WELCOME TO THE WEEK ONE OF 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.
The following discussion is based upon the major weather features appearing on Monday night's surface weather maps:
TRANQUIL WEATHER CONTINUES ACROSS NORTHEAST -- An elongated ridge of high pressure that was centered along the New England coast Monday evening was responsible for a relatively pleasant Labor Day across the Northeastern and Great Lakes States. Except for thunderstorm clouds associated with a weak disturbance that moved into northern Ohio along the shores of Lake Erie, skies were relatively cloud-free from New England westward to the upper Mississippi Valley. Afternoon high temperatures were close to the long-term average highs for the date across New England, while farther west, high temperatures across the upper Midwest were 10 Fahrenheit degrees above average.
While high pressure was expected to remain across the Northeast into Tuesday morning, clouds and rain were forecast to spread northward across the Middle Atlantic States due to a storm moving along the Carolina coast. Additional clouds and rain were to spread eastward from a disturbance from the Midwest.
STORMY WEATHER ALONG MIDDLE ATLANTIC COAST -- A large area of rain was moving northward across coastal sections of the Middle Atlantic States on Monday evening in association with a strong storm offshore of the North Carolina coast. This storm had developed off the South Carolina coast on Sunday and intensified as it moved to the northeast. By Monday evening, the storm's low-pressure center was located just offshore of Cape Fear, NC.
The counterclockwise circulation of winds surrounding the low-pressure center was producing an onshore wind from the east on the north side of the low. These easterly winds were reported northward along the coast of the Del-Mar-Va Peninsula. (NOTE: According to meteorological tradition, winds are named for the direction from which they originate; therefore, an east wind is from the east.) Northerly winds were observed along the coastal plain and Piedmont of the Carolinas on the western side of the storm's center.
The onshore flow of humid air from off the Gulf Stream and the western North Atlantic supplied sufficient water vapor for locally heavy rainfall along the coast. Daily maximum precipitation records were set on Monday at Cape Hatteras, NC (3.37 inches) and at Wallops Island, VA (0.64 inches). As of late Monday, the precipitation shield associated with this storm had extended as far north as the Baltimore (MD) metropolitan area.
The storm was forecast to travel northeastward across the Outer Banks of North Carolina by sunrise on Tuesday. By evening, the storm's center was expected to be nearly 100 miles off the mouth of Chesapeake Bay. The precipitation shield to the north and northwest of the storm was expected to continue producing locally heavy rainfall totals. Up to four inches of rain were forecast to fall across sections of the Del-Mar-Va Peninsula and Chesapeake Bay during the 24 hours ending Tuesday evening. Consequently, flood advisories and watches were posted from eastern North Carolina north to Tidewater Virginia, running through early Tuesday afternoon.
The persistent onshore winds from the northeast with speeds ranging from 15 to 25 mph were expected to pile seawater against the coast, producing waves of up to four to six feet in the surf zone. Coastal flooding of low-lying coastal areas was possible. Piling the water along the beaches could result in deadly rip currents in the water, with a coastal hazards risk running through Tuesday evening.
WET WEATHER IN OHIO VALLEY -- Rain and thunderstorms moved to the northeast across the Ohio Valley toward Lake Erie on Monday evening. This showery precipitation was associated with a weak low-pressure center and trough of low pressure located over southern Indiana. This surface low-pressure trough was a reflection of a disturbance in the wind flow at altitudes between 20,000 and 30,000 feet that helped maintain the precipitation. As of late Monday evening, scattered to numerous rainshowers covered sections of Ohio, Indiana, Michigan and West Virginia, while several isolated thunderstorms were detected in central Ohio and southeastern Lower Michigan. Earlier, thunderstorms traveling across middle Tennessee in association with the low pressure trough became severe late Monday afternoon, generating strong thunderstorm winds and producing large hail. Trees were downed by the strong-thunderstorm winds and up to seven-eighths of an inch-diameter hail fell to the east and northwest of Nashville, TN.
The showers and thunderstorms over the Ohio Valley were forecast to expand to the northeast and east on Tuesday. By early evening, the area of precipitation was to merge with the precipitation shield accompanying the storm off the Carolinas, resulting in widespread rain and thunderstorms from the upper Ohio Valley across the Appalachians to the Eastern Seaboard.
