A Historic Early April “Heat Wave” On The Way

Get ready for record breaking warm (not really hot) weather the 2nd half of this week!

This morning we are under a cool and showery airmass, with snow levels down around 4,000’…that’s quite a change from the past 5 days!  This is a quick-moving cool airmass and it’ll be mainly gone by Tuesday evening.  Then things get real interesting as a pattern we’ve never seen so early in the season develops…

HIGHLIGHTS

• Wednesday-Friday will each likely set a record high temperature in Portland (records: 78, 79, 74)
• The warmest weather we’ve ever seen this early in Spring is coming for one day…Thursday
• Most likely high temperatures will be in the 85-90 degree range in the western valleys on Thursday
• The coast should see 70s and a few 80s Thursday (one day only)
• This isn’t really a heatwave (temps remain below 90), but it IS unprecedented for the first week of April in our area.
• Confidence is unusually high for this event as all models are in agreement

 

What’s going on?

A big ridge of high pressure develops right over the West Coast Wednesday and lingers through Friday.

It happens pretty quickly with the highest upper-level heights over us on Wednesday.  At that time models are in very good agreement showing 500 millibar heights around 584 decameters (5,840 meters) over the Salem sounding location.  Taking a look at the SPC website showing historical soundings:

notice the yellow area is the forecast, and the red lines are the highest number we’ve seen for each date in the past…we’ve never seen heights this high in early April!  As the ridge moves slightly east Wednesday night and Thursday, upper-level flow goes southerly (warm!) and the low level flow turns sharp easterly.  The WRF-GFS surface map for Wednesday PM shows north/northeasterly flow:

and then straight east wind midday Thursday:

Onshore flow in the valley does not kick in until Thursday evening/night.  As a result we get maximum warming under full sunshine both days.  It’s a relatively easy forecast, since models are in great agreement.

Then the next question is “how warm could it get?”  Hard to believe I see these numbers in the first week of April:  All models are right around +18 to +19 degrees Celsius at 850 millibars.  That’s upper 60s at 5,000′ in the atmosphere overhead!  Take a look again at the SPC sounding climatology and you see how far out of the norm this is going to be (in yellow):

Crazy isn’t it?  We haven’t see 850mb temperatures this warm in our history until about 3 weeks from now!  And remember the red line is highest on record.  The normal isn’t even visible on the chart.  The atmospheric temperature overhead will be above normal for even July/August in our area.

As a result, the extremely high (for early April) temperature forecast is a bit tricky.

At this point, if it were the last week of April, I’d forecast 90 degrees for Thursday.  Instead, accounting for lower sun angle a few weeks earlier, we’ll go for somewhere in the mid-upper 80s.  That’s still 25-28 degrees above normal!  Wednesday will be somewhere around 78-81, and Friday with stronger onshore flow should quickly push us back down below 80, so the extreme temps are just for the one day…Thursday.

Chief Meteorologist Mark Nelsen