How unusual will this next episode of warm and wet weather be for the Inland Northwest?

By now you have likely heard that this is going to be a wet and warm remainder of the week across the Inland Northwest. This wet spell of weather is expected to continue into early next week. So why is this? Well, most of it can be attributed to yet another atmospheric river event. While most atmospheric rivers come and go, this event will be a little different, as we will likely be impacted by a trio of rivers. The first arrived yesterday into last night. The next is expected to unleash its torrent of moisture on Friday and the last will likely impact the region on late Sunday. The video below shows the expected sequence of events. The atmospheric rivers are denoted by the blue, purple and red colors

So what does this mean for the area. Well, it will translate to several things. The most obvious is it will result in very wet weather. Here's the estimated rainfall we are forecasting between today and Sunday night.

Rainfall forecast between today-Sunday night

These are some pretty hefty totals. Notice the reds and oranges spread across the Cascade Crest and extreme northeast Washington/north Idaho. These locations could potentially amass a few spots of 3-6" of precipitation, most of which will fall as rain. While these amounts are impressive, the question is just how rare are they? We get atmospheric rivers all the time this time of year. Certainly it can't be that unusual, can it? Well, if put into perspective for the time of year and amount of precipitation it actually turns out to be a very rare event. One of the new tools in our weather toolkit consists of quantifying an event based on climatology. In other words, how common is a weather event for a given time of year? For this event, we will look at the amount of rainfall expected over a 3-day period (between today-Sunday). Below you will see an image from one of our ensemble models showing the approximate amount of precipitation (black contours or isohyets) combined with the rarity of the forecast amount (the color shading). What's interesting, in this case, is northern California is covered with isohyets in excess of 3 inches, while over the Inland Northwest we see a small area of 1.5"-2+"  inches over extreme northeast Washington and north Idaho (the amount is smoothed and will not catch the locally higher amounts we forecast over the mountains). Despite the lighter rain totals over our neck of the woods, it's actually a less common event as shown by the blue and dark green shading. The blue shaded areas are locations where this event places in the 99.9 percentile (or 1 out of 1000 events)  for all rainfall events between December 22 and March 22nd (between 1985-2012). The dark green shaded areas, which cover a good portion of the Inland Northwest represent a 99.5 percentile event (1 out of 500 events). So technically speaking we will experience the most unusually heavy precipitation in the western US this weekend.

Ensemble precipitation forecast (Thu-Sun) combined with the relative rarity of the amount.

The other unusual weather phenomena we are expecting this weekend will be the warmth. If we once again choose to utilize a percentile ranking of the warmth for this event it's rare, but nowhere close to as rare as the rainfall. Here's a look at the forecast temperature and the respective percentile (for the period between January 27-February 17)  for the temperatures at 850 mbs (around 4500' above sea level) on Friday afternoon. The image below is showing temperatures in the lower to middle 40s (4-6°C)  pale yellow which indicates roughly the 90th percentile (1 out of 10 events). So really it's not rare at all. Temperatures at this level will be significantly cooler than what we experienced on the 25th of January when our 4500' temperature surged into the lower to middle 50s.

Climatology rarity of 850 mb temperature for Friday afternoon (2/6/15)

What will make this an unusual event though is the duration of the mild temperatures. Right now for the Spokane area we are forecasting 4 consecutive days of high temperatures right around 50° (see image below). The last time we saw that many 50°F or warmer days between December and early February (2/10) was in 1963! The longest stretch on record is 6 days set in 1917. A 3-day stretch of low-temperatures of 40°F or warmer is also rare for this time of year. Between tonight and Saturday night, we are forecasting lows at or above that mark. Surprisingly, the last time we saw three nights of lows ≥ 40°F was earlier this winter (in early December). Before that, we had only seen 3 or more consecutive nights of 40°F or warmer lows 7 times since 1881.

7-day outlook for the Spokane area Thursday (on left) through Wednesday (right)

Just the latest chapter in what has been an unusually mild and relatively snow-free winter.