textproduct: Great Falls

KEY MESSAGES

- Thunderstorms, some strong to severe, are expected to move through Montana tomorrow, particularly north of a Helena to Lewistown line. - Widespread soaking rainfall expected tonight through Tuesday morning, with widespread rainfall amounts over an inch across the plains, with lighter amounts across Southwestern Montana. - Drier conditions return Wednesday, but precipitation returns Thursday night into Friday.

DISCUSSION

Deep upper level troughing located over Washington and Oregon will continue slowly drift eastward over the coming days, which will bring a prolonged period of unsettled weather to the Treasure State. Overnight radar has continued to show some showers and thunderstorms drifting from SW to NE in two areas: the first stretching from around Missoula to Cut Bank, and the second along and east of a line from Lewistown to Havre. These two areas will likely continue to persist through late this morning, before a break arrives for early afternoon. A second, stronger round of showers and thunderstorms will push through the area, with some of the storms becoming strong to severe (more on this below).

Another round of precipitation will arrive on Monday as a mix of convective and stratiform precipitation, which will generally hang across the plains through Monday night and into Tuesday morning. The convective elements within this precipitation will result in some rather impressive rainfall totals, with most areas across the plains likely to see at least 1 to 2 inches through early Tuesday, with isolated to scattered amounts in excess of 3 or 4 inches along and west of Interstate 15 possible where some of the heavier convection repeatedly moves over the same areas. While flooding is not expected to be a major concern given the overall gradual nature of the rainfall, some field/poor drainage flooding cannot be ruled out.

Weak upper level ridging will return to the area Wednesday and Thursday, bringing a reprieve from the rainfall for a brief period, but another, albeit weaker and faster moving, trough will swing through Friday and Saturday, bringing another round of widespread rainfall.

- Forecast Confidence & Scenarios:

Thunderstorms today: A potent vort max embedded within the upper level flow will push into Southwestern Montana late this afternoon, kicking off a few showers and thunderstorms as it does so. As these storms reach the plains, they will encounter one of two environments: The first is a heavily capped environment, albeit one with still enough shear to allow the storms to remain organized and produce some hail as they move northeastward. The second possibility is that these storms will encounter a weakly capped atmosphere, which would allow the storms to tap into stronger shear and have the potential to become surfaced based, allowing both wind and hail to reach the ground as the storms evolve into supercells. The Storm Prediction Center has placed most of the area in a slight risk for severe thunderstorms, primarily due to the threat of large hail.

Heavy rainfall through Tuesday: Heavy rainfall through Tuesday will result in a widespread 1 to 2 inches of rainfall across the plains, with around a 10% chance of seeing over 4 inches along and west of Interstate 15 north of Great Falls. So far, soils in this area seem to have readily accepted the copious amounts of rainfall, as areas along the Rocky Mountain Front that have received over an inch of rainfall over the past 24-36 hours have not really seen rivers respond in any meaningful way. This gives hope that soils will continue to be receptive to precipitation, though as soils continue to absorb rainfall it is possible that the soils will saturate and result in some field and poor drainage areas, along with some rises on rivers and streams. A Flood Watch was considered, and may still be considered in future updates, but for now soils appear to be capable of handling this long duration rainfall event.

AVIATION

06/06Z TAF Period

North-Central and Central Montana: Low stratus will move in and linger through most of the night with rain showers possible, particularly along and west of the I-15 corridor. Generally MVFR/IFR conditions are expected during this time but there is a potential for temporary reductions into LIFR should fog/mist start to develop. A larger concern will be the thunderstorm potential tomorrow afternoon and evening with the chance for gusty winds and gusty winds. Thunderstorms are expected to develop after 07/00Z, starting in the south and quickly moving to the northeast. Mountain obscuration will be a concern throughout the entire period.

Southwest Montana: VFR conditions are expected to prevail over the period with gusty south winds expected to develop in the Madison River Valley Sunday afternoon. There is a low probability for some fog developing at KBZN tonight but confidence was not high enough to include it in the TAF.

For KGTF: MVFR conditions for lower ceilings develop by 06/08Z with periods of IFR conditions possible between 06/10Z and 06/15Z. Winds start to gradually pick up out of the north in the early afternoon and become more northeasterly by 06/23Z. There is a chance for scattered thunderstorms across the region Sunday afternoon with a few expected to turn severe. For the KGTF vicinity there are especially concerns for hail and gusty winds, with the greatest chance for severe weather expected after 07/02Z. The crosswind potential is low at this time but the greatest concerns would be related to the thunderstorm develop expected later in the day. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 72 51 63 46 / 20 90 100 60 CTB 62 46 57 44 / 90 100 100 90 HLN 76 52 65 46 / 10 80 100 20 BZN 82 49 69 41 / 0 60 90 10 WYS 75 40 59 30 / 0 90 70 10 DLN 80 46 66 37 / 0 70 80 0 HVR 76 51 68 46 / 50 90 100 90 LWT 78 50 69 45 / 10 90 90 20

TFX WATCHES/WARNINGS/ADVISORIES