DATASTREME DAILY SUMMARY

Tuesday, 23 October 2001


00Z Weather Systems


The following discussion is based upon the major weather features appearing on Monday night's surface weather maps:

ACTIVE WEATHER MOVES ACROSS THE MIDWEST -- A warm front extending eastward across the Midwest from a weak low pressure center over the Missouri Valley near Kansas City, MO was the focal point for strong to severe thunderstorms on Monday. Hail-producing thunderstorms developed along this warm front starting during late Monday morning in the vicinity of Omaha, NE and Council Bluffs, IA and then continued through the late afternoon in the vicinity of Ottumwa in southeast Iowa. Some of the hail reached 1.75 inches in diameter. Thunderstorms continued well into Monday night along the warm front. In addition to the hail, heavy rains fell across eastern Iowa during the evening hours, where Doppler radar estimated rainfall totals of up to 3 inches within 2.5 hours. The heavy rains produced flooding of numerous streets in Rock Island, IL. As a result, an urban and small stream flood advisory was posted from eastern Iowa into northwest Illinois. Farther east, the thunderstorms that continued to train across north central and northeast Illinois prompted an urban and small stream flood advisory through the nighttime hours of Tuesday morning for the region.

The heavy rain and thunderstorms were the result of warm and humid Gulf air to the south of the warm front being carried up and over cooler and more dense air located to the north of the front. Temperatures during the late evening were in the low 70s as far north as central Illinois in the warm air south of the front, but in the 50s across southern Wisconsin to the north. As the humid air was lifted, it was destabilized, thereby enhancing vigorous vertical motions associated with thunderstorm activity.

A slight risk of severe thunderstorms continues across the mid-Mississippi Valley on Monday night. On Tuesday the area under a slight risk of severe weather is expected to expand so as to include portions of the Great Lakes and the Ohio Valley. The low pressure center in the Plains is expected to move eastward with a cold front sweeping southeastward across the Mississippi Valley.

Between 0.75 to 1.4 inches of rain could fall during the 24 hours ending on Tuesday evening across the mid-Mississippi Valley and the western Great Lakes. Dense fog developed across southeastern Nebraska and southwest Iowa late Monday night as a result of the moisture from the afternoon thunderstorms.

QUIET AND WARM WEATHER ACROSS THE SOUTH -- High pressure located across the Southeast continued to pump warm and humid air northward across the South. As a result, much of the nation to the east of the Rockies experienced relatively mild autumn weather, as high temperatures were at or above the average highs for late October. The largest anomalies (arithmetic differences between the actual and average highs) were found across the western Plains, particularly in the Texas Panhandle, and across Virginia, where highs were on the order of 15 degrees above average. Record high temperatures were either tied or set in Midland TX (91 degrees) and in South Carolina at North Myrtle Beach (83 degrees) and Florence (83 degrees).

With clear skies and weak winds along with sufficient near-surface atmospheric humidity, dense fog became a problem across the South late Monday night as visibility was reduced to less than 0.25 miles across portions of South Carolina and Georgia and along the Gulf Coast to include southern Alabama, Mississippi and Louisiana. Consequently, dense fog advisories were issued for these areas.

MORE RAIN FOR THE SUNSHINE STATE -- Rainshowers and thunderstorms continued to develop along a weak trough of low pressure over southern Florida. More than an inch of rain fell at some locations on Monday. Three waterspouts were spotted approximately 10 miles southwest of Key West during the afternoon. As much as 0.8 inches of rain could fall by Tuesday evening.

A POWERFUL STORM HITS THE NORTHWEST -- A potent cold front accompanied by strong winds and wintry precipitation moved into western Washington State and Oregon late Monday afternoon. This front extended southward from a low pressure center located along the British Columbia coast between Vancouver Island and Alaska's southeast Panhandle. The storm was the first major storm of the upcoming winter season to hit the Pacific Northwest.

A tight pressure gradient surrounding this deep storm system produced the strong southerly winds ahead of the cold frontal passage, followed by strong westerly winds behind the front. With winds expected to range between 39 to 54 mph, gale warnings were in effect for marine interests along the Oregon and Washington coasts extending from Florence, Oregon northward to include the Strait of Juan de Fuca. Wind-driven ocean swells with heights reaching 25 feet were expected to arrive late Monday night. A variety of coastal flood watches and warnings were posted for the Washington State coast. Beach erosion is anticipated along the central Washington coast. Heavy surf advisories were also in effect from Cape Shoalwater, WA to Point St. George, CA.

The cold front is expected to continue moving eastward, crossing the northern Rockies into Montana on Tuesday morning. By Tuesday evening this system is forecast to have moved across the Plains to the eastern Dakotas. A high wind warning was in effect for interior sections of Washington State and Oregon, the Snake Valley of Idaho, northern Montana east of the Divide and southeastern Wyoming. Wind gusts to 60 mph were expected in some locales, especially in mountain passes and valleys. Blowing dust could also reduce visibility across Oregon and in eastern Idaho.

