9/3/13

Summer of 2013: Projections of a More Comfortable Summer Come to Pass Along with Quite Variable Rainfall Amounts!

Overview

Certainly one of the more comfortable summers than in recent years was enjoyed across Southeast Lower Michigan in 2013. Projected temperatures in my Summer Outlook of around normal readings came to fruition along with why we would see a less hot and humid summer (see upper air pattern discussion).  Careful, that above average temperature for the summer is misleading.


*Summer 2013
Average Temperature (Departure from Normal)
Normal
Immediate Detroit Area
71.9 / +1.1*
70.8*
Flint
70.5 / +1.1*
69.4*
Saginaw
69.7 / +0.7*
69.0*
                                             * SE Mich Ave                            70.7/+1.0                                                          69.7

* Data from NWS DTX  - Summer Normals are computed based on each site's period of record, which is different than the 1981-2010 period used to compute individual monthly statistics 

 Comparing this summer's temperature averages to the past several shows one of the main reasons this summer felt cooler!

Top 20 Coldest/Warmest Summers in Southeast Lower Michigan
 
Rank
Detroit Area*
Flint Bishop**
Saginaw Area***
Coldest
Warmest
Coldest
Warmest
Coldest
Warmest
Temp
Year
Temp
Year
Temp
Year
Temp
Year
Temp
Year
Temp
Year
1
66.5
1915
74.8
2005
65.4
1992
74.2
1933
64.8
1915
73.0
1933
2
67.0
1992
74.5
1995
66.1
2009
74.0
1934
65.1
1992
73.0
1931
3
67.3
1927
74.5
1955
66.2
1958
72.7
1936
65.5
1982
72.5
1955
4
67.5
1875
74.4
2010/11
66.3
1960
72.6
1939
65.8
1945
72.3
1995
5
67.6
1903
74.2
1988
66.5
1969
72.6
1931
65.9
1950
72.1
1930
6
67.8
1985
74.0
1933
66.6
2004
72.6
1921
65.9
1924
72.1
1921
7
67.9
1912
73.8
1949
66.7
1985
72.3
2010
66.1
1985
72.0
2010
8
67.9
1907
73.7
1921
66.8
1972
72.3
1949
66.4
2009
72.0
1988
9
68.1
1982
73.6
1952
66.8
1967
72.2
1955
66.4
2004
72.0
1937
10
68.2
1972
73.5
1991
66.9
1962
72.0
1935
66.4
1979
71.9
1936
11
68.3
1979
73.5
1959
66.9
1927
71.9
1938
66.5
1977
71.7
1998
12
68.3
1902
73.5
2002
67.0
1982
71.8
1988/11
66.6
1951
71.5
1934

                                                    36th   warmest   71.9      2013    25th warmest  70.5  2013   36th warmest  69.7   2013



Perhaps no month displayed the temperature difference between this past summer and several previous summers than July. Note the marked contrast in temperatures and rankings this past July when compared to the past several Julys.


July 2013               Actual    Normal    Departure 
 Detroit                       74.0         73.6          +0.4       
 Flint                           73.0         70.5          +2.5       
Saginaw                     71.7         71.0          +0.7       



Rank & Average Temperature Records for July

Detroit
Rank
Average Temp.
Year
1
79.3
2011
2
79.0
1955, 2012, 1921
5
77.9
1916
6
77.7
1931
38TH       74.0       2013


 
Flint
Rank
Average Temp.
Year
1
78.0
1921
2
77.7
1935
3
76.8
2011
4
76.7
1934
5
76.5
1955
6
76.1
2012
 22ND       73.0       2013



Saginaw
Rank
Average Temp.
Year
1
77.5
1921
2
76.8
1916
3
76.4
2012
4
76.1
2011, 1935
  36TH      71.7     2013


Digging Deeper into the Summer Temperatures 

Ok, so all these statistics show why this summer was considerably cooler and more comfortable when compared to the past several. While it's a given the average temperatures for the cities/summer were still a bit above normal, one must deep a bit deeper to really get the full picture. Where all temperature statistics really above normal over the region? And if not, what statistic wasn't above?

Looking across ALL of Southeast Lower Michigan paints a bit of a more complete picture. Note, average highs this summer actually averaged below normal. So it wasn't the highs that were above normal...and another reason it felt cooler this summer.  During the day (where summer is typically measured by the public) on average, it was actually cooler than normal. It was the average low temperatures that came in above average for the summer. Enough above normal to push the summer averages just above normal statistical threshold when computed.



Even with the heat island influencing Detroit Metropolitan Airport temperatures, check out each monthly average high temperature compared to normal at Detroit. Every month failed to make its normal high. The average high for the summer comes out to be 80.6, nearly a full degree below normal. And, what does this say about areas away from the city?

