Summer Forecast (July through September)

National Weather Service Summer 2010 Outlook

For Southwest Lower Michigan

By William Marino
Forecast Overview

The official National Weather Service forecast for July through September of 2010 for Southwest Lower Michigan from the Climate Prediction Center (CPC) calls for an equal chance (EC) for above, below, or near normal temperatures (Fig.1) and for precipitation (Fig.2). 

Normals

Normal temperatures are calculated averages from the 30-year period of record from 1971 to 2000. Table 1 lists normal values of temperature and precipitation for the summer months of July through September for Grand Rapids, Lansing, and Muskegon.

 

Forecast Reasoning

 

The forecast for this summer is primary based on the interaction of ENSO with the polar jet stream, whose dominant position affects the weather patterns over Southwest Lower Michigan. Beside the ENSO factor, the persistence of seasonal trends is strongly considered in these forecasts as was the dynamical models and soil moisture anomalies.         

 
Temperature
 

            An El Niño prevailed through the winter months of 2010, but as of early May it had weakened and was expected to fade to neutral by late spring.  By midsummer La Niña conditions were expected to prevail across the tropical Pacific.  Warmer than normal temperatures typically prevail over Southwest Lower Michigan during La Niña summers (Fig. 3).  However the ensembles of the Global Coupled Ocean and Atmosphere Model (CFS) show below normal temperatures (Fig. 4) over most of central and northern United States for July through September, with the center of the cold anomaly just south of the Great Lakes. The CPC consolidation tool shows above normal temperatures across most of the contiguous United States (Fig. 5). Lower Michigan has some highest probabilities of any part of the contiguous United States for above normal temperatures.  Note that the skill of this model is the best of any of the tools used by CPC and typically verifies better than their hand drawn forecasts. Above normal soil moisture over the central United States was the primary reason the hand drawn temperature forecast shows any area below normal.  Combining all these tools together CPC is forecasting an equal chance of above normal, near normal or below normal temperatures for Southwest Lower Michigan.

 

 

 

Fig.1. The CPC forecast summer 2010 temperature anomaly.

Fig. 2.  The CPC forecast summer 2010 precipitation anomaly

 


Fig. 3. CPC composite of temperature trend for a La Niña summer.

 

Fig. 4. The CFS temperature anomalies forecast for July through September.

 

Fig. 5. Consolidation forecast from CPC for July through August of 2010.

 

  Looking at our local tools for this summer’s forecast temperatures we first considered all years when there was a moderate or strong El Niño during the previous winter that faded to neutral by late spring then to a La Niña by late summer (Fig. 6.). This tool shows above normal temperatures will prevail across the Southwest Lower Michigan. There are nine years in this analog set. Of those nine years, six had warm summers, one had a near normal summer and two had colder than normal summers. One of the cold summers, 1992, was the year that Mt Pinatubo’s dust in the stratosphere caused global cooling. That summer was over four degrees colder than normal area wide. It was the coldest summer of our 115 year database for all of Southwest Lower Michigan.  The other summer that was below normal, 1958 was only 0.4 degrees colder than normal. The mean anomaly for all nine summers is 0.7 degrees above normal. The mean for the six warm summers is 1.7 degrees above normal, which is more than one standard deviation from normal (1.4 degrees).

 

 

 

Fig. 6.   Summer temperature anomalies for all Southwest Lower Michigan when a moderate or strong El Niño in the winter that faded to either neutral or a La Niña by the summer.

             The composite temperature anomalies from the reanalysis database from NCDC (Fig. 7) using those same years featuring the same ENSO trend (9 years) shows warmer than normal temperatures prevailing across all of Lower Michigan with the warmest anomalies near the Lake Michigan shore. This matches the CPC composite (Fig. 3) over the Great Lakes nicely. Figure 7 also shows no below normal areas, which is very similar to the CPC consolidation forecast (Fig. 5).

