DATASTREME DAILY SUMMARY

Thursday, 28 September 2000



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TRANQUIL WEATHER OVER THE NATION -- Very few locations across the continental United States experienced precipitation on Wednesday night, as relatively high pressure dominated a relatively featureless weather map from coast to coast. The locations experiencing precipitation were found in Florida, across Upstate New York and in the Southwest.

The most intense precipitation appeared to be located across south Florida where rainshowers and some embedded thunderstorms were found along a stationary front that bisected the peninsula, separating a warm tropical air mass to the south from a slightly cooler, but drier air mass to the north. During the late afternoon, a severe thunderstorm warning was issued for the region west of Lake Okeechobee as Doppler radar indicated that some of the thunderstorm cells had become severe. A waterspout was also spotted on the eastern portion of the lake in midafternoon. Earlier, a waterspout was detected near Pompano Beach on the Atlantic coast. This stationary front is expected to remain in place through most of Thursday before gradually dissipating. As much as 0.5 inches could fall by Thursday evening across some locales in the southern half of the Florida Peninsula.

Another area of light rain was found in Upstate New York along a cold front that was the leading edge of a cool air mass associated with high pressure moving southeastward across the Great Lakes. The cold front extended southwestward from the Maritime Provinces across New England and New York State before curving to the west, crossing northern Ohio, Indiana, Illinois, and then turning to the northwest across Iowa to become a stationary front across the Dakotas. The lack of sufficient available moisture meant little precipitation along the front. As the cold front continues to move slowly toward the Ohio Valley, some light precipitation could fall along the front, especially in the central Appalachians on Thursday. The western portion of the front could become a warm front across the Dakotas as a low pressure system is forecast to move eastward across the Canadian Prairie Provinces and draw warmer air from the south.

Widely scattered precipitation in the form of rainshowers and thunderstorms were detected across southern Arizona and New Mexico. This precipitation was associated with what would typically be considered the southwest monsoon. In this monsoon regime, southerly winds carry moisture from the Gulf of California and the Gulf of Mexico northward around the thermal low pressure located over the lower Colorado Valley. This seasonal semi-permanent low pressure system results from the heating of the surface.

A COOL START -- The widespread high pressure has provided relatively clear skies, low atmospheric humidity and weak winds -- all ingredients that enhance nighttime cooling. As a result, most locations east of the Rockies had minimum temperatures on Wednesday morning that were at or below the long-term average low temperatures for the date. The greatest departures were found across the South, where record low temperatures were either tied or set across the Southeast in Jackson, TN (39 degrees), Jackson, KY (40 degrees), Asheville, NC (48 degrees), along the Gulf at New Orleans-Audubon Park, LA (55 degrees) and the Texas cities of Austin-Bergstrom (45 degrees), Longview (45 degrees), Beaumont (52 degrees) and Corpus Christi (52 degrees). The low temperature at Corpus Christi also tied the record low for the month of September.

With little significant change in the weather patterns, a repeat of Wednesday morning's cool start could be in store for Thursday. Frost and freeze advisories were posted for Michigan's Lower Peninsula on Wednesday. Frost warnings were also issued for western North Carolina.

A WARM DAY ACROSS THE WEST -- With no major storm systems, the West continued to remain warm and dry on Wednesday. Afternoon high temperatures from the northern Rockies to the Pacific Coast were as much as 10 degrees above the climatological average high temperatures for the end of September.

TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 12 degrees at Wisdom, MT, while the Wednesday afternoon highest temperature was 104 degrees at Death Valley, CA.

ALASKAN WEATHER -- An elongated ridge of high pressure stretching eastward from Siberia across much of Alaska on Wednesday continued to bring cold air into the state. As of late Wednesday afternoon, most of the observing stations were reporting northerly winds. Earlier, Cold Bay and Palmer experienced gusty north winds. Relatively clear skies were found across most of northern and western Alaska, except for coastal communities. With onshore winds, light snow and freezing drizzle were reported along the Arctic Coast.

A storm system located over the northeastern Gulf of Alaska south of Cordova spread clouds and precipitation across the southeastern quadrant of the state. Snow also fell across the state, with the most snow falling over eastern and southeastern interior sections of the state, to include Anchorage, Delta Junction and Northway. Along the northeast Gulf coast, the air was sufficiently warm to allow heavy rain to fall at Yakutat and Cordova.

Windy conditions were found across the Aleutians because of the strong pressure gradient produced between the high over the Bering Sea and a potent storm system over the North Pacific south of the Aleutians.

The lowest temperature in Alaska on Wednesday morning was 18 degrees at Galbraith Lake. The highest statewide temperature as of Wednesday afternoon was 57 degrees at Juneau.

HAWAIIAN WEATHER -- An area of showers associated with a disturbance in the trade winds moved westward across the islands on Wednesday afternoon. The disturbance was south of the Big Island. By Thursday these showers should be west of Kauai. A ridge of high pressure located to the north of Hawaii produced southeasterly winds across most of the islands, except over the Big Islands, where winds with a more easterly direction were found.

The buoy moored to the northwest of Kauai continued to monitor a relatively steady ocean swell generated by a slow moving storm system in the North Pacific. This swell is expected to produce 10 to 15 foot waves. Consequently, high surf advisories were posted for the north and west shores of the islands.

EYE ON THE TROPICS -- Hurricane Isaac reintensified to major hurricane status on Wednesday as it moved across the tropical Atlantic. As of late Wednesday night, Isaac, with sustained near-surface winds of 115 mph, was located 1050 miles east-southeast of Bermuda and moving to the northwest at 16 mph. Tropical Storm Joyce was upgraded to hurricane status on Wednesday morning as winds strengthened to speeds in excess of 75 mph. Joyce, the sixth hurricane of the Atlantic hurricane season, had 95 mph gusts on Wednesday night and was moving to the west at 13 mph approximately 1200 miles east of the Windward Islands.


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 (one week ago tomorrow on Friday, 22 September 2000), 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 - 28 September

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


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URL: datastreme:/learn/r_sum.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.