Friday’s Severe Threat Has Increased

Before sunrise this morning, the Storm Prediction Center pushed much of the Tri-State into an enhanced risk for severe storms Friday:


This is an upgrade to the risk from yesterday when the enhanced risk only came into Fayette, Union, and Franklin County of southeastern Indiana. The risk is lower in the southeastern Tri-State.

A line of rain and storms will likely sweep into and through the Tri-State Friday night with discrete, individual cells (potential troublemakers favoring areas along/west/north of I-71 and along/west of I-75).

Much of this bump up in the risk is driven by the tornado threat. SPC’s Short Range Ensemble Forecast “Tornado Ingredients” composite (a mush-mash of tornado parameters with varying weights) has highlighted the Tri-State at 11pm Friday:


Note that 11pm Friday is not the specific time severe weather is expected in the Tri-State; there is a window, and this time is near the center of that window.

I talked about the SHERB parameter earlier this week in a blog post earlier this week in a blog post; as a review, it has skill in highlighting severe weather areas where there is limited instability and high wind shear (a similar environment to what we have Friday). SHERB values of 1+ signal and elevated threat for severe storms in this mode. Here’s what this morning’s NAM model has for SHERB at 7pm Friday:


Those values are higher and positioned more to the east than runs earlier this week. Does this morning’s NAM model agree?


It does, and these values are not extreme, but high. This suggests confidence is rising in strong or severe storms late Friday.

I could show you additional parameters and signals, but ultimately, you need to know the threats and timing. My latest thinking is below:


The damaging straight-line wind threat, large hail, and tornado threat have increased since yesterday, but this threat may lower or increase again later today or Friday. Stay alert!

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Friday Severe Threat Update

Early Wednesday morning, the Tri-State was placed in a slight to enhanced risk for severe storms for Friday. This should not be a surprise if you saw my blog post from yesterday. The enhanced risk is northwest of Cincinnati, and the rest of the Tri-State is in the slight risk:


The enhanced risk is the higher risk, and the slight risk is the lower risk.

There will be a main line of rain and storms sweeping through Friday night, but there is also a concern – especially northwest of Cincinnati – for discrete, individual cells or “troublemakers” ahead of the main line late Friday afternoon and early Friday evening. These cells can sometimes be supercellular, but not always. In this case, guidance supports tornadic thunderstorms and supercells over northern and central Indiana, but the threat is more nebulous and conditional in southeastern Indiana.

Temperatures will likely be in the mid 50s early Friday morning:


…and the dewpoint will likely be within a couple of degrees of the temperature. Warm, humid air supports the storm threat. Wednesday morning’s NAM model future radar product suggests there will be stray showers – especially northwest of Cincinnati – early Friday morning:


Temperatures will rise into the low and mid 70s Friday afternoon. This kind of warmth fuels storms, especially in February:


Tuesday morning’s NAM model suggests a line of showers and storms will develop to the west of the Tri-State early Friday afternoon:


This line is likely to intensify during the afternoon as it pushes east:


Let’s stop here, and introduce the supercell parameter. What’s that? It’s an index that shows the support for supercell storms and tornadoes. Higher values of the supercell parameter suggest a higher threat for tornadoes, and the NAM model has these high values in northwestern Indiana at 4pm Friday:


…and a lower, but still significant threat here. The GFS model is not as impressed at 5pm Friday:


…but it, too, has us in a relatively low threat. Here’s what the NAM model thinks the radar will look like at 7pm Friday:


The highest radar echoes are still over Indiana, but the line is marching east. This line will gradually weaken as it moves east, but strong and severe storms are still a possibility Friday evening. My overall thinking on the severe threat late Friday is here:


Damaging straight-line, large hail, tornadoes, and flooding are late Friday’s severe weather threats, in that order. Stay alert and be prepared!


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Let’s Talk About Friday’s Severe Weather Threat

Strong and severe storms on Friday have been a threat in the Ohio Valley since Sunday. That threat is still there, and the Storm Prediction Center has highlighted much of the Ohio Valley and lower Great Lakes in their latest severe weather outlook, which was released before sunrise Tuesday:


What does 15% mean? It shows the risk of a severe weather report within 25 miles of point. 15% is the equivalent of a slight risk. The Tri-State is in this risk.

