071  
FXUS65 KBYZ 250228  
AFDBYZ  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BILLINGS MT  
828 PM MDT MON APR 24 2017  
   
UPDATE
 
 
LATEST UPPER AIR ANALYSIS SHOWS 100+ KT H3 JET NOSING FROM THE  
GREAT BASIN TOWARD THE CENTRAL ROCKIES, WITH INCREASINGLY  
DIVERGENT FLOW EXTENDING THRU WY INTO OUR CWA. THE COMBINATION OF  
INCREASING SYNOPTIC SCALE ASCENT AND DEEPENING UPSLOPE WINDS IN A  
MOIST AND UNSTABLE AIRMASS HAS ALLOWED FOR PLENTY OF SHOWERS  
ACROSS OUR WESTERN CWA THIS EVENING. THERE HAS BEEN SOME LIGHTNING  
BUT THAT IS DIMINISHING SINCE SUNSET. COLD ADVECTION ASSOCIATED  
WITH CANADIAN SURFACE HIGH TO OUR NORTHEAST HAS DROPPED TEMPS TO  
THE 40S...WITH LOW-MID 30S NOTED IN NE MT ALREADY.  
 
UPPER AND SURFACE LOWS WILL BECOME STACKED LATER TONIGHT IN  
NORTHEAST WY, ALLOWING FOR SIGNIFICANT PRECIPITATION ALONG THE  
EASTERLY UPSLOPE SIDE OF THE BIGHORNS, EXTENDING ACROSS OUR FAR  
SOUTHEAST. TRAILING CONVERGENT AXIS IN CENTRAL MT WILL KEEP PCPN  
GOING IN OUR WEST TONIGHT INTO EARLY TOMORROW AS WELL. OUR  
NORTHEAST PARTS FROM MILES CITY TO BAKER WILL REMAIN DRY WITH THIS  
SYSTEM.  
 
HAVE MADE NO CHANGES TO THE INHERITED WINTER HIGHLIGHTS, WHICH  
INCLUDE A WINTER STORM WARNING FOR THE BEARTOOTH/ABSAROKAS AND  
ADVISORIES FOR THE SHERIDAN FOOTHILLS AND ACROSS OUR FAR  
SOUTHEAST ZONES. LOW LEVEL COOLING WILL BE A RESULT OF ADVECTION  
AND DYNAMICS, AND LATEST RUC WET BULB ZERO HEIGHT FORECASTS  
SUGGEST A CHANGE TO SNOW OVERNIGHT, PERHAPS NOT TIL AFTER 09Z FOR  
THE LOWER ELEVATIONS OF SHERIDAN COUNTY. SO, THESE AREAS WILL SEE  
A GOOD DOSE OF RAIN BEFORE SNOW BECOMES A CONCERN. FURTHER WEST,  
THE BEARTOOTH/RED LODGE FOOTHILLS WILL ALSO SEE A CHANGE TO WET  
SNOW LATE TONIGHT OR EARLY TUESDAY, BUT MOISTURE WILL BEGIN TO  
LESSEN BY THEN. SO ALTHOUGH SOME WET ACCUMULATION IS POSSIBLE IN  
THE PARADISE VALLEY AND AT NYE AND RED LODGE, BELIEVE THESE AREAS  
WILL SEE MOST OF THE PRECIPITATION IN THE FORM OF RAIN. MELVILLE  
ON THE EAST SIDE OF THE CRAZY MTNS SHOULD ALSO SEE SOME WET  
ACCUMULATION. ACCUMULATIONS OF 3-6 INCHES STILL SEEMS A GOOD CALL  
FOR OUR FAR SOUTHEAST, ESPECIALLY OVER THE HILLS AND NEAR THE WY  
BORDER. WILL NEED TO KEEP AN EYE ON SHERIDAN COUNTY PER THE  
UPSLOPE. IF SHERIDAN/DAYTON/BIG HORN CHANGE OVER EARLIER THAN  
EXPECTED, HIGHER SNOW AMOUNTS ARE POSSIBLE. ALSO, THE BIGHORNS  
WILL SEE HEAVY SNOWFALL AND THIS LOOKS TO BE A HIGH END ADVISORY  
WITH UP TO A FOOT OF SNOW ABOVE 6KFT. TRAVEL ON I-90 THRU SHERIDAN  
AND ON HIGHWAY 212 THRU SOUTHEAST MT WILL BE MESSY TOMORROW  
MORNING.  
 
