textproduct: Great Falls

KEY MESSAGES

- Rain will gradually end along the Hi-Line this evening.

- It will be dry and breezy on Wednesday and Thursday.

- Precipitation and cooler temperatures return Friday through this weekend.

DISCUSSION

- Meteorological Overview:

An upper-level trough is moving over North-central, Central, and Southwestern Montana this afternoon. As a result rain showers will linger across the Hi-Line through this evening with cooler temperatures across the area. On Wednesday an upper-level ridge moves over North-central, Central, and Southwestern Montana. This will bring dry and warm temperatures (near seasonal averages) to the area on Wednesday. On Wednesday the pressure gradient increases which will bring breezy conditions to the Rocky Mountain Front. On Thursday the upper-level ridge begins to break down. This will keep warmer temperatures with mostly dry weather for the area on Thursday. Thursday evening a cold front will move through North- central, Central, and Southwestern Montana.

On Friday an upper-level trough is over North-central, Central, and Southwestern Montana. This combined with the cold front that moved through will bring cooler temperatures (well-below seasonal averages) to the area. On Friday there will be rain showers across North-central, Central, and Southwestern Montana. Saturday through Monday there will be an upper-level closed low that will bring cooler temperatures and widespread precipitation to the area. Higher elevation locations have the potential to receive snow this weekend. See the Forecast Confidence & Scenarios Section for details. -IG

- Forecast Confidence & Scenarios:

On Wednesday there is a 60 - 80% chance for wind gusts of 40 mph or greater. On Thursday there is greater than a 60% chance for wind gusts of 40 mph or greater across the Rocky Mountain Front and Plains west of 1-15, the Hi-Line, Cascade to Judith Basin County, and the North-South orientated valleys of Southwestern Montana. On Thursday along the Rocky Mountain Front there is a 60 - 80% chance for wind gusts of 55 mph or greater.

Saturday through Monday deterministic and ensemble models differ on the track and strength of the upper-level trough/closed-low. On Saturday two clusters have a closed upper-level low in the vicinity of Washington/Oregon. This scenario would keep temperatures similar to Friday with widespread precipitation and breezy conditions across North-central, Central, and Southwestern Montana. On Saturday one cluster has an upper-level trough over eastern Washington and Oregon. This scenario would keep similar weather to the closed low scenario but it would be slightly less windy. A third cluster has a upper-level trough over Montana. This scenario would bring cooler temperatures and widespread precipitation across North-central, Central, and Southwestern Montana. On Sunday three out of four clusters have an upper-level trough over North-central, Central, and Southwestern Montana. This scenario would bring cool temperatures and widespread precipitation to North-central, Central, and Southwestern Montana. On Sunday the fourth cluster has a closed upper-level low off the coast of Washington/Oregon with an upper- level ridge over Montana. This would bring warm and mostly dry weather to North-central, Central, and Southwestern Montana. This all shows that this weekend through Monday there is high uncertainty in the overall weather pattern. However, in whichever scenario pans out there is at least a moderate chance for snow across the higher peaks of North-central, Central, and Southwestern Montana this weekend. -IG

AVIATION

08/18Z TAF Period

Rain showers will continue over North-Central MT through this afternoon, dissipating around 21Z for KCTB and 01Z for KHVR. There is a chance of weak thunderstorms over KBZN from 22Z to 01Z. Near any showers and weak thunderstorms, mountain obscuration is possible. VFR conditions will prevail throughout this TAF period with the exception of KHVR, which could see MVFR visbilities through 01Z.

For KGTF: Due to the varying wind directions, a TEMPO group was added with -SHRA to reflect the sporadic nature of the winds, as well as the potential for light showers near the terminal. While VFR conditions will prevail throughout this TAF period, cloud decks will remain low, near MVFR levels at times, through the afternoon, before beginning to lift around 00Z. Due to the strong northwesterly component of the wind, there is high confidence of periodic wind gusts exceeding the cross-wind threshold through 02Z, at which a southwesterly wind shift will create a less than 10% chance of the cross-wind threshold being exceeded. -Dzomba

PRELIMINARY POINT TEMPS/POPS

GTF 44 76 54 81 / 10 0 0 0 CTB 44 69 50 72 / 10 0 0 0 HLN 42 76 49 84 / 0 0 0 0 BZN 39 77 45 84 / 0 0 0 0 WYS 30 70 30 72 / 0 0 0 0 DLN 37 78 41 81 / 0 0 0 0 HVR 43 75 49 81 / 40 0 0 0 LWT 41 73 50 82 / 20 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.