BLUSTERY WEATHER ACROSS THE NORTHEAST -- The storm system that moved slowly from the Great Lakes into the Northeast over the last several days finally moved into eastern Canada on Wednesday. However, the storm system had deepened as it moved toward the lower St. Lawrence Valley, with the central pressure falling from approximately 1000 mb on Tuesday evening to 990 mb on Wednesday night. As a result of the tight pressure gradient between high pressure moving across the Midwest and the departing low pressure system, relatively strong northwesterly winds continued across the Northeast on Wednesday evening, with speeds on the order of 20 mph.
In addition to the windy conditions, scattered precipitation continued to fall across the region, especially over New York State downwind of Lake Ontario. This precipitation was lake-enhanced, as cold air was being carried across the relatively warm lake surface. While some snow showers had been detected earlier, most of the lake-enhanced precipitation was falling as rain as air temperatures were not sufficiently low to support snow. Earlier in the day, as much as one inch of snow fell at Snowshoe, WV, which represented the first snow of the season. Some light rain was also falling across northern Maine.
COLD AIR MAINTAINS ITS GRIP -- A large cold high pressure cell continued to drift eastward across the Ohio Valley. As of Wednesday evening this high was centered over Kentucky and middle Tennessee. Relatively clear skies and low humidities were found across much of the country east of the Mississippi River. Earlier, record low temperatures were set on Wednesday morning at Charlotte, NC (35 degrees) and Macon, NC (36 degrees) in the Southeast. In addition, record lows were either tied or set in Texas at Austin/Bergstrom (35 degrees), Longview (37 degrees), San Antonio (40 degrees), College Station (42 degrees) and Victoria (45 degrees). Afternoon high temperatures across the much of the eastern half of the country were below the average highs for mid October. Many locations in the region stretching from the eastern Great Lakes across the Ohio and Mississippi Valleys to the Gulf Coast experienced highs that were on the order of 10 degrees below the average highs.
The high pressure center is forecasted to continue drifting eastward, reaching Tidewater Virginia by Thursday evening. As a result of the dominance of the cold high pressure system, coupled with the clear skies and light winds, many locations from the Ohio Valley to the Eastern Seaboard should experience significant nighttime radiative cooling. A variety of freeze warnings and frost advisories cover portions of the Ohio Valley, the Cumberland Plateau and the Appalachians, stretching from southern Ohio and Indiana southward to northeastern Georgia. Frost advisories were also in effect for the Chesapeake Bay area of Maryland and Virginia.
UNSETTLED WEATHER MOVES ACROSS THE NORTHERN PLAINS -- A cold front moving eastward across the northern Plains was accompanied by clouds and some widely scattered precipitation. Most of this precipitation was falling in the colder air behind the front, across the western Dakotas, Montana and Wyoming. The cold front trailed southward from a low pressure center located over central Manitoba.
The air behind the cold front was not significantly colder than the air being displaced since this new air mass had been of Pacific origin rather than from northern Canada. Farther to the south, the air located over the Plains was unseasonably warm, in part due to the southwesterly winds that were flowing downslope from the Rockies. Afternoon highs across the western Plains and the central Rockies were on the order of 15 to 20 degrees above average. Valentine, NE had a record high temperature of 86 degrees. In the Southwest, Douglas, AZ reported a record high of 90 degrees.
With the low pressure forecast to move eastward from Manitoba to Hudson Bay on Thursday, northern portions of the cold front are expected to travel eastward across the Plains and the Upper Mississippi Valley, reaching the western Great Lakes by evening. Between 0.1 and 0.2 inches of rain could fall in the 24 hours ending on Thursday evening in a band running from South Dakota eastward to northern Wisconsin and Michigan's Upper Peninsula.
TRANQUIL WEATHER RETURNS TO THE WEST -- High pressure located off the Oregon coast is expected to push inland on Thursday. As a result, relatively quiet weather should return to much of the Pacific Northwest through Thursday. With relatively clear skies and weak winds, frost warnings were issued on Wednesday night for southwest Oregon.
