WEATHER OVER THE WEEKEND -- A winter storm that had developed late Thursday across the central Rockies was the main weather system that dominated the nation's weather over this past Super Bowl weekend. This storm moved eastward across the southern Plains on Friday, the lower Mississippi Valley on Saturday and the Southeast on Sunday. This storm was accompanied by locally heavy amounts of snow in the cold air to the north of the storm track. By midafternoon on Saturday, storm-total snowfall across the Rockies and western Plains were as much as 50 inches in Colorado and 27 inches in Wyoming. Farther to the east, snowfall totals ranged from four to six inches in Indiana to 17 inches in Nebraska.
On the storm's southern side, strong to severe thunderstorms developed across the southern Plains and the lower Mississippi Valley on Friday and Saturday. Some of the severe thunderstorm cells generated damaging winds and produced large hail across Texas, Oklahoma and southern Kansas. Two unconfirmed tornadoes were spawned in Texas.
WEATHER FOR THE START OF THE NEW WEEK -- The following discussion was based upon the major features appearing on the surface weather maps of late Sunday night.
The weather across the 48 coterminous United States was uncharacteristically quiet for early February late on Super Sunday. Radar detected only a few widely scattered areas of precipitation. Satellite imagery indicated most areas of the country had relatively few low clouds. Those areas that had clouds appeared to be under generally thin high clouds. In addition, most areas of the nation, especially across the northern tier of states, experienced unseasonably mild weather. As of late Sunday evening, none of the non-mountain stations reported temperatures that were in the single digits above zero.
The generally tranquil weather across the nation on Sunday evening was due to several expansive high pressure cells and the lack of any major low pressure systems that are typically accompanied by inclement weather. The dominant high pressure cell was centered over the central Plains of Nebraska, Kansas and Oklahoma. Additional areas of high pressure extended northwestward along the northern Rockies into southwestern Canada and eastward to the Middle Atlantic States. The atmosphere was also relatively dry and stable across much of the nation, resulting in a lack of major clouds or areas of precipitation.
Florida and the western Gulf Coast were two of the areas of the nation that had unsettled weather on Sunday evening. Rainshowers and a few isolated thunderstorms were found across south Florida in the warm, humid and slightly unstable tropical air mass located to the south of a cold front moving southward across the Southeast. At least one inch of rain fell across the Florida Keys on Sunday according to 24-hour radar estimated precipitation totals. These showers and thunderstorms were associated with a trough of low pressure that was situated over the southeastern Gulf of Mexico off the western tip of Cuba.
The low pressure trough off Cuba could develop on Monday with a better developed surface circulation pattern and increased shower activity. Forecasters at NOAA's National Hurricane Center foresaw the possibility that this trough had a medium chance of becoming a subtropical depression or a subtropical storm by Tuesday evening before the system would merge with a midlatitude cold front. Regardless of future development, the low pressure trough could bring locally heavy rainfall and gusty winds in showers and thunderstorms to South Florida and the Florida Keys on Monday and Tuesday.
To the west, most of the showers that had been along the western Gulf Coast earlier on Sunday had either weakened or moved out over the western Gulf of Mexico. By late evening, a few remnant showers continued across the lower Mississippi Valley in Louisiana and along the Texas coast. According to 24-hour precipitation totals estimated by radar, many locations along the Texas coast had received up to one half of an inch of rain on Sunday, with a few locations receiving as much as one inch.
Widely scattered areas of light snow were detected across the Great Basin and the western slopes of the central Rockies late Sunday evening. The snow was associated with a disturbance in the upper tropospheric flow pattern over the Great Basin. This disturbed flow, which was circulating around a pool of cold air across the region, was providing sufficient lift to produce clouds and snowshowers. A weak trough of low pressure that extended from central Colorado across the Four Corners region into northern Arizona was the surface reflection of the presence of the disturbance in the upper troposphere.
Some snow was expected to fall on Monday across the central Rockies of Colorado and northern New Mexico. This snow would be associated with a weak low pressure system that would form across the southern Rockies from the weak surface trough of low pressure. A winter weather advisory was posted through early Monday evening for the San Juan Mountains of southwestern Colorado as up to nine inches of snow were expected. Strong southerly winds that would shift to a northwesterly direction by early afternoon could create blowing snow conditions that would limit visibility.
Farther to the north, light snow could fall by late Monday along the northern Rockies of northern Wyoming and western Montana. This upslope snow would develop on easterly winds as cold air circulating around a large high pressure cell in Saskatchewan would move up along the sloping terrain of the western Plains toward the Rockies.
Aside from the anticipated shower and thunderstorm activity across south Florida and the snow along the Rockies, the weather across the rest of the nation would appear to remain relatively tranquil into Monday due to the continued presence of high pressure across the Plains, the Southeast and the interior Northwest. However, the presence of high pressure could result in unpleasant weather conditions in the form of dense fog and the degradation of air quality.
Dense fog was developing across sections of the Midwest and eastern Plains late Sunday evening, as visibility levels were falling to less than one quarter of a mile due to the large number of fog droplets. The proximity of high pressure created clear skies and light winds across this region, which can often lead to dense fog. The clear skies would permit relatively unimpeded nocturnal radiative cooling of the air to the dewpoint, or to a temperature were saturation of air with respect to water vapor would occur. Formation of fog would occur with subsequent cooling. The light winds or calm conditions under high pressure would permit fog formation by preventing the mixing of drier and warmer air down to the surface. Dense fog advisories were posted through Monday morning across central Illinois westward across northern Missouri to eastern Nebraska and western Iowa. With temperatures expected to fall to below freezing in some sections of the Midwest, freezing fog could form as liquid water fog droplets could freeze on contact with cold surfaces such as roadways. Freezing fog advisories were also issued for sections of west central Indiana, southwestern Iowa and northwestern Missouri.
