The first official day of summer arrives on June 21 this year for our region although we have been in meteorological summer since June 1. After a cool and wet start to our Spring and a very warm and dry end to it many are wondering what the summer looks like for our region. Will we see a repeat of the last 3 years with scorching temperatures and extreme dryness, or possibly a mild and yet very wet summer like 2012 or perhaps a "normal" summer for a change? The answer lies with two important features that influence our weather in Northwest Montana and the position of each one. First let's take a look at the official summer outlook from the Climate Prediction Center (CPC for short) for the entire country. The outlooks are in the maps below.
The maps above show the probability of above average, average or below average temperatures with the bottom map showing the probability of above average, average or below average precipitation. For our region, the CPC official outlook is calling for greater odds of above average temperatures (meaning warmer than normal a 40 % to 50 % chance of that happening as evidenced by the orange shading) with greater odds of below normal precipitation (meaning a 40 % chance of below normal precipitation as evidenced by the dark brown shading). In plain ordinary English that simply means that CPC expects our region to have a greater likelihood of a warmer than normal and a drier than normal summer. In general they are expecting that for the entire Northwestern US.
For our region specifically I would like to show a few more charts that I believe will determine what our summer will end up with and will share shortly what I expect for Lincoln and Sanders Counties. Take a look at the map below.
This map was created showing the upper air pattern across our region on August 3, 2017 of last year and summarizes the overall pattern. The important thing to note here is the position of the two "H"s over Nevada and Utah and the axis they are on which stretches from offshore of the Washington Coast southeast to New Mexico. This is the upper level ridge sometimes referred to as the "4 Corners High" or "Continental Ridge." It builds over the Western US every summer and is generated by the heating of the land beneath it with the core of the upper level high sitting at around 18,000 feet. This ridge is considered to be a negative tilt ridge as it's axis runs from northwest to southeast. We are located on the northeast edge of the upper ridge, not directly underneath it but also far enough away from the jet stream which is well to the north and east as to not have any weather or cooling provided by the jet stream or passing shortwave disturbances. Also the position of the negative tilt of the upper ridge does not all for any disturbances generated by thunderstorm outbreaks to our south associated with the Southwest Monsoon to rotate up into our region but instead they are deflected out into the Pacific off the California coast. The result of this type of pattern is like the one we had last summer. Weeks and weeks of hot, dry and smoky conditions from forest fires and relentless heat and drought. Now look at the chart below.
This chart is from July 3, 2012 and looks very different from the one above it. The two "H"s are much further south and centered over Nebraska and New Mexico with an axis stretching from Minnesota to Baja California on a northeast to southwest axis. Of perhaps greater importance are the tightly packed heights (lines) over our region and the northwest along with an "L" just off the British Columbia coast in the corner of the map which is the Gulf of Alaska low, noticeably absent from the first picture. This ridge axis is positively tilted and suppressed further to the south due to the strength of the Gulf of Alaska low. The result for our region in these situations is similar to the summer of 2012 which was one of the wettest in recent history with an abundance of rain and cool temperatures in both June and July before a warmer and drier August set in without rain until the end of the month. Two very different outcomes. There were very few fires in 2012 as much of the Northwest was wet and cool.
Typically for our region one of these two patterns will dominate with a third pattern not listed or shown here where the ridge axis is more egg shaped and runs north to south across the Western US. These type of patterns typically bring our region hot conditions with lots of thunderstorm outbreaks as disturbances and upper level low pressure areas rotate up from the southwest and interact with the jet stream and instability across our region. Typically though these patterns do not dominate during the summer as the ridge axis almost always takes on either a negative or positive axis.
From what CPC says they believe that the ridge will take on a negative tilt to it once again for the summer of 2018 thus keeping our region hot and dry. From what I have seen I can't say that I can disagree with that but I am not so sure that it will play out that way. My feeling is that we are going to see a bit of a split between the two this summer with the first half of the summer seeing more of a positive tilt ridge axis and stronger Gulf of Alaska low which may bring our region some bouts of cool and unsettled weather followed by a warmer to hotter and drier second half of the summer with a ridge axis bouncing back and forth between neutral (north/south) to negative tilt. How the ridge axis sets up and how the Gulf of Alaska low sets up are the key to what type of a summer we will have. So in conclusion after all that I do think that this summer overall will be a bit warmer than normal though not as hot as the last two summers and also drier than normal though not as dry as the last two summers. As for fire season, the potential is there for more large fires unless we end up with a substantial amount of rain in July and August which, while possible, is not likely. We will see thunderstorm development this summer but how many and how often will be key to fire starts and also how big the rain cores will be with the storms. One difference I do believe we will see this summer is for a warmer late summer/early fall around late August into September with the 4 Corners High likely to hold on a little longer and stronger than the past two years which saw it get knocked down early in September and bring good rains, high elevation snows and much colder weather in September. I don't believe that is as likely to happen this year but time will tell.
So there you have it. I hope this helps to explain and give some guidance on what the upcoming summer outlook for Lincoln and Sanders County looks like. As always there is a big bust potential but will stick my neck out for now and hope all works out. Enjoy the summer and feel free to comment on this article.