DATASTREME DAILY SUMMARY

Tuesday, 9 October 2001


00Z Weather Systems


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

COLD AIR MOVES TO THE EAST COAST -- The cold high pressure system that chilled the nation's midsection over the weekend drifted eastward on Monday from its position over the Ohio Valley and the lower Lakes. Record low temperatures were either tied or set on Monday morning in Michigan at Alpena (21 degrees) and Detroit (25 degrees); in Ohio at Mansfield (27 degrees); in Pennsylvania at Pittsburgh (29 degrees); in New York State at Buffalo and Rochester (both 31 degrees); in West Virginia at Elkins (22 degrees) and Charleston (30 degrees); in Virginia at Blacksburg (25 degrees) and Lynchburg (27 degrees); in New Jersey at Atlantic City (32 degrees); in Delaware at Wilmington (33 degrees) and in South Carolina at North Myrtle Beach (42 degrees).

Northwesterly winds around the eastern flank of the high spread cold air southward across the Northeast. Some widely scattered rain and snowshowers from the eastern Lakes accompanied this northwesterly flow. A few snowflakes and ice pellets were observed in the Boston, MA metropolitan area on Monday morning. The trace of snow at Blue Hill Observatory in nearby Milton tied the earliest occurrence of snow in the 116 year period of record at this historic observatory.

In the region along and to the east of the axis of the ridge where cold-air advection was the result of northerly and northwesterly winds, high temperatures along the Eastern Seaboard from New England south to the Carolinas were on the order of 10 degrees below the long-term average highs for the first week of October. To the west of the ridge axis, much of the Mississippi Valley experienced highs near average, while across the Plains where southerly winds produced warm-air advection, highs were between 10 to 15 degrees above average, particularly over western Kansas and the Panhandles of Oklahoma and Texas.

As of Monday evening an elongated ridge of high pressure extended from Upstate New York southward to the Carolinas. Much of the Eastern Seaboard was experiencing tranquil weather with relatively cloud-free skies and weak winds. The cold air mass, coupled with the clear skies and weak winds were prime ingredients for another night where radiative cooling would cause temperatures to plunge. As a result, a variety of freeze and frost advisories were posted for the Middle Atlantic States, extending from southern New England south to North Carolina. By late Monday evening the temperature at Atlantic City fell to 30 degrees, eclipsing the record low temperature for the date set earlier in the morning.

UNSETTLED WEATHER MOVES TOWARD THE MIDWEST -- The southerly winds that brought warmer air northward across the Plains also brought moisture into the Upper Mississippi Valley and the western Great Lakes. A warm front extended southeastward across Iowa and Missouri from a weak area of low pressure located over South Dakota. An area of rain and some imbedded thunderstorms was found to the north and east of the warm front, extending primarily from western Illinois northward across Wisconsin to Michigan's Upper Peninsula. This precipitation was associated with "overrunning", where warm and relatively humid air is carried up and over a relatively shallow dome of colder air, such as along and to the north of the warm front.

Farther to the south and west, some thunderstorm activity developed across the southern Plains in association with a weak trough line that extended southward from the area of low pressure in South Dakota. One-inch diameter hail was reported to have covered the ground in the Texas Panhandle on Monday afternoon.

The warm front is expected to move northeastward across the western Great Lakes on Tuesday. A cold front should follow, crossing the northern Plains. As much as an inch of rain could fall across the eastern Nebraska and western Iowa in the 24 hours ending on Tuesday evening, the result of the rain and the imbedded thunderstorms ahead of the advancing cold front.

WINDY WEATHER ACROSS FLORIDA -- A relatively tight pressure gradient located to the south of the high pressure ridge over the Carolinas produced a strong easterly flow across the Florida Peninsula. Because of these strong easterly or onshore winds, a heavy surf advisory was in effect through Tuesday for the Florida coast from Fernindina Beach to Jupiter Inlet.

In addition to the winds, thunderstorms were also found across South Florida in association with a cold front that was located to the south and east of Miami.

COOL WEATHER MOVES INTO THE PACIFIC NORTHWEST -- Relatively cool air associated with high pressure from over the North Pacific moved onshore across the Pacific Northwest. This cool air was behind a cold front that stretched across Montana, Idaho and northern Nevada. Afternoon high temperatures across the West were as much as 10 degrees below average. Scattered light rain was found across the Northwest and the northern Rockies as a result, in part, due to the onshore flow of maritime air from the Pacific interacting with the barriers provided by the various mountains such as the Olympics, Cascades and the Rockies.

As the cold front continues to move eastward on Tuesday morning, some of the precipitation should fall as snow in the colder air over the mountains. A snow advisory was in effect into Tuesday morning for the northern Wasatch Front of northern Utah and southeastern Idaho, where accumulations of 3 to 6 inches of snow are anticipated at elevations above 7000 feet.

With high pressure moving onshore along the Oregon coast during the early hours of Tuesday morning, skies should clear. As a result, frost advisories were in effect for southern Oregon and northern California.

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 16 degrees at Pellston, MI, while Monday's highest temperature was 99 degrees at Death Valley, CA.

