Strengthening Tropical Storm Barry could dump 25 inches of rain on Louisiana

Tropical Storm Barry is strengthening and edging ever closer to Louisiana, where it could unload dangerous amounts of rain this weekend, in some places in excess of two feet. It is also predicted to push ashore a "life-threatening storm surge," or a rise in water levels above normally dry land at the coast that can inundate low-lying roads, homes and businesses.

Water levels are already rising along the Louisiana coast, and winds will start picking up Friday afternoon, followed by bands of heavy rain Friday night. The strongest winds, heaviest rain and highest water levels are expected Saturday.

On Friday morning, Barry's maximum sustained winds increased from 50 mph to 65 mph, just 9 mph from hurricane strength. The National Hurricane Center projects it will become a hurricane by Saturday morning, when it is approaching landfall along the east coast of southern Louisiana.

The storm, centered 100 miles southwest of the Mississippi River, is crawling west-northwestward at just 5 mph. "The slow movement of Barry will result in a long duration heavy rainfall and flood threat along the central Gulf Coast," the Hurricane Center said.

The situation in New Orleans remains precarious. The city is protected from flooding by a system of levees and pumps. The pumps can be easily overwhelmed by extreme rainfall rates which results in minor flooding, but if any of the levees should fail (even without getting topped), parts of the city could become engulfed by floodwaters.

The Mississippi River water level forecast is perilously close to the 20-foot height of some of the protective levees on Saturday morning. The river is predicted to crest at 19 feet, the highest level since 1950, but the water level was already running about half a foot higher than forecasts on Friday morning.

A hurricane warning is in effect for much of the central and eastern coast of Louisiana, with tropical storm warnings covering most of the rest of the state's coastline. The tropical storm warning also covers Lake Pontchartrain and New Orleans.

In addition to the hazards posed by high water, damaging winds are expected in the narrow zone where the storm center makes landfall, with winds sustained over 70 mph and gusts possibly exceeding 80 mph. Farther inland and even considerable distances away from the storm center, tropical-storm-force winds over 40 mph and gusts over 60 mph are possible.

"Winds of #Barry will be strong enough to take down some trees - easier on already saturated soils - that could be life-threatening by falling on cars, roads (and cars later hit) or homes," tweeted Rick Knabb, The Weather Channel's hurricane expert.

Power outages are likely.

Tornadoes are also possible within the storm's rainbands as they move inland off the Gulf of Mexico.

Although Barry is only expected to become a Category 1 hurricane and could even remain a tropical storm, hurricane specialists frequently say "there's more to the story than the category."

The category rating of a hurricane only refers to its peak sustained wind speed - it says nothing about how big the storm is, its storm surge, and how much rain it will produce.

The rainfall forecast is ominous for places like New Orleans, Baton Rouge, Lafayette, and southern Mississippi.

"Barry is expected to produce total rain accumulations of 10 to 20 inches over south-central and southeast Louisiana along with southwest Mississippi, with isolated maximum amounts of 25 inches," the Hurricane Center writes. "These rains are expected to lead to dangerous, life threatening flooding over portions of the central Gulf Coast into the Lower Mississippi Valley."

Heavy rain bands may pass over some of the same areas repeatedly, with rates as high as three to four inches per hour.

The heaviest rain will progress from south to north through southeastern Louisiana and southwest Mississippi on Saturday, progressing into northern Louisiana, southwest Arkansas and west central and northern Mississippi on Sunday. By Monday, heavy rain is likely in western Tennessee and central and northern Arkansas.

During the middle of next week, between Tuesday and Thursday, Barry's remnant rains are likely to spread through the Tennessee and Ohio Valleys potentially to the mid-Atlantic.

The storm surge, or water pushed ashore by the storm's wind, could reach 3 to 6 feet above normal tide levels in the most at-risk areas.

"There is a danger of life-threatening storm surge inundation along the coast of southern and southeastern Louisiana, portions of Lake Pontchartrain, and portions of coastal Mississippi where a Storm Surge Warning is in effect," the Hurricane Center writes.

Here are some specific forecasts from the Hurricane Center:

- Mouth of the Atchafalaya River to Shell Beach: 3 to 6 feet

- Shell Beach to Biloxi MS: 3 to 5 feet

- Intracoastal City to the Mouth of the Atchafalaya River: 3 to 5 feet

- Lake Pontchartrain: 3 to 5 feet

- Biloxi, Mississippi, to the Mississippi/Alabama border: 2 to 4 feet

- Lake Maurepas: 1 to 3 feet

Two ingredients that shape a storm and where its heaviest rainfall occurs are the vertical wind shear (change in wind speed and direction with height) and low-to-mid-level relative humidity (moisture content). For the past day or so, the northern Gulf Coast has lucked out and Barry's wet side has remained south of its center, over water. That remains the case Friday - so far.

The forecast indicates that the midlevel dry air will get nudged to the west by the storm's circulation even as the wind shear remains northerly for at least the next day. What does that mean? If the forecast is right, the heaviest rain will be displaced almost entirely to the east of the center while areas west of the center see far less rainfall.

There's a small chance a sharp cutoff in the heavy rainfall sets up near New Orleans such that instead of seeing 10 to 15 inches of rain, the city sees significantly less (on the order of a few inches). But forecasting where the heaviest rain starts and stops is difficult and New Orleans should prepare for the worst.