Winter forecast for the U.S.; Typhoon Mirinae threatens the Philippines

Expect a warmer than average winter in the north central U.S., and cooler than average over the Southeast U.S., according the latest winter forecast issued last Thursday by the National Oceanic and Atmospheric Administration (NOAA). Wetter than average conditions are likely from Texas across the Gulf Coast and across the Southeastern U.S., with California also more likely than not to get increased precipitation. Those are the temperature patterns observed about 60% of the time in the U.S. during an El Niño event. As I discussed in yesterday's post, El Niño--the periodic warming of the equatorial Eastern Pacific Ocean that occurs every 2 - 7 years--has just crossed the threshold from weak to moderate, meaning that that ocean temperatures are 1.0 - 1.5°C above average along the Equator off the Pacific coast of South America. Moderate El Niño conditions are expected to continue through the winter months of December - February, with some climate forecast models predicting intensification into a strong El Niño. The presence of all that warm water alters the path of the jet stream, affecting the winter weather across a large portion of the globe. In particular, the southern branch of the jet stream--called the subtropical jet--is expected to intensify, bringing increased cloudiness and storminess along its usual path, from south Texas, across the Gulf Coast, through Florida, and into South and North Carolina. The increased cloudiness typically leads to cooler temperatures and more rainfall across the entire Southeastern U.S., especially Florida and South Texas. This should give significant drought relief to the South Texas.

The heat in the Eastern Pacific Ocean during an El Niño winter makes for more intense mid-latitude low pressure systems off the U.S. west coast. As a result, these storms are able to pump abnormally warm air into western Canada, Alaska and the extreme northern portion of the contiguous United States. With this warm air typically comes drier than normal conditions. With the forecast for Vancouver, Canada--home to the February 12 - 28 2010 Winter Olympics this winter--calling for warmer and drier conditions than average, lack of snow may be a problem for some of the venues. If the El Niño manages to strengthen to the strong category (greater than 1.5°C), California will most likely experience much increased precipitation, and the North Central U.S. will see winter temperatures 3 - 6°F above average. NOAA maintains a web page that shows the typical U.S. temperature and precipitation patterns for any season for historical EL Niño and La Niña events. Golden Gate Weather takes the idea a step further, and separates the effects by whether it is a weak, moderate, or strong El Niño.

How good is this forecast?
Not all El Niño winters follow the usual pattern. For instance, the El Niño winter of 2004 - 2005 had above-average temperatures over the entire U.S., and drier than average conditions across Florida, which are the opposite of the typical pattern observed during most El Niño winters (Figures 1 and 2). There are other natural variations in the weather and climate, such as the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) that can act to counteract or re-enforce the typical El Niño pattern. In addition, the climate is changing due to human-caused emissions of greenhouse gases, and its possible that what we've come to expect an El Niño winter to be like based on past history will no longer be true as often. It is likely the dramatic loss of arctic sea ice in recent years is already significantly affecting precipitation and temperature patterns during winter, and that the patterns we expect to see during El Niño winters are now changing to a new mode. I'll discuss the possibilities in a future post.


Figure 1. Wintertime (December - February) temperatures for the U.S. over the past nine years. The past three winters with El Niño conditions (yellow highlighted years) were 2003, 2005, and 2007. The 2003 El Niño was moderate strength, and 2005 and 2007 were weak. Image credit: National Climatic Data Center.

Figure 2. Wintertime (December - February) precipitation for the U.S. over the past nine years. The past three winters with El Niño conditions (yellow highlighted years) were 2003, 2005, and 2007. The 2003 El Niño was moderate strength, and 2005 and 2007 were weak. Image credit: National Climatic Data Center.

Philippines prepare for Typhoon Mirinae
Category 2 Typhoon Mirinae continues to trek westward towards the Philippine Islands, but appears to have leveled out in intensity. Latest infrared satellite loops show a large, well organized cloud pattern, but an eye is not evident, and the areal extent, organization, and temperature of the cloud tops has not changed much over the past six hours. Satellite estimates of Mirinae's intensity have held steady over the past twelve hours. Wind shear is a moderate 15 knots, and the ocean temperatures are very warm, 29°C. These warm waters extend to great depth, and the Tropical Cyclone Heat Potential (TCHP) is 80 - 100 kJ/cm^2 along Mirinae's entire path to the Philippines. Values of TCHP in excess of 90 kJ/cm^2 are frequently associated with rapid intensification. With wind shear expected to remain in the low to moderate range over the next three days, intensification of Mirinae into a major Category 3 or higher typhoon before its expected landfall at 06 UTC October 31 on Luzon Island is a good possibility.


Figure 4. Mirinae on October 27, 2009, as a tropical storm with 55 mph winds. Image credit: NASA.

Quiet in the Atlantic
There are no threat areas in the Atlantic to discuss, and the the Caribbean is dominated by dry air and high wind shear. However, the extreme Southwestern Caribbean near the coast of Panama is expected to gradually moisten early next week, and several recent runs of the GFS and NOGAPS models suggested that a tropical depression could form in the Southwest Caribbean off the coast of Panama 7 - 8 days from now.

Next post
In tomorrow's port, I'll discuss the impact of El Niño on wintertime tornado activity in Florida, plus have an update on the Philippines typhoon, and the latest model predictions of a possible tropical depression in the Caribbean next week.

Jeff Masters