Will Portland see a rain trend this May?


PORTLAND, Ore. (KOIN) – Why don’t we start by mentioning that the last seven years of the month of May have all been below the current average rain for Portland.

We have had some dry May months of recent years. You may recall the 0.17 inches of rain in 2018 or the 0.59 inches of rain in 2015 (figure 2). There is hope: the early 2010s, brought in some hefty rain totals.

Will we find ourselves in a similar pattern to start the early 2020s? The average rain for Portland is 2.47 inches in the month of May. The extreme most to come out of the month of May was 5.55 inches back in 1998. Don’t look now, but the extreme driest year on record was 1992 with 0.10″ (we weren’t far behind that in 2018).

Figure 1

One of the large issues at hand right now, we have had the driest start to the meteorological spring (March, April and May) in Portland. If you take the accumulated rain from the months of March and April and combine them, we have a whopping 1.94 inches (spring deficit of -4.47 inches). It is clear that we need as much rain as we can possibly get to try to fight back from this dry meteorological spring.

We have 28 days to do our best job before the meteorological summer comes flying in (June, July and August). I mentioned above that we are on a poor stretch of May months over the last seven years. 2020 was closer to our average, but still below. It looks like we would need to flirt with one of our wettest May months on record to even come out at a wash for the spring months.

Figure 2

Right now, there isn’t a lot of rain in the forecast. We do have a chance to pick up on some minor accumulations Monday afternoon and evening. For the most part, many in the Willamette Valley will finish the day dry.

However, there’s a chance for a more moisture-laden system by the end of the week. Does this mean we are looking at an alteration to our weather pattern? Not necessarily, but it will be our first opportunity for measurable rain in Portland that may help out some of our local gardens and yards. As the month of May nears June, we really start to see a transition to our summer conditions. Temperatures are warmer and more consistently in the 70s. We have few chances at soaking rain across the state. We also start to pick up on more convection and storms that may produce lightning.

I mentioned the temperatures start to take a turn for the warm summer by the time we hit the end of the month. Check out how the high temperature continues to step to the lower 70s around May 23 the end of the month before we start working up to the 80s. The low temperature starts to trend to near 50 degrees as well. Just overall, we are nearing the end of spring and working our way to those warmer and drier months.

Figure 3

I want to briefly show you the May temperature and precipitation outlook from the Climate Prediction Center (CPC). The outlook is projecting many areas in the United States to be warmer than normal (probability) with a small sliver to the north to likely have a chance for a colder than normal month. Portland happens to be in the area that is an equal chance for below, normal, or above. The first week of May is coming out to be very much that scenario where we have just about everything. If you cycle through the slideshow and check out the May precipitation outlook, you will notice that we are also outside any zones that may have high probability for drier or wetter than normal conditions. We again, are in an equal chance for below, normal and above. A section of the state is in the area that may end up drier than normal.

We know that it is tough for that portion of the state to get consistent moisture. The climate is different and the average rain totals do not match what we experience west of the Cascades. However, the system that moves through Thursday and Friday is going to drop some needed rain for the state. Not the case for those from about Klamath county and east. Coincidently, the same area that the CPC is projecting to be drier than normal this May. Going back to the original question, will May be the month to bring the rain back? It doesn’t look like there is a strong case at this time for a robust month of rain, but we at least have rain in the forecast this week.

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