Mid-Summer 2003 Update for Southeast Lower Michigan

Written by: William R. Deedler, Weather Historian
National Weather Service Detroit/Pontiac, MI
July 28th, 2003

This summer's weather thus far has been fairly comfortable across Southeast Lower Michigan with more generous rains than last summer. This is in keeping with our local Summer Outlook which overall, continues to be on track.

RECENT TRENDS:

Temperatures across Southeast Lower Michigan have, on the whole, averaged normal to below for the first six weeks of the summer months. While there have been some hot days, thus far this summer has been not as warm (nor as uncomfortable) as last summer. In fact, overall temperatures for June averaged below normal across the region and so far in July, readings have oscillated from above normal to below producing near normal results. This pattern of the first part of the summer starting off cool did indeed materialize, thus matching the trend noted in the in-house study and stated earlier in the Summer Outlook:

"While most of the past several months have averaged cooler than normal due to an aggressive northwesterly flow, the summer's average temperature is still expected to fall within the normal range. The local data...also suggests the best chances of normal to above normal temperatures will come mid to late summer. Generally, the coolest weather (or best chances for below normal temperature departures) was in June or in some cases, even into early July, where the dominant cool upper flow persisted the longest. The warmest weather (or best chances for above normal temperature departures) was generally seen in July and/or August. Still, this summer is not expected to be as warm as last summer nor contain as many ninety degree days."

The convective rainfall pattern so far this summer still reflects the busy winter and spring storm track into the Ohio Valley and Southern Great Lakes. Though seasonally weakening this summer, the storm track continues to be impressively active. Several systems have brought rounds of heavy convective rains into the Northern Ohio Valley and occasionally further north, into Southeast Lower Michigan. Due to the expected only gradually weakening of the northern jet stream and an active storm track in the Great Lakes and Northern Ohio Valley, the following scenario was proposed in the Summer Outlook as far as expected precipitation trends:

"The dominant storm track still favors the Northern Ohio Valley (wet conditions have dominated across the Ohio Valley) . As a summer pattern establishes itself, it is likely this pattern will retreat further north into the Great Lakes (similar to that of 1973). This would support rainfall across the region to continue to more generous (but more scattered) into the summer months...unlike the past few summers. This, along with more intermittent and weaker northwest upper flow (that would still supply the occasional clashing of air masses) and this would point to a normal, if not locally above normal, rainfall scenario"

What about the remainder of the Summer?

Late spring, the Climate Prediction Center /CPC /initially spoke of La Nina conditions developing (or increasing) during the summer months. Recently however, the CPC stated latest sea surface temperatures /SST/ over the central and eastern Pacific Ocean have once again warmed to near normal levels (or a Neutral state) and thus rescinded the earlier outlook:

"Current atmospheric and oceanic conditions do not support the development of La Nina in the next few months. Negative sea-surface temperature anomalies weakened across the central and eastern equatorial Pacific during June, resulting in an upward trend in SST anomalies in all of the Nino regions "

It was stated in the Summer Outlook that an overall Neutral pattern had the least affect on our summer. The rest of the summer still looks to be on track, with temperatures overall around normal, if not a bit above. The in-house study did suggest that mid to late summer had the best chance to be the warmest and bring the summer average temperature up a bit, closer to normal, with the first half of the summer the best chance to be the coolest (or below normal).

Little change is anticipated in our summer precipitation outlook with most areas receiving near normal to locally above normal rainfall. As with most summers with convective type precipitation, there may still be areas of below normal rainfall but they are not expected to be as extensive or notable as in the past few summers. It should be noted, however, we have yet to see our " normally" (87% of the time) occurring summer dry spell, when in a two to four week span, less than a half inch of rain falls. In another in-house study, it was found that the best chance for a dry spell historically is approaching and is from mid August into early September with mid July to early August a close second. (For more information, check our this earlier study titled "Summertime dry spells in Southeast Lower Michigan are more common than you might think".)

Look for the summer review write-up (including a preview to the fall) in September.


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