A Tornado WATCH or a Severe Thunderstorm WATCH may be issued for most of northern Kentucky and southwestern Ohio shortly.

Carroll County and all of southeastern Indiana are under a Tornado Watch through 9pm:

Additional watches and warnings may be posted for the Tri-State later this afternoon and this evening. Be alert!

The radar composite as of 3pm shows a bowing line of storms to our west. These storms are moving east at roughly 45mph and are quite capable of producing damaging straight-line wind in excess of 60mph:

Additional round of rain and storms are likely tonight, but the round approaching from the west is likely to be the main event. It will zap a lot of the instability available for storms this evening.

The visible satellite snapshot as of 2:15pm shows breaks in the cloud both ahead of the line of storms around Louisville and also behind it:

Breaks in the clouds will support to an increase in instability and stronger storms. Temperatures have been slow to rise today due to a cloudy sky, but strong southerly flow has pushed temperatures into the mid 60s for most:

There is still plenty of time for the record high today in Cincinnati of 66° set in 1933 to be broken or tied.

The main severe weather threat through tonight continues to be damaging straight-line wind, but tornadoes are also a secondary threat:

Most of the Tri-State is still in an ENHANCED risk for severe storms through tonight, with the highest threat being southwest of Cincinnati:

You are encouraged to have multiple ways of getting warnings and watch information, including from a NOAA weather radio!

While it is most common in the spring, severe weather can happen at any time of the year in the Tri-State. A Tornado Warning was issued for Adams County at 12:30am three days before Christmas in 2013. 8 Severe Thunderstorm Warnings were issued that month. Severe weather happens in December.

The risk for strong and severe storms on Wednesday night has increased in the last 48 hours. There are several things for which we have a good understanding, and there are other things that have yet to become clear.

Specific Threats

Based on recent guidance, damaging straight-line wind is the most likely severe weather threat Wednesday night. Tornadoes and large hail are secondary threats, and flooding is the least likely threat. Here is a summary of forecast severe weather impacts for Wednesday night:

Timing

As the graphic above suggests, the most likely time for strong to severe storms to occur in the Tri-State is 9pm Wednesday to 2am Thursday. There are consistent signals that this is the most likely window for strong storms, but storms may develop earlier based on some recent computer guidance.

Locations Affected

The threat for severe weather will be highest in the western half of the Tri-State Wednesday and lower for the far eastern parts of the Tri-State. All of southeastern Indiana, most of northern Kentucky, and much of southwestern Ohio is in a SLIGHT risk for severe storms per the Storm Prediction Center:

The rest of the Tri-State is a lower, marginal risk for severe storms.

Storm Modes

Initially, storms that develop in the Ohio, Tennessee, and Mississippi Valley Wednesday will be cellular but will likely coalesce into a line or lines of storms. The threat for damaging straight-line wind is typically higher with lines of storms, while discrete, individual storms can pose a higher tornado or hail threat.

Tuesday morning’s high-resolution WRF model suggests showers – rain that starts and stops – will be will develop late Tuesday night and early Wednesday:

Showers and rain will let up during the afternoon, but showers and storms will redevelop near and after sunset:

Notice that some discrete cells are developing in the forecast radar snapshot over Kentucky, Illinois, and Tennessee. The high-resolution WRF model suggests discrete cells are a possibility Wednesday night in the Tri-State:

While likely overdone, this radar snapshot suggests an elevated tornado threat. However, there are other models not as supportive of this elevated tornado threat, including Tuesday morning’s lower-resolution WRF model forecast for Wednesday afternoon:

It is far more aggressive with the development of storms to our west Wednesday afternoon. It also has that line to the west meeting up with clusters of rain and thunderstorms ahead of the line near Cincinnati very early Thursday morning:

Notice the differences between the high-resolution and low-resolution WRF models. This leads to a needed discussion on…

Uncertainty

While there are discrepancies in timing and severe weather threats tomorrow night,  there is little doubt that thunderstorms will develop in the Ohio, Mississippi, and Tennessee Valleys tomorrow and continue into Wednesday evening. Tuesday morning’s WRF model suggests the jet stream (in blue) will be strong from the Plains to the Great Lakes around midnight Thursday, with upper-level divergence (lift) strongest along the Ohio River from Cincinnati to Cairo, Illinois.

As I blogged about in a similar, colder season severe weather setup on December 19, 2013 (see mention of Tornado Warning and severe weather notes above):

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.

This morning’s NAM and GFS models produce SHERB values in excess of 1 Wednesday night:

Values of 1.4 to 1.6 are rather high for this time of the year. This does not guarantee severe weather will occur, but it certainly suggests the severe weather threat is elevated.

High temperatures will be in the mid to upper 60s tomorrow. The record high Wednesday in Cincinnati is 66° set in 1933. That record forecast to fall tomorrow. Record warmth, dewpoints in the 50s and 60s, and strong wind will be signals of a rising storm threat.

Be weather aware tomorrow and tomorrow night! Remember, the forecast and severe risk may change tonight and tomorrow. Stay alert for updates!

There has been no accumulating snowfall so far this season or this month in Cincinnati. Accumulating snowfall is defined as 0.1″ or more at the Cincinnati/Northern Kentucky International Airport where official records are kept. From 1915 to 1947, records for Cincinnati were kept at the Abbe Observatory in Clifton; from 1870 to 1915, records for Cincinnati were taken downtown. Snowfall records before 1893 for Cincinnati are considered unofficial, but I will use them use here.

With the lack of snowfall so far in December, many are wondering if we will see snow or if winter is – essentially – cancelled. Yes, we will see snow before June comes, and no – winter is not cancelled.

At this time, there is no clear window of opportunity to see accumulating snow in Cincinnati for the rest of December. That may change; for now, however, accumulating snow is unlikely in the next two weeks.

Since 1870, there have only been 14 Decembers without accumulating snowfall: 1875, 1877, 1882, 1885, 1888, 1889, 1891, 1908, 1931, 1940, 1941, 1971, 1982, and 2011. Because of the warm, non-snowy start to summer, some are incorrectly assuming that cold and winter will avoid the Tri-State this winter. While this winter overall will likely be warmer, drier, and with less snow than average, there will be bouts of snow and cold through next spring.

In the 145 years where no accumulating snowfall was recorded during December in Cincinnati, there were only 2 (out of 14) without accumulating snowfall in the January that followed:

An average of 4.4″ of snowfall accumulated in the Queen City during January when it failed to accumulate in December.

Of the 14 years in Cincinnati where snow didn’t accumulate during December, there was only one where snow didn’t accumulate in the following February:

The only meteorological winter (December, January, and February) where no accumulating snowfall was recorded in Cincinnati was 1931-1932. Snow, however, did come in March and also in November of 1931.

Since 1870, there has never been a year in Cincinnati where snow didn’t accumulate in the months following a December without accumulating snowfall:

On average, historical records suggests an average of 9.3″ of snowfall accumulates from January to June after a 0.0″ snowfall total for December. Based on these records, accumulating snow is extremely likely in January, February, March, April, May, or a combination of those months even if we make it to the end of 2015 without accumulating snow.

While there is little room for debate on whether we will see snowfall in the coming months, the discussion about when and how much snow will fall is endless. Cincinnati averages 18.1″ of snow each December, January, and February combined and 22.5″ from one summer to the next. For perspective, here are the least snowy June to June periods on record in Cincinnati:

It is important to note that most of the Decembers since 1870 without measurable snowfall came prior to 1915, where weather records were kept in downtown Cincinnati where the urban heat island was strong. This bubble of warmer air near the city center made it more difficult for snow to accumulate.

History suggests snow will return eventually. High temperatures are forecast to be in the 30s this weekend; highs in the 60s and 70s usually come and go quickly in December.

The official high temperature in Cincinnati (at the International Airport where records are kept) today was 70°. Since official records for ‪#Cincinnati‬ began on December 1, 1870, there have only been 13 December days with a high temperature of or above 70°. Today was one of them.

When compared to today, there have only been 6 days in Cincinnati on record with a higher high temperature during December:

The last time Cincinnati made it to 70° during December was on December 3, 2012. There have been 7 December days in the Queen City with a high temperature of exactly 70° (most recently in 2008).

Based on the average temperature (the sum of the high and low temperatures divided by 2) through 4pm today of 66.0°, today ranks as the 4th warmest December day on record:

It is important to note that today’s average temperature is not final until midnight. Temperatures will drop near 60° near midnight Sunday, which may bring the average temperature down a degree or two.

