Rain is Coming to the Northwest During the Driest Time of the Year

 Every meteorologist knows that climatology only provides typical or average conditions and that natural atmospheric variability can provide some major excursions from normal conditions.

That will certainly be true next week.  Climatologically, the last week of July is the driest of the year in western Washington (see plot at SeaTac below).  Best time of the year to plan a barbecue or an outdoor wedding.

But this year, the last week of July will bring rain...and even significant rain in some locations.

Let me show you.

The weekend will be generally dry, with a wet system moving in Sunday night/Monday morning.

The 24-hour totals ending 5 AM Monday show lots of rain along the coast, with moisture extending into NW Washington.

The next 24-h totals ending 5 AM Tuesday show western Oregon and Washington getting thoroughly dampened with as much as a half inch of rain.  A lot for this time of the year.  Even eastern WA gets a piece of it.

And light rain continues into Tuesday.

How certain are we of this rain?  

Take a look at an ensemble of many predictions for rain accumulation in Seattle for the next week using the National Weather Service's GFS model. Each gray line is a different forecast with the blue line being the high-resolution prediction.  Considerable variation (and thus uncertainty), but nearly all the forecasts predict rain.

And don't forget the temperature.  The highly accurate Weather.com website has the high on Monday not even getting into the 70s (see below).

This cool/wet period should have minimal thunderstorm activity and will help reduce subsequent wildfire activity.

Northwest wildfires are most frequently grass fires. Climate change is not an important contributor to grass fires.

When many folks think about Northwest wildfires they visualize a burning forest.

But in reality, most Northwest wildfires are grass or range fires, with trees playing a minor role.

As we will see, grass fires have little to do with climate change, but a LOT to do with flammable, human-spread invasive grasses and human ignition.   And such grass fires can subsequently contribute to wildfires in adjacent forests.

Consider a visible satellite image from early this afternoon (from a NASA MODIS satellite).  You can clearly see the smoke from some fires over eastern Washington and Oregon.  This satellite also can sense the heat from the fires (orange dots).   

Look closely and you will notice that few fires are over elevated terrain with dense forests.  Most are associated with extensive grasses and range vegetation (including small shrubs).

For reference, here are the current fires from the Northwest Fire Coordination Center.

One of the most active Washington fires is the Black Canyon fire northwest of Yakima, which has spread over 4500 acres in grass (see photo below).

The biggest fire in the region is the Lone Rock Fire of northeast Oregon, which has now spread over 131,000 acres.  Grass and shrubs.   Most of the fires in our region...and most of the smoke production..is from grass and range vegetation.

In fact, the USDA Fuelcast website diagnoses how much grass/range fuel is available (see latest determination below).  Bountiful, above-average values are in place over eastern Oregon and Washington (dark green colors) thanks to a moist spring and leftover dead grass from last year.

It is important to note that large dangerous levels of light fuels such as grasses are NOT due to drought, but rather above-normal precipitation during growth periods.

Summer heat and drought have little impact on grass fire potential since our normally dry summers are virtually always able to cure and dry out grasses and the like.

Consider the Columbia NWR RAWS site near Othello (a picture of the nearby terrain is shown).

The ten hour fuel moisture percentage (the moisture level of light vegetation) for this site for the past several months is shown below.   Under roughly 12% is dry enough for fire.  Clearly, such fuels have been dry enough to burn for months. During every summer such fuels are ready to burn.  With or without global warming.

But something HAS changed that makes grass/range fires more likely:  the replacement of native (less flammable) bunch grasses with invasive, flammable grasses such as cheatgrass (also known as grassoline).

Here is a map of where cheatgrass has moved in.  Large areas of our region.

In addition, these human-spread flammable grasses are more likely to be ignited these days, with large increases in population as well as fire-inducing human activities (from fireworks and off-road vehicles to gunnery practice to deficient power infrastructure, to name only a few.)

Strong winds play a critical role in spreading grass fires, something of particular note on the windy, lower eastern slopes of the Cascades.

Too many media outlets blame grass and range fires on climate change without factual bases or supportive information.   Grass/range fires also play a large role in inducing forest fires, something I will discuss in a future blog.

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.

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.

 

Precipitation?

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.

Thunderstorms Return to the Northwest

 Thunderstorms have been relatively rare this summer, but today will see some boomers over the Cascades and eastern Washington.

In fact, there were quite a few thunderstorms yesterday over southwest Oregon (see below, each dot is a lightning stroke) and this activity is moving north as I write this.

So what is going on?  Let's start with the visible satellite image around 7 AM this morning (below).  I have a red arrow pointing to Seattle.    

Wow.  An impressive-looking band of clouds stretches north of the Oregon/Washington border from the ocean to eastern Oregon.  That is a change.

This band even has some lightning in it (see the lightning flashes at 6AM below)

The radar (below) indicates some precipitation with it, although little is reaching the ground.

Why is this cloud/rain band there?   

The key is that the persistent upper-level ridge (high pressure) has moved inland and an upper-level trough is approaching (see upper-level map--500 hPa pressure/roughly 18,000 ft) below).  The heights (think pressure) are shown by the solid lines and the shading shows relative humidity at that level, with white being the highest.

A trough is moving northward along the Oregon coast.  The approach of a trough causes upward motion that increases relative humidity in front of it and releases instability in the atmosphere.  Such instability can cause thunderstorms.

