WELCOME TO THE WEEK ONE FOR AMS WEATHER STUDIES-This Daily Weather Summary file will describe the current weather pattern across the U.S. Additional Supplemental Information…In Greater Depth files will provide optional background material when appropriate.
Sincerely,
Ed Hopkins
HAPPY LABOR DAY!
Today, the first Monday of September, is a Federal holiday as passed by Congress and signed by President Grover Cleveland in 1892. This holiday often represents the traditional end of summer.
WEATHER OVER THE WEEKEND -- The Northeast and sections of the Midwest along the Great Lakes and upper Mississippi Valley experienced relatively tranquil weather conditions for the first two days of the three-day Labor Day weekend. A large area of high pressure drifted eastward across this region over the weekend. Rainshowers and thunderstorms seemed to be common place across sections of the Southeast and the Midwest along the mid-Mississippi and lower Ohio Valleys. A weak area of low pressure traveled eastward from the southern Plains into the lower Mississippi Valley, accompanied by showers and thunderstorms. Widespread scattered showers and thunderstorms were also found across the Rockies and Intermountain West over the weekend.
WEATHER FOR THE START OF THE NEW WEEK -- The following highlights of the national weather have been extracted from the surface weather map for late Sunday night.
Relatively quiet weather was reported across the Northeast and the Great Lakes States on Sunday evening, courtesy of a large and elongated high-pressure system centered over the Gulf of Maine off the Downeast coast of Maine. This ridge of high pressure extended westward across the Great Lakes to the upper Mississippi Valley. Satellite imagery along with surface reports showed generally cloud-free skies from New England westward to the upper Mississippi Valley. Winds were weak near the center of the high-pressure ridge, a characteristic associated with most highs. Farther away from the high-pressure center, a clockwise circulation in the surface winds could be seen. Winds from the southeast were found across the Middle Atlantic States and the Ohio Valley. Farther west, winds were from the south across the Mississippi Valley, but from the southwest across sections of Ontario that were to the northwest of the surface high-pressure ridge.
The high-pressure center situated along coastal New England was forecast to move eastward across the Canadian Maritimes and out over the North Atlantic on Monday. Sections of the Northeast should expect relatively tranquil weather on Labor Day despite the eastward movement of the high. However, some clouds and widely scattered showers associated with a weather system over the Ohio Valley could spread northeastward by late afternoon and evening.
Farther south, clouds, rainshowers and some isolated thunderstorms were found along the coast of North Carolina and over the coastal waters of the western North Atlantic on Sunday night. These clouds and precipitation were due to a low-pressure system that was centered off the South Carolina coast. The counterclockwise circulation surrounding this low-pressure center produced north to northeast flow of air across coastal sections of the Carolinas.
The low-pressure system was forecast to remain along the South Carolina coast on Monday, bringing some clouds and precipitation to the coastal sections of the Carolinas. Nearly 3.75 inches of rain across the Outer Banks of North Carolina were forecast to fall during the 24 hours ending on Monday evening.
Sections of the Midwest, primarily along the Ohio Valley, together with the Cumberland Plateau along the western slopes of the central Appalachians continued to experience widespread clouds, rainshowers and thunderstorm activity on Sunday evening. The clouds and showery precipitation were associated with a diffuse area of low pressure that stretched across the mid-Mississippi and Ohio Valleys, from the Ozarks on northwestern Arkansas to southwestern Ohio. (Note: The orange dashed line appearing on the accompanying fronts-satellite insert is a trough line, which represents the axis of a weak trough of low pressure.) A pool of cold air at altitudes ranging from 20,000 to 30,000 feet helped destabilize the atmosphere sufficiently to permit formation of clouds and showery precipitation across the region.
The area of showers and thunderstorms was expected to shift northeastward on Monday, moving up the Ohio Valley toward the eastern Great :Lakes. As much as one inch of rain could fall across sections of the Ohio Valley in the 24 hours ending Monday evening.
Rainshowers and some thunderstorms were found across the high Plains and sections of the central Rockies on Sunday evening, primarily across southern Saskatchewan, eastern Montana, central and eastern Wyoming, eastern Colorado and western Nebraska. This showery precipitation was out ahead of a cold front that was traveling eastward across the region as it trailed southward from a low-pressure system moving across central Alberta. Earlier in the evening, large hail with diameters up to one inch fell from thunderstorm cells traveling across the high Plains of the Nebraska Panhandle.
