Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 14:52 GMT le 30 septembre 2011
The devastating Texas drought that has already cost over $5 billion could continue for nine more years, predicted Texas State Climatologist John Nielson-Gammon in an interview with Reuters yesterday. "It is possible that we could be looking at another of these multi-year droughts like we saw in the 1950s, and like the tree rings have shown that the state has experienced over the last several centuries," Nielson-Gammon said. Drought statistics released yesterday by the U.S. Drought Monitor showed that over 96% of Texas is experiencing the two worst categories of drought, extreme and exceptional. The past 12 months have been the driest one-year period on record in Texas. The main blame for this year's drought can be put on La Niña, the cooling of equatorial Pacific waters that deflects the jet stream and takes rain-bearing low pressure system away from Texas. Other large-scale atmospheric/oceanic patterns called the Pacfic Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) have also favored dry conditions for Texas this year. When the AMO brings warm ocean temperatures to the North Atlantic, as it has since 1995, Texas is typically dry. Texas also tends to be dry when the PDO brings cool ocean temperatures to the coastal North Pacific next to North America. This has been the case since 2007 (except for late 2009 and early 2010.) In a post earlier this month in his excellent blog, Climate Abyss, Nielson-Gammon has this to say about the influence of global warming on the 2011 drought:
Precipitation: The balance of evidence does not support the assertion that the rainfall deficit since October 2010 was made larger or more likely by global warming.
Temperature: Compared to long-term averages of summer temperature,the rainfall deficit accounted for about 4°F of excess heat and global warming accounted for about 1°F of excess heat. Warmer temperatures lead to greater water demand, faster evaporation, and greater drying-out of potential fuels for fire. Thus, the impacts of the drought were enhanced by global warming, much of which has been caused by man.
Figure 1. This week's Drought Monitor showed that over 96% of Texas was experiencing the two worst categories of drought, extreme and exceptional. Image credit: U.S. Drought Monitor.
High fire danger for Texas today
Strong winds, temperatures in the 90s, and relative humidities in the 15 - 25% range will bring critical fire conditions today to Texas in the Austin-San Antonio region today, according to the Storm Prediction Center. Austin set a record high for the date yesterday, when the mercury climbed to 99°. Dry weather will dominate Texas for the coming week, but an increasing flow of moist air off the Gulf of Mexico next weekend may allow for a little drought relief 7 - 10 days from now. Texas' hurricane season is pretty much over; it is rare for tropical storms to affect Texas this late in the season. There is the potential the state could get moisture from an Eastern Pacific tropical storm, but there are probably only going to be 1 - 3 more storms in the Eastern Pacific this year, since activity in the basin is sharply lower during La Niña events.
Ophelia strengthens into the season's fourth hurricane
Hurricane Ophelia has strengthened into an respectable Category 3 hurricane with 115 mph winds, making it the season's fourth hurricane and third major hurricane. The arrival of the season's fourth hurricane on September 29 is a week later than the average date for the season's fourth hurricane, which is September 21. This is a remarkably late date for a season boasting the 2nd greatest number of named storms ever recorded before October (sixteen). Typically, over 60% of all named storms intensify into hurricanes; this year, the percentage has been only 25%. This bizarre combination of a near-record number of named storms but only four hurricanes has led to a near-average year for total destructive potential, as measured by the Accumulated Cyclone Energy (ACE). We are about 7% above average for ACE for this point in the season, according to stats compiled by Dr. Ryan Maue. The combination of near-record warm sea surface temperatures but exceptionally dry, stable air over the tropical Atlantic responsible for this year's odd hurricane season shows no signs of changing over the next few weeks. However, it's worth pointing out that the ocean regions north of 20°N latitude where Ophelia and Philippe are positioned have near-normal levels of atmospheric stability and dry air.
Figure 2. Morning satellite image of Ophelia.
Recent satellite loops show that Ophelia continues to intensify. The hurricane has a prominent eye, good upper-level outflow, and solid lower-level spiral banding. Moderate wind shear due to upper-level west-southwesterly winds is slowing intensification, and shear is expected to remain moderate through Sunday. The models agree that Ophelia will track far enough to the east of Bermuda that the island should see sustained winds below 45 mph, since it will be on the weak (left) side of the storm. The 11 am wind probability forecast from NHC gave Bermuda a 30% chance of receiving tropical storm force winds, and just a 1% chance of receiving hurricane force winds. Ophelia's closest approach to the island will be late Saturday night and early Sunday morning. Ophelia is likely to bring high winds and heavy rains to Southeast Newfoundland Monday, though the models have been trending towards keeping Ophelia farther offshore from Newfoundland in their recent runs. The 11 am wind probability forecast for Cape Race, Newfoundland gave it a 36% chance of receiving tropical storm force winds, and a 1% chance of hurricane force winds.
Tropical Storm Philippe
In the middle Atlantic, Tropical Storm Philippe continues to struggle against dry air and wind shear. Satellite loops show Philippe is a small system with little heavy thunderstorm activity, with the surface circulation exposed to view by wind shear. Wind shear is expected to increase to a very high 30 - 40 knots Saturday through Monday, thanks to the upper-level outflow from Ophelia. This shear may be high enough to destroy Philippe by Monday, though most of the models predict Philippe will survive the shear for the next five days. In the event Philippe does survive the shear, the storm could penetrate far enough west that Bermuda might need to be concerned with it, since Philippe will be forced almost due west by a strong ridge of high pressure early next week. There is even a small chance (perhaps 5%) that Philippe could make a comeback from Ophelia's high shear and affect the U.S. East Coast or Bahama Islands late next week.
Elsewhere in the Atlantic, none of the computer models is calling for a new tropical storm to form in the coming seven days. The large-scale environment over the Atlantic currently favors sinking air, due to the current phase of the Madden-Julian Oscillation (MJO). This situation will likely last well into next week, and will discourage formation of new tropical storms. The MJO is a 30 - 60 day cycle of thunderstorm activity that affects the tropics.
Figure 3. Satellite-predicted rainfall amounts for the 24-hour period ending at 2 am EDT Saturday October 1, 2011, for Typhoon Nalgae. The typhoon is expected to dump 4 - 8 inches of rain over Northern Luzon in the Philippines. This is probably an underestimate, given Nalgae's recent rapid intensification from Cat 2 to Cat 4. Image credit: NOAA/NESDIS.
Typhoon Nesat drenching Vietnam; Category 4 Typhoon Nalgae drenching the Philippines
Typhoon Nesat hit Vietnam today near Hanoi as a strong tropical storm with 70 mph winds. Nesat roared across Luzon Island in the Philippines Monday as a powerful Category 3 typhoon with 120 mph winds, leaving 43 people dead and 30 missing. The Philippines has a very bad case of deja-vu today, with Typhoon Nalgae already beginning to dump heavy rains on the main island of Luzon. Nalgae is expected to follow a track almost exactly the same as Nesat's, passing over Luzon just to the north of the capital of Manila. Nalgae is expected to maintain its Category 4 strength until landfall, and will likely bring at least 4 - 8 inches of rain to Northern Luzon. While this may not be as wet a storm as Nesat, Nalgae's rains will be falling on soils already saturated from Nesat's heavy rains, and the potential exists for high loss of life due to extreme flooding and mudslides. Wind damage is also a huge concern; Nagae's 135 mph winds are capable of causing widespread destruction on Luzon.
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