National Situation Update: Monday, March 26, 2007

Homeland Security Threat Level: YELLOW (ELEVATED).

National Weather Update

Midwest: Another day of record-setting high temperatures is expected across the region on March 26, 2007. As temperatures soar into the 60s, 70s, and even a few 80s, look for some records to fall across the Mississippi Valley.

From the northern Plains to the Great Lakes, skies will be partly cloudy. A few late day showers or storms may intrude on the western Dakotas.

As moisture surges north, showers and thunderstorms are expected to develop across Missouri and will move into Illinois later in the day.

South: An upper-air low churning northeastward across Texas on March 26, 2007 will encourage downpours and thunderstorms from the Lone Star State northward through Oklahoma and northwest Arkansas. Parts of western and central Texas could be drenched by as much as two or three inches of rain.

Elsewhere in the South, a weak stationary front may trigger isolated showers and storms over eastern Tennessee, northeast Alabama, northern Georgia and the southern Appalachians.

West: Scattered showers, thunderstorms, and mountain snow will speckle a good chunk of the West March 26, 2007, as a trough aloft amplifies along the coast and low pressure develops over the Great Basin. Some of the rain could be locally heavy in southern Oregon and Northern California; precipitation is not expected in Washington state, Southern California, much of Nevada, southern Utah, most of Colorado, Arizona and New Mexico.

Snow will coat the higher terrain. A winter storm watch is in effect for the California Sierra, where up to a foot of snow is possible. More snow will fall in the mountains of Nevada and Utah as the storm progresses eastward for midweek.

Northeast: A weak frontal system will deliver scattered showers and thunderstorms to much of the Northeast March 26, 2007.

Areas from southeastern Pennsylvania and New Jersey southward through Virginia are expected to remain dry. Southwest breezes will be prevalent, and should keep high temperatures in the near- to above-average category. (NWS, Media Sources)

IN-Flooding Update

The State EOC reports that flood conditions in Hamilton, Marion and Madison Counties, IN have improved. The flood warning for the White River in Marion County has been lifted. The State EOC reports that conditions in Marion County have improved. The NWS is predicting river levels to drop significantly next week.

There are no issues to report in Madison County. This will be the last Incident Report for this event unless new issues arise. (FEMA Region V)

Tornado Climatology

Because a tornado is part of a severe thunderstorm, and thunderstorms occur all over the Earth, tornadoes are not limited to any specific geographic location. In fact, tornadoes have been documented in every one of the United States, and on every continent, with the exception of Antarctica (even there, a tornado occurrence is not impossible). In terms of absolute tornado counts, the United States leads the list, with an average of over 1,000 tornadoes recorded each year. A distant second is Canada, with around 100 per year.

Tornado Alley
In the United States, there are two regions that get proportionately more tornadoes than anywhere else. Florida is one and Tornado Alley is the other. Florida has a lot of tornadoes simply because it is home to almost daily thunderstorms. In addition, the Florida peninsula also is impacted by tropical cyclones. When these cyclones move ashore, the embedded thunderstorms often will produce tornadoes. However, despite the violent nature of a tropical cyclone, most of the tornadoes they spawn (some being water spouts) are normally relatively weak.

Strong to violent tornadoes (those of F3 or stronger on the Fujita Tornado Damage Intensity Scale), are relatively rare, and are not usually experienced outside of the central United States. Although its boundaries are debatable (depending on which criteria you use - frequency, intensity, per unit area), the area from central Texas, northward to northern Iowa, and from central Kansas and Nebraska east to western Ohio is often collectively known as Tornado Alley. Climatologically, Tornado Alley is ideally positioned for the formation of super-cell thunderstorms, and therefore is also home to many violent tornadoes.

Overall, for the U.S., most tornadoes (around 83%) are considered weak (F0), and around 98% of all U.S. tornadoes are below F3 intensity. That leaves just about 2% of all U.S. tornadoes to be categorized as violent (F3 and above). Of these violent twisters, thankfully just around 0.4% achieve F5 status, with winds over 261 mph and nearly complete destruction. However, given that on average over 1000 tornadoes hit the U.S. each year, that means that 20 can be expected to be violent and around 4 might be incredible. Fortunately, recent years have not seen as many F4 and F5 tornadoes as those probabilities would indicate.

Daily and Seasonal Peak Occurrences
Because most tornadoes are related to the strength of a thunderstorm, and thunderstorms normally gain most of their energy from solar heating and latent heat released by the condensation of water vapor, it is not surprising that most tornadoes occur in the afternoon and evening hours, with a minimum frequency around dawn (when temperatures are lowest and radiation deficits are highest). However, tornadoes have occurred at all hours of the day, and nighttime occurrences may give sleeping residents of a community little or no warning.

In addition, tornadoes occur throughout the year. Because a tornado may occur at any time of the day or year somewhere in the U.S., there really is no national tornado season (as there is with Atlantic hurricanes). Instead, each region may experience increased tornadic potential at different times of the year. The months in which tornadoes are most likely corresponds to the times of year when solar heating is at is maximum. In the Northern Hemisphere, that is August and September. Regionally, however, the frequency of tornadoes in the United States is closely tied with the progression of the warm season. Most of the early spring tornadoes in the U.S. tend to occur in the lower latitudes of the Southeast and south Central regions. Gulf States, such as Mississippi and Louisiana are the frequent recipients of tornadoes from February to April. Late spring tornadoes migrate a bit farther north, often into Kansas, Nebraska and the Tennessee Valley region. By mid-summer, most of Tornado Alley is active and tornadoes may occur throughout the U.S. Late summer tends to bring some of the stronger tornadoes into the upper Midwest and Ohio valleys, and the pattern shifts back southward into the late autumn. The fewest tornadoes are documented during the winter months. (NOAA )

Earthquake/Tsunami Activity

No significant activity to report. (USGS, Earthquake Hazards Program, Alaska Earthquake Information Center, Pacific Tsunami Warning Center, and West Coast/Alaska Tsunami Warning Centers)

Fire Management Assistance Grant (FMAG)

No new activity to report. (FEMA HQ)

Tropical Weather Outlook

Atlantic/Caribbean/Eastern Pacific: No significant activity to report.

Western Pacific: No significant activity to report. (NOAA, National Hurricane Center, Central Pacific Hurricane Center and the Joint Typhoon Warning Center)

Preliminary Damage Assessments

PDAs for the Tornadoes in New Mexico will begin starting with a planning meeting on March 26, 2007.

PDAs will be conducted in Clovis, Logan, and Portales counties March 27 - 28, 2007. (FEMA Region VI, FEMA MOC/Denton)