textproduct: Great Falls

KEY MESSAGES

- Warming trend begins Friday with temperatures peaking on Saturday.

- The risk for showers and isolated weak thunderstorms increases Saturday afternoon.

- Cooler and wetter weather moves in later Sunday through Monday with mainly high elevation snow.

DISCUSSION

- Meteorological Overview:

A few light snow showers/flurries are possible across portions of north-central MT through early Thursday morning in association with a shortwave disturbance embedded within northwest flow aloft that exits across eastern MT by Thursday afternoon. Surface high pressure slides across the State behind the wave, bringing one more day of near to below average temperatures across the plains while slightly above average temperatures persist across southwest MT.

Noticeable warming begins Friday as upper level ridging briefly builds across the Northern Rockies ahead of troughing that moves onshore this weekend with additional warming likely Saturday as southerly flow increases with the approaching trough. Initial moisture advecting northward out of the Great Basin and terrain heating across southwest MT may support some shower development and perhaps an isolated weak thunderstorms late Friday afternoon and evening. Deeper moisture and an embedded shortwave ejecting out of the western upper trough will bring more widespread showers and a risk for isolated/weak thunderstorms to the area Saturday afternoon and evening.

A cooler and wetter period of weather follows Sunday and Monday as the upper level trough shifts east across the western US. The bulk of the upper level energy moves by to our south, but several weaker disturbances and waves of moisture are likely to lift across the region for periods of more widespread precipitation across the area late Sunday through Monday. There is still some uncertainty with smaller scale waves moving through the trough Sun/Mon and these differences increase early next week, especially regarding how much of a break there may be in the unsettled conditions before the next upper level system arrives around mid-week. Hoenisch

- Forecast Confidence & Scenarios:

Isolated thunderstorm potential is mainly limited to areas near the ID border late Friday with a 15-20% risk of thunderstorms as far north as central MT Saturday afternoon/evening.

There is 30-50% probability of precipitation amounts exceeding 0.50" across much of central and southwest MT Sunday through Monday with a 20-30% probability for amounts of 1.00" or more mainly limited to the central MT ranges near the continental divide and Little Belts. Snow levels are currently forecast to lower to around 6000-7000ft by Sunday and further lower to around 5000 ft on Monday. This would limit the potential for impactful snow accumulation at pass levels to mainly the Monday morning period, but there is still considerable uncertainty at this point. Hoenisch

AVIATION

09/06Z TAF Period

Mid- to higher level clouds will pass through a west to northwesterly flow aloft tonight into Thursday. A shortwave and weak surface cold front will bring a northerly wind shift, scattered to broken low VFR/MVFR clouds, and a few isolated light snow showers/flurries between 09/09 and 09/22Z, mostly for central and north-central areas. These lower clouds will obscure mountains, particularly near the central MT island ranges.- RCG

PRELIMINARY POINT TEMPS/POPS

GTF 28 53 28 66 / 0 0 0 0 CTB 23 51 24 63 / 10 0 0 0 HLN 32 59 32 68 / 0 0 0 0 BZN 30 60 30 68 / 0 0 0 10 WYS 27 59 27 64 / 0 0 0 30 DLN 33 64 33 69 / 0 0 0 10 HVR 25 53 27 68 / 20 10 0 0 LWT 27 51 27 64 / 10 10 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.