The cold front marked the leading edge of somewhat colder air. Temperatures late Sunday evening across eastern Montana out ahead of the front had remained in the mid to upper 70s, while temperatures behind the front in western Montana were in the 50s and 60s. Although the higher elevation was playing a factor in the lower temperatures across the western Montana, these temperatures were at least 10 Fahrenheit degrees lower than readings taken 24 hours earlier, which was prior to frontal passage.
The air across the northern and central Plains was sufficiently warm, humid and unstable to provide two of the ingredients for thunderstorm development. The eastward moving cold front helped provide the trigger, or the needed lifting mechanism to thunderstorm development.
Northern sections of the cold front were expected to move eastward across the northern Plains on Monday as the parent low pressure system was forecast to travel eastward into central Saskatchewan. By evening, the front should have reached central North Dakota. A secondary low pressure center could form along southern sections of the front over northwestern Wyoming early Monday, causing this section of the front to move relatively slowly, remaining over Wyoming through the evening hours. Showers and thunderstorms could develop in the warmer, more humid and somewhat unstable air mass out ahead of the advancing front. However, atmospheric conditions do not appear favorable for development of organized severe weather event on Monday across the Plains.
The frontal passage should also result in windy conditions across those sections of the West behind the front. Lake wind advisories were in effect for much of Monday in the Snake Valley of southeastern Idaho, but through mid-Monday morning over northeastern Montana.
Farther south, rainshowers and thunderstorms that developed during the mid to late afternoon across the southern Rockies and adjacent high Plains had diminished during the mid to late evening. Only a few isolated thunderstorms continued across the mountains of New Mexico. This showery precipitation was also associated with warm, humid and unstable atmospheric conditions, but the trigger for additional lift was the daytime heating of the mountain slopes that provided the added buoyancy.
Unsettled weather was found across the Pacific Northwest on Sunday evening. Rain had spread across the Olympics, the Puget Sound area and the west-facing slopes of the Cascades of western Washington. This rain was associated with an approaching area of low pressure that had moved across the eastern North Pacific toward British Columbia's Vancouver Island. Farther to the east, thunderstorms traveling across eastern Washington State generated strong thunderstorm winds that caused some property damage north of Spokane, along with quarter-sized hail (three-quarter of an inch diameter) late Sunday afternoon. Daily maximum precipitation records for the date were set on Sunday in Washington State at Olympia (1.54 inches) and Seattle (0.93 inches).
An onshore flow of humid air from off the Pacific Ocean should continue through Monday. This onshore flow will be forced aloft by the Olympics and Cascade Mountains, helping to produce additional rainfall totals. As much as one inch of precipitation was forecast to fall by late afternoon on the west-facing slopes of the Olympics and Cascades during the 24 hours ending late Monday afternoon. The air was sufficiently cold that some light snow could fall at elevations above 6000 feet in the Cascades east of Seattle.
Farther south, the cold air along with clear skies could result in cold conditions across the interior sections of the Northwest. A freeze warning and frost advisories were posted for sections of central and southern Oregon and northeastern California.
Quiet weather conditions were found across the Southwest and California on Sunday evening. These conditions should continue on Monday.
TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Sunday, the lowest temperature reported in the continental U.S. was 33 degrees (Fahrenheit) at Saranac Lake, NY and Whitefield, NH, while Sunday's highest temperature was 111 degrees at Death Valley, CA.
ALASKAN WEATHER -- Three low-pressure systems were responsible for clouds covering southwestern, northwestern and southeastern sections of Alaska on Sunday. A storm traveling eastward along the central and eastern Aleutians spread clouds across these sections of this island chain, along with much of the Alaska Peninsula and the Pribilof Islands in the southeastern Bering Sea. Some rain and fog were also found across the region. As of late Sunday afternoon, the low pressure center was located southwest of Adak, while an accompanying frontal system stretched eastward and southward across the eastern Aleutians and the North Pacific. Farther to the north, clouds covered northwestern Alaska, stretching from the northern Bering Sea northeastward across the Seward Peninsula and the North Slope to the Arctic coast. These clouds were spreading to the northeast along a frontal boundary that contained a diffuse area of low pressure located over northwest Alaska. Clouds were also spreading northwestward across the Alaska Panhandle and southeast Alaska in association with broad area of low pressure located over the eastern Gulf of Alaska. This low pressure area contained tow separate low pressure centers, with one low pressure center located west of Ketchikan, while the other low was centered off the British Columbia coast near Vancouver Island. Some snow also fell across the central and eastern Arctic coasts, while light rain fell farther south along the North Slope. Winds reached 30 mph along the Seward Peninsula and St. Lawrence Island in the northern Bering Sea. A ridge of high pressure stretched northward across the western Gulf of Alaska and into sections of south central and southwestern Alaska. This ridge was accompanied by relatively cloud-free skies across south central, southwestern and interior sections of the state. Unseasonably warm weather continued across sections of southern Alaska on Sunday, as daily high temperatures for the date were set at McGrath (72 degrees) and Juneau (72 degrees).
