WINTER ARRIVES -- Arctic air continued to spread southeastward across a large portion of the country to the east of the Rockies on Tuesday, accompanied by unseasonably chilly weather and lake-effect snow downwind of the Great Lakes. Afternoon high temperatures across much of the nation's midsection were below the long-term average highs for the date, with the greatest departures centered in the Upper Mississippi Valley, where highs were on the order of 20 degrees below average.
High pressure along with the cold air meant that several record low temperatures were either tied or set on Tuesday morning across the Southeast to include Jackson, TN (21 degrees); Florence, SC (21 degrees); in Georgia at Savannah (21 degrees) and Macon (22 degrees); in North Carolina at Asheville (18 degrees), Raleigh-Durham (19 degrees) and at Norfolk, VA (23 degrees)
The arctic air was associated with a large high pressure system that was centered over northern Iowa on Tuesday evening. The leading edge of this cold air mass was defined by a cold front that trailed southward from a low in Quebec along the Eastern Seaboard to the Carolinas, where the front curved westward, traveling across the central Gulf Coast states to the lower Mississippi Valley where it became part of a warm front associated with a low pressure system in the Plains.
This high pressure system is expected to drift to the south and east on Wednesday, with the center reaching northeast Missouri by morning.
A freeze warning was in effect over interior southeast Georgia, much of interior north Florida and some counties in central Florida.
MORE LAKE-EFFECT SNOW -- The north to northwest winds that followed the cold frontal passage across the Midwest on Monday and Tuesday produced lake-effect snow initially across northern Wisconsin and the Upper and Lower Peninsulas of Michigan. As much as 10 inches of snow had fallen across northern Lower Michigan by Tuesday evening. However, the region affected by the lake-effect snow shifted eastward on Tuesday as winds behind the cold front turned to a more westerly direction. Bands of heavy snow fell across New York State as of late Tuesday night. As of that time, a secondary cold front associated with low pressure over Quebec was traveling across Lake Ontario, enhancing the snowfall in addition to turning the winds to a westerly direction to maximize the fetch. The fetch, representing the distance that air travels across a water surface, is greatest for a west wind, yielding a greater amount of lake-effect or lake-enhanced snow.
A variety of lake-effect watches, warnings and advisories were in effect for many of the locations in the traditional snowbelt that are downwind from one of the lakes. An advisory remained in effect for the Keweenaw Peninsula of northern Michigan and across portions of northern Lower Michigan downwind of Lake Superior and northern Lake Michigan. Additional advisories remained in effect for southwestern Lower Michigan near Grand Rapids. Most of these locations were expecting from 3 to 5 inches. Watches and warnings were posted for as much as 3 to 6 inches of snow for much of western New York from near Buffalo eastward through the Mohawk Valley and then northward to the Thousand Islands. Additional watches were posted for portions of western Pennsylvania, where as much as 10 inches could fall along the higher terrain, the result of orographically enhanced lifting and for the mountainous regions of western Maryland and West Virginia.
THE EFFECTS OF THE WINDS -- In addition to the cold and the snow, the Northeast also experienced windy conditions associated with a low pressure system located over southern Quebec. A shift to northwest winds across Lake Erie, along with a gradual weakening, allowed water that had been piled in the eastern part of the lake on Monday night to return to the west, bringing water levels up to safe navigable levels.
A gale warning was posted for the New England coast from Merrimack River, MA to Eastport, ME.
SNOW ACROSS THE PLAINS -- A weak low pressure system developed along the western portion of the cold front that had moved southward across the Plains ahead of the advancing arctic air. This area of low pressure that was located over western Kansas on Tuesday night developed a counterclockwise circulation that caused a portion of the front to begin moving back north as a warm front, which extended southeastward from the low pressure center to the lower Mississippi Valley. An area of snow spread northeastward from the low across Kansas, Missouri into southwest Iowa.
This system is expected to travel to the east-southeast on Wednesday. A snow advisory for southeastern Kansas, southwestern Missouri and northwest Arkansas was posted as 1 to 3 inches of snow were expected by Wednesday morning.
THE FOG PERSISTS -- Foggy conditions continued across some of the major valleys of the West on Tuesday and into Tuesday night as high pressure, coupled with a temperature inversion provided a cap to the saturated air that collected in the valleys. Dense fog advisories were once again posted on Tuesday night for the Sacramento and San Joaquin Valleys of California and in Idaho for the Snake and Treasure Valleys.
UPPER AIR -- The main feature of interest on the upper air charts for 00Z Wednesday was the broad height trough across the northern portion of the country, reflecting the cold air that had spread southward across the region. A ridge has formed across the West, resulting in an amplified meridional flow, especially over the Rockies.
YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE LOWER 48 --Tuesday morning's lowest temperature was 17 degrees below zero at Orr, MN and Fargo, ND, while the highest temperature on Tuesday was 90 degrees at San Diego/Gillespie, CA.
ALASKAN WEATHER -- High pressure remained anchored over the Arctic Ocean north of Barrow on Tuesday. Modified arctic air was found across the northern two thirds of the state, to the north of an east-west running stationary front that ran along the Alaska Range. However, a large low pressure system moved across southwest Alaska, with a center located near Kenai. The tight pressure gradient between the arctic high and the low over southwest Alaska produced strong easterly to northeasterly winds with gusts to 50 mph across northern Alaska. These winds produced blowing snow and blizzard conditions. In addition, these winds caused wind-chill equivalent temperatures to as low as 55 degrees below zero at Barrow.
The low also produced southwesterly, onshore flow across southeast Alaska and the Panhandle, where heavy rains fell. By midafternoon, Juneau had received 2.45 inches of rain and Sitka had 2.21 inches. To the west of the low, snow was reported at some stations across the Aleutians and western Alaska.
The state's lowest overnight temperature on Tuesday morning was 27 degrees below zero at Umiat. The highest temperature by mid afternoon of Tuesday was 49 degrees at Klawok and Sitka.
HAWAIIAN WEATHER -- Light trade winds returned across the islands on Tuesday following the dissipation of a shear line during the morning. The southeastward moving shear line, representing the remnants of a midlatitude cold front, reached the western shores of Oahu during the night. A weak ridge of high pressure following the shear line to the northwest of the islands was expected to cause an increase of the trade wind speed to between 10 to 20 mph by Thursday. Except for the usual trade showers over the mountain slopes, skies should be relatively cloud-free.
Ocean swell produced by a storm over the northwest Pacific several days ago arrived in the vicinity of the islands from the northwest on Tuesday. As a result, a high surf advisory for the northwest shores of the islands was posted on Tuesday evening as 8 to 12 foot surf was expected beginning late Tuesday evening.
IT'S SURE DARK! - Have you noticed that the sun is setting early these days? During the first ten days of December, many locations throughout the country will experience their earliest sunset times of the year. The exact day for the earliest sunset depends upon the latitude, so you may want to check the date in your locale from the sunrise tables appearing in on-line, interactive service available for the entire year at most cities in the United States. The reason for the earliest sunsets occurring in early December rather than on the winter solstice (early morning of Thursday, 21 December 2000) is because the sun is not as precise a time-keeper as our watches. Because of a combination of factors involved with the earth's elliptical orbit about the sun and the tilt of the earth's spin axis with respect to the plane of the ecliptic, the sun appears to "run fast" by as much as 15 minutes as compared with clock time in November. However, with the approach of the winter solstice and perihelion (4 January 2001), the apparent sun slows during December and finally lags the clock by 12 minutes in February. As a result, a noticeable and welcome trend toward later sunsets can be detected by the end of December, especially by those residents in the northern part of the country. However, the latest sunrises occur at most locales in early January, meaning that early risers will continue seeing dark and dreary mornings for another month.
A WHITE CHRISTMAS --The first week of December has not been very wintry across a large portion of the country east of the Rockies. As we approach the holiday season, many of us get in the mood only when they see a snow cover. What is the probability that your area will experience a White Christmas? Consult Wednesday's optional Supplemental Information for additional information.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.