James Spann forecasts an increase in showers, storms for Alabama from Alabama NewsCenter on Vimeo.

SUN AND STORMS: A moist, unstable air mass will sit over Alabama for the foreseeable future, and with an upper trough north and west of the state that means scattered to numerous showers and thunderstorms daily through the weekend. It won’t be an all-day rain or a washout, but you will deal with a few passing showers or storms, especially from about 1 until 11 p.m. The chance of any one location seeing rain each day is 65-75%, and the storms that form will be very efficient rain producers with this kind of air mass. Otherwise, look for a mix of sun and clouds through the weekend with highs in the upper 80s Friday and mid 80s Saturday and Sunday.

NEXT WEEK: The weather won’t change very much. The days will be warm and humid and the sun will be out at times. Expect a number of showers and thunderstorms, especially during the afternoons and evenings. Temperatures will remain below average, with highs mostly in the mid to upper 80s.

UHI STORM?: An isolated, lone thunderstorm developed over downtown Birmingham around 7 p.m. Thursday and dropped more than 2 inches of rain in a short amount of time. A Flash Flood Warning was issued and some streets temporarily were impassable because of high water. Considering the circumstances, there is a good chance that the urban heat island (UHI) effect was partly to blame for the development of this storm.

An urban heat island occurs when a city registers higher temperatures than the surrounding rural areas. Cities like Birmingham heat up because of all the cars, air-conditioning units, idling engines and miles of asphalt and concrete that either produce or retain heat. The added heat causes air to ascend over the city center and sink along its periphery, aiding in thunderstorm development.

TROPICS: Shower activity associated with a non-tropical area of low pressure several hundred miles south-southwest of Newfoundland is becoming less organized, and recent satellite wind data indicate that the circulation is weakening. The low is expected to degenerate to a trough later today south of Newfoundland, and its chances of tropical cyclone formation have diminished. Tropical storm formation is not expected across the Atlantic basin through early next week.

ON THIS DATE IN 1979: The most damaging tornado in Wyoming history touched down 3 miles west-northwest of the Cheyenne airport. This strong tornado moved east or east-southeast across the northern part of Cheyenne, causing $22 million in damage and one fatality. In all, 140 houses and 17 trailers were destroyed and 325 other homes were damaged. Four C-130 aircraft and National Guard equipment sustained $12 million damage.

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