April 9-10 Potential Severe Weather Outbreak

We appear to be facing the season's first severe weather outbreak on April 9th and 10th.

Model guidance indicates a strong storm system will eject out of the Southwest and into the Southern Plains to start off the second week of April. As it does so, a band of strong winds in the jet stream (pictured above) will shoot northeast on the east flank of the storm system. It is these upper level winds that will aid in what I am predicting to be the season's first severe weather outbreak. I anticipate the upper level wind field to exceed 100 knots, as the forecast graphic above shows. For future reference, this and all forthcoming graphics are valid the evening of April 9th. With the upper level wind support, we have one of many pieces to the puzzle of making a severe weather event.

We now continue on to something called Deep Layer Shear, or DLS. As you know, shearing is one of the necessary ingredients for tornado formation, and also for severe thunderstorm formation. The higher the shearing, the stronger the storms. Looking at the layout on the forecast image, we see the highest shearing extending from central Texas into Missouri and even into Iowa. The highest shearing appears to be along a forthcoming cold front that will kick the nation's first round of springtime temperatures out. With springtime temperatures present when the cold front comes through, the cold front should react like a springtime cold front- strong and possibly severe. With the highest shearing centered to the south, the largest tornado threat would be confined to the Southern Plains at this time.

Continuing to work our way down the atmosphere, we arrive at the 700 millibar layer, best known for the layer which holds the nocturnal lower level jet stream, or LLJ. The lower level jet stream is just what the name says- a smaller, weaker jet stream at a lower level of the atmosphere. Looking at the chart above, we see our storm system shown in Nebraska. We can identify that the storm is roughly in that region, as the pressure contours begin to close off and center themselves, indicating the presence of lower pressure and thus our storm system. The wind field is rather quiet around the north and western sides of the storm system, but it is the southeastern flank of the system where we find high wind speeds. A glance at the legend on the left tells us we are dealing with winds above 55 knots. As a rule of thumb, when you have high lower level winds and high upper level winds, the chance of severe weather is greatly increased. Of course, other atmospheric factors have to be present to provide for the severe weather, but a solid wind field certainly helps. These high lower level winds are most prevalent in Arkansas, north of the Gulf Coast. I want you to take note of the location of those lower level winds, as we will discuss it a bit later on in this post.

We now arrive at the surface. On the top are predicted dewpoint values for the evening of April 9th, and on the bottom are precipitable water values in colors, high and low pressure system denotations in their respective letters, and 850 millibar wind forecasts in the light gray wind barbs. The general idea of these two charts is clear; we are dealing with the availability of moisture. I pull these two up together, as they are closely intertwined and both are required for severe weather production. Basic meteorology states higher dewpoints means higher moisture. In the same sense, higher precipitable water (PWAT) values indicate higher humidity (you may have heard your TV weatherperson say that there is the equivalent of 1.3 inches of water in the air on a humid day- that is precipitable water). There are a few things we can draw from these two images. The first is that this storm system, wherever it ends up, will have the Gulf of Mexico open to draw loads of moisture north to provide a moist base for severe weather. This is clearly reflected in the high dewpoints and PWAT values across the Plains and southern Midwest. One other thing we can deduce from the second image in particular is how strong the lower level wind field will be. We already discussed the lower level jet stream, but these wind barbs are for the 850 millibar level. You may recall the 850 millibar level is 5,000 feet above ground, while the 700 millibar level is 10,000 feet above ground. As far as finding out what wind speeds those wind barbs are predicting, each long line is 10 knots, each shorter line is 5 knots, and each triangle is 50 knots. For example, we can see a few triangle wind barbs in Louisiana, Arkansas, Texas and Oklahoma. Not only does that mean 50 knot wind speeds just 5000 feet above ground (AG), it also means a very windy day here at the surface if this forecast were to verify.

So we've figured out that the models are pretty excited about this event- they've been showing it (albeit not extremely consistently) for multiple forecast runs now. But is it possible we could get some clues from the past on where this severe weather event may go?

The answer is yes.

If you think back to Christmas Day 2012, there was a severe weather outbreak that spanned much of the Gulf Coast. The event began in Texas with a tornado reported in Houston, continuing and strengthening on east through Louisiana, Alabama and Mississippi. A very wild day as most of the severe weather reports were tornadoes and damaging winds, as the archived Storm Prediction Center report chart shows on the bottom image. The top image illustrates the 500 millibar overview on the evening of Christmas Day. The storm system was plowing through the Gulf Coast, with the severe weather out in front.

Many of you know me as an advocate of the repeating pattern of the Lezak Recurring Cycle (LRC). This year's cycle length is approximately 53 days long, and if we count back two cycles (106 days), we arrive a within a day or two of Christmas Day. What does this mean? This means that this upcoming severe weather event could be happening in the same pattern that the Christmas Day severe weather outbreak occurred. Ergo, this upcoming severe weather potential could echo the Christmas Day event (not specifically in strength, but in overall pattern). One thing that should be noted is that the April 9-10 storm looks to be further north than the Christmas Day storm. This would mean the severe weather should then be further north as well. It's too early to name states and areas that could be affected, but my faith in this event's potential is growing by the day.

