FastCast--Tuesday, July 16 to Thursday, July 18:
An interesting period is ahead for the region, with a shortwave trough bringing a regionwide chance of thunderstorms on Wednesday, which will bring high fire danger, especially because the thunderstorms will have little to no rain in them. But first, the heat will return on Tuesday, with highs across the lowlands reaching the mid 80s to low 90s, except cooler near the water, especially north of Everett. There is a chance of thunderstorms across the lowlands, mainly Wednesday morning, but this is more fickle and unpredictable than the potential thunderstorms over the Cascades. On Wednesday, clouds are expected to start the day west of the Cascades, with skies getting sunnier, and highs reaching the low to mid 80s (upper 80s possible for some areas). Thursday will likely begin with morning clouds, with highs cooling a bit into the low to mid 80s (upper 70s to low 80s from Everett northward, mainly near the water). Overnight lows will only drop to the low to mid 60s on Wednesday and Thursday mornings, but will likely drop back to the upper 50s after that. Continue reading below for an update on the active weather ahead.
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Continue reading the full blog below!
A weather system, known as a "shortwave trough" will move through the Pacific Northwest from Tuesday through Wednesday, bringing a chance of widespread thunderstorms to the region, especially the Cascades. This brings a high risk of many new fire starts, especially due to the risk of dry lightning. Let's take a look at the forecast!
Below is the forecast showing forecasted instability (CAPE index) across the region on Wednesday evening, from the HRRR high-resolution forecast.
This forecast shows high CAPE across the region, with values of 500-2500 (high by PNW standards) in the Cascades and Olympics.
Let's take a look at how this event will play out. We'll start with the European model forecast for lightning flash density (amount of potential lightning in an area), for Tuesday afternoon.
On Tuesday afternoon, thunderstorms are expected over the Cascades and highlands of Southern Oregon. This is concerning as there are already large wildfires in this area. Additionally, there is a lesser chance of thunderstorms on Tuesday afternoon for the Cascades and Olympics.
As we go into Tuesday night and early Wednesday morning, thunderstorms will move northward. Below is the European model forecast for where thunderstorms could be early Wednesday morning.
Before sunrise on Wednesday, thunderstorms will be moving north, with a potential area of thunderstorms moving north up the Washington and Oregon coasts, and additional thunderstorms moving north through the Oregon Cascades. There is potential for thunderstorms to impact the Willamette Valley early Thursday morning.
Next, let's look at thunderstorm potential for Wednesday morning (from approximately sunrise to midday).
As we move through Wednesday morning, thunderstorms will continue to impact the northern Oregon Cascades, likely spilling into the southern Washington Cascades. Additional storms are possible along the coast and in NW Oregon, including the Willamette Valley.
The situation will change immensely on Wednesday afternoon, as seen below.
From early to mid afternoon on Wednesday, thunderstorms will become quite active over the southern Washington Cascades and over parts of northern Oregon, mainly east of the Cascades. There is also a lesser chance of thunderstorms over the Olympics.
Then, by the middle to later part of Wednesday afternoon, into Wednesday evening, is the highest chance of thunderstorms, likely across the majority of the Washington Cascades.
By this time, there is a potential for high amounts of lightning across the entirety of the Washington Cascades, the Olympics, and parts of the Blue Mountains from north central Oregon into extreme SE Washington.
Going into Wednesday night, thunderstorm activity will likely decrease and move east. The biggest threat with this thunderstorm activity is the lack of precipitation with these storms. Below is the European model forecast for total rain through Wednesday night.
Notice that the maximum amount of precipitation in the Cascades is around 0.15", with similar maximum amounts along the coast, the Olympics, the Oregon Cascades, and the Blue Mountains.
This combination of the potential for a large amount of lightning strikes and little to no precipitation will create a very dangerous environment for new wildfires. Below is the National 7-Day Significant Fire Outlook from the NWCG (National Wildfire Coordinating Group), valid for Wednesday.
Any areas in red have a high risk of fire potential, with orange/tan areas being a moderate risk of fire potential. "L" represents lightning, and "B" means "critical burn environment." It's safe to say that there is a high risk of new wildfires and growth of existing fires.
Finally, let's take a look at expected temperatures on Tuesday and Wednesday. It's going to heat up across the region over the next few days. The NWS NBM forecast for highs on Tuesday is below.
On Tuesday, lowland highs will reach the upper 70s to mid 80s from Seattle north and the upper 80s to low 90s from Seattle south. The coast will reach the upper 60s near the beaches to the upper 70s inland. The Willamette Valley will reach the mid to upper 90s, and Eastern Washington will reach the upper 90s to low 100s (isolated mid 100s possible).
Wednesday's temperatures will be a bit different due to the thunderstorms and associated clouds moving through the region at times. In Western Washington, this will also bring muggier conditions.
On Wednesday, highs across the lowlands will reach the mid to upper 80s, with areas near the water in the upper 70s to low 80s. The coast will reach the mid 60s to low 70s, while the Willamette Valley will reach the upper 80s to low 90s. Eastern Washington will reach the upper 90s to low 100s, with some areas reaching the mid 100s. Again, temperatures west of the Cascades and in the mountains themselves are highly dependent on how much cloud cover there is across the region. Thick clouds associated with the moisture moving through will lower temperatures a few degrees.
Stay tuned for an update by Tuesday night regarding the forecast for thunderstorms, fire danger, and temperatures.
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