Tuesday, October 9, 2012

Long Range Lookout: Very Active Pattern Setting Up

Hour 228 of 12z October 8 Ensembles

Hour 324 of 12z October 8 Ensembles
This is your weekly edition of Long Range Lookout, published October 10.

The ensembles from October 8th (when this post was made) are all showing very strong storms in the long range affecting the United States. While they all have very different scenarios, let me tell you why you should take notice.

You should know if you are an avid weather person that models have trends, i.e. forecasting a heat wave for the last 5 days. Well, ensembles have a similar 'rule'. A general rule of thumb for ensembles is to look for a general pattern. If we look at the ensembles above, we can definitely see that they are all very different, so one would think there is no pattern. However, we aren't looking for a specific storm- we are looking for something they all have in common. A reanalysis of these images reveals that the ensembles all have strong storm systems in their forecasts, meaning we have a pattern.

Seeing as the ensembles are forecasting at least two unusually strong storm systems to come through, one would think that there is some evidence to back this possibility up. However, models have been known to drastically overestimate storms over the ocean. Models typically have a tough time with systems not on North American land, where more data on the system can be gathered.

In conclusion, the next 7-14 days and beyond are looking unusually active. I would not be surprised to see two or more unusually strong storms hit the West Coast and produce severe weather on one side of a frontal boundary and a blizzard on the other side when it moves into the Central and Eastern US.

The North Atlantic Oscillation, or NAO, also looks to be taking a dive, clearing the way for some cold in the Northeast. But that's not all- the Arctic Oscillation (AO) may also plummet as a ridge forms over the North Pole.

The NAO and AO are both oscillations that bring cold to the nation in a negative phase (ridge over Greenland for NAO, ridge over Arctic Circle for AO) and supply warmth to the nation in a positive phase (stormy over Greenland for NAO, stormy over Arctic Circle for AO). This ridge formation over Greenland and the North Pole should create a double-whammy, where some chilly weather should enter the nation. Also, take note of the ridge over the northeast Pacific. This symbolizes a positive PNA, which means more cold and storms for the central and east regions of the country.

Things are looking up, snow lovers!


Tornado Outbreak Threat Looms For Midwest and Plains on Saturday

Preword: What follows is STRICTLY my personal opinion and SHOULD NOT be used to make personal decisions regarding your safety. This is ADVICE, NOT TRUTH. **Use the following information at your own discretion.**

I am becoming increasingly concerned with the potential Saturday has to cause a widespread severe damaging wind and tornado threat throughout the Midwest and portions of the Plains. I have been manually tracking certain parameters of the past several GFS model runs, and have found that the model is trending stronger with shearing and moisture in the region, enhancing severe and possibly dangerous weather. Let's get right into it. All charts below are valid for Saturday evening.

At the 500mb level, a strong disturbance is progged to dig into the Plains and Midwest with a negative tilt and winds upwards of 80 knots accompanying the system. A negative tilt means the system has its winds pointed towards the southeast, like is shown above. Systems with a negative tilt possess more severe weather potential than regular systems, and some of the worst weather has come from negatively tilted storm systems.

This model chart is valid for Saturday evening, and is extremely worrisome to me. The presence of such strong upper level winds means that the atmosphere will be charged up and rearing to go. Such mid level winds enhance storm strength, already setting the stage for a dicey evening. But things only go downhill from here.

This is a map of 850mb winds, commonly known to enhance tornado potential and severe thunderstorm strength if they are high. On Saturday evening, the lower level winds look to be absolutely screaming, with winds maxing out at 70 knots, with some spots going into the 80 knot threshold.

It is uncommon to see such strong wind values, and such values are only associated with the stronger types of storm systems, including negatively tilted systems like this one. I worry that the presence of such high winds will only encourage severe thunderstorms to form and drastically increase tornado potential.

The Storm Prediction Center has already set up a 'Day 5' risk area for much of the Midwest and eastern Plains, indicating that confidence is relatively high that some form of severe weather will occur. An examination of the discussion issued by the SPC on this matter shows that the concerns are severe winds and tornadoes. The threat does indeed exist, as we saw in the 850mb chart above.

This is a map of surface-850mb wind shear. Wind shear is, in simple terms, wind going in different directions at different strengths. The more wind shear there is, the higher the chance of some tornadic activity. One glance at the shearing chart above, and we can already define that the Midwest may be in for a rough night, with high shearing combined with strong lower level winds to give a potentially blockbuster show for the region.

I believe that the Midwest will get the worst of this event, but a fairly large area, including part of the Plains, is at risk for a tornado outbreak. THE RISK GETS HIGHER CLOSER TO THE CENTER OF THE CIRCLE. Places like northwest Illinois, Iowa and northern Missouri may face some serious trouble on Saturday night.

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