Steady rain this week is expected to cause a sharp rise in rivers and streams across Northwest Montana.

According to the National Weather Service in Missoula, steady rainfall will persist through at least Friday, with periods of heavy downpours possible Tuesday and Wednesday.

The most significant accumulations will be found in the mountains around Glacier Park, totaling over 2 inches for some locations. Even lower elevation valleys, such as the Missoula Valley, have an opportunity of accumulations well over 1 inch in the Tuesday to Wednesday time-frame, the Weather Service noted in its forecast discussion Monday.

The widespread rain, combined with mountain snowmelt will cause “dramatic increases” to small streams and main rivers, the Weather Service cautioned. Flooding is possible beginning Wednesday through the weekend.

“Watch for rapid rises in river levels across the region, with the greatest concerns on the Clark Fork, Flathead, Bitterroot, Clearwater, Yaak, Fisher, and Swan Rivers,” the Weather Service warned.

The Flathead River at Columbia Falls is projected to crest at 13.17 feet on Friday morning. That height is considered minor flood stage.

The North Fork of the Flathead at the Canadian border is projected to hit its action stage, reaching 9.64 feet. The Swan River also is anticipated to hit action stage at 6-feet Friday afternoon.

Projections for the Stillwater River at Lawrence Park show the river cresting at 5.65 feet, below the 7-foot action stage.

Hungry Horse Reservoir will rise dramatically through Memorial Day, filling from a water elevation of 3,530 feet on Monday, to 3,538 feet on May 25. Full pool is 3,560 feet.

Flathead Lake is also expected to see a steady increase in its pool level. The lake water elevation was at 2,888 feet on Monday, and will fill to 2,889 by May 25. Energy Keepers, which manages the dam at the south end of the lake, anticipates Flathead Lake being full for summer recreation sometime in June.

Mountain snowpack in the Flathead Basin remains well above normal for this time of year, registering at 125 percent of average.