Friday, January 29, 2021

Winter Set to Grab Headlines - Snow On the Way

Snow enthusiasts rejoice! After a long gap between the December dose of winter precipitation and now, we are poised to get a visit from Old Man Winter! Despite lowest sun angle and shortest day length occurring in December, many people forget that our true "peak" winter weather climatology is January/February. Check your calendar - this storm is no exception! 

Speaking with a bit of technical weather jargon - a trough in the atmosphere will approach the area from the west. Exactly how this transpires will determine how the weather down at the surface level will unfold. That's the super "easy" way to explain things. In reality, the nuances that will go into producing a snowstorm for the region are much more complicated. 

The models have seemingly come into better agreement in terms of there being a snow event. However, key differences remain (as usual) in the specifics. Specific small scale features will only be resolved closer to "game time" and some features like heavy snow bands are always narrow and will potentially mean the difference between one location receiving a few inches and another receiving double-digit snowfall. 

The timing of the storm system is expected to come in two parts. The first part, which should be decent for the entire area, will be a slug of snowfall that will arrive Sunday between the very early hours and 7am or so (depending on exactly where you are). This will arrive from southwest to northeast. This portion of the storm appears to be fairly well agreed upon on most guidance and may drop between two and six inches. 

At that point, there may be a tapering off or lull in the storm. During this time, there is a chance that areas south and east (and even perhaps into the close in suburbs mix with sleet or rain. However, precipitation may be so light during this time, it may not appreciably bring down totals. 

Then, during the day on Monday, as the low pressure relocates off the coast, potentially significant bands of snowfall will likely form in or near our area to the northwest of the low pressure center. These bands WILL occur, but the exact orientation and geographical location of them is uncertain. Areas that get under sustained bands of snow will add significant snowfall to the totals from Sunday. Areas that lie between snow bands in areas of subsidence (sinking air) may receive lower totals. 

The early evening run (18z) of the GFS model showing a possible progression of the system. Note that the GFS often does not resolve temperature profiles well during setups like this. Thus, the mixing/changeover to rain may not be as pronounced as shown. (Source: PivotalWeather)

The snowfall from the Monday portion could add double (or more) the totals from the Sunday portion. Ultimately, this should become more clear as we get closer in time to the start of the storm. 

The early evening model runs have shown a general theme that we are getting a snowstorm. But as mentioned above - the details are scattered around and focused in different parts of the region depending on what model you're looking at. 

Storm totals (from both portions of the storm) are expected to be AT LEAST three or four inches across most of the area. The exceptions will of course be to the south and east of the I-95 corridor as usual. However, totals in isolated locations (that get under heavy bands) could be double-digits. 

Early evening (18z) GFS model run total snowfall for both portions of the storm. Note that the resolution of the GFS model may not 100% capture exact banding locations. (Source: PivotalWeather)

Early evening (18z) RGEM (a regional Canadian model) snow totals. Note that this model is higher resolution and can show areas where banding may setup more exact than the GFS model above. However, the banding may not end up being in those spots at all. (Source: PivotalWeather)

I'll be posting a more refined snow map tomorrow morning with some additional details once we have a few more cycles of the models to analyze and digest.

Essentially, you can see above that while the general idea of a significant snowstorm is seemingly "locked in" - the details are going to be what determines the winners and losers in this system. With more model data, and observations of how the storm develops tomorrow night and Sunday, we'll be able to determine who is going to "jackpot" 

Stay tuned! 

Monday, January 25, 2021

Taste of Winter On Tap

 Snow lovers will be disappointed by how the next two potential winter weather events have trended in the models. What appeared to be a week where winter might have roared back with a vengeance, has trended to a much more tame situation. We'll go in-depth below. 

Late in the work week last week, it appeared that a minor to moderate winter weather event would impact the area tonight into tomorrow. Additionally, it seemed that there was significant potential for a larger storm later in the week. However, due to some complex factors in the mid and upper levels of the atmosphere, both events have trended downwards. 

With regards to the winter weather event tonight and tomorrow - temperatures are quite marginal. While favored areas to the north and west (think the I-81 corridor) may see some icing and snowfall, the overall theme of this storm system is minor. There may be a a slushy inch or two even close to the metro area, along with a little dose of icing as well. However, I would think that by tomorrow afternoon, most areas will have safe travel close to the cities. This will be more of a nuisance storm than anything. 

High resolution NAM model showing potential snowfall primarily along and north of the Potomac River. This may even be a bit on the generous side. A slushy inch or two is certainly possible, however. (Source: College of Dupage)

You can see above that the high resolution NAM model shows a little snow accumulation focused north of the Potomac River. Even so, that accumulation should be relatively minor and slushy. 

Freezing rain forecast from the high resolution NAM model. A little icing is possible, especially to the west in the favored colder areas. (Source: College of Dupage)

Icing may be a story as well. Of course, even the tiniest amount of freezing rain accumulation can cause mayhem on the roads. The good news is that temperatures aren't frigid for this storm. Icing is still a concern, but as soon as temperatures rise above the freezing mark, things should improve. Perhaps the I-81 corridor will see a more prolonged period of icing conditions. Regardless, use caution on the roads during the precipitation, and be careful particularly on the bridges and overpasses. 

The attention will then turn to later this week. What appeared to be our first chance of a big snowstorm has petered out. Instead, a storm system looks likely to pass to the south. We may see some flurries around, but at the moment, the chances for substantial snow are going way down. Snow lovers will be stuck waiting again. 

The 06z GFS model run from Monday morning. You can see a low pressure area and its associated precipitation well off the coast. This storm system was forecast to be much closer to the coast a few days ago. (Source: TropicalTidbits)

There are indications that the overall Northern Hemispheric pattern may remain semi favorable for potential snow events going forward, but the problem so far has been a lack of cold air across the entire continent. We'll see where February takes us!