weatherblog

Cooler with isolated thunderstorms Friday afternoon

Thursday’s high of 73 degrees in Seattle was not a record but it was the warmest day since back on Sept. 25. The average date of the first 70 degree day is April 15, so just a tad ahead of schedule.

Light rain is moving into the coast Thursday night ahead of a quick-moving cold front. Ahead of that, we’re still in the 50s and low 60s around Puget Sound at 10 p.m. after a warm day, but as the front moves inland tonight with its rain – passing Puget Sound before daybreak – temperatures will cool significantly. Expect a tenth of an inch or less of rain most areas through the morning hours.

In the afternoon Friday, we’ll be in the more unstable post-frontal environment, with cooling temperatures aloft. That combined with an expected Puget Sound convergence zone in the general Seattle-Everett region later in the day and some limited sunshine will increase instability enough to get some heavier downpours in the convergence zone (and to a less extent, elsewhere) with showers of small hail and even the possibility of lightning strikes. The current forecast model blend has about a 1-in-4 chance of a thunderstorm within earshot of Seattle and surrounding areas tomorrow afternoon. Highs will be 15-20 degrees cooler than today with low to mid 50s common.

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In the Cascades, a little light snow is possible from morning through mid-afternoon but a Winter Weather Advisory will go into effect at 5 p.m. Friday for the Cascades above 3,000 feet. Snowfall will become heavier at times into Friday night, particularly in the eastward extent of the Puget Sound convergence zone. Stevens Pass could be a little more favored for heavy snow than Snoqualmie Pass, but pass travelers will want to check before heading out all the way through Saturday. Expect up to a foot of snow in the locations that receive heavier snowfall with 6-12 inches a good bet elsewhere Friday evening through Saturday.

On Saturday, the lowlands will have showers and sunbreaks with highs in the 40s. I expect fewer areas of precipitation on Sunday with showers and more sunbreaks likely, though worth noting snow levels will be 500 feet or below Sunday morning so we could see some wet snow on higher hills in spots to start Sunday but with little to no accumulation expected as a general rule. However, we will watch any areas of heavier precipitation at any time this weekend as wet snow could occur briefly in lowland locations.

Next week’s forecast is somewhat uncertain, but low rain chances each day seems like a good bet but still with most of the time likely to be dry. Highs are expected to remain in the low 50s in Seattle – cooler than average.

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