Tuesday, 2 September 2014

00Z Weather Systems

WELCOME TO THE WEEK ONE OF AMS WEATHER STUDIES - This Daily Weather Summary file will describe the current weather pattern across the U.S. Additional Supplemental Information…In Greater Depth files will provide optional background material.

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

ACTIVE WEATHER ACROSS THE NATION'S MIDSECTION -- The Midwest and eastern sections of the central Plains continued to experience locally heavy rains along with strong to severe thunderstorms on the Labor Day holiday. Several large clusters of thunderstorms continued into the late evening across the eastern Plains and the Midwest, with the largest clusters stretching from southeastern Kansas across Missouri to southern Illinois.

These thunderstorms were in the warm, humid and slightly unstable air out ahead of a cold front that stretched from the northern Great Lakes southwestward across the Midwest to the southern Plains of the Texas Panhandle. The cold front was trailing from a low pressure system that was moving across to the northeast Ontario toward James Bay in central Canada. The counterclockwise circulation of winds around the low pressure system was producing a generally southerly wind flow (or winds from the south according to the meteorological tradition for naming winds for the direction of origin) across the Mississippi Valley out ahead of the front. The clockwise circulation of winds around high pressure centered across the Southeast reinforced the southerly flow of warm and humid air from the Gulf of Mexico. Northwesterly to westerly winds found behind the cold front were bringing cooler and drier air eastward.

The slow moving front has had a history of severe weather, extending back into this past weekend. By early Monday afternoon, severe thunderstorms had formed out ahead of the northern sections of the cold front. A squall line that represents a linear arrangement of strong to severe thunderstorms formed over western Lower Michigan. Some of these thunderstorms generated strong straight-line thunderstorm winds across Lower Michigan that downed trees and power lines. Afternoon thunderstorms moving across northern sections of Lower Michigan spawned two EF-1 tornadoes (on the Enhanced Fujita Scale). These tornadoes uprooted trees and caused some damage to several residences.

Later in the afternoon the focus of the severe thunderstorm activity shifted farther to the south across the mid-Mississippi Valley, the Ozark Plateau and the eastern Plains. As many as six unconfirmed tornadoes were reported to have made brief touchdowns in southeastern Kansas during the early evening. Damaging straight-line thunderstorms were also reported across the region. Two people were injured during the late evening when several camper trailers were blown over in Missouri. Thunderstorm winds gusted to 80 mph in Kansas and Oklahoma. A few thunderstorm cells also produced hail that had diameters ranging from one to two and a half inches.

In addition to the severe weather, the showers and thunderstorms produced locally heavy rainfall totals especially across Kansas, Missouri, Illinois and Indiana. According to 24-hr radar estimates, between one and six inches of rain fell across a swath that ran from near Wichita, KS eastward to near Indianapolis, IN. Numerous rainshowers and thunderstorms formed and remained across this region for extended time intervals. Columbia, MO received 4.50 inches of rain on Monday, which represents a new daily maximum precipitation record for the date. Farther to the north, earlier rains in the vicinity of the front produced radar estimated rainfall accumulations that ranged from one half of an inch to nearly two inches across sections of northwestern Wisconsin and Michigan's Lower Peninsula.

Northern sections of the cold front were forecast to travel eastward across the Great Lakes and the upper Ohio Valley on Tuesday while southern sections of the front were expected to travel southward across the mid-Mississippi Valley. Thunderstorms were expected to form on Tuesday across many areas in the eastern half of the nation. Much of the thunderstorm activity would be out ahead of the advancing cold front. Severe thunderstorms could develop across sections of the eastern Great Lakes southwestward across the Ohio Valley to the Mid-South. The region extending from the Mohawk Valley of Upstate New York southwestward across the Appalachians and upper Ohio Valley to eastern Kentucky was considered to be under a risk of severe thunderstorms. Large communities within the slight risk area include Syracuse, NY; Pittsburgh, PA; Columbus, OH; Cincinnati, OH and Lexington, KY. These thunderstorms could produce damaging straight-line winds. Elsewhere, large hail approaching quarter size could move across sections of Kansas, northern Oklahoma and western Missouri during the evening.

The showers and thunderstorms were forecast to produce one to two inches of rain across sections of the Ozark Plateau and the mid-Mississippi Valley during the 24 hours ending Tuesday evening. This anticipated rain plus the recent heavy rain has necessitated the continuation of flash flood watches across sections of southeastern Kansas, southern Missouri and southern Illinois through Tuesday morning. Farther east, the flash flood watches would continue into Tuesday afternoon across southwestern Ohio.

HOT WEATHER ACROSS THE SOUTH -- Unseasonably warm weather was found across the Southern States running from the southern Plains of west Texas to the Atlantic Coast of Florida and the Carolinas. Afternoon high temperatures across the southern Plains were at least 10 Fahrenheit degrees above the average highs for the start of September. High temperature records for the date were either tied or broken on Monday in Texas at Lubbock (104 degrees) and Midland (104 degrees); in Florida at Fort Myers (96 degrees) and Sarasota-Bradenton (96 degrees) and in South Carolina at North Charleston (95 degrees).

A large high pressure system that maintained relatively clear skies and southerly winds was responsible for the continued warmth across a large section of the South.

With high pressure forecast to remain in place across the Southeast on Tuesday, unseasonably warm weather should continue across the region.

COOL AND DRY ACROSS THE WEST -- A large area of high pressure centered over the interior Northwest, the northern Rockies and the Great Basin on Monday evening was responsible for relatively quiet weather conditions across most of the country west of the Continental Divide. Except for a few clouds that formed as winds were flowing over Washington's Cascades and Montana's northern Rockies, skies were relatively cloud free across the West.

The air mass accompanying the high pressure system was not only dry but also cool. The winds circulating in a clockwise direction around the eastern flank of the high were bringing cooler air southward from Canada. Afternoon high temperatures across Montana, Wyoming and the western Dakotas were 5 to 10 Fahrenheit degrees below long-term average highs for the date.

Generally quiet weather conditions were expected to prevail across the West on Tuesday as the high pressure ridge was forecast to drift slowly to the southeast along the western slopes of the Rockies. By evening the center of the high should be located over western Colorado.

By Tuesday afternoon a low pressure system and accompanying cold front was forecast to reach the coast of the Pacific Northwest. By late evening this cold front should have traveled across the Cascades and the interior Northwest, reaching the northern Rockies of Idaho and Montana by early Wednesday. Some light precipitation was expected to spread across Washington, Idaho and Montana in the cold air behind the front.

The approach of the low pressure system and accompanying cold front coupled with the slow southeastward drift of the high pressure system across the Intermountain west would result in a tightening of the pressure gradient (or difference in pressure over a given distance) across sections of the northern Rockies. Tightening of the pressure gradient would result in a corresponding strengthening of the west winds across the interior Northwest. A high wind warning was posted across the northern Rocky Mountain Front near Glacier National Park that would run through Tuesday afternoon. Westerly winds with sustained speeds ranging between mph could gust to 60 or 70 mph.

The dry weather across the West should exacerbate the wildfire weather situation. Dry and warm winds from the west-southwest were expected Tuesday across southern and central sections of Wyoming. The anticipated combination of very low atmospheric humidity levels and strong gusty winds near the surface during the afternoon has necessitated the posting of red flag readings across a large area of the Cowboy State. These red flag warnings, which would run on from midday through midevening on Tuesday, are meant to warn the public of the critical wildfire weather conditions involving low humidity and strong winds that could enhance the spread of wildfires. Red flag conditions could develop again on Wednesday before a cold front would move to the southeastward across the state turning winds to a northwesterly or northerly direction, resulting in slightly lower temperatures.

TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature on Monday was 25 degrees (Fahrenheit) at Daniel, WY, while Monday's highest temperature was 116 degrees at Death Valley, CA.

ALASKAN WEATHER -- Many of the surface weather observation stations across mainland Alaska reported mostly cloudy to overcast skies on Monday afternoon. Fog was also reported at some locations. Satellite imagery animations revealed an extensive cloud shield moving generally eastward across most of the 49th State. However, the western edge of the cloud shield was moving across coastal sections of western Alaska by early evening. This cloud shield was accompanying a cold front that was traveling toward the east across mainland Alaska. The front was trailing southwestward from a low pressure center located over the Arctic Ocean to the north of Point Barrow. As of midafternoon this cold front stretched from near Deadhorse southwestward to the Seward Peninsula and St. Lawrence Island in the northern Bering Sea. This front, which marked the leading edge of colder air, was also accompanied by rain and snow. Farther to the south, a stationary front stretched across coastal sections of southern Alaska, from Cook Inlet across the Kenai Peninsula and the northern Gulf of Alaska to the southeast Panhandle before extending into northwestern Canada. Clouds and rain were also along this frontal boundary. Although a weak ridge of high pressure extended along the Alaska Range, remnant clouds remained across this area.

Fairbank received 1.43 inches of rain on Monday, which easily broke the daily maximum precipitation record for the date.

Snow was expected along the Brooks Range and also along sections of the Alaska Range around Denali National Park overnight. Therefore winter storm warnings and winter weather advisories were posted across sections of northern Alaska through early Tuesday morning. Three inches of snow could fall along the Brooks Range, while three to eight inches were likely near Denali.

The state's lowest temperature on Monday morning was 25 degrees at Gulkana. The highest temperature across Alaska as of midafternoon on Monday was 67 degrees at Seward.

