WET WEATHER ADVANCES INTO THE NATION'S MIDSECTION -- A large area of precipitation spread northward across the Mississippi and Ohio Valleys into the southern Great Lakes on Wednesday evening. This precipitation, consisting of heavy rains, with some light snow and freezing rain on the northern and western periphery, was associated with a warm front that stretched northward from a low pressure center over the central Gulf Coast. Southerly winds to the east of the front brought abundant Gulf moisture northward to supply the precipitation. The "overrunning" of this southerly flow over the dome of cooler to the north of the warm front provided the lifting mechanism to produce the precipitation.
Thunderstorms moving across east Texas, southwestern Louisiana, southern Mississippi and western Tennessee produced large hail and caused some wind damage. During the predawn hours, an unconfirmed tornado was reported to have caused tree damage in southwest Louisiana near Lake Charles.
Daily precipitation records were set in Tennessee at Memphis (5.86 inches), Jackson (3.59 inches) and Dyersburg (2.55 inches); in Texas at Longview (3.15 inches), Austin-Camp Mabry (1.91 inches), Tyler (1.66 inches) and Lufkin (1.09 inches); in Louisiana at Monroe (2.88 inches) and Shreveport (1.59 inches) and in Arkansas at Jonesboro (2.68 inches), Texarkana (2.43 inches) and El Dorado (2.37 inches). The rains were welcomed in many areas of Texas that have experienced below average precipitation for the last several years. However, the excessive rainfall rates, coupled with recent heavy rains, have produced major flooding on Wednesday night in the Memphis, TN metropolitan area, with roads flooded elsewhere in Tennessee, Arkansas and Mississippi.
The surface low is expected to move northward along the Mississippi Valley on Thursday, accompanied by the precipitation shield. By evening the low pressure center is forecast to be located near Memphis, TN. Between 1.0 and 2.7 inches is expected to fall in the 24 hours between Thursday and Friday mornings over the mid-Mississippi and Ohio Valleys. As a result, flood and flash flood watches remained in effect for an area that stretched from the Gulf Coast of east Texas across much of Louisiana, northern Mississippi, eastern Arkansas, northwest Alabama, western Tennessee, western Kentucky, southern Indiana and southwestern Ohio. While a slight risk of severe thunderstorms continues across the lower Mississippi Valley through Thursday morning, the lower Mississippi and Tennessee Valleys are expected to be under a moderate risk of severe weather on Thursday.
With the moisture streaming toward the Northeast, some of the precipitation could fall as rain, snow, ice pellets or freezing rain on Thursday. As a result, winter weather advisories were posted for northern Upstate New York, much of Vermont, and the northern half of New Hampshire and Maine.
WINTRY WEATHER ACROSS THE LONE STAR STATE -- A wintry mix of snow and freezing rain continued to fall across portions of Texas and northeastward into Arkansas and Missouri. The combination of overrunning moisture and a pool of sufficiently cold air was responsible for the relatively heavy snow that fell across portions of west Texas on Wednesday. As much as 10 to 12 inches of snow fell from near Midland to north of Abilene. Midland had record snowfall (6.0 inches) on Wednesday. In those areas where the cold near-surface layer was not too thick, freezing rain fell, such as near Dallas-Fort Worth. Winter storm warnings remained in effect for the Hill County of central Texas, while winter weather advisories were posted across north Texas and portions of eastern Oklahoma, northwest Arkansas and southwestern Missouri. A freezing rain advisory was also in effect for portions of north central Arkansas.
WARM WEATHER CONTINUES ACROSS THE SOUTHEAST -- A large high pressure center located off the coast of the Carolinas dominated the weather across the Southeast, yielding another unseasonably warm day. High temperatures across the Southeast and Middle Atlantic States ranged from 15 to 20 degrees above average. Record high temperatures were either tied or set in North Carolina at Raleigh-Durham (81 degrees), New Bern (81 degrees) and Greensboro (77 degrees); in Tennessee at Knoxville (77 degrees), Bristol (77 degrees) and Chattanooga (76 degrees); in Kentucky at London (73 degrees); in Virginia at Lynchburg (74 degrees) and Blacksburg (73 degrees); in West Virginia at Beckley (71 degrees); in New Jersey at Atlantic City (69 degrees) and in Connecticut at Bridgeport (64 degrees).
The proximity of the high pressure center also has meant that dense fog has formed as the result of relatively clear skies and weak winds. With fog reducing visibility to less than 0.25 miles at some locations, dense fog advisories have been issued for portions of Maryland, the Virginias and North Carolina.
COLD WEATHER TAKES HOLD ACROSS THE WEST -- Cold high pressure was centered over Nebraska and dominated the weather across the central Plains. High temperatures across the Rockies and the Plains ranged from 20 to 30 degrees below average. With record snowfall, Midland, TX also reported a record low temperature of 16 degrees and record low maximum temperature of 25 degrees.
In southern California, the low temperature of 38 degrees at Chatsworth was a record low for the date.
