As we mentioned yesterday, we’re tracking a band of snow that will swing along an arctic front from NW to SE through the state tomorrow morning, bringing a brief period of moderate to heavy snow to the region. While we aren’t expecting more than a couple of inches of snow at the max, this event has the potential to be a high impact one due to the intensity of the precipitation and the timing at which it arrives(That is, during the morning commute). This update is broken down into two sections. If you’re interested in the technical discussion, read on, otherwise, scroll down to the snowmap and continue reading the impacts and sensible forecast from there.
Technical Discussion and Model Analysis
An arctic front swings into the area from the NW tonight, and it’s accompanied by a shortwave which will bring some snows to the area. These snows will be caused by what is known as Warm Air Advection (WAA), which is essentially when warm air blows into a cooler area. Given that warm air usually contains more moisture than colder air, the instability created from the temperature gradient serves to release that moisture, and as a result, produce snowfall in the area. The snow does not turn to rain however because the colder air is still entrenched in the low levels, and as a result, the precipitation can remain as snow all the way to the surface.
All models agree that this WAA will occur and that a band of snow will form, but the differences between the models lies in where exactly the band will be heaviest and how much moisture it will carry with it. The guidance is generally split into three main camps, with the first(ECMWF, SREF), being driest overall, the second(NAM, 4kmNAM, GGEM, UKMET, GFS, HiresRGEM), being more broadbrushed with QPF and generally having the highest amounts in the hills but also being a bit stronger overall with the band, and the third(RGEM, Parallel GFS), jackpotting SWCT on a stronger band that extends down into the NYC metro, with little elsewhere.
Here are total precipitation maps from basically all models except the ECMWF(Which we can’t show due to copyright issues). For this event, snow ratios should hover around 12:1, meaning that for every inch of liquid equivalent, we’d see around 12” of snow. We won’t be seeing numbers anywhere near that, but use that ratio to interpret these maps. 0.10” of LE = 1.2” of snow, 0.15” = 1.8”, 0.05” = 0.6”, etc.
First off, the general trend with WAA events is that they come in a little wetter than modeled(As is usually the case with most little intense events, it’s hard to have heavy snow and a total of a dusting). Therefore, am inclined to lean slightly towards the wetter solutions in general and up numbers slightly on the map as a result. Models have also been trending wetter, usually a sign for a small overperformer, and the system looks very robust on radar so far, all signs to go towards a slightly more bullish forecast.
Next, positioning. The band will be coming in from the NW, and given that it will be strongest first when it hits the NW hills, I think it’s a reasonably good bet that the mountains will work their magic and be able to squeeze a tenth or two of an inch of liquid out of the band. For that reason, and that the band will be stronger when it hits that area, I’m comfortable with forecasting one to three inches for the NW hills with moderate to high confidence.
The next question is the SW corner of the state. Most models were of the opinion that the area is too far southwest to be in the best banding, and by the time the band reaches that area, it’s already weakening. However, today we’ve seen the NAM and 4km NAM move towards extending the band further south towards that area and the NYC metro, and they are joined by the RGEM, which has been consistent in that depiction for several runs now. Think it’s a relatively low probability chance that they are correct and a coating is most likely, but the chance that they do end up being right (maybe 30-40%?), is high enough that I can’t discount it considering the increased impact. As a result, we’ll forecast a coating to two inches for the SW corner.
The band will almost certainly impact the rest of the state, so it then comes down to how strong the band is as it progresses and how much liquid it has in it. Generally inclined to lean towards the wetter solutions for the reasons described above, and given that models have been trending towards holding onto the strength of the band for a little while longer and the enhanced ratios, will forecast an inch to two inches of snow for most of the eastern ¾ of the state or so, with a local maximum of up to three inches possible in far eastern areas, especially the NE hills where models are showing an area of enhancement from the higher terrain and the hills.
Bust potential with this event is moderate in both directions, and is almost exclusively tied to the strength of the band and it’s positioning. Precipitation type will not be an issue as temperatures are forecast to remain at levels well below what would be needed to introduce liquid precipitation into the forecast.
When we put that all together, we get this snowmap.
As you can see from the above map, we aren’t forecasting that much snow for the area, but what makes this a potentially high impact event is the intensity of the precipitation and the timing of it. Most models only have 2-4 hours of snow for the area, but that snow will be moderate to heavy, with accumulations of up to an inch per hour possible and visibilities dropping below half a mile in the heaviest bands. Furthermore, the band will move through during the morning commute when traffic is heaviest, leading to potential traffic jams and slowdowns given that volume will already be high. Also, because temps have been so cold, all surfaces including roads are plenty cold enough for accumulations to occur right from the start of the event and in light precipitation as well.
Here are hourly simulated radar images from the hi-res NAM for tomorrow morning showing the timing and progression of the band of snow. Ignore it’s precipitation type forecast as it is a computer error, all precipitation will be snow.
To summarize, a quick hitting, light amount but high impact snow event is expected for tomorrow morning. We’ll be up early tomorrow morning and posting by around 7 AM with updates on the band and short range modeling, and will continue to update throughout the commute and morning as needed. We’d really appreciate it if you let us know what you’re seeing in your area by leaving a comment on our Facebook page(www.facebook.com/southernconnecticutweather), tweeting us @SouthernCTWX, or emailing me at spencer at southernconnecticutweather dot com. Thanks in advance for your help!
The next extended forecast update will be tomorrow evening or on Saturday. Thanks for reading SCW!
-Spencer (Map by Tim)