Severe Weather: When's it Coming Back?

The atmospheric pattern has been pretty bad as of late, but there's a few things looking up for a rebound by mid to late May.

Just a couple of days ago, a sizable closed low sat itself down in the southern Plains and confirmed what has been a week of waiting and watching to see if this atrocious pattern would actually come to fruition, as models had been saying. Unfortunately for us, the models were spot-on with the development of this closed low, and its presence has declared the beginning of May essentially a dead zone for severe weather. The image above, taken from Saturday, shows the Northern Hemisphere pattern with the closed low in the Southern Plains and massive ridging of high pressure across Canada and into the Northeast Pacific. This ridging created a Pacific pattern that was just oppressive to getting any storm systems into the United States. At this point in time, there is a storm system sliding into the Southwest that could make for a little activity in days to come; I don't see any major opportunities for that storm system, however.

The key to getting out of this bad pattern is what's called the Madden-Julian Oscillation, MJO for short. The MJO has eight phases, each of them centering tropical convection at different locations along the Equatorial Pacific. The above image shows infrared imagery over the Indian Ocean, and we can see the outlined area of enhanced tropical convection stretching from Southern Indian into the waters to the south of South Asia. The location of this convection tells me the Madden Julian Oscillation is most likely in Phase 1, shifting into Phase 2. Typical effects of MJO Phases 1 and 2 on the United States in May include a rather wet pattern over portions of the Plains and Midwest, with below normal temperature anomalies spread across the Central and Eastern United States.

Model guidance sees this MJO wave along the Phase 1 and Phase 2 borderline, with the latest forecast having the MJO extend through Phase 2 before plowing through Phases 3 and 4. Guidance begins to diverge by the time Phase 5 rolls around, but it is Phase 4 we will be looking at for the first opportunity for a severe weather event in some time. The American model suite is looking pretty confident when approached with the idea of a moderately-strong Phase 4 MJO wave. Because there are eight phases, each one is different from the other.

The 500 millibar atmospheric pattern for Phase 4 in the month of May is shown above, provided courtesy of Americanwx. In the typical Phase 4 pattern, we see a low pressure pattern in the waters off the West Coast into the Southwest US. The troughing pattern in these areas indicates the idea of multiple storm systems. If storm systems were to take this route through the Southwest, it would be expected that the severe threat would shift to the Southern Plains. High pressure over portions of the Arctic could open the door for a larger presence of cold air in Canada, which could then aid in the strength of a cold front. However, I am skeptical of the idea- to assume that ridging over the Arctic will automatically provide more cold weather is a dangerous assumption to make, and one I'm not willing to elaborate on.

The thing that is probably the more hopeful indication (outside of the progressively strengthening MJO) for severe weather prospects is the Climate Prediction Center's analog dates for the 8-14 day forecast period. The Climate Prediction Center will take the projected atmospheric flow for the 8-14 day timeframe, find dates with a similar set-up to the forecasted one, and list them on the image above. If you look closely, you can see that the first date of interest is May 2nd, 2009. Of course, 2009 was a year with an unusually active tornado season, and one that many storm chasers would give a lot for. I wouldn't put too much stock into the premise of 2009 being the top analog- many a time I have seen favorable analog dates turn out to be duds. What you should take from this all is that an improvement is on the way. Time will tell if this improvement is small or significant, but it will certainly be better than this absurd closed low in the Plains for days on end.

Andrew

Some info provided by: Stormgasm.com, AmericanWx

Arctic Outbreak Threat Looms

The threat for an arctic outbreak of cold air still looms, although forecasts have backed off on its intensity. While intensity is down, the consensus of a cold air shot has risen.

This is the temperature anomaly forecast from the ECMWF model for 5,000 feet off the ground, with wind vectors superimposed on the anomalies. For this image (valid for the morning of May 12th), we see a large swath of cold air exiting Canada and making its way down through the Great Lakes, plowing into the Eastern US after affecting everyone from central Canada down to Mexico. Wind vectors indicate the deepest cold pocket will be shifting southeast from Michigan/Wisconsin, likely moving towards the eastern Great Lakes in the few days following May 12th. Analysis of temperature forecasts from the European model suggest the northern Plains would easily drop below freezing temperatures, while cities like Chicago, Detroit, Des Moines and Milwaukee may be looking at upper 30s.

The prestigious ECMWF ensemble set, known as the ECMWF EPS (we'll call it the EPS for short), also sees the evolution of this cold wave. In strikingly similar location, the EPS has the coldest of the cold skirting the Midwest and plunging into the heart of the western Great Lakes. At the same time, abundant cool air flows down into the southern Plains and shifts east to allow for a major cool-down across the Eastern Seaboard. Not all that different from the European model alone, the combination of these two forecasts proves to be one that will be hard to beat.

I posted the other day on how the idea of an arctic shot of cold air was having trouble with the models, and that the chance it happens at that intensity was slim. As expected, models have dialed down the intensity of the cold wave, but model uncertainty has been drastically reduced. We could be looking at a nationwide heavy rain event (I hesitate to say severe weather event, because the pattern has been so poor as of late) if current model forecasts go as planned.

Andrew

Atmospheric Analysis - May 4, 2013

This post will address the synoptic atmosphere from May 4th to May 20th.

Current state of the atmosphere is rather bizarre in my opinion, with the United States (henceforth abbreviated as CONUS) under a deep closed low centered in the Plains at this hour. This closed low attains a minimum pressure of over 1000 millibars, although that value is deceiving. Observed temperatures show 60 and 70-degree temperatures being thrown up across the Southeast and Ohio Valley on the eastern side of this storm system as it meanders in the Plains. Further to the north, massive high pressure acts as a barrier to protect Canada from this closed low and bring a stretch of rather quiet weather to the region. Greenland is under a solid storm system, indicating that the North Atlantic Oscillation (NAO) is in its positive (warm weather for the CONUS) phase. A lack of defined height anomalies across the Arctic doesn't help me as far as trying to figure out synoptic temperature trends over the next 48-72 hours. Dynamic situation set up in East Asia, as a strong storm system is going to battle with two ridges of high pressure. Bering Sea is void of any significant height anomalies, complicating the act of figuring out just what the atmosphere wants to do in the aspect of warm or cold temperature trends in coming days.

We first look at the short/medium range forecast, with the American GFS ensembles forecasting 500 millibar height anomalies- the same anomalies as the left-hand panel in the top image. This forecast is valid for the morning of May 10th. On this day, we observe a highly meridional flow across the United States. Elongated high pressure extends from the Pacific Northwest along the western Canadian coastline and into Alaska. The ridge, with its full length spanning from the northwest Plains to extreme northeast Russia, acts as a barrier between a strong area of low pressure in the northeast Pacific. This ridge also allows the formation of a Rex Block in the Western US, with high pressure in the Pacific Northwest opposing low pressure in the Southwest and Southern Plains. Common traits of the Rex Block include a relatively zonal flow downstream of the blocking pattern, and this is somewhat evidenced into the central and eastern parts of the nation. Weak high pressure forms a bridge between the Rex Block ridge and moderate high pressure east of the Canadian Maritimes, and this forms the zonal flow east of the Rex Block. Low pressure in the Southern Plains acts to create some disruptive weather across the Gulf Coast states, but nothing significant in the synoptic aspect is anticipated. Around the rest of the Northern Hemisphere, strong low pressure envelops northern Canada into Greenland, symbolizing a positive Arctic Oscillation (AO)- if you recall, the positive AO helps high pressure and warm weather transpire across the contiguous United States. Low pressure extends into Greenland, defining the positive NAO. Rather stormy East Asian region indicates the United States may see some stormy weather after May 16th, although that is to be determined.

Examination of the long range brings us to the morning of May 18th, using the same ensemble set and same 500mb height anomaly index. General atmospheric flow is much more zonal across the board in the long range, with high pressure dominating the United States and western Canada. Such a zonal flow would certainly allow for at least seasonable temperatures to develop across the nation. I am skeptical of low pressure trying to push south through Canada; I fear that the ensembles are overdoing the zonal flow in the CONUS, and cooler weather may be in store for the Upper Midwest in the long range. Arctic Oscillation remains positive in the long range, and North Atlantic Oscillation follows suit. As we get closer to this date of May 18th, I expect the ensembles to see this highly zonal flow turn more meridional- ensemble sets have a natural tendency to make the atmospheric flow far too calm than what ends up happening.

Andrew

Arctic Outbreak May Strike Mid-May

It looks like an Arctic Outbreak may be in the offing for the middle days of May, just as many began to think that the chilly spring would finally retreat.

The European model is shown above, projecting temperature anomalies 5000 feet off the ground for the morning of May 14th. A large swath of well below-normal temperatures is seen swinging into the northern Plains, Midwest and Northeast. It does appear that this outbreak would occur following the passage of a storm system across southern Canada, which is the typical way an arctic cold event transpires. This outbreak is not yet showing up on the American GFS modeling system, and is only shakily supported by the ECMWF model and its ensemble set. That said, the synoptic set-up isn't all that impossible.

A strengthening Madden-Julian Oscillation wave currently making its way across the waters north of Australia will push east into the open waters of the Pacific. As this transition occurs, the MJO will enter Phases 2 and 3, the latter of which we are watching for cold weather potential. The projected timeframe for this arctic outbreak falls on roughly May 14th, which equates to Phase 3 in this European MJO forecast. Ensemble members from the European forecasting system are shown in yellow lines as being in a decent consensus on a moderate-strength Phase 3 MJO status for May 14.

The typical atmospheric set-up for a Phase 3 MJO during the month of May is shown above. We tend to see high pressure building over the Bering Sea into western Asia and Siberia. The result includes low pressure being elongated into two main centers; one east of Greenland and one in north central Canada. With the presence of another high pressure system in the Canadian Maritimes, the second low pressure area can be coaxed to squeeze south into central or even south central Canada. The situation may then turn towards producing low pressure in the northern US in response to the southern shifting of the Canadian low pressure system. Altogether, it comes down to cold weather being more obligated to push towards the US. The cold weather does not necessarily penetrate the United States, but the likelihood of a cold weather event with a synoptic set-up like the one shown above is increased.

Minimum temperatures for the morning of May 14th tell the story, with widespread temperatures in the 20s and 30s extending across Canada and well into the Plains, Great Lakes and Midwest. We even see the freezing line pushing towards northern Missouri and southern Nebraska. As I told above, model guidance is uncertain on the evolution of this scenario. American model guidance is unsupportive, and European guidance that is supportive of such an incident happening is rather inconsistent from run to run. I prefer to leave this event at the 'possible' level rather than elaborating on how this could all go down- I really want to see more model support for this situation before going ahead with potential implications and other effects of this arctic outbreak.

Andrew

Severe Weather Prognosis - May 3, 2013

This discussion concerns the next week and how severe weather may evolve within that timeframe.

Madden-Julian Oscillation is predicted to strengthen and move into the middle phases as the next 7 days evolve. Current stage of MJO is in weak Phase 1 as the above image shows. The GFS Ensembles predict the MJO wave to move through Phases 2 and 3, both of which are known to ignite severe weather concerns. Phase 2 has been judged by multiple meteorologists to have an atypically high risk of tornado outbreaks. Main concern for this discussion is centered around the time that the MJO spends in Phase 3. Time spent in Phase 2 is of little interest, as unusual weather pattern involving a massive closed low in the Plains situates itself in the region for the period of time the MJO spends in Phase 2. By the time the Madden Julian Oscillation wave reaches Phase 3, the closed low will have moved out of the contiguous United States and another storm system will be forcing itself through the Southwest and Southern Rockies. It is this timeframe when our first severe weather risk arises.

Vorticity map shows closed low on the Eastern Seaboard and the next storm system pushing through the Southern Rockies. Future forecasts going into the overnight hours of May 8-May 9 include high instability values and extreme supercell composite values. Capping inversion will likely prove problematic should the current forecast pan out, but the event's details will be addressed in a future post.

Madden-Julian Oscillation will be in Phase 3 during this timeframe, and analogs were examined for springtime Phase 3 dates. These dates were gathered from 2011 to 2006, with candidate months including April through June. Storm Prediction Center storm reports from all dates in these five years were examined, and 17 of 19 analog days included a synoptic severe weather event. The conclusion indicates that Phase 3 of the MJO does include severe weather potential.

Concern then falls to the aforementioned May 8-9 potential severe weather event. Current forecasts have this event progged to be relatively significant, but further analysis is needed in coming days to assure or disprove this estimation. Not willing to examine any dates beyond the May 8-9 timeframe, uncertainty in both model predictions and MJO forecasts are too great to give any confidence.

Andrew