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On the March 3, 2019 EF4 Tornado in Lee County, AL

March 10, 2019Outside of NYC, Tornadoesalabama tornadoes, cold front, ef4 tornado, march 3 2019 tornado outbreak, severe thunderstorms, severe weather, spc, storm prediction center, storm relative helicity, tornado, tornado outbreak, tornadoes, warm front, warm sectorLei Zhao

Last Sunday, while I was preparing my post on the snowstorm that was about to hit NYC and the Northeast, the southern side of this same storm system was starting to produce a serious severe weather event in portions of the Deep South. A large, violent, and ultimately deadly EF4 tornado hit parts of Lee County, AL during the afternoon. The tragic toll of 23 confirmed fatalities due to this tornado was more than double the total deaths due to tornadoes in all of 2018. This was also the deadliest single tornado since the EF5 tornado that hit Moore, OK on May 20, 2013. In this post, I’ll share some thoughts and observations about the meteorology behind this event, and about what made this tornado so powerful.

Storm Prediction Center’s Forecasts

One aspect of the event that impressed me was the prescient, geographically accurate, and timely Mesoscale Discussions and convective outlooks that the Storm Prediction Center issued during the course of the day. The SPC already had a handle on the risk for severe weather in parts of the Deep South as evidenced by the convective outlooks they issued Sunday morning.

Storm Prediction Center’s Day 1 Convective Outlook issued at 7:52AM Sunday Mar 3, 2019. The black dot within the orange “ENH” enhanced risk area shows the approximate location of Lee County, AL
Storm Prediction Center similarly recognized the risk for tornadoes on this day. The black dot indicates the location of Lee County, AL. within the hatched yellow area representing a 10% or greater probability of EF2 – EF5 tornadoes within 25 miles of a point risk area

Regarding the enhanced risk area that the SPC identified as possibly being affected by tornadoes:

The most favorable … space for tornadic potential … still appears to be within the enhanced-risk area, where strong deep shear, large low-level hodographs, and at least low-end surface-based buoyancy will juxtapose. Forecast soundings show rapid prefrontal destabilization …. [a]s that occurs, severe potential will steadily ramp up…. a few tornadoes also are possible. Tornado-event density, and risk of significant tornadoes, still is somewhat unclear — being strongly dependent on existence/number of preceding supercells that can develop…
Storm prediction center, day 1 convective outlook issued 7:52AM EDT Mar 3, 2019

SPC foresaw that the energy (instability) and spin (shear, imparted by strong winds at different levels of the atmosphere) required for strong tornadoes would have a chance to come together in the enhanced risk area. They also identified that the greatest risk would be with any supercells that could form ahead of the main line of thunderstorms that would accompany the cold front later on.

Weather Prediction Center’s surface analysis valid for 1PM EDT Mar 3, 2019, an hour or so before the EF4 tornado struck Lee County, AL

As it turned out, supercells did form ahead of the cold front – one in particular drew the attention of astute SPC forecasters, and this would end up being the supercell responsible for the tornado that hit Lee County. In follow up Mesoscale Discussions regarding the tornado watches over the enhanced risk area, SPC forecasters were remarkably accurate and timely in identifying the risks associated with this supercell and the favorable conditions it would encounter.

Lee County is circled in green in SPC’s Mesoscale Discussion #0145 surface map.

MCD #0145 was issued at 1PM CDT (local time), and contained the following text. The forecasters cited favorable conditions for a strong tornado to form within 30-60 minutes. Just around 2PM, about 60 minutes after this MCD was issued, the EF4 tornado hit Lee County.