Looking at all the summers that followed cold winters, only 21% were warmer than normal. Cold summers followed cold winters about 35% of the time, while near normal summer temperatures followed cold winters 48% of the time. This seems to suggest that near normal summer temperatures are the most likely outcome for this summer. 

 

Fig. 7. Composite temperature forecast for the summer of 2010 based on years when there is a moderate or stronger El Niño during the previous winter that fades to neutral by late spring and finally to a La Niña by late summer or early fall.

Fig. 8. Summer temperature anomaly if the winter is warmer than normal.

However, warm summers followed warm winters 77% of the years. Cold summers followed warm winters about 13% of the time. None of those four years were years with the same ENSO trend as this past winter.   Near normal summers follow warm winters 10% of the time. Like the cold summer that followed warm winters, none of those happened when a moderate or strong El Niño during the winter that then faded to neutral or La Niña by summer.  If there is a warm winter, a cold summer would appear to be a very unlikely outcome considering the correlation to ENSO.   Given that Southwest Lower Michigan had a warm winter in 2009/2010 season we therefore expect a warmer than normal summer seems to be the most likely outcome.

Since January of 2006 sixty two thirds of the months were warmer than normal, tenth of the months were near normal, and about a quarter of the months were colder than normal. This tells us warm months occurred nearly three times a frequently as cold months and nearly seven times more frequently than near normal months. This too would suggest a warmer summer is more likely

            Both the CPC data and our down scaled forecast tools overwhelmingly suggest warmer than normal is more likely than colder than normal or near normal. This is also supported by the CPC Consolidation forecast.

 
 
Precipitation
 

            Precipitation trends are less obvious than temperature trends. La Niña summers that followed El Niño winters were to be drier than normal for most of Southwest Lower Michigan. However, areas south of Interstate 96 and east of Route 131 (Fig. 9) tended to be closer to normal.

 

 

 

Fig. 9.   Composite Precipitation anomalies in inches versus the 1971-2000 long term average for all ENSO neutral summers that followed a El Niño winter that faded to either neutral or a La Niña.

Precipitation over the past fifteen years compared to the 1971 to 2000 normal period show no significant trend. This can be seen in Figure 10 shows the summer precipitation departure from normal for the 36 long term climate stations from 1995 through 2009. There were four summers with below normal precipitation, three summers with above normal precipitation and eight summers with near normal precipitation. 

 

 

 

 

 

 

 

 

Fig. 10. Summer precipitation departure from normal from 1995 through 2009 for all available Southwest Lower Michigan climate stations.

 

            Comparing the past fifteen years to the 1971 to 2000 normals period, we see a trend toward drier summer closer to the Lake Michigan shore (Fig. 11).  

 

Fig. 11. Summer composite precipitation anomalies for 1995 through 2009 versus 1971 to 2000.

 

            The CPC consolidation forecast for precipitation matches our analog year, in that it shows drier than normal over most of Southwest Lower Michigan except for those areas south of Interstate 96 and east of US-131. 

 

 

 

Fig. 12. Consolidation forecast for precipitation from CPC for July through September

 

 

Combining the ENSO signature with the trend, its concluded that while there is a slight preference for dry summers, there really is no particular preference one way or the other,   Hence our summer precipitation forecast calls for the “EC” forecast. 

 

Severe Storms

 

For the severe weather frequency, during the analog years, there is a slight increase in the frequency of summer storms but a decrease in fall storms. If you add the entire year’s severe storms together for the analog years the average is 110 severe storm events, while for the analog years it’s 112. Clearly there was not a significant difference between the analog years and the mean for all years in our severe storm data base.

 

 

Fig. 13 . Total severe storm events for the analog years compared to the mean from 1985 to 2009.

 

Summary
 

When all of these influences are considered together for Southwest Lower Michigan, the result is a preference for temperatures to average above normal. The precipitation forecasts is for an equal chance for above, below, or near normal.

 

 

 

 

 

 

 



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