(CBS SF / CNN) — The White House coronavirus task force is warning states about an uptick in coronavirus test positivity rates in a number of new cities this week and raised concerns about California’s Central Valley.

Task force coordinator Dr. Deborah Birx said there are encouraging signs across the South, a region hit hard by a surging pandemic in recent weeks, but she outlined new areas of concern in a private phone call with state and local officials Wednesday, according to a recording of the call obtained by the journalism non-profit Center for Public Integrity.

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“We are concerned that both Baltimore and Atlanta remain at a very high level,” Birx said on the call. “Kansas City, Portland, Omaha, of course what we talked about in the Central Valley (in California).”

“We are seeing a slow uptick in test positivity in cases in places like Chicago, Boston and Detroit and DC,” she said, adding that the virus has entered a new phase.

“This outbreak is different from the March, April outbreak in that it’s in both rural and urban areas,” Birx said.

Birx told CNN on Sunday that the deadly virus is now more “extraordinarily widespread” than it was in the early days of the pandemic.

In Wednesday’s call, she said the concern last week centered on increasing numbers in Tennessee, Kentucky, Indiana, Ohio, Illinois and Virginia.

“Although we’re seeing improvements in some of the red states and some of the states have actually moved from being in a red category — that was more than 10% test positivity — to under 10%, Kentucky, Tennessee and Virginia moved back into the yellow states status,” Birx said. “Their work needs to continue to intensify to continue to bring down case counts.”

Now, Birx said, Nebraska and California have moved into the red category, with more than 10% of tests coming back positive. She said Los Angeles may have seen improvements but that there’s significant movement of the virus up California’s Central Valley.

“Although L.A. may be looking a little bit better, there is significant movement of virus from Bakersfield, all the way up the Central Valley, to Stockton,” Birx said.

Despite Birx’s quick reference to 10% positive test rates, it’s not entirely clear which states the task force has designated as “red,” “yellow” or “green,” how often that label may change or what the criteria may be for the designation because the panel hasn’t released the information on the classification system.

In another private call last month, Birx warned of a concerning rise in coronavirus cases in 12 cities, including Miami, New Orleans, Las Vegas, San Jose, St. Louis, Indianapolis, Minneapolis, Cleveland, Nashville, Pittsburgh, Columbus and Baltimore.

Birx’s previous assessment prompted San Jose Mayor Sam Liccardo to refute the doctor’s statement, while allowing that the city still had room for improvement.

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“I won’t speculate why the White House selected the cities that it did, as hundreds of US cities have higher rates of transmission, hospitalization, and death than does San Jose,” Liccardo said in a statement on July 24th. “Regardless, there’s no question that our County’s coronavirus indicators are rising, and we all need to double down on what our public health authorities know works: wearing masks, socially distancing, washing hands, and taking the precautions necessary to take care of each other.”

Fauci: Test positivity upticks are a ‘predictor of trouble ahead’

By alerting officials to upticks in positivity rates in the nine cities and California’s Central Valley, Birx essentially was warning those areas to act now to prevent an undesirable surge in cases, another member of the White House task force told CNN on Thursday.

Test positivity rates can give early indication that a surge in daily case counts will come if nothing is done, Dr. Anthony Fauci said.

“It’s a pretty good predictor, usually before people become aware of it,” Fauci told CNN’s “New Day.”

“So what Dr. Birx is saying, is now is the time to accelerate the fundamental preventative measures that we all talk about: Masks, social distancing, avoiding crowds, outdoors greater than indoors, washing hands, et cetera.

“Those kinds of simple things can actually prevent that uptick from becoming a surge. So she was warning the states and the cities to be careful, because this is a predictor of trouble ahead.”

Birx said Wednesday that she has crisscrossed the country over the past five or six weeks, driving about 6,000 miles, and has seen a few trends for herself.

“We’ve really seen that America is on the move and people are going on vacation,” she said. She urged officials on the call to draw attention in their state to the dangers of exposing older family members to returning vacationers because of wide asymptomatic spread.

Birx again blamed “super spreader” events for driving the virus and said mitigation efforts should continue.

Birx ended Wednesday’s call on a hopeful note.

“We are watching each county, each city across the United States very carefully and are providing the best advice we have based on models … showing that these mitigation efforts are working,” she said.

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