Friday, April 18, 2014

Long Range Forecast for Late April, Early May

Let's examine the long range forecast for late April into the beginning days of May.

Long range analog guidance from the ESRL/PSD division, a special physics-based meteorology branch of the government weather service, indicates we will see troughing setting up in Western North America in late April as a strong upper level low drops into the Southwest, provoking high pressure out ahead of it in the Central and East US. This ridge out ahead of the upper level low will likely make for anomalously warm weather across the aforementioned sections of the country, a real treat in the face of such a nasty winter.

Beyond the last days of April, it is expected that the weather will take on a slightly cooler tone. In the wake of a Kelvin Wave currently pushing across the Pacific, enhanced tropical convection is expected to develop near the 60E Longitude demarcation, a classic Phase 1 MJO signal. When we see enhanced tropical convection in this Phase 1 signal, it typically means we can anticipate cooler than normal weather here in the United States.

I am a bit skeptical of this cold weather forecast, due to the response we're looking to see in East Asia around April 26th. 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. In this image above, we see projected tropopause pressures, vector winds, and wind speeds way up in the middle-upper regions of the troposphere into the stratosphere. If we look to this forecast image, valid April 26th, and find Japan in the top left corner of the image, we can make out a bulge of orange pushing towards the center of this image. That orange bulge signifies the presence of a Rossby Wave. In simple terms, this Rossby Wave will 'break' over Japan and initiate an intensive warming spell. This may continue for some time, but if it does happen in late April, we would likely see the cooling effects of the Phase 1 MJO hurt, as this East Asian development would likely overrule it.

To summarize:
• A warm end to April is expected.
• A cool start to May is possible, but there are hints that the late April warmth may just carry over into May. More time is needed to investigate this potential.
• A severe weather event is possible in the final 7 days of April, due to the upper level low in the West US.

Andrew

Thursday, April 17, 2014

2014-2015 Winter Preliminary Analysis of ENSO

We're going to take a look at the forecast for the El Nino Southern Oscillation (ENSO) phenomenon for the Winter of 2014-2015 in today's post.

The image above shows a composite of multiple global modeling systems, and their projections for water temperature anomalies in the eastern Pacific. In this forecast, we see the heavy majority of the pictured models preferring to rapidly warm the East Pacific over the summer, as we see the general linear trend from neutral-ENSO temperatures (anomalies nonexistent, near zero) to a moderate El Nino (anomalies of +1C to +1.5C) from now until August. Beyond that point, the model guidance and ensemble members begin to diverge and lose their consensus, but there does appear to be quite a bit of room for a strong El Nino (+1.5C or higher water temperature anomalies) to form later on in the fall.

So, with an El Nino expected heading into the fall, what if it continues on into next winter?

If we look at the image above, we can get a good idea of what temperature anomalies may be like for the upcoming winter due to the El Nino. This image shows you the probability of extreme warm or cold winters during El Nino years. As the image shows, the North US experiences a higher chance of an extreme warmth winter, while the South sees their risk for an extreme cold winter rise. Thus, we can anticipate that the North might see a warmer than normal winter this year and in to 2015. However, that will be looked at again later this year.

If we check out a similar graphic for precipitation anomalies, we get a feel-good story. The West US, currently suffering through one of the worst (if not the worst) droughts on record, typically experiences a wetter than normal winter during El Nino winters. The winters near Montana are typically a bit drier than normal when in an El Nino state, and this is also found along the Ohio Valley. The Plains and East Coast then tend to see wetter than normal winters. This spells good news for not only winter weather fans in the East, but those hurt by the drought in the West.

Let's summarize this.
• Model guidance is strongly leaning towards an El Nino event this winter.
• During El Nino winters, the North US is typically warmer than normal, while the South is cooler than normal.
• During El Nino winters, the East Coast, Plains and Rockies are typically wetter than normal, while the North Plains and Ohio Valley are drier than normal.
• Many more factors affect seasonal outlooks than just the El Nino Southern Oscillation phenomenon- this does not constitute a winter forecast.

