Wednesday, January 31, 2018

Winter 2017-2018 Update - It's Not Over Yet

We are two-thirds of the way through climatological winter (December-February), and it has been a rather unusual one so far in many different ways. Winter weather won't necessarily stop in March, or even April (or in one or two recent cases, May), but for statistical purposes December through February is our target period.

Winter weather got off to a late start in most of the country as much warmer than normal weather prevailed in much of the country. The big news in December prior to Christmas was the tinder dry weather in the west and the devastating wildfires in California. There was also some heavy snow across parts of the southeast the second week of December.

The weather pattern changed dramatically the last week of December, and cold Arctic air spilled into much of the eastern half of the country. It persisted through the the middle of the month with one or two brief breaks. This pattern change also brought much needed rain to California. Unfortunately the rain triggered deadly mudslides as it washed away hillsides left bare by the earlier wildfires.

It was during this period the snow cover across the U.S. was at the highest of the season so far with 53.6 percent of the lower 48 states with snow on the ground. This was also the time when all 50 states had measurable snow on the ground. There was a notable lack of snow in the Cascades and Sierra Nevada.

The Accumulated Winter Season Severity Index (AWSSI) in mid-January depicted a severe to extreme winter in much of the eastern U.S. as well as in the northern Rockies. Below normal snowfall in the Midwest and Plains contributed to the average to moderate categories there, despite cold weather.

So, here is how we stand at the end of January. Temperatures have been normal to below normal across most of the country east of the Rockies., while it remains warm in the western U.S.

Season-to-date snowfall is an odd picture, with much above normal snowfall in the Gulf Coast states the southeast, and in the northern Rockies. Snowfall is near to above normal around the Great Lakes and New England. Snowfall is well below normal in the northern Plains, central and southern Rockies, and much of the Midwest.

As of today snow cover was down to about 25 percent across the U.S., half of what it was two weeks ago.

The precipitation map demonstrates the disparity in winter precipitation across the country to date. It has been dry in much of the west, and very dry from eastern New Mexico through the Texas and Oklahoma panhandles.

The dry weather in California, in what is normally the wet season, is having a significant impact on snow in the Sierra Nevada, crucial for water supplies the rest of the year. Snow water equivalent (SWE) is approaching record lows for a number of locations for this time of the season. SWE is also very low further east in the central and southern Rockies.

The last two weeks of mild weather is also reflected in changes in the AWSSI with a drop of one category in the Midwest and in the east.

Winter weather looks like it will make a return during the first two weeks of February, at least, so there will be more to factor in before this winter is over. A broad scale trough is forecast to reestablish over much of  the U.S. in the next week, funneling more Arctic air into the country.

500 millibar forecast map for 6:00 p.m. Thursday, February 8, 2018.

Monday, January 15, 2018

Singing Ice

Many people don't find a lot to like about winter weather. I'm good with everything about winter except the occasional howling winds, and freezing rain. Even if you aren't a fan of winter weather, there are a lot of interesting phenomena to observe during cold weather. Many of you have probably seen the "throw boiling water into the air when it's -20°F" video as one example. Here is a video I came across the other day which describes the physics behind the phenomenon of "singing ice".  This video was produced by National Public Radio science and hosted on their Skunk Bear YouTube channel. It's a worth a watch!