Saturday, May 18, 2019

Potentially Significant Severe Weather Event Monday

A potentially significant severe weather event is being highlighted by the Storm Prediction Center (SPC) for Monday, with further threats emerging on Tuesday. Click on any image to enlarge. Check out the post on this weekend's severe threat by clicking here.

Monday, May 20th
The Storm Prediction Center's categorical (left) and probabilistic (right) outlook for severe weather on Monday.
Source: Storm Prediction Center
The Storm Prediction Center has issued a Moderate Risk (level 4 out of 5) of severe weather for the western half of Oklahoma (not including the panhandle), north-central Texas into the Texas panhandle, and extreme south-central Kansas for Monday, with an Enhanced Risk (level 3 out of 5) surrounding that area through much of west Texas, the majority of Oklahoma and southern Kansas. On a probabilistic level, viewing the probability of severe weather within 25 miles of any given point, the SPC has placed a 45% chance of severe weather in the same areas first mentioned, thereby green-lighting the issuance of a Moderate Risk. Across both the Moderate and Enhanced risk areas, as well as a small part of the Slight risk region in western Texas and the Oklahoma panhandle, a hatched area is outlined, which delineates a 10% or greater probability of significant severe weather within 25 miles of any given point.

The threat for Monday looks to be twofold over the course of the day, with the first threat appearing early in the morning.

Forecasted 250 millibar wind speeds and heights at 4am central Monday.
Source: TwisterData
By 4 am central time on Monday, weather models expect a strong upper-level low to move into the Southwest, heading up a robust extension of the Pacific jet stream that will include a jet streak exceeding 140 knots rounding the base of the low. It is this jet streak that may help fire off a substantial severe weather event later in the day. For the time being, though, notable divergence in the southern Plains should provide for an early-morning threat of strong to severe storms.

Forecasted 250 millibar wind speeds and heights at 7am central Monday.
Source: TwisterData
A little later in the morning, model guidance sees a reservoir of moisture readily available across the primary risk area, with dewpoints in excess of 60 degrees across Oklahoma and Texas. Further, a dryline is already evident in extreme western Texas and eastern New Mexico, which will provide the focus for thunderstorm development later on in the day.

Forecasted radar reflectivity at 6am central Monday.
Source: College of DuPage
By the time the early morning commute on Monday is beginning, strong to severe thunderstorms are projected to be firing off the northern part of the dryline in the Texas and Oklahoma panhandles into southwest Kansas. Some scattered development is possible further south, but is not expected to be as widespread as areas further north by Amarillo. With limited instability and a material inversion expected to be in place at this point in time, thunderstorms are not expected to be significant, though with the upper air dynamics forecasted to be in place, severe weather cannot be ruled out.


By the afternoon hours, the potential for a significant severe weather event becomes more tangible.

Forecasted 250 millibar wind speeds and heights at 7pm central Monday.
Source: TwisterData
As briefly noted at the start of this post, the jet streak that had been rounding the base of the low in the morning is expected to curve around to the east side of the trough by the evening, seemingly in an effort to maximize the severe weather threat. Once again, upper-level divergence is forecasted to be present across Texas and Oklahoma, encouraging the development of thunderstorms in those areas.

Further, note that the positioning of the trough has moved from looking as if it's tilting towards the bottom-left of the picture in the morning to now almost pointing straight down. This portrays a maturing and strengthening trough, an additional factor suggesting substantial severe weather is a possibility.

Forecasted instability (CAPE) at 7pm central Monday.
Source: TwisterData
Also in the evening, instability is expected to have both moved northward to the Oklahoma/Kansas border and increased quite a bit throughout the day. Indeed, convective available potential energy (CAPE) on the order of 3,000 to 4,000+ joules per kilogram (j/kg) of CAPE is forecasted across western Texas and much of Oklahoma, even into western and southern Arkansas. For reference, a general rule of thumb holds that severe thunderstorms can function in only 2,000 j/kg of CAPE. The area will also be uncapped by the evening hours, removing any barriers to potentially explosive thunderstorm formation.

