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FXUS65 KBYZ 042044 AFDBYZ

Area Forecast Discussion National Weather Service Billings MT 244 PM MDT Sat May 4 2024

DISCUSSION

This afternoon through Sunday night...

Main short term impact will be gusty to locally strong southerly winds over the eastern half of the forecast area on Sunday, continuing into Sunday night. These gusty winds combined with temperatures in the 80s and low Relative Humidity values will bring an elevated grass fire danger Sunday afternoon for areas east of Billings into the western Dakotas. The strongest wind gusts may arrive Sunday night over SE Montana, just ahead of a cold frontal passage, with probabilities for gusts over 45 mph peaking late evening at 35 to 45% along the eastern state line.

Ridging over the area this afternoon will keep skies sunny and temperatures pleasant for this time of year, mainly in the 60s.
Mostly clear skies on tap for tonight with temperatures 5 to 10 degrees warmer than this morning as southerly winds begin to increase across the forecast area ahead of an approaching storm system.

As mentioned above Sunday will be a transition day with a cold front and well organized storm system poised to move into the area by late in the day. Ahead of the late day cold front, winds will increase out of the south pulling in warmer air and boosting temperatures into the 70 to 85 degree range for most areas. The warmest temperatures will be over the Bighorn and Tongue river valleys where probability for highs over 80 degrees are well above 60 percent (78% Miles City). Winds will be gusty at times in the 35 to 45 mph range, likely strongest from eastern Sheridan county north into southern Rosebud and Powder River county during the afternoon, shifting to the rest of SE Montana into the evening as the approaching cold front compresses the flow. At this time do not anticipate needing a Red Flag Warning as the southerly flow will also be bringing increased low level moisture with it.
Combined with increased greening up of grasses across the area the threat just doesn't quite rise to the level of needing a warning.
May issue a rangeland fire danger statement tonight, and will continue messaging through other formats for the need to be careful not to start a wildfire tomorrow.

Along and behind the cold front will see precipitation chances increase along and behind the cold front. There looks to be good moisture associated with this system, but downslope winds kick in quickly tomorrow night as the front moves through, limiting precipitation chances and amounts from what they could be. Overall precipitation trends for tomorrow afternoon and night have trended lower due to this downslope wind effect. Freezing levels drop from around 9500 feet Sunday afternoon to around 5500 feet over the western mountains and foothills by Monday morning. Current forecast shows 2 to 4 inches of snow Sunday night, mainly on west/south facing slopes, for the Beartooth/Absaroka and Crazy mountains. Chambers

Monday through Friday...

Deterministic models have come into better agreement with the 500mb low track on this mornings model runs. The system is currently forecast to come across the forecast area Monday morning as an open trof, developing a closed low by the afternoon over east central Wyoming and then pivot northward into NW SD by Monday evening. After that the low drifts northward into W ND through Tuesday, and back into E MT Tuesday night. Additional upper energy pushes in from the northwest Wednesday, phasing with the upper low and kicking it quickly eastward on Thursday. General trofing and northwest flow persists over the area Thursday into Friday keeping conditions unsettled to end the week.

There are a couple of things that could keep precipitation from being as heavy as is currently advertised over our neck of the woods. First, with the system coming through as an open trof we negate the normal wrapped up storm system easterly winds in the low to mid levels, and instead go straight to downslope west/northwest winds that persist through the event. While there is good dynamics aloft to force precipitation, upslope and isentropic ascent will be missing for most of the area, especially central and west. The dynamics will also quickly traverse the area and set up mainly east of the area near the core of the upper low limiting residence time over our area. The downslope winds are being advertised as strong as the storm system sets up shop to our east and deepens, it will pull winds downhill toward it with models suggesting gusts in the 40 to 70 mph range Monday and Tuesday (strongest Tuesday). Winds this strong will disrupt upward motion and make precipitation production difficult, especiall over the eastern half of the forecast area. Another issue is that the NBM has a time lag for precipitation, and while models have trended lower with precipitation chances on the 12z run due to the above factors, it will take some time for the NBM precipitation accumulations to catch up with the current model trend.