Location                    June                July               August        Summer
Detroit Ave High      78.5               82.5              80.9              80.6
Normal                      79.3               83.4              81.4              81.4       
Departure                  -.8                  -.9                  -.5                 -.8


Another way people judge the summer is how many 90 degree days occurred? Normally Southeast Lower Michigan "feels" about 8-12 ninety degree days (depending on location). Here's another reason why it felt cooler than the statistics might want you to believe. Detroit had only five 90 degree days less than half its 100 year normal of 12. In addition, those 90s occurred all in one week in July when it was hot and humid. There were no 90 degree days in June nor August...highly unusual, especially August. Flint fared better with twice as many 90s /10/ than Detroit...which looks a bit strange at first glance but then add Saginaw's number of 90s to the mix /8/ and maybe not. (Just for the record, I do believe Flint's ASOS temperature runs about a degree or so hot when compared to surrounding sites). There is another reason Flint and to a lesser extent Saginaw chalked up more 90s than Detroit...moisture in the ground due to wide variances in rainfall between the cities. Simply put; Detroit had heavy amounts of rainfall this summer (see rainfall) whereas Flint just crept up to near normal while Saginaw was substantially below normal. Wet grounds tend to hold temperatures in the summer down a bit...not to mention the increased cloud cover due to more rain.

Number of  90s This Summer
City            M     J      J     A
Detroit        0      0     5      0
Flint           1       2     6      1 
Saginaw    1       1     5      1


 *Hot Spell - Detroit                          *Cool Spell - Detroit 



            July Hi    Lo  Av  Dep                      Aug  Hi   Lo   Av   Dep

  15     93    75    84    10                            12     74    65    70     -3                      
  16     90    77    84    10                            13     69    55    62    -10                     
  17     94    75    85    11                            14     72    51    62    -10                     
  18     94    75    85    11                            15     75    51    63     -9                     
  19     95    73    84    10                            16     79    56    68     -4                         
                                                                                                   
*There were other hot & cool spells during the summer but these two were two of the most notable.

Summer Rainfall Tallies

Summer 2013        /Rainfall/    /Norm/   /Depart/    /Summer 2012/
Detroit                       16.13           9.89          +6.24         3rd wettest summer           
Flint                             8.83           9.57           -0.73               
Saginaw                      6.53           8.94           -2.34          27th driest 
 

The vast majority of the region saw suitable amounts of sunshine along with generous to abundant amounts
of rainfall during the summer. I say the vast majority since there were still some notable pockets of below 
normal rain creating a minority of drier regions. All in all, good weather for gardens and vegetation was had
across the majority of the region with limited areas having rainfall deficiencies, those being mainly in portions
of the Saginaw Valley into parts of the Thumb (see second map; departure map). Ok then, how about too
much rain?

If anything; the occasional circumstance brought too much rain from thunderstorms with heavy to torrential rains which led to briefly saturated grounds. Heavy rains across extreme Southeast Lower Michigan brought predominately wet conditions to the far corner of the state. There was quite a difference in rainfall across Southeast Lower Michigan this summer! Totals (first map) ranged from only four to six inches over portions of the Thumb (mainly Bay and Tuscola Counties) to as much as 15 to 20 inches over parts of Monroe County. Note the departures above normal (second map below; based on radar rainfalls) of up to a foot, mainly over Monroe County. Heavy rains led to Detroit's third wettest summer on record. This summer was a classic example of where rainfall projections of below to above normal are most suitable (see Outlook performance)

                                                           SUMMER RAINFALL AMOUNTS - Radar Estimated





The last few heavy rainfall events were near the close of the summer with heavy rains on August 27-28th and again on the 30-31st. See maps below;







June Rainfall & Departure Map

June rainfall is one of the most important monthly rainfall for vegetation; note the wide variance this past June!


                                                                         

 


Upper Air Patterns of the Summer

The upper air pattern of the summer consisted of strong summer sub-tropical ridge that dominated over the Southern Plains and Desert Southwest which occasionally built east into the Upper Mississippi Valley and Great Lakes Region. This ridge oscillated as far east as the East Coast at times connecting with the summertime Bermuda High. Just as as important the ridge was the other main feature of the summer; a strong, below normal height upper low pressure system that dominated over northeast and eastern Canada. This upper low occasionally retrograded or redeveloped over south-central Canada. It was the interaction of these two patterns that created our comfortable summer. 

