textproduct: Great Falls

KEY MESSAGES

- Early morning showers over North Central MT will give way to more widespread thunderstorms this afternoon, some of which could be severe.

- Widespread rain and high mountain snow expected Saturday through Monday, along with cooler temperatures.

- Temperatures slowly warm late next week, with scattered showers and thunderstorms continuing.

DISCUSSION

- Meteorological Overview: A slow moving upper level low will affect our CWA over the next several days. There will be a few morning showers over North Central MT, otherwise expect another warm afternoon in the Southwest and a hot afternoon over North Central MT. Showers/thunderstorms redevelop this afternoon over the Southwest and push northward into North Central MT by evening. A few storms could be severe over North Central MT, with damaging wind gust the main impact, along with some heavy rainfall.

For Saturday through Monday, the upper level low will continue to drift northward and move from south to north across MT. This will allow for widespread rain to affect much of the CWA this weekend, with the heaviest rainfall likely over the western half of North Central MT. Temperatures will be much cooler starting Saturday. The airmass will still be unstable enough for some lightning strikes during the afternoon/evening hours over the weekend though. Snow levels will lower, but I am thinking not quite as low as previously forecasted for most areas. Overall, most of the accumulating snowfall will remain above 7000 feet or so.

For Tuesday through next Friday...expect a gradual warming trend by later next week. There will continue to be a chance for scattered showers/thunderstorms most days next week. The latest GFS/EC are showing signs of another fairly good weather system later next week, but the models differ on timing/strength a bit, thus confidence is low at this time for precip amounts late next week.

- Forecast Confidence & Scenarios: With a southwest flow aloft this afternoon, there is a good chance for another round of showers/thunderstorms over North Central MT. The best time frame for any severe storms will be from about 4 PM until 8 PM, along the Highway 2 corridor from Cut Bank to Havre.

Over the weekend, the probability of 2 inches of rainfall is about 60 percent west of a line from Great Falls to Havre. The probability for 3 inches of rain is generally less than 10 percent. Thus a few locations will receive close to 3 inches of rain, but overall there is a 90 percent chance most areas will see less than 3 inches of rain.

The probability for 2 inches of snow is only 50 percent, and that is for elevations above 7000 feet. Brusda

AVIATION

29/12Z TAF Period

Isolated thunderstorms will affect areas mainly north of Highway 2 through about 14z this morning. Otherwise scattered thunderstorms will develop over Southwest MT this afternoon and then move northward through North Central MT during the late afternoon/evening hours. Strong gusty surface winds/heavy rain likely with stronger storms this afternoon. A more widespread rain starts to develop after 09z Saturday over Southwest MT, moving northwest towards the Helena/Great Falls area by 12z Sat. Widespread rain likely the rest of Sat and much of Sun over the CWA. Mountains will be obscured at times this afternoon by passing thunderstorms, then become obscured once again over Central/Southwest MT after 09z Sat as the widespread rain begins. 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.

In terms of flood statements, an ESF will be issued this morning to talk about some of the expected rainfall over North Central MT. Some of the bigger rivers are forecast to rise near or go into action stage this weekend/early next week, however, no river is currently forecast to go into flood stage. That could change as newer information comes in. 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. Thus there is the potential that some river flood statements might be needed.

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. Those 10 percent locations that receive near 3 inches of rainfall could have some impacts, and thus some areal flood statements might be needed. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 88 55 66 47 / 30 60 90 90 CTB 84 52 64 42 / 40 40 80 90 HLN 86 53 62 46 / 30 50 90 90 BZN 85 47 60 41 / 20 40 100 100 WYS 78 39 52 35 / 10 30 90 90 DLN 80 47 58 40 / 10 40 100 90 HVR 92 54 79 48 / 30 40 60 90 LWT 86 49 67 44 / 30 40 90 90

TFX WATCHES/WARNINGS/ADVISORIES

None.