1. A Discussion of “Synoptic Climatology of the Arizona Summer Precipitation Singularity”
Reid A. Bryson and William P. Lowry
Bulletin of the American Meteorological Society, Vol. 36, No. 7, September 1955

2. Singularity
A characteristic meteorological condition which tends to occur on or near a specific calender date more frequently than chance would indicate.

3. Arizona Precipitation Singularity
It is observed in Arizona’s precipitation record that there is a dramatic increase in areal coverage and amounts of precipitation on or about 1 July

4. What other meteorological phenomena are contemporaneous with the singularity?
Dramatic northward movement of Pacific (subtropical) High (anticyclone)
Increase in deep easterlies over Arizona

5. Dramatic northward movement of Pacific (subtropical) High (anticyclone)
Study of long term behavior of Pacific High indicates that its position shifted rapidly northward on or about 1 July, from about 34N to 41N.

6. Increase in easterly component to surface wind over Arizona
In this figure, the more negative the number, the stronger the easterly component

7. Arizona Precipitation Singularity
Does it relate to these other observed phenomena?
If so, how?

8. Role of Behavior of Subtropical Highs In Altering Surface Wind Direction
Shift of surface subtropical highs northward with the season changes surface wind directions in Arizona

9. Other Phenomena Observed at the Time of Singularity
Profound change in air mass over Arizona (warmer and moister at all levels)
Large change in wind directions both surface and aloft (aloft--southwesterly to southerly or southeasterly; surface-- southerly to southeasterly or easterly)

10. Role of Seasonal Fluctuations in Mid and Upper Tropopsheric Flow
Shift from “Low Zonal Index” to “High Zonal Index” Accompanied By Profound Wind Shifts in Mid and Upper Troposphere

11. Simultaneous Shift in Surface and Upper Air Wind Directions Related to Seasons
Midtroposphere--
Shift to moist southeasterly flow from Gulf of Mexico
Surface--Shift to moist southeasterly flow from Gulf of Mexico

12. Thus, the shift and upper level features with the season brings with it a profound change in the deep layer air mass in Arizona.

13. Source Area for Southerly and Easterly Winds in Arizona
Gulf of Mexico
Gulf of California
Thus, a very humid air mass is brought into Arizona at surface levels.

14. Air Mass Is Prone To Thunderstorms
In areas of strong local heating
In areas where topography provides substantial lift.
Thus, the elevated northern portions of Arizona and other mountainous areas receive the most precipitation.

15. Late May/June vs July

16. Monsoon
A seasonally-reversing wind system that results from the differential heating and cooling of the continents and the oceans.

17. Arizona Monsoon
The monsoonal wind circulation that is associated with the onset of widespread convective rains in Arizona and the Southwest.

18. Bryson and Lowry Study
Original scientific impetus for understanding circulation patterns associated with Southwest summer convective precipitation.

19. Problems With The Study
Location of Pacific High based on subjective hand placement of “H” on weather maps
Precipitation “singularity” not rigorously defended on the basis of statistical testing
Causes for evolution of surface and upper air features not put into proper context of what was known at the time