Monday, 1 September 2014

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.

WELCOME TO SEPTEMBER -- The name September appears to be derived from "Septem", the Latin word for the number seven, representing the seventh month of the year in the old Roman calendar that started with the month of March.

WEATHER OVER THE WEEKEND -- Many areas across the nation's midsection experienced wet weather at the end of last week and over the first two days of the third-day Labor Day weekend. Areas along the upper Texas and Louisiana Gulf Coasts received rainfall totals that exceeded 6 inches between early Friday and late Sunday that resulted in flooding. This rain was associated with an area of low pressure that remained along the lower Texas coast at the start of the weekend. This low brought a flow of humid air from off the Gulf. Farther to the north, areas across the Midwest and the Ohio Valley reported between two to three areas of rain. Some areas across the upper Mississippi Valley and the western Great Lakes were the recipients of some of the heaviest rainfall totals. The locally heavy precipitation was the result of a series of weak, but complex low pressure systems and accompanying frontal boundaries that moved slowly from the Plains across the Mississippi Valley.

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.

Several areas of active weather continued through Sunday evening along the along the Atlantic Seaboard. Showers and thunderstorms produced unsettled weather along the Atlantic Seaboard, with the Middle Atlantic and southern New England coasts receiving the brunt of some of the thunderstorms, while coastal regions of the Southeast also experiencing severe weather.

Strong to severe thunderstorms developed in the warm and humid air across the Northeast during the late afternoon and continued into the evening. Several of the thunderstorm cells became severe as they generated strong straight line winds that downed trees in the New York City and Washington, DC metropolitan areas along with Worchester, MA. A lightning strike from a thunderstorm cell moving across the New York City metropolitan area early Sunday evening injured three men on a beach in the Bronx. Some of these thunderstorms also halted play at the US Open tennis tournament in Queens. By late evening many of the thunderstorms had moved off the Middle Atlantic Coast and had dissipated. However, a few scattered thunderstorms continued into the late evening across sections of eastern Maryland and southern New Jersey. According to 24-hour radar estimates, nearly three inches of rain fell across New Jersey and sections of Maryland and northern Virginia.

Farther to the south, strong to severe thunderstorms developed across sections of the Southeast during the mid to late afternoon. Damaging straight-line winds toppled trees in the Carolinas, Georgia and Florida. Many of these thunderstorms across the Southeast a cluster of late evening thunderstorms was found across sections of southern Georgia and northern Florida. Additional thunderstorms were also located over the northeastern Gulf of Mexico.

The shower and thunderstorm activity along the Atlantic Seaboard was due to a warm and humid air mass that was situated across the eastern half of the nation. Winds were carrying this warm and humid air northward along the coast, and providing two of the needed ingredients needed for thunderstorm development, namely warm, humid air at low altitudes and unstable atmospheric conditions. These southerly winds (from the south in accordance with meteorological tradition) were circulating in a clockwise circulation regime around a large and elongated ridge of high pressure that was centered over the western North Atlantic several hundred miles offshore. A disturbance in the upper-level winds at altitudes between approximately 18,000 and 30,000 feet was producing an environment that helped lift and destabilize the near surface air sufficiently to trigger the shower and thunderstorm activity.

Showers and thunderstorms were expected to redevelop along the Middle Atlantic and New England States on Labor Day. Nearly one inch of rain was anticipated during the 24 hours ending Monday evening along coastal areas extending from southern New Jersey northward to southern Maine. Farther south, showers and thunderstorms could produce nearly one half of an inch of rain along the Florida Peninsula's Gulf Coast.

Numerous thunderstorms continued into late Sunday evening across sections of the central Plains and Midwest. These thunderstorms developed in the warm and humid air that was being carried northward ahead of an advancing cold front. Southerly winds transporting this deep layer of humid air extended upward through the lowest 10,000 feet of the atmosphere. Some of the thunderstorm cells turned severe across the Plains beginning in the late afternoon and continuing through the late evening. Large hail with diameters ranging between one and two and a half inches fell from thunderstorms moving across Kansas, Nebraska and Iowa. Thunderstorm winds that reached at least 60 mph in some locations caused damage across these states. Trees and utility lines were downed and semitrailer trucks overturned on I-29 near Sioux City, IA. A person was injured by lightning in northwestern Iowa. The largest clusters of late-evening thunderstorms were detected running across Kansas, northwestern Missouri and central Iowa. A squall line, or a linear array of strong to severe thunderstorms, developed across Iowa during the mid to late evening. Farther to the north, a few isolated thunderstorms continued across southeastern Minnesota and western Wisconsin.

