The following highlights of the national weather have been extracted from the surface weather map for Wednesday night:
UNSETTLED WEATHER MOVES EAST FROM MIDWEST -- Although high pressure was centered across Upstate New York and New England on Wednesday evening, clouds and precipitation were spreading eastward across this region from the Midwest. Rain and some snow were moving eastward across Lower Michigan and northern Ohio, as well as across Upstate New York. Farther south, some rainshowers and some isolated thunderstorms were detected along the coasts of Virginia and North Carolina. While the evening was cold in New York City with game time temperatures in the upper 40s, skies were overcast and the weather held as the New York Yankees won the 6th game of Major League Baseball's 2009 World Series to claim their 27th championship. Interestingly, the date matched the latest date a World Series game had ever been played.
This precipitation spreading eastward from the Midwest was associated with a surface weather feature and a disturbance in the upper tropospheric flow pattern. A weak surface low-pressure center was moving across northern Indiana. Westerly winds in the mid and upper troposphere were being carried over the dome of cold air across the Northeast. A disturbance in this flow was helping produce the clouds and precipitation as the humid air was being carried eastward.
The weak low-pressure center was expected to move eastward and intensify after moving out over the warm waters of the Gulf Stream in the western North Atlantic on Thursday. A cold front accompanying a low-pressure center located over northern Ontario on Wednesday evening was expected to sweep southeastward across the upper Midwest. The low-pressure center would have moved eastward into Quebec by evening. By that time, the cold front should have moved across the Great Lakes, followed by a cold bubble high-pressure system that would move southward across Michigan's Upper Peninsula. A large area of lake enhanced rain and snow should have spread across the Northeast, merging with the precipitation associated with the system moving eastward from the Midwest. Precipitation totals for the 24 hours ending Thursday evening were as much as one quarter of an inch across New York State and Pennsylvania.
COOL AND QUIET ACROSS NATION'S MIDSECTION -- Relatively cloud-free skies were found across much of the nation's midsection on Wednesday evening due to high pressure centered over southeastern Nebraska. Cool air was accompanying this high-pressure system, primarily on the eastern flank.
The high-pressure center was forecast to drift to the east-southeast on Thursday, passing across southern Missouri by sunrise and then into western Kentucky by evening. The dry air mass along with weak winds and clear skies near the center of high pressure should permit vigorous radiational cooling that would send predawn temperatures into the lower to mid 30s across a large area. For those areas that had not experienced frost or freeze conditions, freeze warnings were posted for sections of the mid-Mississippi Valley north of the St. Louis (MO) metropolitan area and across middle Tennessee. Farther west, frost advisories were in effect across the Ozark Plateau of southwestern Missouri, northwestern Arkansas, southeastern Kansas and northern Oklahoma.
MORE RAIN FOR SOUTH FLORIDA -- Scattered rainshowers were found along southern Florida's Atlantic Coast and along the Florida Keys on Wednesday evening. Several thunderstorms were detected out over the Gulf Stream between Florida and the northern Bahamas. This precipitation was associated with the remnants of a stationary front that had stretched from northeast to southwest across south Florida and adjacent waters of the western North Atlantic and the Gulf of Mexico.
Showers and thunderstorm activity should continue through Thursday across south Florida. Nearly two tenths of an inch of rain could fall across the region during the 24 hours ending during the evening.
THE HEAT CONTINUES ACROSS THE WEST -- A dome of warm air remained across much of the western half of the nation on Wednesday. Afternoon high temperatures from the high Plains westward to the Pacific Coast were either at or above the seasonal average highs. Sections of the Southwest and the Rockies had the largest temperature anomalies or departures from average, with highs that were 10 to 15 Fahrenheit degrees above average. More than one dozen high temperature records were either tied or broken across Arizona, Utah, California and Oregon.
WET WEATHER TO REACH PACIFIC NORTHWEST -- Clouds remained across Washington State and western Oregon on Wednesday night as a cold front approached the coast of these two states. The cold front, which was approximately 200 miles offshore during the early evening, trailed southward across the eastern North Pacific from a large storm over the northern Gulf of Alaska. A warm front associated with this storm was located along the coast of British Columbia's Vancouver Island.
The cold front was expected to reach the coast late Thursday after stalling momentarily earlier in the day. A southwesterly wind flow ahead of the front should cause an onshore flow of humid air that would be lifted along the Coastal Ranges, the Olympics and the Cascades. While heavier precipitation was forecast for Vancouver Island to the north, over one inch of liquid equivalent precipitation was expected to fall along the Washington's Olympic Peninsula during the 24 hours ending late Thursday afternoon. Slightly less than one half inch was expected along the western slopes of Washington's northern Cascades, while farther south, nearly two-tenths of an inch of precipitation could fall along the Oregon and northern California coast. Since the onshore flow of air was to be sufficiently warm, the precipitation should fall as rain as snow levels would be too high to support snowfall.
UPPER AIR -- The mid-tropospheric 500-mb and upper-tropospheric 300-mb constant pressure charts for 00Z Thursday both exhibited a wave-like pattern in the wind field that was associated with the height of the pressure surfaces. A height ridge on both pressure surfaces was found over the Intermountain West that associated with the dome of unseasonably warm air located over the West. To the east, a height trough was situated over the eastern Lakes and the Appalachians due to the cooler air mass near the Earth's surface. Consequently, the mid and upper tropospheric westerly flow entering the nation from off the Pacific was deflected northward around the Western height ridge. Once east of this ridge, strong northwesterly winds were found across the Plains. The northwesterly winds became westerly winds as they moved across the Southeast on the southern side of the height trough over the Lakes. To the east of this trough, southwesterly winds were found along the Eastern Seaboard. The strongest winds on the 300-mb surface reached speeds in excess of 110 knots across the northern Plains and upper Mississippi Valley between the ridge and trough. A second area of 110-knot winds was across the Middle Atlantic Coast to the southeast of the trough.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 9 degrees at Daniel, WY, while the Wednesday afternoon highest temperature was 97 degrees at Mesa, AZ.
ALASKAN WEATHER -- Clouds had spread across essentially all of mainland Alaska on Wednesday afternoon as a large and complex area of low pressure moved across southern sections of the state. This low-pressure area contained two relatively deep low-pressure centers that were located over the northern Gulf of Alaska southeast of Kodiak Island. By late afternoon, the minimum central pressures in these double-barreled low-pressure centers were as low as 952 mb. A frontal boundary accompanying the northern low-pressure center curved eastward across the eastern Gulf of Alaska reaching the coast of British Columbia coast southeast of Alaska's Panhandle. Several trough lines radiated outward across the state from the low-pressure centers. One of these trough lines stretched northwestward to the Bering Strait and into the Chukchi Sea. In addition to the clouds, a variety of precipitation types fell across the state in response to the low-pressure system. Rain fell along the Panhandle along with the Gulf coast of south central Alaska. Inland and along the western coast, rain was mixed with snow. Farther north, show fell across central interior Alaska as well as across the North Slope and northwest Alaska. Since the two low-pressure centers over the Gulf were relatively deep, a tight pressure gradient surrounding the system produced strong and gusty winds. Middleton Island reported wind gusts to 60 mph, while Cold Bay and Atka had gusts that reached 40 mph. Winds gusted to between 30 and 40 mph along the North Slope.
The strong storm over the Gulf of Alaska was expected to continue generating strong winds on Thursday. Therefore, high wind warnings and wind advisories were in effect across much of the southern half of the Panhandle through early Thursday evening. Easterly winds in the coastal areas under the warnings could gust to over 60 mph. In addition, a coastal flood advisory was also posted for a section of the coast because of a one to three-foot storm surge on top of astronomical high tides that remain relatively high due to the recent full moon. The storm surge was a dome of water that formed over the Gulf of Alaska by winds piling the water.
The lowest temperature in Alaska on Wednesday morning was 29 degrees below zero at Arctic Village. The highest statewide temperature as of Wednesday afternoon was 56 degrees at Annette.
HAWAIIAN WEATHER -- Variably cloudy skies were found across Kauai and Oahu late Wednesday afternoon because of two weather features. Low clouds, along with some rainshowers, were moving across these western islands due to the remnants of a frontal boundary that stretched across these islands. High clouds associated with an upper tropospheric pool of cold air approximately 950 miles west of Kauai were moving across the western islands. The low clouds and showers were forecast to move southward across Maui and the Big Island on Thursday morning as the remnants of the front continue to weaken and eventually dissipate. Showers should affect the windward sides of these islands. High pressure located approximately 1300 miles to the northwest of the Aloha State was drifting eastward. While some weakening of the high-pressure cell was expected, moderate to fresh easterly trade winds should remain thorough the weekend.
PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Skies were beginning to clear across Puerto Rico, Culebra, Vieques and the Virgin Islands late Wednesday night, after the islands experienced numerous rainshowers and thunderstorms during the afternoon and early evening. Some locations along northern coastal Puerto Rico had rainfall totals that exceeded one inch. Flooding occurred along several rivers on Puerto Rico. A thunderstorm cell over San Juan produced a wind gust to 29 mph. Rainfall across the Virgin Islands was generally less than two-tenths of an inch. By late evening, a few isolated showers were traveling westward across the islands, while radar and satellite imagery detected a large cluster of thunderstorms located over the Atlantic waters well to the northwest of Puerto Rico. The clouds and precipitation were associated with a tropical wave with an axis that had passed across the Virgin Islands and as approaching Culebra, Vieques and the eastern coast of Puerto Rico during mid-evening. Winds ahead of the wave axis were from the northeast, while winds east of the axis were from the east to east-southeast. With the tropical wave moving westward away from the islands, drier air was forecast to spread across the region. Some afternoon showers and thunderstorms could develop over the islands due to daytime surface heating.
EYE ON THE TROPICS -- The ninth named tropical cyclone of the 2009 North Atlantic hurricane season, Tropical Storm Ida, formed on Wednesday afternoon from a low-pressure trough that organized into a tropical depression over the southwestern Caribbean. As of late Wednesday evening, the maximum sustained winds surrounding the center of Tropical Storm Ida had intensified to 65 mph, as it was located approximately 60 miles northeast of Bluefields, Nicaragua. At that time, Ida was moving toward the northwest at 7 mph. Forecasts indicate that Tropical Storm Ida could intensify into a minimal hurricane as it gradually turns to the north-northwest early Thursday as its forward speed decreases. With this projected track, Ida should make landfall along the eastern coast of Nicaragua early Thursday. Since Tropical Storm Ida could become a hurricane before landfall, a hurricane watch was posted for a portion of the eastern coast of Nicaragua, meaning hurricane conditions could be possible in the watch area within 36 hours. Tropical storm warnings remained in effect for the entire Nicaraguan coast as tropical storm conditions could occur within 24 hours. In addition to the strong winds, rainfall totals from Ida could reach 20 inches over eastern Nicaragua and Honduras, causing life-threatening flash flooding and mudslides. The winds could produce a storm surge of up to three feet along the Nicaraguan coast that could produce dangerous battering waves north of the point where the center of Ida makes landfall.
While no organized tropical cyclones were detected across the eastern North Pacific on Wednesday evening, the National Hurricane Center was monitoring an area of low pressure several hundred miles south of the border between Mexico and Guatemala. This low-pressure area contained rainshowers and thunderstorms. Forecasters gave this system a low chance of becoming a tropical cyclone before Friday evening because the upper tropospheric wind environment appeared to become less conducive for cyclone development.
Television weathercasts are often not complete without an arrow or two indicating the position of the "jet stream". What are jet streams?
Prior to World War II, little was known of atmospheric conditions above the Earth's surface. A few instrumented airplane and kite measurements had been made but cloud patterns proved the most complete guide. Combatants during the war first encountered high-speed winds at great altitudes on bombing missions. Reid Bryson, a wartime meteorologist on Saipan in the Pacific in November 1944 (and later noted climatology professor at the University of Wisconsin-Madison), first predicted 175 mph winds near the 30,000-foot flight level based on arguments similar to those presented in the DataStreme Pressure Blocks Investigation 5B. Aircraft actually encountered those winds, confirming the forecast! Carl-Gustav Rossby named these winds the "jet stream" after high-speed jets of water that had been studied in the laboratory. Knowledge of these winds is obviously critical for aircraft operations!
Decades of radiosonde data and more recent computer models of atmospheric flows have confirmed that the strength and location of what has become known as the Polar Front jet stream in upper level motions are linked to the formation, movement, and evolution of the Highs and Lows of the surface weather maps. The polar front is the boundary between the cold air masses of high latitudes and the warm air masses of lower latitudes. For more detail on some of these relationships, see the Thursday Supplemental Information…In Greater Depth.
To be submitted on the lines for Thursday on the Study Guide, Week 9 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast