La Nina Springs and Severe Weather in Southwest

MichiganDoes having a La Nina

Impact Severe Weather Frequencies?

What We Will Look At

• Tornadoes frequency since 1950• Trends of Tornadoes since 1950• Mean Number of Tornadoes by Phases of ENSO• Tornado Frequency as a function of ENSO phase• Severe storms by season for La Nina and El Nino• Severe Storms by Season for ENSO phase• Possible reason for difference in numbers by ENSO type– Mean Jet Stream for La Nina– El Nino in winter and spring

Tornado Trends By Year for the GRR CWA

19861987

19881989

19901991

19921993

19941995

19961997

19981999

20002001

20022003

20042005

20062007

20082009

2010

02468

101214161820

Tornaodes By Year and Strength

F0 F1 F2 F3 ALL

Year

Num

ber b

y Ye

ar

Tornado Numbers and Winter NAO

1950195219541956195819601962196419661968197019721974197619781980198219841986198819901992199419961998200020022004200620082010

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

0

5

10

15

20

25

30

35

40

45

50

NAO compared to Tornado Numbers

NAO for the Winter Polynomial (NAO for the Winter)Tornado Numbers Per Year Polynomial (Tornado Numbers Per Year)

Axis Title

Axis Title

Tornado Trends Since 1950

19501952

19541956

19581960

19621964

19661968

19701972

19741976

19781980

19821984

19861988

19901992

19941996

19982000

20022004

20062008

20100

5

10

15

20

25

Tornado Trends Since 1950

All Tornadoes Moving average (All Tornadoes) F2 Tornadoes F3 Torandoes

Year

Num

ber P

er Y

ear

All peak years for Tornadoes, since 1950, had a moderate or strong La Nina the preceding winter.

Tornado – ENSO Index – NAO IndexTornado Peaks - NAO - ENSO

NAO ENSO Year Severe TORNADO0.67 -1.2 1956 41 23-0.08 -1.6 1976 90 230.47 -0.8 1965 51 20-0.20 -0.7 2001 159 190.21 -0.4 1967 38 16-0.76 0.6 1977 35 15-0.39 -1.8 1974 58 140.42 0.4 1991 106 14-0.49 -0.1 1979 52 13-0.30 -0.7 1968 54 13-0.91 -1.8 1989 45 130.31 -0.6 1975 97 11-0.11 -0.5 1986 37 11

Tornado Trends

• No statistically significant change in tornado numbers, by year since 1950– There is an average of 12 years between peaks and

minimums in yearly Tornado Number• Seems to be some relation to the NAO – NAO negative , tornadoes are more frequent– NAO positive , tornado numbers are suppressed

• La Nina actually is a bigger player for increased tornadoes

Compare Mean ENSO for High and Low Numbers of Tornadoes

Low T Avg High T Avg

-0.80

-0.70

-0.60

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20ENSO Mean Compared to Tornado Frequency

Yearly Tornadoes

El Nino

La Nina

Tornadoes and ENSO

• Now we will see if ENSO Phase makes any difference to tornado frequency– We will look at this by season for each phase of

ENSO

Tornado Frequency By ENSO Event

La Nina Neutral El Nino0

1

2

3

4

5

6

7

8

Avg # TOR

Avg # TOR

Type of ENSO Event

# of

Tor

ando

es

Results

• For La Nina, there is a statistically significant increase in tornado frequency at the 95 % confidence level

• All of the peaks in tornado frequency since 1950 have been years with a moderate or stronger La Nina during the winter preceding the event

• For La Nina we average about 8 tornados per season since 1950

• For El Nino we average around 5 per year since 1950

Now Let’s Consider Sever Storms in General

• Does ENSO phase have an impact of severe storm frequency?

• Does ENSO phase impact which season of the year severe storms are most prominent?

Severe Storm Frequency for the GRR CWA since 1986

19861987

19881989

19901991

19921993

19941995

19961997

19981999

20002001

20022003

20042005

20062007

20082009

20100

50

100

150

200

250

# of Severe Storms by Year

# of Severe Storms by Year Moving average (# of Severe Storms by Year)

Axis Title

Num

ber o

f Eve

nts

Severe Weather Trends

• There is a statistically significant increase in severe storm frequency since the 1950’s to the past 10 years

• Most of this increase is related to how we gathered the data– Different MIC’s had different methods to gather the data

• Charles Snyder …1970’s and 1980’s• Dean Gulezian…..1990’s• Dan H. ……………..1996-2007• Dan C. ……………..since @2007

La Nina And Storm Frequency By Season

Winter Spring Sum Summer Fall Sum Sum0

20

40

60

80

100

120

140

La Nina Severe Storm Frequency

Ave Strong Avg Mdt Avg Weak Normal

Season

# of

Sto

rms

EL Nino Storm Frequency By Season

Winter Spring Sum Summer Fall Sum Sum0

20

40

60

80

100

120

140

El Nino Severe Storm Frequecy by Season

Ave Strong Avg Mdt Avg Weak Normal

Season

# of

Sto

rms

Does ENSO Impact Seasonal Storm Frequency ?

Winter Spring Summer Fall Sum

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

Severe Storm Frequency for the GRR CWA /ENSO Type by Season

La Nina Mdt El Nino Mdt

Season

Freq

uenc

y Co

mpa

red

Nor

mal

Trends in Severe Storms Due to ENSO Phase

• For both winter, spring, and summer La Nina has a significant increase in the numbers of events compared to El Nino– Average 6 more in the winter, compared to El Nino– Average 8 more in the spring, compared to El Nino– Average 10 more in the summer

• Fall sees a suppression for both compared to ENSO neutral

• For the entire season there is typically 24 more severe storm events during La Nina years

Let’s see why there would be a difference

We will look at the 300 mb mean jet core for

both La Nina and El Nino years

Moderate or Strong La Nina Winter Mean 300 mb Scalar Wind Speed

Moderate or Strong El Nino Winter Mean 300 mb Scalar Wind Speed

Moderate or Strong La Nina Spring Mean 300 mb Scalar Wind Speed

Moderate or Strong El Nino Spring Mean 300 mb Scalar Wind Speed

*Spring is March through May, but most of our severe storms are May

through August

*So, lets look at the jet core in May to see how far north it gets during a typical La Nina Spring*

300 mb Jet Core in MaySW MI in the entrance region of Jet

Note the 850 mb jet location for May during Moderate La Nina’s

*It would seem this may be what we are looking for

• The low level jet aimed at SW MI• The exit region of the polar jet is just

to our NE

Conclusions

• There is a statistically significant increase in severe storms and tornados during La Nina years

• Seems the jet core is farther north in both the winter and spring during La Nina years

• Since severe storms are typically related to high winds and jet level, it would make sense the Grand Rapids CWA would see more frequent severe storms during La Nina years than for El Nino year