Wednesday, April 9, 2014

April 14-15 Potentially Significant Snowstorm

Model guidance is suddenly converging on a potentially significant late-season snowstorm.

The image above shows the most recent forecast from the GFS model, highlighting something I discussed the other day: a potential late-season snowstorm that may drop significant amounts of snow. We had seen the ECMWF model dropping similarly high amounts of snow, as the hand-drawn graphic below shows. Remember, this was yesterday's ECMWF MODEL forecast, NOT my forecast.

Yesterday's ECMWF model snowfall map
While these late-season snowstorms are typically weaker, due to warmer soil temperatures that reduce sticking snow, as well as a genrally unfavorable environment, the Gulf of Mexico can provide a basis for intense amounts of moisture and fuel. If there's enough cold air in place, a strong snowstorm can result.

Tropical Tidbits
There is some merit to the idea that we may see a strong storm system, as the ECMWF is suggesting. Shown above was the observed 500mb height anomaly chart over the Western Pacific on April 6th, where cool colors depict negative height anomalies/stormy weather, and warm colors represent quiet, warm weather. On the morning of April 6th, we saw a trough moving through Japan, possessing a pretty decent strength. This is significant, as it directly relates to what we may see in this April 13-15 timeframe .There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. Thus, we can expect a storm in the US on an April 12-16 period, and the April 13-15 ECMWF snowstorm falls right within this timeframe. Additionally, this trough in Japan brought along some pretty cold weather, which COULD contribute to additional chances for snow in this potential storm system. 

The image above shows the projected mean surface pressure by the ECMWF ensembles, as well as the spread among all ECMWF ensemble members in the shown colors for April 14th. We first observe how the ensembles are much less enthusiastic about the idea of a storm system for this timeframe, only bringing a minimum 1004-1002 millibar swath of low pressure over an area from northeast Indiana into southeast Michigan. This sort of weaker projection is to be expected, as these ensemble means take the average of 52 ensemble members, unlike the ECMWF model, which only shows one solution. The next item we observe is the swath of oranges and yellows over the Plains and Midwest. These warmer colors indicate a higher spread in the ensembles; in other words, the ensemble members are in higher disagreement with one another over this higher spread envelope, thus indicating increased uncertainty. However, looking back at the post on this storm from the other day, we note that the uncertainty has decreased, with the ECMWF ensembles showing mainly yellows (decent uncertainty), rather than a swath of oranges, which depicted even higher uncertainty. This bodes well for the idea of a snow event.

If we look at the projected ECMWF ensemble mean 850mb temperatures over North America for April 14th, we find a startling scene. The ensemble mean actually seems to support an upper Midwest snow event, with the freezing line located just west of the dashed black line (marked as the number 0 ). I don't have access to precipitation products that display each ECMWF ensemble member, but just analyzing this map alone, it does seem like the ECMWF ensembles would support at least a slight snowfall event.

The image above shows each individual ensemble member from the GFS Ensemble set, valid for this timeframe. In many of the ensemble members, we see substantial precipitation located to the west of that yellow-ish/brown-ish line, which defines the line between freezing and non-freezing temperatures about 5,000 feet above the ground. It's important to note that the ensembles are still pretty spread out, despite decent agreement on the precipitation location, and it is because of this spread, among other things, that I'm very hesitant to support a snowfall event for this time period.

Here's the caveats I'm currently concerned with about this storm.

• This event would be happening in mid-April, rather than mid-January. Thus, the environment is much more hostile to snowfall.
• Model guidance, including ensembles, remain inconsistent with this storm on differing scales.
• Model guidance is notorious for over-projecting cold air coverage and intensity.
• The pattern has been very "wintry" this year, meaning the models may still want to hold on to this "wintry" feel, whereas the actual pattern may be much warmer.

Let's summarize this.
• A potentially major storm system is expected in the April 12-16 timeframe.
• Colder than normal weather is expected to arrive with this storm system.
• Model guidance is hinting at a potentially significant snowfall event during the April 14-15 timeframe, in conjunction with this potentially major storm system.
• There is high uncertainty with this potential event. Caution must be used.