convergence Archives - LZ

Tropical weather is not often on our minds at this time of the year, since the Atlantic Hurricane season is still months away. But there are other active tropical basins at this time – last week Cyclone Idai spawned in the Southwest Indian Ocean and brought devastation to vast swathes of Mozambique, Zimbabwe, and Malawi. With 700 deaths already attributed to it, Cyclone Idai is one of the deadliest tropical cyclones to have impacted southern Africa on record. Sadly, that death toll could continue to increase as floodwaters recede, revealing the full extent of damage from the storm. In this post, I’ll take a look at why Cyclone Idai was able to grow so powerful, and why it caused such serious flooding.

Meteorological Synopsis

Idai formed as a tropical depression in the Mozambique Channel on March 4, 2019. It moved onshore shortly thereafter and spent several days over Mozambique as it executed a cyclonic loop. During this time, it managed to retain tropical characteristics (keeping a warm core), before it exited again over the open waters of the Mozambique Channel. As it continued moving east, it encountered increasingly favorable conditions for intensification with very warm sea surface temperatures and decreasing vertical wind shear.

Visible satellite image of Cyclone Idai close to when it made landfall on March 14, 2019
Track map of Cyclone Idai. The purple dot indicates where Idai first formed, and the green dot is where it lost tropical characteristics

For more details on the images above see the Wikipedia article about Idai.

A subtropical ridge began forming and strengthening over eastern South Africa, causing the cyclone to start turning west and heading towards the coast of Mozambique again.

As Idai moved west, it underwent 2 distinct periods of rapid intensification in response to the favorable conditions found over the Mozambique Channel during this time. In between, it experienced an eyewall replacement cycle, where it temporarily weakened as concentric eyewalls formed, and the inner one eventually collapsed as the outer one took over.

Upper level divergence at 18 UTC on March 14, 2019
Low level convergence at 18 UTC on March 14, 2019
Vertical wind shear analysis at 18 UTC on March 14, 2019

Unfortunately for those in Idai’s path, it regained strength after a second eyewall replacement cycle as it approached landfall because conditions continued to be quite favorable. The analyses above with satellite overlays valid for 18:00 UTC March 14, 2019 show the cyclone near its peak intensity under nearly ideal conditions (lower left of the image). The first two analyses show strong upper level divergence and low level convergence around Idai respectively. This indicates that upper level outflow was well established, which is also visible from the symmetrical presentation of the cyclone visually, with outflow channels evident. Upper level outflow is required for “ventilating” intensifying cyclones and helping them grow/maintain their intensity. That’s because as the storm gathers strength, low level convergence intensifies as winds in the storm pull air from surrounding areas inwards towards the eye of the storm. This convergence is what powers the intense thunderstorms in the eyewall. Without upper level divergence, the air converging at the storm’s core would continue building up, eventually resulting in rising pressure and a weakening storm – pressure, after all, is a measure of the mass of air over a given area.

The last image shows that Idai was also in a zone of very low vertical wind shear, between 5-10 knots during this time before landfall. Low vertical wind shear helps preserve the structure of a tropical cyclone. Unlike with severe thunderstorms, wind shear can actually shift the core of the strongest thunderstorms away from the center of a tropical cyclone’s center of circulation. That often marks the beginning of the end for a tropical cyclone. In the case of Idai, low wind shear let it strengthen considerably close to landfall, although it did weaken a bit right before landfall due to increased shear and interaction with land.

Why Such Devastating Flooding from Idai

While powerful storm surges accompanied Idai, the worst impact from Cyclone Idai was widespread catastrophic flooding in Mozambique and in Zimbabwe. Part of this comes down to the geography of the area impacted, with a broad, flat flood plain between Pungwe and Buzi Rivers, and with parts of Beira, the largest city in the area, lying below sea level.

European Space Agency satellite image, with areas in red indicating inundation taken earlier last week

As Dr. Jeff Masters pointed out in a post on Weather Underground, another big reason why Idai caused such serious flooding was because of the storm’s slow forward progress. This came down to the storm being embedded in an environment with weak steering currents.

CIMSS deep layer mean steering (250-850 mb) analysis loop of the Indian Ocean. Cyclone Idai is visible in the lower left corner.

