DATASTREME DAILY SUMMARY

Tuesday, 16 October 2001


00Z Weather Systems


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

ONE WEATHER SYSTEM DEPARTS -- The cold front that produced record rains across the Midwest over the weekend moved to the Eastern Seaboard early on Monday and by nightfall was off the Coast. As of Monday evening, this front trailed southward across the Canadian Maritimes and the western Atlantic Ocean from a low pressure system located to the northeast of Hudson Bay in northern Quebec. The southern portion of the cold front had stalled as a stationary front across South Florida, where some thunderstorm activity continued into Monday night. A waterspout was spotted moving toward the west approximately 5 miles off Key West early Monday evening.

High pressure spread across the Atlantic Seaboard behind the departed cold front. Relatively cloud-free skies and weak winds were found under the high that stretched from southern New England to the Southeastern States. As a result, freeze advisories were posted for Monday covering the higher terrain of northern Georgia.

Farther to the north, cold air continued to move southeastward across the Great Lakes, circulating around the large low pressure system over eastern Canada. This system represented a large cold pool of air that extended upward through much of the troposphere. Some widely scattered convective showers continued across the western Lakes as the cold air moved across warmer surfaces, resulting in a destabilization of the air mass.

A NEW SYSTEM MOVES ACROSS THE MIDWEST -- A storm system that developed early Monday morning over the Oklahoma Panhandle moved eastward across the Plains, reaching the Bootheel of southeast Missouri by evening. A warm front extended eastward along the Ohio River from the low center, while a cold front trailed to the southwest across the Ozarks to west Texas. A large precipitation shield was situated to the north of the system, extending from the Ohio Valley north to northern Illinois and eastward to northern Ohio. Most of the precipitation was steady rain associated with non-convective clouds, resulting from warm Gulf air moving northward and being carried aloft above the cooler air to the north of the warm front in a process often called "overrunning".

This storm system is expected to move toward the northeast on Tuesday. As it moves, the system should continue to undergo the typical occlusion process, with the cold front pivoting more rapidly around the low pressure center than the warm front. By sunrise the occluding low pressure center is forecast to be located near Dayton, OH and by evening, a secondary low is forecast to have formed in the vicinity of the point of occlusion, where the cold and warm fronts merge to form the occluded front. At this time the primary low pressure center is expected to be located along the northern shore of Lake Erie, while the secondary low pressure center should have reached the Del-Mar-Va Peninsula. The warm front should extend eastward off the East Coast, while the cold front stretches southward along the Coast to north Florida. A broad area of precipitation is anticipated to spread eastward across the Great Lakes to the north of the storm track. As much as 1.3 inches of rain could fall in the 24 hours ending on Tuesday evening from the lower Ohio Valley across the Great Lakes.

CHILLY AIR PUSHES SOUTHWARD -- A large cold high pressure system was building southeastward across the Plains in the wake of the low pressure system. Afternoon high temperatures from the mid-Mississippi Valley westward across the Plains to the Rockies were between 10 to 15 degrees below the long-term average highs for mid-October. As of Monday evening, the high pressure ridge extended southeastward along the eastern slopes of the Rockies, stretching from northern Wyoming into the Texas Panhandle.

This high is expected to drift southeastward on Tuesday, with the center of the high located across central Texas by evening. The cold dry air, coupled with weak winds should enhance nighttime radiational cooling. Just minutes before midnight on Monday night, the temperature at Oklahoma City fell to 38 degrees, tying a low temperature record for the date. In addition, A variety of freeze warnings and advisories were posted across portions of Oklahoma, Kansas, Missouri and Arkansas.

WARM WEATHER ALONG THE WEST COAST -- With a weak offshore flow of air blocking the cooling effects of a west wind off the ocean, high temperatures across central California were at least 10 degrees above average.

