Tag Archives: warm sector

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

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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 New Year’s Eve Weather – Dec 31, 2018

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New Year’s Eve this year looks like a washout, but temperatures will be very mild. In fact temperatures will maintain a non-diurnal trend, warming through the night. New Year’s Day could see the possibility of record warmth before a return to more normal conditions mid-week. Another storm system approaches for the end of the week.

Rest of today – cloudy with high temperatures in the upper-40s. A warm front approaching from the south and west will start to bring steady rain to the area by late afternoon. A favorable low-level jet, along with robust isentropic lift (overrunning of warm air above cold air along the warm front) should enable plenty of moisture and enough instability to trigger bouts of heavy rain later in the evening and overnight. Unfortunately, the bulk of rain looks to fall right during periods when revelers will be out on the town celebrating the new year. Winds will pick up as well with the pressure gradient tightening and efficient downward momentum transfer of low-level jet winds during periods of precipitation. As mentioned above, temperatures will follow a non-diurnal trend, with the warm front and warm sector of this storm advecting milder air from the southeast. Temperatures will actually rise into the mid-50s overnight.

NAM (North American Model) forecast of 850 mb relative humidity and winds for 10PM tonight. The blue hues indicate saturated air, while we can see some wind barbs showing winds > 60 knots. These are classic signs of a low-level jet that is often implicated in producing heavy precipitation events.
Weather Prediction Center surface forecast for 7AM Tuesday. Note the tight packing of isobars (black lines) around the parent low that’s going to bring us rain. This indicates a tight pressure gradient that will drive strong winds. This surface forecast also suggests that our time in the warm sector of this low (outlined in light orange above) will be brief. Warm advection (transport of warm air) won’t last long into Tuesday.

Tuesday (New Year’s Day) – I’ll follow up with a detailed forecast, the start of 2019 will be quite interesting for a couple reasons, one being possible record warmth and the other being strong, gusty winds. Temperatures will be rising throughout the night into Tuesday such that high temperatures will peak early in the day. Depending on exactly when this happens, temperatures could push above 60ºF, which would be nearing or exceeding records. The record high for January 1st at Central Park is 62ºF set in 1966. A lot will hinge on whether clouds clear up early enough to allow for a few hours of solid sunshine. This warmth will be short-lived though, because we won’t spend that much time in the warm sector of this parent low. As a trailing cold front moves through, winds will shift to the northwest and result in cold advection. As a result, temperatures will be falling during the day into the upper-40s by sunset. Temperatures are forecast to continue dropping overnight into Wednesday with lows bottoming out around freezing in the city.

Wednesday – a rude awakening to normal temperatures for this time of year. There will be a cold start to the day with high temperatures 20ºF cooler than Tuesday, in the upper-30s. Partly cloudy skies. Overnight lows in the mid-30s.

Thursday – warmer than Wednesday with another storm system approaching. So far, looking at a mostly cloudy day with highs in the low-40s. Rain may start as early as the overnight hours. A lot of uncertainty still at this time with the timing, scope, and precipitation type of this next storm though.

WxChallenge for Omaha, Nebraska (KOMA) | Oct 16-19

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Last Week’s Results

Tough week last week, with the major forecast bust I had on precipitation for Oct 9 for Omaha, plus generally underwhelming performances on temperatures the remainder of the week. Since I did pretty well the on the first city, and am better this week, my cumulative rank hasn’t gone down too much and I’m sitting at #263 nationwide (out of 1500) as of this writing.

Verifications

Below are the verified readings for each metric we were asked to forecast during last week, and a column showing error points I was assessed.

 

Forecast Day Max Temp (°F) Min Temp (°F) Max Sustained Wind (kts) Precipitation (inches) Error Points Assessed
10/09/2018 57 50 16 0.74 22.7
10/10/2018 50 40 23 0.01 7.9
10/11/2018 51 38 19 0.00 8.0
10/12/2018 47 39 10 0.12 5.3

Post-Mortem

Day 1: Major bust across the board. Was off by more 5 degrees or more on both the high and low. This made up about half of the error points. The rest came from being off on max precipitation by more than 1.20”. MOS was considerably off the mark with temperatures – though NAM was closer. This was all due to the fact that a stationary frontal boundary never made it close enough to KOMA to realize warmer temperatures, and heavier precip. The axis of the 850 mb LLJ stayed south and east, along with the best surface convergence. As a result, nearly all of the model guidance was horribly wrong. The closest forecast in the class was still nearly 0.50” too high. I put too much credence into the upwards trends on the models – there were some indications already that KOMA was trending to be on the drier side of this event. Still, even if I had nailed the precipitation forecast, I would have busted hard on the temperatures. Need to be a lot more cautious going forward about situations where a frontal boundary and precipitation are around, and probably hedge down on temperatures during transitional seasons when a forecast site could be on the cold side of a front with rain.

Days 2-4: Much better forecasting overall, especially in regards to precipitation. However, got a lot of error points for being too cold across the board with temperatures. Buying into the colder temperatures in MOS made for an overall forecast that was too cold.

