NAIONAL WEATHER SUMMARY

 

AUGUST 2012

 

1^st-4^th…An active weather pattern developed across the nation on Wednesday as monsoonal activity continued in the West, frontal disturbances and low pressure trekked across the North, and a trough of low pressure lingered over the eastern third of the nation. In the West, monsoonal moisture streamed inland from the southwest across southeastern California and the Four Corners through the day. Ample moisture over these areas combined with waves of energy pushing through the region translated into areas of rain and thunderstorms during the afternoon. Flash Flood Watches remained in effect for areas from southeastern California and northwestern Arizona through southern Utah, while Flood Advisories were in effect for north-central Arizona and parts of western Colorado. To the east of this activity, high pressure ridging continued over the south-central U.S. with scorching hot temperatures. Areas from the Southern and Central Plains through the western Tennessee Valley and the mid-South experienced daytime highs ranging from 100 to 110 degrees. A few showers and thunderstorms formed in the Mid-Mississippi Valley as disturbances from the West were guided through the Central Plains, but provided little relief from the heat. A variety of Heat Advisories and Excessive Heat Warnings remained in effect for the region. To the north, frontal disturbances with low pressure supported showers and clusters of thunderstorms in the Northern High Plains and the Northern Plains. Areas form parts of the Central and Northern Plains into the Upper Mississippi Valley remained at slight risk of severe thunderstorms with severe wind and hail. In the East, a trough of low pressure lingered over the eastern third of the nation and supported more showers and thunderstorms. Much of this weather activity was focused in the Southeast and Mid-Atlantic.

The nation saw another active weather day as multiple weather features developed on Friday. A trough of low pressure dipped into the Northern High Plains from central Canada. This system created a series of frontal boundaries that produced moderate to heavy showers and thunderstorms from central Montana through the Dakotas. The system advanced eastward throughout the day and reached into the Upper Midwest. To the south, scattered showers and thunderstorms developed from the Central and Southern Plains through the Tennessee and Ohio Valleys, as a few waves of low pressure moved through the region. These storms also had a slight chance of severe weather development across Mississippi and Arkansas. Main threats in these areas were strong winds and large hail, with possibly a tornado or two. Further south, a tropical wave of low pressure over the Bahamas pushed abundant moisture into Florida as it slowly approached from the east. This created heavy rains and strong winds for most of Florida. Mid-day total rainfall reached up to 1.73 inches in Opa Locka, Florida. Meanwhile, out West, monsoonal moisture over the Southwest maintained shower and thunderstorm activity across across Arizona, New Mexico, and into the Central Rockies and Great Basin.

 

5^th-11^th…A long cold front moved through the eastern third of the country on Monday, producing the most active weather in the country. The front began the day stretched from Arkansas through western New York before moving slowly eastward. There was not a tremendous amount of moisture associated with this front, but scattered showers and thunderstorms were triggered throughout the Southeast and Mid Atlantic. These thunderstorms developed particularly in the afternoon. Early morning rain and thunderstorms also moved through New Jersey, but these thunderstorms had pushed out into the Atlantic Ocean later in the morning. Meanwhile, drier conditions moved into the Upper Midwest and Ohio Valley behind the aforementioned cold front. With the general lack of rain in the country, perhaps the biggest story was the continuing heat in the Plains. While temperatures were not as hot as last week, Oklahoma experienced widespread temperatures in the upper 90s and 100s once again. Meanwhile, a moderate amount of monsoon moisture continued to stream into the Southwest, allowing a few thunderstorms to pop up in the Four Corners region. Dry weather greeted the Intermountain West and Northern Rockies. The Northeast rose into the 70s and 80s, while the Southeast saw temperatures in the 80s and 90s. The Plains were hot once again with temperatures soaring into the 90s and 100s, while the Southwest saw similar temperatures.

Showers and thunderstorms developed across the eastern U.S. as a low pressure system lingered over the Great Lakes. This system created a warm front that extended northeastward into Maine, while a cold front stretched south of this system and moved through the Eastern Valleys. These systems pulled abundant moisture in from the Gulf of Mexico and Atlantic Ocean, which allowed for showers and thunderstorms to develop over most of the eastern U.S. Heavy rains developed in the North, from Michigan to Maine with heaviest rainfall reported in Belmar-Farmingdale, New Jersey. Some of these storms turned severe with strong winds and hail. Multiple trees and power lines were blown down from New York through Massachusetts. To the south, warm and humid conditions ahead of the cold front produced severe thunderstorms across the Gulf states and Tennessee Valley. Rainfall totals ranged around an inch with strong and damaging winds. Behind this system, cooler conditions persisted across the Upper Midwest as flow from the north continued. Highs ranged in the 70s with overnight lows in the 50s. Meanwhile out West, a dominant ridge of high pressure maintained hot conditions for the Southwest. This allowed for offshore flow to continue and kept the region hot and sunny. Excessive heat warnings remained in effect as highs surpassed 110 degrees again across the deserts of California, Arizona, and Nevada. Just to the east, a weak trough of low pressure covered the Great Basin, Intermountain West, as well as the Northern and Central Rockies. This kicked up widespread scattered showers and thunderstorms throughout the day. Due to prevailing dry conditions, this increased fire danger for most of the Western US.

 

19^th-25^th…The most active weather in the country occurred along a front that stretched from the Gulf Coast through North Carolina. This front allowed moisture to stream along it, producing areas of heavy rain and thunderstorms through the panhandle of Florida. This represented the only significant rain in the country as several inches of rain fell. Some rain did fall along the eastern seaboard from North Carolina through Delaware, but this precipitation was not widespread in nature. Additional scattered showers and thunderstorms did develop in New England in the afternoon. Meanwhile, monsoon moisture streamed into the Southwest and produced a few thunderstorms in the Four Corners region. More thunderstorms were expected into the afternoon, as is typical this time of year. Coastal clouds along the West Coast kept the immediate coast cool, while inland areas of the Southwest once again warmed up to the 90s and some 100s. Cooler weather is on the way as a low pressure system from the Gulf of Alaska approaches the West. The Northeast rose into the 70s, while the Southeast saw temperatures in the 70s and 80s for the most part. The Southern Plains rose into the 80s and 90s, while the Northwest saw temperatures in the 60s, 70s, and some 80s with the coolest temperatures at the coast.

Active weather returned to the Plains on Wednesday as a trough of low pressure moved off the Northern Rockies and into the Northern Plains. This system pushed a frontal boundary eastward and kicked up scattered showers and thunderstorms across the Dakotas and Minnesota throughout the day. Some of these storms have turned severe with large hail and gusty winds. One inch in diameter hail was reported in Stewart, Minnesota, while 1.25 inch in diameter hail was reported in Dassel, Minnesota. Strong winds ahead of this front increased fire danger across the Central Plains, due to prevailing hot and dry surface conditions. In the South, a frontal boundary lingered over northern Florida and the Southeast and triggered more heavy rainfall with strong thunderstorm development. These storms have not yet turned severe but a mid-day total of 2.69 inches of rain was reported at Perry, Florida. Behind this system, high pressure built over the Eastern US from the Great Lakes through the Tennessee Valley. This brought sunny and dry conditions to most of the Eastern states. In the West, monsoonal moisture lingered over the Southwestern US and allowed for thunderstorm activity to continue across the Southwest, Great Basin, and Southern Rockies. Periods of heavy rainfall and flash flooding were expected in parts of Arizona and southern California.

