Winter of 20 - '21 was a somewhat boring, slow and mild winter with February the only "real winter month" of the three.
There was a rather mild intro to winter from the get-go in December that carried right on through January. Temperatures averaged several degrees above normal both months across Southeast Lower Michigan and it looked as though we were going "skate" through another relatively balmy winter as we had the winter before /Winter of 2019-2020/. All three winter months in that winter averaged above or well above normal. This left Detroit with its 9th warmest winter with 32.6 degrees; Flint had its 5th warmest winter with 31.5 and Saginaw checked in at 29.8 degrees/7th warmest winter.
Our most recent winter didn't change direction until early in February with much colder weather; heralded in by a drastic shift in the predominant upper air pattern to the Arctic. Before the change; a rather persistent upper air troughs routinely pushed into the West Coast; as discussed in my Winter Outlook;
Pacific/North American /PNA/ Pattern
A negative /-PNA/ generally reflects a trough over the western portion of the US with ridging and warmer weather in the East.
The Pacific Jet Stream remained dominant much of the early to mid winter and thus; only occasionally allowed colder, polar air into the eastern part of the country. For much of December and January; colder and stormier weather slammed into the northeast Pacific; then down into the Rocky Mountain range (note white path and arrows in the negative PNA pattern above). This also encouraged a mild to warm, upper and surface wind flow from the southwest and south into the Ohio Valley and Great Lakes.
During February; the PNA shifted to a dominant positive pattern bringing the stored up, Arctic cold in the form of several polar vortexes pushing down into the center part of the country, then spreading out east and south. It was at this time; notable record cold and snow occurred mid month with the influx of the "blue norther".
Statistics for the Winter 2020-21
WINTER TEMPS
2020-21 SEASON SNOWFALL
Winter Forecast & Analogue Performance
Closer to home; let's take a look at the winter statistics for Detroit, Flint and Saginaw and compare it to the Winter Analogue statistics...and most important; the forecast.
Forecast was for:
Temperatures - Normal to Above (or -1.0 to +2.5 temp departure)
Most analogue winters averaged normal to slightly below with an average sitting
at 26.5 (30 year norm @ 27.9 or -1.4 below). Taking into account the
more recent La Nina winters (including Modoki La Ninas), computer guidance
and recent trends; I look for a normal to above normal temperature winter.
This is both for the winter (Dec-Feb) and cold season (Nov-Mar).
This projected milder winter average/departure verified well with the average for Southeast Lower Michigan coming in at 27.6. The normal for the area is 25.8; therefore the departure for the winter in Southeast Lower Michigan was +1.8 degrees. Keep in mind the normal range is for a departure -1.0 to +1.0 for our purposes. Expanding to the cold season, November thru March temperatures were also above normal with November averaging just above normal and March well above normal. Therefore; with the exception of only February; the entire cold season period averaged above normal.
Not only did the Winter Outlook temperature departure verify for the winter; so did the temperature and snow trends.
The Forecast Winter Trend read:
Temperatures and Snowfall:
Most analogue winters averaged normal to slightly below with an average sitting at 26.5 (30 year norm @ 27.9 or -1.4 below). Taking into account the more recent La Nina winters - Modoki La Nina winters- computer guidance and recent trends; I surmise the average of the winter analogues are on the cooler side and therefore; I look for a normal to above normal temperature winter. This is both for the winter (Dec-Feb) and cold season (Nov-Mar). The analogues contained front-end loaded, mid-loaded or back-end loaded winters regarding temperatures (coldest relative to norms) and snowfall (relative to averages). There were twice as many snowier winters (8/4) as there were snowless (see analogues and legends below). Potential for a major snowstorm or two in the snowy La Nina analogues was slightly greater in every month Dec-Mar with a lean toward late season.In descending order; back-end loaded (roughly late Jan-Mar) winters were favored in regard to colder temperature and/or heavier snowfall.
And one more thing; there are enough well below normal (or "snowless")
months that showed up in every set of analogues to be concerned about a
bust on the snowfall prediction but I'll hang tough with the normal to
above - for the time being anyway.
Both colder temperatures and heavier snows did fall in all areas in February; especially across the Flint area. Detroit more than doubled its normal snow for February with 21.6" this was 11.6" above the norm /10.2'/. Up around Flint; snowfall for February was a whopping 29.7. This was nearly triple the norm of 10.8" /+18.9"/. Saginaw recorded 18.1" of snow for February; which was more than double /+9.5/ their normal amount of 8.6"
In spite of the heavy snows in February; the snow totals for the season just caught up to around normal at both Detroit and Flint (see analogue chart - stats for Winter 2020-21) - which shows just how dry and snowless much of the winter was up until February. Saginaw's seasonal snowfall still came in well below normal with 32.3" or 9.2" below normal. Snowfalls at both Detroit and Flint came close to their analogue means (Detroit - 44.9/48.8" & Flint 48.6/52.4"); whereas Saginaw was well below its snowy mean of 52.0" with just the 32.3". The storm track clipped the Flint and Detroit regions with the best snows of the season.
Analogues & Actual Winter 2020-21 Data
Precipitation and Snow Amount Forecast:
The most surprising trend of the winter was the lack of precipitation and a lesser extent; snowfall leading to below normal precipitation for the winter right into the spring and normal to below normal snowfall. This part of the forecast did not verify. La Nina winter's are notoriously wet in the Ohio Valley into the Lower Lakes but not this time with much of this area being drier than average.
Reasoning and deductions as to why the La Nina precipitation and in some areas, snowfall didn't verify along with a longer, mild winter
Map II
Another sideline but just as important was the lighter westerly QBO never dominated during the winter as expected. In fact at one point; it looked as though the QBO may start shifting to an easterly phase (negative) toward spring. Negative QBO's are notorious for aiding colder and stormier winters in the eastern two-thirds of the country. As of this writing; next winter looks more promising for a easterly QBO
Map I
Map II
Map III
Next up shortly the Summer Outlook
Making weather fun while we all
learn,
Bill Deedler
- SEMI_Weather Historian
