textproduct: Great Falls

KEY MESSAGES

- Widespread rain showers today, with some weak thunderstorms possible this afternoon.

- A cold front will lower snow levels down to around 6000 ft tonight, allowing for light mountain snow accumulation through tomorrow morning.

- Warm and dry conditions Thursday night through the holiday weekend.

- Breezy conditions Sunday through Tuesday, with the strongest winds being in Southwest MT.

- Chances for precipitation and afternoon thunderstorms return on Monday.

DISCUSSION

- Meteorological Overview:

Widespread rain showers will continue today, with enough instability to support a few afternoon thunderstorms. While these thunderstorms will generally be weak in nature, some of the stronger cores could contain brief downpours, gusty winds in excess of 40 mph, and small hail. Rain will remain showery through this evening, before becoming stratiform during the overnight hours. Lingering rainfall will be showery tomorrow morning, before precipitation chances taper off tomorrow afternoon. A cold front will make its way through Montana tonight, lowering snow levels to around 6000 ft and allowing for light mountain snow accumulation. The exceptions are the Gallatin and Madison mountains, in which isolated areas could receive a few inches of snow.

Ridging begins to build in the west CONUS tomorrow evening, and continues through the holiday weekend. This will allow for warm temperatures, dry conditions, and sunny skies tomorrow evening through Monday. Westerly flow aloft will help mix down the strong upper level winds to the surface, allowing for breezy conditions Sunday and Monday. Temperatures will peak on Monday, with many locations seeing highs in the upper 70s to mid 80s.

This peak will be cut short by another cold front early next week, causing temperatures to decrease. An upper level trough moving west from the Olympic Peninsula will bring back chances for precipitation and afternoon thunderstorms starting Monday night. It will also bring increased windy conditions, with winds being the strongest in Southwest MT. The trough will bring unstable southerly flow, which, combined with low surface pressure and sufficient moisture, will increase chances for some thunderstorms to be strong to severe. However, the models are still divided on the timing, strength, and exact position of this trough, making the severity of these storms uncertain. -Dzomba

- Forecast Confidence & Scenarios:

Widespread rain showers are expected to continue throughout the day, with chances for thunderstorms in the afternoon/evening hours. While these thunderstorms are generally expected to be weak in nature, there may be localized instances of gusty winds over 40 mph, small hail or graupel, and heavy downpours over a quarter inch with the stronger cores.

This activity will expand in coverage and become more stratiform as it slides southward into southwest MT tonight into Thursday morning. H700 temperatures falling to around -4 to -7C combined with lingering convective processes may drop snow levels as low as 5000 ft at times, though accumulating snow looks to primarily be confined to areas above 6000 to 6500 ft and even higher. Snowfall amounts look to be most significant over the northern portions of the Madison/Gallatin ranges where the probabilities for at least 3 inches of snow or more are currently running above 70% tonight through Thursday morning. The probabilities in this area for at least 4 inches of snow are 60-70%, whereas for at least 6 inches of snow its 20-30%. Other mountain areas of the southwest, along the Continental Divide, and over the central island ranges will generally see trace amounts up to around an inch or two on the high end. Overall, cold and wet outdoor recreation will be the primary impact with this system. Total rainfall/liquid equivalent amounts will generally range between a tenth to a quarter inch for most lower elevation locations with a sharp decreasing gradient to just a few hundreths or so along the Hi-Line and the drier southwest valleys along and west of I15. Of course rainfall amounts in excess of a half inch will be more common for areas over and near higher terrain. Precipitation diminishes and becomes more scattered in nature by Thursday afternoon. -RCG and Dzomba

AVIATION

20/18z TAF Period

A cold front working through the region will continue to result in wind shifts to a more northerly direction through the afternoon and evening. Numerous showers will develop across the region this afternoon, with greatest coverage slowly shifting southward into the evening before precipitation becomes more stratiform in nature near sunset. Scattered thunderstorms will be around this afternoon, but overall coverage was too low confidence to warrant mention in TAFs at this time.

Precipitation diminishes from north to south late afternoon through the night, but low clouds will remain into Thursday morning. Mountains will become increasingly obscured as low clouds develop through the remainder of the day. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 36 60 37 70 / 60 20 0 0 CTB 33 62 37 69 / 20 10 0 0 HLN 36 61 37 70 / 90 20 0 0 BZN 33 56 31 66 / 90 50 0 0 WYS 28 51 23 62 / 90 40 0 0 DLN 33 58 32 66 / 80 30 0 0 HVR 34 65 35 73 / 10 20 0 0 LWT 33 55 33 66 / 60 40 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.