A mature supercell located near Montgomery is favorably located within a region of maximized surface pressure falls (3-4mb per 2 hours) immediately east/southeast of the surface low. KMXX VAD shows 500 m2/s2 0-1km SRH when accounting for the observed Montgomery County supercell’s storm motion. Given the ample buoyancy and intense shear profile in place, it appears tornadogenesis will likely occur within the next 30-60 minutes with the possibility of a strong tornado occurring.
Storm prediction center Mesoscale discussion #0145, issued 1PM CDT mar 3, 2019

Why Conditions Were So Favorable for a Strong Tornado

The following analysis about the mesoscale conditions that favored strong tornadoes on this day came about from a discussion I had with Steve Corfidi, my instructor for the class I took on mesoscale forecasting (severe weather forecasting) as part of Penn State’s Undergraduate Certificate in Weather Forecasting. Steve Corfidi also used to be the Lead Forecaster at the SPC. Suffice to say, I am quite privileged to have been able to glean some insights about this storm from him. These observations are related to another MCD from SPC that day, MCD #0147.

Storm Prediction Center’s MCD #0147, issued at 2:06PM CDT, right around the time the EF4 Lee County Tornado was on the ground. Lee County is circled in green.

In this MCD, the SPC highlights an area of localized surface pressure falls in dashed blue. Steve Corfidi commented this effect is related to “rise and fall pressure “waves” that move across the earth twice-daily in response to solar heating”. As the earth heats up, air warms and rises, and this generates a thermal low since there’s less air over a warmed up spot of the earth than surrounding areas. In this case, this resulted in a localized area of surface pressure falls over the area circled in dashed blue as the day progressed. In response, surface winds will have a tendency to deflect towards the center of the lowering pressure. You can see this by looking at the wind barbs in the chart above: those that are closer to the cold front are more southwesterly, but the ones closer to the blue dashed area are actually more southerly, since they are deflecting towards the north and the localized pressure falls. This is known as the isallobaric effect. This had direct impacts on the favorability of the environment for tornadoes, as Steve Corfidi helped me understand.

My illustration of the situation over Lee County during this tornado.

As winds near the localized pressure falls became more southerly in response to isallobaric effect, this actually increased the vertical wind shear values in the area of the pressure falls (green here, blue dashed area in the SPC analysis, the red 300 mb wind profile barbs are approximated from this sounding). Since vertical wind shear is measured by looking at both the difference in direction and speed of winds at different levels, a change in wind direction at the surface, all else being equal, will result in higher wind shear. Relative to other areas in the warm sector of this storm, this produced an even higher value of storm relative helicity (SRH, as alluded to in MCD #0145) as well as the aforementioned vertical wind shear. I don’t have space to elaborate on why SRH and vertical wind shear are important for tornadoes, I will say that it has to do with enhancing storm rotation, and tornadoes are intense, vertical circulations of rotating air.

One other observation worth mentioning is that the “geometry” of the warm sector maximized the amount of time the supercell could spend in an extremely favorable environment. If you look at the large blue arrow in my illustrated diagram, check out how the approximate mean storm motion was largely parallel to the orientation of the warm front and axis of the maximized surface pressure falls. That meant that as the tornado formed, it was able to keep moving through a favorable environment for much longer than if the storm motion had been more northeasterly, or say southeasterly.

NYC Weather Update – May 15, 2018

May 15, 2018NYC, Rain, Storms, Thunderstorms, Tornadoes, Weather Updatecold front, hail, new york city weather, nyc weather, nyc wx, severe thunderstorms, storm prediction center, thunderstorms, tornadoes, tstormsLei Zhao

The story of this week will be the continuation of an unsettled weather pattern from this past weekend. Today, we will see a chance for some severe thunderstorms, and more rain chances lurk for us during the rest of this week, even extending into the weekend. Temperatures during this time will vary between normal and below normal depending on the extent of cloud cover and rain.

Rest of today – warm, humid, with a high in the mid-80s. Chances for severe thunderstorms increasing late in the afternoon. The Storm Prediction Center has placed our area in an enhanced risk for severe thunderstorms, with primary threats being damaging “hurricane force” wind gusts, as well as heavy rain. Some large hail and an isolated tornado are not out of the question. High resolution rapid refresh model simulated radar suggests the best timing of when the storms hit NYC will be around 6PM during the evening rush hour. This will make for a messy road home.

