A MATTER OF CONTRASTS -- Once again, high temperatures across the nation to the east of the Rockies were well above the climatological averages on Wednesday. Some locales in the central Plains were at least 30 degrees above their long term average high temperature for the date. Numerous record high temperatures were either tied or exceeded from the central Plains across the Midwest to the Northeast. The record high temperature of 76 degrees set Wednesday at North Platte, NE marked the highest temperature for so early in the season.
However for much of the West, colder than average conditions continued as some locations especially along the Coast were at least 10 degrees below their average high for the date. Record low temperatures were either tied or set on the West Coast on Wednesday morning at Burns, OR (11 degrees below zero) and in California at Oakland (37 degrees), Redding (27 degrees) and Stockton (30 degrees).
Cold air to the west of the cold front spread across the Rockies and into the high Plains late Wednesday. Numerous locations experienced a dramatic cooling following passage of the cold front. For example, Salt Lake City had a record high temperature of 62 degrees on Tuesday, but snow and temperatures in the 20s on Wednesday morning. The temperature at Denver, CO dropped by 24 Fahrenheit degrees within two hours during mid afternoon when the winds shifted from a southwesterly to a northeasterly direction, and by a total of 40 degrees in four hours. Passage of the front through Denver was also marked by blowing dust.
On Wednesday Interstate 80 reopened over the Sierras after being closed because of eight feet of snow from the last storm.
A POTENT STORM MOVES ACROSS THE PLAINS -- A two-part storm system was moving across the Plains late Wednesday night. Part of the system had taken a northern route across the Rockies, while the other part had moved across the southern Rockies. One low pressure center was situated over the lakes region of southern Manitoba while the other low center was developing and intensifying along the southern portion of the cold front over southeastern Colorado. Scattered precipitation was found across the northern Plains and the upper Great Lakes in association with the low over Manitoba. Heavy snow, with accumulations of at least a foot, had fallen across the mountains of Wyoming and Colorado as the southern branch of the storm moved across the Rockies.
A strong counterclockwise circulation was developing around this storm system as indicated by the tight isobar backing that suggests a strong pressure gradient. A broad southerly wind flow could be found across the Plains and the Mississippi Valley to the east of the storm system. Winds to 30 mph were common across Texas. To the west and north of the system, strong northerly winds, with speeds to 40 mph and gusts to over 55 mph were sweeping southward across the foothills of the Rockies and the high Plains.
This storm system is forecast to move eastward. The southern low pressure system is expected to move northeastward along the front and become a dominant influence on the weather across the nation's midsection. A variety of advisories, watches and warnings have been posted across the Plains in anticipation of the wintry mix of precipitation and the strong winds. In the cold air to the west of the storm system, snow was expected to spread in a northeast direction across the Plains toward the Dakotas. A band of freezing rain and ice pellets (sleet) could also form across the Plains.
The eastward movement of the cold front could also signal an outbreak of severe thunderstorms. The southerly winds across the southern Plains has brought a influx of warm, humid Gulf air northward near the earth's surface. As the dome of cold air pushes eastward behind the cold front, the warm air is lifted, which could trigger thunderstorm development. The region from north Texas to Kansas was anticipating the possibility of thunderstorms with large hail, high winds and possible tornadoes. On Thursday, the risk for isolated thunderstorms shifts eastward, and extends from the southern Plains and Mississippi Valley into the southern Great Lakes. This area encompasses a region of moderate risk of severe thunderstorms over the lower and mid Mississippi River Valley.
ANOTHER STORM ON THE WESTERN HORIZON -- Despite the scattered precipitation especially along the Cascades and the Olympics because of the persistent onshore flow of air, the Pacific Northwest had a short respite from the stormy weather on Wednesday. A high pressure system moved eastward and was positioned over the Great Basin. However, the next storm system over the north Pacific was approaching the Pacific Northwest, with a cold front making landfall on Thursday morning. Strong southwest winds had begun to increase in speed ahead of the cold front. Storm warnings for winds in excess of 54 mph were in effect for the Washington State coast, while gale warnings were posted for the Oregon coast and the Strait of Juan de Fuca and interior waters of Puget Sound.
The combination of a cool air mass, coupled with the clear skies, low humidity and the weak winds associated with he high pressure could cause frost damage to agricultural interests in the San Joaquin Valley of California, where a frost advisory has been posted.
YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US -- The lowest temperature on Wednesday was 9 degrees below zero at McCall, ID. Wednesday's highest temperature was 91 degrees at Lajitas, TX.
ALASKAN WEATHER -- The two week "cold snap" of 1999 may be coming to a close by the end of the week as the ridge of high pressure containing arctic air moved east across northern Alaska toward the Yukon Territory. Temperatures across the interior were roughly 20 degrees warmer on Wednesday morning as compared with 24 hours earlier. Bettles tied its daily low temperature record with a reading of 58 degrees below zero.
As the high moved eastward, temperatures began rising along the Bering Sea coast as winds on the west flank of this high turned to come from the south. However, with southerly winds moisture increased due to the wind flow from the oceans to the south, and the approach of a storm system. This large storm was moving northeastward from across the western Bering Sea, causing an increased chance of snow and windy conditions for St. Lawrence Island and the west coast of Alaska. Consequently, blizzard warnings were posted for various locations in western Alaska to include the regions surrounding Kotzebue and Norton Sounds, the Yukon Delta and St. Lawrence Island. On Wednesday morning, the air temperature at Nome finally rose above zero for the first time in last 16 days, the ending the third longest subzero string at this Gold Rush City. However, it also had received 5 inches of snow by early afternoon. Winds gusting to 30 mph produced blizzard conditions as the visibility fell to less than one-half mile, and wind-chill equivalent temperatures dropped to 45 degrees below zero.
Elsewhere, the storm system in the Gulf of Alaska continued to weaken. This system was responsible for the clouds and precipitation that fell across the Panhandle and the Southeast. A weak trough of low pressure over the Chuckchi Sea produced light snow and a winter storm warning on the North Slope eastward to Prudhoe Bay.
The state's overnight lowest temperature on Wednesday was 60 degrees below zero at Huslia and Tanana . The mid-afternoon high was 36 degrees at Hydaburg, Metlakatla, Sand Point and St. George Island.
HAWAIIAN WEATHER -- The high pressure ridge that had been responsible for the strong northeast trade winds earlier in the week moved southward toward Hawaii as it weakened on Wednesday, resulting in a weakening of the winds as the center of the high got reasonably close to the islands. The southern portion of a cold front trailing from a midlatitude storm system had approached the islands. As of Wednesday afternoon, this front was within 400 miles to the northwest of the islands and loosing some of its frontal characteristics, becoming a shear-line. This shear-line, distinguished by a change in wind direction and band of clouds and precipitation, should reach the islands by Friday.
INTERPRETING A SOUNDING -- For an explanation of how the sounding information retrieved from a radiosonde or rawinsonde is displayed on a specially prepared chart, call up the Thursday's optional 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.