DATASTREME DAILY SUMMARY

Thursday, 12 October 2000


00Z Weather Systems


MODIFYING AN AIR MASS -- The arctic-like air mass that has dominated the weather across the eastern two-thirds of the nation for the last five days appears to have undergone a noticeable modification. This air mass is associated with a large sprawling high pressure system that was centered over the Ohio Valley as of Thursday night. With the exception of some clouds associated with precipitation along the Texas Gulf Coast, few clouds were reported from the east slopes of the Rockies to the Atlantic Seaboard.

Several signs of air mass modification were evident. The number of locations experiencing record low temperatures decreased from more than 80 per day last weekend to roughly a dozen on Wednesday morning that were located primarily in the Southeast. More critically, the afternoon high temperatures across much of the country east of the Rockies were more seasonal, with readings that were within several degrees of the average highs for mid-October -- a far cry from the 20 to 25 degree departures below the average experienced by residents of the Southeast on Monday. The only major exception to Wednesday's warming trend could be found along the Rio Grande in south Texas, where afternoon highs were on the order of 20 degrees below the average. Fewer locations experienced daily low maximum temperature records as had been the case earlier. The turn toward more seasonal afternoon temperatures suggests an air mass modification with warming from below as the cold air mass moved across warmer ground. Also, the air mass was relatively dry meaning that most of the solar heating under relatively clear skies could be used to heat the air. A dry air mass also meant that nighttime temperatures could fall rapidly as the lack of water vapor permitted greater nocturnal radiative heat loss.

The northern Plains and the eastern slopes of the Colorado Rockies had afternoon highs that were on the order of 10 degrees above the average highs for the season. The winds across the Plains had turned more southerly as the high pressure cell moved to the Ohio Valley. Warm air advection with these southerly winds along with downslope compressional heating from air flow over the Rockies contributed to the return to the mild weather.

The forecast models indicate that the high pressure center should remain relatively stationary across the Ohio Valley through Thursday evening.

INCLEMENT WEATHER IN THE WEST -- Widespread clouds and precipitation were found across the West from the Pacific Coast to the Continental Divide. Most of the precipitation was in the form of rain across Arizona and New Mexico, while farther north, snow was reported at higher elevations of the Colorado and Montana Rockies. Relatively heavy rains were reported across normally dry southern Arizona, as Tucson had a record 1.34 inches of rain, which ranked as the ninth wettest October day on record. Douglas received a record 1.31 inches, making Wednesday the sixth wettest October day at that location.

A large cold pool of air extending up through the troposphere and the accompanying counterclockwise circulation regime were responsible for generation of the clouds and precipitation. Some residual moisture from former Tropical Storm Olivia was carried northward across Baja California and the Gulf of California into the Southwest. At the surface, a complex set of low pressure systems were found across Montana and Wyoming. A cold front trailed essentially southward across Arizona. To the west of the cold front, many locations across the Great Basin had afternoon high temperatures that ranged between 10 to 20 degrees below the average high temperatures for the date.

The broad region of low pressure and accompanying precipitation shield are not expected to move quickly to the east on Thursday. Rain should continue across Arizona and New Mexico, with valley rain and mountain snow across the central and northern Rockies. Because of Wednesday's record-setting rains, flash flood watches were posted for southeastern Arizona and southwestern New Mexico. Snow advisories for southwestern Montana for snow accumulations of 2 to 5 inches at elevations above 5000 feet, and for the San Juan Mountains of southwestern Colorado.

TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 19 degrees at Hibbing and Mora, MN, while the Wednesday afternoon highest temperature was 84 degrees at Gila Bend, AZ.

ALASKAN WEATHER -- High pressure situated over interior Alaska produced relatively clear skies and chilly weather across interior and southwest portions of the state. A tight pressure gradient across western Alaska and the Bering Sea was produced between the high and a strong storm system over eastern Siberia, producing windy conditions over the region. A front extending southward from the low pressure in Siberia across the Aleutians produced clouds across the island chain. An east-west stationary front extended from south central to southeastern Alaska, separating the cold air in the interior from warmer air along the coast. To the south, a storm system was located over the Gulf of Alaska to the south of Prince William Sound. The front as well as the storm system combined to spread clouds and precipitation across southeast Alaska and the Panhandle. The heaviest rain was found over the southern Panhandle.

Clouds were also found across northwest Alaska, where strong winds produced areas of blowing snow. Red Dog had peak winds near 50 mph, along with snow and blowing snow. Locations north of the Brooks Range had wind-chill equivalent temperatures reaching 20 degrees below zero.

The lowest temperature in Alaska on Wednesday morning was 8 degrees below zero at Arctic Village. The highest statewide temperature as of Wednesday afternoon was 54 degrees at Port Alexander, Adak, Kake and Gustavus.

HAWAIIAN WEATHER -- The ridge of high pressure located to the northeast of Hawaii has maintained northeast trade winds across the islands on Wednesday. These winds are expected to continue through the weekend with speeds ranging from 15 to 20 mph. A westward moving tropical disturbance roughly 500 miles south-southeast of Hilo was expected to pass south of the Big Island, increasing the chance of showers over that island on Thursday and Friday. Small craft advisories remained in effect for all state waters.


CONCEPT FOR THE DAY - PRESSURE ALTIMETERS

Even before humans learned to fly in balloons or in aircraft, we ventured up into the atmosphere by climbing mountains. But how high were we? Lacking a long tape measure, another procedure was needed! Pascal and Perier had noted back in the 17th Century that barometer readings decrease as one climbs a mountain. This relation between pressure decrease and altitude increase can be used to find your altitude using an instrument, called an altimeter, which displays the altitude. A pressure altimeter consisting of an aneroid barometer mechanism is used in aircraft and by hikers. Even some digital wristwatches have altimeters. The scale of the altimeter is calibrated in terms of the altitude at which one would expect to find a particular atmospheric pressure under average conditions as described in the Standard Atmosphere.

Of course, atmospheric conditions are constantly changing and are seldom at average values. Movements of large scale weather systems cause pressure changes. One can compensate for these pressure variations by setting the beginning of the altimeter scale using the "altimeter setting", provided in hourly weather reports available from the National Weather Service. Another deviation in altitude readings from the average occurs when temperatures vary from those of the Standard Atmosphere. A column of air warmer than average will give altimeter altitude readings that are lower than the actual altitude, while a cold air column will give readings higher than the actual altitude. Pilots have to be especially mindful of these temperature variations when flying at low levels or in reduced visibility conditions.

For more information on pressure altimeters and their readings, see Thursday's optional Supplemental Information .

QUESTIONS:

To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 5 Chapter Progress Response Form, under section B. Daily Summary.

  1. A pressure altimeter indicates the correct altitude[(in the Standard Atmosphere) (whenever it is used)].
  2. During a hot summer, a pressure altimeter would probably indicate an altitude reading that was [(lower) (higher)] than the actual altitude.

HISTORICAL WEATHER EVENTS - 12 October

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


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URL Address: datastreme/learn/r_sum.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.