textproduct: Great Falls

KEY MESSAGES

- Warm, breezy and mostly dry conditions for much of the region today, with some isolated showers and thunderstorms possible in Southwest MT this afternoon.

- Monday through Friday will see daily chances for showers and thunderstorms in the afternoon and evening hours. Some of these storms could be strong to severe.

- Warm temperatures will continue through Tuesday, with a brief cool down Wednesday and Thursday.

DISCUSSION

- Meteorological Overview:

Due to zonal flow aloft, today will be warm, breezy and mostly dry across the region, with the exception of a few showers and weak thunderstorms possible in Southwest MT this afternoon. Tomorrow, an upper-level trough begins to move inland from the west coast, bringing widespread chances for showers and thunderstorms. The upper level trough will transition the zonal flow to southwesterly flow, advecting warm air into Montana. This warm air advection, combined with upper level ridging to the east, will make for the warmest day of the week for most of the region, with highs in the 80s to low 90s. The warm temperatures will last through Tuesday, before a cold front Tuesday night will bring a brief cool down Wednesday and Thursday.

On Tuesday, the axis of the upper level trough moves southeast down western Oregon and and northern California, resulting in southeasterly flow into the region. The trough settles into a spin over California and Nevada Wednesday and Thursday, creating a shift to a more steep southerly flow. On Friday, the trough begins to break down and the axis moves northeast towards Montana, putting the region back into southeasterly flow.

With the daily influence of this trough spinning to the southwest of Montana comes an ushering of moist and unstable air into the region each day. Combined with the warm afternoon temperatures, there is a daily chance for showers and thunderstorms through the rest of the approaching work week. Some of these storms could be strong to severe, depending on the position of the trough and the timing of the moisture each day. See the "Forecast Confidence & Scenarios" section for more details. -Dzomba

- Forecast Confidence & Scenarios:

Monday Thunderstorms:

Overall, these showers and thunderstorms look relatively mild compared to what is expected later in the week. With CAPE generally expected to stay below 1000 J/kg, there is not a lot to work with. Some isolated instances of strong, gusty winds are certainly possible across Southwest Montana but nothing reaching the level of concern compared to what arrives later in the week.

Tuesday Thunderstorms:

One of the limiting factors on Tuesday will be moisture. The 00Z run of the GFS has continued the trend of pushing back the timing for when the moist air gets pulled up into Central Montana. The Euro still has dewpoints getting close to 50 degrees starting at around 6 PM. Either way, the trend is that the moist air is not expected to arrive until much later in the day than what would normally be favorable for severe weather.

This not to say that severe weather is not possible. With highs across the Northern Plains in the mid 80s to mid 90s, there is still a potential for isolated, strong, diurnally-driven convection. Because moisture is going to be lacking, especially in the early afternoon hours, the main concern will be dry downbursts which could produce isolated instances of strong winds.

All things considered, the timing of the moisture will determine the dynamics of the afternoon. If the moisture is able to arrive earlier in the day, the chances for stronger convection increases. However, if the current trend of the GFS pushes the arrival further into the evening/overnight hours, then the threat lessens significantly. Despite only being a couple days out, there is still uncertainty as to how Tuesday will play out.

through Friday. Wednesday Thunderstorms:

By Wednesday, moist air is expected to filter into Central and North- Central Montana with the latest Euro pushing afternoon dewpoints into the low to mid 60s across the Northern Plains. The current areas of concern are focused more on Central Montana, particularly around Judith Basin, Fergus, Chouteau, Hill, and Blaine counties. The latest model runs continue to show a modest 1000-1500 J/kg CAPE across the region. The limiting factor on Wednesday will be how warm temperatures get. Right now, east of a line between Havre and Lewistown, highs in the upper 70s to mid 80s are expected, with locations to the west of that line only reaching the low to mid 70s. Widespread cloud cover throughout the day is possible, which could further limit afternoon instability.

Simply put, it is not unreasonable for a couple of the storms on Wednesday to be near-severe or severe. However, right now there is too much uncertainty in the morning setup to make any deterministic calls on how widespread or significant the threat will be.

Thursday and Friday Thunderstorms:

While there is still a bit of uncertainty as to exactly how this upper level trough will set up across the western CONUS, there is increasing confidence with the latest model runs that both Thursday and Friday will be widespread thunderstorm days. As of now, the Euro is a lot more generous with CAPE than the GFS on both days. The Canadian opted more for a confined area of higher CAPE across the central third of the CWA on Thursday, roughly between Great Falls and Bozeman. On Friday, the Canadian shows higher CAPE values in the northern half of the CWA. -thor and Dzomba

AVIATION

24/18Z TAF Period

VFR conditions will prevail through the period. There will be isolated thunderstorms through 03z over Southwest MT, mostly south of a Dillon to Big Sky line. Thunderstorms will also redevelop after 21z on Monday over Southwest MT and along the divide from Dillon northward to the Helena area. Mountains/passes will be obscured at times over Southwest MT by passing showers/thunderstorms. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 48 88 56 88 / 0 0 10 50 CTB 45 84 52 84 / 0 0 10 50 HLN 47 85 51 84 / 0 10 20 40 BZN 44 85 48 83 / 0 10 30 60 WYS 36 78 39 75 / 10 0 10 60 DLN 43 81 46 79 / 0 20 30 40 HVR 49 91 58 96 / 0 0 10 10 LWT 46 84 53 88 / 0 0 0 10

TFX WATCHES/WARNINGS/ADVISORIES

None.