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FXUS65 KBYZ 160133 AFDBYZ

Area Forecast Discussion...Updated National Weather Service Billings MT 733 PM MDT Mon Apr 15 2024

UPDATE
Made some adjustments to PoPs. Adjusted PoPs to lower them west of Powder River county and took some of the edge off of the QPF over western and central zones too, based on trends. Convection was not as extensive over western and central zones, with cooler northwest surface winds in place limiting instability. Showers and a few thunderstorms were around late this afternoon, with some small hail in the stronger cells, but these have weakened quite a bit, with mainly widely scattered showers west of a Melstone to Fort Smith line. Activity over southeast Montana has been slow to develop too. Showers and thunderstorms were now starting to develop and work into Carter county from northeast Wyoming. Only just recently have showers started crossing the state line from Wyoming. This activity should fill in the rest of the night and agree with high PoPs in Carter county and the southeast part of Powder River county going forward. Updated PoPs in southeast Montana to account for trends. TWH

DISCUSSION

Today through Tuesday night...

Weak thunderstorms and rain showers have continued across the northwestern counties of the forecast area today. The strongest storms produced several lightning strikes this morning. With a low pressure system moving across the Central and Northern Plains today, convective activity is expected to increase this afternoon and evening, with models showing 500-2000 J/kg of CAPE. The highest amount of instability will be confined to the far southeast, where there is a small area included in the Storm Prediction Centers Marginal Risk for severe thunderstorms. Along with the instability, an upper level low over Wyoming is producing lots of divergence, as a result an increase in surface level convergence will provide ample forcing. The main concerns for these storms is hail and lightning, as well as some strong winds.
There is a decent amount of moisture available across much of the area tonight as well, with PWATs up to 1 inch in SE MT. Currently, total rainfall amounts tonight into tomorrow morning range from a few hundredths to a few tenths from Miles City to Fort Smith and west. As for locations south and east of this line (including Sheridan), 0.25-1" of total precipitation is possible, with much of Powder River and Carter Counties in a 0.5" to 1" range.

Tomorrow, as the upper low departs WY, a diffluent pattern setups from W-E as a trof digs into the northern Rockies from Canada. As this trof progresses south and east, cold air and moisture will surge into the region. Precipitation type will be snow in the mountains while rain is expected in the foothills and lower elevations initially. For the foothills, a transition to snow is expected Tuesday night, as temperatures drop into the 30s. Snow levels will drop down to ~3000ft by Wednesday morning, allowing for some potential snowfall in lower elevations including Billings. Up to an inch of snowfall is possible in Billings, however with highs in the 60s to low 80s lately, and warm soil temperatures, accumulations would be hard to see for non- foothill/mountain locations. As a result, Winter Storm Warning is in place for the Absaroka/Beartooths, Pryors, and Big Horns beginning Tuesday evening and ending Thursday afternoon. A Winter Storm Watch is in place for the Beartooth and Red Lodge Foothills will also go into effect 9pm MDT Tuesday.

Latest snowfall probabilities can be found at weather.gov/byz

With the departing surface low in the Plains, strong winds will wrap around it, resulting in breezy to gusty conditions tomorrow.
Wind gusts across much of the area in the 20-40 mph range with gusts into the 40-50mph in Sheridan County. The strongest winds will begin to diminish by mid-morning, coming back down into the 30-40 mph range.

High temperatures for Tuesday will be in the 50s to mid 60s.

Matos

Wednesday through Sunday...

Upper level storm system is centered over SC Canada on Wednesday morning with a trof digging down the backside into W MT/ID during the day and into Wednesday night. Normally having the low centered that far north and east would put our part of the world in a downslope dry and windy quadrant. But, there is some QG forcing aloft that slides across the southern half of the forecast area during this period associated with a strong cyclonically oriented jet stream along the MT/WY line. This jet with just a bit of curvature will provide the upper support for what is mainly a low level upslope/isentropic lift type of event. The bulk of the precipitation will occur along the north facing foothills and into the mountains.