This lack of a cap also presents a potential fly in the ointment for severe weather potential, however. Model guidance projects the cap to be eroded by 1pm, and instability building to more than 2,000 j/kg by that same time. This presents an opportunity for storms to fire earlier in the day, potentially corrupting a more substantial severe threat that could evolve in the evening hours. I find such a development possible, particularly if the early morning storms are able to lay down outflow boundaries that could then provide a lifting mechanism for those potential mid-day storms. Such an outcome must be monitored closely, as it could materially change the forecast.

Forecasted surface-500 millibar wind shear at 4pm central Monday.
Source: TwisterData
If thunderstorms are still able to fire in the late afternoon and evening hours, the atmosphere appears primed for at least a decent severe weather event. Combined with abundant moisture, instability and a  lifting mechanism by way of the dryline and any possible outflow boundaries from earlier storms, wind shear is expected to be sufficient for the development of rotating thunderstorms in western Texas, much of Oklahoma and especially in Kansas, which will be in close proximity to the surface low in eastern Colorado.

More details will need to be ironed out in coming model runs today and tomorrow, but at this point in time it appears Monday evening will hold the potential for a significant severe weather outbreak in portions of Texas, Oklahoma and Kansas.

Tuesday, May 21st
Forecasted severe weather threat for Tuesday, May 21st.
Source: Storm Prediction Center
On Tuesday, the severe weather threat is expected to shift to the north and east, seemingly to a less-intense degree than Monday. The progression of the trough to the east will bring Missouri, much of Arkansas, southwest Illinois, extreme southeast Iowa and northeast Texas under the gun for potential severe weather. At this time, a significant severe weather event is not anticipated, but this may change depending on the evolution of Monday's severe weather episode.

A potentially significant severe weather event is forecasted for Monday across portions of the southern Plains, with all modes of severe weather (including tornadoes, which could be strong) possible. The threat area will then shift eastward for Tuesday and likely decrease in intensity relative to the day before.


Multi-Day Severe Weather Outbreak Continues This Weekend

A multi-day severe weather event that began at the end of this last workweek will continue through the weekend and into the start of next workweek. This post will address the Saturday and Sunday threats, with a forthcoming post addressing the Monday and Tuesday threats. Click on any image to enlarge it.

Today: Saturday, May 18
Severe weather outlook for Saturday, May 18.
Source: Storm Prediction Center
For today (Saturday), the Storm Prediction Center has outlined a rather large area of severe weather risk, spread from the Gulf Coast across Texas and Louisiana up through the Plains and Midwest, tapering off in the Great Lakes and Upper Midwest regions. Out of five levels of severe weather risk, the SPC has assigned an Enhanced Risk (level 3/5) of severe weather to northeast Texas, northwest Louisiana, much of Arkansas and southeast Oklahoma. This means that thunderstorms (not necessarily severe) should be expected, and severe thunderstorms are relatively likely, with some potentially posing a threat for significant severe weather (i.e. very large hail, significant damaging winds, and even tornadoes).

Latest surface analysis as of 10:30am central time on May 18.
Source: Weather Prediction Center
A look at frontal positions and surface observations as of this typing reveals a low pressure system in southeastern South Dakota, with a strength of 998 millibars. As a general rule of thumb, low pressure systems below 1000 millibars are considered relatively strong, and this is no exception. A stationary front is seen draped across the Midwest, a boundary which has been the focus for thunderstorm development over the last couple of days already. To the south, another frontal boundary is identified through Nebraska, Kansas and Oklahoma, which is then attached to a second low pressure system in the Texas Panhandle.

We can glean a few focal points for today's severe weather from this chart. First, the Weather Prediction Center (WPC) has identified an outflow boundary positioned in the northern half of Texas, west of Dallas. You may notice how this lines up well with the Enhanced risk of severe weather, and this is no mistake. It seems plausible that, with the movement of the dryline eastward, there will be more than enough forcing to ignite thunderstorms, particularly in the absence of a substantial cap. Indeed, this has proven to be the case, with strong to severe thunderstorms ongoing along that corridor in Texas.

Current (as of this posting) radar view.
Source: College of DuPage
The highest severe weather threat for today appears to rely on the continued eastward progression and consolidation of the aforementioned dryline. Model guidance has this dryline diffusing somewhat during the evening hours over central Texas, which may diminish some of the impetus for severe storms, but with the main event already ongoing this does not seem to be a significant concern.