ACTIVE WEATHER ACROSS THE PLAINS -- Numerous thunderstorms developed during the afternoon and early evening across the Plains on Monday afternoon and evening. Some of the thunderstorms became severe, as the threshold criteria were exceeded pertaining to strong thunderstorm winds (exceeding 57 mph) and large hail (three quarters of an inch or greater). Thunderstorm winds gusted to 70 mph across western North Dakota, south central Nebraska and western Kansas. Up to golfball-sized hail fell in Nebraska, Kansas and Oklahoma.
Several clusters of thunderstorms continued across the western Plains into late Monday evening. One cluster was across eastern Montana extending southward across eastern Wyoming. These thunderstorms were in the cooler air behind a nearly stationary cold front that stretched from western North Dakota southwestward to southern Idaho and northern Nevada. Two clusters of thunderstorms were detected across western and south central Kansas. These thunderstorms were in the warm and humid air out ahead of the cold front. Another cluster of thunderstorm cells was located over sections of west Texas. In addition to the thunderstorms on the Plains, several isolated thunderstorms were also found across the mountains of Colorado and western New Mexico.
Thunderstorm activity was anticipated across the Plains on Tuesday, especially in the region with warm and humid air out ahead of the advancing cold front. The National Weather Service's Storm Prediction Center has forecast a slight risk of severe thunderstorm activity across a section of the central and northern Plains running from northeast Colorado to northwestern Minnesota. Hail and damaging winds appear to be the primary threats from the thunderstorm cells that should develop during the afternoon and extend into the evening.
COOL AIR SPREADS INTO NORTHWEST -- A cool and dry air mass associated with a ridge of high pressure had moved into the Northwest on Monday, in the wake of the cold front that stretched across the western Plains. The invasion of the air mass resulted in clearing skies across Washington State, Oregon, Idaho and northern California. By early evening, the center of the high pressure was located off the mouth of the Columbia River. The cool air, clear skies and weak winds permitted early morning temperatures to fall rapidly due to the loss of thermal radiation to space. The temperature at Klamath Falls, OR fell to a record-tying low temperature of 32 degrees on Monday morning.
High pressure was forecast to continue spreading across the interior Northwest on Tuesday. The clear skies and weak winds along with low humidity levels should permit vigorous heat loss to space by radiational cooling with little mixing during the nighttime hours, resulting in temperatures that could fall to near or below the freezing point. Therefore, a freeze warning and frost advisory were posted for the early morning hours of Tuesday across the valleys of south central Oregon.
TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 28 degrees (Fahrenheit) at Stanley, ID, while Monday's highest temperature was 109 degrees at Death Valley, CA.
ALASKAN WEATHER -- Several areas of low pressure across Alaska and surrounding waters on Monday were responsible for widespread clouds across western and southern sections of the state. A stationary front that lay in a trough of low pressure stretched from northern sections of Canada's Yukon Territory southwestward across Alaska to the Pribilof Islands in the southeastern Bering Sea. A weak low-pressure center had formed along the front and moved to the northeast along the front. A more intense low-pressure center was traveling eastward along the Aleutian Islands. By late Monday afternoon, this storm center was located just to the south of Cold Bay near the western tip of the Alaska Peninsula. An attached frontal system stretched southeastward across the Gulf of Alaska. Light rain along with the low clouds and fog stretched from the eastern Aleutians and the Pribilofs across Bristol Bay and the Alaska Peninsula to Kodiak Island. Winds surrounding this relatively intense low-pressure center off the Alaska Peninsula gusted to over 40 mph at Dutch Harbor in the Aleutians. Weak ridge of high pressure centered off the Washington and Oregon coast stretched northward along the British Columbia coast and Alaska's Panhandle to southeastern sections of mainland Alaska. Clear to partly cloudy skies were reported across eastern sections of the state, from the Panhandle northward along the Alcan border to the Brooks Range in northern Alaska. High pressure across the western Bering Sea near Russia's Kamchatka Peninsula helped maintain partly cloudy skies across the western Aleutians.
The temperature at McGrath reached a record-tying high of 69 degrees on Monday. At Barrow, the record streak of 68 consecutive days in which the temperature remained above freezing ended on Monday morning when the temperature dropped to 31 degrees.
