Super Dew

Last night, I was struck by how wet some surfaces were...without any rain.

For example, here is an area of a brick patio around 6 PM...quite wet.

The origin of this wet bounty was the very moist air over the region, with dew points rising well into the fifties.   At the same time, the weather had been relatively cool, allowing the ground surfaces to cool below the dew point.

Dew Point 101

The dew point or dew point temperature is the temperature at which water vapor starts to condense as the air cools.

So when the surface temperature declines below the dew point, water vapor condenses onto the surface, producing a wet sheen.

Yesterday, a plume of warm, moist air from the subtropics caused local dew points to climb.

Below are the dew points around noon yesterday.  Many were in the low to mid-50s.

A plot of the dew point at Boeing Field (ending yesterday evening) illustrates that the dewpoint yesterday was the highest of the winter so far,  reaching nearly 55 F.

Ground temperatures were relatively cool after a fairly cool late December, with temperatures at SeaTac shown below (the blue line shows the daily range. 

 In general, temperatures were below 50F, allowing surfaces not in direct sunshine to cool to that or cooler temperatures.  That means dew.

Soil temperatures in Seattle near the University of Washington are shown below.  Generally below 50F.

So, with a cool surface and moist air, abundant condensation occurred, something that was apparent if one looked around.

Finally, some folks have expressed concerns about air quality under high pressure.   

At this point, there are no issues, with the latest vertical sounding over SeaTac not indicating any low-level inversion (with temperature increasing with height).  Inversions trap low-level pollutants.

And air quality measures are generally quite favorable (low), as indicated by the green dots below.  You can breathe easily.

One Day "Heat Wave" Followed by a Week of Dry Weather

 If you have an umbrella, put it away...you won't need it for a while.

 Find your sunglasses...you will need them.  

A major mid-winter break from the rain and murk has now begun...and tomorrow, Tuesday, will be the warmest day of the period.   Some locations will even get to 60°F, but perhaps not where you would expect it.

Consider the forecast surface air temperature at 2 PM tomorrow over Washington--close to the time of warmest temperatures in winter (shown below).

Western Washington is warm, with some locations reaching the upper 50s...and some lucky folks into the lower 60s.

Since such warmth is so attractive this time of the year, here is a close-up map at the same time.  Head north and east of Seattle!

I know what you are wondering.  Can you trust this forecast?  

Good question...particularly after the disastrous forecasts of a big windstorm last week.  As I have discussed before, we need to consider the multiple predictions using ensemble systems, and the UW has one of the best.

Here is the prediction of the many forecasts of the UW system for Seattle.  Nearly all are on board for the warmth.   You can bank on this one-day wonder.

We will cool off after tomorrow, but persistent upper-level high pressure will move in, with at least a week of totally dry weather.  To show this, below is the ensemble of many forecasts of precipitation in Seattle from the NOAA/NWS GFS model.

Precipitation is flatlined at least through January 20.  Other forecast systems are doing the same thing.

Interestingly, long-term climatology shows that amid-month drying is not unusual.  To illustrate, here is the climatological probability of receiving at least 0.10 inches at SeaTac in a day.  You will see a drop in mid-January.

But don't worry, extended model forecasts show the precipitation coming back.   

And if I were irresponsible, I would show the forecast total snowfall by the end of the month.

A forecast that shows lots of snow over the western lowlands.  It would get lots of clicks and attention.   

This is the graphic I am not going to talk about, so please don't look at it and don't talk about it on social media. 😁