A WINTER STORM MOVES ACROSS THE NORTHWEST -- Widespread precipitation associated with a storm system just off the Washington State coast was found across the Pacific Northwest and northern California. The low pressure center was located approximately 400 miles to the west of Cape Flattery, WA, with an occluded front that curved eastward to a point of occlusion located near the southern end of Puget Sound. A warm front continued eastward into interior Washington, while a cold front trailed southward along the Washington and Oregon coasts before curving toward the southwest. Rain was falling along the coast and across the lower elevations, while snow was found over most of the higher elevations and in the interior. Precipitation records were set in Washington at Olympia (1.75 inches), Hoquiam (1.44 inches), Sandpoint in North Seattle (1.20 inches) and Ephrata (0.64 inches). By early Wednesday morning, the southern Oregon Cascades had as much as 12 inches of snow.
The storm is expected to continue to move inland on Thursday. A variety of winter storm warnings and snow advisories covered much of the Cascades in Washington and Oregon, the Sierras and northern mountains in California, and the mountains in Idaho, Montana, Wyoming and Utah. A foot of snow is expected above 3000 feet in the Olympics, while one to two feet of snow are expected above 8000 feet in the Sierra.
In addition to the precipitation, the tight pressure gradient also generated strong and gusty winds. Winds gusted to 81 mph at Seaside and to 70 mph at Bandon on the Oregon Coast. High winds also downed several trees near Walla Walla in eastern Washington. Gale warnings were posted for marine interests along the Washington and Oregon coasts from Cape Flattery, WA to Point St. George, CA and along the California coast from Point Arena to Santa Cruz Island. Heavy surf advisories were in effect for the coast from Cape Flattery to Point Arena. High wind warnings were posted for the Sierras of California and for adjacent portions of Nevada. Southwest winds could gust to 100 mph along the ridge tops near Lake Tahoe. High wind warnings were also in effect for the Laramie Range of southeast Wyoming, where southwest winds could gust to 70 mph.
A winter storm watch was posted to extend from late Thursday into Friday for the San Juan Mountains of southwest Colorado, where a foot of snow could fall.
UPPER AIR -- The mid-tropospheric 500 mb constant pressure chart and the upper-tropospheric 300 mb chart for 00Z Thursday both indicate a deep height trough located above the cold air over the eastern slopes of the Rockies.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 15 degrees below zero at Laramie, WY and Alamosa, CO, while the Wednesday afternoon highest temperature was 85 degrees at New Orleans, LA.
ALASKAN WEATHER -- An arctic air mass associated with a large high pressure system located over northwest Canada continued to dominate the weather across much of mainland Alaska. Much of the state had relatively cloud-free skies. Early afternoon temperatures across interior Alaska ranged from 29 degrees below zero at Northway to 12 degrees at Huslia. The southern periphery of the arctic air mass was the stationary front that stretched eastward from Bristol Bay across the Alaska Peninsula and the north Gulf coast to southeast Alaska before curving southeastward and following the coast of the Panhandle and British Columbia. A storm system over western portions of the Alaska Peninsula spread clouds and light snow across southwest Alaska. Snow was falling at Dillingham and Cordova, while rain was reported at Kodiak. Another developing storm was located over the western Aleutians near Adak Island. Strong winds were reported across the Panhandle, the result of a tight pressure gradient that developed between the high pressure center over northwestern Canada and a strong storm system located off the coast of Vancouver Island. Juneau had wind gusts to over 50 mph. A high wind warning was posted for the Copper River Delta in the vicinity of Cordova and Prince William Sound, where winds could gust to between 60 and 100 mph.
The lowest temperature in Alaska on Wednesday morning was 42 degrees below zero at Northway. The highest statewide temperature as of Wednesday afternoon was 41 degrees at Dutch Harbor.
HAWAIIAN WEATHER -- The Kona low that had produced heavy rains across Hawaii at the beginning of the week had moved westward away from Kauai. The rains and isolated thunderstorms finally ended and skies began to clear from the east. High clouds remained over Kauai late Wednesday afternoon. As the low moves westward and high pressure builds to the north, the southerly winds that have dominated the islands for the last several days are expected to be replaced by more typical east to northeast trade winds. Small craft advisories were in effect for all state waters.
EYE ON THE TROPICS -- Hurricane Olga continued to move erratically across the North Atlantic on Wednesday. As of late night, Olga was moving toward the southwest at 13 mph and was approximately 425 miles east-southeast of Bermuda. Maximum sustained surface winds were 75 mph. Eight to ten foot long period ocean swell generated by Olga was being reported in the coastal waters surrounding Puerto Rico and the US Virgin Islands.
REPORT FROM THE FIELD -- Jeff Chapman, LIT Member from Sioux Falls, SD reports that the marked change in the upper tropospheric jet stream pattern during the last week has had a profound effect on the weather in eastern South Dakota and southwestern Minnesota. After the first 22 days of November, Sioux Falls appeared to be heading for the warmest and driest Novembers on record as the jet stream was well to the north. But since Thanksgiving, the southward shift in the jet stream produced an abrupt change from warm and dry to wet and cold, with two major storms in five days. As a result of a storm that brought record rainfall, which was followed by a 12 inch snowstorm, Sioux Falls now will have the wettest November on record.
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 contributing 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 optional Supplemental Information 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, Part B, Applications', Week 11 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast