The following highlights of the national weather have been extracted from the surface weather map for Wednesday night:
WET WEATHER CONTINUES ACROSS THE GREAT LAKES -- A storm system moved northeastward into the Great Lakes spreading locally heavy rains across the Midwest. The low pressure center that formed this storm system traveled from near St. Louis, MO on Tuesday to the northern portions of Lower Michigan by Wednesday evening. A warm front extended east-southeastward from the low pressure center across Lakes Huron and Ontario into the Middle Atlantic States. This warm front represented the northern advance of warm, humid tropical air carried northward on southerly winds. A cold front trailed southward from the low pressure center across the Ohio Valley and then curved southwestward over the lower Mississippi Valley to terminate in a stationary front over west Texas. This cold front represented more of a wind shift than a marked boundary between air masses of significantly different temperatures and humidities. Behind the front, winds shifted from southerly to northwesterly.
While most of the rain was found across the eastern Great Lakes in the vicinity of the warm front, rain was also wrapped around the northern portion of the low, extending westward into the eastern portions of Michigan's Upper Peninsula. Rain and some embedded thunderstorms were also found across the Appalachians to the east of the advancing cold front. As the storm passed by, O'Hare Airport in Chicago, IL received 1.72 inches of rain on Wednesday, a record for the date.
This storm system is expected to undergo occlusion, a stage in the life cycle of a midlatitude storm system where the more rapidly moving cold front merges with the slower warm front to form an occluded front. The low pressure center is forecast to slowly move toward the Straits of Mackinac separating the Upper and Lower Peninsulas of Michigan by sunrise on Thursday. An occluded front is expected to curve eastward from this low center to near the western end of Lake Ontario, where the warm front should continue eastward to the New York City metropolitan area, while the cold front curves southwestward along the spine of the Appalachians. By evening the low pressure center is forecast to have moved to the north of Georgian Bay in Ontario, with the occluded front curving eastward to the central Hudson Valley in New York State before splitting into a warm front that stretches across southern New England and a cold front that traverses southward across the Piedmont of Virginia and the Carolinas. More than an inch of rain is anticipated to fall during the 24 hours ending on Thursday evening across the Northeast, stretching from the eastern portions of the Upper Peninsula across Upstate New York and Pennsylvania. Accumulations ranging from 0.50 to 1.0 inches are also expected southward along the Eastern Seaboard to the Carolinas. Unfortunately, this rain is not welcome by those working in the recovery operations that continue in New York City and Washington, DC.
MORE SEVERE WEATHER IN THE SOUTHERN PLAINS -- Several severe thunderstorms moved across southwest Kansas and the Panhandles of Oklahoma and Texas, spawning a tornado, producing high winds and dropping large hail. These thunderstorms were associated with a weak trough of low pressure that extended in a north-south direction across the western Plains. The tornado was spotted in the Oklahoma Panhandle on Wednesday evening and the strong thunderstorm winds caused some property damage in Kansas. The largest hail fell in the Texas Panhandle, with a report of diameters reaching 2.50 inches. Several other locations reported high winds, with a 70 mph wind gust at Midland, TX.
A slight risk of severe weather continues through sunrise on Thursday over the western Plains, stretching from the Texas Panhandle northward into western Kansas. On Thursday, the area that could be under a slight risk of severe weather shifts to the east and north, to stretch from Oklahoma into southern Iowa.
ACTIVE WEATHER ACROSS THE NORTHERN PLAINS -- A cold front moving eastward across eastern Montana into the western Dakotas was accompanied by some strong to severe thunderstorms. These thunderstorms contained winds that gusted to 60 mph near Glasgow in northeast Montana. Large hail fell from those thunderstorm cells as well as thunderstorms that moved across North Dakota, to the southwest of Bismarck. The cold front trailed southward from a low pressure center located over Saskatchewan.
This cold front is expected to move eastward across the Plains on Thursday, reaching the Missouri Valley in the central Dakotas by morning. A low pressure center could develop along this front and move eastward into northwestern Iowa by evening.
CONTINUED QUIET WEATHER ACROSS THE WEST -- The Intermountain West was dominated by high pressure that produced another day of relatively tranquil weather. The lack of rain-producing weather systems continues to plague sections of the West that have experienced drought conditions and wildfires this summer. While the interior West has been relatively warm, a trough of low pressure stretching the length of California has maintained an onshore flow onto the central California coast from off the cool waters of the Pacific Ocean. For the fourth consecutive day, Monterey, CA set a new low high temperature record as Wednesday's high only reached 61 degrees.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 25 degrees at Big Piney, WY, while the Wednesday afternoon highest temperature was 110 degrees at Death Valley, CA.
ALASKAN WEATHER -- A complex storm system moved eastward across the Bering Sea and the Aleutians. This storm was undergoing the occlusion process in its life cycle and consisted of one primary low pressure system in the western Bering Sea near the coast of Russia and a secondary low that was just to the north of Dutch Harbor in the eastern Aleutians. An occluded front, representing an amalgamation of a cold front with a warm front stretched southeastward from the primary to the secondary low pressure centers. A warm front continued to the southeast into the Northern Pacific, while a cold front curved to the southwest from the secondary low. Clouds were found across western Alaska. Another weak low pressure system was located over the Gulf of Alaska near Cordova and Yakutat. This system spread clouds and rain across south central and southeast Alaska. Hooper Bay had received 3.02 inches of rain during the 24 hours ending on Wednesday morning. Several weak troughs of low pressure were situated over the northern two-thirds of mainland Alaska.
The lowest temperature in Alaska on Wednesday morning was 21 degrees at Eagle. The highest statewide temperature as of Wednesday afternoon was 59 degrees at Eielson AFB.
HAWAIIAN WEATHER -- Moderate trade winds continued across the islands courtesy of a ridge of high pressure located to the north of Hawaii. Some trade showers were found on the windward slopes of the islands. Some high clouds spread northward across Oahu and Kauai from thunderstorms associated with disturbances passing to the south. This pattern is expected to continue through the remainder of the week.
EYE ON THE TROPICS -- An area of clouds and precipitation organized into a tropical depression over the western Caribbean for a brief time on Wednesday. As this depression was the ninth tropical depression of the 2001 hurricane season in the North Atlantic, it was identified as TD 9. After forming, TD 9 moved westward into Central America, causing it to become disorganized. As of late Wednesday evening, TD 9 had maximum sustained winds of 30 mph and was located 60 miles west of Puerto Cabezas, Nicaragua. Movement of the system was 18 mph to the west. Earlier, tropical storm watches had been posted for the coasts of Guatemala, Nicaragua, Honduras and Belize. Heavy rains were expected.
Simply watching clouds grow and change during any time interval brings home the idea that weather is not just limited to conditions measured at the earth's surface. To fully understand weather systems we must make measurements of various weather elements up through the atmosphere as well as over widely separated distances at the surface. Balloons carrying instruments and a radio transmitter are released twice each day (00Z and 12Z) at about 70 stations across the continental US. These instruments measure the atmospheric temperature, humidity, and pressure as the balloon ascends, until finally bursting at an altitude of about 20 kilometers or higher. The balloons are also tracked by ground based direction finding equipment to infer winds at various levels. These instruments radio their observations of the vertical patterns ("soundings") back to the launch site as they travel, hence the name radiosondes, or when tracked for winds, rawinsondes.
On the local scale, atmospheric conditions aloft may determine if the air were "unstable", resulting in vertical motions that could lead to thunderstorms or even tornadoes, or "stable", a condition that would suppress vertical cloud formation. On the larger scale, the moisture supply for major storm systems can be judged, or circulation patterns for continued fair weather or even forecasts of hurricane movements can be made. To fully know the weather now and for the near future, the total atmospheric picture must be seen. And long-range forecasts can only be attempted with knowledge of the complete three-dimensional structure of the global atmosphere.
For an explanation of how the sounding information retrieved from a radiosonde or rawinsonde is displayed on a specially prepared chart, call up the Thursday's optional Supplemental Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 2 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.