Tuesday, 6 September 2011

00Z Weather Systems

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:

LEE MAKES SOGGY MARCH NORTHEASTWARD -- Clouds, rainshowers and isolated thunderstorms that spread northeastward from the Mid-South across the Appalachians and into the Northeast resulted in a soggy Labor Day holiday for many residents along the East Coast. Radar estimated 24-hour rainfall totals for Monday indicated that many locations from the Virginias north into Maine had received one to three inches of rain, while farther to the southwest, sections of east central Mississippi, central Alabama, northern Georgia and east Tennessee had recorded between six and eight inches. More than 15 stations across the Southeast, the Middle Atlantic and New England States using conventional rain gauges reported setting daily maximum precipitation records on Monday. Some of these records were impressive, especially the 9.85 inches of rain that fell at Chattanooga, TN, a total that not only set a record for the date, but also breaking the all-time 24-hour precipitation record for the city that had been set back in 1886. Birmingham, AL received 7.11 inches of rain, which was a daily record.

The heavy rains across sections of the Southeast, the Appalachians and the Northeast were due to two separate weather features that consolidated. Former Tropical Storm Lee that had made landfall along the Louisiana coast early Sunday morning lost its tropical characteristics by early Monday as it merged with a midlatitude frontal boundary over Louisiana. This frontal boundary was the southern end of a slow moving cold front that trailed southwestward along the Appalachians from northern New England to the Mid-South. By Monday evening, the low pressure center that represented "post-tropical cyclone" Lee had traveled to the northeast and was situated over central Alabama near the capital city of Montgomery. (Post-tropical cyclone is a term that describes a low pressure system that had initially developed as a tropical cyclone but had subsequently lost its tropical characteristics as it took on those that are similar to midlatitude low pressure systems.) This somewhat vigorous low pressure center, with a minimum central low pressure center of 993 millibars (or 29.32 inches of mercury), was located along the frontal boundary that extended across the Southeast. (Millibars are the traditional meteorological unit of air pressure.)

The counterclockwise circulation surrounded post-tropical cyclone Lee resulted in a portion of the front north of the low pressure center to have become a slow moving warm front as warm and humid air was being carried northwestward on southeasterly winds (from the southeast in accordance with the meteorological nomenclature for naming winds according to the direction of origin) to the east of the low. A cold front extended to the south of the low pressure center, as north and north-westerly winds circulated around the western flank of the low. Farther north, the frontal boundary was nearly stationary. This front separated a warm humid air mass to the east and a cooler and drier air mass located to the west.

The presence of former Tropical Storm Lee over the Southeast contributed to the flow of humid tropical air northward along the nearly stationary cold front. A relatively deep southwesterly flow of air supplied water vapor and unstable atmospheric conditions that helped fuel thunderstorms across the Southeast and Middle Atlantic States. Some of the thunderstorm cells that developed across the Southeast on Monday afternoon and evening became severe, as thunderstorm winds or hail exceeded the minimum criteria for classification as a severe thunderstorm. Strong straight-line thunderstorm winds, some of which were observed to have had speeds ranging from 59 to 70 mph, downed trees across Alabama, Georgia, Florida, the Carolinas and southwestern Virginia. As many as 14 unconfirmed tornadoes were reported across the Florida Panhandle, in the Atlanta metropolitan area of northern Georgia, and across the Piedmont of North Carolina. Several of these tornadoes were responsible for damaging property. Large, quarter-sized (one inch diameter) hail also fell from some of the thunderstorm cells. Thunderstorms continued into the late evening across sections of Virginia, just to the south of the Washington, DC metropolitan area. These thunderstorm cells were below severe limits.

The low pressure center that represents post-tropical cyclone Lee was forecast to travel to the northeast on Tuesday, passing the Atlanta, GA metropolitan area during the morning hours and reaching east Tennessee by evening. Sections of the slow moving cold front located across the Middle Atlantic States and New England were expected to have moved eastward and off the coast during the day. However, the front was to remain stationary across the Carolinas. Thunderstorm activity was to be expected along the Atlantic Seaboard on Tuesday, extending from Florida northward to New Jersey in the warm and humid air along and to the east of the front. Sections of the Carolinas and southern Virginia were considered to be at a slight risk of severe thunderstorms. Supercell thunderstorms could develop by afternoon along the stalled frontal boundary that would extend across this slight risk region. These supercell thunderstorms could produce damaging straight-line winds and possible tornadoes.