What time am I expecting storms? It depends on which model you believe. Here’s how much thunderstorm-related precipitation Tuesday morning’s NAM model (I’ll just call it NAM from now on) is predicting between 7am and 1pm Friday:


But Tuesday morning’s GFS model (I’ll just call it GFS from now on) is predicting little if any thunderstorm-related precipitation in the Tri-State from 7am to 1pm Friday:


Notice the GFS develops the line of rain and storms to the west of the Tri-State Friday morning and early Friday afternoon. It actually has it over the Tri-State Friday evening, between approximately 9pm Friday and 2am Saturday.

Clearly, there are timing differences that need to be worked out. For now, let’s not worry about the timing. Instead, let’s focus on the environment at 1pm Friday for all of the graphics below based on Tuesday morning’s NAM and GFS. Cincinnati is the black dot in all of the graphics below.

Where will the upper-level disturbances be? Remember: when it comes to Friday storms, the NAM is the faster model, and the GFS delays the onset of storms until the evening. Here’s where the disturbances will Friday afternoon per the NAM:


The best lift in the atmosphere is on the nose of these disturbances (green, yellow, and red) moving east and northeast. The NAM shows small disturbances over the Ohio Valley (triggering storms) with a bigger disturbance over Wisconsin.

What does the GFS think?


It has a similar idea to the NAM over the Ohio Valley, but it has the large disturbance to the west farther south larger (deeper red).

Wind fields differ for the NAM and GFS models. The NAM has a strong wind (45-60mph) 5,000 feet over the Ohio Valley Friday afternoon:


…but the GFS suggests a stronger wind field:


The red colors are severe wind speeds 5,000 feet above the ground, not at the surface. These strong speeds, however, can be brought down to the ground in heavy rain or strong downdrafts.

A lapse rate is the rate at which temperature decreasing with increasing altitude. High low- and mid-level lapse rates support storms, including strong and severe storms if values are very high. Here are the low-level lapse rates for Friday afternoon from the NAM:


These are high values. Does the GFS agree?


No, it does not. It strengthens lapse rates slightly late Friday but not to the levels of the NAM at 1pm Friday. How about the mid-level lapse rates? We want those high, too, if we’re getting severe storms. Here’s what the NAM thinks:


This are okay, but not high. What does the GFS think?


These are higher than the NAM. Clearly, there is uncertainty in the strength and timing of the highest lapse rates. The lapse rate is important when assessing storm strength.

How about the SHERB parameter? First, what is SHERB? To review from one of my previous blogs:

While instability can often have a big influence on the chance for thunderstorms, it isn’t as important this time of the year. If thunderstorms are likely […], the SHERB parameter or index can be very helpful to a meteorologist in the colder months when looking a threat for severe weather. The SHERB parameter is helpful for getting a handle on a severe weather threat in the colder months because it focuses on temperature changes near the ground, lift in the atmosphere, and wind shear instead of instability (instability tends to be low in the winter even when we get severe weather).

Why is SHERB important? Unlike summer severe weather events which are driven by high instability and less of everything else, cold season events are driven by everything else and not often by instability. SHERB is a special blend of “everything else” that is important when gauging a severe weather threat…which makes it valuable when we don’t have summer-like heat and humidity. When SHERB values are high and the chance for rain and storms is high, severe weather is often a concern.

Here’s what the NAM thinks for effective (storm-based) SHERB at 1pm Friday:


In low-instability, high wind shear environments (like Friday’s), the SHERB values greater than 1+ often signal support for strong and severe storms. What does the GFS think for SHERB values at 1pm Friday?


Again, the GFS develops the line of rain and storms to the west later Friday, whereas the NAM develops them closer to Cincinnati earlier Friday. What does SHERB look like later in the day Friday per the GFS?


We’re almost in the sweet spot (1+), but most of those values stay west of the Tri-State through Friday night.

Clearly, there are timing and strength issues with Friday storms, but the environment is supportive of strong and severe storms near or west of the Tri-State. The SPC has pinpointed the most likely area for storms well.

Here’s my overall thinking on Friday’s severe threat:


The timing is open-ended on purpose…for now.

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Update on Tuesday’s Severe Storm Threat

This morning, the Storm Prediction Center placed the entire Tri-State in a slight risk for severe storms on Tuesday and Tuesday evening:


A review of the latest model guidance suggests there will be modest instability (in purple) over the Ohio Valley Tuesday:


The black dot is Cincinnati. The highest amount of instability will likely be positioned west and southwest of the Tri-State.

The jet stream will be overhead Tuesday afternoon, with the highest wind speeds to the west of the Tri-State. Favored areas for lift in the atmosphere are highlighted in blue:


This is not impressive lift over the Tri-State, but it helps. How about the placement of upper-level disturbances? Here’s what Sunday morning’s NAM model things by 4pm Tuesday:


Disturbances are highlighted in blue. Lift in the atmosphere will be highest ahead of these disturbances (which will move from west to east).

How is the tornado threat looking? Let’s assess this threat using the Significant Tornado Parameter (STP). Values of 1 or highest suggest an elevated risk for tornadoes. Here’s what the NAM projects for STP at 4pm Tuesday:


Values of 1+  are in light yellow, so there is a risk for tornadoes Tuesday…BUT it is a secondary threat at this time based on review of other data.

Do we have moisture available to storms? Here’s the dewpoint forecast during the work week:


We don’t need to be humid in February to get strong or severe storms, but higher dewpoints suggest an increased likelihood for storms. Dewpoints in the mid 50s is high enough to strong strong storms in the cold season.

After a review of these parameters and a bunch of other data, the slight risk seems appropriate for Tuesday, with damaging straight-line wind and large hail being the main threats:


I’ll update this threat as time permits on Monday and Tuesday.

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Severe Storm Threat Tuesday: Knowns & Unknowns

The Storm Prediction Center has placed the Tri-State in a risk for severe storms on Tuesday, highlighted here:


For this and all graphics below, Cincinnati is labeled with a green dot. What type of risk is this? Think of it as a “slight” risk for now. A formal classification of “marginal,” “slight,” “enhanced,” “moderate,” or “high” is only used for a given day and the two days that follow it, so we will get the category tomorrow.

What is contributing to this risk? First, strong or severe storms need access to instability. Warm, moist air at the ground and cold, dry air aloft will help air to rise and create storms. Here’s what Saturday morning’s NAM model thinks for instability at 4pm Tuesday:


Modest amounts are in the purple, and higher amounts are in the blue. Even the purple shades can support storms, but other ingredients need to be in place (especially in the colder months of the year) to get thunderstorms.

Will it be warm? That’s important. I see temperatures only dropping to near 50° early Tuesday…


…and rising into the 60s Tuesday afternoon:


That’s more than warmth enough to see thunderstorms, including strong and severe storms.

How about moisture? Here’s the dewpoint forecast for the next few days:


We don’t need to be humid this time of the year to get strong or severe storms, but the dewpoint almost always needs to be above 50° to get severe storms in February.

How about lifting mechanisms for the assist? The NAM model (like others) has several compact disturbances (orange/red colors) over the Ohio Valley Tuesday afternoon:


…this supports storms, but it’s not impressive lift. How about the jet stream? Here’s what the NAM thinks at 4pm Tuesday:


Jet stream level wind speeds are in blue, and upper-level divergence of air (promoting lift) are in contours of pruple. There is also lift in the “left-exit” region of the jet streak (part of the jet stream) over the Tri-State. This lift overhead will support storms, but it’s not big time lift.

How about wind shear (the change of the wind speed or direction with increasing altitude)? The NAM has plenty of that:


Remember this is SPEED shear shown here, not directional shear (a lot of the latter supports tornadoes).

How about helicity (a measure of how likely storms are to rotate)? The NAM has us in a 200 m2/s2 area:


What the heck does that number mean? The threshold for what supports severe storms can vary from one season to the other, but this is significant to mention a tornado risk somewhere in the Ohio Valley. Let’s not discuss the level of this risk for now, but instead, acknowledge that this threat should be watched.

How about the SHERB parameter? What’s that? See more on this in this blog post from 2015. This is a helpful parameter in severe weather setups when instability is low. Now that that’s been explained, here’s what the NAM thinks for SHERB at 4pm Tuesday:


Any value over 1 with thunderstorms in the area suggests those storms may be strong or severe. How about effective SHERB (in simple terms, SHERB but accounting for storm depth) at 4pm Tuesday?


Again, values are near or just above 1, which supports a strong to severe storm threat.

So what do I think about Tuesday’s severe storm threat? Here’s my latest thinking based on the setup and acknowledging uncertainty:


Stay tuned. This threat may change, and new guidance will help to target areas that have a better threat versus other areas.

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Snow Likely Thursday

While snow is coming Thursday, the sky will be sunny to mostly sunny this afternoon. A 10:15am cloud snapshot of the Ohio Valley shows some clouds to the northwest moving southeast:


Despite sunshine, it is still cold. As of noon, temperatures are in the mid to upper 20s for most:


A sustained wind between 10 and 20mph means it feels colder. Wind chills in the Tri-State are in the teens and 20s:


Temperature will barely rise this afternoon thank to a westerly wind dragging cold air in from the Plains and Mississippi:


The influence of low pressure will begin tonight in the form of high- and mid-level clouds increasing. Plan for temperatures fall through the 20s:


Snow is likely Thursday. The coverage will increase in the morning, be steady for most of the afternoon, and end Thursday evening:


Temperatures in the low to mid 20s will mean snow-to-liquid ratios will be higher than usual. This means that liquid-equivalent precipitation (all forms of precipitation melted down) will need to be multiplied by a factor that is higher than usual. A baseline ratio for temperatures in the upper 20s and low 30s is 10:1. Given that temperatures will be in the low to mid 20s, this snow-to-liquid ratio is more like 18:1. Remember, 10:1 or 18:1 are snow-to-liquid ratios, NOT forecast snow totals. I am explaining the meteorology of taking what forecast models produce to a snow forecast. Models are producing 0.1″ to 0.18″ of liquid for this event (all snow), so totals of 2-4″ totals look appropriate for most, but not all. Here’s what I’m thinking for event total snowfall (Thursday and Thursday night):


This is a significant increase in amounts compared to 24 hours ago. This will not be a winter storm, per se…but it will be significant winter weather event. Roads will at least be wet on Thursday, and they will likely be snowy in spots with pavement temperatures in the 20s. Snow squalls are unlikely tomorrow, but the visibility will be reduced by snow…especially in the late morning and first half of the afternoon Thursday:


Red lights for travel conditions are somewhat rare for me. Again, this does not appear to be a winter storm, but impacts will be significant!

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Rain Showers Today, Snow Showers Thursday

Scattered showers are forecast through the lunch hour. Regional radar shows rain showers moving from west to east through the Tri-State:


Notice the break in showers over west-central Indiana. That break is over the Tri-State by mid-afternoon, but the area of rain showers in Illinois will overspread the Tri-State late in the business day and this evening. The amount of lift in the atmosphere is increasing to the west, so plan for more showers later today and tonight after a brief break this afternoon.

It’s warm outside! As of 11am, temperatures are in the 50s:


Temperatures will be gradually falling through the 50s this afternoon with showers favored early and late:


Showers will continue this evening as temperatures fall through the 40s:


Showers will diminish late tonight, briefly mixing with flurries, especially northwest of Cincinnati:


Notice the quicker fall in temperatures late tonight compared to this evening! We’ll start Wednesday in the upper 20s and low 30s. Little if any warm up is forecast Wednesday, even with decreasing clouds:


Wednesday will be a much colder and breezy day. The next wave of precipitation (Thursday) will be in the form of snow showers. Models are getting more aggressive with the strength of this wave, so up to 1″ of snow is forecast Thursday:


Notice that no Tri-State community is favored for more or less snow. Snow showers implies snow that starts and stops. Here is a summary of late week impacts:


Snow showers will increase in coverage Thursday morning, decrease in coverage late Thursday, and cold air will plunge into the Ohio Valley behind this wave. Lows will be in the low to mid teens and highs will be in the mid to upper 20s Friday through Sunday.

Snow squalls are unlikely Thursday, but there will be drops in visiblity underneath snow showers.

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