After a week of hot, humid weather and several bouts of strong to severe thunderstorms, we will finally get a break from both the heat and the rain just in time to enjoy the weekend. Behind a cold front that’s slowly pushing south, high pressure will build in and usher in a Canadian polar airmass that will bring a noticeable change to high temperatures – 10ºF cooler in most cases. Rain chances return late in the weekend period.
Rest of today – showers associated with a wave of low pressure moving along the cold front that passed last night should end by noon. Mostly cloudy with gradual clearing. High temperatures in the upper-70s and overnight lows in the mid-60s.
NAM (North American Model) forecast for 2 meter above ground level temperature Saturday at 2PM
Saturday – mostly sunny, high temperatures in the upper-70s, noticeably drier feel with dew points much lower than the past few days with Canadian high pressure taking over. Overnight lows in the mid-60s.
Sunday – below average temperatures continue with high temperatures in the mid-70s. By this time, the high pressure center will have shifted to the northeast and will thus enable cooler, more moist onshore flow to influence the weather. We should see more clouds and could even see some scattered rain showers. Overnight lows in the low-60s.
Weather Prediction Center surface forecast for 8AM Sunday
Monday – high temperatures in the upper-70s with partly sunny or mostly cloudy skies. An area of low pressure (which could become a tropical depression this weekend) off of Florida will track along the East Coast and should at the very least bring cloudier weather. Overnight lows in the mid-60s.
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
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
In lieu of an early week forecast this week, I’m opting to share some observations about weather I experienced in Mexico last week while on vacation. I stayed in Isla Mujeres, a small island located about 13 miles off the coast of Cancun. Temperatures were of course quite warm. It was also unusually windy for this time of year down there, though nothing quite like the windy weather NYC experienced today with some peak winds recorded at over 50 mph nearing 60 mph.
METAR READINGS FROM JFK and LGA showing peak winds of 56 mph and 58 mph respectively recorded at 12:04AM and 8:31AM respectively todaY
I’m only just getting adjusted back to cold temperatures, and am not looking forward to snow possibly falling Wednesday night and another storm bringing wintry precipitation Friday. On the bright side, we are now only about 3 weeks off from the vernal equinox and the start of spring!
On the Servicio Meteorológico Nacional (SMN) – National Meterological Service of Mexico
Since I was going to be in Mexico, I started checking out the Mexican government’s weather service page. Check out the surface analysis below that’s overlaid on what looks like a GOES East image from last Thursday (02/21/2019).
Click to enlarge this surface analysis from the Mexican SMN
Unlike our own National Weather Service, the SMN numbers frontal systems that move through Mexico. Notice the stationary front in the center of the image is labeled as “Frente No. 38” (Front #38) and you can see “Frente Frio No. 40” (Cold Front #40) crossing from southern California into northern Baja California in the upper left corner of the image. They also number their winter storms. The “B” (representing a low pressure center) over Nevada is labeled as “Octava Tormenta Invernal” (Eighth Winter Storm). “Corriente en Chorro Polar” (polar jet stream), “Corriente en Chorro Subtropical” (subtropical jet stream) are familiar features to us, which we seen streaking across the northwest and central portions of Mexico respectively. A “Corriente de Bajo Nivel” (low-level jet) is seen flowing from the east towards the Yucatan. Here’s a translation of the text in the lower left panel:
Systems affecting Mexico The Eighth Winter Storm over the southwestern US combined with cold front #40 in northwestern Mexico will favor showers with some strong storms, very cold temperatures, and wind gusts over 60 km/h in the northwest and northern Republic, and also the potential for snow or sleet in mountainous areas of Baja California, Sonora, and Chihuahua, extending gradually towards Durango. Front #38 with stationary characteristics extends over the western Gulf of Mexico and will generate clouds with isolated rain in the eastern and northeastern parts of the country.
Servicio Meteorológico Nacional of mexico
Synotpic Conditions – the Tropical Big Picture
The consistent breezy south-southeasterly winds I felt on Isla Mujeres were tied to that low-level jet (LLJ) pictured above. This LLJ enhanced the general easterly trade winds in the area. This was a result of the influence of a high anchored over the Western Atlantic, and a low over northern Colombia pictured in the OPC surface analysis below (issued Friday 2/22 02:35Z) “funneling” the winds.
In this analysis of the Western Atlantic from NOAA’s Ocean Prediction Center, you can see that a broad high pressure center was anchored near Bermuda. Meanwhile, a low sat over northern Colombia. The clockwise flow around the high and counterclockwise flow around the low in proximity to each other act to enhance the easterly trade winds found in the tropics.