NBM Probabilities for warning criteria foothills snow (6+ inches):

Red Lodge 75% Nye 85% Livingston 25% Big Timber 15% Columbus 5% Harlowton 0% Sheridan WY 0% Story WY 80%

Current forecast shows 10 inches from Red Lodge to Nye, with 3 to 4 inches from Columbus to Big Timber and Livingston. Also have 2 to 4 inches for the Sheridan Foothills along I-90, with 10 for Story.

Given the time of year and ground temperatures, not expecting much impactful accumulation during the daytime hours, and even in the higher foothills would expect roads to melt during the afternoon hours. Thinking the mountains are a high probability (75% greater than a foot of snowfall) warning, but for the foothills not as confident in the impacts. The ground temperatures/time of year/and low level winds being a bit northwesterly (doesn't favor Red Lodge upslope) are a concern for warning type impacts, and an advisory might be good enough. Plan to Upgrade to warnings for the mountain zones, but will hold off on the foothills and let the mid shift take another look tonight as the snow holds off until after midnight Tuesday night.

The heaviest precipitation will taper off Wednesday night as dry air pushes in from the north dropping PWATs under a quarter of an inch by Thursday morning. The northerly winds in the lower levels will continue into Friday before turning east and then south heading into the weekend. During this northerly wind period will continue to see upslope enhanced rain/snow showers in the foothills, with light additional snow accumulations for the mountains each day. The northerly winds will keep temperatures cool across the forecast area, with highs staying mainly in the 40s through Saturday. Shortwave ridging builds in for Sunday for a warmer day with more sun. Chambers

AVIATION

VFR conditions will prevail tonight. Scattered showers and isolated thunderstorms could bring local MVFR conditions at times through the night tonight over southeast Montana. Small hail is possible with thunderstorm over Powder River/Carter counties through 06z tonight, but heavy rainfall will be primary threat with the storms over southeast Montana. From Roundup to Billings to Fort Smith, activity should be mainly in the form of showers.
Winds will be northerly with gusts 15 to 30kts for most of the forecast area, with far southeast Montana holding on to a gusty southerly wind until just before sunrise. Wind gusts along the Bighorn mountain and foothills in Wyoming could gust 35-40kts at times through tonight, impacting KSHR at times through sunrise Tuesday. Chambers/TWH

PRELIMINARY POINT TEMP/POPS

Tue Wed Thu Fri Sat Sun Mon ----------------------------------------------------------- BIL 047/064 035/044 028/045 025/043 026/049 030/064 038/065 42/W 76/S 22/S 12/S 11/U 02/R 22/R LVM 038/059 031/041 021/041 019/039 020/047 030/060 034/062 44/W 87/S 33/S 21/B 11/U 02/R 22/R HDN 047/066 036/046 027/047 023/044 023/052 028/067 035/067 41/B 66/S 22/S 11/B 11/U 01/B 22/R MLS 046/064 037/046 028/043 022/041 022/048 029/064 038/065 31/N 11/N 11/N 01/B 00/U 01/U 11/B 4BQ 047/062 037/046 027/045 023/041 023/047 029/066 037/064 62/W 12/S 11/N 11/B 01/B 01/U 11/B BHK 042/062 033/046 024/041 019/039 019/045 024/059 033/062 54/W 01/N 11/N 01/B 01/B 01/U 11/B SHR 044/062 032/041 022/041 020/041 019/047 025/062 032/062 61/N 78/S 52/S 22/S 22/S 11/B 21/B

BYZ WATCHES/WARNINGS/ADVISORIES
MT...Winter Storm Watch remains in effect from Tuesday evening through Thursday morning FOR ZONES 56-66.
Winter Storm Warning in effect from 6 PM Tuesday to 6 PM MDT Thursday FOR ZONES 67-68-171.
Winter Storm Watch remains in effect from Tuesday evening through Thursday afternoon FOR ZONES 67-171.
WY...Winter Storm Warning in effect from 6 PM Tuesday to 6 PM MDT Thursday FOR ZONE 198.
Winter Storm Watch remains in effect from Tuesday evening through Thursday afternoon FOR ZONE 198.

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