Multiple areas of active weather developed across the nation on Friday. A low pressure system moved through the Northern Plains and pushed a warm front through the Dakotas and Upper Midwest, while a cold front developed behind the system across the High Plains. To the south, another area of low pressure moved off the Central Rockies and into the Central Plains. This pushed a warm front eastward into the Mid-Mississippi River Valley, which created periods of heavy rain and strong thunderstorms. Thus, showers and thunderstorms developed across most of the Plains on Friday as these weather features moved off the Rockies. In the South, areas of wet weather developed from Texas through the Southeast. Onshore flow from the Gulf of Mexico created heavy rains and scattered thunderstorms from eastern Texas through the Lower Mississippi River Valley. Heaviest rainfall was reported in Lake Charles, Louisiana with a mid-day total of 2.59 inches. In the Southeast, a frontal boundary slowly moved offshore and into the Atlantic Ocean. However, this triggered a few more showers and thunderstorms across parts of Florida as well as eastern North Carolina and South Carolina. In the West, monsoonal moisture continued to shift eastward throughout the day. This allowed for thunderstorm activity to diminish across Arizona and the Great Basin, but continued for most of the Central and Southern Rockies.

Isaac continued to lash Louisiana, Mississippi, and Alabama with extremely heavy rain, strong wind, and life-threatening storm surge and flooding as it weakened from a category 1 hurricane to a tropical storm in the afternoon. As of Wednesday afternoon, Isaac was located about 50 miles southwest of New Orleans, Louisiana and continued on a northwestward track with maximum sustained winds near 70 mph. Storm surge values of about 8 feet were reported at Shell Beach, Louisiana and Waveland, Mississippi, while dangerous surf and rip current conditions continued along the West Coast of Florida and the northern Gulf Coast. In regards to rainfall, rain totals from Monday through this afternoon have been quite significant. For this time period, New Orleans Lakefront Airport reported a rainfall total of 9.26 inches, Boothville, Louisiana reported 6.58 inches, and Gulfport, Mississippi reported 5.05 inches. Isolated tornadoes also remained a concern along the central Gulf Coast region and parts of the Lower Mississippi River Valley. Elsewhere in the East, showers and thunderstorms developed in the Carolinas as the tail of a cold front became nearly stationary across the region. In the West, light, isolated rain showers developed near the coast and northern areas of the Pacific Northwest as a trough of low pressure in the jet stream pushed inland through the region. Meanwhile, a dry cold front pushed through the Northwest and heightened fire weather conditions in the region as gusty winds up to 40 mph and relative humidity levels as low as 5 percent persisted across much of the region. Critical fire weather conditions continued in areas of northeastern Idaho through eastern Montana and into areas of the western Dakotas, as well as areas from eastern Colorado through Nebraska during the afternoon and into the evening.

Isaac reduced to a tropical depression on Friday with maximum sustained winds less than 39 mph. The system lost energy and strength as it slowly moved northward and further inland over Arkansas and Missouri. Flow around this system continued to pull abundant moisture in from the Gulf of Mexico and produced extremely heavy rains for the Mid- and Lower Mississippi River Valley. Mid-day rainfall totals for Friday ranged from 2 to 4 parts of Arkansas and Missouri, with the heaviest rainfall was reported at 5.17 inches in Alexandria Esle, Louisiana. Strong winds persisted in the vicinity of the storm with gusts up to 54 mph at Tin City, Arkansas. Since Monday, rainfall totals have reached up to 20.08 inches in New Orleans, Louisiana, 17.04 inches in Kiln, Mississippi, and 16.60 inches in Vero Beach, Florida. Thus, the mid and lower Mississippi River Valley remained under severe flooding conditions. To the North, a frontal boundary moved into the far Northeast and obtained moisture from the Atlantic Ocean. This produced showers and thunderstorms, some of which have turned severe with strong winds and hail in the afternoon and evening hours. Out West, monsoonal moisture brought more thunderstorm activity to the Southwest, Great Basin, and parts of the Rockies. To the south, Hurricane Ileana maintained category 1 strength with maximum sustained winds at 75 mph. The system was about 355 miles west of the southern tip of Baja California and moisture from this system made its way up the coast, and brought clouds into northwestern Mexico and far southern California.

Jim G. Munley, jr.
http://www.jimmunleywx.com