A searing heat wave rare even for the Desert Southwest sent temperatures soaring to record levels on Monday, with Needles, California tying its record high for the date of 118°F (47.8°C). The temperature might have gone higher in Needles, but a thunderstorm rolled in at 3:20 pm, and by 3:56 pm PDT, rain began falling at a temperature of 115°F (46.1°C). Most of the rain evaporated, since the humidity was only 11%, and only a trace of precipitation was recorded in the rain gauge. Nevertheless, Monday’s rain at 115° in Needles sets a new world record for the hottest rain in world history. I don’t think many people were outside to experience to experience the feeling of rain falling at 115°, but if they were, it must have been an uncomfortable, sauna-like experience! Thanks go to Dr. Warren Blier of the NWS Monterey office for pointing out this remarkable event to me.

It is exceedingly rare to get rain when the temperature rises above 100°F, since those kind of temperatures usually require a high pressure system with sinking air that discourages rainfall. Monday’s rain in Needles was due to a flow of moisture coming from the south caused by the Southwest U.S. monsoon, a seasonal influx of moisture caused by the difference in temperature between the hot desert and the cooler ocean areas surrounding Mexico to the south. According to weather records researcher Maximiliano Herrera, the previous record for hottest rain, which I blogged about in June, was a rain shower at 109°F (43°C) observed in Mecca, Saudi Arabia on June 5, 2012 and in Marrakech, Morocco on July 10, 2010. The 11% humidity that accompanied Monday’s rain shower at 115° in Needles was the lowest humidity rain has ever occurred at anywhere on Earth in recorded history, according to Mr. Herrera.

Figure 1. True-color MODIS satellite image of California and Arizona taken at 1:25 pm PDT August 13, 2012. Developing thunderstorms surround Needles, CA, and the line of clouds to the southwest of the city would develop into a thunderstorm that brought rain to the city at 4 pm PDT, at a temperature of 115°F. Image credit: NASA.

A “very rare” heat wave for Phoenix
The heat wave that brought Needles’ record hot rain has broken an exceptional number of heat records in Phoenix, Arizona the past two weeks. According to the Phoenix NWS office, the “almost unbearable heat” of the first two weeks of August is a “very rare” event, and August 1 – 14, 2012 was the warmest such 2-week period in city history. The average temperature on August 6 – 13 was 100°F or higher each of the eight days, tying the record for most consecutive days with an average temperature of 100°. The temperature peaked at 116° on August 8, just 6° below Phoenix’s all-time record of 122° set on June 26, 1990. The forecast for Phoenix call for a bit of relief–highs are expected to be a relatively modest 105° today, and down near 100° by Friday.

Figure 2. Morning satellite image of 93L over the Central Atlantic.

93L close to tropical depression status
A large tropical wave (Invest 93L) is located in the Central Atlantic about 700 miles east of Bermuda. Satellite loops this morning show a surface circulation has formed, and heavy thunderstorm activity has increased to the point where 93L should be considered a tropical depression, if the heavy thunderstorms can persist through this afternoon. Wind shear is light, and ocean temperatures are warm, near 28°C. The latest Saharan Air Layer Analysis from the University of Wisconsin shows that 93L has moistened its environment considerably, and dry air should no longer be a significant impediment to development. The 8 am EDT run of the SHIPS model predicts that wind shear will remain in the low range through the weekend, and I expect this system will become Tropical Storm Gordon by Friday. The storm will not affect Bermuda, but residents of the Azores Islands should keep an eye on 93L, which could pass through the islands as early as Sunday night. In their 8 am Tropical Weather Outlook, NHC gave 93L an 80% chance of becoming a tropical depression by Friday morning.

Elsewhere in the tropics
In the Gulf of Mexico, a fall-like cold front is expected to stall out early next week, and the GFS model is predicting something could start to spin up near the Texas/Mexico border on Monday. Wind shear is predicted to be low to moderate, and cold fronts stalled out over the Gulf of Mexico often serve as the seed for tropical storms.

Most of the models predict development of a new tropical wave off the coast of Africa 6 – 7 days from now.

From:    http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2186