textproduct: Great Falls

KEY MESSAGES

- Warm and dry weather persists through Friday with windy conditions Friday afternoon bringing elevated to critical fire weather conditions.

- A drastic change to much colder temperatures occurs Saturday with widespread precipitation developing and changing to snow for most locations by late Saturday.

- Impactful winter weather and snow accumulation is possible over some mountain passes this weekend, especially Saturday night through Sunday morning.

DISCUSSION

A broad upper level ridge south of the area and zonal/westerly flow aloft maintains warm and dry conditions across the area today with generally light winds. Meanwhile, upper level troughing in the Gulf of AK will deepen today as additional energy arrives from the west, setting the stage for a significant pattern change as the the large scale flow amplifies and the trough digs southward into the western US this weekend. Increasing west-southwest mid- upper level flow ahead of the approaching trough along with surface low development in southern AB will lead to windy conditions Friday as well as a few additional degrees of warming and increased fire weather concerns through Friday evening. An initial Pacific cold front looks to move east across central/north- central MT Friday night with stronger cooling from north to south on Saturday as an airmass from Canada surges south through the area.

Precipitation develops Saturday night along the continental divide with the initial Pacific front with more widespread precipitation developing across central MT Saturday, both along and following the surge of colder air and in association with a plume of deeper moisture and frontogenic forcing. Models are in reasonable agreement with the evolution of the trough this weekend as it splits/elongates with much of the energy digging south along the west coast through Sunday. This maintains a flow of deep moisture across the area in SW flow aloft, while the lower level cold air continues to deepen and spread south through Sunday. The focus for steady precipitation will likely continue across central MT into Saturday night then shift southward through SW MT later Saturday night through Sunday. Beyond this, models diverge with how long precipitation may linger but an overall decrease from NW to SE is likely later Sunday through early Monday.

The combination of cooling aloft with the upper trough and surge of lower level cold air from the north will rapidly cool temperatures and lower snow levels across western portions of north-central/central MT on Saturday with daytime temperatures likely in the upper 30s to around 40 by Saturday afternoon as snow levels lower to around 3000-4000 ft. While snow levels are also likely to lower to near the valleys in SW MT Saturday night, confidence in timing is lower. Potential exists for significant snowfall amounts over some mountain passes this weekend with main impacts likely in the Saturday night through Sunday morning period when snow is most likely to accumulate on surfaces that will take time to cool after the recent warmth. A few inches of snow are also likely at lower elevation locations including Helena and Great Falls. The drastic change to colder temperatures this weekend will be an impact to anyone with outdoor plans, especially after the recent extended period of mild weather. Winter weather statements are likely for portions of the area this weekend as confidence in snowfall amounts and timing increases. Hoenisch

AVIATION

08/06Z TAF Period.

VFR and dry conditions prevail through the TAF period. Winds remain on the lighter side throughout the TAF period. High clouds begin to increase towards the end of the TAF period. -Wilson

FIRE WEATHER

Consecutive days of unseasonably mild temperatures, low afternoon humidity and poor overnight recovery have lead to increased fire danger indices, particularly across central MT where overnight moisture recovery has been lowest. Unseasonable warmth and continued dryness on Friday will combine with increasing southwest winds with gusts as high as 30-40 mph to create elevated fire weather conditions across much of the area with the most critical combination of winds and dryness across central MT, where a Fire Weather Watch is in effect. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 78 54 80 38 / 0 0 0 30 CTB 74 52 72 31 / 0 0 0 30 HLN 77 47 82 42 / 0 0 0 20 BZN 76 43 81 43 / 0 0 0 10 WYS 72 31 70 37 / 0 0 0 10 DLN 77 41 80 43 / 0 0 0 0 HVR 73 47 82 34 / 0 0 0 10 LWT 75 51 82 38 / 0 0 0 10

TFX WATCHES/WARNINGS/ADVISORIES

Fire Weather Watch Friday afternoon for Central and Eastern Lewis and Clark National Forest Areas-Helena and Townsend Ranger Districts of the Helena National Forest.