Fall-like weather pattern ahead

Could there be a chance for the first frost and freezing temperatures in September?

A cold front marking the boundary of the coolest air mass of the season thus far will pass through the Upper Great Lakes region late Monday (September 17) into Tuesday (September 18), bringing a period of rain and possible thundershowers in Michigan. The frontal passage signals the development of a deep upper air troughing-feature across the region that will lead to cool, unsettled and very fall-like weather for much of the upcoming week. Look for temperatures Wednesday through Friday (September 19-21) to fall back to well below normal levels from the upper 60s far south to the low and mid-50s north. Low temperatures will generally be in the 40s except for Wednesday morning, when relatively clear, calm conditions may lead to frost and freezing conditions across areas of the Upper Peninsula.

Given northerly flow and subfreezing temperatures only a few thousand feet above the ground surface, lake effect rain showers are likely on a daily basis through the end of the week, especially in the favored lee sides of Lake Superior and Lake Michigan. Rainfall totals through Saturday morning (September 22) are expected to remain less than 0.25 inches in most areas, with somewhat heavier, 0.25 to 0.50 inches totals across northern sections of the state. Another cold frontal passage and reinforcing shot of cold air is expected by late Friday or early Saturday, possibly leading to a more widespread threat of frost and freezing temperatures across at least northern and central sections of the state Sunday and Monday morning (September 23-24), given the development of clear, calm overnight conditions (monitor latest forecasts for more details).

Recent medium-range forecast guidance has been consistent in calling for the upper air trough across the Great Lakes region and this week’s cool weather. Both the current NOAA Climate Prediction Center 6-to-10- and 8-to-14-day outlooks (for September 22-26 and September 24-30) call for below normal mean temperatures and near to above normal precipitation totals. However, the latest version of the medium range forecast guidance hints that the upper air trough may shift eastward and out of the region by near the end of next week. Should that occur, we would be looking at warmer (potentially even above normal) temperatures.

Despite the outlooks calling for relatively cool weather in the next week or two, the majority of recent, long lead forecast guidance suggests that the next few weeks and the majority of the fall season will be warmer than normal. NOAA Climate Prediction Center 30-day (for September) and three-month (for September through November) both call for increased odds of above normal mean temperatures for Michigan and the entire Midwest region. Note that even during relatively warm falls, there are nearly always occasional outbreaks of cold, Canadian-origin air into the region (such as during the upcoming week), which means it is very difficult if not impossible to forecast the first freezing temperatures of the season with any accuracy more than several days in advance.

There is relatively less direction in the precipitation outlooks with a forecast of the climatology “equal odds” scenario of below-, near- and above normal levels during September for all of the state except the western half of the Upper Peninsula, where below normal totals are expected. During the September through November period, the equal odds scenario is forecast state- and region-wide. As noted in earlier articles, warmer than normal sea surface temperatures across eastern sections of the equatorial pacific suggest the potential development of an El Nino episode during the next several weeks. If the event does materialize as expected, and is at least moderate intensity, the climatological statistics favor the upcoming winter and early spring seasons to trend milder and drier than normal.

This is the current thinking at the Climate Prediction Center, with the official outlooks for the winter season currently calling for milder than normal mean temperatures across nearly all of the northern continental USA including Michigan, and for normal to below normal precipitation totals across much of the Ohio Valley and southern Great Lakes regions.

Dr. Andresen’s work is funded in part by MSU’s AgBioResearch.

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