Record low and high temperatures may be broken Sunday. Low temperatures will center in the mid 50s Sunday morning, just below the record maximum low temperature for tomorrow (December 13th). Sunday’s record high temperature of 67°, however, is likely to be broken tomorrow:

The last time there were 2 or more 70°+ days in the same December in Cincinnati was in 1998 (December 4th and 6th). The last time where were 2 consecutive 70°+ days in the same December in the Queen City was in 1982 (December 2nd and 3rd).

55° is the record maximum low temperature for Monday (December 14th). While this record may appear to be broken Monday morning (with temperatures in the upper 50s and lower 60s), temperatures will fall through the 50s Monday morning and afternoon; with this forecast on the table, Monday’s maximum low temperature record is unlikely to be broken.

By the way, no snow fell in Cincinnati today, so the list of dates where we had to wait longer for the first accumulating snowfall of the season (historically) is growing shorter:

At this time, no snow is forecast in the next 7 days, and there is too much uncertainty in model guidance to say when our first window for accumulating snowfall will be in the next 2-3 weeks. Colder air is returning later this week, but the combination of precipitation and temperatures near or below freezing is unlikely through this coming weekend.

High temperatures in Cincinnati are likely to rise above 60° Friday, Saturday, and Sunday; in fact, highs Saturday and Sunday are forecast to be in the upper 60s to near 70°. Is it uncommon to reach into the 60s in December? The answer depends on how high the temperature goes.

On average, the high temperature hits or rises above 60° 2 days each December. We have yet to hit 60° in December 2015. While it will be abnormally warm the next few days, there were 5 days in both December 2012 and December 2013 where the high temperature was 60° or higher:

The records for the most number of 60° days in Cincinnati during December is 15 set in 1889.

Making it to or above 65° in Cincinnati during December is less common. On average, there is 1 65°+ day in the Queen City each December; there have only been three of these days since 2009:

The temperature went above 65° 6 times in December of 1982.

70°+ days in December are rare; there have only been 12 of them since 1870 and 1 since 2009:

The record high temperatures in Cincinnati for Friday, Saturday, and Sunday respectively are 70° set in 1931, 64° set in 2007, and 67° in 1927. The record high for Friday will not be broken, but the record highs Saturday and Sunday are forecast to be tied or broken.

The bigger story continues to be the lack of snow in Cincinnati, especially accumulating snow. There was no snow in Cincinnati today, so we are down to 21 years of the 122 years where we had to wait longer (than this season) for the first accumulating snow of the fall or winter:

Snow is unlikely in the next 7 days. The Climate Prediction Center takes that forecast one step further, suggesting December 16 through 23 will be warmer than average; this forecast works against the chance for snow nearing Christmas.

While you may have seen flurries at some point this fall, there have been no flurries or snow showers reported in Cincinnati (at the International Airport, where official records are kept) since summer ended. If you’re thinking it is unusual to make it to December 1st without any snow, you’re right.

In the last 100 years (since 1915), the first flakes of the season (whether the snow accumulates or doesn’t) were most commonly observed in the first 10 days of November, although it was not uncommon for the first flakes of the season to fall in mid to late November or late October:

There have only been 7 years since 1915 where the first snow flakes of the fall came after December 1st; the most recent year that this happened was in 1998 (which is also the record latest):

Before 1915, trace amount of snow were not included in official Cincinnati weather records. In other words, if snow fell before 1995 and the accumulation was under 0.1″, the daily snowfall amount was listed as “0.0” not “trace.”

Official snowfall records for Cincinnati date back to 1893, so we have the dates when the first accumulating snow of the fall or winter occurred for the last 119 years (this seasons’ first accumulating snow has not yet occurred).

Of these 119 years, the first accumulating snow (0.1″ or more) of the fall or winter in Cincinnati most commonly occurred between November 20th and 30th:

The first day of the season with accumulating snow has only come on or after December 1st 39 of the last 119 and after December 1st 38 of the last 119 years. The first day of the fall or winter with any snow (accumulating or not) is usually not the first day with accumulating snow; in the last 100 years, there were only 21 years in Cincinnati where the first day with any snow was also the first day with accumulating (0.1″ or more) snowfall. On average (1893 to 2014), 1.2″ of snowfall is recorded on the first day of the fall or winter with accumulating snowfall.

Using the mean and the median, the first flakes and first accumulating snow of the fall/winter season will come late this year (the dates have already passed).

Our only hope for seeing snowflakes in Cincinnati in the week ahead is Wednesday afternoon and evening. Long range forecast guidance suggests December will be warmer than average, so the likelihood for snow in the next 2-3 weeks is not high.