JKL  
 

 
   
SHORT TERM...VALID FOR TUE AND WED
 
 
A FEW SHOWERS WERE MOVING E THROUGH THE FORECAST AREA EARLY THIS  
AFTERNOON. ABUNDANT CLOUD COVER WAS KEEPING THUNDER AT BAY.  
 
A PACIFIC JET, SEEN ON WATER VAPOR, WAS DIVING SE INTO THE FOUR-  
CORNERS REGION AND WILL BRING UPPER DIVERGENCE TO THE REGION  
THROUGH TUESDAY AS IT CARVES OUT A DEEP TROUGH. AN UPPER LOW WILL  
FORM OVER NE WY BY 12Z TUE. AND WILL PUSH E INTO THE DAKOTAS TUE.  
EVENING. ASSOCIATED SURFACE LOW WILL DEEPEN OVER E WY TONIGHT.  
MODELS AGREED THAT A BULLSEYE OF QPF WOULD DEVELOP N OF THIS LOW  
AND AFFECT AREAS FROM THE SHERIDAN FOOTHILLS N AND NE INTO SE MT  
TONIGHT THROUGH TUE. MORNING. STRONG ENE WINDS WILL WRAP AROUND  
THIS LOW INTO THE FORECAST AREA. BACKDOOR COLD FRONT WILL PUSH W  
INTO THE AREA BRINGING NEGATIVE 850 MB TEMPERATURES AS FAR W AS  
YELLOWSTONE AND MUSSELSHELL COUNTY TUE. MORNING. QPF AMOUNTS WILL  
BE GREATER THAN A HALF INCH IN SOME AREAS AROUND THE LOW.  
SOUNDINGS SHOWED THE PRECIPITATION CHANGING TO SNOW AROUND THE LOW  
BY 12Z TUESDAY. SNOW RATIOS COMBINED WITH EXPECTED QPF WILL ALLOW  
FOR SEVERAL INCHES OF SNOW. THE SNOW, ALTHOUGH WET, WILL COMBINE  
WITH THE WINDS TO SUPPORT ADVISORY CONDITIONS. THEREFORE WILL HAVE  
AN ADVISORY FOR SNOW TONIGHT THROUGH TUESDAY IN SE MT AND THE  
SHERIDAN FOOTHILLS.  
 
TIME-HEIGHT CROSS SECTIONS CONTINUED TO SUPPORT HIGH SNOWFALL  
AMOUNTS IN THE BEARTOOTHS/ABSAROKAS AND NE BIGHORNS INTO TUESDAY.  
THE BIGHORNS MAY SEE WARNING CRITERIA SNOW, BUT WILL LEAVE THE  
ADVISORY IN PLACE FOR NOW. OTHERWISE, RAIN WILL MIX WITH SNOW  
ACROSS THE AREA OVERNIGHT THROUGH TUE. MORNING. SOME RAIN/SNOW MIX  
WILL LINGER IN THE SE TUE. AFTERNOON. SNOW AMOUNTS LOOKED TOO  
LIGHT TO SUPPORT AN ADVISORY OVER REMAINING AREAS, BUT THIS WILL  
NEED TO BE WATCHED IN SUBSEQUENT SHIFTS. HIGHS WILL REACH ONLY THE  
40S ON TUESDAY. DID MENTION SOME THUNDER OVER WESTERN ZONES TUE.  
AFTERNOON AND EVENING WITH LI'S BELOW ZERO DEGREES C.  
 