Andrew

Monday, April 14, 2014

April 16-17 Potentially Significant Snowstorm

I'm monitoring the potential for a significant snowstorm on April 16-17.

We begin by looking at the GFS model. The GFS model shows a strong storm system dropping down from Canada, taking advantage of the cold air already displaced south in the United States and producing an intense snowstorm in the Upper Midwest. The GFS indicates we may see amounts over 12 inches across northern Wisconsin and upper peninsula of Michigan, with spotty amounts nearing or exceeding 18 inches possible. Given model guidance tendencies to be too snowy/too cold in their forecasts, as well as the fact that models may be overdoing snow because they're stuck in a wintry pattern when we're really transitioning to spring (though the snow falling out my window begs to differ), I'm skeptical of these 18 inch-plus amounts. However, if the storm is as strong as projected, a big snowstorm would be expected. I'm just skeptical of temperature profiles right now.

The NAM model, a short-range, higher-resolution sibling of the GFS model, is actually showing similar amounts. I'm expressing surprise here because the NAM is notorious for over-doing snowfall guidance, in that it can (and frequently does) forecast more snow to fall than what actually does fall. However, comparing the GFS and NAM models, they are pretty similar. While this raises confidence in this outlook, it's nothing to get too excited about. I'm still very skeptical of this event, and I want to emphasize how this has a high 'bust potential' (meaning the forecast has the potential to 'bust', or fail). However, if the forecast ends up to be on target, then I am on board for a big snowstorm. With spring, model guidance starts to struggle due to the pattern changing, and that messes up model guidance. This could be a flash in the pan, or something more.

Shown below is a hand-drawn image of amounts as depicted by the GFS and NAM, for easier interpretation.


To summarize:
• There is the potential for a significant snowstorm in the Upper Midwest on April 16-17.
• Amounts may exceed 12 inches in many places.
• I am skeptical of this forecast, as model guidance may be too cold and may be forecasting too much snow to fall.

Andrew

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.

Andrew

Monday, April 7, 2014

April 13-15 Potential Late-Season Snowstorm

WARNING: The chances of a significant snowstorm in April are VERY low. This post is only showing one model's output, and is made to alert people that there could be some snow in this timeframe. I do NOT expect we see these amounts verify exactly.

I'm watching the April 13-15 timeframe for what could be a strong storm system resulting in, yes, snow.


Shown above is an image depicting the most recent ECMWF model snowfall forecast for a potential April 13-15 snowstorm. The exact image showing the ECMWF snowfall output cannot be displayed for copyright purposes. In this most recent forecast, we saw a swath of 2 to 6 inches of snow stretching from northern Missouri into western Iowa and central Illinois, before moving northeast into both land masses of Michigan. Inside that accumulating snow swath, we then see a rather large area of 6 to 12 inches-plus of snow from western Iowa and northern Illinois into southeastern Wisconsin, before continuing into the northern LP of Michigan. As I mentioned above, I do not expect we see these snow amounts hitting for this timeframe; this post is merely to alert people about the potential for snow.

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, 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.


Looking at the raw ECMWF surface pressure forecast for the morning of April 14th, we see the storm system at a pretty low minimum central pressure, likely below 1000 millibars, in the middle of Missouri. The ECMWF model may be locked on to this snowstorm idea, but its ensembles are much more uncertain.


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. We first observe how the ensembles are much less enthusiastic about the idea of a storm system for this timeframe, only bringing a minimum 1006 millibar swath of low pressure over an area just to the east of the ECMWF model. 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. While this would initially disprove the idea of a storm system in this timeframe, note how the highest spread is located further west of the ensemble low pressure alignment, more in line with the ECMWF model. All in all, this means that the ensembles are seeing this idea of a storm, but aren't biting yet.

To summarize:
• A potentially major storm system is expected in the April 12-16 timeframe.
• Colder than normal weather may arrive with this storm system.
• Model guidance is hinting at a snowfall event during the April 13-15 timeframe, in conjunction with this potentially major storm system.
• There is high uncertainty with this potential event. Caution must be used.

Andrew