Locally heavy rain was expected to continue across the Southeast and along the Appalachians. Expected rainfall totals for the 24 hours ending Tuesday evening ranged from over one inch across New England and New York State to nearly four and three quarters of an inch over the mountains along the Virginia and West Virginia border. These rains could result in flash flooding and mudslides, especially in those regions where the soil had become saturated by the heavy rain that accompanied Hurricane Irene last week. Numerous flood and flash flood watches covered all or portions of 17 states, extending from northern Alabama to Maine.

QUIET WEATHER ACROSS A LARGE SECTION OF THE NATION -- While locally heavy rainfall and severe weather were continuing across the Appalachians and the Atlantic Seaboard on Monday evening, many of the other 48 contiguous states experienced relatively tranquil weather along with few clouds. However, one exception was found across the Southwest and Intermountain West where afternoon thunderstorms developed.

The nation's midsection, extending from the Mississippi Valley west across the Plains to the eastern slopes of the Rockies experienced the quietest weather. A large ridge of high pressure centered over northern Wisconsin and extending southwestward to the Texas Panhandle was responsible for cloud-free skies and slightly cool, but pleasant weather conditions.

The high pressure located over the western Great Lakes was forecast to drift slowly eastward on Tuesday. By evening, the center of the high was expected to be situated over the eastern sections of Michigan's Upper Peninsula. Pleasant weather was to continue across the nation's midsection on Tuesday.

THUNDERSTORMS DOT THE WEST -- A south to southwest flow of air extending upward through the first 20,000 feet of the atmosphere was bringing sufficiently humid air across the Southwest to produce afternoon clouds and thunderstorms. As of late Monday evening, isolated to scattered thunderstorms continued across Arizona, New Mexico and southern California. Partly to mostly cloudy skies extended northward into western Wyoming.

Thunderstorms should develop during the afternoon across sections of the West, extending from southern Arizona northeastward across the Four Corners area to the western slopes of the central Rockies and into the Utah's Great Basin. Rainfall totals from these thunderstorms during the 24 hours ending early Tuesday evening could range from one tenth of an inch over some areas of Arizona to over one half of an inch along the western slopes of the central Rockies in Colorado.

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 22 degrees (Fahrenheit) at Dillon, MT, while Monday's highest temperature was 106 degrees at Chandler and Phoenix, AZ.

ALASKAN WEATHER -- While skies ranged from clear to partly cloudy across central and western sections of Alaska on Monday afternoon, overcast skies prevailed across southern Alaska and surrounding waters. The overcast skies across southern sections of the 49th State were associated with a powerful storm that was moving eastward across the northern Gulf of Alaska. As of late Monday afternoon, the storm's low pressure center was located approximately 150 miles to the southeast of Kodiak Island. The storm's minimum pressure was approximately 954 millibars (or 28.17 inches of mercury). With high pressure located to the west of the storm over the western Aleutians and to the east over north central Canada, a tight pressure gradient had developed around the storm. This tight pressure gradient was responsible for strong winds across the Gulf of Alaska and southern Alaska, leading to a relatively potent storm. Wind gusts reached 75 mph at Portage and 67 mph at Middleton Island in south central Alaska and to 46 mph at Dutch Harbor in the Aleutians. In addition to the relatively intense low pressure system that formed the storm's center, several trough lines that represented the axes of weak troughs of low pressure extended well out from the storm. One of these trough lines extended northward across interior sections of Alaska well to the north of the storm's center. Rain also accompanied the storm, with rain being reported across many locations in southern Alaska, stretching from the Alaska Peninsula eastward to the southeast Panhandle.

The storm moving across the northern Gulf of Alaska on Monday was forecast to linger over this region on Tuesday and Wednesday. Some weakening of the storm was anticipated. Wind advisories were continued through midmorning Tuesday along most of the southeast Panhandle as strong winds with gusts to 45 mph would shift slowly from easterly to southeasterly. Locally heavy rain was also expected along sections of coastal south central Alaska, with some locations receiving between three and five inches of rain. An urban and small stream flood advisory was to remain in effect through midmorning on Tuesday across eastern sections of the Kenai Peninsula and the area around western Prince William Sound.