Andrew

*Note- the connection to the LRC was made by the members of the AccuWeather Forums.

First Severe Weather Outbreak On The Horizon

The first solid severe weather outbreak of the severe weather season appears to be on the horizon.

Medium range model guidance is indicating that a storm system will come onshore from the West Coast to kick off the second week of April. As the image above (valid for April 9) reveals, the storm will then travel east and into the Southern Plains. As this happens, high pressure in the East US will be pushed away towards the Atlantic Ocean, leaving warm air spread out east of the Mississippi with nowhere to go. Since spring is around the corner, a good cold front should arrive and could very well bring the first severe weather outbreak of the season. Let's look closer at different factors going into this event.

We'll start with the upper layers of the atmosphere and work our way down. Pictured above is the forecasted jet stream position for the same timeframe as the first graphic. We see our storm system clearly outlined by the large dip in the jet stream, beginning to push east. At this particular timeframe, our storm system is beginning to wrap a band of enhanced jet stream winds into its southern flank. This is shown by the darker greens and light yellows found in southern Arizona, New Mexico and Texas. As the storm system pushes to the east, this band of higher winds will also. I anticipate these higher winds to push into Arkansas and into the Gulf Coast/Midwest regions as time goes on. With these strong upper-level winds pushing into the region where the cold front may ignite thunderstorms, the storms could gain a stronger stance if the upper level winds cooperate.

We now move onto the projected shearing values for the evening of April 9th. It is worth noting that a rule of thumb in severe weather forecasting is that higher shearing values means a higher tornado risk if the rest of the atmosphere is cooperating. We see the highest shearing levels placed from north central Texas into Missouri and Illinois, forming an upside-down horseshoe in a similar formation as the projected jet stream position for nearly the same time. Because we are seeing the highest shearing values just along the projected cold front, there is certainly reason to think any potential thunderstorms would get a boost of energy from both the favorable jet stream winds and enhanced shearing values.

Continuing to work our way down the atmosphere, we arrive at the 700 millibar mark, the area best known for watching the lower level jet stream. The lower level jet stream (LLJ) is the main driver of severe weather when you find out some nasty weather may be headed your way overnight. If you know that the sun heats the Earth during the day to give energy to the storms, you may ask how they can keep going at night. The nocturnal lower level jet stream is your answer. This graphic is for the 700 millibar level, showing projecting winds for the evening of April 9th. We see the strongest winds are centered in southern Illinois, with still-formidable with speeds stretching across Missouri, Arkansas and into eastern Oklahoma. You can see the location of the storm system with the multiple circles in the central Plains, and the location of these high winds to the immediate east of those circles tell me these high winds will be right in the area for thunderstorm formation. These high winds increase my concern that this could be a solid severe weather event, possibly even reaching outbreak level. I will discuss how that outbreak level will become more likely as we keep going.

Now shown are projected precipitable water values in millimeters for the morning of April 9th. We see our storm system in the central Plains (denoted with the L) and our precipitable water values in colors. Winds 5,000 feet above the ground are shown in the gray wind barbs. Take a look at what you see:  those pale yellows are high precipitable water (PW) values being drawn directly from the Gulf of Mexico. With abundant moisture being a crucial ingredient in severe weather, this system could certainly have enough moisture to put on a show of severe weather. An analysis of instability maps for this same timeframe reveals enough energy is projected to be present for the season's first real opportunity at a synoptic severe weather event. The wind barbs for 5000 feet above ground are screaming at a maximum 67 knots, meaning the moisture flow will be intense. If this forecast verifies, we will most likely be looking at a good severe weather day for storm chasers.

Natural caveat is we are 8 days away and the forecast will change, but with the pattern looking more active in the long range, this sort of situation could happen more than once!

Andrew

Pattern Looking Warmer, More Active

The overall weather pattern for the United States looks like a solid warm-up is on the way, and with it a more active storm pattern.

We start with the forecast for the Arctic Oscillation, valid over the next several days. The black line shows the current forecast, with the other colored lines indicating past forecasts. We see that the forecast for the Arctic Oscillation is another dive to deep negative territory, followed by a tremendous spike into positive territory. If you'll notice, we have had a negative Arctic Oscillation for a while, which means high pressure is stationed over the Arctic. When the Arctic Oscillation (AO) is negative, cold air is more encouraged to hit the United States, and this effect has been shown greatly over the past few weeks. If you want something to blame for the chilly start to spring, this is your culprit. The spike to positive territory means we should see low pressure building back over the Arctic, allowing for warmer air to flow back into the nation.

Another index we can look to for temperature guidance is the Madden-Julian Oscillation, or MJO. The MJO involves the location of convection along the equator in the Pacific Ocean. Each phase indicates a different location for this convection, and each phase affects the US differently. For example, when the MJO is in Phases 8-2 in the winter, we can expect cold weather. We see the index moving into Phases 4 and 5 in coming days, as shown above by the European model. These phases in the springtime tend to favor a slightly warm/slightly cool nation, switching back and forth depending on the strength of the phase. We see that the MJO is close to the circle at the center of the image, meaning it will be weak when it enters these phases. Nonetheless, I expect the incoming warm trend to be assisted at least slightly by the MJO. With the MJO in Phases 4 and 5, storms in the West can also become more common, which then forces high pressure formation in the East. This trait usually shows itself in the winter, but could also become present in early mid-spring.

