Natural gas and oil operators in the Gulf of Mexico (GOM) were removing some nonessential personnel and keeping a wary eye on a disturbance that "very likely" is to become Tropical Storm Bill by Tuesday.

According to the National Hurricane Center (NHC), a U.S. Air Force Reserve unit Hurricane Hunter aircraft investigated the broad area of low pressure Monday morning "and found that the circulation was too poorly defined to qualify the system as a tropical cyclone," said Senior Hurricane Specialist James Franklin. However, thunderstorm activity was becoming better organized in the afternoon, and "very likely" will form a tropical storm as it moves to the northwest. Formation of a tropical storm within 48 hours was estimated at 90%.

The NHC was not planning to update its forecast at 5 p.m. ET. The next aircraft was to be in the disturbance before 8 p.m., officials said.

Chevron Corp. and Royal Dutch Shell plc were among the operators that removed nonessential staff from offshore facilities as the low pressure system began to intensify. Many others were monitoring operations and were prepared to bring employees ashore. Many GOM operations for producers and pipeline operations are handled remotely from Houston.

"Shell has initiated efforts to reduce nonessential personnel on some offshore assets as a precautionary measure in addition to normal preparations for heavy weather," spokeswoman Kayla Macke told NGI. "No impact to operations or product is anticipated. We will continue to monitor the situation and will take further precautions as warranted." She did not indicate how many employees would be impacted, nor the facilities.

Chevron also was monitoring the storm, spokesman Kent Robertson said. “Some nonessential personnel have been evacuated from some offshore facilities in the Gulf of Mexico. There have been no impacts to production."

BP plc, which operates one of the largest fleets in the U.S. offshore, was "closely monitoring the disturbance in the Gulf to ensure the safety of our workers and operations in the deepwater Gulf of Mexico,” a spokesman said Monday. "Normal operations" were continuing, but "we remain prepared to respond as conditions warrant and as part of BP’s year-round efforts to plan, train and prepare for the Atlantic hurricane season.”

Houston Ship Channel (HSC) pilots ceased boarding outbound vessels around 2 p.m. CDT, and they expected to shut down operations before dark Monday. The HSC, which runs 52 miles through Galveston Bay, provides access to various ports and other cities with industrial facilities, including the massive petrochemical and refinery complex. The Port of Houston Authority said "normal terminal hours" were scheduled unless conditions changed.

The system, dubbed Invest (investigation) 91L, formed on Saturday. Maximum sustained winds were estimated to be near 45 mph Monday afternoon.

The biggest threat isn't to the offshore, but to the onshore, said meteorologists. The National Weather Service (NWS) in Houston/Galveston said there was an increasing risk for heavy rainfall through Wednesday that could lead to a “dangerous flood threat for parts of Southeast Texas,” which still is recovering from record rainfall and deadly floods.

“Expect bands of showers and thunderstorms containing very heavy rainfall Tuesday and Wednesday associated with the tropical disturbance that will be moving across the Texas coast and then off to the North,” NWS forecasters said. “If everything lines up correctly rainfall totals through Wednesday for parts of the area could approach a five- to seven-inch range, with some amounts exceeding 10 inches…”

Even if it doesn't become a named storm, "tropical storm conditions are likely: along portions of the middle and upper Texas coast and in extreme southwestern Louisiana through Tuesday,” said Franklin. meteorologists Bob Henson and Jeff Masters urged the region’s residents to "keep a wary eye" on the disturbance. An offshore rig at South Marsh Block 268, about 50 miles south of the Central Louisiana coast, measured sustained winds of 40 mph, gusting to 47 mph at 8:35 a.m. CDT Monday, they noted.

"Sea-surface temperatures [SST] are increasingly warm ahead of 91L, with unusually high readings" for mid-June of 82-86 degrees along the Texas coast. "These warm SSTs and the improved upper-level outflow should give 91L a brief window of potential intensification before landfall on Tuesday morning..." Based on two models, landfall was expected along the Texas coast, with the threat of "serious" flooding for East Texas into Oklahoma.

"Both states just experienced the wettest single month in their history, and soils remain near saturation," said Henson and Masters. "Even without such a worrisome precondition, systems like 91L are notorious for producing enormous amounts of rain, sometimes with tragic results. 91L has a large and very moist circulation, and steering currents will be weak as the system slowly moves around a strong, hot dome of high pressure over the Southeast."

Three of the five wettest tropical cyclones on record for the U.S. mainland occurred in Texas, including Allison and Claudette, and none of the five attained hurricane strength, according to The Weather Channel's Michael Lowry. Allison was one of the most destructive storms ever to hit the Houston area, and Claudette produced at least 40-plus inches of rainfall in Alvin, east of Houston. meteorologists expected water levels  to continue to build along the Gulf Coast into Tuesday as the system organizes and strengthens. The heaviest rain and biggest risk of flooding is forecast for Tuesday through Thursday along the upper Texas Coast, northeastern Texas, Louisiana and Oklahoma. Cities most at risk included Houston, Dallas, Oklahoma City and Shreveport, LA.