Western Washington faces its inaugural rainfall of the autumn season this Tuesday. Significant precipitation is expected in coastal and mountainous regions, though a brief period of sunshine is forecast to return later in the week.
An inch of rain for the coast and mountains
Western Washington is entering its first major weather system of the fall season this Tuesday.. According to forecasters, the incoming Pacific weather system will deliver up to an inch of rain to coastal and mountainous regions, while inland areas can expect roughly half an inch. This precipitation coincides with a noticeable temperature drop across the state.
High temperatures on Tuesday are expected to fall between five and 10 degrees below the averages seen in late September, with highs likely ranging from the mid-50s to the lower 60s. This shift marks a definitive end to the warmr patterns seen earlier in the month.
Radiational cooling and the struggle to clear morning fog
As the region transitions into autumn, residents will experience more pronounced temperature swings due to a process known as radiational cooling. This occurs as heat escapes the earth's surface after the sun rises above the Cascade crest. In the early morning,the sun's angle is often too low to stop this cooling process, meaning temperatures may actually dip a few degrees further before the sun climbs high enough to begin the warming cycle. This can lead to significant lows, with the South Sound and outlying areas potentially dropping into the upper 30s.
The recent rainfall adds a layer of complexity to morning visibility and comfort. The report notes that when fog forms following recent rains, the sun must reach a higher and sharper angle to dissipate it. in extreme cases during the late fall and winter, thick fog can become so persistent that daytime temperatures struggle to reach even 40 degrees.
A window for fall colors following the equinox
The fall equinox occurred at 5:05 p.m., officially marking the start of the new season.. Following the Tuesday rain, higher pressure aloft is expectd to build over the Pacific Northwest , which will act as a stabilizer for the region. this shift will bring a return to dry conditions and early fall sunshine, with high temperatures anticipated to rise into the mid-60s to mid-70s.
This upcoming stretch of clear skies provides a strategic opportunity for residents. The dry weather and sunshine will offer an ideal window for viewing the vibrant autumn hues as tree leaves change color across the state, making it a perfect time for outdoor viewing.
The 90% probability of a strong El Niño winter
While the immediate forecast shows a return to sunshine, the long-term outlook for the Pacific Northwest remains volatile. Forecasters indicate there is a 90% chance of a very strong El Niño this winter, with a 70% chance that the event will be record-breaking.
Such a significant climate pattern could heavily influence weather trends in the coming months. A record-breaking El Niño could fundamentally alter the frequency and intensity of winter storms, potentially leading to much more unpredictable patterns for residents in Western Washington compared to a standard seasonal transition.
Uncertainty over the intensity of upcoming thunderstorms
Despite the predicted sunshine later this week, a stronger fall storm is currently moving through the region. While the report mentions this system will bring a combination of wind and rain, it also flags the potential for thunderstorms.
It remains unclear exactly how widespread these thunderstorms will be or if they will coincide with the heaviest rainfall predicted for the coast. Additionally, while the El Niño probability is high, the specific impact of a "record event" on Washington's local precipitation levels remains a point of scientific anticipatin that residents should watch closely.
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