textproduct: Great Falls

KEY MESSAGES

- Another round of showers and thunderstorms this afternoon with additional precipitation tonight into Monday.

- Breezy Tuesday and Wednesday.

- Much cooler precipitation producing system Wednesday into Thursday, with lower elevation snow by Thursday morning.

DISCUSSION

- Meteorological Overview:

A south to southwest flow aloft remain in place associated with a weakening upper level trough near northern CA this afternoon. An initial wave ejecting from this trough is moving across the region now, aiding in the development of showers and thunderstorms near the ID border. This convective activity moves north and northeastward through the remainder of the afternoon and early evening. Gusty winds and lightning across Southwest and Central Montana will be the primary concern today.

An additional wave from the weakening troughing near CA lifts toward the region tonight resulting in a stratiform type precipitation band tonight into Monday. Some light snow near and above pass level is forecast, but impacts look minimal at this time. Additional showery activity looks to develop Monday afternoon in areas that are able to eek out some sunshine and develop weak instability.

Zonal flow aloft develops Tuesday into Wednesday ahead of troughing shifting southeastward from the Gulf of Alaska toward the Pacific NW. Breezy west to southwest winds develop as a result. While a brief stretch of stronger wind cannot be ruled out, this just looks to be a breezy timeframe at this point.

The upper level system approaching the Pacific NW Wednesday shifts eastward and over the Northern Rockies Wednesday evening into Thursday. Much cooler air works in with a cold front, bringing snow levels down to lower elevations. There is still some uncertainty with respect to overall amounts with this system (Largely related to the storm track), but at least brief light snow at lower elevations is a reasonable expectation at this time by Thursday morning.

Cluster guidance favors brief ridging developing Friday into the weekend, which would favor a drier stretch, with temperatures slowly trending warmer. By early next week approximately 90% of cluster guidance members have some sort of troughing returning across the west. -AM

- Forecast Confidence & Scenarios:

Thunderstorms today and precipitation through Monday:

Inverted-V soundings across Southwest Montana for the remainder of the afternoon suggest gusty outflow winds will be the main concern today. The risk for severe thunderstorms today across Southwest Montana is 5% within 25 miles of a given point.

Heading into tonight showery activity diminishes, with a banded type of stratiform precipitation taking over. The overall areal extent of this band appears to be narrow, which has resulted in probability of precipitation decreasing in many areas for tonight as confidence in its placement has increased. Given the cooler air aloft going into Monday, areas that clear out sufficiently Monday morning and afternoon will see at least a low-probabiltiy for afternoon showers. All precipitation diminishes by Monday night.

Winds Tuesday into Wednesday:

Ridgetop stability has weakened slightly over the timeframe in which the strongest zonal winds aloft are currently timed. As such, the probability for a 55 mph gust along the Rocky Mountain Front Tuesday and Wednesday has decreased ever so slightly. Further east in Cut Bank the probability for a 55 mph gust is between 40 and 50% over this timeframe.

Precipitation Wednesday into Thursday:

Volatility is still present, but confidence remains high in a trend cooler, with at least light lower elevation snow by Thursday morning across the region. Areas most favored for impactful snow in terrain are south of I-90 and east of I-15. The probability for 9 inches of snow in these mountains for this event is greater than 50%. -AM

AVIATION

12/18Z TAF Period

Scattered to widespread showers and isolated thunderstorms are expected to develop across Southwest MT this afternoon. These showers/thunderstorms will bring MVFR conditions at times, with gusty erratic winds possible with a few stronger storms. This band of showers will lift north throughout the afternoon through Monday morning, switching to stratiform rain mainly in North- Central MT. There remains some uncertainty in how far north this band of rain lifts, which gives low confidence in end timing for rain at terminals in North- Central MT. MVFR ceilings will be persistent through the TAF period across North-Central MT, with IFR clouds developing Monday morning. Mountain obscuration will be possible at times due to precipitation and low ceilings. Morning fog at KCTB is currently dissipating, but IFR ceilings may linger for another hour or two. -Wilson

PRELIMINARY POINT TEMPS/POPS

GTF 38 58 39 59 / 70 50 20 0 CTB 32 58 37 54 / 30 30 10 0 HLN 36 57 35 59 / 80 40 20 0 BZN 31 56 31 60 / 30 50 40 10 WYS 26 44 26 47 / 60 50 50 20 DLN 32 52 30 56 / 40 40 10 0 HVR 36 57 37 63 / 60 60 30 0 LWT 35 56 34 59 / 90 30 40 10

TFX WATCHES/WARNINGS/ADVISORIES

None.