The following discussion is based upon the major weather features appearing on Tuesday night's surface weather maps:
COOL AND QUIET -- High pressure moving into the northern Plains from the Canadian Prairie Provinces was producing relatively quiet weather across the region. The air mass associated with this high pressure system was cool, as compared with recent weather, but not significantly cold by long-term standards for this time of year. Late evening temperatures across the upper Midwest and the northern Plains were 10 to 15 Fahrenheit degrees lower than what they had been 24 hours earlier.
While temperatures were slightly below seasonal averages on Tuesday across the Plains, colder air should spread southward into the central Plains on Wednesday as the high pressure system along the US-Canadian border would reach northern Kansas by midday.
UNSETTLED WEATHER MOVES ACROSS NATION'S MIDSECTION -- A large area of scattered precipitation was moving to the east-northeast across the lower and mid-Mississippi Valley on Tuesday evening. Light rain was falling across sections of Arkansas, west Tennessee and Mississippi, while farther north, snow and mixed precipitation (rain and snow) were reported across Missouri along with southern and central Illinois. Precipitation totals were generally light, with several hundredths of an inch of rain reported. Several tenths of an inch of snow fell around the St. Louis metropolitan area by late evening.
The precipitation was associated with a weak area of low pressure moving along a cold front that stretched across the nation's midsection. As of early evening, this front stretched from the Middle Atlantic States westward across the Ohio and mid-Mississippi Valleys to the Ozark Plateau and the southern Plains. A low pressure center formed along a section of the wave in this frontal boundary over southeastern Missouri. Rain was found in the warm air to the south of the front, while snow was falling in the colder air to the north of the front.
The new low pressure center that had formed late Tuesday night over southeastern Missouri was forecast to travel eastward to the Cumberland Plateau of east Tennessee by Wednesday morning. A weak area of low pressure that had been situated along the front over the mountains of western North Carolina was expected to move northeastward into West Virginia. The movement of these low pressure centers would cause eastern sections of the frontal boundary to move northward across the Middle Atlantic States, while western sections of the front would travel southeastward across the Southeastern States. By evening the two low pressure centers would have consolidated off the Middle Atlantic coast. Southern sections of the front also would have moved offshore.
The precipitation associated with the low pressure centers and the frontal boundary was forecast to spread across the upper Ohio Valley into the Middle Atlantic States on Wednesday. Snow would fall across the northern fringe of this precipitation shield, while rain would be closer to the front and low pressure centers. With anticipated snowfall totals ranging between one and six inches across the central Appalachians and the adjacent Piedmont, winter weather advisories were posted across sections of south central Pennsylvania, the northeast Panhandle of West Virginia, northern Virginia and northern Maryland. Most of these advisories were to run through mid to late Wednesday evening. Up to two inches of snow could fall in the Baltimore (MD) metropolitan area.
WET WEATHER CONTINUES OVER SOUTH FLORIDA -- A few scattered rainshowers continued across the southern Florida Peninsula into late evening. These showers represented the remnants of earlier showers and thunderstorms that had developed across the Sunshine State in association with a trough of low pressure that drifted slowly across the Florida Peninsula. By evening, this low pressure trough was along the Atlantic coast. Heavy rainshowers in the morning resulted in 1.71 inches of rain falling at Miami on Tuesday, which set a new maximum daily precipitation record for the date.
Showers and thunderstorms could form across south Florida on Wednesday afternoon with the development of an afternoon sea breeze, primarily over the region between the Ft. Lauderdale and the Miami metropolitan areas south to the northern Florida Keys. Temperature differences created between land and sea in response to surface heating would produce an onshore flow of air called a sea breeze. The shower and thunderstorm activity could occur along the advancing sea breeze front, the leading edge of the sea breeze regime.
RAIN IN SOUTH TEXAS -- Scattered rainshowers and several isolated thunderstorms remained across the Lower Rio Grande Valley of south Texas and the lower Texas Gulf Coast. Some locally heavy rain fell across the region. Daily maximum precipitation records were set in south Texas on Tuesday at McAllen (0.64 inches) and Harlingen (0.61 inches).
