DATASTREME DAILY SUMMARY

Thursday, 27 September 2001


00Z Weather Systems


The following highlights of the national weather have been extracted from the surface weather map for Wednesday night:

A WEAKENING LOW -- The slow-moving storm system that has lingered over the eastern half of the nation since the beginning of the week has finally weakened. By Wednesday evening, this storm has nearly undergone a complete occlusion process, with the primary low pressure system remaining over Ontario near Georgian Bay. A secondary low pressure center that had formed along the occluded front was situated over northern Maine. This secondary low pressure system has also begun occluding with an occluded front extending northeastward into the Gulf of St. Lawrence, where a warm front continues eastward across Newfoundland, while a cold front extends southward over the western Atlantic Ocean to the Florida Peninsula.

As of Wednesday evening, some areas of light rain were moving across Lakes Erie and Ontario and into northern Ohio, western Pennsylvania and Upstate New York as part of the counterclockwise circulation around the primary low pressure center. Some of this precipitation, especially that found along the downwind shores, was lake-enhanced precipitation. This lake-enhanced precipitation was the result of cold air being carried across a warmer lake surface, collecting moisture and becoming destabilized.

This storm system will continue to weaken while the primary low pressure system is expected to remain over the eastern Great Lakes through Thursday. Less than 0.5 inches of rain are expected during the 24 hours through Thursday evening across portions of northern Ohio and into the St. Lawrence Valley.

A RAINY SUNSHINE STATE -- A broad area of rain and embedded thunderstorms was found across the Florida Peninsula and out over the Gulf and Atlantic on Wednesday night. This precipitation was associated with the southern portion of the cold front that stretched southward from the storm system affecting the Northeast. The cold front has stalled and become a stationary front across the Florida Peninsula. During the late afternoon, at least one waterspout and several funnel clouds were spotted off the Florida Keys near Key West, while another waterspout was observed off the Atlantic coast north of Ft. Lauderdale.

The stationary front is forecast to remain across southern Florida through Thursday. Nearly 1.9 inches of rain could fall along the front during the 24 hours ending on Thursday evening. As a result, a flood watch was in effect for portions of southern Florida extending from the Gulf Coast south of Tampa Bay to Lake Okeechobee.

UNSEASONABLY COLD AIR REACHES THE SOUTHEAST -- A cool high pressure system spread southward and eastward across the eastern half of the nation on Wednesday. As of Wednesday evening the center of this high was located over the lower Mississippi Valley. Since this chilly air mass was relatively dry and locations near the center experienced weak winds, nighttime radiational cooling has been enhanced. As a result, morning temperatures have fallen to near record low readings across the Southeast. Record low temperatures were either tied or set in Kentucky at London (34 degrees), Bowling Green (34 degrees), Paducah (37 degrees), Lexington (38 degrees) and Jackson (41 degrees); in Indiana at Evansville (37 degrees) and Indianapolis (37 degrees); in Tennessee at Bristol (36 degrees), Nashville (38 degrees), Dyersburg (43 degrees) and Memphis (46 degrees); in Virginia at Lynchburg (35 degrees); in West Virginia at Bluefield (37 degrees); in Alabama at Huntsville (38 degrees); Birmingham (41 degrees), Montgomery (43 degrees) and Mobile (50 degrees); in Arkansas at El Dorado (42 degrees), Jonesboro (43 degrees) and Pine Bluff (44 degrees); in Georgia at Athens (45 degrees); in Mississippi at Tupelo (40 degrees) and Meridian (44 degrees); in North Carolina at Asheville (40 degrees) and Charlotte (42 degrees); in South Carolina at Greenville-Spartanburg (42 degrees); and in Texas at Bergstrom Field in Austin (47 degrees). Afternoon high temperatures from the Great Lakes southward to the Gulf Coast and Southeast ranged from 10 to 15 degrees below the average highs for late September.

Little change in the location of the high pressure system is expected on Thursday, leading to another chilly morning in store for Thursday. As a result, a frost advisory was in effect for western North Carolina.

THE ROCKIES REMAIN HOT -- Unseasonably warm conditions were found across the Rockies and the Great Basin on Wednesday, where afternoon high temperatures were on the order of 15 to 20 degrees above the average highs. Record high temperatures were in Utah at Moab (96 degrees) and Vernal (84 degrees); in Wyoming at Sheridan (91 degrees), Worland (88 degrees), Casper (86 degrees), Cheyenne (85 degrees) and Rock Springs (81 degrees); in Nevada at Wells (87 degrees); in Colorado at Colorado Springs (87 degrees) and Dinosaur National Monument (86 degrees); and in Nebraska at Chadron (93 degrees).

UNSETTLED WEATHER IN THE NORTHWEST -- A cold front that moved into the Pacific Northwest brought more widely scattered light rain over the mountains of Washington, Idaho and western Montana.

This front is expected to stall across the northern Rockies and the Great Basin. Some light rain is expected to continue across the region on Thursday, with amounts totaling less than 0.1 inches.

TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 28 degrees at Bigfork, Cook and Pine River, MN, while the Wednesday afternoon highest temperature was 111 degrees at Death Valley, CA.

