997  
FXUS65 KGGW 151529  
AFDGGW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GLASGOW MT  
829 AM MST SUN DEC 15 2024  
   
DISCUSSION  
 
*MORNING UPDATE:  
 
UPGRADED EASTERN ZONES TO A WINTER STORM WARNING DUE TO  
SIGNIFICANT IMPACTS OBSERVED FROM ICING AND SEVERAL HOURS OF  
FREEZING RAIN TO COME. SOME STATIONS HAVE RECORDED OVER A QUARTER  
INCH OF PRECIPITATION SO FAR IN WESTERN PORTIONS OF DAWSON AND  
RICHLAND COUNTIES WITH A PORTION OF I-94 CLOSED. TO THE WEST THE  
WINTER WEATHER ADVISORY WAS CANCELLED, AS A SHARP CUTOFF HAS  
CONTINUED TO KEEP PHILLIPS AND VALLEY COUNTIES MOSTLY DRY.  
 
KEY MESSAGES:  
 
- WINTER WEATHER ADVISORY THROUGH 11 PM TODAY FOR ALL COUNTIES  
EXCEPT PETROLEUM COUNTY, PRIMARILY FOR A LIGHT GLAZE UP TO 0.2"  
OF ICE AND MINOR SNOW ACCUMULATION.  
 
- MULTIPLE DISTURBANCES ARE FORECAST FOR MONDAY NIGHT INTO  
TUESDAY, AND WEDNESDAY EVENING INTO THURSDAY. LIGHT SNOW IS  
EXPECTED WITH THE FIRST DISTURBANCE.  
 
WEATHER PATTERN OVERVIEW:  
 
IN THE MORE RECENT DEVELOPMENT OF A SURFACE LOW PRESSURE WITH A  
STRONG LOW LEVEL INVERSION AND SUFFICIENT MOISTURE TRANSPORT, HIGH  
RESOLUTION AND GLOBAL ENSEMBLE MODELS HAVE COME INTO AGREEMENT.  
ALTHOUGH THE MAJORITY OF THE GGW CWA WILL SEE A GLAZE OF FREEZING  
RAIN, THERE IS A SWATH SET UP FROM EASTERN GARFIELD NORTHEAST TO  
PLENTYWOOD OF UP TO TWO TENTHS EXPECTED THROUGH THE LATE AFTERNOON  
TODAY. THERE IS HIGH (60 PERCENT) CONFIDENCE PRECIPITATION WILL  
TRANSITION TO SNOW BY EARLY EVENING FOR MOST OF THE AREA BEFORE  
ENDING ON SUNDAY NIGHT. FOR SOUTHWEST PHILLIPS COUNTY, A LESS  
DRAMATIC INVERSION WILL ALLOW MOST OF THE PRECIPITATION TO  
ACCUMULATE IN THE WAY OF SNOW.  
 
A SHORT TIME PERIOD OF DRYING IS EXPECTED WITH A SHORTWAVE RIDGE  
MOVING ON MONDAY, FOLLOWED BY ANOTHER DISTURBANCE MONDAY NIGHT  
INTO TUESDAY. A MORE SIMPLIFIED SNOW EVENT THAN TODAY WILL PASS  
THROUGH THE AREA. THERE IS LOW TO MODERATE (30 TO 50 PERCENT)  
CONFIDENCE OF SNOW ACCUMULATION SINCE THERE IS NOT MODEL  
AGREEMENT ON THE STRONGEST FORCING PLACEMENT. GENERALLY HIGHER  
AMOUNTS ARE EXPECTED CLOSER TO CENTRAL MONTANA.  
 
ALTHOUGH THERE IS LOW CONFIDENCE ON TIMING AND LOCATION OF  
PRECIPITATION, THERE APPEARS TO BE ANOTHER SHORTWAVE TROUGH THAT  
WILL MOVE THROUGH FAIRLY ZONAL FLOW WEDNESDAY EVENING INTO  
THURSDAY. ALTHOUGH DETERMINISTIC MODEL SOUNDINGS FROM THE GFS AND  
ECMWF INDICATE A STRONG LOW LEVEL INVERSION WILL BE PRESENT AT THE  
ONSET OF THE EVENT, THERE IS STILL LOW (20 PERCENT) CONFIDENCE ON  
THE SPEED AT WHICH THIS INVERSION ERODES, AND THEREFORE  
PRECIPITATION TYPE.  
 
FORECAST CONFIDENCE & DEVIATIONS:  
 
THERE IS HIGH CONFIDENCE PRECIPITATION WILL TRANSITION FROM  
FREEZING RAIN TO SNOW BY MID AFTERNOON TODAY. THERE IS HIGH  
CONFIDENCE SOUTHWEST PHILLIPS COUNTY WILL SEE ACCUMULATING SNOW  
RATHER THAN FREEZING RAIN. THERE IS MODERATE CONFIDENCE ON LIGHT  
SNOW ACCUMULATION OCCURRING ON MONDAY NIGHT INTO TUESDAY, WITH LOW  
CONFIDENCE ON PRECIPITATION AMOUNT AND TYPE FOR THE WEDNESDAY  
EVENING INTO THURSDAY SYSTEM.  
 
 
   
AVIATION  
 
LAST UPDATED: 0930Z  
 
FLIGHT CAT RANGE: VFR TO IFR.  
 
DISCUSSION: ALTHOUGH VFR CEILINGS AND VISIBILITY WILL CONTINUE  
THROUGH ABOUT 12Z, A MIX OF FREEZING RAIN AND SNOW HAS BEGUN FOR  
EACH TAF SITE BESIDE KGGW IN RESPONSE TO A SURFACE LOW MOVING  
THROUGH THE AREA. IFR CONDITIONS ARE EXPECTED FROM AROUND 14Z TO  
00Z MONDAY FOR KSDY AND KGDV. THERE IS MODERATE (50 PERCENT)  
CONFIDENCE OF IFR CONDITIONS OCCURRING FOR KOLF. FOR KGGW, THERE  
IS MODERATE (50 PERCENT) CONFIDENCE OF IFR CEILINGS BETWEEN 14Z  
AND 19Z. THERE IS HIGH (60 TO 80 PERCENT) CONFIDENCE OF AT LEAST  
A LIGHT GLAZE OF FREEZING RAIN OCCURRING BEFORE 18Z, UP TO AROUND  
A TENTH OF AN INCH FOR THE TAF SITES. REGARDLESS OF AMOUNT, THERE  
IS MODERATE TO HIGH CONFIDENCE THAT ICING AND SLIPPERY RUNWAYS  
WILL OCCUR. FREEZING RAIN CHANCES ARE AT THEIR HIGHEST PRIMARILY  
BETWEEN 09Z-15Z BEFORE TRANSITIONING TO SNOW.  
 
WIND: LIGHT AND VARIABLE, BECOMING NORTHWEST AT 15-25 KNOTS WITH  
GUSTS UP TO 35 KNOTS AFTER 18Z.  
 
 
   
GGW WATCHES/WARNINGS/ADVISORIES  
 
WINTER WEATHER ADVISORY UNTIL 6 PM MST THIS EVENING FOR DANIELS-  
GARFIELD-WESTERN ROOSEVELT.  
 
WINTER STORM WARNING UNTIL 6 PM MST THIS EVENING FOR DAWSON-  
EASTERN ROOSEVELT-MCCONE-PRAIRIE-RICHLAND-SHERIDAN-WIBAUX.  
 
 
 
 
 
WEATHER.GOV/GLASGOW  
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