ALASKAN WEATHER -- A large storm system over western Alaska Peninsula was responsible for clouds, rain and winds across southwestern Alaska. The storm consisted of a low pressure center near Cold Bay with a central pressure of 969 mb and an associated occluded front that extended first eastward across Bristol Bay and the Alaska Peninsula before curving southward over Kodiak Island and into the North Pacific Ocean. During the afternoon rain fell at Cold Bay, Dillingham, King Salmon, Sand Point, Kodiak and Petersburg. Winds gusted to 39 mph at Iliamna, to 38 mph at Saint Paul, and to near 30 mph across the Alaska Peninsula and the eastern Aleutians. An east-west oriented stationary front extended across Alaska from near Eagle along the Alcan border to the Bering Sea off the Yukon-Kuskokwim Delta. This front separated relatively mild air to the south from colder arctic air was found to the north. High pressure was located over the Arctic Ocean north of eastern Siberia. Snow fell at Bettles and Eielson AFB reported freezing fog.

The state's lowest temperature on Monday morning was one degree above zero at Noatak. The highest temperature across Alaska as of midafternoon on Monday was 54 degrees at Hydaburg, Kake, Ketchikan and Metlakatla.

HAWAIIAN WEATHER -- High pressure located to the northeast of Hawaii maintained relatively strong northeast trades across the islands on Monday. These trades had speeds that ranged between 15 to 30 mph. Trade showers were expected across the windward slopes of the islands particularly at night and during the morning. Because of the strong trades, gale warnings were in effect on Monday for the Pailolo and Alenuihaha Channels, while small craft advisories were continued for all other state waters. High surf advisories were posted for the south and east facing shores of the islands.

EYE ON THE TROPICS -- As it moved westward across the western Caribbean Sea, Hurricane Iris became a major hurricane on Monday, reaching a Category 4 status on the Saffir-Simpson Scale, as maximum sustained winds reached at least 145 mph, just prior to landfall. Heavy rain (5 to 8 inches), high winds and a storm surge (a mound of wind-driven ocean water) of upward of 18 feet were battering the coast of Belize. By late Monday night Iris made landfall approximately 80 miles south of Belize City and by early Tuesday morning after moving inland, Iris had weakened to a minimal hurricane as the maximum sustained surface winds had dropped to 80 mph. At that time, it was located over central Guatemala moving toward the west at 22 mph. Central pressure was 990 mb.

Tropical Storm Jerry weakened to a tropical depression as maximum sustained surface winds dropped below 39 mph. The National Hurricane Center issued its last advisory for Tropical Depression Jerry late Monday afternoon when the system was located 240 miles south of Ponce, Puerto Rico. Movement was to the west-northwest at 20 mph and maximum sustained surface winds were at 30 mph.

REPORT FROM THE FIELD -- Dr. Michael Passow, a former DataStreme LIT Leader from the New York City metropolitan area, reported on the effects of the cold front that passed across the Northeast over the weekend. He was witness the frontal passage from a kayak on Lake Placid in the Adirondack Mountains of New York State, complete with snow squalls, ice pellets (sleet) and strong winds. As much as 3 inches fell on nearby Whiteface Mountain, closing down the highway to the top. Two hikers in the Green Mountains of Vermont were caught overnight and had to be rescued the next day by ski patrols.


CONCEPT FOR THE DAY - AIR PRESSURE CHANGES

Highs and Lows, with their comings and goings, are our major weathermakers. Being in the paths of these migrating weather systems, local measurements of air pressure often can be our major forecasting guide to the weather heading our way. Changes in air pressure due to passing weather systems are not great compared to the total air pressure. They are tens of millibars at most within a range from about 970 to 1040 mb corrected to sea level. This change is only a few percent of the total pressure, a small amount compared to the change going straight up where a one millibar drop occurs for approximately every 10 meter ascent in the lower atmosphere! But the change in pressure along the horizontal, and the distances and times involved still create strong storms such as the low pressure system off the Maine coast on Monday. On the other hand, small horizontal pressure changes over large distances are associated with the expansive high-pressure system that dominated the weather over the eastern two-thirds of the nation during the last several days.

Atmospheric pressure readings, corrected to sea level, are reported by the National Weather Service each hour with changes over three hour intervals (0Z to 3Z, etc.), also listed as a "pressure tendency". The DataStreme meteogram, (short for meteorogram), provides a graphical display of the hourly weather conditions at some station over the past 24 hours. The bottom panel of the chart is a plot of the time series of sea-level pressure readings, similar to the trace provided by a barograph, such as that found in Figure 5.4 of Part A. Falling pressure values usually denote approaching storms (a low or front) while rising values accompany clearing or continued fair weather associated with an approaching high pressure cell.

These changes have long been the basis of forecasting when based purely on local observations, "when the glass (barometer) is low, expect a blow"!

For more details describing variations in pressure with time, see Tuesday's optional Supplemental Information .

QUESTIONS:

To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 5 Chapter Progress Response Form, under section B. Daily Summary.

  1. If a Low were moving away from you, the atmospheric pressure at your location would probably be [(increasing), (decreasing)] .
  2. Being near the center of an expansive High, you would expect [(little or no) (a rapid)] change in pressure over the next several hours.

HISTORICAL WEATHER EVENTS - 9 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.