Rainshowers and isolated thunderstorms could develop across the lower Rio Grande Valley of south Texas on Wednesday morning. However, the cold front located across central Texas on Tuesday evening should travel southward and cause most of the showers to move into northeastern Mexico.
RAIN IN SOUTHERN CALIFORNIA -- Light to moderate rainshowers continued across sections of southern California into Tuesday evening. These showers were the remnants of more widespread coastal rain and mountain snow that stretched from northern California southward to the Mexican border. While over one half of an inch of precipitation fell across central and northern California, several tenths of an inch of precipitation were reported across southern California.
The precipitation across southern California was associated with a low pressure center that had formed during the late morning along a frontal boundary offshore of Point Conception on the southern California coast. This front was part of a storm moving across the North Pacific toward southern Alaska. During the afternoon, the newly formed low pressure center moved southeastward. By late afternoon, this low pressure center was less than 100 miles to the west of California's San Clemente Island offshore of San Diego. The precipitation farther to the north diminished as sections of the front to the north of this new low pressure center dissipated along the central and northern California coasts.
In addition to the rain, strong winds accompanied this system. Winds gusted to over 40 mph in the mountains around the Los Angeles Basin and northward along the coast to the San Francisco Bay area. Large ocean swell developed in the waters of the eastern North Pacific because of the strong winds associated with frontal boundary and the low pressure system. This ocean swell was producing large waves and high surf along the beaches of southern California.
The low pressure center was forecast to continue traveling to the southeast on Wednesday. By late afternoon, the low pressure center was expected to have moved onto the central coast of Mexico's Baja California Peninsula. Some rain should continue to fall across southern California. Up to four tenths of an inch of rain was forecast to fall along the coast between Los Angeles and San Diego during the 24 hours ending late Wednesday afternoon. Light rain should also spread eastward into the lower Colorado River Valley of southeastern California and western Arizona.
Strong and gusty winds from the northeast were expected to develop across southern California on Wednesday as the low pressure center continues to travel to the southeast. A tight pressure gradient, which would maintain the strong winds, should remain across the region because of the pressure difference between the forecasted location of high pressure over the southern Rockies and low pressure offshore of southern California and northwest Mexico. Wind advisories were in effect for the mountains around the Los Angeles Basin through midday, as sustained northeast winds could reach 30 mph with gusts to 45 mph. High surf advisories remained in effect through Friday afternoon along those exposed beaches extending from San Diego to San Luis Obispo because of the large waves and surf that would have heights to ten feet.
UNSETTLED WEATHER OVER THE NORTHWEST -- Some coastal rain and mountain snow continued across western sections of Oregon and Washington early Tuesday evening. This precipitation was associated with the weakening frontal boundary that was between 100 and 200 miles off the Pacific Northwest Coast.
The front was expected to dissipate after reaching the coast of Oregon and Washington in the predawn hours of Wednesday. However, orographically enhanced precipitation should continue along the western slopes of the Olympics and the Cascades in Washington and the northern Rockies of Idaho as onshore winds would be lifted along these mountain ranges. Up to one half of an inch of liquid-equivalent precipitation was forecast to fall across the Olympic Peninsula during the 24 hours ending late Wednesday afternoon.
YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US -- Tuesday morning's lowest temperature was 17 degrees below zero at Crane Lake, MN, while the highest temperature on Tuesday was 88 degrees at Edinburg Intl, TX (unofficial).
ALASKAN WEATHER -- Skies cleared across most of mainland Alaska on Tuesday afternoon, leaving clouds along the western coast of Alaska, stretching from the Chukchi Sea coast southward to the Alaska Peninsula. The clearing occurred as a storm moving to the north-northwest across the northern Bering Sea moved across Far Eastern Russia's Chukchi Peninsula and over the Chukchi Sea. A front accompanying this storm stretched southward across the Bering Strait and the eastern Bering Sea to near the Pribilof Islands. The clouds that remained across western Alaska were associated with this frontal boundary. Some locations along the coast also received snow, especially along the Alaska Peninsula. Rain fell on Kodiak Island. A ridge of high pressure centered over Canada's Yukon Territory was expanding westward into eastern interior Alaska, which helped dissipate the clouds across interior sections of the 49th State. Clouds and snow were found along the North Slopes. Some clouds were also moving northward along the state's southeastern Panhandle. These clouds were associated with a large storm that was moving to the north-northwest across the North Pacific and into the northwestern Gulf of Alaska.
