Tuesday, May 20, 2014

Memorial Day Weekends in the Inland Northwest...Nice or Not?

 Now that Memorial Day is approaching, our most common question we get at the National Weather Service is "what's the weather going to be like this weekend?" After all, this is the start of the summer camping season for many of the denizens of the Inland Northwest. Before folks hear the answer, they often have the preconception that weather in this part of the country really doesn't become conducive for prolonged outdoor activities until early July. With that notion in mind, we decided to take a climatological glimpse of what Memorial Day weekends in the past truly delivered.

Usually looking up weather over holidays is easy. The 4th of July always falls on the 4th day of July, and Christmas always falls on December 25th. For Memorial Day though, the days varied from year to year, however before 1971 and the passage of the Uniform Monday Holiday Act, Memorial Day always fell on May 30th. Consequently, what constituted the Memorial Day Weekend isn't exactly clear. To make things simple we decided to only look at Memorial Day Weekends from 1971 and beyond. These dates can range from May 23rd through May 31st.

First what is the normal weather like for this period? Lets begin by looking at the mean 500 mb (18k ft MSL) maps. Sure enough, moist southwest flow typically is still pointed at western Washington and the Cascades. Meanwhile there is a ridge that sets up just west of the Continental Divide along the Alberta-British Columbia border extending into central Montana.

500 mb mean map 5/23-5/31

That map alone suggests the region still might be prone to wet weather so what are the climatological temperatures and chances for precipitation for this time of year? We will start by looking at data for Spokane. But knowing that many people like to get away from the metro area and go camping in the woods, we also decided to look at the weather at the Priest River Experimental Station in north Idaho. Here's a map of where that station is located

And here are the averages:

So not bad. An average high around 70°F with rain expected on roughly 1 out of 4 days doesn't sound too terrible. But that's climatology. Now here's reality. Lets start on a positive note and look at the top 10 warmest Memorial Day Weekends. Here is the list for Spokane.

Top 10 warmest Memorial Day weekends for Spokane. 

For those who like warm and dry weather, the holiday weekend in 1983 was hard to beat. Average highs were above 90° and every day was dry. In fact of the top 10 warmest holiday weekends, 7 of them were bereft of precipitation. At Priest River, the results were similar as far as temperatures. 1983 was the year of the warmest and driest conditions. However note that only 60% of those weekends were dry, and one of those weekends (1985) saw rain on every day.

Top 10 warmest Memorial Day weekends at Priest River experimental station. 

Now let's look at the list of least desirable Memorial Day weekends. For Spokane there were 5 extended weekends which saw the average temperature stuck below 50°F or roughly 10°F colder than normal. The worst year (by temperature) was 1975, however the rainfall that year was light (there were trace amounts of rain which fell on 2 of the 3 days). Perhaps 1989 could be considered a worse year for outdoor activities considering an average temperature of 48°F and just over an inch of precipitation.

Top 10 coldest Memorial Day weekends for Spokane. 
For Priest River, the bad weather Memorial Day weekends were  worse than what was experienced in the Spokane area. 1977 was the coldest year, by far with an average temperature just below 45° although that year was dry. Campers in north Idaho certainly woke up to frost on their tents as lows dipped to 30° on two of the three mornings. From a comfort standpoint, the worst weekend might be the one in 1994 with an average temperature about 8° cooler than normal and measurable rainfall on each day.

Now lets look at a few daily extremes:

Hottest High Temperature: Spokane     94° (May 29, 1983)
                                              Priest River 91° (May 24, 2003)

Coldest Low Temperature: Spokane 35°* (May 26, 1991, *last of several occurrences)
                                               Priest River 29° (May 30, 2011)

Wettest Day:   Spokane      0.79" (May 28, 1988)
                         Priest River  0.96" (May 29, 2010)

Wettest Weekend: Spokane 1.20" (May24-26, 1997...it rained all three days)
                                 Priest River 1.58" (May 29-31, 2010...it rained all three days)

Percent of entirely dry Memorial Day weekends: Spokane 19%
                                                                                  Priest River 23%

This last tidbit is a little unexpected since Priest River is typically a wetter location than Spokane. The answer may lie in the fact that the Spokane observation is taken at least once an hour through the day, whereas the Priest River one is taken once a day. Thus if a trace of rain fell at sometime during the day and it wasn't witnessed it might be recorded as a dry day. Nonetheless, the odds of seeing an entirely dry Memorial Day weekend in the Inland Northwest (especially the eastern third of it) is not good.

Will this weekend be one that sees dry and warm weather? Looking at the latest weather maps, the answer is a resounding no. Here is our latest forecast issued at 3pm today (Tuesday 5/20)

For Spokane it looks like at least a 20% chance of precipitation each day with temperatures cooling to right around the seasonal normals.

Spokane Outlook

For north Idaho things look a little cooler and a little wetter.

Sandpoint outlook...Priest River graphs are not available
Based on the temperature and precipitation forecasts this does not look to be a top-10 Memorial Day weekend weather-wise.