The following highlights of the national weather have been extracted from the surface weather map for Tuesday night:
COLD AIR DOMINATES THE EAST -- Unseasonably chilly air spread across much of the nation east of the Mississippi River on Tuesday as a large sprawling high pressure system drifted to the southeast across the mid-Mississippi Valley. This high was accompanied by a cold and dry air mass that brought some of the chilliest air of the season to the East. By evening the center of this high was centered over southern Illinois near the confluence of the Ohio and Mississippi Rivers. The clockwise circulation of winds around the high produced northerly winds across a large area of the East, resulting in cold air advection on the eastern flank of the ridge of high pressure. High temperatures for Tuesday across the region east of the axis of the ridge were well below the long-term average highs for the date, with many locations in the upper Ohio Valley and the Appalachians reporting highs that ranged between 20 and 25 Fahrenheit degrees below average. In some locations across the Southeast the afternoon high temperatures either tied or broke low maximum temperature records for the date, including Bluefield, WV (27 degrees), Blacksburg, VA (30 degrees) and in South Carolina at Greenville-Spartanburg (42 degrees), Columbia (45 degrees) and Charleston (49 degrees).
The high-pressure system located over the Midwest was forecast to continue traveling to the southeast on Wednesday. By evening, the center of the high was expected to be located along the western section of the Florida Panhandle. The presence of the cold dry air mass across the Southeast was continuing to cause concern for freezing conditions and for wildfires across the region. Freeze warnings were posted again across a large area of the Southeast, extending across coastal sections of South Carolina, nearly the entire state of Georgia, southeastern Alabama and northern Florida, including the Panhandle. Additional freeze warnings were also issued for coastal sections of Mississippi and eastern Louisiana north of Lake Pontchartrain. Red flag warnings were issued across several areas of the Southeast, including the northern two-thirds of Alabama, along with several scattered areas in Florida and northeast Georgia.
LAKE EFFECT SNOW CONTINUES -- The cold northerly winds circulating around the northeastern side of the high pressure ridge brought a continuation of lake-effect snow to the Midwest and the Northeast on Tuesday evening. Some lake effect snow continued across the eastern sections of Michigan's Upper Peninsula, while a larger band of snow stretched the length of Lake Michigan, as northerly winds ran the length of the lake, resulting in a sufficiently long fetch (travel over water) to produce additional snow at the southern end of that lake. Additional light lake-effect snow was falling across Ohio, Pennsylvania and New York State downwind of Lakes Erie and Ontario. However this snow was relatively light, as the winds were oriented across these lakes, resulting in a shorter fetch.
This lake effect snow event has been memorable. Some locations in the Upper Peninsula had received more than 20 inches of storm total snowfall by early Tuesday. Marquette, MI received 5.8 inches of snow on Tuesday, which set a maximum snowfall record for the date. As of Tuesday morning, several locations in northern Indiana downwind of Lake Michigan had received between six and ten inches of snow.
Lake effect snow warnings and advisories were continued across sections of New York State south of Lake Ontario into midday on Tuesday as several inches of additional snow could bring snowfall totals for the current storm to 18 inches.
A BRIEF WARMING -- Southerly winds on the western flank of the high pressure ridge acting in concert with an approaching Alberta Clipper low pressure system were responsible for what should be a brief warming across sections of the northern Plains and the Upper Midwest. Late evening temperatures across the Dakotas, Minnesota and Wisconsin were 5 to 15 Fahrenheit degrees above what they had been 24 hours earlier, due mainly to a shift from northerly to southerly winds, which resulted in warm air advection. Any increase in cloud cover ahead of the Alberta Clipper also contributed to the higher nighttime temperatures because of the lack of significant radiational cooling.
The Alberta Clipper low-pressure system located over the southwestern Manitoba as of Tuesday evening should supply another round of cold air to the Midwest over the next several days. This low-pressure system formed during the predawn hours of Tuesday on the eastern slopes of the Canadian Rockies in southern Alberta, after a frontal system had moved across the mountains. As its name implies, this low moved quickly from Alberta eastward along the US-Canadian border to Manitoba by early evening. At that time, a slow moving warm front extended southward across the Dakotas into Nebraska and Kansas. This front separated relatively cool air located over the eastern Dakotas from warmer air to the west. While the winds ahead of the front were from the south, resulting in warm air advection and temperatures in the 20s and 30s that were above what they had been on Monday night, the air west of the front was even warmer, with late evening temperatures that were in the mid to upper 40s. Westerly winds across the northern Plains were downslope, warmed by compressional heating during their descent from the higher terrain of the Rockies to the west.
However, a cold front trailing westward from the low-pressure center along the US-Canadian border was about to bring more cold air southward. On Wednesday, the Alberta Clipper was forecast to travel eastward along the border, reaching the eastern section of Michigan's Upper Peninsula by evening. Eventually, this system should continue eastward to near Buffalo, NY by Thursday morning. The cold front trailing the low-pressure center was expected to sweep southeastward across the Plains on Wednesday, bringing another round of cold air as far east as Lake Michigan and as far south as Oklahoma by evening. Another frontal boundary located over central Canada was forecast to travel south as a cold front, bringing a third wave of cold air to the upper Midwest by Thursday.
The two waves of cold air approaching the upper Midwest should result in more lake-effect snow for the region south and east of Lake Superior, as winds turn to the northwest and north. Lake effect snow watches and warnings were posted for northern Wisconsin and the Upper Peninsula from Wednesday evening through early Friday morning, as up to a foot of new snow could fall downwind of Lake Superior. Winds accompanying this system could result in blowing and drifting snow that could reduce visibility.
ANOTHER DAY OF RECORD HEAT -- A large dome of warm air that extended up through the troposphere remained entrenched across the West on Tuesday. Afternoon high temperatures across the western half of the nation were above the long-term average highs for mid November. However, the surface ridge of high pressure shifted slightly southeastward on Tuesday to become centered over near the Four Corners area of southeast Utah, southwest Colorado, northeast Arizona and northwest New Mexico. With this southeastward displacement, the region experiencing the largest temperature anomalies (differences between observed and average temperatures) had shifted eastward across the Great Basin, the central and northern Rockies and the adjacent high Plains. Highs temperatures were as much as 25 Fahrenheit degrees above average across Colorado, Wyoming and Montana, together with adjoining western sections of the Dakotas and Nebraska. Farther to the west, the return of an onshore flow resulted in afternoon high temperatures along the entire Pacific Coast to remain relatively close to seasonal averages.
UPPER AIR -- Both the 500 and 300 mb charts for 00Z Wednesday showed the continuation of an amplified wave pattern, which had been displaced eastward from the previous evening. The height trough associated with the cold air had moved eastward to over the Northeast, while the height ridge has over the Plains and Rockies, associated with the record warm air over this region. Farther to the west, a new trough was located over the eastern North Pacific. The wavelike jet-stream pattern resulted in a southwesterly flow across the Pacific Northwest, westerly winds across the northern Rockies, and northwest flow from the Plains east to the Southeastern States.
YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US -- Tuesday morning's lowest temperature was 2 degrees at Upson, WI, while the highest temperature on Tuesday was 93 degrees at Riverside (UCR), CA.
ALASKAN WEATHER -- A large ridge of high pressure centered over western Alaska late Tuesday afternoon brought cold air along with fair to partly cloudy skies across a large area of mainland Alaska. Subzero temperatures were reported during the afternoon across sections of northern Alaska. While relatively cloud-free skies were reported across western Alaska, some snow was reported across eastern interior sections of the state along the Alcan border. Clouds were spreading northward across southeastern Alaska and northern sections of the Panhandle, out ahead of a complex storm system that was advancing to the northeast from the North Pacific into the Gulf of Alaska. To the north, fog was reported along the Arctic coast. Clouds were also found along the central and western Aleutians due to a low-pressure system traveling eastward across the North Pacific south of the central Aleutians and another system near Russia's Kamchatka Peninsula.
The temperature at Nome fell to 7 degrees below zero on Tuesday morning, the first time during this young winter season that the temperature fell below zero. This occurrence was only three days after the average first date of first subzero temperature at that city. For comparison, the earliest subzero temperature in Nome occurred on 12 October 1996, while the latest was 17 December 1912.
The sun set at 1:32 PM Alaska Standard Time yesterday (18 November 2008) at Barrow, the northernmost city in Alaska, for the last time this year. The sun will rise above the local horizon in Barrow at 12:57 PM AST on 23 January 2009. While the sun will be below the horizon for the next 66 days, residents of this city will have roughly three hours of some diffuse sunlight each day that is equivalent to civil twilight, provided the cloud cover is not too thick.
The state's lowest overnight temperature on Tuesday morning was 22 degrees below zero at Bettles. The highest temperature by mid afternoon of Tuesday was 46 degrees at Adak and Hydaburg.
HAWAIIAN WEATHER -- Clouds and showers were beginning to dissipate across the Aloha State on Tuesday afternoon as the remnants of a wind shear line was weakening across the region. The shear line, a former midlatitude cold front that lost its thermal contrast, represented a region where wind speeds and directions changed across a relatively short distance. Winds across areas south of the shear line were from the east, while slightly stronger winds to the north of the line were from the northeast. The remnants of this shear line were expected to drift southeastward, producing some showers. The pool of cold air in the upper troposphere that had reached the islands early Tuesday moved off to the northeast buy late in the day. This cold air aloft had destabilized the lower troposphere, contributing to the shower activity. The mid and high clouds associated with the southwesterly winds circulating around this upper tropospheric feature had moved to the northeast away from the Big Island.
Ocean swell generated by strong winds well to the northeast of the Hawaiian Islands continued to produce high surf to the east facing shores of Kauai and Oahu. Thus, high surf advisories were continued for these shores through Wednesday. A small craft advisory was also in effect through early Wednesday evening for the waters of many of the inter-island channels and Maui's Maalaea Bay, along with the waters to along the southern coasts of the Big Island.
PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Skies were relatively cloud free across Puerto Rico, Culebra and Vieques on Tuesday evening, while partly cloudy conditions were reported over the Virgin Islands. Radar detected isolated to scattered showers traveling westward across the waters surrounding the islands. Several showers were reaching the eastern coast of Puerto Rico. High pressure was located over the western North Atlantic to the northeast of the islands. Therefore, the prevailing winds across the region were from the northeast at speeds ranging from 10 to 15 mph. The high also was responsible for the maintenance of dry and relatively stable air mass over the region. However, some pockets of humid air being carried by the lower tropospheric flow should continue to produce some clouds along with isolated to scattered showers on Wednesday, with most of the shower activity affecting the north and east facing coasts of the islands. Rainfall totals should be relatively light.
EYE ON THE TROPICS -- No organized tropical cyclones were detected across the North Atlantic or the eastern North Pacific basins on Tuesday evening; no cyclone development was anticipated through Thursday.
REPORT FROM THE FIELD -- Steve LaDochy, a geography professor at the California State University, Los Angeles and a valued member of the AMS Education community, reported that on the wildfire situation in the Los Angeles basin. He said that air pollution advisories were issued for a large area of southern California, including the Inland Empire east of Los Angeles on Monday as a light afternoon sea breeze was spreading smoke filled air that was not fit to breathe. Steve noted that in some sections of Los Angeles, the smell was quite strong and brown clouds of smoke could be seen. Ash also fell across large areas. Santa Ana winds continued over 30 mph in some locations on Monday, as temperatures reached the 90s, while relative humidity levels were exceedingly low. However, he indicated that the forecast called for a break by midweek.
TORNADO SEASON -- With one month remaining in the year, more than an estimated 2100 tornadoes have been reported across the United States during 2008, with 125 fatalities. For more information concerning this year's tornado statistics, consult Wednesday's optional Supplemental Information.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast