textproduct: Great Falls

KEY MESSAGES

- There will be another round of showers and thunderstorms today with some storms producing localized strong wind gusts, large hail, and torrential downpours in addition to lightning, especially over the plains east of I15.

- Showers and thunderstorms continue on Saturday followed by cool,wet, and raw conditions with some mountain snow Saturday night through Monday.

- Uncertainty remains high with the precise locations of the heaviest precipitation for this weekend's storms.

- Temperatures rebound heading towards the middle of next week, but there will be daily opportunities for scattered showers and thunderstorms.

DISCUSSION

- Meteorological Overview:

Southwesterly flow will become firmly established over the Northern Rockies today with PWATS approaching and even exceeding the one inch mark for the northern half of the forecast area according to the North American Ensemble Forecast System (NAEFS). Forcing from an approaching Pacific trough will maintain variable cloudiness and scattered to numerous showers and thunderstorms. Abundant cloud cover/morning showers may compromise diurnal heating and delay or even preclude strong to severe storms by adding CIN and lowering CAPE some. Ensemble guidance continues to highlight central/north- central areas east of I15 with ML CAPE in the 600 to 1200 J/kg range and around 300 to 700 J/kg over the southwest, hence these areas will have higher instances of strong to severe thunderstorms given bulk shear values rising to the 30 to 40 kt range and provided that the cap is eroded in a timely fashion. Strong flow aloft over the southwest will also play a role in transferring any convective gusts to the surface. In addition to the severe threat, the anomalously high PWATS will also add a localized heavy rain threat that may cause localized flooding of normally susceptible areas such as burn scars and flood prone urban areas. Scattered to numerous shower and thunderstorm activity will continue into Saturday via the southwesterly flow remaining in place. Abundant cloud cover should dampen instability, but convectively driven areas of heavier rainfall will still be a threat, especially heading into the afternoon and evening hours.

The main trough will move through the Northern Rockies Saturday night through early Monday. This system is still looking to undergo stretching and shearing as it moves east. Even though it appears the main area of forcing will be close enough to bring at least a period of widespread rainfall to most locations, these transformations continue to make it difficult to decipher the precise areas of heavier rainfall as will the presence of convective enhancement. Additionally, model guidance has not been displaying run to run consistency with the track of the primary forcing, which currently places the heaviest precipitation along and west of the Continental Divide.

The cold core with this system looks to be rather impressive with H700/H500 temperatures running between 2 to 3 standard deviations below average according the the NAEFS. Even though the cold core aloft looks to be centered a little farther south of the forecast area, there will be at least some wet snow over most mountain areas due to dynamic cooling from heavier precipitation banding and or convective enhancement. Accumulating snow will generally remain above 6500 ft, mostly occur during the morning hours Sunday and Monday, and will be most widespread over the southwest, closest to the cold core aloft.

There will also be gusty winds with this system, especially on the backside of it on Monday. Overall, expect a cool, wet weekend for late June standards, especially Sunday and Monday. Once this system exits the region later on Monday, there will be warming and drying heading into the middle and end of next week; however, a rather unsettled west to southwesterly flow aloft will remain in place and passing shortwaves will maintain daily opportunities for scattered showers and thunderstorms. - RCG

- Forecast Confidence & Scenarios:

Periods of strong to severe thunderstorms and localized heavy rain through Saturday...

Showers and thunderstorms will move in a southwest to northeast fashion this afternoon and evening and will be preceded by a weaker showers in the morning. This will result in cloud cover and convective contamination, though the wind shear will be more favorable for storm maintenance if the boundary layer recovers in time for the afternoon round of storms. The current target area for strong to severe storms will be for areas east of I15, particularly for central/north-central areas where timing will be best aligned with peak diurnal heating. While there is still a spread on how unstable the environment will be, the higher end guidance supports sufficient shear and CAPE for rotating supercells and even an isolated tornado (mainly Fergus county). Today's thunderstorm hazards will have a higher ceiling than yesterday which may include strong wind gusts, large hail, and torrential downpours. There will also be a little more focus for heavy rainers and localized flooding of normally flood susceptible areas such as burn scars and prone urban areas. With that being said, storm motion around and over 20 kts should shorten storm residence time and the opportunity for flooding. By Saturday, the most robust instability looks to move east of the forecast area, but the threat for some stronger storms on the eastern fringes of the forecast area and the localized heavy rain threat will continue for most areas east of I15.