The cold front that was moving eastward from the Plains and helping trigger some of the thunderstorm activity was accompanying a low pressure system that was traveling toward the northeast from southeastern Manitoba into western Ontario. The cold front, which marked the leading edge of cooler air, extended from northern Minnesota to the south-southwest to southwest Kansas. The southerly winds ahead of the cold front were pumping warm, humid and unstable air northward across the eastern Plains and the Mississippi Valley. The northern extent of the warm air was indicated by a warm front that extended to the southeast across Wisconsin and Lower Michigan.

Locally heavy rainfall was reported across the Plains and Midwest because of the showers and thunderstorms. Sections of Kansas and Iowa received between one and three inches of rain according to 24-hour radar estimates. Norfolk, NE received 1.34 inches of rain, which set a daily maximum precipitation record for the date.

Northern sections of the cold front were expected to travel eastward across the upper Midwest and to the western Great Lakes on Monday as the associated low pressure system was forecast to continue traveling toward the northeast across Ontario. Southern sections of the cold front were expected to move slowly eastward and southward across the central and southern Plains. Thunderstorm activity was anticipated to continue on Monday across the nation's midsection. Some of the thunderstorm cells could become severe on Monday, primarily across sections of the Midwest and eastern Plains in the region ahead of the advancing cold front. The squall line that had developed during the late evening over Iowa could continue across sections of southern Wisconsin and northern Illinois by sunrise on Monday. An elongated area extending from central Lower Michigan southwestward across the mid-Mississippi Valley and the Ozark Plateau to northeastern Oklahoma were considered to be under a slight risk of a severe thunderstorm. Major metropolitan areas within this slight risk area include Detroit, MI; Milwaukee, WI; Chicago, IL; Indianapolis, IN; St. Louis, MO and Kansas City, MO. Potentially damaging straight-line winds (with speeds in excess of 58 mph) and/or large hail (with diameters one inch or larger) could develop from these severe thunderstorms. An isolated tornado or two could develop near the southern end of the slight risk region.

The showers and thunderstorms were expected to produce locally heavy rainfall totals across sections of the eastern central Plains and the Midwest. Between one and two inches of rain were forecast to fall during the 24 hours ending on Monday evening across areas of eastern Kansas and western Missouri in the Kansas City metropolitan area and northward into northern Iowa and southeastern Minnesota.

In addition to the unsettled weather across the nation's midsection and Eastern Seaboard, much of the region experienced relatively warm weather as widespread southerly winds were bringing warm air northward across the region. Afternoon high temperatures were between 5 and 10 Fahrenheit degrees above the average highs for the date. Temperatures reached record levels in on Sunday in west Texas at Borger (106 degrees), Amarillo (104 degrees) and Lubbock (100 degrees). Farther to the east, the temperature at Tallahassee, FL on Sunday afternoon reached 97 degrees, which tied an earlier high temperature record for the date.

While warm and unsettled weather dominated the eastern half of the nation on Sunday, cool and tranquil weather was found across a large area of the West. Afternoon high temperatures on Sunday across the northern High Plains, the northern Rockies and the interior Northwest were 10 to 15 Fahrenheit degrees were below the long-term late August average highs.

The cool air that spread across the northwestern quadrant of the country was behind the cold front that had moved across the Plains. A large ridge of high pressure that extended eastward from the Pacific Northwest across the Northern Rockies was responsible for the cool and dry air mass. Only a few clouds were detected across the region, primarily over some of the higher terrain of the Rockies and the Cascades.

The cool and generally quiet conditions across the West were expected to continue on Labor Day. High pressure was forecast to remain centered over the Northwest and the Intermountain West. However, a few scattered light rainshowers could move across the higher terrain of the Rockies, producing rainfall totals of several hundredths of an inch.

TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Sunday, the lowest temperature reported in the continental U.S. was 31 degrees (Fahrenheit) at 1 mile east of Dillon, CO, while Sunday's highest temperature was 115 degrees at Death Valley, CA.

ALASKAN WEATHER -- Mostly cloudy to overcast skies were reported at many locations across northern, western and southwestern Alaska on Sunday afternoon, while partly cloudy skies were confined to a few areas in the interior and south central sections of the 49th State. Satellite imagery revealed numerous clouds moving eastward across northern and western Alaska that were associated with a low pressure center located over the Arctic Ocean to the north of Point Barrow and a trailing cold front that curved to the southwest over northwestern Alaska and out over the Chukchi Sea to another low pressure center in Far East Russia. The satellite images also showed a large and fairly dense area of clouds across southern Alaska that were associated with a complex low pressure system with several low pressure centers and an accompanying front that stretched along the southeast Panhandle and the adjacent waters of the northeastern Gulf of Alaska. Fewer clouds were detected across interior and south central sections of the state due to a weak ridge of high pressure that extended into central Alaska from the southwest. Radar at Nome detected rain moving toward the Norton Sound and the western coast of Alaska, while radar revealed light precipitation along the coast of the southeast Panhandle.

