Introduction

Meteorology

Precipitation

Runoff

ALERT System Performance

Summary

Appendix A: Selected Meteorological Services Program Outlooks

Appendix B: Point Rainfall Maps and Isohyetal Rainfall Coverage Maps

Appendix C: NEXRAD Reflectivity Images from Phoenix WFO

Appendix D: Depth/Duration/Frequency Plots and Histograms for Seven

Appendix E: Selected Digital Photos

A number of heavy thunderstorms dropped large amounts of rain in the Wickenburg area, lower Hassayampa and upper Centennial watersheds on August 29^th, 2000. These storms caused flooding along several watercourses in northwestern Maricopa County and eastern La Paz County, including Martinez Creek, Sols Wash and Centennial Wash.

The month of October featured a series of Pacific low-pressure frontal systems which tapped tropical moisture in northern Mexico as they passed through the State, resulting in heavy widespread rain with numerous embedded thunderstorms. Again, the Wickenburg and Centennial areas were affected, along with Grass Wash, Tiger Wash, Jackrabbit Wash and much of northern Maricopa County.

The purpose of this report is to present in a concise manner selected rainfall, runoff, graphical and statistical data from the storm events of August 29, October 10, October 21-23 and October 27, 2000. The majority of the data presented was collected by the Flood Control District's (FCD) ALERT System, with contributions from the Phoenix National Weather Service (NWS) Forecast Office and the United States Geological Survey (USGS) surface-water data collection program.

The goal of this report is to present data and derived products for general information purposes, and as a guide for future prediction of water levels and discharge rates in the watersheds affected by flooding.

DISCLAIMER: Although these data have been reviewed and/or edited they may be subject to significant change. Data users are cautioned to consider carefully the provisional nature of this information before using it for decisions that concern personal or public safety or the conduct of business that involves substantial monetary or operational consequences.

This report and the data contained within it, as well as the entire compliment of real-time and historic FCD ALERT data can be viewed or downloaded from our website at:

http://www.fcd.maricopa.gov/alert/alert.htm

Also, a Compact Disk containing this report and all associated tables and graphics can be ordered from the FCD at the address on the report cover page or by calling (602) 506-8701.

On the 29^th of October the atmosphere over central and southern Arizona was very moist, possibly the most moisture available for shower and thunderstorm development of any day during the 2000 monsoon season. As for a trigger (dynamics) to get storms going, there was a lower/mid level atmospheric disturbance forecast to move north/northwest through the state. Also, in the upper levels of the atmosphere, there was a disturbance off the California coast - aiding the import of moisture from the south, as well as acting as an additional trigger for storm development.

The precipitable water at 5 AM was 1.84 inches, the 850mb dewpoint was >14 degrees, and the 600mb dewpoint depression was <2 degrees - collectively a strong indicator of very moist air from the surface to over 15,000 feet altitude. The only concern, which could inhibit storm development, was existing cloud cover and relatively cool temperatures - but as it turned out these concerns didn't materialize.

A few showers and thunderstorms developed on the 28^th, continued to develop and became stronger late that night and peaked on the morning of the 29^th. Moisture at lower and middle levels of the atmosphere continued to be pumped into Maricopa County from the south (Mexico/Gulf of Mexico) through most of the day. Thus there was no drying of consequence following the initial outbreak of showers. To make matters worse, the storm moved at a relatively slow speed (mostly 12-15 mph), allowing locally heavy rain to remain over an area for a period of time. The storm center finally moved north of the County during the night of the 29^th.

A series of weather disturbances moved south/southeast from the northern Pacific (a pattern referred to as split flow) - for the extended period of mid October into November. Although this is not an overly unusual trend or general track, what was different was the extended period of time that was involved, the strength of the systems and the track's location so far south for the time-of-year (into Arizona). Normally only one or two of these disturbances will become moderately strong, but this time most resulted in periods of moderate to heavy rain for Maricopa County. As a result our (calendar) annual rain accumulation went from a deficit to above normal.

On the 9^th of October weather maps showed a large upper-level atmospheric weather disturbance sliding south along the west coast. At lower and mid levels of the atmosphere moisture from the sub-tropics was advected north, via south to southeast winds, into Maricopa County. And lastly, there was a strong cold front moving east through southern California - located along a north-south line near the western border of Arizona the morning of the10^th, moving east to roughly a Page to Blythe line by around noon.

This resulted in a moist atmosphere that was "triggered" by the approaching disturbance and associated cold front. The end result was short-lived, locally-heavy morning rain in the Phoenix area - which moved east and northeast with time.

Farther to the south, over northern Mexico, there were remnants of tropical storm Olivia, but it's doubtful that this system had much of an impact on what took place over Maricopa County. Most of the moisture associated with Olivia was advected eastward into New Mexico and Texas.

 October 21- 23, 2000

To set the stage for this event a few showers and thunderstorms moved through Maricopa County on the 19^th (Thursday). Local rain accumulations amounted to .25 to .50 inch. This rain was the result of the first in an extended series of upper level weather disturbances that moved south/southeast into our area during October/November.

It was dry on the 20^th (Friday), as Maricopa County was between disturbances. But an approaching strong cold upper atmospheric weather disturbance (from the northwest) resulted in the development of scattered showers and thunderstorms Friday night and Saturday morning.

This Pacific storm had considerable moisture associated with it, and moved into an area that was already moist due to residual moisture associated with the disturbance that moved east across southern Arizona on the 19^th.

Numerical guidance, as early as Wednesday (18^th), suggested the threat of rain for the weekend. And by Friday guidance indicated the threat of heavy rain - to over an inch. With the Saturday "Weather Outlook" issued at 1:10 PM, clients were alerted to break out their Flood Response Plans and prepare for a significant rain event.

The heaviest and most destructive rains fell Saturday evening (21^st) - with portions of western Maricopa County receiving over 3 inches of rain. Especially hard hit, as a result of the heavy rain, were Centennial Wash (flow into Wenden), Wickenburg (Sols Wash) and Skunk Creek.

The associated upper level disturbance was slow to leave the area. As a result additional rains fell Saturday night and Sunday, with the storm exiting to the north and east Sunday night and Monday. But the heaviest rains by Sunday (to over an inch) were over the east and northern fringes of the County - not the same area hit hard Saturday evening.

The major storm that took place October 21-23 left the soil very wet - possibly saturated in many areas. Thus additional rain would become mostly runoff. The weather disturbance that moved toward Maricopa County on the 26^th and 27^th, like previous disturbances, moved into the area from the northwest.

This weather disturbance had an unseasonably strong cold front associated with it. Wednesday morning the front was positioned north-south over far southeast California and eastern Nevada. By Wednesday evening the front was north-south through central Arizona. By Thursday morning (27^th) it had dissipated east of the County, but a secondary front, or trough, developed back near the Arizona/California border. This latter feature was probably a significant trigger that aided development of the locally heavy rain that followed.

The advection of moisture from Mexico was probably not overly noteworthy, compared to the disturbance 5 days earlier, but as mentioned the ground was already very moist. And the upper level disturbance was very strong - resulting in good dynamics to act on the existing moisture.

Showers and thunderstorms moved into the area from the west, again hitting northwest Maricopa County with locally heavy rain. The main band of rain (north-south orientation) was very slow to move east. By late morning (27^th) some areas west and southwest of Wickenburg reported rain accumulations of over an inch, and locally heavy rain remained in the general Wickenburg area into the afternoon hours.

During the afternoon the heaviest rains slowly moved east toward the central portion of the County. By late afternoon the heaviest rains were centered over the central portion of the County; and by evening the heaviest rains were over the far eastern portion of the County - as the associated weather disturbance started to move away from the area. But local showers persisted over much of the County through the evening.

Appendix A contains selected Outlook and Message Products from our Meteorological Services Program (MSP). The MSP provides heavy rainfall and severe weather forecast products to emergency management agencies, public works and transportation agencies, engineering departments, law enforcement, and fire departments and recreation departments within Maricopa County.

A series of heavy thunderstorms dropped large amounts of rain in the Wickenburg area, lower Hassayampa and upper Centennial watersheds on August 29^th, 2000. These storms caused flooding along several watercourses in northwestern Maricopa County and eastern La Paz County, including Martinez Creek, Sols Wash and Centennial Wash. Rain fell in general from 7:00 am through 2:00 PM.

The month of October featured consecutive Pacific low-pressure frontal systems which tapped tropical moisture in northern Mexico and the Gulf of California as they passed through the State, resulting in heavy widespread rain with numerous embedded thunderstorms. The heavy event periods were from 4:00 AM through 11:00 AM on the 10^th, 7:00 PM on the 21^st through 11:00 AM on the 22^nd (with scattered showers on the 23^rd), and 7:00 AM through 6:00 PM on the 27^th. Again, the Wickenburg and Centennial areas were affected, along with Grass Wash, Tiger Wash, Jackrabbit Wash and much of northern Maricopa County, including Skunk Creek, New River and Cave Creek.

The following Table 1 lists selected frequency statistics at seven automated ALERT Stations:

Appendix B contains four pages showing two graphics for each storm. The first is a map of the County showing point rainfall totals measured by the ALERT automated gages. The rainfall period and ending time are shown at the bottom. The second graphic is a contoured rainfall map using the point values from the first map. It covers the same time period but may focus on areas of interest.

Appendix C contains several NEXRAD radar images from the storms in 1-hour, 3-hour and Storm Total formats, courtesy of Phoenix WFO. Appendix D displays depth/duration/frequency plots and histograms for seven selected stations. Additional detailed rainfall information is available on the web at: http://alert.fcd.maricopa.gov/showrain.html or http://alert.fcd.maricopa.gov/alert/raindata.html.

The storm of August 29^th featured significant runoff events in the Wickenburg area and in the Centennial watershed. Flows in these areas had estimated return periods of between 8 and 17 years.

On October 10^th, flows in central Phoenix and Scottsdale were generally around a 2-year return period. But the rains from this storms served to wet many watersheds around the County, which added to the runoff potential for subsequent storms.

October 27^th saw heavy runoff in the northern, central, western and northwestern portions of the County. Centennial Wash experienced what is believed to be a 100-year event, Sols Wash saw a 66-year return period event, Flying E Wash in Wickenburg around a 40, and Hartman and Tiger Washes approximately 16-year events.

Flows during the October 27^th storm were again heavy in the western and northwestern parts of the County. Jackrabbit Wash experienced a 50-year flood, Hassayampa @ I-10 recorded a 70-year event, Casandro Wash in Wickenburg suffered a 40-year, and Sols Wash and Tiger Wash each were deluged by 10-year events.

Table 2 on the following page lists significant discharge peaks and return periods for selected FCD stream gages. Table 3 presents peak stage and discharge values at all FCD water-level stations. Following Table 3 are three selected hydrograph plots showing travel and warning times.

"Q" refers to discharge; Q₁₀₀ is the 100-year or Regulatory Discharge

(a) - Return period estimated from Flood Insurance Study rainfall/runoff modeling

(b) - Return period estimated from structure design report

(c) - Return period estimated from streamflow data recorded prior to these storms

NA - Not Available

Values in Blue are computed from and/or verified by slope/area survey computations and/or high-water marks. Values in Green are provided by USGS. Values in Red are added or changed in the 02/01/2001 revision.

To find the location of an ALERT water-level station, point your browser to http://www.fcd.maricopa.gov/alert/alert.htm and use the "Station Location Maps" tool. Additional stage and discharge information is available on the web at: http://alert.fcd.maricopa.gov/showflow.html

During the storms of August 29^th through October 27^th, 2000 the FCD ALERT System consisted of approximately 238 automated rain sensors and 106 automated water-level sensors.

On the August 29^th storm only one rain/water-level station failed to operate - Agua Fria @ Buckeye Road.