The frontal boundary across the Bering Sea was expected to remain relatively stationary on Wednesday. A blizzard warning was continued through midday on Wednesday for the Pribilof Islands. Snowfall together with blowing snow due to north winds gusting to 50 mph should reduce visibility to less than one quarter of a mile. Southerly winds should continue to bring warm air and rain to Kodiak Island, which should help melt the snow on the ground. With up to one and a half inches of rain forecast to fall through Wednesday night, local flooding or ponding of poorly drained areas was anticipated.
The state's lowest overnight temperature on Tuesday morning was 18 degrees below zero at Northway, while the highest temperature by mid-afternoon was 52 degrees at Metlakatla.
HAWAIIAN WEATHER -- Low clouds and rainshowers associated with a cold front were moving to the east-southeast across the Aloha State late Tuesday afternoon and early evening. The midlatitude cold front trailed southwestward across the central North Pacific from a low pressure system located roughly 1000 miles to the northeast of Hilo. Satellite and radar imagery indicated that most of the clouds and showers were moving across Oahu, Molokai and Maui, as the front was passing across Oahu. Some of the rain was locally heavy, with Wainiha on the north slope of Kauai receiving 1.30 inches by late afternoon. Skies were beginning to clear over Kauai following frontal passage by early evening, while the clouds and showers had not arrived over the Big Island. The approaching front was also marked by gusty winds. Kahuku on Oahu reported winds of 46 mph and Maui had winds to nearly 40 mph. Strong southwesterly Kona winds on the southern coast of the Big Island caused some tree damage and power outages. Winds were gusting to 70 mph on the summits of Mauna Loa and Mauna Kea on the Big Island
The cold front was expected to continue traveling to the southeast across Maui and the Big Island during the late hours of Tuesday night, before exiting the region on Wednesday. Some freezing drizzle could fall over the summits of Mauna Loa and Mauna Kea. The clouds and showers associated with the front should also travel away from the Big Island. The gusty southwest winds that had been ahead of the front would be replaced by the brisk north winds following frontal passage. The cold front will usher in a chilly and dry air mass. Northeast to east trade winds should return on Thursday. The instrumented buoy moored to the northwest of the islands was reporting large ocean swell approaching from the northwest. This swell, generated by the strong winds associated with the front, was expected to generate large waves and surf along the north and west-facing coasts of the islands on Wednesday. The highest surf could occur at the times of high astronomical tides associated with Tuesday's full moon. Consequently, a high surf warning was posted through Thursday night for the north and west facing coasts of Niihau, Kauai, and Oahu, along with the north-facing shores of Molokai and Maui. A high surf advisory was also in effect for the Kona coast of the Big Island. A small craft advisory was continued through Wednesday night for all state waters because of the winds and large seas.
PUERTO RICO/U.S. VIRGIN ISLANDS WEATHER -- Skies ranged from partly cloudy to variably cloudy across Puerto Rico, Culebra, Vieques and the Virgin Islands on Tuesday night. Satellite imagery showed several clusters of low clouds moving westward across the region, while some high clouds traveling to the east across the islands and the Caribbean. Radar detected scattered rainshowers associated with the low clouds traveling westward across regional waters and the islands, especially across the Virgin Islands and the southern sections of Puerto Rico. These low clouds and showers were being carried westward on moderate easterly trade winds in the lower troposphere. These trade winds were on the southwestern flank of a large high pressure cell centered over the North Atlantic well to the northeast of the islands. The moderate trade winds over the islands had speeds ranging from 10 to 15 mph. The high level clouds were being carried eastward on upper tropospheric westerly winds. As of late afternoon, rainfall totals were as much as one half inch across Puerto Rico.
Some low clouds together with scattered rainshowers were expected to pass across the islands on Wednesday. These clouds and showers would be embedded in the easterly trade winds that should continue to prevail across the region.
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.