ALASKAN WEATHER -- Much of the state was cloud covered on Wednesday as the result of several low pressure systems and accompanying fronts. Relatively little precipitation was found across the state. One low pressure system was located over the Gulf of Alaska southeast of the Kenai Peninsula, another low pressure system was situated over the central Aleutians, a third low pressure center was located over the western Bering Sea near the Russian coast, and a fourth low was over the Beaufort Sea to the northeast of Alaska. A stationary front stretched eastward along the Brooks Range from near Kotzebue to the northern Yukon Territory. This front separated arctic air associated with a high pressure center located to the northwest over the Chukchi Sea from warmer air over the interior of Alaska.

The lowest temperature in Alaska on Wednesday morning was 18 degrees at Anaktuvuk Pass. The highest statewide temperature as of Wednesday afternoon was 59 degrees at Cordova and Talkeetna.

HAWAIIAN WEATHER -- The islands experienced clouds on Wednesday that were remnants from an old cold front now called a wind shear line. Scattered showers were found along the windward slopes of all the islands. Once the shear line passes to the south and west of the Aloha State late on Wednesday night, the trade winds should increase in intensity. The showers are also expected to diminish on Thursday.

A FOLLOW-UP -- On-site investigators have reported on the tornadoes that moved from northern Virginia into the Maryland suburbs on Monday. Two tornadic thunderstorms moved across the area, with tornadoes leaving an interrupted path of destruction along the ground. According to the Fujita Scale, the one that tore through Culpepper, VA was rated F-4, with maximum winds ranging from 200 to 220 mph, while the killer tornado that swept across the University of Maryland campus in College Park was rated F3 with maximum winds between 158 and 206 mph. This latter tornado had moved from Alexandria, VA passing the Pentagon, then crossed the Potomac near the Jefferson Memorial before lifting over the District of Columbia. Items from College Park were found 50 miles away near Baltimore.

EYE ON THE TROPICS -- In the eastern North Pacific, Hurricane Juliette weakened slightly as it continued its movement to the northwest along the Mexican coast. As of Wednesday night, Hurricane Juliette had maximum sustained surface winds of 110 mph and was moving to the northwest at 10 mph in the vicinity of 215 miles to the south-southwest of Cabo San Lucas, Mexico. Clouds and moisture from Juliette could reach the Southwestern States by this upcoming weekend.

Hurricane Humberto continued to move toward the northeast across the North Atlantic. As of late Wednesday night Humberto was located 290 miles south of Cape Race, NF, with forward motion to the east at 23 mph. Maximum sustained surface winds were 75 mph.


CONCEPT FOR THE DAY - SUN PATHS and LENGTH OF DAYLIGHT

We all can recall summer's long daylight hours and hot noontime sun that was high in the sky. Now, thoughts of the upcoming winter bring memories of short daylight and weak rays from a midday sun that is low in the sky. The Study Guide Part A: Narrative shows the paths of the sun on the first days of the astronomical seasons in the midlatitudes where most of us live. We are now in the part of the year when the combination of the inclination of the Earth's axis to its orbital plane and the movement of our planet along its orbital path are tilting the Northern Hemisphere more and more away from the Sun. Our daylight periods are getting shorter and the noon sun is ever lower in the sky.

Having passed the Autumnal Equinox (last Saturday evening, 22 September 2001), where we all experienced 12 hours of sunlight (plus a few minutes - see Thursday's optional Supplemental Information file), the length of daytime is decreasing by several minutes from one day to the next. For Washington, DC, this decrease is about 3 minutes less possible sunshine each day. This effect is accentuated the higher the latitude. The period of daylight will continue to decrease, but at a slower and slower rate. On the first day of winter, 21 December, International Falls, MN on the Canadian border will have only 8 hours and 17 minutes of possible sunlight compared to Key West, FL at 10 hours and 37 minutes. (Barrow, AK will have NO sunlight, being north of the Arctic Circle.) For comparison, residents in International Falls basked in a lengthy 16 hours and 9 minutes of possible sunshine on the first day of summer last 21 June, while Key West had 13 hours and 40 minutes of possible sunshine (Barrow conceivably could have had 24 hours of sunshine, barring no clouds). A portfolio of sunrise photos shows the beginning of the path of midlatitude sun over year.

The more indirect solar rays and the shorter intervals of daylight combine to decrease solar heating in the Northern Hemisphere at this time of year. In addition to this decrease in daytime energy gain, the simultaneous loss of infrared radiation to space during both day and night dominates the exchange of energy between Earth and space. The result is an autumnal cooling of the Northern Hemisphere. Some cold air masses already have invaded the country, and more cold air is yet to come!

More details are provided in DataStreme Activity 3B and Thursday's optional Supplemental Information .

QUESTIONS:

To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 3 Chapter Progress Response Form, under section B. Daily Summary.

  1. Currently each succeeding day in our midlatitudes is [(shorter) (longer)] by a few minutes.
  2. At Northern Hemisphere locations at this time of year, [(incoming solar energy exceeds outgoing infrared) (outgoing infrared heat exceeds incoming solar energy)].

HISTORICAL WEATHER EVENTS - 27 September

From the files of the Aviation Weather Center, Kansas City, MO and Intellicast


Return to DataStreme Homepage

URL: datastreme:/learn/r_sum.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.