Additional severe weather is possible north of the Enhanced Risk area, although with a lower reservoir of instability and relatively weaker forcing for storms, the risk is understandably downgraded to Slight for areas in the Plains and Midwest. It should be cautioned that model guidance does have thunderstorms re-developing in northwest Oklahoma by the late afternoon hours, although SPC guidance suggests this will hinge on the development of the atmosphere in the wake of the ongoing storm complex. As such, residents in western Oklahoma and Kansas should keep updated throughout the day as the risk of severe weather may evolve differently than discussed here.

Tomorrow: Sunday, May 19
The threat of severe weather continues into Sunday.
Severe weather outlook for Sunday, May 19.
Source: Storm Prediction Center
The highest level of severe weather highlighted by the Storm Prediction Center for Sunday is a Slight Risk, a level 2 out of 5 on the agency's risk scale. While this is lower than the Enhanced Risk, notable severe weather events have occurred in Slight Risk areas multiple times before. As such, this threat should not be completely brushed aside.

Forecasted precipitable water values for 1pm central, Sunday.
Source: TwisterData
By Sunday afternoon, the system of frontal boundaries and primary low pressure system will move to the east and north, with the low pressure system ending up in the Wisconsin-Michigan vicinity by the afternoon hours. Precipitable water values in excess of 1.0" are expected across the Midwest and Ohio Valley, with a narrow corridor of 1.5"+ values expected ahead of the cold front. For reference, it can begin to feel 'muggy' when precipitable water values exceed 1.0", and it is generally noticeably humid when values exceed 1.5". This suggests not only a muggy day ahead on Sunday for some, but also a readily available area of moisture for thunderstorm development.

Forecasted instability (CAPE) values for 1pm central, Sunday.
Source: TwisterData
At the same time as the above image, a wide swath of elevated instability is seen stretching from southeast Texas and Louisiana north into slivers of western Kentucky, southern Illinois and southeast Missouri. Instability as shown here, defined as Convective Available Potential Energy (CAPE), is generally conducive for severe thunderstorms when exceeding 2,000 joules per kilogram (j/kg).

You may be wondering why there isn't a higher severe weather risk over southern Texas and Louisiana given the high instability forecasted, and that would be not only a very good question but a good way to consider how there are more factors than just instability required for severe thunderstorms to develop. Indeed, in my Severe & Unusual Weather course I took at the University of Oklahoma for my final semester, the teacher identified four necessary ingredients for strong thunderstorms: Instability, a lifting mechanism, moisture, and wind shear.

Looking over model guidance for southern Texas, we certainly have enough instability present, as the above graphic shows. The image above that one shows there will also be enough moisture, with precipitable water values exceeding 1.5". It's the wind shear and lifting mechanism features where the severe risk for southern Texas and Louisiana runs into problems.

Forecasted surface-500 millibar wind shear ("bulk shear") values for 1pm central, Sunday.
Source: TwisterData
There are a number of ways to identify wind shear, and each one has its own merits. For example, there is value in examining wind shear at the lower levels of the troposphere, while there is also value  in checking out wind shear across the majority of the troposphere. For this situation, we'll look at wind shear from the surface to the 500 millibar level. In the above graphic, we see a swath of 30+ knot wind shear stretching from Texas and Oklahoma into the Ohio Valley and Great Lakes regions.

In my opinion, wind shear becomes abundant using the surface-500mb range when exceeding 50 knots, as this should be enough wind shear to separate the updraft from the downdraft and prevent any thunderstorms from popping up, sticking around for less than an hour, and then collapsing because there isn't enough wind shear for the updraft to be tilted away from the downdraft. We don't see this benchmark met in southeast Texas or Louisiana- indeed, values there struggle to exceed 30 knots.This suggests it will be difficult for thunderstorms to sustain themselves and become severe for a prolonged period of time. That's not to say they can't become severe at all, but we would want quite a bit more wind shear to believe that the environment near the Gulf Coast is conducive for severe weather on a larger scale.

