textproduct: Great Falls

KEY MESSAGES

- Daily chances of showers and thunderstorms through Wednesday, a few may become severe.

- Heavy rain is possible Monday and Tuesday which may lead to isolated flash flooding.

- Much warmer and drier conditions towards the end of the week and through the weekend.

DISCUSSION

- Meteorological Overview:

A shortwave combined with moist southwesterly flow will continue to make its way across the region today through tomorrow. A few may become severe but the main threat will be heavy rain which may lead to localized flash flooding. Weak transient ridging will pass through on Tuesday with left over moisture and instability potentially being enough to kick off some afternoon showers and thunderstorms, especially in North- Central Montana. An upper- level trough makes its way across southern Canada Wednesday into Thursday. While the bulk of the instability is expected to stay in Canada there is still a chance for afternoon showers and thunderstorms with a few that may be strong to severe.

Heading towards the weekend upper-level ridging builds over the western CONUS leading to much warmer temperatures across the state.

- Forecast Confidence & Scenarios:

Today and tomorrow: Scattered showers in Southwest Montana will continue to spread further north as the afternoon goes on. The latest hi-res model runs point towards a cluster of storms forming in Idaho and moving up to the northeast through our region this afternoon/evening. There is still some disagreement as to how exactly it will play out and there is a possibility that the morning cloud cover may have been enough to dampen some of the threat for stronger to severe storms. But with the disturbance moving through and bringing ample moisture there is still reasonably high confidence that at least a couple of the storms may be strong to severe.

For Southwest Montana, there will be a greater concern for both wind and hail. Further north above a line roughly between Helena and White Sulphur Springs, precipitable water values are close to the daily climatological max. While some gusty winds and small hail might be present at times the main concern will be heavy downpours. Flash flooding is possible both for slow-moving storms and any training thunderstorm situation. Poor drainage and urban areas will be at the highest risk along with burn scars.

Because confidence is lacking in any one area or zone receiving enough rainfall to cause widespread flash flooding, for now, no flood products will be issued. But those living in Central and North-Central Montana should have a way to receive warnings, especially during the overnight hours in case things end up being worse than what is currently expected.

Tuesday: Rain is expected to continue across Central and North- Central Montana through the day on Tuesday with a chance for some afternoon thunderstorms to develop. Heavy rain will once again be a main concern for any shower or thunderstorm that develops with a chance for isolated flash flooding extending through the evening hours across portions of the region, mainly east of a line between Great Falls and Chester.

Wednesday: Dewpoint temperatures on Wednesday afternoon are slightly lower compared to previous model runs. But with dewpoint temperatures still expected to reach the mid to upper 50s, afternoon highs across the lower elevations reaching the mid 80s to low 90s, and bulk shear in the 30 to 50kt range, there is still a good chance that at least a couple storms turn severe. The difference the final position of the trough will make is whether or not a couple become severe, which happens if the trough stays further north, or if a few more become severe, which will happen if the trough ends up further south. The main threats will be hail and wind with some localized heavy downpours although storm motion should be fast enough to lessen the flash flooding concerns compared to Monday and Tuesday.

Friday and beyond: As the ridge starts to build over the region, temperatures will continue to warm through the weekend with Saturday expected to be the warmest day. A good portion of North- Central Montana has a 20-30% chance for highs exceeding 100 degrees both Saturday and Sunday. In reality, the occurrence of 100+ highs are likely to be a bit more sporadic with some of the typically hotter areas like Fort Benton and the Upper Missouri River Breaks portion of northern eastern Fergus county are the most likely to experience 100+ temperatures.

While drier conditions are expected to persist at least through early next week there is always a non-zero chance for an isolated shower to pop up along the mountainous regions. These showers will generally be confined to terrain areas and are unlikely to last long. But they may pose a risk for those recreating in the area, especially if lightning starts to become a factor.

Longer term guidance from CPC has suggested that the chance for hotter temperatures may linger through most of July. If this occurs the main concern will be compounding heat stress from multiple days of above normal temperatures. This may cause impacts for those recreating outdoors and those without air conditioning. -thor

AVIATION

06/18Z TAF Period

An upper level weather disturbance lifting north from the Great Basin will continue to spread increasing clouds across the area through this afternoon. Showers and thunderstorms developing this afternoon across southwest MT will track north into portions of north-central MT this evening and overnight. The risk for strong thunderstorms with gusty winds, hail and locally heavy rainfall is highest across SW MT late this afternoon. Further north, the risk for showers and generally weak thunderstorms increases this evening with additional showers circulating around the disturbance across central/north-central MT through the overnight period. Locally heavy downpours and brief MVFR conditions are possible with most showers. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 55 81 55 91 / 60 40 0 20 CTB 54 79 53 86 / 60 30 10 10 HLN 56 84 56 88 / 60 20 0 40 BZN 52 83 53 87 / 50 10 10 20 WYS 48 80 46 81 / 50 20 10 10 DLN 50 82 52 85 / 40 10 10 30 HVR 57 76 53 90 / 80 90 20 20 LWT 51 76 51 86 / 80 70 10 20

TFX WATCHES/WARNINGS/ADVISORIES

None.