Today's gusty winds over southwest MT...

The strong southwesterly flow aloft continues to look supportive of H700 winds approaching and even briefly exceeding the 30-40 kt range over will southwest MT this afternoon. This timing will be optimally aligned with peak diurnal mixing and the southerly flow will be favorable for terrain funneling over the the north-south valleys and mountain passes along the Idaho border. Additionally, any showers or thunderstorms that develop in this environment will help transfer stronger wind gusts to the surface. NBM 5.0 probabilities continue to support a 70 to 90% chance that gusts exceed 40 mph along and south of a Dillon, to Ennis, to Big Sky line, with even a 50 to 80% chance that gusts exceed 50 mph. Furthermore, there is even a 20% chance that sustained winds within the Madison Valley reach 40 mph for a period of time from late Friday morning through the early evening hours. While these probabilities and deterministic guidance don't necessarily support high wind highlights, it will windy for so late in the season. This will cause some difficulties for those operating high profile vehicles and for those with with recreation plans in the mountains. Campers over the higher terrain should also beware of their surroundings and watch for falling trees, especially if there is lingering damage from the cool season winds.

Cool conditions with widespread precipitation and areas of mountain snow this weekend...

Widespread rain showers and embedded thunderstorms are in the forecast late Saturday through early Monday. Deterministic guidance has recently shifted the area of heaviest precipitation northwestward towards the Continental Divide, but there is still uncertainty given poor run to run model consistency and continued stretching and shearing of the main low pressure system. The latest NBM probability for 1 inch of rain or greater over the 72 hour period ending 6 am Tuesday continues to run between 60 and 80% for virtually the entire forecast area except for the far eastern portions of central/north-central MT. Also, there is roughly a 50 to 70% chance for 2 inches of rain or greater over the higher terrain of the Northern Rocky Mountain Front and over the Little Belts during the same period. The focus of the heavier rain looks to be generally concentrated over central and southwest MT Saturday through early Sunday before heading northward over the plains and north to the Canadian border Sunday into Monday.

Mountain snow looks to be most widespread over the higher terrain of the southwest, but other mountain areas that see convective or heavier precipitation banding will see snow levels artificially lowering, at least on a temporary basis. This includes the Logan Pass area and the King's Hill area in the Little Belts. Snow levels during this entire event should range between 6500 to 8000 feet, lowest during the overnight and early morning hours Saturday night through Monday morning. Winter weather advisories have only been sent out for the mountain zones south of I90 where there is highest confidence for wet snow; However, anyone with outdoor plans should be prepared for cool and damp conditions for all mountain areas where these conditions may be dangerous to those not properly dressed. Also forest and rural roads may become muddy or even impassible from the wet conditions. Area rivers, creeks, and streams should be able to handle the widespread 1 to 2 inch rainfall totals, although there will be water rises and localized ponding of water or minor flooding, particularly for the normally susceptible low lying areas or other prone areas including sensitive burn scars. - RCG

AVIATION

26/12Z TAF Period

Scattered showers/isolated thunderstorms will affect the CWA through the period. However expect a period of stronger storms on this afternoon over the central and eastern portions of the CWA. All severe weather hazards will be possible this afternoon east of a line from Shelby to Helena. Mountains/passes will be at times through the period by clouds/precipitation. Brusda

CLIMATE

The Great Falls CWA has had 12 years with a winter statement issued during the month of June since 2008. That means around 60 percent of all June's since 2008 a winter weather statement was issued. Thus it is not uncommon for a winter statement to be issued in the Great Falls CWA during June. It does not snow in the same zones every year, but because the Great Falls CWA is quite large, it is not uncommon for a winter statement to be issued in June. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 78 52 70 48 / 70 70 40 80 CTB 70 48 69 48 / 60 40 10 60 HLN 77 51 67 46 / 80 90 80 90 BZN 81 48 65 42 / 60 80 90 100 WYS 75 40 56 32 / 20 50 90 90 DLN 76 46 61 39 / 30 70 90 100 HVR 82 52 75 49 / 50 100 20 70 LWT 78 48 67 44 / 60 100 80 80

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from midnight Saturday night to noon MDT Monday for Gallatin and Madison County Mountains and Centennial Mountains-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains.