After Becoming a Snowstorm, Lala Reinvigorates Itself to Become Category 4 Hurricane
Earlier this week we introduced you to Lala, a Pacific tropical cyclone. In our first post, Lala was a tropical storm that was setting its sights on Hawaii. Some interesting things have happened since then. First, Lala strengthened to hurricane intensity as it approached the big island of Hawaii. Here’s an animation of the true color view from GOES-18 (GOES West) as the eye passed just to the south of the Big Island. Note that this storm has the visual hallmarks of a weaker tropical cyclone, including an obscured eye.
Of course, just because it’s a weaker storm doesn’t mean it’s not a dangerous storm. Given the westerly trajectory of Lala, the strongest winds are going to be be on the north side of the system where the cyclonic flow is pushing with the storm’s motion instead of on the south side where the flow is acting opposite to the storm’s motion. The impact of this flow was notable, and the NCEP storm summary is astonishing: 140 mph wind gusts at Mauna Kea, hurricane-force winds throughout Hawaii, over a dozen stations with more than two feet of rain. Hundreds of thousands of Hawaiians were without power, and flash floods tore through the islands’ rugged terrain. But perhaps no impact captured the imagination of mainlanders quite like the the snowfall experienced on Hawaii’s highest peaks. Hawaiian snow isn’t altogether rare, as we discussed in this post back in February. August is a little different, however, and this was extremely atypical behavior. However, for most deep storms including hurricanes, the clouds are so deep that precipitation starts out as snow before melting into rain as it falls. With tropical cyclones, most of the time there isn’t a 13,800 foot tall mountain in the way to be hit by one of the rain bands and so we never see this snow before it has the chance to melt.
Consider this sounding from Hilo on the Big Island at 0600 UTC on the 16th (8:00 pm local time on the 15th), taken as the hurricane was passing by to the southwest and archived by the invaluable University of Wyoming Atmospheric Science Radiosonde Archive. This is a typical tropical cyclone profile, with saturated air all the way through the troposphere and strong winds all the way to the surface, and wind that shifts from blowing toward the low at the ground (convergence) to blowing away from it aloft (divergence). But look at that freezing level: it’s around 550 mb or 5140 m. While that’s still almost 1 kilometer above the height of Mauna Kea, the slow lapse rate in this saturated and well-mixed environment means that the temperature at the mountain’s peak is only a couple of degrees warmer than freezing. Snowflakes could easily have formed at the higher levels and not completely melted before reaching the surface at this significant altitude.
While Lala dished out a blow to the people of Hawaii, it didn’t emerge unscathed. The interactions with the terrain scoured the storm of some of its momentum, and it dropped back down to tropical storm status. It’s mid-August, however, and so the waters to the southwest of Hawaii are typically warm. This year, those waters are also 0.5 to 1.0 degrees C above normal thanks to being on the outermost periphery of El Niño as can be seen on the below map. This puts the sea surface temperatures in the area around 80-82 F, which is plenty warm for tropical development.
Maneuvering into a warm sea and low-shear environment was beneficial for Lala’s development, and it rapidly intensified. From 0000 UTC on the 18th to 24 hours later, it strengthened from a strong tropical storm all the way to a weak Category 4 hurricane. The time series of automated intensity estimates from the CIMSS D-MINT product tells the story well.
As of 1400 UTC on the 19th, Lala is straddling the line between Category 3 and Category 4. One of the mesoscale sector scans from GOES-18 has been trained on it, so it’s possible to see it evolve on a minute-by-minute scale. Here is an animation from around 1430 UTC of the Band 13 infrared window channel. It’s night at this time, so no shortwave products are available. Still, this is an excellent way to assess just how intense this storm is. There’s a clear, well-defined eye that even has a hint of some mesovortices in the inside. The cold thick clouds have the classic cyclonic rotation to them while the outer cirrus bands are exhibiting the anticyclonic spin of the divergent flow aloft.
Lala’s westward progression is projected to end as it’s forecasted to make a northward turn. Fortunately, while it is still a major hurricane its current path is taking it over the open ocean and its impact on lives and property is expected to be minimal.