textproduct: Great Falls

KEY MESSAGES

- Trending warmer through the week, with Friday into the weekend looking hottest.

- Slow moving storms across Southwest Montana Wednesday afternoon and evening will result in an increased risk for excessive rainfall leading to flash flooding.

- Additional showers and thunderstorms elsewhere Wednesday and Thursday ahead of a drying trend Friday into the weekend.

DISCUSSION

- Meteorological Overview:

Upper level ridging centered over the central and southern US with axis stretched northwestward toward the Northern Rockies will generally be the pattern that persists through much of the upcoming week across the region. The ridge is in its initial stages of rebuilding, with a drier zonal flow aloft still persisting across the region. This overall drier setup, along with cooler surface temperatures behind a cold front will inhibit shower or thunderstorm development through the remainder of the day and into tonight. Wildfire smoke will be around at times through much of the upcoming week.

Monsoonal moisture will begin to move northward through the western periphery of this upper level ridging as a more southerly or southwesterly flow aloft develops Tuesday onward. Afternoon and evening showers and thunderstorms initially look to remain confined to areas across Southwest Montana before spreading further north Wednesday and Thursday. Anomalous moisture associated with the monsoonal push combining with weak winds aloft look to result in slower storm motions by Wednesday, particularly across Southwest Montana. This will yield an increased risk for excessive rainfall which leads to flash flooding. Those in urban areas or near burn scars or steep terrain should be aware of this increased risk for excessive rainfall Wednesday.

Storm motions look to increase slightly for Thursday, though still remain on the slower side. At this point confidence is highest in Wednesday being the day with the greatest risk for excessive rainfall, with Southwest Montana being the area of greatest concern.

The risk for severe weather Wednesday and Thursday looks to be more favorable further north on the plains, where forecast soundings show slightly deeper inverted-V soundings. The lack of a stronger forcing mechanism raises concerns as to how widespread shower and thunderstorm development will be on the plains, but should it occur the risk for strong wind gusts would certainly exist.

By Friday the most robust monsoonal moisture begins to be shunted further south and east again as a drier zonal flow develops across the region. Lingering moisture looks sufficient to result in another round of afternoon showers and thunderstorms Friday, though coverage looks lower at this point and the risk for excessive rainfall looks to decrease significantly. What this does mean however is that gusty westerly winds will begin to work back as temperatures continue to trend hotter Friday. As of now the better monsoonal moisture looks to remain south of the region this weekend, which would keep things on the hotter and drier side. Those with outdoor plans this weekend should plan for plenty of breaks and to hydrate often to help mitigate concerns from the heat. Areas that see little precipitation this week will have increased concern for fire weather this weekend given forecast hot, dry, and breezy conditions. -AM

- Forecast Confidence & Scenarios:

The main source of uncertainty at this point looks to occur on Wednesday and Thursday, associated with the risk for excessive rainfall in southern areas and potential for strong to severe thunderstorms in northern areas. Where this transition in threat occurs will largely be dictated by where the 'strongest' flow aloft resides. Should this stronger flow aloft trend northward, the risk for excessive rainfall leading to flash flooding would edge northward and vice versa. It should be noted that excessive rainfall leading to flash flooding will be a concern in urban areas and burn scars, regardless of where the strongest flow aloft ends up settling. -AM

AVIATION

20/18Z TAF Period

A cold front continues to push south across Central and into Southwest Montana late this morning, with the front providing some improved VIS from the hazy conditions as of late. With that being said reduced slantwise VIS upon ascent/descent can be expected throughout the TAF period, especially across Southwest Montana prior to the aforementioned fronts passage. VFR conditions will largely prevail over the next 24 hours, but some passing showers across Southwest Montana along and south of the I-90 could may temporary reduce CIGS to low-VFR and lead to mountain obscuration. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 60 85 60 90 / 10 10 10 0 CTB 56 80 56 87 / 0 10 10 0 HLN 60 87 62 90 / 0 10 10 30 BZN 57 88 58 90 / 0 10 10 40 WYS 50 87 51 82 / 0 30 30 90 DLN 55 84 56 85 / 0 20 20 60 HVR 58 85 60 94 / 0 10 10 10 LWT 54 82 55 86 / 0 10 10 20

TFX WATCHES/WARNINGS/ADVISORIES

None.