The lowest overnight temperature in Alaska as of Sunday was 28 degrees at McCarthy, while the midafternoon highest statewide temperature was 72 degrees at McGrath.
HAWAIIAN WEATHER -- A large and elongated ridge of high pressure located over the North Pacific Ocean north of the Hawaiian Islands on Sunday was responsible for easterly trade winds across the Aloha State. These trade winds from the east are the typical phenomena of the tropical ocean basins, including the Hawaiian Islands, as the winds circulate out of the equatorward side of the large high-pressure cells usually located over the subtropical oceans. A pocket of humid air that produced low-level clouds and scattered rainshowers was being carried to the west across the region on the easterly trade winds. Some of the showers were affecting the windward (east-facing) slopes of the islands. Thin high level clouds at altitudes above 20,000 feet were spreading northward across the islands due to a pool of cold air located at these altitudes to the west of Kauai.
The high-pressure center northeast of the islands was forecast to weaken on Monday, resulting in a gradually weakening of trade winds. Drier air should result in fewer low-level clouds and diminished rainshower activity.
PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Mostly cloudy skies were reported across most sections of Puerto Rico, Culebra and Vieques late Sunday night, while variably cloudy skies were found over the Virgin Islands. The rainshowers and isolated thunderstorms that had produced locally heavy rainfall totals across sections of northeastern Puerto Rico had either dissipated or moved offshore by late evening. The heavy rain caused flooding of the Rio Fajardo in Fajaro. During the late night hours, a few light rainshowers continued over Puerto Rico, while radar detected numerous showers over the waters of the Atlantic north of the islands and a few showers passing to the west over the Caribbean south of Puerto Rico. The clouds and the precipitation were associated with a tropical wave that had moved westward across the region, passing across Puerto Rico Sunday morning, the Mona Passage during the afternoon and reaching Hispaniola during the evening. A tropical wave is a wave-like disturbance in the typical tropical wind flow. Near surface winds across the islands late Sunday were light and from the east-southeast. The shower activity was expected to diminish and clouds dissipate on Monday morning as slightly drier air spreads across the region. However, daytime heating along with residual moisture should result in the formation of clouds, scattered rainshowers and isolated thunderstorms over the western interior sections of Puerto Rico during Monday afternoon.
EYE ON THE TROPICS -- The National Hurricane Center was monitoring a strong tropical wave with numerous rainshowers and thunderstorms that was located over the tropical waters of the eastern North Atlantic approximately 300 miles east-southeast of the Cape Verde Islands early Monday (local time). This wave was moving to the west-northwest at approximately 15 mph. Forecasts indicate a medium chance that tropical depression could form within the next two days, as conditions appear favorable for additional intensification.
The National Hurricane Center was monitoring a large area of low pressure that was accompanied by rainshowers and thunderstorms over the eastern tropical North Atlantic approximately 650 miles west of the Cape Verde Islands on Monday (local time). Conditions appeared favorable for the development of a tropical depression within the next two days as the low moves westward.
An area of low pressure containing rainshowers and thunderstorms was located over the tropical waters of the eastern North Pacific Ocean approximately 1100 miles west-southwest of Cabo San Lucas at the southern tip of Mexico's Baja California Peninsula. As of late Sunday afternoon, this low-pressure area was moving slowly to the west-northwest at 5 mph. While this low had decreased on Sunday, conditions appeared to remain favorable for the development of a tropical depression by late Monday.
NOTE: For discussion of the movement of tropical weather systems over the past week, consult the current Weekly Weather and Climate News.
TOUR OF THE AMS WEATHER STUDIES HOME PAGE -- The AMS Weather Studies home page contains a variety of weather information. To help you get an overview of this page, consult Monday's Supplemental Information…In Greater Depth.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
Return to AMS Weather Studies Homepage
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2009, The American Meteorological Society.