A third, more complicated index can explain what the models are seeing. Shown above is something called the Earth AAM Tendency chart. Without going too deep into this complicated index, the Earth AAM can help us predict incoming temperature patterns. It appears that when the Earth AAM (EAAM) is negative, colder weather becomes more prevalent. In the same sense, a positive EAAM favors warmer weather. Recently, we began seeing a spike in EAAM Tendency values not seen this year. The fact that the Earth AAM Tendency is strongly positive means the Earth AAM itself should also begin to rise to more positive values, helping along the warmer pattern that will be ushered in.

The Climate Prediction Center is on board with this warm-up, predicting over a 60% chance of above normal temperatures across the central and eastern US. The higher the probability, the more likely a strong warm up will occur. With the pattern beginning to favor a stormy West US, it is only logical that we would see a warm and quiet East. This quiet will only last for so long- the storms in the West have to go somewhere. The storm systems beginning to progress east into this warmth then allows us to confront the potential of multiple severe weather events, the first of which is on the horizon and can be seen on the blog.

Andrew

April 5-7 Potential Winter Storm - Midwest, Plains

The potential exists for a winter storm to impact the Plains and Midwest between April 5th and 7th.

The GFS American model has a winter storm moving through the Plains and Midwest beginning on April 5th and continuing through April 7th. Current indications are that this system will drop from Canada and progress through the northern Plains and Upper Midwest. As the system takes this path, a snowstorm will likely evolve as cold air produced from the abundant snow cover in Canada and continued high pressure presence over the Arctic.

Model guidance suggests snowfall amounts from this system would well exceed half a foot, and many locations would top one foot of snow. The heaviest amounts would be found in west central Wisconsin, central Minnesota, northeast South Dakota and extreme southeast North Dakota. Lesser amounts would be found away from the aforementioned areas, but a sharp cutoff will likely exist south of those areas.

Now, the European model does not believe this will be a winter weather event. The model indicates a rain solution is the more likely scenario, and snow would not be nearly as abundant as the American model indicates. A few notes concerning the models, for those trying to pick out which solution is more likely for themselves: The American model has had recent success with storm systems over the European model. Models do not handle springtime storms well due to the unstable temperature profiles (in the sense that precip. types are more uncertain than in the winter).

I will not pick out a solution right now as too much uncertainty exists for my liking, but this storm does need to be monitored for the possibility of a late-season snowstorm.

Andrew

Long Range Chatter - March 30, 2013

Temperature anomalies from the last week show the lack of spring continuing throughout nearly every region of the nation, save the Southwest. Temperature anomalies, shown in Kelvin, dropped as low as 10 degrees below normal in the Dakotas, with even the Gulf of Mexico experiencing some of this cool weather. This cold weather has been provoked by prominent high pressure over the Arctic Circle and Greenland, as this reanalysis image from March 25th shows:

500mb height anomalies are placed on the left, where warm colors depict high pressure
and cool colors indicate low pressure. Mean sea level pressure contour lines, as well as cloud
cover are shown on the right half of the image.

In response to this extreme high pressure over the Arctic and Greenland, low pressure has been provoked to develop over the central and eastern US, more-so over the latter region. As long as we see that high pressure area over the upper latitudes, chances are more than likely that the tendency for cooler weather will continue to fall upon the central and eastern US. With the cold weather, we also expect a tendency for low pressure in very similar regions, so storm systems moving into the eastern US could be more commonplace than instances of above-normal temperatures.

European ensembles predict that this high pressure will remain stagnant over the Arctic and Greenland from now until at least 10 days away. Consequential low pressure will also stay in place north of the Canadian Maritimes and into eastern Canada. New England will also be affected by this relatively stagnant body of low pressure, and these effects may range from below normal temperatures to unusually cloudy days (both of which I consider likely). High pressure attempting to develop over the Western US will enhance cold weather potential south of Canada, possibly encompassing millions east of the Mississippi.

European model temperature anomalies 5000 feet above ground display this cold weather trend, clearer than day in the Northeast. Below normal temperature anomalies resonate well into Canada, sideswiping the Great Lakes and Upper Midwest. Warmer than normal temperatures will assist the skiing season in coming to a close in regions where resorts remain open. This forecast, valid for forecast days 6-10, is pretty likely to turn out when you look at what has been experienced and comparing it to what is being forecast. High pressure will remain favorably positioned over the Arctic to allow cold weather to hit regions of the US previously hit by the cold, and also spread eastward.

Summary
Next 10 days look to remain below normal in temperatures for the central and eastern regions of the United States. Warmer than normal temperatures should persist in the Western US. I feel that best chances for storm systems will be in the northern Gulf Coast (TN/AL/MS/GA) and north, along the Eastern Seaboard and into the New England area. Probably will not see any significant Nor'easters in this timeframe, although a wayward storm system well offshore may allow coastal cities to get a quick bout of precipitation.

Andrew