DATASTREME DAILY SUMMARY

Tuesday, 2 October 2001


00Z Weather Systems


The following discussion is based upon the major weather features appearing on Monday night's surface weather maps:

UNSETTLED WEATHER ALONG THE MIDDLE ATLANTIC COAST -- A storm system several hundred miles off the New Jersey coast continued to bring clouds, rain and windy conditions along the Middle Atlantic coast. As of Monday evening the rain was found from the Del-Mar-Va Peninsula northward to Long Island Sound.

Locations along the Atlantic Seaboard within roughly 100 miles of the ocean experienced high temperatures that were on the order of 10 degrees below the average highs. These cool readings especially along the New England and Middle Atlantic Coast were the result in part due to clouds and precipitation, as well as a brisk onshore flow of air. Farther south, the cool air mass also contributed to the chill.

COOL WEATHER ACROSS THE SOUTHEAST -- A cool and dry high pressure system located across Arkansas and the lower Mississippi Valley was responsible for relatively tranquil weather across the Southeast. Record low temperatures were set on Monday morning in South Carolina at Charleston (44 degrees at Airport and 54 degrees at Downtown); in Florida at Dayton Beach (58 degrees) and at Orlando and Melbourne (both 61 degrees).

THE HEAT GOES ON IN THE WEST -- A large portion of the nation, extending from the California and Oregon coasts eastward to the Great Lakes experienced high temperatures on Monday that ranged between 10 and 20 degrees above the average highs for early October. Daily record high temperatures were either tied or broken on Monday in California at Red Bluff (105 degrees), Elsinore (104 degrees), Sacramento (102 degrees at Airport and 101 degrees Downtown), Stockton (100 degrees) and Cuyuma (95 degrees); in Oregon at Redmond (91 degrees) and Burns (86 degrees); in Nevada at Wells and Winnemucca (both 90 degrees); in Utah at Zion National Park (97 degrees) and Milford (90 degrees); in Colorado at Glenwood Springs (87 degrees) and in Montana at Helena (87 degrees). In addition to being a daily record, the high temperature in Helena also tied a monthly record.

While most of the weather across the west was rather tranquil, a dissipating thunderstorm generated a 67 mph wind near Colorado Springs, CO on Monday evening.

The unseasonably warm conditions across the West have persisted for much of September. Phoenix, AZ reported that the average temperature of 92.2 degrees for this just concluded month of September was the highest on record, which is 6.6 degrees above the 1961-1990 climatological average.

AN ACTIVE COLD FRONT TO MOVE ACROSS THE PLAINS -- A cold front that stretched along the Canadian border is expected to sweep southward and eastward across the northern Plains on Tuesday bringing rain and thunderstorms. The cold front trailed southwestward from a low pressure system along the eastern shores of James Bay to northern Minnesota and then westward across the Dakotas. A low pressure system was located along the front over eastern Montana.

The low pressure system over Montana is expected to deepen and develop as it moves to the east on Tuesday. By sunrise, the low pressure center is forecast to be located over eastern North Dakota and by evening the low should be over northwestern Wisconsin. A cold front trailing the low is expected to sweep southward and eastward across the Plains and into the upper Mississippi Valley. Much colder air is expected to follow behind the cold front. With northwest winds reaching 30 mph expected following the cold frontal passage across eastern Montana and western South Dakota, wind advisories were posted across portions of these two states through early Tuesday morning. While thunderstorms could form ahead of the cold front, the bulk of the precipitation is expected to the north of the track of the storm. As much as 0.3 inches could fall by Tuesday evening across portions of northern Minnesota.

A REMINDER OF JULIETTE -- Bands of rain moved northward into Arizona from Mexico on Monday as part of the moisture associated with the remnants of former Hurricane Juliette. This storm system that at one time had reached a Category 4 on the Saffir-Simpson Hurricane Intensity Scale was only a weak area of low pressure across northwest Mexico and the southern Colorado Valley of southwestern Arizona and southeastern California.

The low pressure system is expected to drift slowly northward on Tuesday, with a continuation of the precipitation across the lower Colorado Valley. As much as 0.2 inches of precipitation could fall across southern Arizona, with heavier amounts anticipated south of the Mexican border.

DRY CONDITIONS ACROSS THE NORTHWEST -- The ridge of high pressure across the West has persisted for more than a year, resulting in seriously low water levels and enhanced wildfire danger in many locations. Hydrologists use the water year that runs between 1 October and 30 September of the following year for measurement of stream flows and precipitation. Many locations across northwest Oregon experienced the driest water year on record in terms of the total accumulated precipitation. Some of the stations that set record low precipitation records in the just concluded water year include Eugene (20.36 inches), Portland (23.00 inches) and Astoria (44.79 inches).

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 23 degrees at Whitefield, NH, while Monday's highest temperature was 110 degrees at Death Valley, CA.

ALASKAN WEATHER -- A low pressure center was located over the Yukon-Kuskokwim Delta of southwest Alaska with an occluded front that curved eastward, then southward to become a cold front that extended over the Gulf of Alaska in the vicinity of Prince William Sound. The heaviest precipitation in the state was found in the vicinity of Prince William Sound and along the coast near the Panhandle. To the west of the cold front, a weak ridge of high pressure spread across the eastern Aleutians, the Alaska Peninsula and the western Gulf of Alaska. A larger ridge of high pressure centered over the eastern Gulf of Alaska stretched northeastward across the Panhandle and southeast Alaska. A trough of low pressure and a stationary front extended eastward across interior Alaska. To the north of this stationary front, a ridge of high pressure that was located over the Arctic Ocean to the northwest of the state stretched across the Chukchi Sea, northwest Alaska and the North Slope. Bettles and Kotzebue had an inch of snow on the ground at daybreak on Monday.

