FastCast--Sunday, June 23 to Thursday, June 27:
A significant, but brief, cooldown is expected across the region on Sunday, with overcast skies, a chance of light showers (especially Seattle northward and on the coast), and highs dropping to the low to mid 60s. This is caused by a weak cold front moving through, which will bring dangerous fire weather (low humidity and strong winds) to Eastern Washington. Conditions will rebound on Monday, with partly sunny skies and highs returning to the upper 60s to low 70s. Tuesday will be mostly sunny and warmer, with highs in the mid to upper 70s (and possibly the low 80s). However, a significant change is store for Wednesday and Thursday, with troughing bringing highs back down to the mid 60s, with a chance of showers (up to 0.3" for the lowlands). Through Thursday, expect overnight lows to be in the low to mid 50s.
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Continue reading the full blog below!
A relatively weak system moving through the Northwest on Sunday will bring a brief cooldown and dangerous fire weather to Eastern Washington. Let's take a look at the forecast!
We'll start with the NAM high-resolution forecast for 24-hour temperature change across Western Washington on Sunday afternoon.
Notice that most of the region will be 10-20° cooler than Saturday, especially in and east of the Cascades and in the Convergence Zone (between Seattle and Everett).
These significant temperature declines will be fueled by strong onshore flow moving into the area. Below is the European model forecast for peak winds on Sunday.
This forecast shows 20-30 mph gusts for Western Washington and 30-45 mph gusts in Eastern Washington. Those winds are of note, as they will fuel high fire danger.
Next, let's take a look at the forecast for relative humidity on Sunday afternoon, also from the European model.
This forecast shows relative humidity dropping to 10-20% for most of Eastern Washington, with some areas reaching values of 5-10%. These humidity values will make it very easy for fires to spread rapidly. This especially applies to any grass fires that start, which will have explosive rates of spread due to the combination of gusty winds and low humidity.
This combination of fire weather conditions has caused the NOAA Storm Prediction Center to put Eastern WA under a "critical" fire weather risk area.
In addition to this risk area, NWS offices in Spokane and Pendleton have issued Red Flag Warnings due to the fire danger.
This system will also bring a chance of light showers to Western Washington, as seen in the NWS NBM forecast below.
The best chance of showers will be on the coast (0.1-0.2"), in the Cascades (0.15-0.4") and from Seattle northward. Showers will be most likely on Sunday, tapering by Monday.
Finally, let's take a look at temperatures, from the NWS NBM high-resolution model. First, the forecast for Sunday.
This forecast shows Sunday's highs in the lowlands reaching the mid 60s, which is likely too high, as this forecast underestimates the impact of thick cloud cover. I'd expect highs in the low to mid 60s (same on the coast), and in the upper 60s to mid 70s for the Willamette Valley. Eastern Washington will reach the upper 70s to upper 80s, except only the mid 70s in the Kittitas Valley, due to strong west winds.
Temperatures begin to rebound a bit on Monday for Western WA, as seen below.
Expect the lowlands to reach the upper 60s to low 70s, the coast to reach the low to mid 60s, and the Willamette Valley to reach the upper 70s. Eastern Washington will cool to the mid 70s to mid 80s.
Finally, here's a look at the forecast for Tuesday, likely the warmest day of the week.
On Tuesday, expect lowland highs in the mid to upper 70s (isolated low 80s in the South Sound), with the coast reaching the mid 60s to low 70s, and the Willamette Valley reaching the mid to upper 80s. Eastern Washington will reach the mid 80s to mid 90s.
Stay tuned for more details about the forecast for next week, and the impending midweek cooldown.
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