The forecast area lies under an increasingly moist southerly flow regime ahead of a deep offshore weather system poised to move in on us later tonight. Today most of the increasing moisture will be aloft with periods of thick high clouds over the inland northwest. However, there are patches of dense low stratus and areas of fog lying about the northern and western areas of the Columbia Basin this morning and especially in the western basin will be slow to clear out today. We’ll see patchy fog in the Wenatchee Valley until early afternoon today with cloudy skies after that, highs today near 40 degrees. Tonight through Saturday night the offshore low will approach the region and eject through the Western U.S. as a split flow trough with the air mass moistening dramatically and deeply beginning this evening over the Cascades and spreading east during the night. There is a 60% chance of rain mixed with snow showers tonight with lows in the lower to middle 30’s. All of this translates into a forecast featuring an increased chance of valley/basin rain tonight from west to east across the forecast area. As we have seen over the last week, the presence of a pool of cooler air trapped in the basin will be conducive to patches of freezing rain during the late overnight and early morning hours form about Ritzville and Davenport and points west along the I-90 and Highway 2 corridor tonight and Saturday morning.
We have a 50% chance for rain mixed with snow on Saturday with highs in the lower 40’s. In the mountains, a slow and steady build-up of 3 to 6 inches of snow every 12-hour period above 3500 to 4000 feet will create locally slick and wintry conditions on area passes.
The weather system will exit the region on Sunday leaving us with mostly sunny skies and unseasonably warm weather into the lower and mid 40’s.
A sharp ridge of high pressure will set back up for Monday and Tuesday across the Pacific Northwest with a mix of sun and clouds both days with afternoon high temperatures in the upper 30’s.
As we move into Wednesday and Thursday, the ridge flattens a bit for a moister westerly flow. There is a 30% chance of a rain/snow mix both days with afternoon high temperatures in the upper 30’s.