DATASTREME DAILY SUMMARY

Tuesday, 25 September 2001


00Z Weather Systems


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

SEVERE WEATHER STRIKES THE EAST -- A slow moving cold front continued to generate widespread rain along with severe weather across the eastern portion of the country on Monday. A tornado was reported in South Carolina on Monday morning. During the afternoon, at least two tornadoes moved from northern Virginia near Culpepper eastward to near Laurel in Maryland. Two students at the University of Maryland in College Park were killed by what appears to have been a tornado that picked up their car and threw it into a grove of trees on campus. A person sustained an injury while inside a mobile home that was damaged by the tornado in Culpepper, VA. Tornadoes are rare to the Washington, DC area and the severe weather that moved through the region appears to be the worst in 75 years. In addition to the tornadoes, strong thunderstorm winds created a trail of damage that stretched for more than 70 miles. As many as 50 people were injured by the storm in Maryland's Prince George's County. Late afternoon thunderstorms also generated damaging winds in southern Pennsylvania near York and in Upstate New York near Massena. The thunderstorms also produced heavy rain. Dulles Airport near Washington, DC reported a record 2.17 inches of rain on Monday. Flash flood warnings were in effect for portions of central New York State where 3 to 5 inches of rain had fallen.

As of Monday evening, a weak low pressure area was located along the front over northern Pennsylvania. A nearly stationary front extended northward from this low into the St. Lawrence River Valley, while a cold front continued southward along the Appalachians and then across the Florida Panhandle into the Gulf of Mexico. Most of the precipitation that included thunderstorms was falling along and to the east of front, extending from near Montreal, QB southward to the Florida Peninsula. Some wrap-around precipitation was also found across the western side of the low pressure system, carried around the storm and across the Great Lakes on brisk northerly winds. These northerly winds were produced by a tight pressure gradient between high pressure over the western Great Lakes and the low over the Northeast.

The surface low pressure system is expected to move toward the north and become an occluded system on Tuesday. The northern portion of the trailing cold front is forecast to remain over New England, while the southern sections would move off the Middle Atlantic coast by morning. A slight risk of severe thunderstorms continues into Tuesday morning in the region to the east of the cold front, along the Eastern Seaboard from the Outer Banks of North Carolina northward to Upstate New York. Portions of New England are expected to experience a slight risk of severe thunderstorms through the remainder of Tuesday. Between 1 and 1.8 inches of rain could fall during the 24 hours ending on Tuesday evening across portions of much of New England, eastern New York State, New Jersey, the Del-Mar-Va Peninsula and coastal North Carolina.

A CHILLY DAY ACROSS THE NATION'S MIDSECTION -- A large high pressure centered over the Arrowhead of northeastern Minnesota spread cold air southward across the Great Lakes and the Mississippi Valley to the Gulf Coast. The polar air mass accompanying this high was responsible for the low temperatures tied at International Falls, MN (25 degrees) and Sioux City, IA (30 degrees) on Monday morning. High temperatures across much of the nation's midsection were below the long-term average highs for the last week of September, with readings between 10 to 15 degrees below the average extending from the Great Lakes southward to the Florida Panhandle and westward from the western slopes of the Appalachians to the high Plains.

This high pressure system is expected to remain relatively stationary over the western Lakes. Because of the cold air mass, coupled with clear-skies and weak winds, frost advisories were issued for Monday night and Tuesday morning that cover much of Minnesota, Wisconsin, Iowa and adjacent portions of the eastern Dakotas, northeastern Nebraska, northern Missouri and northwestern Illinois. Freeze advisories were also in effect for northeastern Minnesota, northwest Wisconsin, extreme western portion of Michigan's Upper Peninsula and north central Iowa.

MORE RECORD HIGHS ACROSS THE WEST -- Afternoon highs across the northern Rockies and a large portion of the Intermountain West ranged from 15 to 20 degrees above the average highs. Record high temperatures were either tied or exceeded in California at Elsinore (106 degrees), El Cajon (100 degrees) and Mount Wilson (90 degrees); in Nevada at Wells (91 degrees); Elko (89 degrees) and Ely (85 degrees); in Arizona at the South Rim of the Grand Canyon (83 degrees); in Utah at Salt Lake City (92 degrees), Logan (91 degrees) and Blanding (86 degrees); in Colorado at Glenwood Springs (91 degrees) and Dinosaur National Monument (85 degrees); in Wyoming at Rawlins (82 degrees), Rock Springs (81 degrees) and Laramie (78 degrees); in Washington at Hanford (94 degrees); in Idaho at Pocatello (91 degrees) and in Montana at Cut Bank (92 degrees), Great Falls (91 degrees), Missoula (90 degrees), Bozeman, (88 degrees), Helena (87 degrees), Butte and Kalispell (both 85 degrees). The record at Cut Bank was the highest temperature recorded for so late in the season at that location.

RAIN RETURNS TO CALIFORNIA -- A large precipitation shield ahead of a storm system moved onshore along the northern and central California coast. The storm was located several hundred miles to the west of Cape Mendocino with an occluded front that approached the coastline. As of late Monday night, this precipitation stretched from southern Oregon southward to near San Luis Obispo, with some of the heaviest rainfall occurring over the Sacramento and San Joaquin Valleys. This rain is welcome in a region that has experienced dry conditions and wildfires.

The low pressure system along with the front and accompanying precipitation is expected to spread eastward across the Sierras and southern Cascades on Tuesday. As much as 0.9 inches of rain could fall across the Cascades and the mountains of northern California during the 24 hours ending on Tuesday evening. A slight risk of severe thunderstorms is expected over the Snake Valley of southern Idaho on Tuesday.

