DATASTREME DAILY SUMMARY

Thursday, 19 October 2000


00Z Weather Systems


WET WEATHER CONTINUES ACROSS THE NORTHEAST -- A storm system located over the North Atlantic approximately 200 miles south-southeast of Cape Cod in eastern Massachusetts helped produce widespread rain across northern New England on Wednesday night. This low pressure center was along a cold front that had moved offshore earlier on Wednesday.

The low pressure system is expected to continue moving to the northeast, reaching the vicinity of Point Sable, NS by Thursday morning and near Newfoundland by evening. Continued heavy and potentially flooding rain is expected across Maine as moisture continues to be drawn northward by the storm system and flow onshore from the Atlantic. Moisture from Hurricane Michael could also be entrained in this system. Winds with gusts to 30 mph are expected in Boston, MA and in Portland, ME.

UNSEASONABLY MILD WEATHER ACROSS THE NORTHERN PLAINS -- High pressure across the central Plains, coupled with the advance of a warm front across the Dakotas, produced a pleasant, but warm day across the northern Plains. Afternoon high temperatures across the north central portion of the country, extending from the western Great Lakes to the Montana Rockies and south to Kansas were above normal, with the Dakotas experiencing afternoon high temperatures that were at least 25 degrees above the long-term average highs for the date. Record high temperatures were either tied or set on Wednesday in South Dakota at Pierre (90 degrees), Rapid City (86 degrees), Huron (84 degrees), and in North Dakota at Dickinson (82 degrees), Williston (80 degrees) and Minot (80 degrees). By early afternoon Pierre had experienced a 32 degree increase in temperature within a span of 4 hours.

A weak cold front passing across the northern Plains in association with a storm system traveling across the northern Prairie Provinces should go relatively unnoticed, since few clouds, no precipitation and a slight drop in temperatures are anticipated. A shift in the wind should occur with the frontal passage.

QUIET WEATHER IN THE SOUTHEAST -- While a weak low pressure was located along a cold front over the Florida Panhandle on Wednesday night, little precipitation was found across the Southeast. Some clouds associated with the low pressure and frontal system stretched across the Tennessee and lower Mississippi Valleys. Elsewhere, skies were relatively cloud-free.

Of note, Key West, FL had a record low temperature on Wednesday morning of 65 degrees, breaking a 123 year old record, and marking the first time in nearly six months that the temperature had fallen to 65 degrees or below. During the afternoon, Mrytle Beach, SC reached a record high temperature of 80 degrees.

WET WEATHER ACROSS THE SOUTHWEST -- Rainshowers and imbedded thunderstorms were found across southern New Mexico, Arizona and California on Wednesday night. This area of precipitation was associated with a region of low pressure centered over the lower Colorado River Valley.

On Thursday, much of the precipitation should drift farther to the east. At least 0.5 inch of rain is expected in a region extending from the vicinity of Phoenix, AZ to EL Paso, TX during the 24 hours ending on Friday morning.

STORMS AHEAD FOR THE NORTHWEST -- Widely scattered precipitation fell across Washington, Oregon and Idaho behind a cold front on Wednesday night. This cold front, trailing from a storm system in northern Alberta, had come onshore earlier on Wednesday, with the northern section of the front traveling eastward across the Montana Rockies. The southern portions of the front remained over southern Oregon and northern California.

A potent storm system situated approximately 1000 miles to the west of the Washington coast on Wednesday night was expected to move to the northeast, approaching the British Columbia coast on Thursday night. As the storm approaches, windy and rainy weather associated with this system should affect the Washington and Oregon coasts. Gale warnings were in effect through Thursday morning for the Washington coast northward from Cape Shoalwater, to include the Strait of Juan de Fuca. Twenty-four hour rainfall totals could be as much as 1.5 inches along the northwest coast of Washington State by Friday morning.

TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 14 degrees at Fraser, CO, while the Wednesday afternoon highest temperature was 95 degrees at Coolidge, AZ.

ALASKAN WEATHER -- Several areas of low pressure were located just to the south of Alaska on Wednesday afternoon. One low pressure center was located in the Bering Sea just west of Bristol Bay, while the other low pressure center was in the Gulf of Alaska south of the Kenai Peninsula. These systems contributed to the clouds and precipitation found across the Aleutians, the Pribilofs, the Alaska Peninsula and south central Alaska, including the Anchorage Bowl. Clouds spread into southeast Alaska and the Panhandle in association with the trough of low pressure extending eastward from the low in the Gulf.

The building of an area of high pressure over the Chukchi Sea has spread colder air southward across much of interior Alaska, as many locations experienced afternoon temperatures in the teens and low 20s. The onshore flow produced clouds across the North Slope and Northwest Alaska, as well as snow at Barrow.

The lowest temperature in Alaska on Wednesday morning was zero degrees at Anaktuvuk Pass, Northway and Arctic Village. The highest statewide temperature as of Wednesday afternoon was 53 degrees at Shemya.

HAWAIIAN WEATHER -- The weakening of the subtropical ridge of high pressure north of Hawaii produced lighter trade winds on Wednesday. High clouds continued over Kauai. The winds are expected to strengthen again as disturbances in the tropical winds pass to the south of the Big Island, thereby tightening the pressure gradient.

EYE ON THE TROPICS -- Hurricane Michael began accelerating to the north as it passed to the west of Bermuda. As of late Thursday night, Michael continued to have sustained surface winds of 80 mph and it was located 230 miles north-northwest of Bermuda, moving to the northeast at 26 mph. On Thursday it is expected to merge with a midlatitude trough of low pressure and loose its tropical characteristics. Swell waves ranging between 4 to 6 feet generated by Michael are still expected along the North Carolina coast, primarily north of Cape Hatteras.


CONCEPT FOR THE DAY - THE HOT SANTA ANA WINDS

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.

QUESTIONS:

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.

  1. The Santa Ana winds of Southern California are warmed primarily by [(solar radiation) (expansion) (compression)].
  2. If the water vapor content remains the same, the relative humidity of a sinking air parcel [(increases) (decreases) (remains the same)].

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 .


HISTORICAL WEATHER EVENTS - 19 October

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


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