WELCOME BACK: Procedure continued from STUDY GUIDE.
Over the previous weekend, 20 -22 October 2000, a cold front trailing from a Canadian low-pressure system pushed across the West. The southerly flow from the Gulf of Mexico ahead of the cold front and behind the persistent high-pressure system that had prevailed over the Southeastern States for many days supplied moisture for the development of thunderstorms from Arizona to the central Plains states. As the cold front moved eastward, the southerly flow spread from Texas to Minnesota. Extensive and heavy precipitation broke out producing record daily rainfall totals from Arizona to Oklahoma on Sunday, 22 October. Details of this weather pattern can be found in the Monday, 23 October, Daily Weather Summary.
Image 1 is the surface weather map for Sunday, 12Z 22 OCT 2000. The cold front stretched from south-central Canada to Arizona. Three pressure systems dominate the 12Z 22 OCT map. One high pressure center is located in Quebec in eastern Canada. A small local high-pressure center is also shown in southwest Virginia, a remnant of the High that had dominated the Southeast for the past two weeks. A second high pressure system is entering the Northwest. And a low-pressure system anchors the southern end of the cold front at the California-Arizona-Mexico border. A cold/stationary/warm front combination from Virginia to Minnesota forms the southern boundary of the cool eastern air mass.
Wind directions at stations in the Northeastern US from Maine around to Minnesota show the circulation around the High to be generally [(clockwise and outward) (counterclockwise and inward)]. (The circulation patterns of the two western systems are less apparent due to the mountainous terrain.)
Wind directions at stations from Texas to Minnesota ahead of the cold front are generally from the [(southeast) (northwest)] consistent with the flow between the Virginia High and a local low-pressure center analyzed by computer in Colorado. This direction of flow around the "back-side" (western side) of the high pressure center that had persisted over the Southeast for several days was bringing [(cold, continental) (warm, maritime)] air from the Gulf of Mexico into the central US.
Air from the Gulf Coast heading northwestward toward the foothills and mountains to the west would flow across increasingly higher terrain. Air flow over rising terrain produces [(upward) (downward)] motions associated with orographic lift. Rising air cools adiabatically and may become saturated forming clouds and precipitation. The majority of stations from San Antonio in south-central Texas to North Platte in western Nebraska were reporting sky conditions of [(clear or partly cloudy) (mostly cloudy or overcast)].
The presence of precipitation on the surface map is indicated by radar echoes as areas of gray shading (colored, when viewed on screen). The intensity coding scheme is given at the lower left edge of the chart. The major area of precipitation at 12Z 22 OCT 2000 was located from [(southeastern Texas to Nebraska and Missouri) (Minnesota to New York)] Another area of precipitation was in southern Arizona associated with the [(High) (Low)] there. Some isolated precipitation patches remained behind the front in the northern Plains and in eastern Kentucky.
Image 2 is the meteogram for Minneapolis, Minnesota (MSP) from 1300Z 22 OCT 2000 to 1200Z 23 OCT 2000. From 13Z on 22 OCT to 05Z on 23 OCT the wind direction reported at Minneapolis was from the [(south or southeast) (west or northwest)]. During this period the dewpoint was [(falling) (steady) (rising)] meaning [(more) (less)] water vapor was in the air over Minneapolis.
At 00Z 23 OCT the weather condition reported was [(rain) (fog) (snow)]. This precipitation was falling from cloud levels that were obviously saturated. The relative humidity of the near-surface air, as shown by the meteogram conditions, was becoming [(higher) (lower)]. By 03Z the near-surface air, with precipitation falling through it and partially evaporating, [(was) (was not)] saturated and the relative humidity was [(less than) (equal to) (more than)] 100%. At 04Z, while the precipitation had ended, the weather condition reported was [(rain) (fog) (snow)].
Image 3 is the surface map for Monday, 02Z 23 OCT 2000, 14 hours later than Image 1. The portion of the cold front from Canada to Colorado had moved [(eastward) (westward)], while the section from Colorado to Arizona became a [(stationary) ( warm)] front. The cold front in the eastern US had advanced [(southward) (northward)]. (The frontal positions on Image 3 are for 00Z. Frontal times and data coincide only on the specific 00Z -, and 12Z - Isobars, Fronts, Radar, & Data maps, linked from the homepage due to timing of analysis reports from NWS.)
The precipitation areas at 02Z 23 OCT 2000 on Image 3 are generally located [(from east Texas to Minnesota) (along the western cold front) (around the Low in Arizona) (all of these locations)]. Isolated patches of precipitation were located along the eastern cold front also.
Compare the Image 1 and 3 maps. For south-central Arizona and Oklahoma, the location of the precipitation echoes indicated that the rainfall probably [(did) (did not)] persist for much of this 14-hour period. Therefore, one might have expected the potential for [(passing showers) (flooding)] in that region. The DataStreme homepage also delivers a map (Surface section: "24 Hour Precipitation") showing precipitation totals for selected stations over each 24-hour period ending at 12Z that day. The 24 Hour Precipitation map for 12Z 23 OCT 2000 (not shown) indicated Oklahoma City, OK, had 4.52 inches while Tucson, AZ, had 1.31 in. during this period!
Additional details are found in the NWS' National Hydrologic Summary for Monday, 23 Oct. 2000:
"FLOODING IN SOUTHWEST
INTENSE RAINS BROUGHT SIGNIFICANT FLOODING TO PARTS OF ARIZONA, NEW MEXICO,
TEXAS AND OKLAHOMA OVER THE WEEKEND. AMONG THE WORST CONDITIONS:
TEXAS: LATE IN THE DAY ON SUNDAY RADAR ESTIMATED THAT UP TO 18 INCHES OF RAIN
IN LIBERTY COUNTY - SANDBAGGING AND RESIDENTIAL FLOODING WERE REPORTED IN RYE
AREA
OKLAHOMA: SUNDAY NIGHT IN CADDO COUNTY, WATER WAS REPORTED IN SEVERAL HOUSES
AND BUSINESSES, WITH EVACUATIONS AND SIGNIFICANT STREET FLOODING IN ANADARKO
AREA; A CAR WAS SWEPT AWAY NEAR STECKER; SEVERAL BRIDGES WERE WASHED OUT AND
NUMEROUS ROADS WERE CLOSED
ARIZONA: SATURDAY NIGHT IN WICKENBURG AREA OF MARICOPA COUNTY, ROADS WERE
CLOSED, SEVERAL CARS WERE STRANDED AND OCCUPANTS HAD TO BE EVACUATED BY
HELICOPTER; CAMPERS WERE EVACUATED FROM A PARK ALONG SOLS WASH "
Along with the radar information, at 02Z a parallelogram, red on screen, was located in central Oklahoma labelled T759. This figure outlined an area under (T)ornado watch #759 of 2000 where conditions were favorable for the occurrence of tornadoes. The Monday Daily Weather Summary indicated that tornadoes were reported!!
Have your students view local radar displays from weathercasts and reports of precipitation amounts when rainfall occurs in your area. Compare your reports with DataStreme weather maps to see what weather conditions are probably causing precipitation in your area.
Hold this activity until you have completed all applications for this week. Instructions for faxing your LIT mentor will appear at the end of this week's Activity B.
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