And so, summer heat begins!

   In case you haven't heard we are on the verge of a prolonged spell of summer heat set to arrive right after the 4th of July. Years ago, it seemed summer would always begin right after the 4th of July in the Spokane however is that really true? According to statistics, perhaps not. Interestingly enough, we can define summer by looking at the warmest 91 (3-month period) of the year which according to our climate statistics occurs around June 16th. Also interesting is the fact, that the beginning of the summer season has been getting later and later each year, however there is a significant amount of disparity when looking at the chart below.  

Also interesting is noting the abundance of red dots since 2000 signifying a warmer than normal summer. Since 2000, 75% of the summers have been warmer than normal with a current streak of 12 warmer than normal summers. Below is a closer examination of that factoid plotted against the 30-year moving average.  The blue line represents the average summer temperature (based on June-August data and not the 91-day period shown above. 

So enough of the summer background, let's take a look at the upcoming forecast and heat-filled details. This burst of heat will arrive care of a very strong ridge of high pressure. If we look at the 500 mb charts (~18,000') in the atmosphere we can see how this scenario is going to play out. On Friday, the ridge will begin to build off the coast and this should begin the heat event.

500 mb heights Friday

By Sunday the ridge will migrate toward the center of Washington ensuring we warm up even further. 

500 mb heights Sunday

By Tuesday the ridge axis shifts into eastern Washington and then the heat will really get going across the Inland Northwest. 

500 mb heights Tuesday

But that's not the end of it, as it looks like the heat will peak on Wednesday or Thursday with the ridge shifting to the Montana/Idaho border. 

500 mb heights Thursday

So how confident are we this scenario is going to pan out? At this point, quite confident. In the past we only had a few weather models at our disposal and run-to-run changes were common, especially at the latter ends of the forecast (for instance days 4-7). However now we utilize ensemble forecasting which takes multiple models (over 100 for the days 1-2 forecast and 100 for days 3-7) and complies them for us. This method can show us the range of possible outcomes and in this upcoming pattern there just aren't a lot of outcomes other than just plain hot. The easiest way to determine how uncertain the forecast is by examining a chart that shows the different model solutions and plotting them on top of one another. 

Let's start by examining the model possibilities for Sunday.  Note the ridge positioned along the West Coast and the pale-yellow shading over the region. The shading represents the differences in the 500 mb heights between the 100 model runs. In this case, the differences are only around 2 decameters, which signifies good model agreement. If you look farther west, over the Gulf of Alaska note the purple shading which signifies large model differences (13 decameters, which is significant). Seeing model agreement only 2-3 days into the forecast isn't unusual, especially for a ridge. 

500 mb ensemble model differences on Sunday 7/7/24

By Tuesday the model differences with the ridge increase, but not by much (only 1 decameter over the Pacific Northwest). This still suggests high forecast certainty. 

500 mb ensemble model differences on Tuesday 7/9/24

By Thursday the model differences increase slightly, but again not to the point where we would think the ridge will be replaced by the offshore trough, delivering cooler weather. All the significant model differences remain on the eastern or western periphery of the ridge. 

500 mb ensemble model differences on Thursday 7/11/24

If we go out even further to next Saturday, there still is a higher-than-normal chance that the ridge is going to stick around, however we'd have to imagine we'd see some cooling by then. 

500 mb ensemble model differences next Saturday 7/13/24

So far, we've only examined the 500 mb heights, which are indicative of sinking air and generally fair weather, but how does that translate to temperatures on the ground?  First, we will examine the mean temperatures forecast by the National Blend of Models. This represents the most likely outcome. 

NBM mean temperature forecast for the Inland Northwest

However, what would we expect if the warmest 25% of the model solutions came to fruition? That would certainly support a historic heat wave. There's no reason to believe this will occur, however it just hints at a possibility. 

NBM 75th percentile temperature forecast for the Inland Northwest

So, what are our chances of exceeding 100°F during the next week or so? It's a done deal for the LC Valley, as well as the Columbia Basin and Okanogan Valley. There's a slightly lesser chance for Spokane, while the "cool" escape will be to Sandpoint. Below is a list of the chances:

Chances of hitting 100°F or hotter

As you can see, the odds are quite good that a large portion of the Inland Northwest will see a multi-day period of triple digit heat. Currently our forecast doesn't suggest we will break any records for consecutive 100°F days however once again we could easily see 4 consecutive days of triple digit heat in Spokane. Something which occurred in both 2021 and 2022 but not many years other than that. The record is 6 set in 1928. 

Other daily records could be broken beginning on Sunday and continuing at least through Thursday. Spokane's records are as follows: 7/7 99°F, 7/8 99°F, 7/9 100°F, 7/10 102°F, 7/11 103°F. All are certainly within reach. We also got the question is Spokane's all-time record in peril? Just 3 years ago Spokane set a new record of 109°F (6/29/2021). While we don't expect to break this record during this spell, it is not a 0% chance. In downtown Spokane, the chance is around 10% on Thursday (in other words 10% of the ensemble model runs are forecasting temps of 109°F or warmer), while at the Airport (the official forecast spot for Spokane) the chance is around 2%. 

Chance of 109°F or warmer on Thursday 7/11/24. 

So far we've discussed the daytime highs which will be trying enough to those who don't tolerate heat well. But we also have to factor in the nighttime lows. Unfortunately, these won't cool off too much either. The table below reveals overnight lows only dipping into the mid 60s to mid 70s for most locations. That means folks without air conditioning will have a tougher than usual time cooling off their houses during the night. And the length of this event will only exacerbate this issue. 

Nighttime low temperature forecasts

If you add these factors together, we are looking at a trying environment for many denizens of the Inland Northwest. We can characterize this using an index we call HeatRisk. By Sunday a large portion of the region will experience Major heat risk which means the heat will impact anyone without effective cooling and/or having access to adequate hydation. 

Sunday HeatRisk

By Tuesday (the last day available for the HeatRisk as of this blog post), we begin to see extreme values popping up around the area. Remember, we expect the heat to peak on Wednesday or Thursday which suggests the magenta will become more widespread. 

Tuesday HeatRisk

Needless to say, we urge all of our friends to take the necessary precautions ahead of time and prepare for this upcoming heatwave. Here are a few hints from the CDC to help you beat the heat. We also want you to treat your loving pets with the utmost care to keep them safe during this heatwave. Please stay tuned to our latest forecast to see how hot it is going to get. 

Safety advice from the CDC and the NWS for you and your pet