All of the above is to say that this system is very dynamic and strong but still evolving in the computer models, so take the precipitation totals as having significant uncertainty still.
There is still the potential for some heavy precipitation Monday into Tuesday in a band somewhere east of Billings (possibly way east) but the models are showing this potential in various areas.
As such, WPC has a slight risk for excessive rainfall over SE Montana in a broad swath. Expect this potential to tighten up and may exit the area completely over the next 36 hours as model consensus improves further.

Snow levels drop from 9500 feet to 5500 feet by Monday morning and generally hangs out around that elevation through the week. As currently advertised not expecting lower elevation snowfall. For the mountains the wind field supports significant snow mainly falling on west to south facing aspects through Tuesday. Beyond that winds should turn more northerly bringing a better chance for north facing slopes to get in on the snow Wednesday and Thursday. Posted a Winter Storm Watch for the northern Bighorn mountains (Monday thru Tuesday morning) to match up with RIW's watch. As mentioned above forcing for snow in the Bighorns will be mainly on west facing slopes, but enough slop over into the Burgess Junction area to get close to a foot with the latest model run. Additionally, wind gusts in the 35 to 50 mph range will produce near whiteout conditions at times Monday into Tuesday.
Totals for the Beartooth/Absaroka/Crazy mountains look less impressive at this time so decided to go without a watch for these mountain locations for now. However, given the wind speeds forecast may need a highlite with subsequent forecasts if the combination of snow and blowing snow warrants it.

Strong winds Monday and Tuesday will be the main impactful weather for most areas early in the week, with the band of heavier rain somewhere east of Billings being a secondary issue. Will likely need a High Wind Warning for portions, possibly a large portion of the area for Tuesday, and maybe in some areas on Monday as well, with models advertising gusts well above 60 mph in many areas.

Winds look to die down some on Wednesday, though still gusty, as the secondary energy arrives and begins to move the core of the upper low east. This period is when more widespread wrap around precipitation may be maximized for many lower elevation locations, and upslope snow really gets established. The current forecast is showing over a foot of snow Wed-Thu for the western mountains, and close to 2 feet possible in the Bighorns. So, this is a tail of two significant periods with this storm system, Strong winds Mon/Tue with localized heavy rain somewhere east of Billings, and more widespread precipitation with heavy mountain snow Wed/Thu.
One thing looks certain, its going to be a tough week for outdoor activities until probably next weekend. Chambers

AVIATION

Winds are still expected to increase this afternoon for a brief period at all terminals. Otherwise, dry and near-clear sky conditions will persist through Sunday morning. Expected rain showers to move across all terminals late Sunday morning and through the afternoon. Vertz

PRELIMINARY POINT TEMP/POPS

Sun Mon Tue Wed Thu Fri Sat ----------------------------------------------------------- BIL 038/074 045/056 042/054 040/049 040/056 042/064 041/071 02/R 55/R 43/R 57/R 86/R 33/R 11/U LVM 041/066 040/051 035/047 035/047 036/051 037/059 036/068 06/T 65/R 33/O 57/O 66/R 22/R 11/U HDN 037/079 044/057 041/055 039/049 039/057 041/064 038/072 01/B 46/R 64/R 69/R 98/R 43/R 11/U MLS 039/082 049/059 039/051 038/051 041/060 042/064 041/070 01/N 49/T 77/R 78/R 77/R 32/R 11/U 4BQ 041/081 047/058 039/052 037/047 039/056 041/061 040/069 01/N 49/T 75/R 67/R 86/T 43/R 11/U BHK 036/075 047/062 036/050 036/051 038/058 038/062 038/067 00/N 69/T 77/R 67/R 76/R 32/R 11/U SHR 036/078 041/055 037/056 036/048 036/050 037/060 036/067 01/N 48/R 43/R 47/R 87/R 54/R 11/U

BYZ WATCHES/WARNINGS/ADVISORIES
MT...None.
WY...Winter Storm Watch in effect from Monday morning through Tuesday morning FOR ZONE 198.

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