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 15 degrees at Westby, MT, while the Wednesday afternoon highest temperature was 99 degrees at Borrego, CA.
ALASKAN WEATHER -- A major storm system extending across much of southern Alaska on Wednesday afternoon brought significant precipitation and strong winds across the region, to include the Bering Sea, the Aleutians and the Gulf of Alaska. This storm consisted of one low pressure center located near Dillingham on Bristol Bay in southwest Alaska connected to a second low pressure center located near Cordova in southeast Alaska by a trough of low pressure. An occluded front curved southeastward into the Panhandle to a point of occlusion, where a warm front continued southward along the coast into British Columbia, while a cold front curved westward across the southern Gulf of Alaska and then the North Pacific. Some locations along the Aleutians and the Alaska Peninsula reported heavy rain, to include Sand Point (1.78 inches) and Cold Bay (1.16 inches). The Anchorage Bowl received between 6 to 8 inches of snow during a 6 hour span on Wednesday morning, which represented the first significant snowfall of the season. Northway and McGrath also reported snow. The Aleutians and the Alaska Peninsula experienced high winds during the morning. Cold Bay had wind gusts to 60 mph. St. Lawrence had north to northwest winds ranging between 25 to 40 mph. Arctic high pressure centered over the Arctic Ocean to the north of eastern Siberia was pushing southward across the North Slope and the Brooks Range into the interior. An east-west oriented stationary front running from the Yukon-Kuskokwim Delta to the southern sections of the Yukon Territory separated the arctic air from the warmer air to the south. Northeasterly winds off the Arctic Ocean produced snow at Deadhorse. During the early afternoon, Barrow reported a wind-chill equivalent temperature of 25 degrees below zero.
The lowest temperature in Alaska on Wednesday morning was 15 degrees below zero at Barrow. The highest statewide temperature as of Wednesday afternoon was 53 degrees at Kodiak.
HAWAIIAN WEATHER -- While the high clouds that were found across the islands for the last several days began to decrease, low level trade wind clouds were increasing along the windward sides of the islands on Wednesday afternoon. High pressure located to the northeast of Hawaii was expected to maintain the trade winds, with speeds ranging from 10 to 25 mph.
EYE ON THE TROPICS -- After a run of eight days across the eastern North Pacific Ocean, Tropical Storm Manuel weakened and was downgraded to tropical depression status. As of Wednesday night when the National Hurricane Center issued its last official advisory on this tropical system, Manuel had maximum sustained surface winds of 30 mph and was located 765 miles to the west-southwest of Cabo San Lucas, Mexico. Forward motion of this weakening tropical depression was 8 mph to the north-northwest.
Seemingly every fall, parts of Southern California are plagued by firestorms. These major conflagrations are usually driven by hot, dry "Santa Ana winds". Santa Ana winds have also contributed to the occurrence of unseasonably warm weather across the Los Angeles Basin earlier this fall.
These winds consist of air originating over the high deserts of the Great Basin to the east. A persistent high pressure cell located over Utah would drive these winds down from where surfaces are at elevations as high as 3000 m (9000 feet), through the mountain passes and canyons to near sea level along the Southern California coast. As the air sinks, it is compressed by increasing air pressures at lower levels. The compression heats the air at the dry adiabatic rate, ending perhaps 20 to 30 Celsius degrees (35 to 50 Fahrenheit degrees) warmer than it begins. The water vapor content of the air remains near the initial amount so that the warming produces final relative humidities that are extremely low. These hot, dry winds desiccate the already parched, late summer vegetation, especially the chaparral. Any spark creates rapidly spreading flames that feed their own growth by the fire-induced circulations created in addition to the Santa Ana winds.
The Essay on pages 195 and 196 of the DataStreme Part A: Narrative provides background on the discovery of the Santa Ana, its climatology and the impact upon vegetation and humans.
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 6 Chapter Progress Response Form, under section B. Daily Summary.
For more information describing the various sets of lines on a Stüve Diagram and how you can make graphical determinations from a Stüve Diagram, you may consult the optional material in Thursday's Supplemental Information .
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast