Last week was a very interesting pattern, given the season.  We had a full week of “Winter In April” weather – including a historic April snowstorm for Portland/Vancouver and temperatures more typical of late January or early February.  There was also an impressive number of thunder and hail storms across the region, probably the best outbreak in the time I have lived on the west side.

There are several points that stick out here:

1.  Snow.  The Monday morning snowstorm was, by far, the latest significant snowfall in the lowlands of NW Oregon/SW Washington, that we have seen in recorded history.
2. Cold.  We saw some of the coldest April temperatures across the region in more than a decade, when you consider both daytime and nighttime.
3. Longevity.  Temperatures were well below normal for seven consecutive days; we haven’t seen such a prolonged bout of April chill since at least 2008!
4. Thunder and hail.  There was numerous storm activity on several of the days; nearly everyone in the region got to see at least a small piece of the action.

It definitely “felt” like winter last week!  I was wearing my winter coat outside for several days, and even turned the fireplace on during the night before the snowstorm.  I made a large batch of ‘Targaryen curry’ last Sunday evening; my original recipe for a very spicy Thai-style curry that makes the inside of your mouth feel like dragon fire.  In particular I recall getting out of the gym Tuesday night around 9pm, when it was 36 degrees outside with slushy rain – after we had had snow two mornings prior.  That was a clear sign to me as to how unusual this cold pattern was for mid-April.

TEMPERATURES

As far as I can tell, this was the coldest April weather pattern we have seen in the Pacific Northwest since at least 2008.  And in some places to our north and east, it may well be the coldest in modern history.  It lasted for 7 days in earnest, from Sunday the 10th through Saturday the 16th.  The cause was an upper-level pattern similar to an arctic outbreak in winter:  strong upper-level ridging into Alaska, with a deep trough digging over the Pacific Northwest, California and Nevada.

The coldest day at PDX, when you average the daily high and low, was Monday the 11th – the same day as the snowstorm.  The low that morning was 33 and the high 51, for a mean of 42.0 degrees F.  That is the lowest daily April mean in 14 years, tying 4/19/2008 (low 36, high 48).  Of course the 2008 day was later in the month, which makes the departure from normal slightly more impressive.  You have to go all the way back to 1975-76, to find April days that were colder still.

We had two days with a high temp below 50  – Sunday and Tuesday, both at 47.  All the other days were in the low 50s, save for a 56 on Friday.  Mean temperature departures for each of the seven days were -7.0F or colder, with Monday a bitter 10 degrees below normal:

Snow totals at the Parkrose weather station were 1.9″, with 1.6″ from the Monday snowstorm.  Notice how as of this Monday the 18th, we already have 3.91″ of rain.  That’s nearly double the normal for the first 18 days of April and nearly an inch above the entire monthly normal.  With more rain on the way today, tomorrow and Thursday, it’s safe to say that April 2022 will go down as a soggy one!

For comparison, here are the numbers for April 2008:

The “big chill” came a bit later in the month, and featured very similar temperatures to 2022 on the coldest days (hence the bigger departure).  Arguably you could say the cold spell lasted for 8 days; though it seems a bit silly to think of low temps 40-42 degrees as a “cold spell.”  For sure there were 4 very cold days, three of which were exceptionally cold for that time of year.

Most interesting of all?  In 2022 we never got to freezing at the Portland Airport!  Only in April (or October) can you pull that off under such a chilly pattern.  It was cloudy and damp/wet almost every night, and we never got to see skies clear and dewpoints drop off.  Had that happened we might have seen temps down to at least 29-30 in Portland, and probably mid-20s in the rural spots.

SNOW

In April 2008 I actually got to see a 1/4″ dusting of snow, in Eugene, Oregon on the morning of the 20th.  But last Monday morning, I awoke to the biggest April snowfall I have seen in my lifetime:  there were 3.5″ on the ground outside my Orchards home.  Several neighborhoods in Clark County saw 5-7″, all below the 500′ elevation level.  The PDX snow station in Parkrose reported 1.6″.  That was the latest measurable snowfall on record for PDX.  It beat the old record for latest “significant” snow event – 1.0″ or greater – by more than a month, the previous record held on March 8, 1951.

The cause for the rare spring snowstorm was a perfect pattern:  a chilly airmass over the top of us, and a strong and deepening low-pressure system moving onshore near Lincoln City.  Any time between early December and mid-February, this setup would have almost certainly given Portland and Vancouver a widespread 10-15″ snow storm, and the cities would have shut down for 3 or 4 days.  Even in early March, it would have likely been enough snow to disrupt daily life for at least one full day.  That was one advantage of having it in April:  what snow does fall, only stays on the roads for a few hours at most.

Here is an image of the incoming low from the April 11 00z WRF-GFS model, taken the evening before the storm.  It shows the low coming ashore at 11pm, right near Depoe Bay on the central Oregon Coast:

This setup puts Portland in the northern sector of the cyclone, where winds tend to be northerly/easterly rather than southerly.  In fact on Sunday evening I remember thinking it felt like it does just before a winter snow or ice storm:  chilly easterly breezes and temps in the 30s at the surface.  This NE flow, when it clashes with the southerly flow, can create heavier precipitation than would otherwise exist.  That tends to drag the snow level down toward the surface, through the process of dynamic cooling.

By 5am, the zero hour of the 12z model, the low was in this position:

That allowed us to stay in the chilly northern sector during the period with the heaviest moisture.  Had this happened 6 hours later during the daytime, snow would have had a lot more trouble sticking to the ground due to the influx of solar energy.  The timing was just perfect for overnight and early morning accumulation.

THUNDER AND HAIL

Cold spring airmasses tend to be unstable, due to the effects of the strong sunshine.  That was definitely the case last week – in fact, I don’t think I recall ever seeing such a prolonged outbreak of hail and thunder showers before.  Tuesday, Wednesday, Thursday and Saturday were all very active days across the region.  I went out chasing one of the hailstorms Tuesday in northern Clark County.  While driving west along a country road, the rain abruptly started, then turned to hail a few seconds later.  Within ten minutes there was a quarter to half inch of small hail on the roads, and snowflakes were starting to fall.  I even heard a clap of thunder during the snow behind the hail; my first-ever experience with thundersnow!

There was another excellent hail storm at my home in Orchards two evenings later.  Some of the stones were between 1/4″ and 1/3″ in diameter!

Wednesday saw a particularly impressive thunderstorm develop in East Portland and Gresham and Troutdale, with up to 1/2″ hail reported.  I didn’t get to experience that one, but I still got a busy and active week of weather action – way more than we ever get in a more typical spring season.

LOOKING AHEAD

The really cold weather is almost certainly over, but we still have some interesting conditions for tomorrow Wednesday.  It appears that the atmosphere will once again get quite unstable, possibly creating another round of thunderstorms and hail over Western Oregon in particular.  The Storm Prediction Center is showing a marginal risk of severe thunderstorms for the west half of the state tomorrow.  That means a slight chance of massive thunder cells with 1/2″ to 1″ size hail.  If there was ever a good day for a weather geek to call in “sick” and go storm chasing, tomorrow would likely be the day! (Shhh!)

We might see nothing more than a few weak thunder claps.  Or we might luck out and see the best T-storm activity the west side has seen for many years.  Or perhaps, we even get one or two weak tornadoes.  Who knows?  There’s only one way to find out!

Karl