Monday, 5 September 2011

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.
Ed Hopkins

00Z Weather Systems


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 -- A large section of the nation experienced unsettled weather conditions at the end of last week and through the first two days of the three-day Labor Day weekend. A storm system moving eastward along the US-Canadian border brought wet weather to the Midwest beginning on Friday and extending through late Saturday. By Sunday, most of the rain associated with this weather system had moved eastward into the Northeast. Farther to the south, rain spread into the central Gulf Coast early Friday as Tropical Depression 13 approached the Louisiana coast. By midday on Friday, this depression had intensified to become Tropical Storm Lee. Rain associated with this new tropical storm spread northward from the Gulf Coast into the lower Mississippi Valley.

Relatively quiet weather prevailed across the West. High pressure spread southward and eastward across the Rockies and out across the Plains. This high was accompanied by a cool air mass. Over the weekend, this cooler air had spread southward behind a cold front that helped bring at least a temporary end to the heat wave that had gripped a large section of the nation's midsection earlier last week.

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.

Much of the eastern third of the nation was covered with clouds, rainshowers and thunderstorms on Sunday evening as a consequence of two major weather systems. One of these weather systems was a cold front that was moving eastward across the western slopes of the Appalachians. This cold front trailed southwestward from a low-pressure center that was situated over central Quebec. Numerous rainshowers and thunderstorms extended from the Cumberland Plateau and Ohio Valley of eastern Tennessee and Kentucky northeastward across Pennsylvania and New York State into northern New England.

The counterclockwise circulation of winds around the low-pressure center over Quebec was drawing warm, humid and somewhat unstable air northward along the Atlantic Seaboard on southerly winds. (According to meteorological tradition, winds are named for the direction of origin. Hence, a south wind is from the south.) These surface southerly winds, which turned to southwesterly winds through the lowest 15,000 feet of the atmosphere, were supplying two of the necessary ingredients for shower and thunderstorm development, namely a warm and humid low-level layer that was potentially unstable. The eastward moving cold front, together with a wind flow pattern at altitudes around 30,000 feet, provided the necessary lift of the warm, humid and unstable air to trigger the thunderstorms.

Earlier in the afternoon, some of the thunderstorm cells across sections of Kentucky, West Virginia, Pennsylvania, New York State, Vermont, New Hampshire and Maine had become severe. These thunderstorm cells had either generated strong and potentially destructive wind gusts or had produced large hail with diameters in excess of one inch. Numerous trees and power lines were downed by the straight-line thunderstorm winds. One injury was reported in Upstate New York from the thunderstorm winds. In addition to the severe weather, the showers and thunderstorms were also producing locally heavy rain in some areas of New York State and New England that seen flooding conditions last week following the passage of Hurricane Irene. According to radar estimates, many locations across the northern Appalachians from the Ohio Valley northeastward to northern New England had 24-hour rainfall totals that ranged from one half of an inch to nearly two inches.

The low-pressure center over Quebec was forecast to continue traveling eastward on Monday, which would result in the eastward movement of the cold front. By evening, northern sections of the cold front were expected to have crossed the Appalachians and to be approaching the Atlantic coast. Showers and thunderstorms accompanying this front should make for a soggy holiday across the Middle Atlantic and New England States. Over two inches of rain were forecast to fall during the 24 hours ending on Monday evening across sections of the Northeast extending from northeastern Pennsylvania across northeastern New York State and into northern sections of Vermont and New Hampshire. Because of this anticipated rain falling on saturated ground, flood and flash flood watches covered most of this region.

The other weather system responsible for the clouds and wide area of precipitation was former Tropical Storm Lee. Widespread rainshowers along with a few isolated thunderstorms covered the central Gulf Coast, the lower Mississippi Valley and sections of the Southeast. Five unconfirmed tornadoes were reported across southern Mississippi during the evening hours. As of early evening, the center of Tropical Storm Lee was located approximately 45 miles west-northwest of Baton Rouge, Louisiana's capital city. At that time, maximum sustained winds were at 40 mph. The storm's minimum central pressure was 989 millibars or 29.20 inches of mercury. (A millibar, abbreviated mb, is the traditional meteorological unit of atmospheric pressure used on analyzed surface weather maps.) By late evening, Lee had weakened to a tropical depression, as maximum sustained surface winds had dropped to 35 mph, below the 39-mph threshold for a tropical storm. However, some gusts with higher speeds were reported, especially well to the south and southeast of the center out over the coastal waters of the Gulf of Mexico. This tropical depression, which was located approximately 55 miles west-southwest of McComb, MS, was traveling to the east-northeast at 7 mph. As of that time, the National Hurricane Center had issued its last advisory for Tropical Depression Lee, as all coastal tropical storm warnings were dropped. Radar estimated rainfall totals by late Sunday evening indicated that some locations in south central Mississippi had received between five and six inches of rain during the previous 24 hours.

