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Meteorological Influences on Arizona Precipitation. Mark Sinclair Meteorology Dept. Embry-Riddle Aeronautical University. Talk outline. How precipitation occurs Winter storms The summer monsoon El Niño/La Niña Global warming Precipitation trends. What causes precipitation?. We need

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Meteorological Influences on Arizona Precipitation

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meteorological influences on arizona precipitation

Meteorological Influences on Arizona Precipitation

Mark Sinclair

Meteorology Dept.

Embry-Riddle Aeronautical University

talk outline
Talk outline
  • How precipitation occurs
  • Winter storms
  • The summer monsoon
  • El Niño/La Niña
  • Global warming
  • Precipitation trends
what causes precipitation
What causes precipitation?
  • We need
    • An abundance of low-level moisture
    • A lifting mechanism
      • Caused by a combination of atmospheric dynamics and local terrain effects
  • In winter (October-May), most precipitation is from winter storms
  • Summer precipitation comes from the southwest Monsoon (July-Sept)

Two precipitation regimes

Summer monsoon

  • 35% of PRC precipitation occurs during the summer monsoon
  • The remaining 65% comes from winter-type storms

Annual average for Prescott is 19 inches

winter storms
Winter storms
  • In winter, precipitation is from cyclonic storms
  • Lifting occurs east of migratory upper level troughs (next slide)
  • Is helped by warm, moist low-level airflow
  • Winter precipitation supplies much of the water needed to recharge the water storage systems of Arizona and replenish soil moisture
    • Less runoff and evaporation in winter cf. summer
    • Slow snow-melt especially good for recharge

The recipe for winter precipitation in Arizona

An active trough over S. California

Upper-level flow

Lifting and precipitation occurs in shaded region

Warm sea temperatures off coast

Warm, moist low-level flow


Winter-type storms in AZ

Prescott precipitation

Monthly frequency of mid-latitude cyclones

Orange represents monsoon precipitation


Strong cyclonic storms near AZ since 1953

A weak correlation between winter cyclones & Prescott precipitation

Prescott winter (NDJFMAM) precipitation since 1950

Running mean

Individual years

Below normal every year since 1994


  • Note that, although cyclonic storms have been increasing slightly, Prescott winter precipitation has decreased in recent rears
  • A cyclonic storm does not guarantee a major precipitation event
  • Wind direction relative to terrain is important
  • Sea temperatures off California also important
  • Arizona has warmed in the last 100 years, resulting in less precipitation falling as snow

100-year trend

Prescott winter (NDJFMAM) precipitation (in)

Trend is -0.47 in per decade

A clear decrease in winter precipitation at Prescott


Prescott and Crown King snowfall

Crown King trend is –5.0 inches per decade

56 inches in 1915 avg

12 inches in 1993

Prescott trend is –2.0 inches per decade

33 inches in 1915

14 inches in 1993

Snowfall decreases in mountainous regions due to a warming trend (next slide)

Best fit Crown King

Best fit for Prescott


Average temp trend is 0.45 F per decade

50 F in 1912

57 F in 2002

Minimum temp trend is 0.65 F per decade

34.5 F in 1912

42.5 F in 2000

orographic enhancement
Orographic enhancement
  • In mountainous regions, precipitation is enhanced on the upwind side of mountains, with a rain shadow on the leeward side
  • This is why Crown King gets 29 inches and Prescott Airport gets only 13 inches of precip
  • Precipitation amounts increase with
    • Increasing wind speed
    • Increasing relative humidity of upwind flow
    • Increasing dew point of upwind flow
how mountains affect precipitation precipitation max on upwind side rain shadow on downwind side



How mountains affect precipitation– Precipitation max on upwind side – Rain shadow on downwind side

Rain shadow

how precipitation is enhanced on the upwind side of mountains the seeder feeder effect

Seeder cloud

Ice falling from higher overcast is enhanced as it falls though dense orographic cloud over mountain – natural cloud seeding

Feeder cloud



How precipitation is enhanced on the upwind side of mountains – the “seeder-feeder” effect
the summer monsoon
The summer Monsoon
  • During July, August and half of September, precipitation comes from thunderstorms associated with the summer monsoon
  • Convection occurs in moist southerly flow
  • Because of its high intensity/short duration, most summer rainfall runs off quickly and/or evaporates
    • Less replenishment of soil moisture and recharging of aquifers than winter precip

The recipe for monsoon precipitation in Arizona

Middle-level flow

Westerlies well to the north

High pressure belt moves north of AZ, bringing mid-level easterly flow to AZ



Mid-level Gulf moisture enters AZ from the east

Low-level surge of warm, moist air

Convection breaks out over AZ


Prescott monsoon precipitation (in)

July + Aug + Sept

No trend in monsoon precipitation, however it has become more variable

Last four years below normal

which factors impact longer term precipitation trends
Which factors impact longer-term precipitation trends?
  • El Niño/La Niña
  • Pacific Decadal oscillation
  • Climate change

Dry El Nino winters

Wet La Nina winters

WetEl Nino winters

Dry La Nina winters

Only impacts winter precip, not summer



Blue = La Niña Red = El Niño


  • Note that between 1950 and 1976, there were strong La Niña’s and weak El Niño’s
  • Between 1976 and 1998, there were strong El Niño’s and weak La Niña’s
  • Many scientists believe that we are shifting back to a prolonged period of strong La Niña’s and weak El Niño’s, like 1950 to 1976. This slower trend is related to the Pacific Decadal Oscillation
climate change
Climate change
  • Our climate is, and has always been, changing
  • Most year-to-year variability is natural
  • Global warming is one recent component
  • Hypothesized to be caused by increases in CO2 that result from fossil fuel burning and deforestation (“greenhouse effect”)
  • Winners and losers
    • Costs include rising sea levels, changes in climate patterns
    • Benefits include longer growing season, more CO2 for crop growth, increased drought and pest resilience
drought and global warming
Drought and Global Warming
  • A stronger hydrological cycle
    • Estimated 7-15% increase in global precipitation
    • Precip increases at high latitudes, smaller decreases at low and middle latitudes (Arizona)_
    • Estimated 5-10% increase in evapotranspiration
    • Potential for more severe, longer-lasting droughts in continental regions like Arizona
  • Prediction uncertainty remains great
    • Models can not yet resolve details of factors like mountains & vegetation changes that influence regional precipitation changes

Some graphs of Arizona precip …

Prescott annual precipitation (in)


Crown King annual precipitation (in)

Crown King winter precipitation (in)


Flagstaff region annual precipitation (in)

Flagstaff region winter precipitation (in)


Jerome annual precipitation (in)

Jerome winter precipitation (in)


Williams annual precipitation (in)

Williams winter precipitation (in)


Seligman annual precipitation (in)

Seligman winter precipitation (in)

  • Two precipitation regimes – winter precip from cyclonic storms & summer monsoon convection
  • Prescott area winter precip seems to be decreasing slightly, but much local variability
  • Snowfall decreasing because of a warming trend
  • More precip during El Nino, less during La Nina, with more La Nina’s on the way