SEVERE WEATHER ACROSS THE SOUTH -- A large area of thunderstorms, including a squall line, was found in the warm sector of a low pressure system across the Lower Mississippi Valley on Wednesday night. The squall line represented numerous thunderstorm cells aligned in a relatively linear arrangement to the east, ahead of the cold front. The cold front extended southward from the surface low, paralleling the Mississippi River. A warm front stretched to the northeast, toward the lower Ohio Valley. These thunderstorms were fueled by the relatively warm and humid Gulf air that was being carried northward by southerly winds across the warm sector, the region southeast of the surface low that was bounded by the warm and cold fronts. This storm system had developed along a stationary front along the Gulf coast earlier in the day.
On Wednesday night, tornado watches were in effect along the squall line that stretched from middle Tennessee southwestward to near the mouth of the Mississippi River in Louisiana. Earlier, a tornado was reported on Wednesday afternoon north of Tupelo in northeastern Mississippi. An injury was sustained as winds associated with the thunderstorms destroyed 2 mobile homes and damaged another home 30 miles east of Memphis, TN. High thunderstorm winds also caused damage across portions of Louisiana, Mississippi and Alabama. The thunderstorms also produced hail to one inch diameter across Arkansas and northern Mississippi.
By Thursday morning, the surface low is expected to move northward, reaching the confluence of the Ohio and Mississippi Rivers. The system is forecasted to reach southern Lower Michigan by evening. As the system moves northward, it is expected to intensify with a drop in the central pressure before the occlusion stage is reached. A slight risk of severe thunderstorms continues through Thursday morning for an area corresponding to the storm system's warm sector, or an area that extends from the Gulf Coast northward to the Ohio Valley, to include the lower Mississippi Valley. On Thursday, the area with a slight risk of severe storms shifts to the Southeastern states, covering much of the region from the Mississippi to the Carolinas, with the exception of the Florida peninsula.
Between 0.5 and 1.0 inches of rain could fall between Thursday and Friday mornings from Alabama and Georgia north to Lake Huron. This precipitation, along with that which fell on Wednesday, is welcome in many of the areas that have seen serious drought conditions through this autumn.
On Wednesday afternoon, Orlando, FL and North Mrytle Beach, SC tied their record high temperatures for the date at 88 degrees and 77 degrees, respectively.
WINTRY WEATHER FOLLOWS -- A relatively strong temperature contrast was found across the storm system in the lower Mississippi Valley. While temperatures remained in the upper 60s across the warm sector as far north as middle Tennessee late Wednesday night, temperatures in the low to mid 30s were found as far south as central Texas. A broad band of precipitation was found on the cold side of the storm system, extending southwestward from southern Wisconsin to north Texas. The air was sufficiently cold for snow to fall along the western fringes of this precipitation shield. Some freezing rain and ice pellets (sleet) were also found in the transition zone between the rain and snow.
This wintry weather is expected to continue as colder air moves into the region as the storm system moves toward the north. As a result, a variety of winter weather advisories were posted in a long line from northwest Illinois to the Red River Valley of southern Oklahoma, while a winter storm warning was in effect for northeast Oklahoma, where 3 to 5 inches of snow could fall.
The West continues to remain cold. Record low temperatures were established on Wednesday at Miles City, MT (9 degrees below zero) and at several stations in Utah to include Alta (3 degrees) and Cedar City (13 degrees). Afternoon high temperatures ranged between 20 to 30 degrees below the average highs for the date across the Plains, from north Texas to eastern Montana. Record low highs were set at El Paso (44 degrees), Austin (48 degrees at Camp Mabry and 49 degrees at Bergstrom), San Antonio (52 degrees).
UNSETTLED WEATHER ACROSS THE NORTHWEST -- A storm system that came onshore along the Washington State coast early Wednesday had reached northern Idaho by late night. This system had begun to occlude, the occluded front extending southward to the Snake Valley of southern Idaho before becoming a cold front that curved southwestward, to travel across northern Nevada and central California. Several weak low pressure centers were found along this cold front. Coastal rain was detected over western Washington and Oregon, as well as portions of Nevada, while mountain snow fell across the interior of the Northwest.
This storm is expected to move southeastward, spreading additional precipitation across the Rockies, the Cascades and the Intermountain West. Winter storm warnings were in effect across Idaho where as much as 9 inches of snow could fall by late Thursday, and in Utah where 12 to 20 inches of snow were forecast in the Wasatch Mountains. Snow advisories were in effect for the Cascades of Washington and Oregon, the Blue Mountains of northeast Oregon, the Montana Rockies, western Wyoming, and southern Nevada.
UPPER AIR -- The mid-tropospheric 500 mb constant pressure chart and the upper-tropospheric 300 mb chart for 00Z Thursday both contain height troughs over the unseasonably cold air found across the southern Plains. These troughs over Texas produce southwesterly winds across the southern Plains. The diverging 300 mb flow over the lower Mississippi Valley on the east limb of the trough helped produce and maintain the severe thunderstorms.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 9 degrees below at Jordan, MT, while the Wednesday afternoon highest temperature was 88 degrees at Orlando and Sanford, FL.
ALASKAN WEATHER -- High pressure over the Arctic Ocean, northeast Alaska and northwest Canada maintained relatively cold weather across the eastern interior portions of the state until Wednesday afternoon. However, a deep low pressure system in the western Bering Sea near the Kamchatka Peninsula produced southerly winds across western Alaska that began to erode the influence of the arctic air. The relatively tight gradient that resulted from the high pressure center over northeast Alaska and the low in the Bering Sea produced high winds in some locations, such as Healy in the Tanana Valley, where the winds approached 60 mph. Relatively mild air accompanying these southerly winds were beginning to ride over the colder air to the north of a stationary front that stretched from the Yukon-Kuskokwim Delta near Bethel to north of the Anchorage Bowl then to the Copper River Valley. This overrunning produced areas of freezing rain with glaze ice accumulations, especially in the Kuskokwim Valley, where a freezing rain warning was in effect. Snow fell at Nome.
The lowest temperature in Alaska on Wednesday morning was 27 degrees below zero at Arctic Village and Umiat. The highest statewide temperature as of Wednesday afternoon was 52 degrees at Chignik and Port Heiden.
HAWAIIAN WEATHER -- High pressure remained well to the northeast of Hawaii on Wednesday, resulting in fresh northeast trade winds across the islands with speeds ranging from 10 to 25 mph. An area of clouds and rain was moving westward across the Big Island on Wednesday afternoon, and will pass over the other islands later Wednesday night and into Thursday morning. Once these clouds pass, relatively cloud-free skies should return. A weakening front located approximately 500 miles to the northwest of Kauai should dissipate and not affect the weather across the islands.
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 Activity 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 optional Thursday Supplemental Information.
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 9 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast