The following highlights of the national weather have been extracted from the surface weather map for Wednesday night:
STORM CONTINUES TO SPIN OVER MIDWEST -- Clouds and rain were continuing to rotate in a counterclockwise direction around a surface low-pressure center located over northeast Missouri on Wednesday evening. While clouds extended across a large section of the Midwest and the Middle Atlantic States, most of the heaviest rain associated with the storm was located across the Appalachians and the eastern Lakes. The large-scale counterclockwise rotation of clouds and precipitation around this occluding storm indicated that a nearly vertical column of cold air stretched upward through the troposphere.
The storm responsible for the widespread clouds and rain across the Mississippi Valley for much of the current week continued a slow and slightly erratic track northward on Wednesday, with the primary low pressure center reaching near Hannibal in northeastern Missouri by evening. The relatively slow movement of this system to the north, moving a distance of approximately 300 miles since Sunday night, also indicated that this system was detached from the large-scale westerly steering flow in the mid troposphere.
The storm continued to occlude, as the counterclockwise circulation of air around the primary low pressure center wrapped cold air around the system, causing the occluded front to arc around the north side of the low, first to the northwest, then to the north and east before curving southeastward to a point of occlusion over northern Georgia. A secondary low-pressure center formed near this point of occlusion, or the system's triple point where the three fronts (warm, cold and occluded) meet. A nearly stationary front stretched southeastward to the Georgia coast near Savannah, while a cold front continued southward across the Florida Peninsula and out over the Gulf of Mexico.
After moving slowly northward across Missouri during the last several days, the primary low-pressure center was forecast to accelerate on Thursday. By sunrise, the low-pressure center was expected to have reached Dubuque in northeast Iowa and by evening, the low should have reached eastern sections of Michigan's Upper Peninsula. The secondary low-pressure center was to travel northeastward along the coast of the Carolinas, reaching the vicinity of Wilmington, NC by evening. The main area of precipitation was to have moved eastward across the Appalachians and sections of the Eastern Seaboard. Rainfall forecasts for the 24 hours ending Thursday evening had totals ranging between one-half of an inch and one inch for an area running from coastal North Carolina northward to western New York State.
STORMY WEATHER CONTINUES ACROSS THE NORTHWEST -- Moderate to heavy coastal rain and mountain snow were falling across western Washington State and northwest Oregon into Wednesday evening as a frontal system approached the Olympic Peninsula from the northwest. A warm front was located approximately 100 miles off the Washington coast, while a cold front was following behind. The fronts were associated with a strong storm that was approaching the British Columbia's Queen Charlotte Islands.
In addition to the locally heavy precipitation, strong gradient winds were reported across coastal sections of Washington and Oregon on Wednesday. Winds gusted to between 55 and 65 mph during the evening. The southerly winds were due to the tight pressure gradient that had developed across the region between high pressure located over the Intermountain West and the 976-mb low-pressure center near the Queen Charlottes.
The warm front was expected to move onshore during the predawn hours of Thursday, followed by the more powerful cold front. During day, the front should move across the Pacific Northwest. A second low-pressure area was forecast to develop off the southern British Columbia coast on Thursday in the wake of the cold front. This new low should travel across the Pacific Northwest on Friday.
These two storms should be accompanied by locally heavy rain and snow, along with strong and gusty winds. Between three and five inches of liquid-equivalent precipitation were expected to fall across the coastal sections of Washington along with the western slopes of the Olympics and northern Cascades during the 24 hours ending late Thursday afternoon. Because of the recent rain and the anticipated additional four to eight inches of rain that could fall by late Friday, flood watches were in effect for the Coastal Ranges of Washington and northwest Oregon.
Winter storm warnings were in effect for the Olympics and Washington Cascades, running through Thursday afternoon, with additional warnings anticipated. As much as 18 inches of snow could fall across the North Cascades by Thursday morning, with up to 12 inches of snow by late afternoon and followed another foot of snow by Friday. Storm total snowfall could reach three feet in some locations. The heavy snow and the strong winds were also causing the danger of avalanches to remain high across these mountains. Therefore, avalanche warnings were in effect. Farther to the east, winter weather advisories were in effect across the mountains of northeast Washington, where up to eight inches of new snow were anticipated.
The strong southerly winds accompanying the cold front necessitated high wind warnings to be posted along the Coastal Ranges of Washington and Oregon through late Thursday night. Some locations could experience gusts to over 60 mph. In addition, storm warnings were in effect for the coastal waters of Washington and Oregon, extending southward from Cape Flattery, WA to Florence, OR for the southerly winds that could gust to 60 mph. Farther south, gale warnings were in effect through Friday afternoon for coastal waters from Florence southward to Point Arena, CA.
The southern end of the cold front should sweep southward across northern California on Friday. In anticipation of the snow that would accompany the cold front, winter storm watches were posted for the northern mountains of California along with the Sierras, running from early Friday morning through mid evening on Friday. A foot of snow could fall across the higher terrain.
The strong winds were not limited to the Pacific Northwest on Wednesday, as winds gusted to 67 mph near Santa Barbara and Los Angeles in southern California.
A QUIET INTERLUDE -- While unsettled weather continued across the Midwest and the Northwest on Wednesday, the Plains States and the Southwest had relatively quiet weather. High pressure located over these areas were responsible for this tranquil weather. While many areas had relatively cloud-free skies, some clouds moved across the northern Plains and the central Rockies. These clouds were associated with a cold front that traveled eastward across the Rockies and out across the northern Plains. With little available water vapor, no measurable precipitation was associated with this front. Other than the few clouds, the front was accompanied by a change in wind direction and slightly lower temperatures.
UNSETTLED WEATHER COULD DEVELOP OVER TEXAS -- A weak trough of low pressure located along the eastern slopes of the southern Rockies in New Mexico and Colorado could develop into an organized low pressure system on Thursday that would bring rain to sections of Texas. A disturbance in upper troposphere moving across the Southwest could cause this low-pressure system to travel southeastward and intensify as it reaches the Texas Gulf Coast. Rainshowers and thunderstorms could develop across the south Texas on Thursday as warm, humid and somewhat unstable air is carried northward from off the western Gulf of Mexico. Some of the thunderstorm cells could reach severe limits, primarily during the late evening hours of Thursday and into Friday morning.
UPPER AIR -- The large cutoff storm system over the mid-Mississippi Valley remained the main feature on both the mid-tropospheric 500-mb constant pressure chart and the upper-tropospheric 300-mb chart for 00Z Thursday. These charts featured large height troughs that were enclosed by a series of closed circular height contours. These closed height contours indicated a column of cold air that extended upward in a "vertically stacked" pattern from the surface low-pressure center into the upper troposphere. Winds at each level were circulating around the trough in a counterclockwise direction, cut off from the prevailing westerly jet stream winds at 300 mb that remained across central Canada.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 3 degrees below zero at Lake George, CO, while the Wednesday afternoon highest temperature was 88 degrees at Miami (Kendall-Tamiami), FL.
ALASKAN WEATHER -- A large ridge of high pressure centered over the Gulf of Anadyr off Far Eastern Russia extended eastward across northern Alaska and into northwestern Canada. Because of this ridge, skies ranged from partly to mostly cloudy across a most of mainland Alaska. The ridge also extended southeastward across the Bering Sea, resulting in partly cloudy skies across sections of the eastern Aleutians. An arctic air mass accompanied this ridge. Many locations in interior sections of the state had afternoon temperatures that remained below zero. The temperature at Cold Bay only reached 26 degrees on Wednesday, thereby setting a new low high temperature record for the date. Some light snow was also reported from the Seward Peninsula eastward into interior sections of the state. Overcast skies were found along the northern coastal areas and southern Alaska. A weak trough of low pressure extended southwestward across the Arctic coast and the North Slope east of Barrow from a low-pressure center over the Arctic Ocean. Light snow fell across the region. To the south, a broad area of low pressure with multiple low-pressure centers located over the Gulf of Alaska was responsible for clouds and precipitation across southeastern Alaska, including the Panhandle. Two low-pressure centers were over the Gulf waters off Prince William Sound, while a deepening low-pressure system was moving into southeastern Gulf, passing near the Queen Charlotte Islands. This approaching storm was responsible for snow across the Panhandle south of the capital city of Juneau. To the west, a large storm traveling to the northeast across the northwestern Pacific Ocean was spreading clouds and rain across the western Aleutians.
With the strong storm southwest of the Queen Charlotte Islands moving slowly to the northeast, locally heavy snow should spread across southern sections of the Panhandle, where a winter weather advisory has been posted into Thursday morning. Between six to ten inches of snow could fall around Hyder. Wind chill advisories were continued through late Thursday morning across the Susitna Valley north of Talkeetna as low ambient temperatures and strong winds would cause nighttime wind chill temperatures to fall to 40 degrees below zero.
The lowest temperature in Alaska on Wednesday morning was 46 degrees below zero at Bettles. The highest statewide temperature as of Wednesday afternoon was 40 degrees at Annette and Shemya.
HAWAIIAN WEATHER -- With a ridge of high pressure extended across the central north Pacific approximately 1000 miles north of Kauai late Wednesday afternoon, the Aloha State continued to experience moderate to fresh east to northeast trade winds. The air mass over the islands was sufficiently humid and slightly unstable that low clouds and showers were being carried westward by the trade winds, resulting in relatively wet conditions on the windward sides of the islands. Since the trade winds were sufficiently strong, some of the showers were carried to the leeward sides of the smaller islands. The high-pressure ridge was forecast to begin weakening near the end of the week, resulting in a corresponding relaxation of the tight pressure gradient and a weakening of the trade winds.
The strong trade winds continued to produce choppy to rough seas around the islands. The combination of strong trade winds and high seas has caused small craft advisories to be extended through early Thursday morning for all state waters. The rough seas were also producing high surf that had heights exceeding 8 feet on the east facing coasts of the islands, where a high surf advisory was continued into Thursday morning.
PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Skies ranged from partly to mostly cloudy across Puerto Rico, Culebra, Vieques, the Virgin Islands and the surrounding waters late Wednesday night, as scattered to numerous rainshowers and isolated thunderstorms remained across the region. These showers and thunderstorms lingered from earlier showery precipitation that had formed during the late afternoon and continued through the evening. Sections of Puerto Rico, including the San Juan metropolitan area experienced torrential rain along with frequent lightning during the early evening. Some flooding occurred. San Juan received 2.48 inches of rain on Wednesday, which was a new daily maximum precipitation record for the date. By the late evening, most of the showers and thunderstorms had moved offshore. Cold air in the upper troposphere across the region along with a sufficiently humid air mass created unstable atmospheric conditions that permitted development of the convective precipitation. A weak pressure gradient across the region was responsible for light and variable surface winds. Because of continued abundant water vapor and unstable atmospheric conditions, clouds, showers and thunderstorms should develop across the islands on Thursday due to daytime heating of the land surfaces.
EYE ON THE TROPICS -- No organized tropical cyclones were detected across the North Atlantic or the eastern North Pacific basins on Wednesday evening; no cyclone development was anticipated through Friday.
A flood occurs when water overflows the confines of a stream or other body of water and accumulates over low-lying areas. Floods are classified as flash floods, river floods and coastal floods.
A "flash flood" is a dangerous rise in water level of a stream in a few hours or less caused by heavy rain, ice jams, earthquake and earthslide, or dam failure. A flash flood usually occurs within 6 hours of the rain event, typically a thunderstorm. In some cases, the heavy precipitation can produce a wall of water, moving at incredible speeds and with sufficient force to roll boulders, tear out trees, destroy buildings and bridges and scour out new channels. In many cases, flash floods can cause automobiles to be swept away in just 2 feet of moving water. Since the 1970's, flash flooding has caused an average of 200 fatalities per year in the United States.
These flash flood events often differ in rapidity and violence from "river floods" that are seasonal, resulting from spring rains and melting snow. River floods may crest slowly and persist for as much as a week. The record spring 1997 floods on rivers in the Dakotas and Minnesota, notably the Red River of the North, were the result of rapid spring thaw of the heavy snow cover from a record number of blizzard-producing snowstorms. The great Midwest floods of 1993 were essentially a combination of river flooding produced by a persistent weather pattern, but also by the many local flash flood events on tributaries to the Mississippi and Missouri Rivers. All these local floods contributed a large, but slow, increase in the water volume in the entire river basin.
"Coastal floods" are usually associated with tropical weather systems and will be described subsequently.
The Thursday Supplemental Information…In Greater Depth file describes the terminology used in the official statements issued by the National Weather Service to inform the public of floods and flash floods. Also included are major meteorological factors leading to flash flooding.
To be submitted on the lines for Thursday on the Study Guide, Week 11 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast