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

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

Verification of the 2013-2014 Winter Forecast

Hello everyone, this is a look back at my 2013-2014 winter forecast, and an analysis of how well it performed against what we observed from December 1, 2013 to March 1, 2014.

Temperature Verification

In my Final 2013-2014 Winter Forecast, I projected that we would see the core of the cold centered in the Northern Plains, Upper Midwest, Great Lakes, Midwest, Ohio Valley and into the Central Plains. The forecast also called for warmer than normal temperatures in the Southeast, while average conditions were to prevail over the southern Plains and the Northeast. My projection had the West US in for back and forth temperatures, ending up around normal.

Actual temperature anomalies for December - January - February 2013-2014

Looking back at what actually happened this past winter, we saw the core of the cold in the Upper Midwest into the Midwest itself, with still well-below normal temperatures in the Northern Plains, Ohio Valley, Great Lakes, and Southern Plains. This part of the forecast verified quite well, with the only trouble spot being the Southern Plains, which ended up a bit cooler than forecasted. The Northeast observed overall normal conditions, verifying that part of the forecast. The Southeast did end up a bit warmer than normal, but not as much as I thought it would be. The main trouble spot was in the West, where we saw warmer than normal temperatures instead of the average conditions I predicted. This can be attributed at least partially to the drought, as well as the persistent ridge in the Gulf of Alaska.

On a self-grading scale, I might put this forecast at an A-minus, maybe down to a B-plus.

Precipitation Verification

For the precipitation forecast, I anticipated above normal precipitation from the Central Plains into the Midwest and Great Lakes, believing that was where we would see the primary storm track. I also believed a drier than normal winter was in store for the Southeast, with neutral conditions in the East and West.

This part of the forecast actually busted quite badly. The Central Plains saw below normal precipitation rather than the above normal precipitation that was forecasted, though we did see verification in the Great Lakes. Overall, the East Coast didn't stray too far from what I thought would happen, ending up only slightly above normal overall compared to my projected average conditions. The West US definitely busted hard, with a serious drought taking over instead of my forecasted average conditions. The Southern Plains also ended up drier than normal, while the Southeast did not.

On a self-grading scale, this portion of the forecast likely deserves a D-plus.

Snowfall Verification

The snowfall forecast called for snowier than normal conditions over the Plains, Upper Midwest, Midwest and Great Lakes. The rest of the country was pegged for average conditions, with the East Coast possibly in line for some big storms.

I do not have a verification image, but I can affirm that the Midwest and Great Lakes regions verified accurately in this portion of the forecast. The East Coast ended up a bit snowier than I thought it would, and the West definitely got much less snow than what is considered average.

I would put the accuracy of this forecast at a B even.

Overall Forecast Verification

The overall forecast called for some big snowstorm potentials in the Northeast, with a warm and dry winter in the Southeast. The risk for a warm and dry winter existed in the Southern Plains, with a cold and snowy winter outlined for the North Central US. Slightly below average temperatures were anticipated for the Rockies, while a warmer than normal winter was projected for the Pacific Northwest. I feel verification of this graphic went better than I actually thought it would, with most areas seeing at least partial verification of this winter forecast.

For the overall forecast, I believe a grade of B-plus would be appropriate here.

What did you think of the accuracy of this forecast? Comment below with your thoughts!

Andrew

April 12-16 Potentially Major Storm System & Cold

Tropical Tidbits

Model guidance indicates we will see a trough drop through Japan on the morning of April 6th, in the wake of a stronger system just before it. You can read about that storm on this post. 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 case, we see can expect a storm system in the United States, likely accompanied by a brief cold spell, in the April 12-16 time period, 6-10 days after the expected April 6th intrusion in Japan. The post title indicates this may be a potentially major storm due to the deep negative height anomalies in the above forecast, but that may be more for the cooler weather.

Projected PNA Index on top left
Projected NAO index on top right
Projected WPO index on bottom left
Projected EPO index on bottom right

Shown above is a chart of four panels, each showing the projected anomaly of a different oscillation in the atmosphere. Let's begin with the top-left panel, the PNA. The Pacific North American (PNA) index deals with 500mb height anomalies in the West US and into the Northeast Pacific. When we see high pressure dominating this area, we deem it to be a positive PNA. When we see storminess over the aforementioned regions, a negative PNA is in place. The positive PNA tends to lead to cold and stormy weather in the Central and East US, while the negative PNA provokes high pressure and warm weather across the same areas. We see the PNA is to be positive during this expected storm system, which does support the idea that this storm will bring along cooler conditions with it. A potential storm track in the positive PNA is one that goes through the South Plains, which could create severe weather, before shooting north into the Midwest or Ohio Valley.
We see the WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are both projected to be negative during the April 12-16 time period, adding to the chances that we see cooler weather during this timeframe.

To summarize:
• There is the potential for a storm system in the US in the April 12-16 timeframe.
• Cooler than normal conditions may arrive during this time period.
• Severe weather is a possibility. Exact locations are unknown.

Andrew

Long Range Outlook for April 2014

Today's post will look at the expected weather pattern for the month of April, 2014.

We currently see tropical forcing located over the Indian Ocean, specifically southeast of the subcontinent of India. This forcing is associated with a convectively-coupled Kelvin Wave (CCKW), which will begin progressing east in future days. Currently, the placement of the CCKW supports cooler than normal weather, which we have been seeing recently, and should continue to see in the next few days, probably into about April 9th or 10th.

Specialized maps from the Australian Bureau of Meteorology indicate we will see this CCKW push east in coming days, taking the tropical forcing with it into a pattern that should bring more seasonal, possibly even above normal temperatures to the United States.

Typically, when we see enhanced tropical convection entering the western Oceania region, we tend to see the weather pattern favoring warmer than normal weather in the United States. On the Madden-Julian Oscillation scale, this would mean movement into Phases 5 and 6.

500mb height anomalies for Phase 5 MJO events in April (left)
500mb height anomalies for Phase 6 MJO events in April (right)

Temperatures for March-April-May for all eight MJO phases

If we take a look at mid-level height anomalies for MJO Phases 5 and 6, as well as temperature anomalies for phases 5 and 6, we see that the pattern decisively supports seasonal to slightly above normal temperatures. Even though it won't be a Madden-Julian Oscillation wave going into western Oceania, the tropical forcing from the CCKW should suffice to bring about these seasonal temperatures.

Despite this, a recent spike in East Asian mountain torque anomalies means we're likely to see those warm temperatures dampened a bit. In the winter, positive East Asian mountain torque values indicate the perturbation of the upper Arctic, and these +MT events can bring about sudden stratospheric warmings. In this specific case, though, we can just expect those warmer temperatures to be tempered some by this positive mountain torque event in East Asia. These warm CCKW-induced temperatures should come about in the general mid-April timeframe, but again, watch for cold shots in response to the positive East Asian mountain torque in the not-too-distant future.

Moving along, we can look ahead into late April by examining outgoing longwave radiation (OLR) plots from Paul Roundy at Albany. Looking specifically at the bottom plot, valid for April 20th to 27th, we see strong negative OLR anomalies (enhanced tropical convection) present in western Oceania, the same place where the CCKW is expected to travel in just a couple days. If this chart is to be believed, we may expect seasonal to warmer than normal temperatures for the end of April, as these negative OLR anomalies are in the typical Phase 5 and 6 MJO phases, which, as we looked at above, bring about warmer than normal weather in the US.

To sum up, here is a graphical representation of expected temperature anomalies across the month of April.

Andrew