In the animated loop of 250-850 mb deep layer mean steering winds, we see that as Cyclone Idai traversed the Mozambique Channel, it was in an area of very light steering winds (very few streamlines, few arrows on those streamlines). Mature tropical cyclone motion is influenced by winds in this layer of the atmosphere because these winds impact the tall thunderstorms in the cyclone’s core. When steering currents break down as in the loop above (a ridge with anticyclonic flow dissipated north of Idai), a tropical cyclone will start to slow down and sometimes can meander. In this case, with Cyclone Idai, a slower moving storm led to a prolonged period of heavy rain over the impacted areas. Had steering currents remained stronger, Idai would have produced less heavy rain over the same areas during a shorter window, likely reducing flooding impacts considerably.

Consider Donating to Support Recovery Efforts

The affected countries of Mozambique, Malawi, and ZImbabwe simply do not have the financial and physical resources to respond to a disaster of this magnitude. There are many worthwhile NGOs on scene providing relief aid. Please consider making a contribution to support these efforts, as the recovery effort will take years to complete. International Rescue Committee, Doctors Without Borders

I haven’t had time to post about the current week’s WxChallenge forecasts for Caribou, ME, which I’ll have a follow up post about Saturday or Sunday to go over my results. Instead, this week, I’m going to put the same forecast skills I’ve been applying to the upcoming nor’easter that’s going to impact NYC Saturday. I’ll provide a deterministic (exact number) forecast for high temperature, low temperature, maximum sustained wind speed, and total accumulated precipitation for the period starting 2AM Saturday and ending 2AM Sunday, with a less exact forecast for the rest of the weekend. In my later follow up post, I’ll verify my results and see just how close I got. For purposes of verification, my forecast will be for LGA. If you’re only reading this summary, just know that Saturday will be a windy, raw, rainy day, but that conditions will improve as the day goes on.

Friday – partly sunny skies with high temperatures in the low-50s. Light winds from the south-southeast.

Saturday – High temperature: 54°F. Low: 48°F. Maximum sustained winds (2-minute averaged): 34.5 mph from the east-northeast. Total precipitation: 1.25″. The bulk of the rain should end by the afternoon hours.

Sunday – mostly cloudy, high temperatures in the upper-50s with return flow from the west downsloping into the area.

Forecast Discussion

The nor’easter that’s coming to us this weekend has tropical origins as the remnants of Hurricane Willa, which originated in the Eastern Pacific and made landfall in Mexico as an intense Category 3 storm. This means that it will be able to tap to tropical moisture, bringing the potential for a heavy rainfall event. At the surface, a low pressure center will be intensifying as it slides up along the Mid-Atlantic coast. At the mid and upper levels of the atmosphere, the set up is favorable for robust divergence, which should result in to strong surface convergence and lift. At the 850 mb level, there are indications of a strong low-level jet with winds as high as 50 knots from the east to east-northeast during the Saturday morning hours ahead of the warm front attached to the nor’easter. This would enable highly efficient moisture transport and convergence in our area. The net result of this should be periods of heavy rain. This is what justifies rainfall totals over an inch, though because of the relatively fast forward speed of this storm, I don’t believe that totals above 1.50″ are likely.

These aren’t pictured below, but MOS (model output statistics), and forecast plumes (ensembles of many different iterations of the same forecast model) probabilities suggest a consensus range of 1.15″-1.24″. Short Range Ensemble Forecast (SREF) probabilities show a greater than 90% chance for rainfall totals greater than or equal to 0.50″, 70-90% chance of totals greater than or equal to 1.00″, but very low chances of totals exceed 2.00″. The record rainfall for Saturday at LGA is 1.76″, though there are higher record totals surrounding this date, so this record isn’t what we’d call particularly strong.

On the temperature front, the trajectory of the onshore flow accompanying this storm actually brings it over Atlantic waters that are warmer than the air temperatures have been this week. This should bring warmer air into the area, however, because it will also be raining and cloudy, evaporational cooling will be a concern (as it rains, some of the moisture from raindrops will evaporate, which takes heat input to achieve, lowering surrounding temperatures), and that will likely knock temperatures down a bit. Overnight lows should end up being fairly mild in comparison to previous nights because of cloud cover and the aforementioned winds over warmer waters.

The wind speeds should be quite strong tomorrow. MOS is showing winds in the 24-26 knot range. However, given that 850 mb winds will be even strong, in the 50 knot range, and it will be precipitating, meaning downward transfer of momentum by raindrops should mix some of these stronger winds to the surface, which is why I think sustained winds could be as high as 35 mph at some point in the day at LGA (which is exposed near open water, so there’s less of an impact from friction over the land). I think gusts could easily top 40-45 mph tomorrow during the most intense rain squalls.