July 20, 2024

Will this be the driest July on record for the region?

 Several folks have asked me whether we will break the record for the driest July in history.  

Several locations, including SeaTac Airport, have had no precipitation this month, and to put it bluntly, little is predicted for many locations.  

First, how much has fallen so far?   For western Washington, some locations have received a bit, such as the Cascades (from some thunderstorms last week) and along the coast.

Eastern Washington has had less, most of it from a few showers from hit-and-miss thunderstorms last week.

Tomorrow, there could be some very light precipitation across the western portion of the Northwest due to a weak upper-level trough of low pressure moving up the coast.  The latest forecasts suggest that air will be sufficiently unstable for a few convective showers, particularly over the Cascades and the coast, but nothing you would write home about.

To illustrate, here is the latest UW WRF model simulation of clouds at 2 PM tomorrow.  

Looks impressive, right?  You can tell it is convective by the mottled appearance of the simulated clouds.  Lots of mid-level cumulus.

But the rainfall is very modest....here is the forecast total through 5 AM Monday.  A few convective showers in southern Oregon, light precipitation along the coast, but very little in the interior.

The NOAA/NWS HRRR model is similar.

The UW extended accumulated precipitation from this morning through the end of the month (below) predicts considerable rain over coastal BC, but little over much of the Northwest, except for the far southwest that gets a piece of  the monsoonal moisture streaming up from Arizona, Nevada, and Utah.

The bottom line is that SeaTac and other sites around here could easily end up with no July rainfall.

Now the shocker:  this would not be any kind of record.  Several previous Julys have done the same.

To show this, here is the July rainfall at SeaTac going back to around 1950.  Six previous years have had a trace or zero.  There does seem to be a trend towards drier Julys, something that is consistent with global warming simulations.

At least you don't have to worry about carrying an umbrella.  And with dropping temperatures, the weather should be nearly perfect for outdoor recreation or a hearty barbecue.

July 19, 2024

The Upcoming Cooling over the Northwest

We have been in a persistent pattern of above-normal temperatures without major heatwaves for the past week.   

But after a short warming today and tomorrow, a cool-down to normal or even below-normal temperatures is in store for the region.   A week without significant thunderstorm activity, but with coastal drizzle.

 To understand what will happen, consider the forecast upper-level maps for the next week.  

But not just any maps.  Let me show you the average of many forecasts...an ensemble of predictions.  

Why?  Because an average of a diverse collection of forecasts generally has more skill than any individual forecast.   These maps show the upper-level heights (solid lines) at 500 hPa pressure, about half of sea-level pressure.  Alternatively, you can think of them as representing pressure at 18,000 ft.    The shading shows the difference from normal (blue is below normal--troughs, reds indicate above normal or ridges).   

There is an offshore trough and a ridge centered over Alberta.  This pattern results in modestly above-normal temperatures for the NW.

In contrast, the pattern tomorrow is warmer for us, with the ridge strengthening and moving a bit westward.  Expect mid to upper 80s in the western WA lowlands tomorrow.  Around 105-110F in the Columbia Basin.  Hot.

By Wednesday, the trough will move into coastal BC and a substantial cooling will be evident.

And this cool trough continues to move in through Friday.  Not a warm pattern for us.

The latest NOAA National Blend of Models (statistical combination of many forecast models) indicates a substantial cooling from Saturday through mid-week, with highs only getting into the mid-70s on some days.  Mid-50s at night, which means good sleeping!

In eastern Washington, the decline will be dramatic, from roughly 110F to lower 90s in Pasco.

Coastal drizzle and light rain, but little inland (see the European Center forecast for total precipitation through next Saturday morning).  No thunderstorms, which means no more lightning-induced wildfire starts.  This is climatologically the driest time of the year, so nothing unusual.

To put it a different way: expect temperate, moderate, and delightful weather this week after a warm weekend.   The string of 80s at SeaTac will be ended.  And the below-normal wildfire situation over Washington will continue.

NOTE:  I will do a special online zoom session at 10 AM on Saturday for Patreon supporters.  Topics include the upcoming summer weather, wildfire weather, and more.

Will this be the driest July on record for the region?

 Several folks have asked me whether we will break the record for the driest July in history.   Several locations, including SeaTac Airport,...