HAWAIIAN WEATHER -- Regional winds across the Aloha State were relatively light and somewhat variable in direction on Monday afternoon. The winds were light and variable because the high pressure system over the North Pacific that would normally drive the typical trade wind flow across the islands was somewhat weaker and displaced farther to the northeast. A few weak low pressure centers were also situated between the high and the islands. With weak regional winds, local afternoon sea breeze regimes developed in response to daytime heating of the interiors of the islands. These onshore winds were helping enhance the development of afternoon clouds and showers across the interiors of the islands and along the normally dry leeward (or west facing) slopes of the mountains on the various islands. Locally heavy rainfall was reported during the afternoon on the windward (or east-facing) side of Oahu, with some rain gauges reporting nearly one half of an inch of rain falling in 15 minutes. A flood advisory had been posted for a time across Oahu. The air mass across the islands was humid and marginally unstable. By evening, the clouds were beginning to dissipate as the islands were beginning to cool. Light offshore directed land breezes were beginning to develop. Satellite imagery showed a few scattered clusters of puffy clouds that were drifting westward in the weak tropical easterly winds. However, clouds were lingering over the Big Island, Oahu and several other islands. Radar detected isolated to scattered light rainshowers traveling westward across area waters. Some of these showers were banked along the east-facing coast of Oahu.

The high temperature at Hilo reached 88 degrees on Monday afternoon, which tied the daily maximum temperature record for the date.

The regional winds were expected to remain across the Hawaiian Islands into Tuesday. The winds were expected to strengthen beginning on Tuesday, returning to a more typical trade wind flow as high pressure builds to the north of the islands. Clouds and showers embedded in these trade winds would dominate the weather across the windward sides of the islands, leaving relatively dry weather on the leeward slopes.

PUERTO RICO/US VIRGIN ISLANDS WEATHER -- Skies cleared during the evening across Puerto Rico, Vieques, Culebra and the Virgin Islands late Monday night, leaving clear to partly cloudy sky conditions. Animated satellite images showed clusters of low level clouds traveling toward the west across the islands and surrounding waters. Radar detected scattered light to moderate rainshowers moving toward the west across the Virgin Islands and the offshore waters of the Atlantic and the Caribbean surrounding Puerto Rico. Earlier, one to one and a half inches of rain fell across Puerto Rico. These clouds and showers were being carried to the west on generally easterly near-surface winds that had speeds ranging from 10 to 20 mph. A tropical wave, or a wavelike disturbance in the tropical easterly wind flow, had passed across Puerto Rico early Monday. Clouds and showers accompanying this tropical wave were situated to the east of the wave axis that had reached western sections of the Dominican Republic by evening. A large high pressure system centered over the western North Atlantic well to the north-northeast of Puerto Rico was responsible for the easterly winds across the region on Monday night.

The drier air should move across the islands on Tuesday morning. However, the next weak tropical wave that was moving westward across the Lesser Antilles was expected to reach the Virgin Islands and then Puerto Rico on Tuesday. Showers and thunderstorms accompanying this next tropical wave should pass across these islands by Tuesday afternoon.

EYE ON THE TROPICS -- In the North Atlantic basin (that includes the Caribbean Sea and the Gulf of Mexico), a tropical depression formed late Monday afternoon over the Bay of Campeche from a tropical wave that had earlier moved across Mexico's Yucatan Peninsula. By late Monday evening, this fifth tropical system of 2014 hurricane season was located approximately 225 miles to the east of Tuxpan, Mexico. Movement of Tropical Depression 5 was to the northwest at 10 mph. Maximum sustained surface winds around this tropical depression were determined to be 30 mph. Forecasts at that time indicated that Tropical Depression 5 could slowly strengthen to possibly become Tropical Storm Dolly as it would curve toward the west-northwest on Tuesday. This projected track would take the center of this tropical cyclone toward the coast of Mexico by Tuesday night before making landfall by early Wednesday. A tropical storm warning was in effect along sections of the central Mexican Gulf coast, signifying that tropical storm conditions would be expected within this warned area within the next 24 to 36 hours. In addition to possible tropical storm-force winds (39 to 74 mph), rainfall associated with this system could range between three to six inches along the coast through Wednesday evening. Flash floods and mudslides were possible in the region's mountainous terrain.

In the eastern North Pacific basin (extending from the western coast of North and Central America west to 140 degrees West longitude), the National Hurricane Center was continuing to monitor an area of disturbed tropical weather associated with a broad trough of low pressure located off the southwestern coast of Mexico. This trough with its large disorganized array of rainshowers and thunderstorms was expected to travel toward the north and northwest over the next several days. Forecasters give this low pressure trough a medium chance of becoming a tropical depression by Wednesday evening. Regardless of intensification, this trough could produce locally heavy rainfall along coastal sections of southwestern Mexico, resulting in life-threatening flash floods and mudslides.

WHAT TIME IS IT? -- You will find that all AMS Weather Studies meteorological maps and charts are labeled with numbers followed with a "Z", such as 00Z, 12Z, 1915Z, etc. These labels indicate the time of observation in Universal Coordinated Time (UTC). The UTC or "Z" time is used because weather observations must be taken at the same time everywhere to accurately represent the state of the atmosphere. But how can you tell from the reported Z time when the observations were made where you live? For additional Z-time explanation, call up Tuesday's Supplemental Information…In Greater Depth.


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

Return to AMS Weather Studies Sample RealTime Weather Portal

Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2014, The American Meteorological Society.