The state's lowest temperature on Monday morning was 27 degrees at Gulkana and Noatak. The highest temperature across Alaska as of midafternoon on Monday was 70 degrees at Fairbanks and McCarthy.
HAWAIIAN WEATHER -- A large and elongated ridge of high pressure extended in an east-west direction across the central North Pacific Ocean approximately 800 miles north of Honolulu was responsible for moderate trade winds across the Aloha State on Monday. These trade winds, which were from the east and east-northeast, are the typical prevailing winds across these tropical islands due to the clockwise circulation around the high-pressure cell across the subtropical North Pacific Ocean basin. Low clouds and isolated rainshowers embedded in the trade wind flow were being carried westward across the region. These clouds and showers were primarily affecting the windward (east-facing) slopes of the islands. The air mass found across the islands has become drier and more stable, which has resulted in fewer low level clouds and showers. Thin high altitude clouds at altitudes above approximately 20,000 feet were passing across Oahu and Kauai on Monday. These clouds were circulating around a pool of cold air that was situated at these upper altitudes farther to the west of the islands. The trade winds were expected to decrease in intensity later in the week as the high-pressure center to the north shifts eastward and weakens.
PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Partly cloudy skies were reported across Puerto Rico, Culebra, Vieques and the Virgin Islands late Monday night, as isolated to scattered rainshowers were detected moving westward across the local waters surrounding the islands. These showers were being carried by trade winds that were from the east at speeds of approximately 10 mph. The northeastern coast of Puerto Rico was being affected the most with showers that were moving across the local Atlantic waters north of the islands. The easterly trade winds were associated with a large area of high pressure located over the central North Atlantic well to the northeast of the region. The air mass across the region was reasonably dry and stable, so few clouds and showers were expected to develop through midday on Tuesday. However, afternoon heating along with the residual moisture should cause development of the usual afternoon showers and thunderstorms over western interior Puerto Rico.
EYE ON THE TROPICS -- The area of low pressure that had formed over the eastern tropical North Atlantic Ocean on Sunday intensified to become Tropical Storm Fred on Monday afternoon (local time). This tropical storm was the sixth named tropical cyclone (an organized low pressure system that forms over tropical ocean waters and has tropical storm or hurricane force winds) of the 2009 North Atlantic hurricane season. By early Tuesday, the center of Tropical Storm Fred was located approximately 245 miles south-southwest of the southernmost island in the Cape Verde Islands chain. Forward movement of this storm was toward the west at 15 mph. Estimates from satellite imagery indicate that maximum sustained surface winds surrounding Tropical Storm Fred were 40 mph. Current forecasts indicate that Fred should increase over the next several days, with a gradual turning toward the west-northwest and then to the northwest.
The National Hurricane Center was also monitoring the non-tropical area of low pressure with rainshowers and thunderstorms located over coastal waters to the east of Wilmington, NC. Although winds at altitudes between 20,000 and 30,000 feet appeared to be unfavorable for tropical cyclone formation, a low chance did exist that this low could become a tropical cyclone by late Wednesday. The low was expected to move slowly to the north-northeast.
The area of low pressure with rainshowers and thunderstorms that had formed over the tropical waters of the eastern North Pacific Ocean well off the Mexican coast intensified early Monday to a tropical depression and then to Tropical Storm Linda during the afternoon. As of Monday evening, the center of Tropical Storm Linda, the twelfth tropical cyclone of the 2009 hurricane season in the eastern North Pacific, was located approximately 1280 miles west-southwest of Cabo San Lucas at the southern tip of Mexico's Baja California Peninsula. Maximum sustained surface winds 60 mph, with movement to the west near 7 mph. Some strengthening of Linda was possible over the next several days, perhaps reaching hurricane status as it was forecast to gradually turn toward the west-northwest.
WHAT TIME IS IT? -- You will find that all AMS Weather Studies meteorological maps and charts are labeled with numbers followed with a "Z", such as 00Z, 12Z, 1915Z, etc. These labels indicate the time of observation in Universal Coordinated Time (UTC). The UTC or "Z" time is used because weather observations must be taken at the same time everywhere to accurately represent the state of the atmosphere. But how can you tell from the reported Z time when the observations were made where you live? For additional Z-time explanation, call up Tuesday's Supplemental Information…In Greater Depth.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2009, The American Meteorological Society.