The October 10^th storm saw two rain/water-level stations fail due to lightning strikes - ACDC at 14^th Street and ACDC @ 36^th Street. Sunset Dam rain/water-level station was down because of a failed battery. The IBW @ McKellips Rd. and Hesperus Dam raingages did not operate properly because of plugged funnels. The Delaney Wash and Flying E Tank raingages were down because of programming errors, and the Buckeye @ 547^th Ave. and Freestone Basin raingages were down for unknown reasons.

On October 19^th a lightning strike hit a power pole across the street from the FCD office building, sending a surge across data and phone lines which destroyed four modems and two multi-port boards. As a result the ALERT base station was down from approximately 08:45 through 11:30. Steps are being taken to make sure this does not happen in the future.

The storm of October 21^st-23^rd saw inaccurate readings from the water-level sensors at Flying E and Hartman Washes, but did not hamper the generation of warnings for these areas. The Pinacle Peak Vista raingage was down due to transmitter failure, and the ASU South raingage was down due to battery failure. The IBW @ McKellips raingage was still inoperative because of a plugged funnel.

On October 27^th the Hartman and Flying E Wash gages were still sending inaccurate data, and IBW @ McKellips raingage still had a plugged funnel, but all other stations worked properly.

The following Table (4) lists most of warnings and information disseminated during the storms by FCD ALERT personnel. We were unable to log all of the warnings for various reasons, and are working to resolve this for future event logs.

 A number of heavy thunderstorms dropped large amounts of rain in the Wickenburg area, lower Hassayampa and upper Centennial watersheds on August 29th, 2000. These storms caused flooding along several watercourses in northwestern Maricopa County and eastern La Paz County, including Martinez Creek, Sols Wash and Centennial Wash. The storm produced significant runoff events in the Wickenburg area and in the Centennial watershed with estimated return periods of between 8 and 17 years.

The month of October featured a series of Pacific low-pressure frontal systems which tapped tropical moisture in northern Mexico as they passed through the State, resulting in heavy widespread rain with numerous embedded thunderstorms. Again, the Wickenburg and Centennial areas were affected, along with Grass Wash, Tiger Wash, Jackrabbit Wash and much of northern and central Maricopa County. On October 10^th, flows in central Phoenix and Scottsdale were generally around a 2-year return period. But the rains from this storms served to wet many watersheds around the County, which added to the runoff potential for subsequent storms. October 27^th saw heavy runoff in the northern, central, western and northwestern portions of the County. Centennial Wash experienced what is believed to be a 100-year event, Sols Wash saw a 66-year return period event, Flying E Wash in Wickenburg around a 40, and Hartman and Tiger Washes approximately 16-year events. Flows during the October 27^th storm were again heavy in the western and northwestern parts of the County. Jackrabbit Wash experienced a 50-year flood, Hassayampa @ I-10 recorded a 70-year event, Casandro Wash in Wickenburg managed a 40-year, and Sols and Tiger Washes each were treated to 10-year events.

Appendix E contains selected digital photos of flooding from around the County.

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

WEATHER OUTLOOK

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

 WEATHER OUTLOOK

Quite pleasant outside right now, temperatures in the mid-80s with very few clouds. Look for a high today around 87 with light winds and some gusts to 15 mph. Tonight will be clear and cool. Should be dry between now and Saturday evening.

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

WEATHER OUTLOOK

Think wet - very wet. This outlook will also be a heads up, or flash flood watch, regarding expected weather conditions. Storm action plans should be looked at - as the odds are good this storm will result in locally (at least) runoff. Be prepared!

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

WEATHER OUTLOOK

More weather disturbances on the way - main periods appear to be Friday (probably afternoon) through Friday night, then Monday, and then again late in the week. For now will only discuss the next two. The atmosphere remains rather moist from the storm this past weekend - more importantly the ground is also very moist. And due to saturated soil conditions additional rain will mostly become runoff. In other words - a situation than could lead to additional flooding.

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

Message 3

Flood Control District of Maricopa County

Meteorological Services Program (MSP)

Message 3