textproduct: Great Falls

KEY MESSAGES

- Widespread rainfall and light snowfall in the higher terrain today through Monday.

- Flooding due to heavy rainfall are possible for areas near and west of the I-15 corridor across North-Central MT.

- Temperatures gradually warm up next week, with daily chances for scattered showers and thunderstorms.

DISCUSSION

- Meteorological Overview:

An upper level trough with a negative tilt centered over Utah moves northward into Montana this weekend, bringing widespread precipitation across the region. The heaviest rainfall will likely be in Central MT and along and west of the I-15 corridor in North-Central MT. There will be enough instability present to where a rumble of thunder or isolated lightning strike is not out of the question Saturday. Snow levels will decrease, but remain above pass levels, allowing for the possibility of light, slushy accumulations near 7,000 ft. The low-level jet strengthens over North-Central MT and along the Hi-Line on Sunday morning, creating breezy conditions throughout the day. Winds for the most part will stay below High Wind Warning criteria, but the combination of the east/northeast winds and heavy rainfall threat can loosen trees up better, causing them to fall and cause tree damage/power outages. By late Sunday night, the upper level trough turns into a closed low and moves northwest. As the system moves out of the CWA on Monday, precipitation chances decrease. However, they do not fully dissipate, and there will be lingering chances of showers and thunderstorms throughout next week. Temperatures will gradually increase, as ridging aloft begins to build over the region Thursday night. -Dzomba

- Forecast Confidence & Scenarios:

Confidence has been increasing for the risk of 1 to 3 inches of rainfall across the region. The low end amount, which has a 90% chance of occurring, shows between 0.75" to 1.5" across the CWA. The high end amount, which has a 10% chance of occurring, shows between 3.5" to 4" for areas along and west of the I-15 corridor across North- Central MT. There may be some enhanced impacts to rivers and creeks due to snowmelt runoff depending on how quickly the cold air comes in. As a result, a Flood Watch has been issued Saturday through Monday (see Hydrology for more info). Where the corridor of best moisture/low level convergence sets up is ultimately where the heaviest precipitation will fall.

The colder air is not as cold/deep as previous forecasts. Therefore, impacts are expected to be limited to slushy accumulations mostly above 7,000ft. We will closely monitor for any additional impacts in case the trend changes. -Wilson/Dzomba

AVIATION

30/06Z TAF Period

Showers/isolated thunderstorms continue to overspread the CWA late this evening. Coverage will continue to increase throughout the night and into the day on Sat. Confidence is low at this time in thunderstorms at terminal sites. Mountains/passes will be obscured throughout the period by clouds/precipitation. Brusda

HYDROLOGY

Widespread rainfall will affect the CWA this weekend. In looking at NOAA Atlas 14, 2 inch rainfalls in a 2 day period happen about every 2 to 5 years in the Great Falls area. For areas west of Augusta, a 3 inch rainfall in a 2 day period also happens about every 2 to 5 years. Thus the spring storm that we are forecasting this weekend is on par for a fairly common spring storm...it is just that we have not had a storm like this in a few years.

Current river forecasts for Saint Mary, and the Sun River are forecasted to go into flood stage late Sunday into Monday. This forecast is heavily dependent on the precipitation amounts and could change in the coming day(s). Many mountains have melted out the snowpack for the season, but a few areas along the divide do have snow, especially Glacier Park. This will add to run off concerns for rivers like the Sun River, Badger Creek, and the St Mary's River.

Additionally, areal QPF of 1 to 3 inches is possible over the western half of North Central MT. Overall, much of the area should be able to handle the first few inches. Recent model runs have an isolated (10% chance) for 4" of rainfall. The combination of heavy rainfall and impacts from river/creek flooding has resulted in a Flood Watch being issued for North-Central MT along and west of the I-15 corridor. Another impact with be rural, dirt roads could become washed out if the higher end amounts end up happening. Depending on how the rivers respond, additional or extensions of flood products may be needed.

-Brusda/Wilson

PRELIMINARY POINT TEMPS/POPS

GTF 53 66 46 53 / 60 90 100 100 CTB 50 64 42 51 / 60 90 90 100 HLN 53 63 45 57 / 50 100 100 90 BZN 46 60 40 56 / 20 90 100 90 WYS 39 56 34 55 / 20 90 90 80 DLN 46 58 41 60 / 40 100 90 60 HVR 52 77 46 58 / 60 60 90 90 LWT 48 65 42 52 / 40 90 90 100

TFX WATCHES/WARNINGS/ADVISORIES

Flood Watch from Saturday afternoon through Monday afternoon for Cascade County below 5000ft-East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Gates of the Mountains-Northern High Plains- Southern High Plains-Southern Rocky Mountain Front.