Disastrous Rains Possible in Philippines from Typhoon Koppu; Mudslides Wallop SoCal

Heavy rains have begun on the Philippines' main island of Luzon as intensifying Typhoon Koppu heads west-northwest at 11 mph towards the Philippines. At 8 am EDT Friday, Koppu was a Category 2 storm with 105 mph winds, and satellite loops showed that Koppu had an impressive ring of eyewall clouds with very cold cloud tops that extended high into the atmosphere. The combination of low wind shear, warm ocean waters that extend to great depth and the presence of two impressive upper level outflow channels make it likely that Koppu will rapidly intensify to Category 4 status before landfall occurs near 18 UTC (2 pm EDT) Saturday on Luzon. Unfortunately for the Philippines, Koppu will then slow and turn to the north as the storm begins to feel the steering influence of a trough of low pressure passing to its north, and the center of the storm is likely to spend 2 - 2.5 days over Luzon before finally emerging to the north of Luzon on Monday or Tuesday. This will subject the island to an extended period of torrential rains, and some truly prodigious amounts of rainfall are being predicted. The 06Z (2 am EDT) Friday runs of the both the HWRF and GFDL models predicted that Koppu would intensify to Category 4 strength before making landfall, and showed large areas of 24+ inches of rain for Luzon over the next five days. The capital of Manila (population 12 million) was near the southern boundary of the predicted 12-inch rainfall totals. While rainfall forecasts from these two models are often overdone by 50%, it appears likely that a historic rainfall event is likely for the Philippines. Expect widespread damaging flooding capable of causing a top-five most expensive disaster in Philippine history.



Figure 1. Typhoon Koppu (left) and Typhoon Champi (right) as seen by Japan's Himawari satellite on Friday, October 16, 2015. Image credit: Japan Meteorological Agency.


Figure 2. Latest satellite image of Typhoon Koppu.


Figure 3. Predicted 5-day rainfall amounts from Typhoon Koppu from the 06Z (2 am EDT) Friday, October 16, 2015 run of the GFDL model. Widespread rainfall amounts in excess of two feet (orange and red colors) were predicted for the Philippines' Luzon Island, north of the capital of Manila. Image credit: NOAA/GFDL.

Best historical analogue for Koppu's rains: the historic rains of August, 2013?
The best historical analogue for the rains expected from Koppu may be an extreme monsoon rainfall event on August 18 - 21, 2013, which was enhanced by moisture from Tropical Storm Trami. Up to 600 millimeters (23.5 inches) of rain fell during one 24-hour stretch, and about 60% of metro Manila was under water at one point. At least 27 people died, and damage was estimated at $2.2 billion, making it the Philippines' second most expensive disaster in their history.

The top five most expensive disasters in Philippines history, according to EM-DAT (dollar values unadjusted for inflation):

1) Super Typhoon Haiyan, 11/8/2013, $10 billion
2) Monsoon rains increased by Tropical Storm Trami, 8/20/2013, $2.19 billion
3) Super Typhoon Bopha, 12/4/2012, $898 million
4) Super Typhoon Rammasun, 7/15/2014, $821 million
5) Tropical Storm Nina, 9/4/1995, $700 million

Elsewhere in the Pacific: Typhoon Champi, TD 19E, and Tropical Cyclone Two
Category 1 Typhoon Champi passed through the Northern Mariana Islands early Friday, and is expected to intensify into a Category 3 storm and turn to the north by Sunday. By Tuesday, a weakening Champi may pass close enough to Iwo Jima to bring tropical storm-force winds to that island.

In the Eastern Pacific, Tropical Depression 19E is slowly organizing as it heads west at 16 mph towards Hawaii. TD 19E is predicted to reach hurricane strength by Monday, but will turn to the north well before reaching Hawaii, eventually dying in the waters between Hawaii and California late next week. TD 19E is not a threat to any land areas.

In the South Pacific, Tropical Cyclone Two, the second tropical cyclone of the 2015 - 2016 season, has formed in the waters about 300 miles north of Fiji. High wind shear is expected to tear TC Two apart by Sunday.

Tropical development in Gulf, heavy rains on Gulf Coast possible late next week
Our leading models for predicting tropical cyclone genesis are becoming more insistent that a tropical disturbance could develop in the Bay of Campeche toward the latter part of next week, possibly moving into the Gulf of Mexico. The last several sets of GFS ensembles support the idea of tropical or subtropical development in this area, and the 00Z Friday operational runs of the GFS, ECMWF, and UKMET models show potential for least slow, modest development late next week and beyond. The focal point is a sharp cold front that moved across the Gulf of Mexico this week. As the front stalls and slowly returns north as a warm front over the next week, an area of disturbed weather should gradually move from the northwest Caribbean toward the Bay of Campeche, where low pressure of some type may begin to consolidate. In October and November, the most likely areas for Atlantic tropical cyclone formation shift from the open Atlantic to the Caribbean and southern Gulf, so the scenarios painted by the models agree fairly well with climatology. Often these cyclones form along decaying frontal systems pushed into the region by the first large, cool Canadian air masses of the autumn.


