WEATHER OVER THE WEEKEND -- Residents of the eastern half of the country experienced an unseasonably chilly mid-September weekend, while those in across the Southeast, the Rockies and Intermountain West had relatively warm weather.
A large and sprawling ridge of high pressure drifted eastward from the eastern slopes of the Rockies to the eastern Great Lakes over the weekend. This high-pressure ridge accompanied a large cold Canadian air mass that not only brought record low temperatures to the Midwest, but an end to the growing season across many locations across the northern tier of states as temperatures fell to below the freezing point. Nearly three dozen daily low temperature records were set on Saturday morning, with nearly that many on Sunday morning. The occurrence of freezing conditions, which effectively marked the end of the growing season, was some of the earliest on record for the fall season, including Dubuque, IA on Saturday morning. Despite relatively clear skies, the afternoon high temperatures remained unseasonably low, as temperatures near the center of the cold air mass were 10 to 15 Fahrenheit degrees below the average highs for mid-September. Some locations also reported daily low high temperature records, such as Salina, KS, which had a high temperature of only 58 degrees on Saturday afternoon. As the air mass moved eastward from the Plains, it began to moderate, so highs were closer to average across the Northeast on Sunday afternoon than they were over the high Plains on Friday afternoon.
While record low temperatures were set across the north, the temperature on Sunday afternoon reached record highs in Florida at Ft. Myers (95 degrees) and Miami (93 degrees). Farther to the west, Garden City, KS reported a record high of 102 degrees.
Thunderstorms became severe across the Southeast on Friday afternoon and evening, spawning tornadoes, generating strong, damaging winds and producing large hail. As many as 11 unconfirmed tornadoes were reported across the Carolinas and Tidewater Virginia. Fortunately, the tornadoes were either weak or short-lived, as they produced only minor damage to buildings. Damaging straight-line thunderstorm winds accompanied some of the thunderstorm cells that traveled across sections of northeast Alabama, Georgia and the Carolinas. Up to one-inch diameter hail fell in Georgia and South Carolina. Farther to the west, severe thunderstorm cells during the afternoon and evening produced wind gusts to 67 mph in southwestern Wyoming and northern Utah.
Hail-producing severe thunderstorms developed across southeastern Colorado and southwestern Kansas on Saturday afternoon. Additional severe thunderstorms were accompanied by damaging winds in the Phoenix metropolitan area of southern Arizona and in southern California east of Los Angeles.
An unconfirmed tornado was reported at Cape Coral in southwestern Florida late Sunday afternoon. Thunderstorms turned sever e late Sunday afternoon across Colorado, as some of the cells generated straight-line winds to 67 mph and one-inch diameter hail. A severe thunderstorm produced strong winds across the Phoenix area late Sunday afternoon, downing trees.
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 cloud-free skies prevailed across much of the nation to the east of the Mississippi River because of the large ridge of high pressure centered over the Northeast from New York State southward to the Blue Ridge of Virginia. This high pressure ridge was accompanied by the cool and dry air mass that had been responsible for the record low temperatures across the Midwest on Saturday and Sunday mornings. Calm conditions or light and variable winds were reported at many locations across the Northeast near the center of the high.
The high pressure ridge was forecast to drift slowly northeastward on Monday, being centered over New England by sunrise and over the Canadian Maritime Provinces by evening. With a cool and dry air mass along with nearly clear skies and weak winds afforded by proximity to the high pressure center, overnight temperatures should fall rapidly due to radiative cooling and a lack of mixing. Consequently, frost advisories and freeze warnings were posted through Monday morning across sections of the Northeast, extending from across much of northern New England and the higher elevations of Upstate New York, northern Pennsylvania and northern New Jersey. Those regions that were under a frost advisory, as temperatures could reach the freezing point for a brief time, while those areas under a freeze warning could expect temperatures to fall into the low 30s and upper 20s for several hours.