Forecasting for Week 2 at KOMA (Omaha, NE) Oct 16-19

Synoptic Set Up

A much quieter weather week is in store for Omaha. With high pressure building, most of the forecast week will see sunny or clear conditions. Light synoptic flow suggests the possibility for winds decoupling at night, giving rise to highly favorable conditions for strong radiational cooling. Temperatures will be trending up through the week as a upper-level trough moves east and heights continue to build.

Notes

NAM MOS guidance seems to be the preferred alternative with respect to overnight lows, given that forecast soundings show a clear signal for nocturnal inversions. Our class discussion notes that 1.4” of snow fell at KOMA over the weekend. Temperatures are in the mid-40s today, but cold overnight lows may mean that we need to consider patchy snow cover in forecasting overnight lows. Since overnight lows are expected to be quite cold to start the week, daytime highs should also be below normal for this time of year. The NWS official forecast does show a warming trend in overnight lows through the week. Delta Method was suggested as a possible approach, and makes sense with how similar the air masses will be day to day. The trickiest part of the forecast this week will be how cold overnight with conditions conducive to strong radiational cooling, and just how much things can warm up during the day.

Forecasts Submitted

This only reflects finalized forecasts, I’ve made small adjustments to the forecast based on checking MOS daily.

Forecast Day Max Temp (°F) Min Temp (°F) Max Sustained Wind (kts) Precipitation (inches)
10/16/2018 60 33 13 0.00
10/17/2018 58 37 7 0.00
10/18/2018 64 43 15 0.00
10/19/2018 69 49 16 0.00

NYC Weekend Weather – Jun 22, 2018

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We got to enjoy one weekend last week where it didn’t rain at all. Unfortunately that won’t be the case this week with a storm passing through tomorrow. Temperatures will be cool with the influence of an onshore easterly wind before the passage of a warm front that will bring the chance for rain and thunderstorms.

Rest of today – cool, cloudy, with a high in the mid-70s.

Saturday – cloudy, with chances for rain and thunderstorms especially later in the day. The best chances for thunderstorms come with elevated convection late in the day and going into the overnight hours as a warm front approaches from the south. Temperatures will be cool in the mid-70s again.

Sunday – improving conditions with warmer temperatures in the warm sector after the passage of the warm front. Highs should reach into the mid-80s, possibly higher depending how much sun we get.

Monday – Canadian high pressure returns in the wake of this weekend’s storm and gives us sunny weather with mild temperatures in the low-80s.

NYC Weekend Weather – May 4, 2018

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After two days of record-breaking heat in the low-90s, a pattern change is on tap for the NYC metro area going into the weekend. Cooler conditions will prevail in the wake of the passage of a cold front, which could also bring rain and thunderstorms (possibly severe). Saturday should shape up nicely but rain chances return Sunday before a cooler start to the work week.

Rest of today – the Storm Prediction Center has placed portions of the region under a marginal-slight risk of severe thunderstorm activity today. We continue to be an increasingly moist warm sector east of an advancing low pressure center. As the cold front associated with this low pushes east, storms will initiate first along a pre-frontal trough during the day today (late morning-early afternoon), then again along the actual frontal boundary itself later this evening. It is the latter round of these storms that could push severe limits. There are limiting factors to this activity especially near the coast with more stable air present. Cloud cover will limit day time heating, reducing overall instability. And the earlier round of activity should serve to actually mitigate the later round of storms. Aside from the rain, it should be mostly cloudy with some breaks in the sun (that would increase chances for storms), with high temperatures in the mid-80s.


Saturday – cooler air moves in behind the cold front tonight while a weak area of high pressure builds. High temperatures should top out in the mid-70s with partly sunny conditions. Rain chances increase towards the overnight hours.

Sunday – cooler still with mostly cloudy skies and chances for rain. High temperatures only in the mid-60s as a weak disturbance passes to our south.

Monday – skies finally clear up with high temperatures moderating and warming into the upper-60s.

NYC Weekend Weather – Apr 13, 2018

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Despite it being Friday the 13th, if you were looking for some spring weather, this happens to be your lucky day. With our region in the warm sector of a large storm system far off to the south and west (which will bring severe weather to parts of the Mid-Mississippi and lower Missouri Valleys today), warm air will be surging from the south. This will give us a great start to the weekend, though things will shift dramatically by Sunday as a frontal boundary dances back and forth across the area. Wild temperature swings of 30ºF are possible with a much cooler day Sunday.

Rest of today – despite the cloud cover now, temperatures should still reach well into the mid-70s. Any breaks in the clouds would likely result in highs hitting 80ºF. There will also be a steady southerly breeze with gusts up to 20 mph.

Saturday – temperatures remain quite warm with highs again in the mid-70s. The warm front that swept north through the region last night, which brought the warm air along with it, will retreat south of the area as a backdoor cold front late during the day Saturday. Expect temperatures to drop rapidly after this backdoor cold front passes through – between 10-15ºF within as little as an hour. Increasing clouds will accompany this frontal passage.

