textproduct: Great Falls

KEY MESSAGES

- Periods of light to moderate snowfall are expected for the mountains of North Central, Central, and Southwest Montana through Saturday.

- Gusty westerly winds will also persist into Saturday, especially over the plains, and especially later today through tonight into Friday morning.

- Temperatures should remain slightly warmer than normal through Saturday.

- A cooler and less windy pattern is forecast for next week, with the best chance for additional precipitation expected Sunday into Monday.

DISCUSSION

- Meteorological Overview:

Today through Saturday... Two separate shortwave troughs in a westerly flow aloft are forecast to impact North Central, Central, and Southwest Montana during this period; one today into this evening, and the other Friday into Friday evening. Each one is expected to bring a period light to at times moderate snow to area mountains with a chance of rain and snow showers at lower elevations of Central and Southwest Montana. The westerly flow aloft will also keep surface winds gusty and out of the southwest to west, especially over the plains, as temperatures remain 5 to 10 degrees above seasonal averages. These winds are forecast to be strongest late this afternoon and overnight in the wake of the first shortwave.

Sunday through next Thursday... A deep low pressure trough ejecting out of the Gulf of Alaska on Saturday is on track to move into the Pacific Northwest on Sunday, then through Montana on Monday. This should bring a cooler and less windy pattern to the forecast area. The initial push of moisture ahead of the trough will bring the best chance for widespread additional precipitation Sunday into Monday, as temperatures cool back closer to normal. A high pressure ridge is then forecast to move east onto the Pacific coast Tuesday through Thursday, which should decrease the threat for precipitation, but a lingering northwesterly flow aloft will keep temperatures near seasonal averages.

- Forecast Confidence & Scenarios:

Today through Saturday...

Snowfall...Probabilities for at least 2 inches of snowfall in each of the next 24-hour periods (5 am MST Thu through 5 am Fri, and 5 am MST Fri through 5 am MST Sat) for valley areas of Madison and Gallatin Counties along the Wyoming and Idaho borders, including Raynolds and Targhee Pass areas, are above 70%. Probabilities for 4+ inches of snow in the mountains for that same area in each of those periods is similarly at least 70%. Therefore, confidence is somewhat high in maintaining the Winter Weather Advisory there through Friday evening. Have also added a Winter Weather Advisory for Northwest Beaverhead County, but only through early this afternoon, focusing on travel over Chief Joseph Pass, where the probability for at least 2 inches of snow is similarly over 70%. The threat for similarly significant snowfall decreases for Saturday, so additional Advisories are not expected at this time.

Wind... Ensemble forecast probabilities for High Wind criteria to be reached on the Rocky Mountain Front and adjacent plains overnight tonight remains in the 30-50% range, initially leading to only moderate confidence in upgrading the High Wind Watch to a Warning. However, forecast soundings for at least the Cut Bank area indicate that a stable layer will develop at ridge top level, with a period of 50 to 60 kt winds just below that layer. This is a favorable set-up for at least low-end High Wind criteria wind gusts, increasing confidence to between moderate and high, so have upgraded the Watch to a Warning for later this afternoon into early Friday morning.

Sunday through next Thursday...

Snowfall... Probabilities for at least 4 inches of snow Sunday into Sunday night increase to at least 50% in most mountain and foothill areas, so additional Winter Weather Advisories may be needed. The overall chance for even minor snow accumulation for mountain areas decreases quite a bit for the remainder of next week. -Coulston

AVIATION

26/06Z TAF Period

VFR conditions will prevail at all terminals except for the KEKS terminal during this TAF period. However, between 26/13Z and 26/23Z at the KBZN, KHLN, and KLWT terminals there is a 20 - 40% chance for snow, rain, and/or a rain/snow mix. This precipitation is unlikely to reduce visibility or ceilings below MVFR levels. At the KEKS terminal there is a 30 - 50% chance for snow between 26/12Z and 26/14Z and between 26/19Z and the end of the TAF period. Snow will reduce visibility to MVFR levels at the KEKS terminal between 26/14Z and 26/19Z. Across all terminals during this TAF period it will be breezy with the strongest winds at the KEKS, KGTF, and KCTB terminals. There will be instances of mountain wave turbulence and low-level wind shear during this TAF period. During the beginning and middle of this TAF period there will be mountain obscuration in Southwestern Montana. -IG

Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information.

PRELIMINARY POINT TEMPS/POPS

GTF 49 31 44 32 / 20 0 10 10 CTB 44 27 41 26 / 10 0 0 0 HLN 46 26 41 28 / 40 10 40 20 BZN 44 22 39 25 / 40 10 60 20 WYS 29 16 29 17 / 90 60 90 60 DLN 41 21 37 24 / 50 10 50 20 HVR 45 28 43 23 / 30 10 0 0 LWT 44 26 43 26 / 40 10 10 10

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning from 2 PM this afternoon to 8 AM MST Friday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains- Southern Rocky Mountain Front.

Winter Weather Advisory until 11 PM MST Friday for Gallatin and Madison County Mountains and Centennial Mountains.

Winter Weather Advisory until 2 PM MST this afternoon for Northwest Beaverhead County.