Tropical Storm Arlene--a heavy rainfall threat for Mexico

The 2011 Atlantic hurricane season has its first named storm, Tropical Storm Arlene, which rapidly spun up last night from a tropical wave that had emerged into Mexico's Bay of Campeche. An Air Force hurricane hunter aircraft is currently in the storm, and has found that Arlene is strengthening. At the plane's flight altitude of 1,000 feet, top winds reported as of 9:30am EDT were 54 mph, on Arlene's south side. The aircraft's SFMR instrument that remotely measures surface wind speeds found top surface winds of 67 mph on southeast side of the storm. This reading was probably a gust, since top winds of 45 - 50 mph have been more characteristic of the SFMR winds. The measured central pressure at 9am EDT was 1000 mb, which is a 2mb drop from the pressure estimate from 5am. Satellite loops show a marked increase in the intensity and organization of Arlene's heavy thunderstorms, with more prominent spiral bands. Mexican radar out of Alvarado also shows this trend. Wind shear has fallen to the moderate range, 10 - 15 knots, and is predicted to fall to the low range, 5 - 10 knots, by tonight. This should allow Arlene to continue to organize, and storm could be approaching hurricane strength by the time it makes landfall Thursday morning in Northeast Mexico. Arlene has moistened its environment enough so that the dry air over Mexico should no longer be a problem for it, and the topography surrounding the Bay of Campeche tends to boost counter-clockwise air flow, enabling systems there to spin up faster than in any other portion of the Atlantic. Arlene is bigger than most Bay of Campeche tropical storms, though, so it may not have time to spin up into a hurricane because of its large size. NHC's 5am EDT advisory was giving Arlene a 9% chance of reaching hurricane strength before landfall. These odds should probably be bumped up to 30%, in light of Arlene's recent intensification.


Figure 1. Microwave satellite image of Tropical Storm Arlene taken at 4:07am EDT June 29, 2011 showed that several spiral bands had formed. Image credit: N avy Research Lab, Monterey.

Rainfall forecast for Arlene
The major threat from Arlene is heavy rainfall, particularly since the portion of coast that will be affected is under extreme drought. Without much vegetation to absorb Arlene's rains and slow down run-off, the expected heavy rains are more likely to cause damaging flooding. NHC is currently estimating that 4 - 8 inches will fall, with isolated areas of up to 15 inches over the mountains. These predicted amounts will probably need to be revised upwards, given Arlene's recent increase in organization. At this time, it appears that Texas will not see any rain from Arlene.


Figure 2. Cumulative rainfall expected along the path of Arlene, as predicted by this morning's 2am EDT (06 UTC) run of the HWRF model. This model predicts a large region of 8+ inches of rain (yellow colors) will affect Mexico. These rainfall amounts are probably too high, as our other models are not showing such heavy rainfall amounts. Image credit: Morris Bender, NOAA/GFDL.

June hurricane climatology
Long-term hurricane records going back to 1851 show that on average, we see just one Atlantic named storm every two years in June. Only one major hurricane has made landfall in June--Category 4 Hurricane Audrey of 1957, which struck the Texas/Louisiana border area on June 27 of that year, killing 550. The highest number of named storms for the month is three, which occurred in 1936 and 1968. In the sixteen years since the current active hurricane period began in 1995, there have been twelve June named storms (if we include 2008's Tropical Storm Arthur, which really formed on May 31). Thus, recent history suggests the Atlantic hurricane season is becoming more active earlier in the season, in line with recent research that found that the Atlantic hurricane season is getting longer.

Arlene: the most common Atlantic storm name of all-time
This year marks the tenth appearance of a storm named Arlene in the Atlantic, making it the most recycled storm name of all-time. The other nine appearances: 1959, 1963, 1967, 1971, 1981, 1987, 1993, 1999, and 2005. It's pretty likely we'll see Arlene again in 2017--no storm that has formed in the Bay of Campeche and made landfall in Mexico has ever had its name retired. Hurricane Audrey of June 1957 formed in the Bay of Campeche and hit Texas as a Category 4 hurricane, killing 550 people, making Audrey one of only two June storms to get its name retired Hurricane Agnes of 1972 was the other. There have been seven storms beginning with the letter "A" that have had their names retired since 1950:

Allison (2001)
Andrew (1992)
Alicia (1983)
Allen (1980)
Anita (1977)
Agnes (1972)
Audrey (1957)

Of the 21 letters in the alphabet we use to name storms, names beginning with the letters "C" have been retired the most--nine times. Second place goes to "F" and "I" names, with eight retirees. The only letter used that doesn't have a retired storm is "V". There has been only one storm with a name beginning with "V"--Hurricane Vince of 2005. The list of retired hurricane names currently has 76 members.

An amazing low temperature of 107°F in Oman
At Khasab Airport in the desert nation of Oman, a remarkable record was set yesterday--the low temperature for the day was a scorching 41.7°C (107°F). The record was brought to my attention by weather records researcher Maximiliano Herrera. The previous highest minimum temperature for the world he was able to find was set just last year at Khasab Airport, 41.2°C (106°F). The U.S. record high minimum temperature may be a 39.4°C (103°F) taken in Death Valley, California in 1970. Higher record high minimums were set there in the early 1920s, but the quality of the data is suspect. Mr. Herrera notes that Khasab Airport in Oman lies at the base of a mountain range, behind which is desert. Winds blowing from the desert towards Khasab Airport flow downhill, undergoing compression and warming, like the Santa Ana winds in California. Incredibly hot conditions in Oman in late June are common, due to a seasonal shift in winds caused by the onset of the Southwest monsoon in India.

Record California rainstorm
Yesterday was the wettest day ever recorded in the San Francisco Bay Area between June 15 - September 15. All-time precipitation records for the month of June (total for the month) were set at SFO (San Francisco Airport), Oakland Downtown, Oakland Airport, Santa Rosa, Napa, Santa Cruz, and San Jose. Thanks go to Christopher C. Burt for this data.

Internet radio show on Arlene at 4pm EDT
I'll be discussing Tropical Storm Arlene on a special edition of our Internet radio show, the Daily Downpour, today (Wednesday) at 4:00pm EDT. Fellow wunderground meteorologists Shaun Tanner and Angela Fritz will be hosting the show. Listeners can email in or call in questions. The email address to ask questions is broadcast@wunderground.com.

Jeff Masters