As you can feel already, the air is quite saturated (fog earlier this morning confirms this), this moisture-laden air will be one ingredient fueling storm growth later especially as temperatures warm with cloud-free skies allowing solar heating to quickly destabilize this air mass. At the surface, a cold front slowly approaching from the north and west will serve as the trigger that provides the lift necessary for storms to initiate late this afternoon. Aloft, the presence of a mid-level shortwave trough upstream of us will help prime the atmosphere, increasing lapse rates and instability aloft, while providing some additional divergence and lifting for storms. Lastly, late in the day, we will be in the right entrance region of a 300 mb jet streak, another favorable ingredient for divergence and synoptic lift. Winds at all levels above the surface will be fairly strong, producing a shear profile that favors strong winds. However, any discrete storms that form ahead of the main squall line could contain strong rotating updrafts favoring large hail and tornadoes. As usually, best chances for severe weather exist north and west of the city, away from the stabilizing influence of the marine layer near the coast. Storms will likely weaken as they hit NYC precisely due to this.

Wednesday – will be significantly cooler behind the cold front with high temperatures only in the mid-60s. Rain chances continue as the frontal boundary stalls and becomes stationary just to our south, in a similar arrangement to what took place over the weekend. This pattern looks to lock in for the rest of the week, bringing multiple chances for rain with it.

Thursday – a drier day mostly, with some slight chance for showers in the morning, then mostly cloudy with high temperatures rebounding into the upper-70s. Rain moves back in again overnight going into Friday.

Friday – cooler again with temperatures dipping into the mid-60s. Mostly cloudy with more chances for rain during the day as the stationary front mentioned above pushes back north as a warm front.

NYC Weather Update – May 31, 2015

May 31, 2015Climate, NYC, Rain, Severe Weather, Thunderstorms, Weather Updatecanadian maritime, cold front, new york city weather, nyc weather, nyc wx, spc, storm prediction center, thunderstorms, tstorms, weatherLei Zhao

We start the week off with some much needed rain and a noticeable cool down in temperatures. Scattered thunderstorms increase in coverage later today and transition into more widespread showers that are expected to persist into the overnight hours. The start of the week will be rainy, cloudy, and cool under the influence of a Canadian maritime airmass. By midweek, we get back into more fair conditions with the sun returning and temperatures back in the mid-70s.

Rest of today – the Storm Prediction Center has placed areas of Northeastern New Jersey in a slight risk area for severe thunderstorms later. Based on latest radar trends and the High Resolution Rapid Refresh model’s output, it would appear that thunderstorms are likely to develop over portions of New Jersey highlighted in the slight risk area, then track eastwards into the NYC metro area later this afternoon into the early evening hours. With breaks in the clouds for sun, we’ll hit temperatures in the low-80s, which when coupled with ample moisture, should support thunderstorms with the potential for heavy rain.

High Resolution Rapid Refresh model output for 8PM EST

Monday – all the activity occurring today and going into the overnight hours is associated with a cold front that is moving south into our area. As this cold front pushes through, east-northeast winds will set up, allowing a cooler, Canadian maritime airmass in. This will translate into cooler than average temperatures only in the mid-60s with showers likely throughout the day.

Tuesday – wet, cool weather continues Tuesday with highs again only in the mid-60s and the ongoing chance for rain showers throughout the day. This extended period of rain from later today into Tuesday should bring some relief to the moderate drought conditions over parts of the Northeast, with the potential for as much as 4″ of rain over the period in certain parts of the region.

Possible total rainfall over the next 48 hours

Wednesday – high pressure finally builds back in Wednesday, pushing away the rainy weather, and yielding pleasant temperatures in the low-mid 70s under partly cloudy skies.

Thursday – slightly warmer than Wednesday, with highs in the upper-70s in NYC.

NYC Weather Update – Jul 14, 2014

July 14, 2014Flooding, NYC, Rain, Storms, Thunderstorms, Weather Updateflash flooding, flooding, nyc weather, nyc wx, severe thunderstorms, severe wx, storm prediction center, weather, wxLei Zhao

The Storm Prediction Center has placed the entire NYC metro area in an area for a slight risk of severe weather today. A strong low pressure center over the Great Lakes and its associated cold front are slowly making their way towards the Eastern Seaboard. Despite the cloudiness today, forecasters are still fairly confident that atmospheric conditions will be favorable for the formation of severe thunderstorms in the region. Based on current radar trends, these storms would most likely not be arriving in the NYC area until later this afternoon. The primary threat to our area will be damaging winds and torrential rain capable of triggering flash floods.

Tuesday will feature a similar set up for weather over the region, though there is less risk for severe weather tomorrow. However, strong storms are possible throughout the day and the risk of flash flooding is a distinct possibility. High temperatures will be in the mid 80s despite cloud cover, due to the influence of warm, humid air rushing in between the cold front and high pressure offshore over the Atlantic. This same warm, humid air mass is what will feed the storms for today and tomorrow. As you see below, the forecast rainfall total for today and tomorrow is as high as 3.6″ over Northeastern New Jersey, and ranges between 2.5-3″ for most parts of the Tri-State area.

Wednesday – we get a reprieve from the stormy weather as the cold front and storm system finally move east of NYC. Expect a cloudy start to the day with gradual clearing and conditions improving markedly. The next weather story for Wednesday through the end of the week is the cool, polar air mass that will be a dominant feature. High pressure will build in behind this storm system and usher in much cooler temperatures, with highs in the low 80s Wednesday-Friday, and overnight lows in the low-mid 60s in the city, and in the 50s north and west!

Thursday & Friday – expecting these two days to be essentially the same weather-wise, with high temperatures in the low 80s in the city (and upper 70s further north). Sunny to mostly clear skies for both days.

NYC Afternoon Weather Update – Jul 9, 2014

July 9, 2014NYC, Severe Weather, Thunderstorms, Weather Updatenyc weather, nyc wx, severe weather, severe wx, spc, storm prediction center, thunderstorms, weather, wxLei Zhao

The Storm Prediction Center has again placed western and interior portions of the NYC region under a slight risk for severe weather today. The atmospheric setup this afternoon is similar to yesterday, with plenty of solar heating and humid air leading to moderate instability. At this hour, CAPE values (a measure of instability) are already hitting 1500-2500 J/Kg across the area, with greater instability further south.

There are already a few scattered thunderstorms forming over Eastern Pennsylvania. These storms are not yet hitting severe limits, but conditions are favorable for further development and increase in areal coverage. With winds aloft mostly unidirectional from the southwest, damaging wind gusts would be the primary threat. Storms that form to the southwest of NYC near the Philadelphia metro area and Central New Jersey would be the ones to watch for immediate impacts in the city.

These storms are still several hours away, and it remains to be seen whether they will hold up upon encountering the cooler and stabilizing influence of marine air infiltrating along south-facing shores with the southwest winds and a sea breeze. Last night, the line of severe thunderstorms weakened just before it hit NYC, sparing us from some wicked weather for the night. The same case may unfold again today, but the situation bears monitoring.

NYC Weather Update – Jun 25, 2014

June 25, 2014NYC, Thunderstorms, Weather Update, Weekendnws, nyc, nyc wx, spc, storm prediction center, thunderstorms, weat, weather, weekend weather, wxLei Zhao

A warm and humid day out there in the Tri-State this afternoon. You can feel it’s potential thunderstorm weather. In fact, the Storm Prediction Center has issued a mesoscale discussion (analysis of weather conditions conducive to severe weather on a medium scale – 10km-500km). Very moist air, warm temperatures, combined with a slowly approaching cold front set the stage this afternoon for possible development of strong to marginally severe thunderstorms. This is especially the case over inland areas, as depicted below. In general, expect a line of showers and maybe a couple thunderstorms to pass through the area later this evening into the overnight areas when the cold front finally pushes through.

Screen Shot 2014-06-25 at 2.08.45 PM — Storm Prediction Center Mesoscale Discussion http://www.spc.noaa.gov/products/md/md1147.html

Thursday – there could be some lingering showers in the AM, but things will dry out quickly and clouds will slowly give way to sunnier skies. High temperatures will be a bit cooler than today, in the upper 70s to low 80s.

