Unusually Muggy Weekend Forecast, With Some Heavy Rain Sunday

SEATTLE — Puget Sound's weekend weather outlook is shaping up to be a strange one for June, with forecasters predicting remarkably high humidity as temperatures rise, and a good chance for a few soakers. The National Weather Service expects Sunday, in particular, will be uncharacteristically wet for the season. Warmer temperatures and fast-rising dew points could make for one of the region's muggiest Seattle days in years.

Forecasters said the unusual ingredients are thanks to some sub-tropical air getting dragged in from over the Pacific.

For Saturday, afternoon temperatures are poised for a double-digit boost compared to Friday, landing in the low-to-mid 70s across the lowlands. Outside the coast, forecasters said much of the region will see a mix of clouds and sunshine for most of the day, as dew points begin to rise and the air gets heavier.

What is the dew point? The National Weather Service explains:

"The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly affects how 'comfortable' it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more 'humid' on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So if you want a real judge of just how 'dry' or 'humid' it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel."

The dew point is expected to rise to the upper 50s by Saturday and top out above 60 on Sunday — a rare threshold to cross in June. Forecasters with NWS Seattle said the bout of humid weather is likely to break a record, and Q13 meteorologist Scott Sistek noted it could be the muggiest day in 13 years.

As for precipitation, Sunday is likely to see the heaviest rains, with up to a quarter-inch possible in Seattle and higher for the South Sound.

"The heaviest rainfall will likely fall from the south Puget Sound region southward into the Chehalis Valley, where 24-hour rainfall totals will likely range from 0.5 to 1"," wrote Madelyn Kristell, a meteorologist with NWS Seattle. "Isolated spots of 1"+ are not out of the question, particularly where the heaviest banding sets up."

Forecasters said the chance for weekend thunderstorms was "scarce at most," but better odds may be in the cards Tuesday. After the latest round of wet weather passes, long-term forecasts still predict June will take a hot and dry turn in its second half.