The state's lowest temperature on Monday morning was 28 degrees at Eagle. The highest temperature across Alaska as of midafternoon on Monday was 69 degrees at Eielson AFB and Fairbanks.

HAWAIIAN WEATHER -- The trade winds that prevailed across the Aloha State earlier Monday had decreased in speed by afternoon, as the large and elongated ridge of high pressure responsible for the trade wind regime was approaching the islands. Trade winds, which are usually from the east and east-northeast, are the typical prevailing winds across the Hawaiian Islands due to the clockwise circulation around high-pressure cells normally found across the subtropical North Pacific Ocean basin. As of late Monday afternoon, the high pressure ridge extended in an east-west direction across the North Pacific Ocean approximately 400 miles north of Honolulu. The proximity of the axis of the ridge meant that the pressure gradient had relaxed across the islands, resulting in weak trade winds that were generally from an easterly direction. With light trade winds across the region, local sea breeze regimes were beginning to develop during the afternoon, while land breezes would follow during the night. Daytime heating of the interiors of the islands produced differences in temperatures between the air over the land and the air over the ocean. These temperature differences led to subtle pressure differences that permitted the development of sea breezes that were onshore (directed from ocean to land). Afternoon clouds and light showers would develop over the mountains as the sea breezes would converge over the interior. More rapid cooling of the land at night would lead to a reversal in temperature difference, leading to development of a weak land breeze that was directed offshore or from land to sea. Cloud-free skies would prevail across the islands at night.

The air mass that was located across the islands on Monday was relatively dry and stable. Therefore, few showers were moving across the region. Light trade winds should continue through most of this workweek, meaning that local sea and land breeze regimes should alternate between day and night.

PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Generally partly cloudy skies were reported across Puerto Rico, Culebra, Vieques and the Virgin Islands late Monday night. Regional winds were relatively light. During the afternoon, rainshowers and a few isolated thunderstorms had developed across interior sections of Puerto Rico and the Virgin Islands due to daytime heating and the development and convergence of local sea breeze regimes. Some of these showers and thunderstorms were accompanied by moderate to locally heavy rainfall rates. After sunset, nearly all of the shower and thunderstorm activity over the islands had either dissipated or moved offshore. Radar indicated only a few isolated showers moving northward across the Caribbean late Monday evening. The winds across the islands and surrounding waters were generally from the south and light in speed. Hurricane Katia (see next section) was traveling to the northwest across the western North Atlantic approximately 700 miles north of the Virgin Islands. The counterclockwise circulation around the center of Katia resulted in the southerly flow across Puerto Rico and neighboring islands. The pressure gradient across the region was relatively weak, resulting in the light wind speeds.

Except for some early morning rainshowers that should pass across the islands on Tuesday, generally fair weather was to be expected across the islands. Typical afternoon clouds and showers could develop across the interiors of the islands in response to daytime heating. The ocean swell that was generated by the winds surrounding Hurricane Katia should subside on Tuesday, resulting in a decrease in the height of the surf on the north-facing coasts of the islands.

EYE ON THE TROPICS -- Hurricane Katia continued to intensify as it traveled to the northwest across the western tropical North Atlantic on Monday. By late evening, Katia became a major category 4 hurricane on the Saffir-Simpson Scale as maximum sustained surface winds surrounding its central eye had increased to 135 mph, with higher speed gusts. At that time, this major hurricane was approximately 450 miles south of Bermuda, as it traveled to the northwest at a forward speed of 10 mph. Current forecasts indicate that Katia should continue its travel to the northwest on Tuesday and Wednesday. Katia should remain a category 4 hurricane on Tuesday, with some fluctuations in strength prior to a slow weakening. Large ocean swells generated by the hurricane-force winds should continue to 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.

The National Hurricane Center was also monitoring a region of shower and thunderstorm activity associated with an area of low pressure that was located of the eastern tropical North Atlantic approximately 600 miles southwest of the southern Cape Verde Islands. Although this low-pressure area had not become better organized during the early hours of Tuesday (local time), forecasters felt that this system had a high chance of becoming a tropical cyclone by Thursday morning as environmental conditions appeared favorable for formation of a tropical depression.

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

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, 2011, The American Meteorological Society.