Some amateur weather sites claim that "monsoon moisture" is moving up from the southwest, but that is not operative in this situation.

Here in Seattle, you can see some clouds and incipient instability aloft, as illustrated by a view around 7 AM from the Seattle PanoCam (see below). It's not impressive at that time, but you tell see something is happening aloft.

So what is going to happen this afternoon?

With the trough approaching and the ground warming, instability, convection, and thunderstorm activity will increase, particularly over the Cascades and eastern Washington.

Here is the forecast radar reflectivity (a proxy for precipitation) at 5 PM today from the NOAA HRRR model.   Some significant thunderstorms over the Cascades!

The simulated radar image by the UW WRF model two hours later from the UW WRF model shows significant activity over eastern Washington.

The lowlands of western Washington will probably escape the rain and lightning.

One concern, of course, is lightning-initiated wildfires, something that DNR and others will have to be on the lookout for.

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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.

Fog and Smoke: An Intimate Relationship in the Northwest

 One of the oddities of Northwest meteorology is that westside fog/low clouds and eastside smoke are often directly related.   

Yesterday was a good example.  To illustrate, consider a series of visible satellite images from yesterday and this morning. 

Yesterday morning at 9:45 AM there was lots of low clouds along the Washington coast and just a hint of smoke from the Pioneer Fire on northern Lake Chelan.

But by 7:45 PM yesterday evening, the smoke from the Pioneer fire had exploded, with the plume heading eastward.   Clearly, strong westerly (from the west) winds were present.

This morning, the low clouds had really pushed into western Washington and there was extensive smoke over northeast Washington from the Pioneer Fire.

To demonstrate that the Chelan fire was the only fire act in town, here is the satellite-based fire emission map from yesterday--lots of  orange dots (fire locations) near Chelan indicate the fire.

So what happened?    The west-east pressure difference increased (higher to the west) as cool/dense marine air pushed into western Washington.   This is associated with the upper-level ridge of high pressure moving inland.

This westside push of marine air and increased pressure diffenence across the Cascades resulted a substantial strengthening of the winds above the eastern slopes of the Cascades.   

To illustrate this, here are the winds at Ellensburg (times in PDT).  Big surge of winds there yestergy, with gusts nearly getting to  40 mph.

Take a look at the max winds yesterday across the region (below).  Pretty windy over the eastern slopes and downstream of the Columbia Gorge.  No wonder wind energy is up.

This is turning out to be a very benign fire season so far, with burned acreage far below normal.  Importantly, the latest model runs suggest no heatwaves over the next week.

The Weather Regimes of Summer

 Weather patterns tend to get "stuck" for extended periods and we have certainly seen such persistent conditions this summer.    Weather patterns can be self-reinforcing and thus stay in position for many days or weeks.   And oceanic forcing, such as associated with El Nino and La Nina, can cause weather patterns to "lock".

Consider the temperature situation at SeaTac Airport from May 1 to yesterday (below).  The blue bars show the observed highs and lows, brown shading shows the climatological or typical ranges, red shows the record highs and blue the record  lows.

The middle of May through the end of June was associated with below average temperatures, while early July has been warmer than normal.  The cool (and moist) June is a one reason why wildfire acreage in our region has been below normal so far.

Looking at the latest forecast for Seattle for the 10 days, we note that the forecasts are stuck in a very boring pattern, with temperatures remaining in the low to mid 80s.  Average highs for this time of the year are roughly 77-79F.

No major heatwaves are predicted and the nights cool into the lower 60s and upper 50s, so decent sleep is possible without AC.

Why are we stuck in the lower to mid 80s?    To explain, below is the average upper level pattern (at 500 hPa or around 18,000 ft) for the next week. Shading shows the difference from normal.

A ridge is evident (higher heights), but centered inland.  The kind of pattern that makes western Oregon and Washington warmer than normal is not associated with heatwaves warming into the 90s,  such as last week (where the ridge is centered on the coast).

We are now entering the climatologically warmest and driest period of the year.   Enjoy.

The Cooling Has Begun

 The differences in temperature between noon today (Wednesday) and yesterday are quite large west of the Cascade crest (see plot below).  Some locations are 15-20F cooler, particularly across southwest Washington and NW Oregon.

The Columbia Basin temperatures were relatively unchanged.

With cooler air moving in across the coastal region, low-level air pressure is increasing there, producing an enhanced difference in pressure across the Cascades.

Such increased pressure differences increase westerly winds (winds from the west), and we can see the wind strengthening over the eastern slopes of the Cascades.  For example, the wind at Ellensburg is now gusting above 35 mph (see plot below).

Such increased winds result in increased wind energy generation (see green line below), which is good...we need it.

The next few days should bring steadily declining temperatures over western Washington.

Let me show you the state-of-the-art UW ensemble of many high-resolution forecasts for Seattle temperatures.  High around 85F today and 80F tomorrow.  Temperatures at night decline to the upper 50s.  Decent sleep beckons. 

The National Weather Service NBM forecast is similar and very boring.  No heat waves. No cold waves.  Just perfect weather rising into the lower 80s.

In contrast, the Tri-Cities will remain around 100F for the same period  (see below).

No rain is predicted on either side of the Cascades during the next week.  

Enjoy the weather.