theWeatherMatt: December 2023

As we come to the close of the first full month of Meteorological Winter, and the end of 2023, many folks in Utah are wondering, "where is the snow?" In fact, much of the United States has seen near or above normal temperatures. Many areas, especially along the east and west coasts, have experienced flooding rains. Perhaps a bit of this is due to the current El Nino status of the Pacific Ocean. Nevertheless, winter's early hibernation for Utah, and the majority of North America, is likely to end. There are signs of a weather pattern shift of the jet stream on some long-range, global numerical weather prediction (NWP) models

The jet stream is the driving force and steering mechanism for winter weather in the midlatitudes (i.e. where Utah is located - along the 40°N latitude line). There are a few key elements to understand about the jet stream. As excess energy over equatorial regions gets transported poleward (in the northern hemisphere, that would be from south to north), temperature and pressure gradients become stronger. Areas of higher and lower pressure become more pronounced. High and low pressure "systems" are what steer the jet stream. These strong winds at high altitude are the jet stream and are the result of existing thermal and pressure gradients. The jet stream is a driver of, and is co-located with, storm tracks.

For a good portion of December 2023, the jet stream pattern has been what we in the meteorology world call "zonal flow." Zonal flow is west to east without much poleward motion. If a jet stream exhibits a more flat or zonal flow, it's lacking the key element of energy, heat, and moisture transport that is needed for "active weather." The result: moderated, milder conditions.

The earth's atmosphere is in a constant state of imbalance, but during brief periods of (sometimes on the order of days or weeks) zonal flow can inhibit the transport of equatorial air to the north and subsequent arctic air to the south (also known as cold fronts and digging troughs). As previously mentioned, the recent weather pattern of the jet stream has been zonal. Below is a quick example of a 2-day loop from the GFS model over the eastern half of the Pacific Ocean (North America is faintly visible over the right third of the GIF, Alaska is near the top center). The loop depicts a previous forecast of jet-level wind speed (knots) and direction at the 250mb pressure level (or approximately 36,000 ft MSL) for December 29-30, 2023.

Note how the jet stream flow is predominantly west to east, with jet maximums consistently in the 120-160 knot range. Zonal flow has been a fairly persistent pattern for the majority of December, where only brief and weak ridging or troughing depart the mean flow. In contrast, see the following model loop.

The above GIF is a longer, 7-day loop, for Jan 04-11, 2023. Note how the zonal flow erodes and more of the typical north-south motion of the jet stream ensues? This GFS model solution suggests a potential turning point into a more active weather pattern. These jet streaks are not only ridging and troughing, but are maintaining the wind magnitude more efficiently than what was observed in December.

But hold on! Before we start getting out the ink pens and write up a forecast, we must realize that this is just one model and one model run's solution - not to mention that the certainty and confidence in the model's solution decreases the further out one looks.

However, upon monitoring the model solutions for a couple of days now (the GFS releases a new model run every six hours), there does appear to be increasing confidence in a degradation of the zonal flow (at least over the far eastern Pacific and North America). The placement of the jet, cold air aloft, and ultimately, areas of precipitation have not been incredibly consistent. It is especially difficult to pinpoint the location, timing, and magnitude of snowfall, for example, beyond 3-5 days out (in most scenarios).

With a caveat and a word of caution, several weather forecast parameters will follow below for your viewing pleasure. But it is important to note that these are simply a "snapshot" of singular models' runs and ensembles and do not represent an official forecast. A great source for official forecasts includes: National Weather Service - Salt Lake City Weather Forecast Office

To switch gears from the graphical model view to data box plots, here is a snapshot of an ensemble from National Blend of Models (NBM) 1D Viewer. An ensemble shows either a group of models or their members and displays the range between them (where lower spread of data can be indicative of higher confidence in that weather parameter's forecast). Below arre the predicted range of daily maximum and minimum temperatures at KSLC over the next 10 days. Note the gradual temperature trend is decreasing with time. By the start of week two in January, maximum daytime temperatures may fall and remain below freezing, with nighttime lows in the teens on January 08-10, 2024. The take home is that a decrease in temperature at KSLC is possible, perhaps likely, in early January.

Also from the NBM 1D Viewer, precipitation at KSLC looks most probable after Friday, January 05, 2024. The chances for snow (under precipitation type) remain greater than rain or freezing rain and coincides with the timing of greater accumulated precipitation rates over next weekend. Given the falling temperatures, it seems safe to bet on snowfall with this setup. But we won't delve much further on precipitation at this time due to the uncertainty of the placement of incoming troughs that far out in time.

While this was not an exhaustive nor in-depth look at the forecast process, it's enough to show that there is potential for a weather pattern shift in early to mid-January. At a glance, things look favorable for lowering temperatures and increased precipitation (snow) for Utah and the region.

Here is a quick glance at the Climate Prediction Center's 8-14 Day Temperature and Precipitation Outlook, valid January 08-14, 2024:

To get an idea of the early potential for mountain snowfall totals on the Wasatch (at Alta), here is the NAEFS guidance. The lower-left chart shows the various model members for accumulated snowfall at Alta. There is plenty of spread (so certainty is lower at this time). But watching the mean (average in the red line) increasing to approximately 20 inches by Saturday evening (January 06) is promising.