In addition to the high winds, widespread precipitation was moving into the Pacific Northwest. As of late Monday night low elevation rain was falling across Washington, Oregon, northern California, Idaho and western Montana, while snow was beginning to fall over the higher elevations. Between 1.0 and 1.5 inches of liquid equivalent precipitation (rain and melted snow) could fall during the 24 hours ending by Tuesday evening across much of the Pacific Northwest, with the greatest totals along the western slopes of the mountains as westerly winds bring moist air inland and are lifted up the mountain slopes. With cold air being brought into the system on westerly and northwesterly winds, snow levels should fall as the cold front passes. As a result, winter storm warnings were issued for the Olympics and west slopes of the Cascades of Washington State above 3000 feet as a foot of snow could fall on Monday night with another 10 inches likely on Tuesday. In addition to the heavy snow, strong winds could cause blowing snow and near blizzard conditions. Winter storm warnings were also posted for the mountains of northern Idaho and western Montana, where 6 to 12 inches are expected by late Tuesday at elevations above 4500 feet. A blizzard warning was also in effect for the Rocky Mountain Front above 6000 feet in Montana as snow combined with winds gusting to 70 mph could produce white-out conditions. Wind-chill equivalent temperatures could drop to 10 degrees below zero. A snow and blowing snow advisory was in effect for western Wyoming, where 5 to 10 inches of snow along with wind gusts to 40 mph were expected by Tuesday afternoon.

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 19 degrees at Silver Bay, MN, while Monday's highest temperature was 96 degrees at Death Valley, CA.

ALASKAN WEATHER -- Much of the state was dominated by low pressure on Monday afternoon. The deepest low pressure center was associated with a storm system located along the British Columbia coast just south of the Panhandle. This low pressure center had a central pressure reading of 980 mb. A weaker low, with a central pressure of about 1000 mb was located along the Alaska Peninsula. Windy conditions were found across the Bering Sea as a tight pressure gradient developed between high pressure near Siberia and the large low pressure system across Alaska. Gambell had gusts to 48 mph, Savoonga had gusts to 35 mph and Middleton Island reported gusts to 33 mph. Snow was reported at many locations across the state, to include Barrow, Deadhorse and Nuiqsut over the north, Delta Junction, Eagle, Fairbanks and Tanana in the interior and Bethel, King Salmon, St. Paul, Anchorage, Cordova, Kenai, Talkeetna and Whittier across the south. During midafternoon Anaktuvuk had a temperature of 3 degrees, with a wind-chill equivalent temperature of 33 degrees below zero.

The state's lowest temperature on Monday morning was 4 degrees below zero at Anaktuvuk Pass. The highest temperature across Alaska as of midafternoon on Monday was 48 degrees at Port Alexander.

HAWAIIAN WEATHER -- The trade winds were beginning to weaken late Monday afternoon as a cold pool of air in the mid to upper troposphere is expected to move north of the islands and erode the surface ridge of high pressure responsible for the trades. A trough of low pressure located to the east of the Big Island is expected to reach the islands on Tuesday and increase the chances for heavy rainshowers and isolated thunderstorms on Wednesday. This trough will also cause the winds to become light and variable across the islands.

EYE ON THE TROPICS -- In the eastern North Pacific, Hurricane Narda weakened as it moved toward the northwest and was downgraded to a tropical storm late Monday afternoon. As of Monday night Tropical Storm Narda was approximately 1730 miles to the west-southwest of Cabo San Lucas, Mexico and moving to the west-northwest at 12 mph. Maximum sustained surface winds were 70 mph.


CONCEPT FOR THE DAY - CLOUDS

Clouds are composed of liquid water droplets or ice crystals. So why don't these cloud particles fall? They do! Actually, they fall slowly, and the air in which they exist is also moving. The water droplets and ice crystals are almost microscopic and fall slowly in still air. A typical size cloud droplet falls at a speed of about 0.003 meters per second (0.007 miles per hour) in motionless air. This droplet would take days to fall from normal cloud heights! Added to this hurdle is the fact that the droplet has formed because the air is rising (and cooling adiabatically) so that the droplet is falling against the flow. If the droplet ever fell below cloud base into air that often has relative humidities less than 100%, the droplet would begin to evaporate. As a result, clouds are forced to stay up there.

If the cloud droplet were favored to grow to raindrop size however, the resulting drop would contain the water content of a million cloud droplets. This size drop falls through still air at a rate of about 4 meters per second (9 mph). The larger the drop, the greater the fall speed. These fall speeds will overcome the rising flow of air and reach the Earth's surface - rain happens.

So when you see a cloud, you know the air in that region is rising. And the cloud is falling relative to the air flow, yet remains in the sky. In this way the atmosphere transports million of tons of water from the Earth's surface, first as vapor, then as water droplets or ice crystals in clouds, and finally as the never-ending resupply of precipitation.

For more information describing various aids that you can use in identifying clouds, you may consult the optional material in Tuesday's optional electronic Supplemental Information .

QUESTIONS:

To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 7 Chapter Progress Response Form, under section B. Daily Summary.

  1. The existence of clouds indicates [(upward) (downward)] air motions.
  2. When rain is occurring, the larger the raindrops falling from the cloud bases, the [(faster) (slower)] the upward motions within the cloud.

HISTORICAL WEATHER EVENTS - 23 October

From the files of the Aviation Weather Center, Kansas City, MO and Intellicast


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URL Address: datastreme/learn/t_sum.html
Prepared by Edward J. Hopkins, Ph.D., emailhopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.