The state's lowest temperature on Monday morning was 15 degrees at Barrow. The highest temperature across Alaska as of midafternoon on Monday was 56 degrees at Merrill Field.

HAWAIIAN WEATHER -- A cold front that has turned into a wind shear line has been moving southward across the islands on Monday. Clouds and the showers associated with this shear line had just reached the northern parts of the Big Island as of late afternoon, while the trailing edge of the cloud band was across Molokai. Winds to the south of the advancing shear line were from the east-northeast, while they were more northerly behind the shear line. High pressure was located to the northwest of the islands. This ridge is expected to move toward the east with an accompanied weakening of the trade winds and a return to drier and more stable atmospheric conditions. With anticipated surf rising to between 10 to 15 feet by Tuesday, high surf advisories were in effect for the north and northeast facing shores of the islands.

EYE ON THE TROPICS -- In the Eastern Pacific the thirteenth tropical depression of the 2001 hurricane season formed on Monday. As of Monday evening, Tropical Depression 13E was located 375 miles to the south of Acapulco, Mexico and it was moving west at 8 mph. Maximum sustained winds were 35 mph.

REPORT FROM THE FIELD -- Marsha Rich, Project Atmosphere AERA from New Hampshire, reported that Mount Washington, NH has received 2.7 inches of snow so far this season, with most of that falling last Thursday and Friday. She suggests to "start getting those skis waxed!"

HARVEST MOON -- The full moon that you might see tonight is known as "Harvest Moon" since it is also the full moon nearest the autumnal equinox (occurring on 22 September 2001). This full moon (the exact time is 1349 Z or 9:29 EDT, etc.) is called the Harvest Moon because the moon rises soon after sunset for several nights and, according to legend, the moonlight from the nearly full moon provides mid-latitude farmers with extra natural light for harvesting their crops before the first frost that ends the growing season. Romantic songs have also been written of its appearance.

Several astronomical factors involving the earth-moon-sun viewing geometry contribute to making this spectacular late summer (or early fall) event special - a large lunar disk that appears as a pumpkin hanging above the eastern horizon just after sunset for several consecutive evenings. Whenever the moon is full, the moon will appear opposite the sun, leading to a moonrise to occur within minutes of local sunset. At this time of year the moon's path appears to be inclined with respect to the local horizon at a relatively shallow angle. As a result, the moon rises by only about a half hour later each night for several nights running - less than at any other time of the year. The next full moon, called the "Hunter's Moon" (1 November 2001), will not be as spectacular.

If you have cloud free skies, step outside and take a look at the bright and nearly full moon rising in the eastern skies within minutes of local sunset for the next several nights.


CONCEPT FOR THE DAY - WARM and COLD AIR ADVECTION

Air temperatures may vary greatly from day to day. Today's afternoon high temperature may be 5 degrees warmer than that experienced on yesterday afternoon, or tomorrow morning's low temperature is expected to be 15 degrees colder than what was recorded this morning. Some places in the country are experienced falling temperatures across the northern Plains as a storm system affects the temperature patterns across the region. How are these significant changes possible?

Many factors are at work in determining the near-surface air temperature at a given place and time. The daily variation in temperature is driven by the daylight and darkness cycle in solar radiation. Under clear skies, the day's maximum temperature typically occurs within an hour or two after local solar noon. Local noon is that time when the sun at most locations would be directly to the south of you and is at its highest point in the sky for that day. Clear skies will usually cause the daily minimum temperature to occur within an hour of local sunrise. If all other factors were equal, changes in the daily temperature pattern from one day to the next would be small because day to day changes in the incoming solar radiation typically are not large.

Cloud cover could complicate the situation - lowering the afternoon temperatures because of the blocking of at least some of the incoming solar radiation, and reducing the drop in overnight temperatures because the clouds absorb the outgoing thermal radiation from the earth and radiate much of it back to the surface.

A third factor affecting the day to day temperature variations is the role that the general wind flow has upon local temperatures. Many times the observed large swings in temperature from day to day result when warm air from some distant locale replaces the cooler air that has resided in the area. This replacement causes a warming. Conversely, if colder air invades, a noticeable temperature drop results. These examples are termed "warm air advection" and "cold air advection", respectively, and could be more significant than the other two factors identified above. Warm air advection can occur with the passage of a warm front and a warm air mass is ushered into the region. A less dramatic change would occur when the winds turn to blow from a warmer region as a high pressure system pulls away from the region. Conversely, cold air advection would take place after a cold front passed and a cold air mass makes its influence felt. For more details describing how you can use a surface weather map to determine regions of warm or cold air advection, consult DataStreme Activity 4A and Tuesday's optional electronic Supplemental Information .

QUESTIONS:

To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 4 Chapter Progress Response Form, under section B. Daily Summary.

  1. [(Warm) (Cold)] air advection could be the cause of rising temperatures from dusk to dawn.
  2. You would expect [(cold air), (warm air)] advection to occur immediately after the passage of a cold front.

HISTORICAL WEATHER EVENTS - 2 October

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


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