HIGHER TEMPERATURES  
OVER SOUTHERN PARTS OF THE FORECAST AREA WHERE CONCERNS OF ANY  
HIGH LEVEL CLOUDS ARE THE LOWEST. HIGHS ARE STILL WEIGHTED TOWARDS  
SUPERBLEND, WHICH IS CATCHING ON TO THE GFS/ECMWF WARMTH TO A  
SMALL DEGREE, WITH SMALL PERCENTAGES OF THE GFS/ECMWF ALSO  
INCORPORATED. IF IT BECOMES MORE CERTAIN THAT THESE CLOUDS WILL BE  
HIGHER LEVEL AND NOT PERSISTENT, HIGHS COULD EASILY BE 5 DEGREES  
HIGHER.  
 
WHILE THE SHORTWAVE TROUGH MOVES INTO THE GREAT LAKES, SURFACE LOW  
PRESSURE WILL MOVE OVER SOUTHERN CANADA INTO THE GREAT LAKES WITH  
MUCH OF THE PRECIPITATION REMAINING NORTH OF THE AREA. HOWEVER,  
PRECIPITATION MAY BRUSH THE AREA AS GLOBAL MODELS BRING IT CLOSE TO  
NORTHERN IOWA. AS THE COLD FRONT MOVES THROUGH THE AREA FRIDAY NIGHT  
INTO EARLY SATURDAY, COLDER AIR WILL ARRIVE AS COLD AIR ADVECTION  
AND GUSTY NORTHWEST WINDS ONCE AGAIN PREVAIL ACROSS THE STATE.  
 

 
   
AVIATION
 
/FOR THE 06Z TAFS THROUGH 06Z MONDAY NIGHT/  
ISSUED AT 1126 PM CST SUN NOV 19 2017  
 
LITTLE HAS CHANGED SINCE 00Z. CONFIDENCE IS HIGH THAT VFR  
CONDITIONS WILL CONTINUE THROUGH THE PERIOD WITH NOTHING BEYOND  
PERIODS OF HIGH CLOUDINESS. WINDS WILL REMAIN S/SSW AT SIMILAR  
SPEEDS UNTIL INCREASING WITH SOME GUSTS DURING MON AFTERNOON  
MIXING, THEN DECREASING AGAIN INTO THE EVENING.  
 

 
   
DMX WATCHES/WARNINGS/ADVISORIES
 
NONE.  
 

 
 

 
 
SHORT TERM...DONAVON  
LONG TERM...ANSORGE  
AVIATION...SMALL  
 
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