Sunday – significantly cooler with easterly onshore winds in the wake of the backdoor cold front passage. This onshore flow off the still cold waters of the Atlantic will result in high temperatures only in the upper-40s. Periods of rain will also occur, especially later in the day with mostly cloudy skies otherwise.

Monday – the backdoor cold front will stall just south of us, then push back north as a warm front again. Temperatures will rise back towards around 60ºF prior to the arrival of a stronger cold front. This will be accompanied by steadier rain and even a couple rumbles of thunder are possible.

 

NYC Weather Update – Apr 2, 2018

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I had trouble typing the date in the title of this post, given that there’s snow falling as I write this! Unfortunately, this snow is not a belated April Fool’s joke. Didn’t have time to post an update about this yesterday, sorry about that! I’ll also be out this weekend for weddings. Now on to the weather: look ahead to a week of up and downs in temperatures and active weather, but no more snow. The weekend coming up… well, there could be some chance for snow again Saturday.

Rest of today – snow departs the city by the lunch hour. Around 3″ of total accumulation, mostly on untreated surfaces, before the sun breaks out and starts immediately melting the stuff. High temperatures in the low-40s with decreasing clouds.

Tuesday – rain associated with an approaching warm front. Cloudy, with temperatures in the mid-40s. Overnight temperatures actually don’t go down much before warm air surges as we enter the warm sector of this second storm of the week.

Wednesday – more rain, this time with a chance for a rumble of thunder with decent lifting associated with the cold front that’s part of this storm. Significantly warmer, cloudy, with high temperatures around 60°F.

Thursday – much cooler behind the cold front, temperatures only in the upper-40s with mostly sunny skies.

NYC Weather Update – Jan 29, 2018

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This week starts and ends with some active weather. Colder temperatures that swept in after the passage of a cold front this weekend will be here to stay for the remainder of the week. We get a couple chances to see some snow, but not anything of considerable quantity.

Rest of today – partly sunny with high temperatures in the low-40s. A coastal storm passes by but far enough offshore that we shouldn’t see serious impacts from it.

Tuesday – chance of rain and snow in the overnight hours with some troughs rotating around the back side of this departing coastal low. Could have enough snow to make for a messy morning commute, but we’re not looking at any significant accumulation. Cloudy otherwise with temperatures topping out in the low-30s.

Wednesday – mostly sunny with high temperatures around freezing.

Thursday – increasing clouds with temperatures warming into the mid-40s in the warm sector before the next storm system hits us.

NYC Weather Update – Dec 11, 2017

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We got our first round of snow of the winter season in NYC this past weekend, with snowfall totals generally between 2-4″ from the event. We open this week with another chance for precipitation, but this time more likely in a mixed mode. An arctic blast hits for the mid-week period before yet another storm system brings our next chance of snow and rain Friday.

Rest of today – mostly sunny, cool, with high temperatures slightly below normal around 40°F.

Tuesday – a low pressure system moving east from the Great Lakes will impact the region with a mix of rain and snow. This time around we’ll be on the warm side of the storm, so we’ll likely see mostly rain at the coast. A warm front will pass through overnight. Temperatures will rise into the upper-40s in the warm sector for tomorrow. Precipitation most likely will occur in the morning. Mostly cloudy otherwise.

Wednesday – as this low pressure tracks into the Canadian Maritimes and deepens, a strengthening pressure gradient on its back side will lead to much colder and windier conditions. High temperatures will struggle to reach the freezing mark with partly sunny skies after a very cold night with low temperatures in the low-20s. Stiff west winds 20-25 mph with stronger gusts will make wind chills feel much colder.

Thursday – another cold day with high temperatures only around the freezing mark and partly sunny skies.

NYC Weekend Weather – Nov 3, 2017

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This weekend’s weather will be a tale of contrasting temperatures and airmasses. The week ends with warm temperatures more than 10°F above normal. A cold front sweeps through and fall returns for the weekend itself, but another warm up takes place leading into Monday. Also, Daylight Saving Time ends at 2AM Sunday.

Rest of today – increasing clouds for the day. Warm with high temperatures in the mid-70s. Yesterday, a warm front pushed through, putting us in the warm sector associated with a low pressure now over northeastern Quebec.

Saturday – quite a different feel tomorrow once this cold front below moves through the area. Winds will become onshore from the east, bring much cooler air. High temperatures are expected to top out only in the upper-50s with sunny skies (around normal for this time of year).

Sunday – there’s a chance of showers overnight Saturday into Sunday associated with the next storm system to impact the area. An advancing warm front is expected to bring light, stratiform rain ahead of it. Temperatures will be warmer, in the mid-60s with mostly cloudy skies.

Monday – the familiar pattern of warm front followed by cold front continues Monday. We’ll again be in the warm sector with temperatures peaking at about 70°F. Rain should accompany the passage of this cold front.