It’s cold enough…so why no snow?

After an anomalously warm start to the winter for the mid-south, seasonable temperatures (30s/40s) have returned since 1/16. So why have we remained snowless? The primary reason for that has been the pattern in the upper levels of the atmosphere. Since the El Nino sharply weakened mid-month, the overall synoptic pattern across the U.S. has yielded a sharp ridge over the western U.S. with a broad u-shaped trough over the east. This pattern does two things…1) Cold air masses from the arctic easily move southeastward across the U.S. every few days resulting in a temperature pattern of cold, warmer, warmer, cold, warmer, warmer… and 2) any storm energy coming across the northern Rockies or from the sub-tropics is quickly pushed eastward by the fast-moving jet stream before a large storm can develop. The arctic air is able to push all the way into the Gulf of Mexico which prevents any warm, moist tropical air from getting involved with the storm. For a winter storm to intensify, a few things have to happen…

1) The jet stream has to develop a v-shape, which in meteorology is called “shortening the wavelength” (horizontal distance from trough to trough). 2) The v-shape pattern causes the storm to slow down as the storm energy moves through the base of the trough. This allows the storm to gain even more energy from the baroclinic zone (horizontal temperature gradient) that typically separates the cold, dry arctic air from the warm, moist tropical air.

Some computer models show that the v-shape pattern may develop in the middle of next week, with a ridge along the West Coast, a trough over the Plains, and a ridge off the Southeast coast. This pattern could lead to an increased chance of seeing snow over Kentucky in the first two weeks of February. However, other models (see day 6) show the u-shape pattern persisting.

How can you tell which is right? Rather than simply guessing and choosing the model that best fits what you want to happen (wishcasting), you can look at what are known as model ensembles. A model ensemble is a large number of runs of a computer model using slightly different initial conditions. The main reason that weather forecasts fail is because the initial data from the upper atmosphere is only recorded twice daily from a network of less than a hundred weather balloons. The poor spatial coverage of these weather balloons leads to a smoothing process whereby the space in between the observations is estimated. By slightly varying how the missing data is estimated, the computer modelers can produce model runs that will result in a number of different unique solutions. If a pattern shows up in a number of the ensemble model runs, it means that the predicted outcome is more likely to occur. By looking at the ensemble runs of the different models, it becomes apparent that about half of the models predict a u-shape pattern and the other half predict a v-shape pattern. So it still is tough to determine what will actually happen next week.

Another tool the forecaster can use is to examine the climate teleconnections that represent the low-frequency variability of mean ridge-trough positions. For the v-shape pattern to verify, there must be a ridge (high pressure) located off the Southeast coast. This often occurs when the Pacific North American pattern (PNA) is in the negative phase. As you can see, the PNA has been positive since the pattern change on 1/16 and is expected to remain positive through the first week of February. Based on this prediction, I would say that the u-shaped pattern is slightly more likely to verify over the next couple of weeks, which means the cold weather will continue but it will be difficult to get a major snowstorm in our area.

An ominous sign can be found here, which shows the polar vortex, which represents the coldest air in the northern hemisphere, over James Bay. If you look at the MSLP 1000 – 500mb hour: 192 map on the bottom left, you can see that the 32F line (roughly estimated as the dashed blue 540 thickness line) is down to the Gulf Coast. Nearly all of the ensembles show this which suggests that at least the first week of February should be very cold (highs in the 20s, lows near 10) for Kentucky.

The bottom line…
A light rain followed by snow event is expected Saturday/Saturday night as the next arctic air mass moves in for Sunday. After that, the best chance for snow will come in the later part of the week (Thursday/Friday) as a piece of energy comes around the trough. This storm will usher in the coldest air mass of the season for Super Bowl weekend.

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