A
sounding from Philip Goldson International Airport near Belize City
(the closest sounding station I could find to Cancun) showed clear
evidence of a well-mixed layer from the surface to just about 900 mb. It
felt like in Cancun, this mixed layer extended a bit further up into
the 850 mb level where the LLJ sat because the winds were stronger.
Sounding taken above Philip Goldson International Airport near Belize City at 7PM Feb. 20, 2019, showing a well-mixed boundary layer representative of the area around the Yucatan Peninsula during these few days
By
way of brief explanation, well-mixed layers like the one shown above
provide favorable conditions for faster moving winds aloft to transfer
their momemtum downwards, in this case all the way to the surface. It
shouldn’t be a surprise that a deep well-mixed layer also existed today
over NYC – enabling the strong winds aloft to mix down, leading to some
very strong winds and gusts.
Sounding from Upton, NY for at 7AM Feb. 25, 2019, showing a deep well-mixed layer down to the surface from around 800 mb with strong winds 35-50 knots through most of this layer
Aside
from the winds, the weather followed a pretty standard tropical pattern
with clouds building in the afternoon and isolated showers. Despite how
flat the Yucatan Peninusla is, it nevertheless provides at least some
small potential for lift and convergence for air flowing off the
Caribbean Sea. This is because there’s actually a significant difference
in frictional properties of land and water, which makes sense since the
surface of the ocean is considerably “smoother” than the corresponding
forested Yucatan. One other notable trait was that the base of rain
clouds in the area took on a distinctively blue hue, which I imagine was
a reflection of the characteristically blue waters of the Caribbean
Sea.
After a week of gloomy, cloudy, and at times rainy weather, we will get a nice break this weekend as conditions improve. The same high pressure that’s caused Hurricane Florence to make landfall on the Carolinas by blocking its northward progress will bring us sunnier weather. Indeed, some of the cloud cover over our area is actually a result of outflow from Florence. Its remnants will impact us with rain at some point early next week.
Rest of today – mostly cloudy with high temperatures in the mid-70s as winds continue from the east. High surf with waves 8-12 feet are possible at the coast due to Florence.
Saturday – decreasing clouds as high pressure continues to build. High temperatures around 80ºF.
Sunday – sunny, high near 80ºF, high pressure remains in control.
Monday – clouds and the chance for rain returns. More moist air returns as the high pressure that gives us nice weather over the weekend slides east (and the flow on its western side starts to bring moisture from the south up). We’ll need to monitor the progress of the remnants of Florence as they are likely to bring us rain.
This week kicks off with a prolonged period of unsettled weather as a warm front and a cold front both impact the area. Chances for rain will be with us through Wednesday. Conditions finally improve late in the week. We luckily will avoid direct impacts (aside from high surf) from what will be an extremely dangerous Hurricane Florence, which looks set to make landfall on North Carolina late this week. Aside from Florence, two other storms pose threats to US territories this week.
Rest of today – cloudy, with periods of rain, heavy at times, especially later in the day towards the overnight hours. High temperatures will be in the upper-60s with cool, persistent onshore flow from the east. Ahead of an approaching warm front.
Tuesday – chances for thunderstorms will persist into Tuesday, with this slow-moving warm front finally pushing through. There could be thunderstorms overnight. Then, later in the day, as the trailing cold front moves through. Once the warm front passes over, we’ll be within the warm sector of the parent low pressure system, which will allow temperatures to climb into the low-80s with a marked increase in humidity.
Wednesday – the cold front mentioned above will slow down as it approaches us and dissipate. This will lead to continuing chances for rain, and thunderstorms with temperatures again in the low-80s.
Thursday – high pressure will finally begin to build in to our north by Thursday behind the cold front above. This will lead to better conditions, with partly sunny skies and temperatures in the low-80s.
Friday – partly sunny skies with high temperatures in the low-80s again with high pressure to our north.
Florence, Isaac, and Olivia All Pose a Threat to the US
These three storms have the potential to impact the Southeastern US, Puerto Rico, and Hawaii respectively within the next week. Among these, Florence is by far the most dangerous, as it could make landfall on North Carolina as a Category 4 storm with maximum sustained winds of 145 mph.
