Probable Maximum Precipitation Needs Assessment for Montana

Issues

• Probable Maximum Precipitation (PMP) is used nationwide by States and Federal Agencies to design spillways

• Since 1940, the National Weather Service has been providing guidance for determining PMP’s

• Most PMP documents require updating (last updated 1963 to 1994)

• Funding to the National Weather Service to update PMP documents has ceased.

• National “Extreme Storm Events Work Group” looking into problems with outdated PMP documents (last meeting May 2014 – where is this going?)

Recent Actions• States have taken the initiative to develop regional updated PMP

estimation methodologyOhio Texas (underway)Nebraska Virginia (underway)ArizonaWyomingColorado (underway)New Mexico (underway)

• Regional PMP studies are expensive!• Wyoming - $800,000• Colorado - $1,000,000

• The benefits to Montana of a regional PMP study is unclear

• The financial feasibility of a regional Montana PMP study is unclear

• Montana has been advised to complete a “Needs Assessment” to determine if completing a Montana regional PMP study has merit and is feasible.

State of Montana Spillway Standard – based on potential for downstream loss of life

Loss

of L

ife

Recurrence Interval

500 5000

5

PMP1000

State of Montana Spillway StandardLo

ss o

f Life

Recurrence Interval

500 5000

5

PMP1000

Most Montana Dams

State of Montana Spillway StandardLo

ss o

f Life

Recurrence Interval

500 5000

5

PMP1000

• Federal Dams• FERC regulated

dams• Tailings Dams• State regulated

dams with large population at risk

State of Montana Spillway StandardLo

ss o

f Life

Recurrence Interval

500 5000

5

PMP1000

State regulated dams with moderate population at risk

Needs Assessment Components

1. Evaluate benefits of regional PMP for Montana• # impacted dams – is it great enough to warrant expenditure

2. Value of having updated PMP methodology• Other States report updated PMP’s are “better science, but not necessarily

lower values” Makes it a difficult sell to dam owners

3. Obtain perspective of other Montana Dam Owners• Federal Montana dam owner perspective on issue is unknown• FERC regulated Montana dam owner perspective on issue is unknown

4. Tailings Dams – New legislation requiring PMF for all dams (sponsored by Montana Mining Association)• Mining association perspective on issue is unknown

5. Evaluate problems with existing PMP methodology

6. Financial Feasibility of completing regional PMP study• Legislative funding highly unlikely (i.e. 0% chance)• Investigate other funding options• Mining association contribution?