A Gloomy April, But Sunny Days Ahead

After such a prolonged, wet late winter/early spring in 2017, I was hoping that 2018 would feature a more abrupt transition to the warm, sunny stretches of April that give us a taste of our beautiful summer to come, but those wishes have no doubt been in vain. If April sunshine is a runway, we’ve been stuck in a holding pattern of cooler and wetter-than-normal weather due to a persistent, large-scale trough over the region. Thankfully, we’ll get permission to land later this week, but not before we make a few more right hand turns.

Our current pattern is a classic one for above-normal precipitation and below-normal temperatures – the ridge downstream of the large “Aleutian Low” (for more on downstream development, the phenomenon that causes these ridges to form ahead of these large lows, click here) allows a deep upper-level trough to enter the Pacific Northwest and direct cool, unstable air into the region.

We’ve already exceeded our normal April rainfall totals for the entire month in many locations, and while average temperatures are only slightly below normal, the diurnal range of temperatures has generally been less than normal due to cloud cover at night helping prevent infrared radiation from easily escaping to space. As a result, our high temperatures have consistently been several degrees below normal… it’s just that our warmer-than-normal lows have given bumped our average temperatures closer to normal. I’m trying not to confuse “average” (referring to the average temperature) and “normal” (referring to how recent precipitation/temperatures compare to climatology), hopefully y’all followed me with that explanation!

Here are the temperatures at Sea-Tac (I couldn’t find any for KPDX) for the past 4 weeks compared to the normal highs and lows for a given date. Note the narrow diurnal range from mid/late March to now, with the exception of 4/6 through 4/11 (ignoring 4/8, which had a very small diurnal range).

The saving grace from all this gloomy weather is that we’ve made some fantastic progress on our snowpack. All of Washington is now above normal, and perhaps more importantly, Southern Oregon and California have gone from critically low (10-20%) to well-below-average, but far more manageable levels. Though I haven’t done the analysis to back it up, it definitely seems like there is far more year-to-year variability in precipitation/mountain snowfall in Southern Oregon/California than there is here in the Pacific Northwest – the drought of 2011-2016 in California comes to mind, as does the extraordinarily wet winter of 2016-2017 that obliterated it. Anybody who was lucky enough to be studying atmospheric science at UCLA in February 1998 would have witnessed 20.51 inches of rain fall at the campus weather station during that month. For comparison, Portland’s all-time monthly rainfall record is a measly 15.24 inches in December 2015 (we’ve had a lot of rain lately!), and that was with 3 extra days to work with!

Forecast:

Alas, we have another rainy day on tap tomorrow as that aforementioned deep upper-level trough slowly swings through the Pacific Northwest. But with snow levels near 3,000 feet, the mountains will once again pick up plenty of snow. Snoqualmie Pass has picked up 19.5 inches of snow this month, which is close to average, and Stevens has already had 50 inches, which is above-average. As you might expect, the difference in snowfall with elevation is much more pronounced at this time of the year than December/January, due both to steeper lapse rates in the atmosphere and the lack of a cold, insulating east pass flow.

The entire Cascade range will see significant snowfall for the first half of this week, with 2-3 feet occurring above 4,000-5,000 feet. Even Snoqualmie Pass could pick up a foot. Here in the lowlands, expect a wide swath of 1-2 inches of rain extending from Bellingham south into Northern California from tonight through Wednesday, with the bulk of this rain occurring tonight through tomorrow evening.

We’ll finally break out of our troughy pattern on Thursday as a ridge sets up over the region, allowing our bodies to finally host those Vitamin D parties they’ve just been dying to throw. Thursday will feature abundant sunshine with highs in the 60s, with temperatures a touch warmer and skies a tad cloudier on Friday. Another upper-level trough will keep California cool and wet during this time, but they could definitely use the extra mountain snowfall.

Thanks for reading! Only a few more days of wet and cool weather… beautiful springtime weather is right around the corner.

Have a great week,
Charlie