In my Summer Outlook it was these two features I spoke about and why I thought this summer would turn out cooler than the past several. From the outlook;


Hemispheric Patterns

As the summer ridge builds into the center part of the country, I look for it to oscillate over the Great Lakes and the East, routinely bringing warm to hot weather to the region. However; it should be duly noted that the distinct upper air pattern that has been a key player since mid winter into spring, albeit naturally weakening with time and becoming less frequent, shows no signs of leaving during the summer. I'm speaking of the occasional blocking across the northern latitudes in which I expect to play a distinctive role this summer.  By occasionally phasing with weaker mid continental impulses, troughing will dive into the Great Lakes and points east and thus, intermittently deliver enough cooler spells of weather to balance out somewhat the heat and humidity we do get, for a more normal and comfortable summer than the past several.




Based on the above upper wind pattern and analogue discussions below, my temperature forecast read as follows.

Temperatures
Recent hemispheric patterns and analogue summers (discussion below) suggest to me a more comfortable summer than the past several. In fact, temperatures should average closer to normal or average between 1.0 degrees below normal to 1.5 degree above.


Analogue Performance

Again from my outlook after researching other analogue summer;


Analogue Trends Seen


Temperatures


The Neutral analogue summers are really quite mixed on the temperature front this time which is hardly surprising given the hemispheric and SST environment. Even though Detroit’s summer data favors normal to below normal temperatures, one must closely compare this data to Flint and Saginaw. The reason being, the normals at Detroit were increased in the past few data sets (1971-2000 & also 1981-2010) due to mainly the effect of the local heat island that now encompasses the Detroit Metro Airport; whereas Flint and Saginaw saw little change in their normals. This would logically then, skew Detroit’s departure temperature data down somewhat. In any event; analogues suggest normal temperatures will prevail.


Rainfall and Severe Weather

Dominant trends past (previous analogues) and recent (past few months) suggest variability in rainfall trends. April was wet while May was dry relative to normal or average rainfall. While the majority of analogue summers lean toward the dry side; a minority were especially wet!  We've already seen this wider range variability recently and it looks like it will continue. When speaking of summer's total rainfall; I look for a range of below normal to above normal given the present and past trends
 
Rainfall averaged decidedly below normal in the analogue set with about 75% below normal...a strong trend. However; the remaining years were very wet and while a minority of years, it still is an important subset of wet years! This suggests to me that the pattern portrayed in my hemispheric discussion could lead to a stalling of the frontal conflict and storminess and thus, runs the risk of repeating in the same general region. While analogues state drier summers prevailed; we still run the distinct risk of a stormier summer including training echoes if the pattern happens to stall in our neck of the woods. In conjunction with this kind of scenario; there is more likelihood of a wider range of rainfall amounts from below normal to above over Southeast Lower Michigan.


In Summary 

The resultant Summer 2013 statistics reflect well my forecast for the summer including the projected upper air pattern and consensus of analogue data. Temperatures across Southeast Lower Michigan were kept in check which led to a more typical summer than recent summers due to the aforementioned upper air projections. Not to be outdone; projected rainfalls across Southeast Lower Michigan of below normal to above normal also materialized, mainly as a result of the forecasted macro-scale pattern, down-scaled for Southeast Lower Michigan. 

Overall one could say it was a normal summer but with some hidden abnormalities (temperatures) and not so hidden (third wettest summer on record at Detroit and possibly worse points south).


Next Up; What's the Autumn Look Like??


Fall Color Time

Fall leaf color is basically caused by lack of sunlight and to a lesser extent is influenced by the late summer weather. However; drier than normal weather for the entire summer into early fall tends to accelerate the leaf changing process, causing the leaves to fall prematurely. Likewise, a wet late summer into September and October will tend to produce fewer vivid colors and the leaves may also fall earlier due to the rain, wind and storms. 

The prime weather conditions which are conducive for brilliant fall colors are warm, sun dominant days and cool, crisp nights but without frosts or freezes; such as high temperatures in the 60s and 70s with lows in the upper 30s to lower 50s. These sharp, daily temperature swings and more importantly, the decrease in sunlight, play vital roles in the development of the leaf color. This combination of weather and lack of sunlight, creates a blocking effect on the sugars which are manufactured in the leaves and keeps them from reaching the root system. Eventually, these sugars convert to pigments that produce the vivid and brilliant colors seen on many trees in the fall. Evidently, the green chlorophyll in the leaves begins to fade during the shorter fall days with subsequently, less sunlight. Thus, the other color pigments already in the tree leaves are exposed, come out and produce the fall color splendor. The yellow color seen in some leaves is created by the xanthophyll pigment, while the orange-red color is caused by the carotene pigment and the red-purple color can be attributed to the anthocyanin pigment. 

While color peak may vary season to season across Southeast Lower Michigan, generally the maximum leaf color occurs during the second and third week in October.

Making weather fun while we all learn,
Bill Deedler - SEMI_WeatherHistorian