Tropical Depression Lee was forecast to turn to northeastward direction and increase in forward speed on Monday, as it passes across southern Mississippi. Eventually, Lee should interact with the southern end of the cold front that was moving into the lower Mississippi Valley and lose its tropical characteristics. Hence, the remnants of Lee would become a midlatitude low-pressure system, also known as an extratropical cyclone. Heavy rain associated with Lee would result in flash flooding, spreading from the central Gulf Coast northeastward into the southern Appalachians. Between 10 and 14 inches of rain were forecast to fall along the central Gulf Coast through Tuesday, while rainfall totals of between four and eight inches were possible across the southern Appalachians and sections of the Tennessee Valley. Some tornadoes could form along the Gulf Coast through Monday, primarily across southern sections of Louisiana, Mississippi and Alabama, as well as over the western Florida Panhandle and southwestern Georgia. Several of these tornadoes could be strong. The persistent onshore winds due to the slow movement of Lee should slowly weaken, resulting in the slow recession of the elevated water levels that have been piled along the coast from Louisiana to the Florida Panhandle.

Quiet weather conditions were found across the northern Plains and upper Midwest on Sunday evening. A large and elongated high-pressure system that was centered across the Dakotas was responsible for the tranquil weather along with skies that were generally cloud free from the Mississippi Valley west to the Rockies and from the Canadian border south to Texas. Some low clouds and isolated rainshowers were moving southward across the western Great Lakes, as cold air was spreading southward on north winds circulating in a clockwise direction around the eastern flank of the high pressure system. This cold air, which was following behind the second of the two cold fronts stretched across the Midwest, was traveling across the warm ground, resulting in a destabilization of the near-surface air, thereby leading to clouds and instability showers.

The large high-pressure system was forecast to drift east toward the western Great Lakes on Monday, leading to a relatively pleasant, but cool holiday across the Midwest and sections of the northern Plains. The presence of a cool and dry air mass, together with clear skies and weak winds near the center of high pressure, should permit nighttime temperatures to fall across the upper Midwest. These conditions represent ingredients for significant cooling of the surface and near-surface air by the loss of infrared (or "thermal") radiation to space, with little mixing of warmer air downward from aloft. Since temperatures were expected to fall to the low 30s by sunrise on Monday morning, frost advisories have been posted across sections of northeastern Minnesota away from Lake Superior through the first few hours after sunrise.

Skies were relatively cloud free across much of the northwestern quadrant of the nation on Sunday evening. High pressure over the northern Rockies helped maintain the relatively tranquil weather conditions across the interior Northwest. A continuation of this quiet weather across the Northwest was expected through Monday.

Scattered clouds along with isolated to scattered rainshowers and thunderstorms were detected across the Southwest early Sunday evening. These clouds and showery precipitation were due to daytime heating. By late evening, most of the clouds and showers had dissipated, leaving only several isolated thunderstorms across southern Arizona and southern California. A southerly wind flow across the Southwest that extended upward through the lowest 10,000 of the atmosphere helped bring sufficiently humid air into the region. Daytime heating of the higher terrain had helped destabilize the air, which resulted in clouds, showers and thunderstorms. The lack of surface heating after sunset led to a dissipation of the shower and thunderstorm activity. With daytime heating on Monday, afternoon shower and thunderstorm activity should return to the Southwest, extending from southern sections of California, Arizona and New Mexico northward into the Great Basin of Utah. Between one and two tenths of an inch of rain could fall from these showers and thunderstorms during the 24 hours ending early Monday evening.

TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Sunday, the lowest temperature reported in the continental U.S. was 23 degrees (Fahrenheit) at West Yellowstone, MT, while Sunday's highest temperature was 114 degrees at Borrego, CA.

ALASKAN WEATHER -- Mostly cloudy skies and rain covered large sections of Alaska on Sunday afternoon as a broad and complex area of low pressure was located over southwestern sections of the state. Three separate low-pressure centers and associated fronts were found within this large low-pressure area. One of the low pressure centers was located over the northwestern Gulf of Alaska, southwest of Kodiak Island, a second low-pressure center was situated over the southeastern Bering Sea east of the Pribilof Islands and the third low-pressure center was over the North Pacific just south of the western end of the Alaska Peninsula. The latter low-pressure center was the strongest of the three, since this center had the lowest minimum pressure. Remnants of a former weak low-pressure system remained across coastal sections of south central Alaska. A slow moving cold front stretching southwestward across western British Columbia from a low-pressure system in Canada's Northwest Territory was contributing cloudy conditions to Alaska's southeastern Panhandle. In addition to the clouds, widespread rain was reported across southern sections of the 49th State, from the Pribilofs and the Alaska Peninsula eastward to the Panhandle. Only a few areas across central and northern Alaska experienced clear to partly cloudy skies earlier on Sunday.

The pressure gradient surrounding the low-pressure centers over southwestern Alaska was responsible for strong winds circulating in a counterclockwise circulation around the entire system. Northwest winds gusted to 59 mph at Adak, 46 mph at Atka and 36 mph at Shemya along the Aleutians on the west side of the system. Farther east, winds gusted to over 30 mph at Portage and Middleton Island in coastal south central Alaska. The low-pressure system was forecast to consolidate and intensify as it travels eastward across the Gulf of Alaska on Monday and Tuesday. With the eastward movement of this system and its intensification, strong winds should persist across sections of southern Alaska. A high wind warning was in effect along the Kenai Peninsula and the western Price William Sound from Monday through Tuesday mornings, as sustained 40 to 60-mph winds could gust to 75 mph in some locations. Wind advisories were also posted across the southeast Panhandle from Monday afternoon into Tuesday morning. Anticipated rainfall totals of between two and five inches of rain by late Monday across the eastern side of the Kenai Peninsula had led to the posting of an urban and small stream flood advisory from late Monday morning through late Tuesday morning for areas surrounding the western Prince William Sound.

The lowest overnight temperature in Alaska as of Sunday was 36 degrees at Buckland, while the midafternoon highest statewide temperature was 65 degrees at Fairbanks.

HAWAIIAN WEATHER -- A large and elongated ridge of high pressure located over the North Pacific Ocean approximately 700 miles north of the Hawaiian Islands on Sunday was responsible for moderate trade winds from the east-northeast across the Aloha State. These trade winds from a generally easterly direction are the typical phenomena of the tropical ocean basins, including the Hawaiian Islands, circulating in a clockwise direction out of the equatorward side of large high-pressure cells usually located over the subtropical oceans. The air mass located over the region on Sunday was reasonably dry and relatively stable, resulting in some low-level clouds and scattered rainshowers that were being carried to toward the west-southwest across the region on the trade winds. Some of the showers were affecting the windward (east-facing) slopes of the islands. Satellite imagery showed additional low clouds located to the east, or upwind of the islands.

The high-pressure cell located to the north of the islands was forecast to weaken and continue to drift slowly to the southeast. Consequently, the moderate trade winds should gradually decline through at least midweek. Typical trade wind weather should also continue on Monday, with some low clouds and isolated showers imbedded in the trade wind flow affecting the windward sides of the islands. However, if the trade winds weaken sufficiently, local sea breeze regimes could develop during the afternoon due to the temperature differences that would develop between land and water due to daytime heating. A local sea breeze that was directed onshore (from ocean to land) would bring cooler and more humid air inland. Added by interior heating, clouds and showers could develop across the interior mountains. .

PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Partly cloudy to variably cloudy skies were reported across most sections of Puerto Rico, Culebra, Vieques, the Virgin Islands and surrounding waters late Sunday night. The showers and thunderstorms that had developed across the southern half of Puerto Rico during the afternoon had generally dissipated following sunset. Since the atmosphere was relatively humid, the showers produced locally heavy rainfall totals, with some locations receiving a few inches of rain. Because regional winds were generally light, local sea breeze regimes had developed across the islands during the late afternoon, creating a situation where showers and thunderstorms could develop across interior sections of the islands. (Sea breezes are created by temperature differences between land and ocean during the afternoon that result in the development of onshore flow of air known as a sea breeze.) By late evening, radar detected a few isolated to scattered rainshowers that were drifting to the south over the regional waters of the Atlantic and Caribbean surrounding the islands. Due to evening cooling, the winds turned to a light land breeze that was directed offshore. At that time, light surface winds across the islands were from the north to north-northeast. These regional winds were part of a counterclockwise circulation regime surrounding Hurricane Katia (see the following "Eye on the Tropics" section) that was located well to the northeast of the Virgin Islands and Puerto Rico, or equivalently to the north-northeast of the northern Leeward Islands.

Regional winds should remain relatively light, with speeds of less than 10 mph, on Monday. Afternoon showers and thunderstorms could develop across Puerto Rico due to the onset of an afternoon sea breeze regime caused by daytime surface heating of the land. Moderate ocean swell generated by Hurricane Katia were expected to build by midday Monday and affect the local waters of the Atlantic Ocean and the Anegada Passage.

EYE ON THE TROPICS -- After weakening to a tropical storm over the weekend, Hurricane Katia re-intensified into a category 2 hurricane on the Saffir-Simpson Scale as it continued traveling to the northwest across the western tropical North Atlantic on Sunday. By late evening, this hurricane was approximately 385 miles to the north-northeast of the northern Leeward Islands or 655 miles south-southeast of Bermuda. Its forward speed was to the northwest at 13 mph. Maximum sustained surface winds surrounding the central eye of Katia were estimated by satellite observations to have increased to 105 mph. Katia was forecast to continue its travel to the northwest, slowing slightly on Monday and Tuesday. Some strengthening was possible, with Katia potentially becoming a major (category 3 or higher) hurricane on Monday. As this hurricane continues its travels, large ocean swells generated by the hurricane-force winds should propagate outward and affect Bermuda, the Greater Antilles, the east-facing beaches of the Bahamas and finally the East coast of the US over the next few days. These large ocean swells could create life-threatening surf and rip current conditions along the beaches of these locales.

No tropical cyclone activity was detected across the eastern North Pacific on Sunday evening and none was expected before Tuesday.

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, 2011, The American Meteorological Society.