Across the interior Northwest, the presence of high pressure was not only creating dense fog and freezing fog conditions, but also air stagnation problems. The high pressure system had created a thermal inversion condition that created very stable atmospheric conditions as warm air would reside over cold air at the surface. These stable atmospheric conditions would inhibit vertical mixing and the dispersion of atmospheric pollutants. Light winds under high pressure would also limit dispersion of the pollutants trapped in valleys. With these conditions expected to continue through the start of the upcoming week, air stagnation advisories were in effect through Tuesday morning across interior sections of Washington and Oregon. Freezing fog advisories were also posted for the southeastern Washington and northern Oregon through late Monday morning.
TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Sunday, the lowest temperature reported in the continental U.S. was 19 degrees below zero (Fahrenheit) at Houlton, ME, while Sunday's highest temperature was 84 degrees at Sanford, FL.
ALASKAN WEATHER -- Mostly cloudy to overcast skies were found across most of mainland Alaska on late Sunday afternoon due to a large area of low pressure that covered most of the state. A strong storm was moving northward across the Bering Sea. During the late afternoon, the storm's low pressure center was located near the Pribilof Islands. A nearly stationary warm front extended to the north from this low pressure center to the Bering Strait before curving to the east along the northwestern and northern coasts of Alaska bordering the Chukchi Sea and the Arctic Ocean. Winds were generally from the south across mainland Alaska because of the counterclockwise circulation around the eastern flank of this storm. An unseasonably warm air mass northward across interior sections of the state, replacing the extremely cold arctic air mass that had been entrenched across the 49th State for the last week. Some of the warmth was the result of compressional heating due to the Chinook winds descending from travel across the Alaska Range. The temperature at Fairbanks reached 37 degrees above zero, the highest temperature since mid December. The temperature at Healy reached 45 degrees and to 40 degrees at the Denali Park Entrance. The storm remained relatively strong, with a minimum central pressure of approximately 970 mb. A tight pressure gradient had developed strong winds, primarily along the front, with some locations having wind gusts to 40 mph near the Bering Strait. Earlier, strong winds reached 45 mph along the north slopes of the Alaska Range and to 65 mph across the Tanana River Flats of interior Alaska. Most of the state had little precipitation, except for some rain on Kodiak Island and the southern Kenai Peninsula. Some ponding of the rainwater on roadways was reported on Kodiak Island.
The lowest overnight temperature in Alaska as of Sunday was 37 degrees below zero at Barrow, while the midafternoon highest statewide temperature was 54 degrees at Metlakatla.
HAWAIIAN WEATHER -- Low level clouds and light rainshowers were moving across the Aloha State late Sunday afternoon. These clouds and showers were associated with a weak midlatitude cold front that was moving to the southeast across Kauai and Oahu by early evening. Winds across the eastern islands to the east of the approaching front were from the southwest, while winds turned to a northwesterly direction following frontal passage. Winds were relatively strong across the summits of Mauna Loa and Mauna Kea on the Big Island, causing the Summit Road to remain closed.
The cold front was forecast to continue moving slowly toward the southeast before stalling over Maui late Sunday night and dissipating early Monday morning. Light showers accompanying the front were to continue across the islands, except for the Big Island, which should remain dry. A weak ridge of high pressure should follow behind the weakening cold front. A high wind warning was continued for the summits on the Big Island through early Monday morning. Storms that had moved across the North Pacific well to the northwest of the islands late last week had generating ocean swell that propagated across the North Pacific. This ocean swell was expected to arrive in state waters from the northwest late Sunday night, causing high surf to develop around the islands. A high surf advisory was to run through early Tuesday morning for the north and west facing coasts of the nearly all of the Hawaiian Islands. A small craft advisory was also in effect for waters around Kauai and Oahu due to wind and wind-generated waves.
PUERTO RICO/U.S. VIRGIN ISLANDS WEATHER -- Generally partly cloudy skies were found across Puerto Rico, Culebra, Vieques and the Virgin Islands on Sunday night. Satellite imagery showed a few low clouds moving westward across the waters of the western North Atlantic Ocean and the northern Caribbean Sea surrounding the islands. These clouds were embedded in the easterly trade winds in the lower troposphere. Some thin high level clouds were moving eastward across the region, carried along by upper tropospheric westerly winds. Radar detected isolated to scattered rainshowers associated with the low clouds traveling westward across the islands and surrounding waters. The showers were affecting the eastern or windward sides of the islands. The moderate easterly trade winds near the surface were circulating around the southern flank of a large subtropical high pressure cell located over the North Atlantic well to the east-northeast of the region.
A few clouds and isolated rainshowers should continue passing across the islands on Monday. The easterly trade winds were expected to prevail due to the continued presence of the large high pressure ridge located well to the east-northeast of the region. These moderate winds over the open waters surrounding the islands produced wind driven seas. The combination of the moderate winds and high seas resulted in the posting of a small craft advisory for the Atlantic waters and the various local passages.
INFORMATION ON WEATHER RECORDS -- If you would like more background information concerning how various temperature records are identified, please consult Monday's Supplemental Information…In Greater Depth.
[Note: For additional information concerning weather and climate topics, see the links appearing in this week's Weekly Weather and Climate News. EJH]
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, 2012, The American Meteorological Society.