textproduct: Great Falls

KEY MESSAGES

- Scattered shower and thunderstorm activity continues through Tuesday with a few storms producing localized strong, gusty winds, brief heavy downpours, and hail in addition to cloud to ground lightning.

- Warmer, drier, and seasonably breezy conditions move in for the second half of the workweek.

- An approaching Pacific trough will bring increased shower and thunderstorm activity, breezy to windy conditions, and elevated fire weather concerns this weekend.

DISCUSSION

- Meteorological Overview:

A shortwave passage will maintain scattered shower and thunderstorm activity for the next couple of days. Similar to yesterday, ML CAPE levels will be running in the 500 to 1,000 J/kg range with effective shear generally between 25 and 35 kts. Minimum RHs will continue to dip into the 20s, so brief wind strong wind gusts and cloud to ground lightning will be the primary hazards; however, there will be hail and brief heavy downpours with the stronger cores given the previously mentioned modestly elevated shear and PWATs around 0.75 inches.

Ridging aloft strengthens over the Great Basin with heights rising as high as the 588 to 594 dm range over the Northern Rockies by Friday. This will reduce shower and storm coverage while diurnal mixing maintains seasonably breezy conditions. Despite the strength of this ridging, weak surface fronts look to keep temperatures mostly in the 80s over central and north-central MT. These features will have a harder time surpassing the central/SW mountains, so upper 80s and even lower 90s are a safer bet down there.

Subtle shortwaves will begin to move into western/southwestern MT as early as Friday evening in advance of a trough moving onto the Pacific NW coast. This wave will interact with diurnal instability and initiate isolated shower and thunderstorm activity, primarily over and near the higher terrain of W/SW MT with localized gusty winds and lightning being the most logical hazards.

The Pacific NW trough then approaches the Northern Rockies during the weekend and brings increased shower and thunderstorm activity and breezy to windy conditions. The main concern for this weekend looks to be dry and continued very warm conditions and increasing fire weather concerns. Southwesterly flow aloft looks to develop in the wake of the exiting trough early next week with passing shortwaves maintaining daily scattered shower and thunderstorm activity. - RCG

- Forecast Confidence & Scenarios:

Mostly afternoon and evening showers and thunderstorms through Tuesday night...

Scattered showers and thunderstorms will develop in a similar fashion as yesterday given the continued moist and modestly sheared environment. The one exception to this is the lack of a more defined forward moving surface front over the plains acting as a lifting mechanism. For this reason, storm coverage may be a bit more widespread over the southwest, but the more isolated central/north- central MT storms are being highlighted with the better quality storm maintenance and upscale growth, especially as they cross the highway 87 corridor and the Little Belt Mountains. Should one of these storms gain momentum, then localized severe wind gusts and lightning will be the primary hazards followed by brief downpours and hail. Other areas will see similar hazards to monitor, although with less intensity in most cases given the mostly singular or weak multiple cellular storm structure. Peak timing for today's showers and storms will generally fall between 2 and 9 pm with coverage diminishing heading into the overnight hours. Tuesday looks to see a similar set up with scattered storms, mostly of the pulse variety with few isolated stronger storms capable of localized strong, wind gusts, brief downpours, and hail in addition to cloud to ground lightning.

Elevated fire weather concerns this weekend...

A Pacific trough looks to bring breezy to windy conditions to the Northern Rockies this weekend despite increased shower and thunderstorm activity. Much of the forecast area has over a 50% chance for wind gusts exceeding 30 mph while minimum RHs fall to or near critically low levels. This is also reflected in the Hot Dry & Windy Index (HDWI) with probabilities for exceeding the 75th percentile on Saturday running above 50% for most locations. Recent rainfall has tamed the previously maxed out Energy Release Component (ERC) values and they are tentatively forecasted to remain shy of critical levels. If the mid-week warm up exceeds expectations, then elevated to critical fire weather conditions may develop for some areas. This situation will continue to be monitored for the need of fire weather products. - RCG

AVIATION

17/18Z TAF Period

An approaching shortwave trough will maintain scattered showers and thunderstorms, mostly between 17/20Z and 18/03Z, though some isolated activity may persist into the overnight hours. VFR conditions will mostly prevail beneath mid- and high level cloudiness; however, brief low VFR/MVFR conditions and mountain obscuration will be observed near any showers or storms. Additionally, instances of localized strong, gusty winds, brief downpours, and hail may be present with the stronger cores.

For KGTF: Winds will be light and variable during this TAF period with less than a 10% chance for exceeding the 15 kt crosswind threshold away from showers and storms. Between 17/21Z and 18/03Z there is a 30% chance for thunderstorms impacting KGTF. There may be instances of localized strong, gusty winds, brief downpours, and hail with the stronger cores. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 52 83 53 86 / 20 20 10 0 CTB 48 77 51 82 / 50 20 10 0 HLN 54 83 51 85 / 20 20 10 0 BZN 51 80 49 84 / 30 30 30 0 WYS 39 75 36 79 / 60 30 30 0 DLN 49 80 47 83 / 50 20 20 0 HVR 51 84 54 86 / 30 20 20 0 LWT 50 79 51 82 / 50 30 20 0

TFX WATCHES/WARNINGS/ADVISORIES

None.