A QUIET WEATHER INTERLUDE -- A large region of high pressure produced pleasant spring weather across a large portion of the country to the east of the Rockies on Wednesday. The center of the high as of late Wednesday night was located over the Ohio Valley. However, earlier during the day, several isolated severe thunderstorms developed over Georgia, producing large hail. These storms had since dissipated. Windy conditions continued across the Great Lakes and into New England, on the northern flank of the high pressure cell, where the pressure gradient was the greatest. Winds gusted to more than 30 mph.
Morning fog was a problem across portions of the Southeast on Wednesday morning as the nighttime cooling of the air under relatively clear skies and weak winds caused saturation of the air when the air temperature fell to the dewpoint. Along the Gulf coast, the high atmospheric content of the atmosphere, as indicated by high dewpoints, kept overnight temperatures relatively high. As an example, a record high minimum temperature of 72 degrees was recorded Wednesday at Houston (Intercontinental), TX.
Under relatively cloud-free skies, record high temperatures were either tied or exceeded Wednesday at Ft. Myers, FL (89 degrees), New Orleans/Audubon Park, LA (88 degrees) and Burlington, IA (84 degrees)
ACTIVE WEATHER MOVES ACROSS THE GREAT LAKES -- A weak trough of low pressure moved eastward along the US-Canadian border on Wednesday, reaching a position near Lake Nipigon to the north of Lake Superior on Wednesday night. An area of precipitation, to include several imbedded thunderstorms, were found to the east of trough across northern Lower Michigan. Some hail fell from these thunderstorms in Michigan.
By Thursday morning, this low pressure system should have reached as far east as Quebec, with cold front that trails southwestward over the lower Great Lakes. Most of the accompanying precipitation, ion the form of rain and rainshowers, should have reached Upstate New York and adjacent portions of northern Pennsylvania and New England.
COLD WEATHER ACROSS SOUTHERN CALIFORNIA -- On Wednesday, a cold pool of air moved inland, producing rainshowers, cold air funnels and small hail across southern California -- all indicative of the unstable atmospheric environment, with warm air near the surface and cold air aloft. The unstable atmospheric conditions also produced rainshowers that produced record daily rainfall at Chatsworth (0.63 inches) and at Pasadena (0.82 inches). The afternoon high of 55 degrees reached at the Civic Center in Los Angeles, CA was a record low maximum temperature for the date. Likewise, Wednesday's high of 49 degrees at Fresno and 52 degrees at Stockton were also daily record low highs. Farther north, as much as 8 inches of snow fell at Mammoth Lakes, CA in the 24 hours ending on Wednesday morning.
A POTENTIALLY POTENT STORM -- A storm system moving across the Rockies has the potential for producing some severe weather across the southern Plains on Thursday, along with heavy rain farther to the north. As of late Wednesday night, the low pressure center was located near Great Salt Lake in Utah. A cold front extended southward to northern Mexico. Earlier in the day, this system produced heavy snow over the Mogollon Rim and the high terrain of northern Arizona. Thundersnow was reported at Flagstaff, AZ. Some locations in the mountains of the Southwest reported gusts to exceed 30 mph.
This storm is expected to intensify and move rapidly across the Plains on Thursday. During the morning, the low pressure center is forecast to be located in northeast Colorado. High winds are anticipated across the high Plains and southern mountains. As a result, a variety of high wind advisories and high wind warnings extend from eastern Colorado and the Oklahoma Panhandle to the Guadalupe and Davis Mountains of west Texas, where wind gusts could reach 70 mph. A precipitation shield should extended eastward across the Plains to the north of a warm front that would stretch eastward across northern Kansas. A cold front is anticipated to extend southward across the Texas Panhandle to the Big Bend of west Texas. A moderate risk of severe thunderstorms is expected to develop in the warm sector of the storm across the central Plains, especially over central Kansas by Thursday morning . By late Thursday, the region where severe thunderstorms are possible shifts eastward, expanding into the Mississippi and Ohio Valleys.
Snow should continue across the central Rockies. A variety of snow and blowing snow advisories were posted for Colorado and Utah.
ANOTHER STORM FOR THE NORTHWEST -- A cold front trailing a low pressure system in interior British Columbia stretched southward across Puget Sound, western Oregon and western portions of northern California. Rain fell at lower elevations while snow continued across the Cascades.
The cold front and accompanying precipitation are expected move eastward, reaching the interior of Washington and Oregon, reaching as far south as San Francisco Bay area by Thursday morning. A snow advisory was posted for the Olympics and Cascades of Washington State south to the Cascades and Siskiyous of southern Oregon, where as much as 8 inches of snow are expected.
UPPER AIR -- The mid-tropospheric 500 mb constant pressure chart for 00Z Thursday continues to show a large cold pool of air over the Southwest, not only in terms of the isotherm pattern, but also with respect to the height contours. The closed contour over southern Nevada surrounds a region of lower heights of the constant pressure surface, corresponding to the region where a relatively cold atmospheric column that extends upward from the earth's surface to 500 mb. Winds circulate in a counterclockwise direction around this feature, which is also associated with the surface low pressure system moving across the Rockies. To the east, a relatively zonal, or west to east wind flow parallels the contours.
In the upper troposphere, the 300 mb chart for the corresponding time, also contains the same closed trough over the Intermountain West. A jet streak is found to the east of this trough, with winds reaching speeds of at least 110 knots over New Mexico. This jet streak is in a position favorable for the intensification of the surface storm system. The acceleration of air moving through this jet streak produces upper level divergence which over-compensates for the low level convergence.
YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- The lowest temperature on Wednesday was 11 degrees at McCall and Stanley, ID. Wednesday's high was 97 degrees at Fort Stockton, TX.
ALASKAN WEATHER -- A large storm system was moving to the northeast across the eastern Bering Sea on Wednesday afternoon. At that time, the low pressure center was located approximately 100 miles north of Atka in the Aleutians. A front extended southeastward, crossing the Aleutians and extending into the North Pacific Ocean. Widely scattered light precipitation was associated with this system. Another diffuse area of low pressure was found over the southern portions of the Panhandle. Clouds and light precipitation were confined to the southern half of the Panhandle. Sitka reported the greatest precipitation total of the regular state reporting stations, with 0.24 inches.
An elongated ridge of high pressure extended northward from the North Pacific across the Alaska Peninsula and into interior sections of south central Alaska. As a result, relatively cloud-free skies were reported across south central Alaska. An increased pressure gradient between the high pressure and the low over the Aleutians produced gusty winds. Cold Bay reported winds in excess of 55 mph, Dutch Harbor winds to approximately 50 mph, while Kodiak, Seward and Whittier had gusts to between 45 and 50 mph.
The lowest overnight temperature in Alaska as of Wednesday was 17 degrees below zero at Nuiqsut. The highest statewide temperature as of midafternoon was 45 degrees at Annette, Gustavus and Skagway.
HAWAIIAN WEATHER -- The trade winds over the islands continued to strengthen on Wednesday as a large high pressure cell to the northeast of Hawaii. However, several bands of trade wind showers were moving toward the islands, with one band expected to pass across the islands late Wednesday night and the second on Thursday night. The low pressure system that was located to the northwest of the islands over last weekend, remained stationary while weakening. Small craft advisories remained in effect for state waters.
THE POLAR FRONT JET -- 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. For more detail on some of these relationships, see the optional Thursday Supplemental 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, 1999, The American Meteorological Society.