Breaking the Daily High Temperature Record Today

Temperatures are surging in western Washington today (Friday).  Yesterday was warm enough, with highs reaching the mid-80s over Puget Sound, and over 100F near Portland and the eastern slopes of the Cascades.   Look closely and you will see some 90s over the eastern and southern slopes of the Olympics where downslope flow gave a temperature boost.

Today will be warmer.

Temperatures aloft have warmed further and easterly flow has developed over the crest and western slopes of the Cascades, as illustrated by a time-height cross section over SeaTac Airport (time on x axis, height on y axis).  As a result, temperatures will surge over the western slopes of the Cascades (into the upper 90s in places)

Seattle-Tacoma Airport reached 87F on Thursday and currently it is running 8F ahead of yesterday, which suggests strongly it will reach into the lower 90sF today.

To give you an idea of how unusual the temperature aloft are right now, below is the climatology of 850 hPa (around 5000 ft) temperatures at the Quillayute radiosonde site.  The red line is the all-time record temperature and the silver dot is today.  Yes...we are right at record levels.

A strange aspect of the temperature records at Seattle this time of the year is that the the record high are relatively low for the second half of August (upper 80s to around 90F) and go up again in September.  The figure below shows this (red dots) as well as the observed highs and lows.    We will easily set a new daily record today.

Why a drop in the record highs and some recovery in a few weeks?  The sun starts to weaken in August, working against record highs.  An important mechanism for the big temperature surges is easterly flow over the Cascades, with downslope flow on the western slopes causing compressional heating as air descends to higher pressure.  This easterly flow is generally forced by large scale weather disturbances, which are weak during mid-summer, but increase in amplitude by early September.  Thus, we can get some very warm days in September when easterly flow is strong.

But we won't break really big records and temperatures near Seattle won't get into the upper 90s F as some folks were suggesting.  Why?  Because the thermal trough, the area of low pressure associated with low-level warm air,  is centered south of us, resulting in a modest north-south pressure difference (higher to the north).  As shown in the temperature, wind and pressure forecast for later today (below), this will cause northerly winds that will move cooler air into Puget Sound.  Still warm, but not crazy warm.  The easterly flow is also bringing in slightly cooler air aloft.  Furthermore, sinking on the western slopes of the Cascades will produce some leeside troughing  (low pressure) along these slopes, which will increase the north-south pressure gradient (again bringing in somewhat cooler air).

Those poor folks in Portland don't enjoy this natural air conditioning and will burn... heading above 100F.  At least they have fine microbrews to cool down with.