FastCast—Wednesday, May 3 to Saturday, May 6:
Unsettled weather will continue as a complex pattern of showers and thunderstorms remains in place over the Pacific Northwest. A main ingredient for thunderstorms is instability in the atmosphere, which will be constant across the region, especially in the Cascades and in Eastern Washington. Wednesday will be quite a warm day in Western Washington, with highs in the upper 60s to mid 70s. Some locations will reach the upper 70s. Across the state, showers and/or thunderstorms are possible on Wednesday morning, mainly across the state’s northern tier. A more widespread round of showers and thunderstorms will increase in coverage from late Wednesday night through Friday. Rain amounts will be heaviest from Thursday to Friday. Highs in Western Washington will decrease substantially from Thursday onward. Expect highs in the mid to upper 50s on Thursday and Saturday, but only in the low 50s on Friday. Lows will decrease from the low 50s on Thursday to the low 40s by Saturday
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Continue reading the full blog below!
Quite an unsettled pattern is ahead for Washington state, featuring many showers and thunderstorms, which can be quite difficult to predict in our area. Let’s take a look at the forecast.
A good way to see predicted thunderstorms is to look at simulated satellite imagery. On this map, brighter colors mean higher cloud tops, and higher cloud tops mean that thunderstorms are present. The image below is the HRRR high-res forecast at 7 AM Thursday.
Notice the black box in parts of Western Washington. This shows an area of potential thunderstorms on Thursday morning.
Let’s compare this to the NAM high-res forecast, for 6 AM Thursday.
Notice that there is a black box in Western Washington, just a bit further south than on the HRRR. The point here is that showers and/or thunderstorms are predicted for Western Washington, though their location may differ in forecasts.
Another important note is that some thunderstorms may start to dissipate and lose strength once they move into/over the Cascades. However, these dissipating storms can still have strong downdraft winds (strong downward winds in thunderstorms). A case in point is around 2:00 AM Tuesday, when local weather expert Michael Snyder recorded an incredible 63 mph wind gust in Normandy Park due to a dissipating thunderstorm. Even in their weakening stage, thunderstorms can have an impact!
Another indicator of thunderstorms is instability, represented in the CAPE index. Below is the UW WRF CAPE forecast for 12 AM Wednesday.
Early Wednesday morning, instability is mainly present east of the Cascades and in the northern half of the state.
Just over 24 hours later, at 1 AM Thursday, is below.
By very early Thursday morning, instability will have increased to include strong CAPE values in parts of Western WA & OR, potentially including areas around Seattle and Portland. This agrees with the potential for thunderstorms early Thursday morning that we saw in the simulated satellite images at the beginning of the blog.
Finally, below is instability at 7 PM Thursday.
Notice that the atmosphere is still unstable across the central strip of the state, including the Puget Sound area. This shows that multiple rounds of showers/thunderstorms are possible on Thursday into Friday.
The expectation for multiple rounds of storms on Thursday and Friday brings a possibility for significant rainfall. Below is the UW forecast for total rain through 5 PM Friday.
The UW forecast shows most of Western Washington receiving 0.5-1.75” of rain, with the highest amounts from Seattle southward along the I-5 corridor and in the Cascades & foothills from North Bend southward. Eastern Washington will receive 0.3-1”, with isolated higher or lower amounts. (Rain will generally be more sporadic in Eastern WA).
One final note…another warm & pleasant day is ahead on Wednesday (before a significant cooldown on Thursday). Below is the GFS forecast for Wednesday’s highs.
Expect lowland highs in the mid 60s to low 70s from Seattle northward and in the mid to upper 70s from Seattle southward. The immediate coast will reach the upper 50s to low 60s, while inland communities will reach the upper 60s to low 70s. Eastern Washington will be warmer, with highs in the mid 70s to mid 80s, warmest in the Columbia Basin.
Obviously, this is quite a fluid weather pattern that can be difficult to predict, as it is hard for thunderstorms to develop and to be accurately pinpointed ahead of time in this area. Stay tuned for updates on my Twitter page and here on the blog. I will try to get an update out by Wednesday night.
Sounds like indoor weather is on the way.
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