The Atlantic hurricane season is effectively over; heavy rains in the Northwest

Only two weeks remain in the Atlantic hurricane season, but the hurricane season of 2009 is effectively over. While the Western Caribbean is still warm enough to support development of a hurricane--as it is year-round--wind shear over the entire North Atlantic has risen to levels prohibitively high for tropical storm formation to occur. Wind shear is forecast to remain very high for at least the next ten days (Figure 1). This is a fairly typical occurrence in the Atlantic for this time of year, though it usually occurs sooner in El Niño years. As you probably know, El Niño conditions in the Eastern Pacific commonly create high levels of wind shear over the tropical Atlantic. This year was no exception, though the shear created by El Niño was not as strong as we've seen in other recent El Niño years. Wind shear was uncharacteristically low in the first half of November, allowing Hurricane Ida to form. Within the past ten days, though, El Niño conditions over the Eastern Pacific have intensified from moderate to strong, and wind shear over the tropical Atlantic has increased significantly, making it unlikely any significant tropical cyclones that affect land will occur the remainder of this hurricane season. Formation of a subtropical storm over the open Atlantic far from land is still a possibility, but such a storm would only be a concern to shipping.


Figure 1. Wind shear forecast for the Atlantic for ten days from now made by last night's 00Z run of the GFS model. The GFS model is forecasting very high wind shear over the entire North Atlantic over the next ten days.

Late season tropical storms in El Niño years
In the 17 hurricane seasons since 1950 where an El Niño event has been present, only three of those years featured named storms that formed after November 15. Tropical Storm Otto formed on November 29, 2004, from the remains of an extratropical storm that got cut off from the jet stream over the middle Atlantic. Otto meandered for a few days far from land before dissipating. Category 1 Hurricane Frances formed on November 19, 1986 in the open Atlantic south of Bermuda, and died after three days without affecting land. Category 1 Hurricane Martha formed on November 21, 1969 in the extreme southwestern Caribbean, off the coast of Panama. Martha weakened to a tropical storm before making landfall in Panama, and was the only named storm in the Atlantic ever to make landfall in Panama.

Typhoon season not over yet
Note that typhoon season in the Western Pacific is not over--we commonly get typhoons well into December. In fact, the ECMWF model is predicting formation of yet another typhoon east of the Philippines, sometime late this week or early next week. Tropical cyclone season is also not over in the Bay of Bengal near India, where some models are predicting an enhanced chance of a tropical storm forming late this week. And in the Southern Hemisphere, hurricane season is just beginning, with the formation of Category 3 Tropical Cyclone Anja near Maritius Island off the coast of Madagascar.


Figure 2. Category 3 Tropical Cyclone Anja in the South Indian Ocean at 1 am EST Monday November 16, 2009. Image credit: NASA.

Heavy rains hit Washington and British Columbia
A strong branch of the polar jet stream laden with moisture is streaming into Washington State and Canada's British Columbia today, and is forecast to bring heavy rains, high winds, and the threat of avalanches to the coastal mountains today through Wednesday. Heavy rains in the Pacific Northwest this time of year are often dubbed the "Pineapple Express" due to Hawaiian origin of the air, and these events are common during El Niño winters, and can strike from Southern California to British Columbia. However, I've been told by Doug McCollor, a forecaster with BC Hydro, that this rain event is not a true Pineapple Express, since the airflow is more west-to-east, rather than from the southwest. He adds, "also, freezing level at Quillayute WA was only 1800m this morning...not that high. In a Pineapple Express the freezing level would be 3500 to 4000m or so. The media here is calling it a Pineapple Express because they look out the window and it's raining all day. It is raining moderately here, no doubt, but it's because there is a downstream Rex block forming over central North America that is impeding the usual west-to-east progression of these storms".

Rainfall amounts in excess of three inches have already been recorded over Vancouver Island, and rains of up to seven inches (Figure 3) are forecast for the region over the next three days. Wind gusts of 44 mph have been recorded at La Push this morning on Washington's Olympic Peninsula.


Figure 3. Forecast precipitation for the 3-day period ending 7 am EST Thursday, November 19, 2009. Up to seven inches of rain are predicted for Washington State and coastal British Columbia. Image credit: NOAA/HPC.

I'll have a new post Tuesday or Wednesday.

Jeff Masters