textproduct: Great Falls

KEY MESSAGES

- Areas of snow or snow showers decrease this evening.

- Temperatures warm above seasonal averages Monday and Tuesday.

- Very windy conditions are likely Monday and Tuesday.

- Mountain snow redevelops Monday and Tuesday with additional opportunities for mountain and lower elevation snow late next week.

DISCUSSION

- Meteorological Overview:

Fast and moist west-northweerly flow from the Pacific continues across the Northern Rockies and MT, maintaining a very active weather pattern for the region through much of the week ahead. A few areas of mainly light snow across eastern portions of north- central MT should decrease through the rest of this afternoon but may redevelop this evening, mainly along and east of a Havre to Lewistown line as another wave of moisture overruns the deeper cold airmass situated there. Orographically driven snow showers continue to affect the mountains through this evening with areas along the continental divide north of Lincoln and the Madison/Gallatin ranges most likely to see additional accumulations of 2 to 4 inches while most other mountain areas can expect around 1 inch or less from additional snow showers through this evening. Temperatures will be tricky tonight across the north-central MT plains as the low level cold airmass that moved in this morning retreats back to the northeast overnight. Minimum temps are likely to occur early this evening before rising overnight for areas adjacent to the Rocky Mtn Front and east across Great Falls to Lewistown.

A relative lull in between disturbances embedded within the fast WNW flow aloft will bring mainly dry conditions with seasonal to somewhat above average temperatures across the entire forecast area Sunday. Mild and increasingly windy conditions are expected Sunday night through Tuesday as upper ridging attempts to build to our southwest with a stronger mid level shortwave and potent upper level jet streaking east along the US/Canadian border late Monday through Tuesday. This results very strong mid level flow of 70kts or higher above 10000ft which looks to peak early Tuesday morning as surface low pressure tracks east across southern AB and SK. Temperatures will warm to upper 40s to around 50 for most lower elevation locations Monday and Tuesday with windier locations seeing overnight temps remain above freezing. Precipitation redevelops along the continental divide Sunday night and will likely intensify Monday night as the upper level wave passes. Snow levels initially around 5000ft look to rise to around 6500 ft Monday and Monday night before lowering again on Tuesday.

Additional disturbances move through the fast flow aloft Wed/Thu with decreasing confidence in timing and details. Models ensembles are increasingly supporting the idea that the upper level jet sinks south with these features allowing a colder airmass to move back southward by Thursday or Friday. Embedded moisture with these disturbances could bring a period of prolonged mountain snowfall as well as some opportunities for lower elevation snow. Hoenisch

- Forecast Confidence & Scenarios:

The risk for a widespread wind event late Monday through early Tuesday has increased with most deterministic models showing 700 MB flow of 75-85kts with the passage of the wave early Tuesday morning. Timing of the peak mid level wind speeds is not optimal for surfacing strong winds and temperature profiles will be key to assessing the potential for mountain wave enhancement, but probabilities for gusts of 60 mph or more are better than 50-60% for areas along the Rocky Mtn Front and adjacent plains as well as across portions of Cascade and Judith Basin counties.

Snow with the Mon/Tues system is focused across areas near the continental divide with rising snow levels likely to keep significant accumulation above most mountain passes. Hoenisch

AVIATION

06/18Z TAF Period

VFR conditions will prevail across the KBZN, KEKS, and KHLN terminals during this TAF Period. During the first 8 hours of this TAF Period at the KBZN, KEKS, and KHLN terminals there is a 30 - 60% chance for rain, snow, and/or a rain/snow mix. At the KGTF, KHVR, KCTB, and KLWT terminals snow and/or low clouds will reduce flight category to at least MVFR levels for at least the first 6 hours of this TAF Period. Winds across the KGTF, KCTB, KHVR, and KLWT terminals will increase by the end of this TAF Period. There will be mountain obscuration during the duration of this TAF Period across North-central, Central, and Southwestern Montana. There will be isolated instances of low-level wind shear and mountain wave turbulence during the duration of this TAF Period across North-central, Central, and Southwestern Montana. -IG

PRELIMINARY POINT TEMPS/POPS

GTF 21 41 36 47 / 20 10 0 10 CTB 11 37 28 41 / 10 0 0 10 HLN 29 41 32 46 / 30 20 20 20 BZN 23 39 27 45 / 40 10 10 10 WYS 14 30 21 34 / 50 60 70 60 DLN 25 39 28 44 / 10 10 10 10 HVR 6 36 24 41 / 80 0 10 0 LWT 18 38 27 43 / 60 0 10 0

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 9 PM MST this evening for East Glacier Park Region-Gallatin and Madison County Mountains and Centennial Mountains-Southern Rocky Mountain Front.