Forecasted 500 millibar wind speed & height values for 1pm central, Sunday.
Source: TwisterData
We also don't see a prominent mechanism to force the air at the surface higher and both initiate & sustain severe thunderstorms. The above image shows the 500 millibar geopotential height field, as well as wind speeds. This is the main level of the atmosphere meteorologists use to look for ridges and troughs, areas of relatively-calm and relatively-active weather, respectively. Ridges are identified by seeing the height contours (solid black lines) push northward- a great example of this is seen in the Southeast U.S. into the Atlantic Ocean. Troughs are identified by seeing height contours push southward, usually increasing wind speeds near the base of the trough as well. Great examples of troughs are seen here on the West Coast and Upper Midwest.

As just mentioned, Sunday afternoon will see a trough pushing through the Upper Midwest, driving that low pressure system over the same area we discussed a little earlier. However, note the relative lack of any trough along the Gulf Coast. This absence of an explicit forcing mechanism in that area suggests it may be difficult to get storms to push up and sustain themselves, especially with a lack of wind shear. It is likely that having only two of those four 'main' ingredients for severe weather is reasoning behind the lack of a higher severe weather threat outlined in Texas and Louisiana. The presence of those four factors is also the reason why a higher risk *is* outlined further to the north. The absence of stronger instability and an environment with more moisture are likely driving forces as to why there isn't an Enhanced Risk of severe weather in the Ohio Valley and Midwest regions.

Both today (Saturday) and tomorrow look to present opportunities for severe weather before this low pressure system moves out of the picture and another, potentially more intense severe weather event commences on Monday. A forthcoming post will address this new storm system.


Thursday, January 18, 2018

January 23-27 Potential Winter Storm

This is a brief update to the previous post concerning this possible winter storm, which you can find here.

Tropical Tidbits
Shown above is a near-analysis of the meteorological environment over eastern Asia for the morning of January 17th. At this time, we saw a storm system skirting the eastern coast of Japan, intensifying somewhat as it did so. As I highlighted in my previous post, this will pose a threat for a winter storm in the January 23-27 period.

I maintain that the Midwest/Ohio Valley regions continue to have a better chance of seeing wintry impacts as opposed to the East Coast, but long-range model guidance suggests the Plains may be able to get in to the mix, as shown below.

Tropical Tidbits
This 210-hour forecast from the GFS model has a sub-1000 millibar low pressure system moving north through the Plains, laying down accumulating snow in the northern Plains, with showers and thunderstorms likely occurring in portions of the Midwest down to the Gulf Coast. As is the caveat with any model forecast beyond 84 hours, this is bound to change, and likely will do so drastically. That's why long range model guidance is used more for identifying the *signal* of a storm, not yet the exact location of such a storm. As such, it is a good sign for those hoping for some stormy weather that the GFS is sniffing out such a possibility in this January 23-27 timeframe.

Tropical Tidbits
To exacerbate this point, above is the Canadian CMC's model interpretation of this storm. It predicts the storm will crawl near the Gulf Coast, impact the Southeast and eventually ride up the East Coast. That flies directly in the face of what I said earlier here, but I'm posting it to highlight how these model forecasts will drastically change in coming runs, as is typical. Again, the point is that the model is sniffing out a storm at all- the path will be refined in coming days.

Tropical Tidbits
Just to entertain all crowds, the ECMWF model above shows this storm cutting up through the Ohio Valley, though slightly outside of this timeframe. Details will be sorted out, but model guidance is latching on to the idea of a winter storm around the January 23-27 timeframe.

To Summarize:

- Model guidance is sniffing out the potential for a winter storm in the January 23-27 period, as was highlighted in a previous post from January 10th.
- Significant uncertainties exist with regard to the location of any wintry weather, and such impacts will not be known with any degree of accuracy for at least a few days.
- For now, the Midwest/Ohio Valley/Plains regions look most likely to see impacts, but this comes from the synopsis of the atmosphere back on January 10th and these locations could very well not be affected by this system.


Monday, January 15, 2018

Stratospheric Warming Event Projected To End January

Model guidance continues to indicate that the stratospheric polar vortex will come under further stress as we head toward the end of the month, with a wavenumber-1 event potentially unfolding in the stratosphere and opening the chance for February to be colder than normal.

Shown above is the 240-hour forecast of geopotential heights (contours) and temperatures (colored fill) from the ECMWF model, analyzing the 70-millibar slice of the stratosphere. This is around the middle or lower-middle part of the stratosphere, and as a rule of thumb stratospheric warming events tend to be stronger and more able to dislodge the polar vortex when the warming event is seen throughout multiple layers of the stratosphere. While I'm only showing the 70-milliber layer, model guidance has latched on to this warming potential throughout the stratosphere.

In this image, we see the stratospheric polar vortex still pretty much in control, centered almost over the North Pole. The piece to analyze, however, is the swath of warmer air trying to push into the Arctic Circle, with the warmest temperatures located over the Bering Sea. The placement of these warmer anomalies is important, because when stratospheric warming events push into the Arctic Circle from the Bering Sea region, they tend to be more successful at disrupting the polar vortex. I'm sure there's some academic literature on exactly why this happens, but in the absence of reading such literature I take that as a rule of thumb.

We can get a better look at the polar vortex through forecasted potential vorticity charts.

In a nutshell, these charts show the orientation of the polar vortex at different levels of the stratosphere. This map shows potential vorticity values at the 550 K level, which is somewhere around the 20-millibar level. Warmer colors depict areas of higher potential vorticity - in this case, the location of the polar vortex. Colder colors indicate where the polar vortex is not, in essence. At hour 240, we see the polar vortex in a rather elongated shape, dislodged from the Arctic Circle and positioned from Greenland across northern Eurasia. This shape of the polar vortex signals that it will be under duress at this point in time, as the polar vortex is 'typically' seen in a more circular shape. This elongated shape suggests pressure being applied from the Bering Sea area in the form of a ridge, which is confirmed by the deeper blue area stretching from the Gulf of Alaska to the Bering Sea. Similar to how higher levels of potential vorticity indicate the polar vortex, lower values (especially in this case) can be indicative of a ridge of high pressure.

So far, we've gathered that model guidance projects the stratospheric polar vortex to come under substantial pressure in the next several days, likely continuing through the end of January. This pressure looks to be applied across much of the stratosphere, boosting the chances that any disruption in the polar vortex could then show up in the troposphere in the form of colder than normal weather. Let's sum it all up with a few graphs.

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels, as we discussed with the rule of thumb earlier in this post. However, it can still be valuable to look at, as a reversal in wind direction at the 1-millibar level could then filter down to lower levels. In this forecast image, the 1-millibar wind speed is forecast to continue declining slightly from a recent peak, before strengthening again and then once more weakening. This would take it back to roughly the same level we're at right now, if this forecast verified exactly as shown. Let's see if any signals can be identified in other panels.

The second panel shows the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where both slices of the stratosphere look to see the strength of the polar vortex gradually decline as wind speeds slow down. While the deceleration here appears modest, there are signs that it will continue through the end of January. Both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, which will lead to a slight deceleration across the 1-millibar, 10-millibar and 30-millibar areas. This will then revert back to near-zero, but at the end of the forecast period the ECMWF sees these fluxes strengthening again, leading to further wind speed deceleration in the aforementioned levels of the stratosphere.  Perhaps most significantly, for this second acceleration in the two fluxes, you'll note how the EP-flux on the bottom panel is pointing straight up, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is also forecasted for the end of the forecast period). As such, while the ongoing increase in fluxes may not do much damage to the polar vortex, the re-configuration of the EP-flux in time for the second projected warming event in about ten days could do far more damage, especially if it is a sustained warming event.

As such, I expect a broadly seasonal to warmer-than-normal temperature pattern for the eastern 2/3 of the U.S. through the end of January, but the chances of colder-than-normal weather increasing, particularly after the first few days of February.

To Summarize:

- Model guidance continues to indicate a potentially-disruptive warming event in the stratosphere towards the end of January.
- As this warming event is expected to affect nearly all sections of the stratosphere, effects in the troposphere are likely in the first and second full weeks of February.
- Colder than normal conditions are expected to increase in likelihood after the first few days of February.


Wednesday, January 10, 2018

January 12-13 Significant Ohio Valley Winter Storm

A significant winter storm impacting the Ohio Valley is expected to lay down accumulating snowfall, potentially in excess in 12", as well as potentially crippling ice accretion.

This post will cover current model guidance, which appears to be forming a consensus as of the January 10 00z model suite.