The cold front that was stretched across northwestern Alaska late Sunday afternoon was forecast to sweep southeastward during the overnight hours and through Monday. Winds were expected to increase from a southwest direction in advance of the cold front. Therefore, wind advisories were posted that would run from early Monday through Tuesday mornings across central and eastern interior sections of the state, running from the Tanana Valley eastward across the Fairbanks metropolitan to the Yukon Flats and the Alcan border. Southwest winds could reach speeds gusting to 45 mph over the higher terrain.

The temperature on Sunday afternoon at St. Paul Island reached 58 degrees, which established a new high temperature record for the date.

The lowest overnight temperature in Alaska as of Sunday was 25 degrees at Eagle, while the midafternoon highest statewide temperature was 66 degrees at Seward.

HAWAIIAN WEATHER -- Winds across the Aloha State on Sunday afternoon were generally from the east-southeast or southeast. These winds were differed from the usually easterly to northeasterly trade wind regime that prevails across the Hawaiian Islands and other tropical locales. These trade winds were part of the clockwise circulation that encircles large pressure systems that are found across the subtropical ocean basins. The presence of a weak surface low pressure trough located near the islands along with a weak high pressure system centered approximately 1500 miles to the north-northeast of the Hawaiian Islands was responsible for the light winds from the east-southeast. Surface observations indicated partly cloudy skies on many of the smaller western islands, while mostly cloudy to cloudy skies were found across Maui and the Big Island. Satellite imagery indicated several large clusters of clouds spreading to the north across Maui, the Big Island and surrounding waters. Fewer clouds were around the western islands in the chain. Radar also detected a few showers over the Big Island and local waters. Humidity levels had increased due to low level winds from the southeast. The light regional winds resulted in the development of afternoon sea breeze regimes around several of the islands in response to daytime surface heating of the interiors of the islands. The sea breezes are directed onshore because of the slight pressure differences that formed between water and land. The sea breezes also helped develop clouds and showers across the interiors of the islands by late afternoon.

Light regional winds from the east-southeast were expected to prevail across the islands through Monday before a return to light to moderate easterly trade winds on Tuesday. The light winds from the east-southeast should permit the development of afternoon sea breezes, which would result in the development of clouds and a few showers across the interior sections of the islands and over the normally dry leeward slopes on Monday.

PUERTO RICO AND US VIRGIN ISLANDS WEATHER -- Most of the stations across Puerto Rico, Culebra, Vieques and the Virgin Islands reported clear to partly cloudy skies late Sunday night, while mostly cloudy conditions were found along the southern coast of Puerto Rico. Satellite imagery revealed only a few clusters of low clouds traveling toward the west across the region. A few higher level clouds were moving northward away from the islands. Radar detected a few isolated light to moderate showers moving toward the west. These showers, which were associated with the low clouds, were being carried westward across the region on winds in the lowest 10,000 feet of the atmosphere that were generally from the east. The higher level clouds were being transported northward on winds between 20,000 and 30,000 feet that were from the south. A weak tropical wave was moving westward from the Virgin Islands toward Puerto Rico during the mid to late evening. This tropical wave, which represents a wavelike disturbance in the near-surface tropical easterly winds, was responsible for some of the low clouds and light rainshowers moving across the region.

The tropical wave should continue toward the west on Monday and drier air was expected to spread across the islands as surface winds return to a more typical easterly trade wind direction. Typical trade wind weather should continue across the islands on Monday with afternoon showers and thunderstorms over the interior sections of the islands. Another tropical wave was expected to reach the region by late Tuesday.

EYE ON THE TROPICS -- No organized tropical cyclones (low pressure system that develops over tropical ocean waters) were found in either the North Atlantic basin or the eastern North Pacific basin on Sunday evening. However, the National Hurricane Center was monitoring disturbed tropical weather in both hemispheres.

In the North Atlantic basin (that includes the Caribbean Sea and the Gulf of Mexico) the region of disturbed weather was associated with a broad area of low pressure that was in a tropical wave moving westward across southern Mexico's Yucatan Peninsula. The tropical wave is a wavelike disturbance in the easterly winds in tropical latitudes. Forecasters give this low a medium chance of becoming organized into a tropical depression by Tuesday evening when it would move out over the waters of the eastern Bay of Campeche on Monday and then the southwestern Gulf of Mexico.

In the eastern North Pacific basin (east of longitude 140 degrees West) the disturbed tropical weather was a large disorganized array of rainshowers and thunderstorms that was associated with a broad trough of low pressure located several hundred miles to the south of the south-central coast of Mexico. While forecasters gave this low pressure trough a low change of intensifying into an organized tropical system by Tuesday evening.

For additional information concerning tropical weather systems from last week along with other topics concerning weather and climate, please consult the 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 Sample RealTime Weather Portal

Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2013, The American Meteorological Society.