If the weather the past few months seemed unusually erratic -even for spring- with cold spells and frost, hot spells with near record highs, dry spells going on for a couple weeks without rain then only to be deluged in a week’s time or less with a couple inches of rain recently; that was our spring - now how about summer?
Strangely enough, while the summer analogues are initially saying the same thing; variable - the picture becomes clearer as we delve deeper into the analogues and computer guidance. It’s really not surprising that on the surface; the initial data intimates the same thing for a few reasons. We are going into a season that is least likely to be affected by hemispheric patterns, an ENSO that is outright neutral and analogues that seem to reflect various patterns. Since that is the case; further investigation, then the weighted years and trends must be uncovered and decided for this Outlook. After this researching, the following is forecast for the summer...
Above normal temperatures should dominate the overall general pattern with occasional significant cooler shifts in temperatures due to retrograding and/or wavering upper air patterns. Still, I look for temperature departures from normals /1971-2010/ +1.0 to +2.5. For summer trends in temperatures; see Analogue section.
Summer rainfalls and departures are probably the most difficult to predict due to the majority of the rains being convective. That being said; I look for quite variable rainfalls; below normal to above. Current trends suggest the better rains to fall from the Thumb Region to Ann Arbor and southeast to the Michigan/Ohio border. Again; summer trends are discussed in the Analogue section
ANALOGUES FOR LA NINA TO NEUTRAL WINTER TO SUMMER 2020-21
Mainly because of the warming the latter quarter of the 20th century into the early 21st century; analogues from earliest periods have been generally discounted or less weighted than the later period analogues. Basically; many of the earlier analogues have been considered too cool for our present warmer period the past several years. However; all analogues are considered for patterns and timing. Note the average summer temperature of the highlighted warmer summers is 72.5/+0.8
All Monthly trends of the summers reflect cooler Junes, warmer July's and normal August's. When discounting the cooler analogues; then leaning more toward the yellow highlighted years gives a different picture. Preferred, highlighted Junes averaged above normal, 70.6/ +1.2; July's 74.5/+0.9 and August's 72.4/+0.4. All averaged above normal with June's the largest departure at +1.2. July was a close second with a departure of +0.9. while August came in at +0.4. Keeping one eye on the analogue data and the other on the projected upper air maps; early to mid summer has the best chance to average above normal with mid to late summer around normal. This matches well with a retrograding ridge mid to late summer and troughing also shifting west with time. (see maps).
Rainfall is again, like many summers; feast or famine. When all rainfalls are averaged; they average near normal (within a inch), 9.24"/-0.65; but there still was a decided slant to below normal by the departure and four summers being drier than average (more than an inch below normal). Only one summer was wetter and the other three were near normal. I feel there is a stronger than average case for below, above and normal rains this summer - location, location, location.
Generally upper high pressure is a dominant player during the summer; allowing only intermittent low pressure and surface fronts through the country. If upper air projected patterns are correct; this should become more common later in the summer. Upper air guidance from the CFS for the season and monthly shows the following scenario.
High pressure aloft focuses on the east-central area of the country early in the summer with a gradual retrograde westward mid-late summer. Note each month's map.
1- Late May into June; a semi-permanent, subtropical area of high pressure in the North Atlantic off the East Coast, normally spreads west across the Eastern and Central U.S.
2- As the high pressure retrogrades westward; this allows troughing into the Northeast with time.
3- Later in the summer, troughing shifts westward into the Lakes.
Maps from CFS for temps/pcpn for the summer
Enjoy the summer! Look for other notable weather write-ups and trends through the summer if conditions warrant.