With a clockwise circulation around the high pressure ridge, winds were from the east and northeast across the Southeast, extending from the Carolinas southward into Georgia and northern Florida. Some of the strongest winds were offshore of the Carolinas. The persistent east to northeast winds had a long fetch (travel distance) across the coastal and offshore waters of the Atlantic, thereby presenting a coastal hazard along the coast from South Carolina southward to northern Florida. Northeast winds could strengthen to 25 mph by late Monday, resulting in the piling of water against the coast by these onshore winds. Windblown surf could develop and reach heights that would require high surf advisories. The surf and the strong onshore flow could also result in coastal flooding, especially at times of high astronomical tide. Dangerous rip currents could also develop in coastal waters by midweek.
The southern extent of chilly air mass was marked by a nearly stationary cold front that stretched from off Florida's Atlantic coast westward across the Florida Peninsula and the Gulf of Mexico to near the mouth of the Rio Grande. Clouds and widely scattered rainshowers were continuing to the south of the front in the warm and humid tropical air across the Florida Peninsula and surrounding waters. Several thunderstorms could be detected over the Atlantic waters offshore of northeast Florida near Jacksonville and over the eastern Gulf of Mexico west of Ft. Myers. Farther to the west, additional thunderstorms were detected over the western Gulf of Mexico and along the Texas Gulf coast.
Although the front was forecast to move slowly southward across the Florida Peninsula on Monday, showers and thunderstorms were expected to continue across the region as a weak low pressure system develops along the front east of Florida. Showers and thunderstorms were also anticipated along the lower Texas Gulf coast.
To the west of the high pressure ridge, southerly winds had developed across essentially the entire length of the Mississippi Valley and Plains on Sunday evening. These southerly winds were responsible for a warming trend. Because of the southerly flow, temperatures late Sunday evening across this region were 10 to 20 Fahrenheit degrees higher than what they had been 24 hours earlier when the center of the high pressure ridge was present.
The southerly wind flow, coupled with the formation of a trough of low pressure along the eastern slopes of the Rockies was helping bring clouds and precipitation to the Plains. These southerly winds were pumping warmer and more humid air northward. This warm and humid air could supply water vapor and become sufficiently unstable to permit development of clouds, rainshowers and thunderstorms. As of late Sunday evening, several clusters of thunderstorms were detected across the northern Plains, stretching from southeastern Saskatchewan across the Dakotas to south central Nebraska.
A low pressure center was forecast to develop along the low pressure trough by early Monday morning and move out onto the Plains. Thunderstorms could develop out ahead of this low pressure system, extending across the Plains and into the upper Midwest by afternoon. A large area that extended from north central Kansas northeastward to northwest Wisconsin could experience a slight risk of severe thunderstorms. Supercell tornadoes could develop by afternoon that would be accompanied by damaging winds and tornadoes.
In addition to the potential for strong to severe thunderstorms, the development of the trough could result in strong winds due to a tightened pressure gradient across the region (as indicated by a closer packing of the isobars on the surface weather analysis charts). Wind advisories were posted for Monday afternoon across southwestern Kansas as strong southerly winds could develop with sustained speeds to 35 mph and gusts to 45 mph.
Farther to the southwest, late evening thunderstorms were also found across the Great Basin and western slopes of the Rockies from western Colorado south ward across the Four Corners area of southwest Colorado, southeastern Utah, northwestern New Mexico and northeastern Arizona. More thunderstorms had also developed across southeastern Arizona extending southward into the northern Mexican state of Sonora. A severe thunderstorm was detected in southern Arizona near Tucson on Sunday night, prompting a severe thunderstorm warning. These thunderstorms were being fed by a plume of humid air from the tropics that was continuing across the Great Basin. This flow should continue, along with the possibility of precipitation on Monday.
Relatively cloud-free skies continued across southern and central California, along with adjacent sections of western Arizona and most of Nevada. The region should remain relatively hot and dry on Monday.
Farther to the north, clouds and some scattered light rainshowers had spread southeastward across the Pacific Northwest. These clouds and precipitation were associated with a cold front that had moved into the region on Sunday evening. Scattered showers and isolated thunderstorms were expected across interior sections of the Northwest on Monday as the cold front continues to travel to the southeast across the region.
TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Sunday, the lowest temperature reported in the continental U.S. was 24 degrees at Butternut, WI and Pahaska, WY, while Sunday's highest temperature was 108 degrees at Laughlin, NV.
ALASKAN WEATHER -- Clouds and precipitation continued across southeastern Alaska and east central sections of the state due to several weak areas of low pressure that had moved into across the Panhandle and over Canada's Yukon Territory. The rainshowers, along with some mountain snow, fell across coastal sections from Prince William Sound southeastward into the Panhandle and northward across the Alaska Range to as far north as Fairbanks and eastward to the Alcan border. A snow advisory was issued through early Monday morning for elevations above 3000 feet in Denali National Park along the Alaska Range as up to an additional five inches of snow could fall on the five inches of snow that had fallen on the Denali Park Road since Saturday night. A narrow ridge of high pressure extended northward across southwestern and south central Alaska, resulting in relatively cloud-free skies across that region. With clear skies, radiative cooling could be anticipated, resulting in the development of frost across sections of Anchorage and the Mat-Su Valley early Monday. A weak low pressure system moved across the Bering Sea bring clouds and light rain to southwestern sections of the state on Sunday afternoon. With a large high pressure system situated over the Arctic Ocean, a weak onshore flow brought low clouds, freezing fog and light rain to the Arctic coast. Well to the west, a deep storm system was moving across the northwestern North Pacific, bringing strong winds to the western Aleutians. A high wind watch was posted for Monday evening across the central Aleutians as south winds ahead of the storm could have sustained speeds ranging from 45 to 60 mph and gusts to 80 mph.
The lowest overnight temperature in Alaska as of Sunday was 30 degrees at Deering, while the midafternoon highest statewide temperature was 64 degrees at Big River Lake.
HAWAIIAN WEATHER -- A large and elongated ridge of high pressure cell located across the central Pacific well to the northeast of the Hawaiian Islands was building westward, resulting in a continuation of the easterly trade winds across the Aloha State. Following widely light showers across the islands and surrounding waters on Sunday afternoon, trade wind weather was found across the region, with showers favoring the windward sides of the islands. A large band of thick high clouds was moving to the north-northeast from near the equator across the islands from Molokai eastward to the Big Islands. These clouds should de crease on Monday. The trade winds were expected to increase through the week as the high pressure ridge continues to increase in intensity.
PUERTO RICO AND US VIRGIN ISLANDS WEATHER -- Variably cloudy skies were reported across the Virgin Islands late Sunday night, while clouds covered the skies over Puerto Rico, Culebra and Vieques. Scattered to numerous rainshowers and thunderstorms were detected during the afternoon across sections of interior and northwest Puerto Rico , where two to three inches of rain fell. By late evening, most of the showers and thunderstorms had either dissipated or moved offshore. Only a few showers were detected across the Caribbean and Atlantic waters surrounding the islands. The clouds and precipitation was the result of unstable atmospheric conditions caused by a pool of cold air in the mid and upper troposphere over the Bahamas and the Greater Antilles. With a weak pressure gradient across the region, winds were light and variable over the islands. Higher pressure was well to the northeast, while lower pressure was across northern Venezuela. Unstable atmospheric conditions should remain across the region on Monday, resulting in another afternoon of numerous showers and thunderstorms across interior Puerto Rico.
EYE ON THE TROPICS -- After becoming a minimal tropical storm late last Thursday night, Tropical Storm Ingrid, the ninth named tropical cyclone of the 2007 North Atlantic hurricane season, continued its track to the west-northwest across the tropical North Atlantic over the weekend. However, it weakened and was downgraded to a tropical depression on Saturday as it approached the northern Leeward Islands. As of late Sunday night, the center of Tropical Depression Ingrid was approximately 160 miles east of the Leeward Islands, as it was traveling to the west at 9 mph. Maximum sustained surface winds surrounding the depression's center was 30 mph. Ingrid was forecast to continue moving to the west or west-northwest on Monday with little change in strength, passing near the northern Leeward Islands.
NOTE: For discussion of the movement of other tropical weather systems over the past week, consult the current Weekly Weather and Climate News.
ONE FOR THE RECORD BOOKS -- If you would like more background information concerning how various temperature records are identified, please consult Monday's optional Supplemental Information.
From the files of the Aviation Weather Center, Kansas City, MO
From the files of the Aviation Weather Center, Kansas City, MO
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2007, The American Meteorological Society.