DATASTREME DAILY SUMMARY

Friday, 22 September 2000



HELENE TAKES AIM ON THE FLORIDA PANHANDLE -- On Thursday the tropical depression that had moved into the Gulf of Mexico at midweek intensified to become Tropical Storm Helene. As of early Friday morning, Helene was 75 miles south of Pensacola, FL and moving northward at 15 mph. Near-surface sustained winds were 65 mph and the central pressure was 1000 mb. The heaviest rain and the strongest winds were found to the northeast of the center of the system. These bands of rain approaching the Gulf Coast were separate from the widespread areas of rain associated with the stationary front that stretched across the northern counties of Georgia, Alabama and Mississippi.

The winds were beginning to increase along the coast as the storm approached. A tropical storm warning was also in effect for the coastal counties from Pascagoula, MS eastward to the Aucilla River for the impending winds ranging between 39 and 74 mph. A hurricane watch was posted from the Florida-Alabama border to the Aucilla River in Florida for the possibility of hurricane force (75 mph or greater winds). A storm surge, or a mound of sea water piled up by the storm winds, was expected to reach heights of 4 to 6 feet above normal astronomical tide level along the coast to the east of where Helene makes landfall.

Helene is expected to make landfall by sunrise on Friday morning, then weaken as it moves inland across the western Florida Panhandle and curves toward the northeast, reaching northern Georgia by Friday evening. As much as 6 inches of rain are expected locally along coastal Florida, while 3 inches of badly-needed rain can be expected to fall inland along the projected path of this tropical system during the 24 hours starting on Friday morning. However, because the soil may not be capable of handling the excessive rainfall rates, some of the rain could run off, causing flood conditions. As a result, flash flood watches were posted for many counties in southeastern Alabama, southwestern and northwestern Georgia and western Florida.

Tornadoes are possible as Helene makes landfall. A slight risk of severe thunderstorms continues through Thursday night for the Florida Panhandle and a portion of northern Florida as a result of the approach of Helene. On Friday the region with the slight risk is expected to expand northward into southeastern Alabama and southwestern Georgia.

PLEASANT WEATHER ACROSS THE NORTHEAST -- A cool and dry air mass accompanied a large area of high pressure that was centered over the Ohio Valley on Thursday night. The eastern and southern periphery of this air mass was the cold front that stretched from Labrador southward across the Maritimes, then over the western Atlantic Ocean before curving westward and traveling across the Carolinas. It developed into a stationary front that continued westward to northern Louisiana. Pleasant weather with few clouds and seasonal temperatures were found across the northeastern quadrant of the country.

The high pressure system is expected to drift slowly to the east on Friday. The stationary front that stretched across the Gulf Coast States is expected to begin moving northward as a warm front. Clouds and precipitation are expected to spread in from the West with the next storm system.

WET WEATHER SPREADS INTO THE PLAINS -- Areas of rain were reported across North Dakota and western Nebraska on Thursday night. A tornado that was reported by a storm spotter destroyed a mobile home and grain bin in east central Colorado on Thursday evening. This precipitation appears to be the forerunners of a storm system has been forecasted to develop from the complex of low pressure centers located across the northern Rockies and Great Basin. This storm is expected to move eastward across the Plains, reaching north central Kansas by Friday morning and northern Missouri by evening. The rain associated with this system is expected to expand across the northern Plains and into the Upper Mississippi Valley by morning. During the day thunderstorm development could occur in the warm air carried northward ahead of the storm. A slight risk of severe thunderstorms on Friday is forecast for an area that stretches from the vicinity of Kansas City, MO northeastward to near Milwaukee, WI. Between Friday and Saturday mornings, as much as an inch of rain could fall from south central Iowa to western Lower Michigan.

SNOW IN THE ROCKIES -- Some of the precipitation that fell across Montana and Wyoming on Thursday fell as snow as a result of the cold air and upslope conditions. Air carried upwards along a mountain slope cools as it rises, resulting in the formation of clouds and precipitation if sufficient moisture is present. Easterly winds circulating around the northern side of the low pressure centers created upslope precipitation across the northern Rockies. The air was sufficiently cold to support snow at the higher elevations. Afternoon high temperatures across Montana were at least 20 degrees below the long term average highs for this time of year.

Snow is expected to continue through Friday across the northern Rockies. A variety of snow advisories and winter storm warnings were posted for elevations above 5000 feet, to include most of Montana to the east of the Continental Divide, for northwestern counties in Wyoming and for the Black Hills in South Dakota. Snow accumulations of as much as a foot at the higher elevations are anticipated. A winter storm watch was in effect for southeastern Wyoming.

AN UPDATE - After surveying the damage, a team of experts declared that a tornado was responsible for the fatality and many of the injuries along with the destruction in Xenia, OH on Wednesday evening. Much of the community in southwestern Ohio remained without power as of Thursday evening.

TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US -- Thursday's lowest temperature was 26 degrees at Big Piney, WY. Thursday's highest temperature was 111 degrees at Death Valley, CA.

ALASKAN WEATHER -- A major storm system moving northeastward across the northern Bering Sea was the dominant weather feature across the state on Thursday. The primary low pressure center was located near St. Lawrence Island, with a secondary low centered near Kotzebue. An occluded front extended southeastward into interior Alaska, where a warm front stretched eastward into the Yukon Territory, while a cold front continued southward and then southwestward along the Alaska Peninsula and into the North Pacific. Clouds and rain associated with this system were widespread across western and interior Alaska. Fewer clouds were found in the southeast and the Panhandle, which were under the influence of a high pressure system located over the Gulf of Alaska and the eastern North Pacific.

The lowest overnight temperature in Alaska on Thursday morning was 21 degrees at Arctic Village, while the mid-afternoon highest statewide temperature was 70 degrees at Birchwood and Elmendorf AFB.

HAWAIIAN WEATHER -- A weak high pressure ridge located just to the north of Hawaii produced light winds that persisted across most of the islands on Thursday. With light prevailing winds on the order of 10 mph, the localized sea breeze circulation regimes were allowed to develop. Daytime heating of the island land surfaces produced a light onshore breeze, with clouds and a few light showers in the interior. The ridge weakened as a cold front trailing from a low pressure center approached the islands from the northwest. These conditions are expected to continue through the weekend.

EYE ON THE TROPICS -- In the eastern tropical North Atlantic, Tropical Depression 13 developed on Thursday morning, but quickly intensified to become Tropical Storm Issac by evening. As of late Thursday night, Isaac had sustained near-surface winds of 40 mph and was moving to the west-northwest at 15 mph approximately 200 miles southwest of the Cape Verde Islands.

In the eastern North Pacific, a tropical depression was approximately 305 mi southeast of Cabo San Lucas at the southern tip of Baja California and moving northwest at 7 mph. Sustained winds were estimated to be 35 mph.


HISTORICAL WEATHER EVENTS - 22 September

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

23 September

24 September


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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.