RAINY WEATHER MOVING TOWARD THE PACIFIC NORTHWEST -- A cold front approaching the Washington Coast is expected to cross over the Olympic Peninsula on Monday morning. By evening the front should have advanced inland to eastern Washington and Oregon. Precipitation accompanying this front should spread across the Pacific Northwest, with as much as 0.4 inches anticipated on the upwind slopes of the Olympics and Cascades by Tuesday evening.

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 9 degrees at Laramie, WY, while Monday's highest temperature was 99 degrees at Hemet and Borrego, CA.

ALASKAN WEATHER -- A strong storm system developing in the southeast portion of the Gulf of Alaska brought clouds, rain and windy conditions across the Panhandle and portions of southeast Alaska. The low pressure system with a central pressure of approximately 984 mb was located 200 miles to the west of Sitka. An occluded front lay along the coast of the Panhandle and British Columbia before curving southwestward off Vancouver Island. Some locations across the Panhandle reported gusts to over 45 mph. A second strong storm accompanied by gale-force winds (39 to 54 mph) was located over the western Aleutian Islands. A ridge of high pressure was located between these two systems, extending from the North Pacific across the Alaska Peninsula to the west coast of the state. Snow fell at Deadhorse, Galena, Tanana and Haines. Bettles and Barrow reported 8 inches of snow on the ground.

The state's lowest temperature on Monday morning was 8 degrees below zero at Noatak. The highest temperature across Alaska as of midafternoon on Monday was 52 degrees at Adak and Klawock.

HAWAIIAN WEATHER -- Pressure began to increase across the islands on Monday signaling a return to stronger trade winds. The pressure rise was due to the strengthening and southeastward movement of high pressure located far to the north of Hawaii. Fewer rainshowers are expected across the islands.

EYE ON THE TROPICS -- In the North Atlantic, Tropical Storm Karen made landfall along the Nova Scotia coast during the midday hours, accompanied by 45 mph sustained winds. The system was also losing its tropical characteristics and was becoming an extratropical storm.

After moving toward the southwest, Tropical Storm Manuel has begun drifting northward. As of Monday evening Manuel was 560 miles southwest of Cabo San Lucas, Mexico and moving toward the northwest at 8 mph. Maximum sustained surface winds were 50 mph.


CONCEPT FOR THE DAY - SATURATION

The chemical compound that we call water is a unique substance in that it can exist in three phases within the ranges of temperatures and pressures found in the atmosphere. At very cold temperatures, water is a solid, ice, that covers our lakes and puddles. Warm it some and it melts to liquid that covers almost two-thirds of the Earth and falls from clouds above. At all temperatures some water exists in the invisible vapor phase. The transition between these forms is crucial to the conditions we associate with "weather". A volume, even without air in it, has a maximum capacity for vapor that depends on the temperature. As the temperature rises, more ice can sublimate or water can evaporate to provide additional vapor for the volume. As the temperature falls, some vapor must condense back to the liquid or solid form. The maximum amount of vapor in the volume at its associated temperature produces the condition called saturation.

Normally a volume of air is not saturated, meaning that less than the maximum possible vapor is present. Saturation can occur in one of two ways. More vapor can be added to the volume to reach the maximum. This addition is seen just above the spout of a boiling tea kettle, or with so-called steam fog, a version of your breath on a cold day. Generally the atmosphere creates saturation a second way, by cooling the air volume to reach saturation with the amount of vapor originally present. Cooling to saturation can be by one of two processes. Rising air cools as it expands into lower surrounding pressures at higher altitudes. Saturation and clouds can result. Overnight radiational cooling near the ground may create fog.

For more information describing the global hydrological cycle, you may consult the optional material in Tuesday's electronic Supplemental Information .

QUESTIONS:

To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 6 Chapter Progress Response Form, under section B. Daily Summary.

  1. As the temperature increases, the amount of water vapor in a volume at saturation [(increases) (is constant) (decreases)].
  2. Clouds [(are) (are not)] usually formed by air being cooled by expansion.

HISTORICAL WEATHER EVENTS - 16 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., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.