textproduct: Great Falls

KEY MESSAGES

- Scattered shower and thunderstorm activity develops over southwest Montana this afternoon and evening.

- Showers and thunderstorms become more widespread over central and southwest Montana on Wednesday with some storms producing torrential downpours capable of localized flooding, especially for burn scars, urban areas, and other flood prone locales.

- Most locations will see widely scattered storms on Thursday. The stronger cells may contain strong, gusty winds, hail, and downpours in addition to cloud to ground lightning.

- Summer heat builds in for the second half of the week with localized elevated fire weather concerns Friday and Saturday.

DISCUSSION

/Issued 306 AM MDT Tue Jul 21 2026/

- Meteorological Overview:

Monsoon moisture continues to stream eastward on the northern periphery of a strengthening ridge of high pressure centered over the Great Basin. While the moisture aloft thickens from top to bottom, variable mid- and higher level clouds and a few spotty light showers or virga will be the main impact this morning. Widely scattered showers and thunderstorms will begin creeping back northward into the southwest this afternoon and evening; otherwise, mostly dry and warm conditions will prevail elsewhere.

A subtle shortwave will move through the ridge on Wednesday and will provide sufficient forcing for a more concentrated area of showers and embedded thunderstorms over southwest and portions of central Montana. While ML CAPE values look to be around or less than 500 J/kg, the combination of forcing and PWATS well over an inch will increase instances of excessive downpours capable of localized flooding, particularly for recent burn scars, urban areas, and other flood prone locales. Higher CAPE values around 500 to 800 J/kg look to be present on the northern fringe of this area of precipitation, mainly for areas along the central MT highway 87/200 corridor southward to I90. These areas will see spottier rainfall, but higher occurrences of strong, to severe storms with strong, gusty winds, hail, and downpours in addition to cloud to ground lightning.

The exiting shortwave will push the richest moisture to the east and leave a drier boundary layer in its wake on Thursday; however, another shortwave moves in from the southwest and brings more scattered showers and storms to much of the forecast area. Forecast soundings show inverted V soundings, a key indicator of stronger thunderstorm outflow winds. Hail, downpours, and cloud to ground lightning will also be concerns with Thursday's storms. The combination of the drying conditions and the building ridge will also warm afternoon temperatures well into the 90s and even some 100s for most lower elevation locations.

The moisture profile will continue to thin out from west to east on Friday, but there may be enough lingering instability and moisture for some showers and storms over the eastern portions of the forecast area. Otherwise, strengthening dry southwesterly flow will increase surface winds for at least the first half of the weekend while temperatures remain hot. This may introduce elevated fire weather concerns if the mid- week rains are too light or scattered in nature. - RCG

- Forecast Confidence & Scenarios:

Ensemble guidance has been persistent in narrowing down the heavier rain threat to southwestern areas south of I90. NBM probabilistic guidance has increased the chances for a half inch of rain for the 24 hour period ending 6 am Thursday up to around 30% for lower elevations and 60 to 80% over higher terrain. This is significant because these probabilities typically do not reflect the localized heavier rainfall from thunderstorms. The main uncertainty is the lower CAPE and whether these rainfall amounts will fall in a timeframe less than six hours in order to produce higher localized flash flooding of burn scars, urban centers, and other flood prone areas.

Rainfall becomes much more scattered in nature for central and southwestern locations north of I90, but higher CAPE and moisture levels may result in much higher rainfall rates in short periods of time for more impactful flooding, albeit in a highly localized fashion. The Weather Prediction Center has recently raised the flood risk to the 'slight' level (5 to 15% chance) for areas south of I90 while maintaining a 'marginal' risk level for central and southwestern areas north of I90 and south of the central MT highway 87/200 corridor. This situation will continue to be monitored for potential flood products over the next 24 hours. - RCG

AVIATION

21/12Z TAF Period

Monsoon moisture will continue to stream eastward and bring variable mid- and high level cloudiness and isolated light showers or virga for the first half of the TAF period. Widely scattered showers and thunderstorms develop over southwest MT between 21/20 and 22/06Z. Gusty winds over 30 kts and occasional lightning will be the primary concerns with this activity. Lastly elevated wildfire smoke may compromise slantwise visibility again today. - RCG

PRELIMINARY POINT TEMPS/POPS

GTF 85 60 90 62 / 10 0 10 20 CTB 80 56 88 59 / 20 10 0 10 HLN 88 63 90 63 / 10 10 30 60 BZN 88 58 90 59 / 10 10 40 70 WYS 87 51 83 51 / 40 30 70 70 DLN 84 57 85 56 / 30 20 50 80 HVR 85 59 93 61 / 10 0 0 10 LWT 82 56 86 57 / 10 10 20 30

TFX WATCHES/WARNINGS/ADVISORIES

None.