textproduct: Great Falls

KEY MESSAGES

- Hazy, very warm, and dry conditions generally continue for the next couple of days.

- A Pacific weather system and increased moisture will introduce shower and thunderstorm activity and cool temperatures down late Wednesday through Friday.

- While some areas may see beneficial rains, this activity may also initiate a few stronger storms with localized strong wind gusts, hail, and flooding of normally prone areas.

- Scattered shower and thunderstorm activity capable of strong wind gusts and lightning will continue into the weekend while temperatures trend above average again.

DISCUSSION

- Meteorological Overview:

Ridging aloft will maintain the hazy, very warm, and dry conditions for at least the next couple of days. Shortwave energy and instability capable of initiating shower and thunderstorm activity looks to remain north of the Canadian border during this period, though some isolated storms may begin to creep towards the Hi-Line Tuesday evening and night. Seasonably breezy conditions will continue with wind gusts exceeding 20 mph at times, especially this afternoon and evening.

A Pacific trough begins to move southeastward through BC/AB and into the Northern Rockies sometime Wednesday through Friday. Ensembles are coming into better agreement with temperatures falling below average Thursday and Friday, but still differ on how widespread the rainfall will be. Shower and thunderstorm activity looks to increase from north to south with areas along the Hi- Line seeing scattered showers and storms as early as Tuesday evening. The Wednesday night through Thursday time period is generally favored to see the most widespread rainfall. While some areas may see beneficial rains and cooling temperatures, the convective nature of this activity may also bring some strong to severe storms with localized strong gusty winds, hail, and heavy downpours capable of flooding of normally flood prone areas. ML CAPE values look to generally run in the 300 to 800 j/kg range with the highest values gradually moving south into central and southwest Montana by late Thursday into Friday.

The ridging aloft begins to build back in heading into next weekend, but another trough approaching the Pacific NW coast will bring southwesterly flow aloft within striking distance of the Northern Rockies. This will result in temperatures warming back up, along with daily scattered shower and thunderstorm activity. - RCG

- Forecast Confidence & Scenarios:

Breezy conditions, shifting winds, and localized elevated fire concerns...

The combination of very warm, dry, and breezy conditions will result in localized fire weather conditions for the next couple of days. Surface boundaries and or diurnal mixing will bring wind shifts and gusts in excess of 20 mph at times. Afternoon humidity will fall to or near critically low levels with only moderate to poor overnight recoveries, driest over southwest Montana. Those working to contain ongoing fires should continue be mindful of the dry conditions and breezy, shifting winds.

Cooler temperatures, some stronger storms, and beneficial rains Wednesday through Friday...

Ensemble guidance is now coming into agreement with cooler and wetter conditions moving in late Wednesday through Friday. Climate anomaly indices, including the ECMWF Extreme Forecast Index are showing signals for precipitation amounts above climatology for this time of year. While this rainfall is definitely welcome, its convective nature can result in uneven distribution and increased instances of stronger storms capable of localized strong wind gusts, hail, and even flooding of normally prone areas. At this time, the most widespread precipitation looks to occur on Thursday.

Scattered gusty thunderstorms this weekend...

Southwesterly flow aloft approaching the Northern Rockies will maintain mid-level moisture and shortwave energy this weekend while the surface boundary layer warms up and dries out. If this pattern comes to fruition, there will be scattered showers and thunderstorms for both days, mostly in the afternoon and evening. The primary hazards will be localized strong, gusty winds, and lightning. - RCG

AVIATION

10/12Z TAF Period

Overall, a westerly flow aloft will reside over MT through the period. At this time, any thunderstorms should remain north of the US/Canadian border through the period. Otherwise, smoke will be the main impact, with Southwest MT having IFR visibility at times because of smoke. Mountains/passes will be obscured across the CWA through the period because of smoke.

For KGTF: Surface winds should generally be westerly today. Overall wind speeds will will generally be less than 15 knots through the period, with just a few isolated gusts near 20 knots this afternoon. Overall the probability of cross winds over 15 knots is less than 20 percent. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 89 55 85 54 / 0 0 0 0 CTB 82 50 77 49 / 0 0 0 20 HLN 88 54 89 55 / 0 0 0 0 BZN 90 52 90 52 / 0 0 0 0 WYS 84 42 85 44 / 0 0 0 0 DLN 88 50 88 51 / 0 0 0 0 HVR 88 54 85 52 / 0 0 0 30 LWT 85 53 82 51 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.