LONG RANGE FORECAST THROUGH THE END OF DECEMBER - 12/09/2018
December is both the final part of fall and the beginning of winter. Winter begins this year on Friday, the 21st. This is the shortest day of the year by daylight in the Northern Hemisphere. At the same time, it is the longest day of the year, the first day of summer, in the Southern Hemisphere. By the way, we are already in Meteorological Winter. That began on December 1st.
December typically in Oregon is a very cold and rainy month. Average highs are in the 40s west of the Cascades. 50s do happen frequently, and 60s can occur in warm years especially at the beginning of the month. The coldest daytime high ever in Medford was set on December 10th of 1943 when it was just 6 degrees for a high in Medford. East of the Cascades high temps can be anything from single digits to the 50s depending on the main pattern happening at the time. Overnight lows are typically in the 20s and 30s west of the Cascades and teens and 20s east of the Cascades. Lows for the mountains and east of the Cascades can drop well below zero. To as low as 30 below.
December is the wettest month of the year and water year here in Medford. Average rainfall is 3.5 inches in Medford. This is more rain in one month than we will see on average between May and October. Depending on where you are west of the Cascades, average rain can be as high as 8 inches out on the coast, to 6 inches in Douglas County. Portions of Western Josephine County can see upwards of 10 inches. December is when the snowpack in the mountains really begins to build. Seeing storms drop feet of snow at a time above 4000 feet is not uncommon at all. Crater Lake National Park has had Decembers that saw up to 20 feet of snow fall.
So what does the rest of December look like? Thankfully, both warm and wet. It looks as though we are going to be in a pattern that will persist through the beginning of the New Year that will see storms coming through on regular intervals. These storms will tapped into very rich moisture sources. What we see are Atmospheric River events setting up. This is when a fire hose gets pointed right at us and turned on. And this is coming not a moment too soon. This is badly needed rain and snow. We are back into a severe drought. And while this very wet December as it is shaping up will not pull us out completely, it will absolutely make a major dent in the rainfall shortage we have.
Temps are going to be warmer than average. Not be a whole lot, but enough. This does two things of not. #1. makes it very unlikely we see a White Christmas in the valleys west of the Cascades. #2. we get the perfect kind of snow to build our snowpacks. It will be very loaded with high water content. Last winter the big problem for us was not enough snow. Behind that was another big problem. Snow with low moisture values to it. That is not going to be the case this year it appears. Certainly not now. Snow levels in December are usually right around 3000 feet. This year it looks like we will average out around 4500 feet. This gives us snow where we really need it. Especially at the ski areas. But, it also keeps I 5, Highway 199 out to the coast, and other low elevation highways like 227, 238, and Highway 42 in Coos County snow impact free. And with Christmas and New Years travel coming up, that is going to be great news for a lot of people. That said, anybody with travel plans through the Cascades, especially from Highway 140 north is likely going to be encountering big time snow impacts.