NOTE: The yellow dashed trough line running diagonally from Quebec to Baja California across the accompanying satellite-fronts insert appears to be the result of a coding error and should be ignored.
The following highlights of the national weather have been extracted from the surface weather map for Wednesday night:
A ZONE OF CONTRAST -- A cold front trailed southward from a low pressure center near Lake Nipigon to the north of Lake Superior. This front, which extended southward across the western Great Lakes and then southwestward across the Mississippi Valley and into the southern Plains, separated warm and reasonably humid air across the East from colder and slightly drier air over the Rockies and the Intermountain West. In addition, the front represented a wind-shift line, where southerly winds were found to the east of the front, while westerly winds were located to the west of the front. The cold front is expected to move eastward and southward on Thursday. By evening the front should have crossed the Mississippi Valley on the east, while the front should be close to the Texas Gulf Coast on the south.
MORE SEVERE WEATHER ACROSS THE PLAINS -- Thunderstorms continued to develop late Wednesday night across portions of Arkansas, Oklahoma, Texas and Louisiana in the vicinity of the cold front. Some of the thunderstorms earlier on Wednesday became severe, with tornadoes, high winds and large hail. A tornado moved through Springfield in southwestern Missouri on Wednesday morning. The tornado, rated F-0 on the Fujita Scale, produced a damage path that was 100 yards wide and one-half mile long near downtown. In the late afternoon two tornadoes were spotted in northwest Arkansas. Some damage was caused by one of the tornadoes. Straight-line thunderstorm winds also damaged property in approximately two dozen locations across northwest Arkansas and southwest Missouri. The thunderstorms that traveled across north Texas were prolific hail producers, with golfball sized hail (1.75 inch diameter) covering the ground in at least one location.
Some of the rainfall from these thunderstorms was heavy. Flash flood warnings were issued early Thursday morning for several streams in southwest Arkansas when Doppler radar estimated that 4 to 6 inches of rain may have fallen over the area.
A slight risk of severe weather continues through Thursday morning across much of Texas, and portions of Arkansas and Missouri in the vicinity of the forecasted position of the front. During the subsequent 24 hours, the lower Mississippi Valley is expected to be in a region that could be under a slight risk of thunderstorms. Between 1 to 2.5 inches of rain could fall during the 24 hours ending as of Thursday night over an area that extends from north Texas northeast to the Ohio Valley. As a result, flash flood watches were posted for a region that extended from the Red River of northern Texas and southern Oklahoma northeastward to west central Arkansas.
PLEASANT WEATHER ACROSS THE EAST -- A ridge of high pressure found off the Middle Atlantic Coast produced pleasant and tranquil weather along the Eastern Seaboard from the Carolinas to New England. With relatively sunny skies, coupled with a southerly wind flow, afternoon high temperatures across the Ohio Valley and eastern Great Lakes were on the order of 10 degrees above the average highs for mid-October.
The high pressure cell is expected to remain off the coast, maintaining this tranquil weather pattern across the East for another day.
COLD AIR FOUND ACROSS THE WEST -- A chilly air mass had moved into the Intermountain West in association with a large high pressure cell that was centered over Utah and Colorado. With a cold air mass coupled with clear skies and weak winds, record low temperatures were tied or set on Wednesday morning at several locations in Utah to include Randolph (10 degrees), Logan (24 degrees) and Cedar City (25 degrees). Afternoon high temperatures were 10 degrees below the long-term average highs across the West, stretching from the Great Basin and central Rockies into the Panhandles of Texas and Oklahoma. Below average highs were also found over the Pacific Northwest. In Washington State, the high temperature at Winthrop was 43 degrees and at Seattle 52 degrees, both representing low high temperature records for the date at the respective locations.
The high pressure cell was expected to drift slowly to the east-southeast. With another night of enhanced radiational cooling anticipated, freeze advisories were posted for the high plateau of northeast Arizona and northwest New Mexico and for the Colorado River Valley in western Colorado and eastern Utah.
UNSETTLED WEATHER IN THE NORTHWEST -- A cold front was moving into the Pacific Northwest accompanied by several areas of precipitation across western Washington State, Oregon, northern California and central Idaho.
This front is expected to move eastward across the northern Rockies by Thursday evening. A tight pressure gradient was situated across the Montana Rockies, as indicated by a relatively close packing of the isobars on the surface weather analysis charts for Wednesday night. This tight pressure gradient produced strong winds. A high wind warning was issued for portions of Montana east of the Continental Divide as 30 to 40 mph west winds could gust to between 65 and 70 mph for much of Thursday.
The precipitation accompanying the front should spread across the region. As much as 0.8 inches is forecast to fall by Thursday evening especially on the upwind slopes of the mountain ranges in the Pacific Northwest. With cold air in the interior, snow is expected across the higher terrain of the Montana Rockies.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 11 degrees at Stanley, ID, while the Wednesday afternoon highest temperature was 95 degrees at Thermal CA and Presidio, TX.
ALASKAN WEATHER -- A large area of low pressure over southeastern Alaska brought clouds and rain across southern Alaska. Rain also fell across the Panhandle. Notable 24 hour precipitation totals included 1.34 inches at Cordova and 1.30 inches at Yakutat. The low pressure center, located near Galena, also helped draw arctic air southward across the northern two-thirds of Alaska. This cold air was also associated with high pressure over the Arctic Ocean. Afternoon temperatures across the North Slope ranged from the single digits to the low 30s. Snow was falling at Deadhorse, Nuiqsut, Bettles and Saint Paul Island. A strong storm system was moving eastward across the North Pacific. As of late Wednesday afternoon the low pressure center, with a central pressure of approximately 968 mb, was located approximately 600 miles south of Adak in the central Aleutians. Because of the tight pressure gradient surrounding this storm system, strong northerly winds were found across the Aleutians. A ridge of high pressure extended eastward from near the Kamchatka Peninsula across the western Aleutians into the western Bering Sea.
The lowest temperature in Alaska on Wednesday morning was one degree above zero at Galbraith Lake. The highest statewide temperature as of Wednesday afternoon was 50 degrees at Haines, Metlakatla and Skagway.
HAWAIIAN WEATHER -- With a ridge of high pressure located to the north and northeast of the islands, residents of the Aloha State experienced another day with trade winds ranging from 10 to 25 mph. However, the trade winds should weaken slightly on Friday as the high moves toward the northeast and a cold front approaches from the northwest. Widely scattered trade showers should continue over the windward slopes of the islands, particularly at night and early mornings. Small craft advisories remain in effect for all state waters, as well as high surf advisories for the east shores of the islands.
EYE ON THE TROPICS -- In the tropical eastern Pacific, the fifteenth tropical depression of the season formed off the west coast of southern Mexico. As of Wednesday evening Tropical Depression 15E was located 240 miles south-southwest of Acapulco and it was moving to the west at 15 mph. Maximum sustained surface winds were 35 mph.
SELECTED SEPTEMBER EXTREMES -- The National Climatic Data Center (NCDC) has produced a list of various noteworthy extremes in temperature and precipitation across the United States during the just concluded month of September at http://lwf.ncdc.noaa.gov/oa/climate/extremes/2001/september/septemberext2001.html . Updates may be made during the upcoming weeks.
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 .
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.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast