Why The Dewpoint Needs To Be A Part Of Official NWS Weather Records

Everyone knows the word moisture. It is a fundamental ingredient for making precipitation or even keeping it away. It is the reason why it was humid today; the more moisture there is in the air (the higher the dewpoint), the more humid it is. The dewpoint is critical for knowing whether clouds will form or whether they will break apart. The dewpoint has either a direct or indirect impact on the strength, positioning, and timing of all weather systems. The dewpoint (a temperature) can impact the air temperature and what type of precipitation falls at the ground and aloft.

For some reason, though, the dewpoint is not listed in official weather records (kept by the National Weather Service in the United States). When you look at weather records for the month of July 2016 in Cincinnati (so far), the maximum, minimum, or average dewpoint is nowhere to be found:


Records of air temperatures, precipitation, snowfall, wind speeds, and even snow depth are kept every single day. Yet the dewpoint is not here. Why?

The dewpoint is so important that is it listed not once, but twice in airport weather observations:


The red number is the dewpoint to the nearest whole number (°C), and the green number is the dewpoint (°C) in tenths (and the first “0” or 1″ here is used to indicated whether the dewpoint is positive or negative, respectively). It’s important enough to go here, but not in official weather records?

The dewpoint is so important it is measured vertically using a weather balloon (to the top of the troposphere, the layer of the atmosphere that we live in and where weather occurs) twice a day at numerous sites around the country and the world. Here’s the weather balloon “sounding” from 8pm Thursday night from Wilmington, Ohio:


Guess what the green line is? It’s the dewpoint! It’s right next to the temperature in red! Wind directions and speeds (listed as barbs on the far right) are also listed. And that’s it for a weather balloon sampling! Every other variable is derived. If the dewpoint makes the variable list here, it should be in official weather records

You want to know what the record lowest or highest dewpoint was at a pressure of 850 millibars (about 5,000 feet above the ground) back to the early days of when weather balloons were launched? Here you go! It’s all here! Want to know how the average dewpoint this month compared to last month or last July? It’s not easy; even the most data savvy meteorologists will need time to make this calculation.

Keep in mind, temperature ranks for a week, month, year, decade, or ANY stretch of time are done by averaging the average temperatures for each day in that period. The average temperature for the day is simply the high and the low divided by 2. I can calculate the average temperature for a year or month and compare it to a similar length of time easily because high, low, and average temperatures are listed in official weather records. High, low, and average dewpoints are not. Comparing the average dewpoint for one year to another can take hours. It should take minutes.

Part of the argument is that dewpoint records don’t go as far back as temperature records. Hourly dewpoint records for Cincinnati go back to the late 1930s, but temperature, rainfall, and snowfall records go back to 1870, 1870, and 1893 (officially and respectively). So it’s not the same period of record…but so? Records are records. The first half of 1872’s temperature records in Cincinnati are gone; they were lost in a fire. We didn’t throw out the entire record on account of some records being lost. Jackson, Kentucky’s weather records (at the National Weather Service office there) began in 1981, some 100 years after Cincinnati’s records began. Still, we aren’t throwing out weather records from Jackson because their period of record is less than 40 years.

It’s time to add the maximum, minimum, and average dewpoint to official climate records for the United States. Moisture in the air is fundamental to understanding what type of weather occurs and how poor conditions will be. The public, not just meteorologists, should have knowledge of how weather conditions are changing so that we can see how moisture is moving over time and space. With a large enough period of record in place, dewpoint records will add to the ongoing discussion about climate change and give all a better indication of whether certain areas are more humid or less humid over time.

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Update On Wednesday And Thursday’s Severe Weather Threat

As I highlighted in my Sunday blog post, there is a significant threat for severe storms Wednesday, Wednesday night, and early Thursday. Damaging straight-line wind was the main concern per yesterday’s guidance, and it still is the main threat. The tornado threat, though, has come up, but it is still a secondary threat at this time.

Just after 3am on Monday, the Storm Prediction Center placed the Tri-State in a slight to ENHANCED risk for severe storms Wednesday and Wednesday night:


The highest potential for severe storms is focused northwest of Cincinnati.

This morning’s guidance has shifted the main area of severe weather farther north into the Great Lakes. Despite this shift, we are still in a significant threat for severe storms Wednesday and Wednesday night.

In yesterday’s blog, I discussed the derecho parameter. While I am not forecasting a derecho at this time, this parameter gives us an idea of how high the damaging straight-line wind threat is IF storms occur. Here’s what this morning’s NAM model thinks the derecho parameter will be at 2am Thursday morning:


Clearly, these are high values (anything over 5 is very significant, really) over the Great Lakes and Ohio Valley. Here’s what this morning’s GFS model thinks for the same time:


While values are not as high, they are still high.

While computer guidance suggests the complex of storms may stay focused to our north, the Weather Prediction Center suggests there will be warm front (the leading edge of warmer air bisecting the Tri-State) Wednesday night:


The air mass lifting in from the southwest will be warm, humid, and unstable, and the front will help provide shear (the change in the speed and direction of the wind). All of these elements combined elevate the threat for tornadoes and damaging-straight wind locally (despite a higher threat closer to Chicago).

The Significant Tornado Parameter tells us where there is a elevated risk for tornadoes IF storms can form. Number greater than 1 are usually supportive of at least the potential for tornadoes. Here’s what Monday morning’s NAM model has the significant tornado threat at for 8pm Wednesday night:


This is elevated. Let’s see what Monday morning’s GFS model has for the same time:


This is also elevated. Notice that this does not guarantee tornadoes, but it does show us the magnitude of the tornado threat.

It is also important to note that model guidance is NOT is not a forecast. A meteorologist must review other guidance to make a forecast, and here is what I have come up with for now regarding late Wednesday and early Thursday’s severe weather threat:


This forecast may change as model guidance changes and as we near the event. Know that the damaging straight-line wind threat and tornado threat are both in play for late Wednesday and Thursday. I’ll update this blog as needed.

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What We Know And Don’t Know About Wednesday Or Thursday’s Severe Weather Threat

In the Ohio Valley, it is rare to be put in a severe weather risk area by the Storm Prediction Center more than 3 days out. This week will feature one of those rare occasions.

The Storm Prediction has put the Tri-State in the 15% severe storm risk for Thursday. The 15 percent risk means SPC feels there is a 15% probability of a tornado, severe hail, or severe wind event occuring within 25 miles of a point:


This is – essentially – a “slight” risk for severe storms on Thursday. The 30% area would be the equivalent of an “enhanced” risk. “Marginal,” “moderate,” and “high” risk equivalents are not issued on forecast days 4 through 8.

The takeaway here is that the Storm Prediction Center feels there is a significant severe weather threat in the Great Lakes and Ohio Valley Wednesday.

Or will it be Thursday? Models disagree on the timing of this mid-week storm threat.

There is already a buzz on social media about a complex of storms racing through the Midwest and Ohio Valley Wednesday. Some are predicting a “derecho” (a long-lived complex of storms that causes wind damage over a large area), which predicting at this range is not worth our time and ridiculous. The definition of a derecho is loose. The SPC believes a derecho must go 250 miles and contain separated 75mph wind gusts. Others disagree that a derecho needs to fulfill this requirements.

Regardless, let’s investigate this severe threat using the derecho parameter, which suggests where derechoes may grow or evolve IF storms are able to occur. The higher the derecho parameter, the better the support for them. Here’s what Sunday morning’s NAM model suggests the derecho parameter will be at 2pm Wednesday:


Clearly, there is a significant risk for storms with damaging wind to our west Wednesday afternoon if you believe this model. Let’s compare it to Sunday’s morning’s GFS model:


The GFS is not as impressed with the threat compared to the NAM, especially for Indiana. It is a model model with a different engine, but it is still showing a significant risk for storms with damaging straight-line wind. The GFS model actually holds the better threat off until Thursday:


The NAM model produces a future radar product. Here’s what Sunday’s morning’s NAM model suggests the radar might look like Wednesday morning:


…and here’s what it suggests the radar will look like 12 hours later:


Even history suggests we need to watch for a damaging straight-line wind threat. The Storm Prediction Center’s database suggests the historical odds of seeing severe weather on June 22 is focused over the Tri-State, central Plains, and Carolinas:


When severe weather occurs in the Ohio Valley on June 22nd, it is very likely to be a damaging straight-line wind report versus a tornado or large hail report:


It is apparently that we need to monitor the damaging straight-line wind threat later this week, although tornadoes and large hail are possible with any storms that sweeps through the Ohio Valley. Our confidence, however, is muted when we see discrepancies in model guidance. The last two runs of the European forecast model (ECMWF) suggest the damaging straight-line wind threat will peak late Wednesday night or early Thursday. It is the “middle of the pack” right now.

Stay tuned. Wednesday and Thursday are days to be prepared for strong and severe storms.


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A Breakdown Of Today’s Severe Weather Potential

Morning showers and storms have stabilized the low-levels of the atmosphere and have prevented temperatures from rising significantly. As of 11am, most in the Tri-State are in the 70s:


Sunshine is, however, returning to the Tri-State per the 10:45am visible satellite image:


Note a boundary (in the form of clouds) from southern Indiana back close to Dayton. The Ohio Valley radar image as of 11:15am shows showers and storms moving east and southeast of Cincinnati:


While the boundary now from southern Indiana into west-central Ohio is moving southeast and will trigger isolated to scattered storms early this afternoon, the main axis of instability is in northwestern Indiana and is dropping southeast. This secondary front will be the one that triggers storms later this afternoon and early this evening.

The Storm Prediction Center has the entire Tri-State in a slight risk for severe storms through this evening:


This is mainly for the potential of damaging straight-line wind. Secondary threats include large hail and localized flooding, especially with dewpoints in the upper 60s and lower 70s as of 11am. Here’s a breakdown of severe weather threats through this evening:


The most likely time for severe weather today centers between 3pm and 9pm, especially between 5pm and 8pm.

Clusters of rain and storms will develop to our northwest later today, gradually coalescing into a line as it moves southeast. The afternoon forecast calls for showers and storms to redevelop this afternoon. Highs today will be in the mid to upper 80s, and it will be steamy:


Showers and storms will diminish and end this evening. Strong and severe storms will be favored early:


Be alert, friends!

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Was It A Storm That Wasn’t In The Forecast? Nope.

If you were watching Ohio Valley radar yesterday afternoon, you might have noticed radar returns in southern Indiana. Were these showers and storms bubbling up due to temperatures rising through the 70s? Did I bust a forecast?

Despite what you may think, the same device that suggests I blew the forecast also suggests I got it right. Modern radar is a power thing when used to it’s fullest potential.

First, is it realistic to have radar returns in this area? A visible satellite snapshot of the Ohio Valley shows cumulus clouds in southern Indiana at 5:15pm yesterday afternoon:


We need clouds to have showers and storms, so the returns on radar could be precipitation. Notice there are no towering clouds in the area though (unlike in northern Ohio were showers and storms were in progress with higher instability); satellite imagery leads us to believe there was just a sea of shallow cumulus clouds yesterday afternoon in southern Indiana.

Here are the radar returns as viewed by the National Weather Service Doppler radar in Indianapolis late yesterday afternoon. The reflectivity (commonly shown on television or apps) is on the left, and correlation coefficient (showing the relationship between the height and width of objects like raindrops, hail, or debris sampled by radar) at the right. Blue values of correlation coefficient here suggest these returns are particles of different shapes and sizes and likely not raindrops, hailstones, or something related to meteorology:


Radar cross sections show these returns did not extend well into the atmosphere. Here’s the reflectivity cross section from NWS’ Indianapolis’ radar at 5pm ET yesterday:


These returns are relatively close to the ground (below 10,000′). Here’s the correlation coefficient cross section at the same time:


Blue values here suggest these radar returns are likely not raindrops or hailstones. So what could it be?

The radar returns become apparent around 4:40pm in the afternoon. See the green dot in the reflectivity at the left with blue (low) correlation values in the same area at the right:


This is surprisingly close to Camp Atterbury in Indiana. The military is known for dropping chaff to scramble radar and serve as a countermeasure. Low (blue) correlation coefficient values on radar are common with smoke plumes and military chaff. The meteorological environment did not support showers and storms, so military chaff is the most likely bet.

Need some additional evidence? Fast forward to this morning. Here’s the visible satellite snapshot as of 8:45am Wednesday:


Notice no clouds in the sky. There were still no clouds over southern Indiana as of 10:30am ET.

Yet there is something on radar in the same area near Camp Atterbury as of 10:20am ET:


Reflectivity values are high and correlation coefficient values are low in the image attached. It looks like chaff is the culprit here. Remember, we need clouds to have showers and storms, and there were no clouds in the sky when these returns were on radar.

Radar is a powerful tool if you review the data thoroughly. Even as a meteorologist, it’s easy to fall into the trap of seeing radar returns and assuming showers and storms are developing. Radar can sample dirt, bugs, plumes from wildfires, tornado debris and precipitation. Things aren’t always as they seem!


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Historical Perspective On This Morning’s Freeze

A freeze in the second half of May in Cincinnati is rare. While rare, freezes and frosts and do occur in late May.

The official low temperature this morning at the Cincinnati/Northern Kentucky International Airport was 31°. This sets a new record low temperature for May 16th; the previous record for May 16th was 32°, set in 1997. Low temperatures in the low to mid 30s were widespread this morning:


Inside the I-275 loop, temperatures were slightly warmer due to the urban heat island effect (more building/concrete to retain heat):


Notice Okeana dropped to 28°. Two weather stations just north of Lebanon reported low temperatures of 29° and 28°. Several sites reported a low of 30° or 31°.

How rare is a low temperature of 32° in mid or late May in Cincinnati? Here are the latest spring dates in the Queen City with a low temperature of or below 32°:


Notice that today’s low of 31° is the 3rd latest spring freeze date on record (back to 1871).

Frost can occur at various temperatures, but 36° or below is a good threshold to use for frost. Using 36° as a baseline, here are the latest spring frost dates in Cincinnati back to 1871:


Today’s low of 31° ties as the 9th latest spring frost date in the Queen City. A freeze (which we had this morning) implies widespread frost.

I am cautiously optimistic that this morning will be the last freeze and frost of the season. There are no forecast overnight lows in the 30s through this weekend. Additionally, history suggests we’re safe from a freeze or frost once we get into June!

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What You Need To Know About Today’s Severe Weather Threat

The Tri-State is in a slight to enhanced risk for severe storms this afternoon and evening. This is the highest threat for severe storms we’ve had so far in 2016.

The latest Storm Prediction Center outlook has the enhanced risk (the highest risk for severe storms over and southwest of Cincinnati this afternoon and evening; strong and severe storms are (slightly) less likely to the northeast and north of Cincinnati:


I encourage you to focus not on the severe threat category for which you live; instead, know that the threat for severe storms is higher in Cincinnati and points south and southwest of Cincinnati and slightly slower from Middletown to Hillsboro.

Based on radar, satellite, and model trends, it appears the highest threat for severe storms will be highest between 3pm and 10pm today:


What might the radar look like at various times today? One model, the High-Resolution Rapid Refresh, seems to have a decent grasp on rain and storm placement now. Here’s what it suggests the radar will look like at 2pm today:


Notice the round of showers and storms that came through late in the morning exiting the Tri-State completely (to the northeast) by 2pm.

Forecast radar at 5pm shows showers and storms becoming more numerous:


Forecast radar at 8pm shows showers and storms bowing and gradually pushing east of Cincinnati:


By 11pm, the High-Resolution Rapid Refresh model shows storms diminishing and pushing east of I-75:


Be prepared! Some storms may produce damaging straight-line wind and large hail. The strongest storms in the Ohio Valley will be capable of producing flash flooding and tornadoes.

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