Hurricane Florence
Over the past day or so, the picture for Florence has become much clearer. Various forecast models have come into remarkable alignment on both the overall track and intensity of Florence over the next 5 days, which means bad news for the Carolinas. The picture looks grim, with Florence set to make landfall as a very strong Category 4 storm packing sustained winds of 145 mph. Florence has nothing but warm water ahead of it, and there should be minimal impact ot this storm from vertical wind shear. Even if the storm weakens due to eyewall replacement cycles prior to landfall, that would only serve to increase the radius of its damaging winds. No matter how you slice it, Florence is likely to bring deadly storm surge and risk of life-threatening flash flooding and inland flooding. The fact that its forecast track slows to a crawl after landfall makes the flooding risk inland especially serious.
Hurricane Isaac
Isaac is a compact storm, which is leading to some uncertainty regarding both its track and intensity forecast. Small tropical cyclones are subject to wild swings in intensity both ways, which can be unpredictable at times. As of now, Isaac is forecast to cross the Windward Islands of the Lesser Antilles as a Category 1 storm, then continue into the Caribbean where hostile shear conditions should cause it to weaken. It is worth noting, despite the UKMET model being an outlier, that there are some signals that the storm could turn to the northwest later in the period, which could mean a threat to Hispaniola and Puerto Rico.
Hurricane Olivia
This storm continues to track towards Hawaii. Although weakening due to the influence of moderate wind shear, it is still set to hit Hawaii later this week as a strong tropical storm. The primary threat to Hawaii from Olivia will be in the form of heavy rain and flash flooding regardless of whether it makes direct landfall on one of the main islands.
Cooler temperatures are finally upon us after a very hot and humid week. However, the cooler weather also brings a prolonged period of cloudy conditions, with chances for showers each day this weekend. Rain chances continue into the beginning of next week with a storm system approaching. Long-term, we look towards the tropics, as Tropical Storm Florence continues to have the potential for bringing some impacts to the area.
Rest of today – overcast, with much cooler high temperatures in the mid-70s. Chances for showers, especially later in the day.
Saturday – cloudy with high temperatures remain cool in the low-mid 70s with northeasterly onshore flow behind a cold front draped west-east south of us. This front (which passed through yesterday) will become stationary. Multiple impulses of energy will run along this boundary and allow for continued slight chances of showers.
Sunday – yet another cool, cloudy day with chances for rain. High temperatures around 70°F.
Monday – the stationary front stalled out south of us will return north as a warm front. As this happens, chances for rain increase, along with temperatures rising back into the mid-upper 70s.
Will Tropical Storm Florence Impact the East Coast?
Tropical Storm Florence has been active over the eastern and central North Atlantic for over a week now. Within the last few days, Florence underwent a rapid intensification that brought it up to Category 3 status, making it the first major hurricane of the 2018 Atlantic hurricane season. Subsequently, it has weakened to a tropical storm under relentless strong shear. However, it is expected to reintensify into a major hurricane once shear subsides. Worrying trends in the track forecast for Florence continue to allow for the possibility of landfall on the East Coast, or at the very least, substantial coastal impacts like dangerous surf and beach erosion. However, the National Hurricane Center continues to note that there is considerable uncertainty to the track forecast for Florence, such that it is still too early to determine what effects it will have on the East Coast. This storm will warrant careful observation over the coming days and I will have subsequent updates on it.
A brief break in the heat over the weekend has led back into another warm spell to begin this week. This latest blast of heat will be short-lived as a backdoor cold front sweeps through tomorrow, followed by another, stronger cold front later this week. Temperatures for the upcoming weekend will once again be comfortable, seasonable to below average. In the tropics, things are starting to get busy with tropical storms Florence and Gordon active and another tropical wave poised to become a tropical cyclone off the coast of Africa. Gordon will make landfall overnight on the Gulf Coast east of New Orleans.
Rest of today – hot and humid with temperatures in the low-90s, mostly sunny skies.
Wednesday – cooler, though still warm with high temperatures in the upper-80s and partly sunny skies. A backdoor cold front will bring cooling onshore southeasterly breezes that help keep a lid on temperatures.
Thursday – temperatures rise back a couple degrees into the low-90s ahead of an approaching cold front from the northwest. The warm air ahead of the cold front could help set the stage for a few scattered strong thunderstorms as the front passes through.
Friday – temperatures cool off quite a bit into the low-80s under mostly cloudy skies. Chance of showers during the day with the cold front lingering south of the area.
Tropics Heating Up
We have 2 active tropical cyclones (Florence, Gordon), and another incipient one in the Atlantic Basin. Tropical Storm Gordon could strengthen into a hurricane before landfall on the Gulf Coast between Mississippi/Alabama overnight into Wednesday. Heavy rain could induce flash flooding in southern portions of these states. The immediate coastline will be at risk of storm surge flooding. Florence doesn’t pose an threat to land. The disturbance shown below that’s furthest east is a tropical wave that’s just emerged off the coast of Africa. This is worth watching as it has a high chance of becoming a tropical cyclone and its eventual track could pose a threat to portions of the Lesser Antilles.
This week will see mostly average temperatures, with the exception of a very warm day tomorrow. Later on in the week, remnants of Subtropical Storm Alberto will have an impact on our sensible weather, bringing on an extended period of unsettled weather that carries into next weekend.
Rest of today – high temperatures in the low-70s with decreasing clouds. If clouds break up sooner in the afternoon, we could see warmer temperatures in the upper-70s.
Tuesday – a warm front will move through overnight into Tuesday. Temperatures will be significantly warmer with mostly sunny skies behind the frontal boundary, with high temperatures close to 90°F in the city away from south-facing shores.
Wednesday – mostly sunny still, with high temperatures falling back to normal levels in the mid-70s as a backdoor cold front moves through. This will cause a shift in winds to the east, bringing cooler, marine air onshore.
Thursday – increasing clouds with chances for rain late in the day, high temperatures holding in the mid-70s.
Memorial Day weekend in NYC kicks off with two incredible days of summer-like weather with excellent conditions for all outdoor activities, including hitting the beach. The weather shifts markedly for Sunday and Monday as a cooler air mass takes hold of the region behind a backdoor cold front. Temperature could be 15ºF-20ºF cooler between the first and second half of the weekend. Rain will accompany the passage of the backdoor cold front Saturday night into Sunday. No day will be a washout though.
Rest of today – sunny with breezy southwesterly winds bringing in warm air. Temperatures should soar into the upper-80s with a few spots reaching the 90ºF mark outside of coastal areas where sea breezes should set up fairly quickly.
Saturday – warm again with high temperatures likely reaching to around 90ºF away from south-facing shores with the influence of sea breezes. Late in the day towards the evening hours, a backdoor cold front will be approaching from the north. Ahead of this frontal boundary, clouds will increase throughout the day, and some pop up showers and thunderstorms could occur. Sea breezes may help initiate some of these storms closer to the coast. Given that there will be some instability in the atmosphere, a couple of these storms could approach severe limits, with strong wind gusts and heavy rain.
Sunday – overnight from Saturday into Sunday is when the bulk of the rain associated with this cold front will fall. There could be a scattered thunderstorm overnight. Rain will impact the beginning of the day Sunday, but conditions should improve later in the afternoon. Behind the backdoor front, easterly onshore winds will predominate, leading to much cooler temperatures only in the low-70s under cloudy skies.
Monday (Memorial Day) – cool with partly sunny skies and high temperatures in the low-70s.
First Tropical Storm of the 2018 Atlantic Hurricane Season
Although our weekend weather won’t be ideal, we are lucky not to have to plan for anything worse. Residents of the Gulf Coast between Louisiana and Florida Panhandle will need to contend with the arrival of the first named storm of the 2018 Atlantic hurricane season, Subtropical Storm Alberto.
This Mother’s Day weekend will unfortunately feature mostly unsettled and cool weather, though no day will be a complete washout. A frontal boundary will just linger in our vicinity during this period, moving north as a warm front, then retreating south as a cold front. This will allow multiple subtle upper level disturbances to move along this boundary, and each of these will bring a chance for rain.
Rest of today – splendid day with sunny skies and just about average high temperatures around 70ºF.
Saturday – chance for rain early between 7-11AM, then a period of mostly cloudy skies until another round of showers with some thunderstorms rolls through in the evening between 8-10PM. Cooler with high temperatures in the mid-60s.
Sunday (Mother’s Day) – chance for showers in the morning, with steadier rain possible through early afternoon. Cool, with northeasterly onshore winds, and high temperatures in the low-60s. Rain chances should diminish later in the day.
Monday – drying out with temperatures in the low-70s and partly sunny skies.
Possible Tropical Storm Impacting the US During Memorial Day?
Consecutive runs of the GFS forecast model continue to suggest that a subtropical/tropical storm could develop in the western Caribbean off the coast of Nicaragua in the next 10 days or so. It’s far too early to be certain of this, but the trend is worth noting. Recent runs suggest that this storm could make landfall on Florida and then travel up the east coast.