Figure 4. A large area of rich moisture and disturbed weather, designated Invest 92L by NHC on Friday afternoon, now covers much of the northwest Caribbean, as shown in this infrared satellite image from 1445Z (10:45 am EDT) Friday, October 16, 2015. Image credit: NOAA/NHC.

Next week’s projected development would be aided to some extent by a strong upper low settling into the southwestern U.S. early next week. This low is predicted to cut off over southern Arizona late in the week before moving northeast into the central U.S. It’s too soon to know whether any development in the Gulf would be tropical or subtropical, but the time of year and projected values of wind shear would tend to favor the latter, especially if strong jet-stream winds associated with the low manage to extend as far east as the western Gulf. Sea-surface temperatures, though cooling with the season, remain warm enough to support tropical or subtropical development over most of the Gulf. Any system that does develop would most likely track toward the north or northeast, steered by the flow between the upper low over Arizona and a strong upper-level ridge over the central Gulf. If model trends are any indication, this could be a very slow-moving feature. Should the area of low pressure stay too close to the western Gulf Coast--a possibility strongly suggested by the models--then interaction with land would hinder tropical development. Another potential complication would be any tropical development in the Pacific just south of the Bay of Campeche, as suggested by the 00Z Friday run of the ECMWF.

Apart from any tropical development, the overall set-up and its slow evolution will support a prolonged period of heavy rain over and near the western Gulf. Rains should begin early next week along the Texas coast, potentially shifting into the central Gulf Coast later in the week. The 7-day precipitation forecast from the NOAA Weather Prediction Center is already calling for 1” – 3” from Houston to Brownsville through Friday, October 23, with most of that projected to fall from Monday onward.



Figure 5. Vehicles are stopped in mud on California's Interstate 5 after flooding Thursday, Oct. 15, 2015. Image credit: Caltrans, via AP.

Southern California walloped by heavy thunderstorms, mudslides
Parts of Interstate 5 were still closed on Friday morning after an intense cluster of thunderstorms dumped torrential rain in the Antelope Valley just north of the Los Angeles metro area on Thursday afternoon. The rains took out several roads and generated mudslides and floods that engulfed nearly 200 vehicles on a state highway east of Tehachapi in mud up to 20 feet deep. Some motorists stayed in their vehicles overnight. No injuries were reported, but the road closures are a major headache for this populous, auto-dependent region. The mudslides and floods (dramatically captured by traffic helicopters) also serve as an unsettling preview of what we might expect from the heavy rains El Niño will likely bring to the region this winter.

Thursday’s intense thunderstorms--large and strong enough to resemble a Midwestern mesoscale convective complex--produced golf-ball-sized hail and thousands of lightning strikes along with torrential rain. The storms were triggered by a tenacious upper-low located just off the Southern California coast. This low carried out a very unusual 12-day loop-de-loop over the last few days, moving in clockwise fashion from Southern California on October 5 to the El Paso area by October 8, Baja California by October 10, well offshore by October 13, and then back around to Southern California. Meanwhile, low-level winds pumped very sultry air into the region over several days; this air mass became highly unstable as it was overtopped by cold upper-level air associated with the upper low. On Wednesday morning, dew point temperatures (which indicate the amount of water vapor in the surface air) hit 69°F in downtown Los Angeles and 72°F in San Diego’s Lindbergh Field—very high values for both locations. Record- or near-record warm sea-surface temperatures from the Southern California coast to Hawaii, partly associated with El Niño, are helping push up moisture values across the region.


Figure 6. Powerful thunderstorms are clustered just north of Los Angeles in this visible satellite image from 2310 GMT (4:10 pm PDT) Thursday, October 15, 2015. Image credit: NEXLAB/College of DuPage.

Two-day precipitation totals ending at 6:00 pm PDT Thursday included 2.37” at Poppy Park and 3.58” at Leona Valley in northern LA County. Rainfall rates near Leona Valley were briefly as high as 4”-6” per hour. While the storms raged, it was fairly tranquil on the other side of the San Gabriel Mountains: no rain was reported in either Los Angeles or San Diego.

Flash flood watches and warnings remained in effect on Friday for large parts of Southern California east and north of the LA Basin. Normally parched Death Valley was under a flash flood warning Friday morning for the second consecutive day. Death Valley’s Furnace Creek observation site received 0.70” of rain this month through Tuesday. Data for Wednesday and Thursday are unavailable, but it is possible Furnace Creek is approaching or has already topped its all-time October rainfall record (1.09”, from 1972). Thanks to weather.com’s Jon Erdman and Nick Wiltgen for details on the Death Valley rains. The embedded Facebook clip below shows flooding on Thursday in the Tehachapi area north of Los Angeles.

Wunderblogger Steve Gregory has more to say on the situation in his Friday afternoon post.

Jeff Masters and Bob Henson