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 23 degrees at Hibbing, MN, while Monday's highest temperature was 113 degrees at Parker, AZ.

ALASKAN WEATHER -- A strong storm system moving into the Gulf of Alaska continued to strengthen on Monday. The low pressure center, with a central pressure of 980 mb, was located to the southeast of Kodiak Island. An accompanying occluded front lay along the Gulf coast from the Kenai Peninsula to near Sitka on the Panhandle and then, as a cold front, curved southwestward across the North Pacific Ocean. Ultimately, the front traveled westward to the Dateline and then northwestward, becoming a stationary front to the south of the western Aleutians. With a ridge of high pressure located over the north central Pacific, a tight pressure gradient was maintained to the west of the storm system. This tight gradient produced strong northwesterly winds across southwestern Alaska. Cold Bay and Dutch Harbor had winds that gusted to over 40 mph. High pressure that extended westward across northern British Columbia also kept a relatively tight pressure gradient and windy conditions across the Panhandle. Heavy rains associated with this storm system fell across southern and southeast Alaska. Yakutat reported 2.25 inches of rain in the 24 hours ending on Monday afternoon, while 1.48 inches fell at Seward. Relatively cloud-free skies were found across the interior. However, snow fell at Deadhorse and Nuiqsut as overnight temperatures fell to the teens across the North Slope.

The state's lowest temperature on Monday morning was 25 degrees at Eagle and Cantwell. The highest temperature across Alaska as of midafternoon on Monday was 64 degrees at Healy.

HAWAIIAN WEATHER -- A wind shear line, marking a zone of marked wind change and showers, was approaching the islands from the northeast. This shear line, located approximately 450 miles to the northeast of Maui, was expected to reach the islands by late Tuesday night or early Wednesday, bringing much needed rain. The trade winds that had been relatively strong and gusty over the weekend had weakened on Monday.

EYE ON THE TROPICS -- In the western North Atlantic, Hurricane Humberto began curving toward the northeast on Monday. As of late Monday night, its position was approximately 505 miles south-southwest of Cape Sable, NS and it was moving to the northeast at 14 mph. Maximum sustained surface winds around Hurricane Humberto were 75 mph.

In the eastern North Pacific, Hurricane Juliette continued to move parallel to the Mexican coast on Monday. As of Monday evening Juliette had maximum sustained surface winds of 125 mph and was located approximately 215 miles south of Manzanillo, Mexico with forward motion to the northwest at 8 mph. Tropical storm advisories remained in effect along a large portion of the west coast of Mexico. At the same time, Tropical Storm Kiko was located 1170 miles to the west-southwest of Cabo San Lucas, Mexico and was moving westward at 8 mph. Maximum sustained surface winds around Kiko were estimated at 50 mph.

In the Central North Pacific, Tropical Depression 2 C moved to the south of Hawaii.

REPORT FROM THE FIELD -- Roger Perry, a former DataStreme participant and currently an eight-grade science teacher in Plymouth, MA reported on the monitoring of Hurricane Humberto by his Weather Club. He said that they used various satellite tracking systems to observe high level clouds streaming out of the west side of the storm system. These high level clouds above altitudes of approximately 20,000 feet, which are called cirrus clouds, are ice crystal clouds that would blow off the tops of the thunderstorm cloud clusters that surround the eye of the hurricane.


CONCEPT FOR THE DAY - WEATHER SATELLITE IMAGE INTERPRETATION

Meteorological satellites are our "weather eyes in the sky". The perspective of space is unique; a satellite image can often provide views of broad-scale weather systems in their entirety. A geosynchronous satellite, high above the equator at an altitude of 23,000 miles and revolving in the same direction as the earth rotates, remains fixed over the same scene on earth. Individual views of the planet below or a rapid sequencing of such images as animation, can display surfaces and clouds associated with weather systems.

Weather satellite sensors are basically tuned to two types of radiation: visible light and infrared. Visible light views are like black-and-white television. Dark surfaces reflect little sunlight and appear dark, while clouds and snow cover are highly reflective, appearing bright. However, clouds, fog, and surface features (forests, mountain ranges, major rivers) are usually visible from space only during daylight hours.

Infrared (heat) radiation is emitted by a surface at a rate directly proportional to its temperature. Consequently, thermal infrared images can be interpreted as temperature maps of the underlying surfaces and clouds. Because heat emissions are continuous, these satellite images are available day and night. Water and ground surfaces are generally warm and appear dark while middle level clouds are cool and look gray. The highest, coldest clouds such as thunderstorm tops are bright white. These temperature ranges may be enhanced by assigning various color schemes for television and computer display.

Specially tuned infrared sensors can even detect invisible water vapor in the middle troposphere. Regions of the atmosphere with little water-vapor content appear dark on "vapor channel" images while high vapor content areas are milky white. Clouds also show as bright white in vapor images.

To view satellite images, and for more details concerning satellite imagery, turn to the satellite views and the Tuesday optional electronic Supplemental Information available on the DataStreme Homepage.

QUESTIONS:

To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 3 Chapter Progress Response Form, under section B. Daily Summary.

  1. Visible satellite views [(would) (would not)] be available during the overnight period.
  2. Detailed daytime views that show small fair-weather clouds and snow cover are probably [(water vapor) (visible)] images.

HISTORICAL WEATHER EVENTS - 25 September

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


Return to DataStreme Homepage

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