As we wrap up the winter season and head into the heat and humidity of summer, let’s take a look back at what was expected this past winter and how the season unfolded.
Going into the winter, a weak El Nino was expected to develop in the Pacific Ocean, but there was some uncertainty as to how soon the warming would occur this past winter. To refresh, El Nino is an abnormal warming of the waters in the Equatorial Pacific, while La Nina is a cooling of those same waters. Typically a weak El Nino pattern will bring the following,
Warmer temperatures in the Western US, with cooler temps in the North-Central and Northeast.
Near normal temperatures in the Central Plains.
Dry weather in the Northwest and South-Central U.S.
Wet weather along the East Coast and across the Southeast.
The Iteris Winter Outlook called for temperatures similar to the typical weak El Nino pattern, with above normal temperatures in the west, and below normal temperatures in the North-Central and northeast. Cooler than normal temperatures were expected in the Southwest. Near normal temperatures were forecast from the High Plains into the Southern Plains and eastward into the Mid-Atlantic. As for the Precipitation Outlook, the Eastern Seaboard and Southeast were forecast to see above normal precipitation, with below normal precipitation across the West, Midwest and Southern Plains. The Northern Plains, western High Plains, and Great Lakes areas were expected to see near normal precipitation.
So what happened?
Well, as predicted, a weak El Nino pattern did develop during the winter. The developing El Nino had the expected effect on the overall weather pattern across the US for the first part of the winter, however the Pacific/North American Pattern (PNA) shifted from a slightly positive phase to a slightly negative one from January into February. This pattern shift caused much colder than normal temperatures across the West and Northern/Central Plains for the latter part of the winter, and also brought warmer than normal temperatures to the southeast. (The PNA is another one of a handful of atmospheric patterns or indicators we look at to determine long range weather outlooks). This unexpected pattern change for February completely threw the temperature outlook for a loop, especially across the West, Northern Plains, and Southeast. The cold was so extreme that Montana and North Dakota saw the 2nd coldest February since the late 1800’s, and South Dakota saw its 3rd coldest February. On the flip side, the Southeast U.S. had a top ten warmest February.
On the precipitation side of things, the vast majority of the country received above normal precipitation. The only areas receiving below normal precipitation were portions of the Western Plains that are in the rain shadow of the Rocky Mountains, from eastern Wyoming, south into western Texas and eastern New Mexico. This was a big change from what was expected going into the winter season. The predominant ridge that was set up across the western US heading into winter (which pushed the jet stream well to the north) broke down, allowing the jet stream to flow right into California and on into the Central and Northern Plains, before pushing back to the southeast into the southern Midwest and then lifting up the East Coast. The shift in the jet stream allowed for numerous storm systems to move through the southwestern US, and on into the central US before exiting through the mid-Atlantic and northeast. The shift happened in accordance with the PNA, as much of the western US had received below normal precipitation until late in the winter season. Then February hit and record snowfall/precipitation was recorded in many spots in the west. Some of the mountains in the Sierra Nevada’s recorded over 25 feet of snow in February. Snow, along with strong winds, were prevalent across the Plains in February as well, which saw several blizzards move through the region. Further to the east, the warmer temperatures created less snow for portions of the southern Great Lakes into the mid-Atlantic region, but heavy rainfall kept the precipitation above normal.
In general, the outlook looked good during the early portion of the winter, but due to one unexpected change (the PNA phase shift), the outlook performed poorly in terms of both temperature and precipitation.
Looking towards the summer
So what is possibly in store for the summer? A neutral or weak El Nino is expected to continue into the summer months, and possibly into the fall as well, although this is less certain. There will be higher probabilities for above normal temperatures across the West, South, and East, while the Central/North-Central will see equal chances of above/below normal temperatures. Chances for above normal precipitation stretch from the Interior West, eastward into the Central/Southern Plains and the Southeast/Mid-Atlantic regions. The Southwest, Upper Midwest, and New England regions will see equal chances for above or below normal precipitation. There is usually a decrease in tropical storm activity in the Atlantic Basin when an El Nino is in effect. That doesn’t mean that hurricanes or tropical storms won’t affect the U.S. this summer and fall, it just means that there will likely be less storms overall, and therefore a slightly lower chance that a given area will be affected.
For a more detailed look at last winter and summer’s forecast, join us for our Winter Recap and Summer Outlook webinar on May 16, 2019 at 10:00 am CT. Register here.
About the Authors:
Rob Pinard has over 20 years of surface transportation weather forecasting experience and is currently the Forecast Operations Manager for Iteris. Mr. Pinard sees to the day to day operations within the weather operations group and continues to work rotating forecast shifts.
Stewart Carpenter joined Iteris in 2014 as a Lead Forecast Meteorologist. In this role, Mr. Carpenter supervises the meteorologists on shift, while working closely with our partners at the Virginia DOT and Maryland SHA.