Pivotal Weather - GFS
We begin with the GFS model's outlook for snow accumulation. The 12z GFS appears to anticipate a stripe of accumulating snow falling from western Tennessee and Kentucky into southern Indiana, much of Ohio and portions of West Virginia. Particularly heavy accumulations are forecasted for northwestern Pennsylvania and western New York, especially near the Great Lakes. Per this model forecast, accumulations look to be in the 2-4" range in western Tennessee and Kentucky, increasing to a broad 3-6" range for Indiana, especially in the southeast portion of the state. Ohio may receive anywhere from 6-12" of snow, with the higher end of this range most likely in the northeast part of the state.

Pivotal Weather - NAM
Next is the NAM's snowfall forecast. As is typical for this model, snowfall accumulations are enhanced, likely beyond what will actually occur. Western Tennessee and Kentucky are projected to receive 4-8" of snow in this solution, a notable bump from the GFS' guidance. Southeast Indiana and a good swath of Ohio are outlooked for a 12-18" snowstorm as per the NAM model, again a substantial increase from the GFS solution. I do not expect the NAM's solution to work out, partially because of its inherent bias of over-doing snowfall accumulations but also the possibility (if not the likelihood) that at least a portion of this projected snow is actually sleet for some areas. This sleet-forecasted-as-snow problem is present in all models for the winter season (and can vary depending on the website you use to get snowfall accumulation graphics), but is particularly maximized in the NAM model. As such, I would take the NAM snowfall graphic with a grain of salt at this point and refer to the GFS and CMC models, the latter of which is shown below.

Pivotal Weather - CMC
The CMC model seems to take a middle ground between the GFS and NAM in a broader sense. On one hand, the CMC anticipates a similar snowfall accumulation story for Indiana/Ohio/Pennsylvania/New York as the GFS, but also sides with the NAM in the idea that snowfall accumulations may be heavier in Tennessee and Kentucky. Adding to the variance is how the CMC's accumulations for Kentucky and Tennessee appear to be slightly west of other guidance. I personally would side with the GFS for accumulations in Kentucky and Tennessee - there will be a very warm air mass in place as this storm gains strength, and I'm not confident in the storm being able to cool off the temperature profiles enough to lay down over half a foot of snow in those areas. I do believe a broad 8-14" range of snow for southeast Indiana, much of Ohio, northwest Pennsylvania and western New York is preferable for now, with higher amounts likely in places nearer to the Great Lakes for Pennsylvania and New York.

Freezing Rain

Pivotal Weather - NAM

Pivotal Weather - GFS
I don't want to discuss freezing rain too much in large part because freezing rain is often a "nowcast" situation, or can only be accurately forecasted immediately prior to the event. I will, however, show the NAM and GFS forecasts for freezing rain to give an idea of where accumulating ice is most likely from this storm. I want to emphasize that, with the above graphics, you should *not* focus on ice accumulations, but *should* focus on the forecasted placement of ice accretion.

The NAM and GFS agree that an ice storm is possible for western portions of Tennessee and Kentucky into southeastern Indiana, as well as south and east Ohio. Further ice accretion is also a possibility for central Pennsylvania, as well as parts of central and eastern New York. The remarkable similarity between projected ice accretion and projected snowfall accumulation makes me more suspicious of the snowfall forecasts, particularly for Kentucky, Tennessee and southeast Indiana, as model guidance could be falsely interpreting freezing rain and/or sleet as snow. As I said, this will likely become a "nowcast" situation for that reason.

To Summarize:

- A significant winter storm is expected this weekend in Kentucky, Tennessee, Indiana, Ohio, Pennsylvania and New York.
- Snowfall amounts of 2-6" are possible in Kentucky and Tennessee, while a range of 6-12" is possible for southeast Indiana into western and central Ohio. Northeast Ohio and western New York look to receive 8-14" of snow by the time this event is finished.
- An ice storm does look possible from this system. While I am not confident in forecasting accumulations, it looks as if central Tennessee, western Kentucky, southeast Indiana, central Ohio, central Pennsylvania and central/eastern New York will be at risk for non-negligible ice accretion.
- The location and intensity of freezing rain will most likely only be accurately determined immediately prior to the event, if not determined during the event (i.e. "nowcasting").