Friday – a splendid end to the work week with mostly sunny skies and comfortable, less humid conditions and temperatures in the low 80s.

Canadian high pressure will move slowly to our east, ushering in some warm air and calm skies

Saturday and Sunday will both again feature excellent weather as a high pressure system works its way over our region following this cold front. As the high pressure slowly slides to the southeast, it will bring us southerly winds that will usher in another period of warm and humid weather. Saturday will be the better day of the weekend with high temperatures in the low-mid 80s and lower humidity. By Sunday, temperatures will begin ramping up into the mid-80s with an increase in humidity. Early next week, we’ll be looking at a continuing warming trend with high temperatures in the mid-upper 80s Monday and near 90 Tuesday before our next chance of rain.

NYC Weather Update – May 27, 2014

May 27, 2014NYC, Rain, Storms, Thunderstorms, Weather Update, Weekendbackdoor front, cold front, noaa, nws, nyc, saturday, severe thunderstorms, spc, storm prediction center, thunderstorms, tornadoes, weather, weekend, wxLei Zhao

What fantastic weather for the Memorial Day weekend! We topped off in the mid-80s across the area yesterday, and we’re likely to see another day with temperatures in the mid-upper 80s across the region (esp in interior portions of New Jersey).

Sadly, our stretch of summer-like temperatures is about to come to an end as a backdoor cold front (a front that does not approach from the west like most fronts do over this area) approaches the area from the north. Ahead of this cold front, temperatures are in the mid-80s and there is robust moisture in the atmosphere. This will provide the right ingredients for the possibility of severe thunderstorms later this afternoon and evening.

Possibility of Severe Thunderstorms This Afternoon

Current thinking is that these storms approach the NYC metro around 4-7PM during the evening rush hour. The Storm Prediction Center has placed NYC, western Long Island, Northern New Jersey, and much of the Hudson Valley under a slight risk area for severe weather this afternoon. By the way, slight risk in this context does not indicate that storms which occur will not be that severe, rather, it is denoting that the probability for severe storms is on the low side, and that coverage is not expected to be widespread. Just so you all are clear, the National Weather Service defines a severe thunderstorm as producing one or more of the following:

Wind gusts greater than 58mph
Hail greater than 3/4″ in diameter
A funnel cloud or tornado

A severe thunderstorm or tornado watch means that conditions are favorable over an area for the development of these types of weather hazards, however, no immediate threat has yet been identified. A warning means that a severe thunderstorm or tornado is imminent or already ongoing, which is understandably a much more dangerous situation. Anyway, the most likely threat for our immediate area are damaging wind gusts and hail, although a few isolated tornadic supercells cannot be ruled out.

Storm Prediction Center Day 1 Convective Outlook – we are in the slight risk area highlighted in yellow

Wednesday – by tomorrow, the cold front responsible for the possible thunderstorms later today will have moved through our area, however, there is still a chance for lingering showers on the backside of this frontal boundary. Also, since this is a backdoor cold front, temperatures will be significantly cooler tomorrow than today with an onshore easterly wind keeping high temperatures only in the mid 60s (about 20 degrees cooler than today).

Thursday – an area of high pressure will build into the area, but due to its position to our northeast, it will also continue to usher in onshore easterly/northeasterly winds that will keep our high temperatures below normal in the upper 60s. Otherwise, it will be a nice, sunny day.

Friday – another cold front will approach the region from the west and bring in the possibility for afternoon showers and thunderstorms. Slightly warmer with cloudy conditions and a high temperature near 70.

Saturday – high pressure will once again build in across the region and remain in place through the weekend. Therefore, am expecting pleasant, sunny conditions with about average temperatures in the mid-70s.

About that meteor shower… so as it turned out, the Camelopardalis meteor showers were a dud. Astronomers may have miscalculated the timing and/or amount of debris accumulated. Hey, it was the first time this particular shower was supposed to go down, so I guess you can’t blame them for not getting it right.

LZ | Weather

Weather updates & posts for NYC region