PRECIPITATION WINDS DOWN TUE. NIGHT INTO EARLY WEDNESDAY UNDER WNW  
FLOW/WEAK RIDGING. ANOTHER PACIFIC JET DIVES SE INTO THE WESTERN  
U.S. WED. AFTERNOON THROUGH THE END OF THE PERIOD, LOWERING  
HEIGHTS AND BRINGING MORE SHOWERS. WEDNESDAY WILL BE A BIT WARMER  
THAN TUESDAY WITH HIGHS IN THE 50S. ARTHUR  
   
LONG TERM...VALID FOR THU...FRI...SAT...SUN...MON
 
 
EXTENDED PERIOD THURSDAY AND FRIDAY IS COOL WITH UPSLOPE FLOW  
WITH LOW HEIGHTS ALOFT BUT FAIRLY DISORGANIZED STRUCTURE TO THE  
UPPER TROUGH OVER THE REGION. THIS WILL YIELD A COUPLE OF DAYS  
WITH MODEST PRECIPITATION AND PERSISTENT COLD TEMPERATURES WITH  
ACCUMULATING MOUNTAIN AND LIKELY FOOTHILL SNOWFALL. BUT  
ACCUMULATION RATES WILL BE SLOW AND DAYTIME MELTING COULD OCCUR  
BELOW 6000 FEET SO IMPACTS MAY BE LIMITED FROM A WINTER  
PERSPECTIVE. THE AMOUNT OF PRECIPITATION WILL CUMULATIVELY CAUSE  
SLOPPY CONDITIONS FOR NON PAVED ROADS AND WILL CONTINUE TO  
INCREASE THE AMOUNT OF MOISTURE IN THE GROUND AND IN THE SNOWPACK  
SO STREAMS AND RIVERS WILL REMAIN AT ELEVATED LEVELS FOR THIS TIME  
OF YEAR.  
 
UPSLOPE FLOW BREAKS ON SATURDAY AND A TRANSITION TO AN UNSETTLED  
NORTHWESTERLY FLOW SETS UP FOR THE REMAINDER OF THE EXTENDED. THIS  
WILL ALLOW FOR A MINOR WARMUP BUT WITH STILL BELOW NORMAL  
TEMPERATURES AND MORE SHOWER CHANCES BUT AMOUNTS WILL NOT BE AS  
GREAT AS THE EARLY WEEK OR LATE WEEK EVENTS. BORSUM  
 

 
   
AVIATION
 
 
INCREASING UPSLOPE FLOW THIS EVENING AND OVERNIGHT WILL CAUSE  
WIDESPREAD LOW CLOUDS AND A MIX OF RAIN AND SNOW. MOUNTAIN AND  
FOOTHILL OBSCURATIONS WILL BECOME WIDESPREAD AND ALL TERMINAL  
SITES WILL ENCOUNTER IFR CONDITIONS OR WORSE TONIGHT IN LOW CIGS  
AND REDUCED VIS. AAG/BORSUM  
 

 
   
PRELIMINARY POINT TEMP/POPS
 
 
TUE WED THU FRI SAT SUN MON  
-----------------------------------------------------------  
BIL 037/047 035/056 039/049 036/046 034/052 036/058 040/058  
85/O 32/T 45/W 75/R 32/O 11/B 33/R  
LVM 036/050 032/054 035/047 030/046 029/051 030/056 034/055  
87/T 46/T 75/W 55/O 42/O 22/R 33/R  
HDN 037/049 034/058 039/050 037/048 035/054 035/060 039/060  
85/O 21/E 45/W 56/R 32/R 11/B 33/R  
MLS 033/046 031/055 036/053 036/050 035/055 036/060 041/060  
11/E 11/B 23/W 33/R 21/B 11/U 23/R  
4BQ 033/044 027/053 032/048 034/046 032/050 033/057 037/059  
77/O 11/B 24/W 44/O 33/O 21/B 33/R  
BHK 027/043 024/051 028/051 030/050 031/053 031/057 035/057  
01/E 11/B 12/W 12/O 11/B 11/B 24/R  
SHR 034/043 030/052 035/045 034/042 030/046 030/053 036/054  
++/O 22/T 45/W 66/O 43/O 11/B 32/R  
 

 
   
BYZ WATCHES/WARNINGS/ADVISORIES
 
 
MT...WINTER WEATHER ADVISORY IN EFFECT UNTIL 6 PM MDT TUESDAY FOR  
ZONES 36>38-58.  
WINTER STORM WARNING IN EFFECT UNTIL 6 PM MDT TUESDAY FOR  
ZONE 67.  
WY...WINTER WEATHER ADVISORY IN EFFECT UNTIL 6 PM MDT TUESDAY FOR  
ZONES 98-99.  
 

 
 

 
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