textproduct: Great Falls

KEY MESSAGES

- Showers and thunderstorms develop this afternoon and persist through the evening hours tonight.

- A cold front will bring breezy and gusty north winds to most areas tonight and through the day on Sunday.

- Additional opportunities for showers and storms on Sunday and Monday before turning drier and warmer through the upcoming work week.

DISCUSSION

- Meteorological Overview:

Warm advection ahead of a compact shortwave diving southeast within quasi-zonal flow aloft will allow temperatures to warm into the upper 70s to low 80s today across Southwest through North Central Montana, with showers and thunderstorms developing during the late morning/afternoon hours and persisting through the late evening hours tonight. At the surface a cold front will dive south from Alberta and across the plains of North Central Montana by the late evening hours tonight, with this cold front continuing to push south through Central Montana through the early morning hours on Sunday and Southwest Montana by the late morning hours on Sunday. Strong 6hr pressure rises on the order of 6-12mb and H850 winds of 20-30kts will help to support strong and gusty north winds in wake of the immediate frontal passage tonight, especially along and west of the US Hwy 87 corridor. Breezy and gusty conditions will linger into the day on Sunday over the plains, with high temperatures falling into the mid-60s to low 70s over the plains of Central and North Central Montana and the 70s across the valleys of Southwest and Central Montana. Northwest flow aloft in wake of the compact shortwave today/tonight will help to support additional opportunities for showers and thunderstorms on Sunday, most notably across the Glacier National Park region and along and north of the MT Hwy 200 corridor. Well below normal temperatures will linger into the day on Monday as northwesterly flow aloft remains in place, with yet again afternoon/evening shower and thunderstorms opportunities.

Transient ridging looks to then slide east and over the Northern Rockies through the remainder of the work week, with high temperatures warm to near normal on Tuesday before climbing well above normal on Thursday and Friday. While an isolated shower or thunderstorm can't be ruled out over the higher terrain through the period, especially on Thursday, dry conditions are largely expected beneath the transient ridging. - Moldan

- Forecast Confidence & Scenarios:

Showers and Thunderstorms Today...

Confidence is high that showers and thunderstorms will initiate across Southwest Montana between 10AM and 1PM, initially over the higher terrain before overspreading the adjacent valleys, with the areal coverage of precipitation increasing through the remainder of the afternoon as the showers and storms drift to the northeast. Further north over the plains of Central and North Central Montana convective initiation will occur later in the afternoon, generally between 2 to 5PM, initially along and west of the I-15 corridor. Areal coverage of these showers and thunderstorms over the plains will be lower than that for areas south of the Montana Hwy 200 corridor, with this activity drifting from west to east through the remainder of the afternoon and evening hours. While instability is quite low/marginal across Southwest through North Central Montana through the day today the presence of dry air in the low to mid- level, as evident by inverted-V soundings, I can't rule out some stronger wind gusts on the order of 45-55 mph with the strongest storms. Additionally, tall skinny CAPE profiles, PWATs ranging from 0.5" to 0.75", and slower storm motions of 20-30 mph will help to support periods of heavy rain. These burst of heavy rain, should they occur over recent (i.e. last 2-3 years) burn scars, would be capable of producing localized flash flooding. - Moldan

AVIATION

20/06Z TAF Period

Scattered showers across Southwest Montana will become more numerous heading into the late morning/early afternoon hours. Precipitation is expected to start off more stratiform in nature and transition to a more convective nature by the afternoon. The main concerns throughout the day will be locally heavy downpours, gusty, erratic winds, and a chance for small hail with afternoon thunderstorms.

Across North-Central Montana there is a chance for afternoon thunderstorms ahead of a cold front descending from Canada. The main concerns will be gusty, erratic winds and lightning.

Generally VFR conditions are expected to prevail across the region but periods of MVFR/IFR conditions are possible under heavier showers and thunderstorms. Mountain obscuration will remain a concern through much of the period, particularly across Southwest Montana. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 82 52 71 47 / 20 30 20 30 CTB 80 46 65 43 / 20 20 50 40 HLN 83 52 77 49 / 20 20 0 0 BZN 77 48 77 44 / 60 50 0 10 WYS 69 39 69 37 / 80 70 30 20 DLN 74 46 77 43 / 90 80 0 0 HVR 82 50 72 45 / 0 50 10 20 LWT 77 48 69 43 / 20 40 20 40

TFX WATCHES/WARNINGS/ADVISORIES

None.