Ernesto disorganized; more fires, extreme heat for Oklahoma

A disorganized Tropical Storm Ernesto continues to speed westward at 23 mph across the Caribbean. Ernesto has brought sporadic heavy rains to Jamaica today, and Kingston has picked up 0.51" of rain as of noon, and recorded top sustained winds of 22 mph. Ernesto looks very unhealthy on visible satellite loops, with its low-level circulation center a naked swirl exposed to view with almost no heavy thunderstorm activity near the center. However, the storm does have some rather far-flung spiral bands, and these bands are bringing occasional heavy downpours to Haiti, western Cuba, Jamaica, and the southwest Dominican Republic. This morning's flight by the Hurricane Hunters found that Ernesto had a very high central pressure of 1006 mb and top winds near 50 mph. The latest wind shear analysis from the SHIPS model shows moderate shear of 10 - 15 knots affecting the storm, but there must be some wind shear the satellites are not able to detect affecting Ernesto, given its disorganized appearance. Water vapor satellite loops show a large area of dry air to the west, and this dry air is also interfering with Ernesto's organization.


Figure 1. Visible satellite image with 375 meter resolution taken of Ernesto by the new Suomi NPP satellite at 1:39 pm EDT August 4, 2012. At the time, Ernesto had a flare-up of intense thunderstorms, and had top winds of 60 mph. Image credit: University of Wisconsin Madison CIMSS.

Forecast for Ernesto
Ernesto's rapid forward speed of 23 mph has been part of the reason for its lack of intensification, but the storm is expected to slow down Monday and Tuesday in response to a trough of low pressure passing to the north. This slowing, in combination with low wind shear, a moister environment, and increasing heat energy in the ocean, may allow Ernesto to strengthen some before making landfall in Belize or Mexico's Yucatan Peninsula on Tuesday night. However, the storm will be passing very close to the north coast of Honduras, putting a portion of its circulation over land and limiting intensification potential. It is unlikely Ernesto will become a hurricane in the Caribbean; NHC is giving just a 19% chance that this will occur. The main threat from Ernesto will be heavy rains over Honduras, Belize, Mexico, and Jamaica. The track forecast for Ernesto has become a bit easier, since the storm's current disorganization and more southerly path make will make it more difficult for the storm to make a northwesterly turn into the Gulf of Mexico like the UKMET and GFDL models are predicting. A stronger Ernesto would have been more likely to turn northwest under the influence of a trough of low pressure passing to the north. If Ernesto survives its crossing of the Yucatan Peninsula, the potential exists for it to re-strengthen over the Bay of Campeche, and make a second landfall on Mexico's coast on Friday, between Tampico and Veracruz. It's pretty unlikely that Ernesto will hit the U.S.-- though Brownsville, Texas could see some rain from Ernesto's outer spiral bands on Friday, if the storm survives that long.


Figure 2. Morning satellite image of Ernesto, showing the exposed low-level center--a swirl due south of Jamaica--and only very limited heavy thunderstorm activity surrounding the center.

Tropical Storm Florence
Tropical Storm Florence continues to plow westward at 14 mph over the Eastern Atlantic, and is not a threat to any land areas for next five days. The SHIPS model is predicting a moderate 5 - 15 knots of wind shear for Florence Sunday and Monday, but the shear will increase to the high range as Florence encounters an upper-level trough of low pressure on Tuesday. The latest Saharan Air Layer Analysis shows that a large area of dry air lies to the north and west of Florence, and this dry air will likely cause problems for the storm. Ocean temperature are near 26 - 26.5°C, which is right at the threshold for where a tropical storm can typically exist. It is possible that Florence could pose a threat to Bermuda next weekend, if the storm survives that long. Both the GFS and ECMWF models dissipate Florence before then.

Historic heat wave in Oklahoma
A second day of destructive fires affected Oklahoma on Saturday, thanks to extreme heat and drought, low humidities, and strong winds in advance of an approaching cold front. At 3 pm CDT Saturday, Oklahoma City had a temperature of 107°, a humidity of 19%, and winds of 16 mph gusting to 22 mph. The Oklahoma fires have destroyed at least 125 homes. The high temperature in Oklahoma City on Saturday reached 109°, the 12th warmest temperature recorded in the city since records began in 1891. Friday's high of 113° tied for the warmest temperature in city history.


Figure 3. Highway 48 is covered in smoke as flames continue, Saturday, Aug 4, 2012, east of Drumright, OK. Image credit: Associated Press.

The only comparable Oklahoma heat wave: August 1936
The only heat wave in Oklahoma history that compares to the August 2012 heat wave occurred during the great Dust Bowl summer of 1936, the hottest summer in U.S. history. Oklahoma City experienced three days at 110° that summer, and a record streak of 22 straight days with a temperature of 100° or hotter. Those numbers are comparable to 2012's: three days at 110° or hotter, and a string of 18 consecutive days (so far) with temperatures of 100° or hotter. The weak cold front that passed though Oklahoma Saturday will bring temperatures about 10° cooler over the next few days, but high temperatures are still expected to approach 100° in Oklahoma City Sunday through Tuesday. It's worth noting that Oklahoma City has experienced only 11 days since 1890 with a high of 110° or greater. Three of those days were in 2011, three in 2012, and three in the great Dust Bowl summer of 1936.

Wunderground's weather historian Christopher C. Burt has a new post on July's heat extremes in the U.S.


Figure 4. Most of Oklahoma has experienced nine consecutive days with highs of 100° or more, and many regions, including Oklahoma City, have had a streak of eighteen such days. Image credit: Oklahoma Mesonet.

Severe thunderstorm complex forces evacuation of Lollapalooza
A organized complex of severe thunderstorms developed over Eastern Iowa and Northern Illinois late Saturday afternoon, forming a dangerous bow echo that swept through Chicago, forcing the evacuation of the Lollapalooza music festival. NOAA's Storm Prediction Center (SPC) logged over 150 reports of wind damage from the storm, with five of the thunderstorms containing winds in excess of hurricane force (74 mph.) And just yesterday, my daughter was bemoaning her misfortune at not being able to get tickets to the show! The thunderstorm complex traveled about 300 miles from Eastern Iowa to Ohio, generating winds gust in excess of 58 mph along most of its path, meeting the definition of a derecho.


Figure 5. Radar image of the severe thunderstorm complex that spawned a dangerous bow echo over Chicago, which forced the evacuation of the Lollapalooza music festival.

Jeff Masters