textproduct: Great Falls

KEY MESSAGES

- Scattered showers and thunderstorms this afternoon capable of briefly heavy rainfall and gusty winds.

- Additional scattered showers and thunderstorms Sunday afternoon and early evening ahead of a cold front bringing drier and breezy conditions.

- Cooler in most areas Monday before temperatures trend closer to, if not above average by mid-week. A few showers and thunderstorms will be around most days next week, primarily across Southwest MT.

DISCUSSION

- Meteorological Overview:

High amplitude upper level ridging is slipping east of the region this afternoon ahead of an upper level trough moving onshore across the Pacific NW. Resulting flow aloft is southwesterly, which in combination with an embedded wave lifting northeastward from the base of the upper trough will result in quite a few showers and thunderstorms today. This activity looks to develop across SW MT initially, spreading northeastward into Central MT and vicinity with time. PWATs are elevated, signaling the risk for some heavy rainfall in the strongest thunderstorms. Modest inverted-V profiles suggest strong wind gusts will be the primary concern from a severe weather perspective today, should thunderstorms be able to congeal into at least a broken line. Looking toward tonight forcing increases, especially west of the Continental Divide, as the core of the upper trough approaches the Northern Rockies. There looks to be quite a bit of showers or thunderstorms that develop west of the Continental Divide as a result, with a few of these leaking east of the divide through the overnight.

Forcing and flow aloft increases further for Sunday as the upper level system moves overhead and eventually east of the region Sunday night. A cold front moves through associated with this system, bringing cooler temperatures, much drier air, and gusty winds. Ahead of this front there looks to be ample instability, forcing, and marginal kinematics to result in at least a few loosely organized thunderstorms Sunday afternoon. Both strong wind gusts and large hail will be a concern in the most robust thunderstorms. This activity largely looks to develop east of I-15 and continue eastward Sunday.

Heading into next week the trend is for weak ridging now across the NW CONUS, which will keep a slightly anticyclonic flow aloft in place. This looks to inhibit showers and thunderstorms for a day or two in most areas, though monsoonal moisture stuck in the southern periphery of the ridge will still result in a few showers and thunderstorms in SW MT near the Idaho border. The ridge axis slowly slips east through the week, with flow aloft becoming slightly more cyclonic each day as another Pacific trough begins to approach the Pacific NW. Showers and thunderstorms gradually increase in coverage mid-week onwards, focused mostly in the afternoon and evening. -AM

- Forecast Confidence & Scenarios:

Thunderstorms today:

Clearing is being noted on latest satellite imagery across much of ID and into SW MT. This will result in at least weak instability through the remainder of the afternoon and evening. The degree to which the area destabilizes will play a large role in how widespread the risk will be for strong wind gusts. Longer periods of sunny skies would increase this risk over shorter periods of sunshine.

Precipitable water (PWAT) values near and in excess of an inch will yield at least a localized risk for briefly heavy rainfall today in the strongest thunderstorms. Should a strong thunderstorm move across an urban area or a burn scar, excessive rainfall leading to flash flooding will be a concern. Overall, the risk for this across any specific area is too low to warrant any flash flood watches. Although this is the case, those in urban areas and near burn scars should be aware of this risk today.

Thunderstorms Sunday:

Stronger flow aloft arrives with he core of the upper level disturbance ahead of the cold front Sunday. Lingering moisture ahead of the cold front looks to combine with daytime destabilization to result in modest instability by the early afternoon across many areas. Forcing does not look to be lacking Sunday, with the primary limiting factor on organized thunderstorms being slightly sub- optimal shear (Closer to 30kts than 40kts). That said, at least loosely organized thunderstorms look to develop Sunday, mainly east of I-15, which will be capable of strong wind gusts and large hail. Should shear trend much higher, concern would increase for longer lasting, more organized severe thunderstorms.

Mid to late week:

Guidance has been inconsistent with respect to the progression of Pacific troughing moving in mid to late week. There is a roughly 50/50 split in cluster guidance members for above average heights (Warmer temps) vs below average heights (Cooler temps) across the region by the end of the week. -AM

AVIATION

22/18Z TAF Period

VFR conditions will prevail throughout this TAF period, with the exception of periods of MVFR conditions due to a passing thunderstorm. Thunderstorms will begin to develop in Southwest MT around 21Z, moving northeast throughout the evening. While confidence was too low in thunderstorms reaching KHVR to include in the TAF, an isolated thunderstorm cannot be ruled out. Any thunderstorms that develop will be capable of producing gusty, erratic winds, brief, heavy downpours, frequent lightning and hail. Periods of mountain obscuration will develop due to passing thunderstorms.

For KGTF: Thunderstorms are expected to begin moving over KGTF as early as 01Z, and will be capable of producing gusty, erratic winds greater than 40 kt, frequent lightning, brief, heavy downpours and hail. Thunderstorms are expected to exit the area around 06Z. Outside of any gusty, erratic winds a thunderstorm may produce, the chance of a cross-wind greater than 15 kts is less than 10% due to a southwesterly direction. -Dzomba

FIRE WEATHER

Although precipitable water is above average today, a few lightning ignitions cannot be ruled out in locally dry areas, particularly in terrain.

The most concerning fire weather day will be on Sunday when a surface front brings gusty westerly winds amid continued very warm and dry conditions. The timing of this front will ultimately determine how long the dry and windy conditions persist. The latest NBM probabilistic guidance continues to highlight large areas of north- central and portions of southwest Montana with over a 70% chance for wind gusts over 30 mph. Although the fuels aren't quite as dry as they were in earlier in the month, elevated fire conditions will be present and the wind may be enough to ignite any lightning caused hold over fires from Friday and Saturday's storms. In addition to the winds, there will be more widely scattered showers and thunderstorms, some with strong, erratic wind gusts. -AM/RCG

HYDROLOGY

The combination of increased monsoonal moisture aloft and lower level moisture moving in from the east today will result in a 10% chance for localized heavy thunderstorm rains producing flash flooding over and near burn scars, urban areas, and other normally flood prone locations. While this activity will be monitored for the aforementioned flash flooding, a flash flood watch will issued with this forecast package per the more scattered nature of higher rainfall rates and progressive storm motion. The moisture profile begins thin on Sunday, which will reduce the threat for excessive rainfall through early next week. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 62 89 54 81 / 20 20 0 0 CTB 58 81 50 77 / 20 20 0 0 HLN 57 87 50 85 / 60 20 0 0 BZN 54 86 49 85 / 50 40 20 0 WYS 46 78 40 79 / 50 30 20 0 DLN 52 85 47 84 / 40 20 0 0 HVR 62 93 56 82 / 10 40 20 0 LWT 60 89 53 79 / 20 50 30 0

TFX WATCHES/WARNINGS/ADVISORIES