Weekend Severe Weather Threat Could Be Significant

It's looking more and more like this weekend could hold a potentially significant severe weather event for portions of the country.

Situation begins unfolding with a storm system digging into the central Plains Saturday afternoon. 500mb vorticity values from the latest GFS model indicate that the event should begin in Nebraska, Kansas and possibly into Oklahoma. Current indications are that this event will be the beginning of a multi-day severe weather event. This Saturday portion appears to be aligned with a damaging wind and hail threat, with the tornado threat sitting in the background. Storm system will be pushing into the Plains getting ready to go towards a negative tilt, which should happen on Sunday.

Instability parameters for Saturday are impressive across the Plains, with as much as 4000 j/kg of capped instability spread across the western portions of Kansas and Nebraska. Capping inversion is certainly a concern with this event, especially on Saturday, but as the storm begins to get a negative tilt we should see instability increase and the risk of nasty storms increase as well.

Sunday becomes a much more significant day in my opinion, as the storm system begins to dig hard and achieve a negative tilt in the process. Significant vorticity lobes are draped across the Plains and Upper Midwest along the sharp height gradient between storm and ridge. This gradient appears to be centered in the Midwest, and it is here where we may see the strongest storms of the day hit. Cities like the Quad Cities in Iowa, Chicago and Rockford in Illinois, and Milwaukee and Madison in Wisconsin may see some nasty weather on Sunday. Instability parameters show over 5000 j/kg of instability overnight Saturday, decreasing into the mid-3000's by evening on Sunday.

When the trough begins to tilt negatively, it looks like the storm formation style will shift towards a more linear system. My gut tells me we'll see either a squall line or at least some form of linear convective system that moves through the Midwest. I find it hard to believe the environment presented by the GFS above supports an individual cell-type storm over a linear storm system.

Another thing to be watchful of is the massive amount of moisture that will be flooding the Plains and everyone on eastward. With the quantity of moisture that will be present in the United States as the severe weather event blows through, I would bet on at least a marginal flooding risk evolving across portions of the nation over the weekend and into the beginning of next workweek. Now, we're a while out from this event, so I can't get into the depth or expanse of this flooding potential, but when you have this amount of moisture just waiting to be converted to rain, it's easy to find a little flooding potential that could escalate quickly.

As of the current time, my map for this multi-day event is below.

Andrew

Weather Overview for May 11, 2013

Midwest: Two cold fronts are pushing through the region at this hour, with the northernmost frontal boundary being forced south by a vorticity center that is bottoming out into the Great Lakes. Progression of these cold fronts will lead to a chilly weekend before a gradual warm-up builds into the mid-week. No severe weather expected.

Northeast/Mid-Atlantic: Cold front progressing east will interact with a region of heightened moisture in the Mid-Atlantic and initiate showers and thunderstorms. Current Storm Prediction Center outlook highlights this area for a Slight Risk of severe weather. Cooler temperatures expected as cold fronts are moved east in coming days. Severe weather is possible.

Southeast/Gulf Coast: Low pressure system that initiated strong thunderstorms in Texas last night has shifted into Mexico as the complex of active weather skirts the Gulf Coast, mainly confined over the Gulf of Mexico waters. Isolated strong weather risk along the coastline before the complex of showers and thunderstorms makes landfall in Florida. Severe weather is possible during that time.

Pacific Northwest: Disturbance is being pushed north into Canada as a result of high pressure development in the Western United States. Despite the movement of this disturbance away from the region, precipitation remains possible. Most likely scenario is a cloudy day with precipitation entering the region this evening as the back-end of this disturbance is far enough south to interact with the Pacific Northwest. No severe weather expected.

Andrew

East Pacific Hurricane Season Starts Soon

Long range tropical forecast models are hinting at favorable conditions for tropical cyclone development in the East Pacific.

Shown above is a long range probability model for tropical cyclone formation from Paul Roundy. The top image shows general probabilities of formation, while the bottom image displays anomalies of tropical cyclone formation probabilities. Recently, this long range model has been hinting at some long range development west of the Mexican coast, but it is only as of May 5th that the model has upped the ante and added enhanced probabilities of development in those waters.

It's looking like we will see the development of a Phase 1 Madden-Julian Oscillation (MJO) wave towards the first week of June. As is shown above, the eight phases of the Madden-Julian Oscillation and occurrences of tropical cyclone development in each phase in the month of June are shown. If we look in the top-left panel, which shows tropical cyclone development in MJO Phase 1, we see a solid presence of cyclone development in the waters to the west of Mexico. What this means is that we very well could be seeing this long range projection of enhanced development potential in the Eastern Pacific verify- in the form of tropical cyclone development.

Exciting things are happening, folks!

Andrew

First Tropical Cyclone of Atlantic Hurricane Season on Horizon

It appears that the first tropical cyclone threat of the Atlantic Hurricane Season is showing up on long range computer models. The American GFS ensembles have been showing cyclogenesis occurring in the Caribbean, to the point that tropical development takes place. Three of the past four GFS model runs have had the development of a tropical cyclone in the Caribbean. While the ensembles don't go out farther than the cyclone reaching the entrance to the Gulf of Mexico, it looks like the Gulf Coast would be at risk for a potential landfall, should this forecast verify. The good news is that forecasts going out this far rarely verify. What should be noted is a building consistency in the model projections, as well as how an ensemble set is showing this development, not just a single model.

The reason the GFS wants tropical development to occur is due to its idea that the current Madden-Julian Oscillation (MJO) wave in the Indian Ocean will die off and transition to Phase 8, the phase that finds enhanced tropical convection in the Atlantic and Caribbean waters. The American GFS Ensembles project below normal 200mb velocity potential anomalies across northern South America well into the Caribbean and Atlantic. If you recall, I warned about the potential of tropical cyclone development in early June just a few days ago- it looks like that threat may be materializing. The placement of these anomalies indicates that this would indeed be Phase 8 of the MJO. However, the GFS may be taking this MJO wave too quickly to Phase 8.

This image shows the projected Madden-Julian Oscillation forecasts from three forecast systems- the GFS Ensembles (green), the Canadian Ensembles (maroon), and the European Ensembles (yellow). Looking at the end of the GFS ensembles forecast, it is clear that the MJO wave is pushed into Phases 8 and 1 at the end of the forecast timeframe, echoing what I described in the velocity potential image above. The Canadian ensemble set is onboard with this idea of a fast-paced MJO wave progression, landing the wave in Phase 8 by the end of its run. What should be noted here is that the Canadian forecast system is notorious for being horribly inaccurate at this forecast range. We then see the European ensemble set in yellow, which seems to die off as it enters the circle in the middle of the image, called the Circle of Death (COD) The COD means that the MJO wave is too weak to have a defined phase, and thus too weak to have a noticeable effect on synoptic weather here in the US and around the world. The European ensembles prefer to have the MJO wave linger in this Circle of Death rather than progress it east and into the Caribbean and Atlantic seas.

After looking around other model solutions, it certainly still looks like the threat for tropical development (either in the East Pacific or Caribbean) is a logical one. I will monitor this threat closely over coming days and weeks and wil update as is needed.

Andrew

Stratospheric "Final Warming" in Full Swing

It's true ladies and gentlemen, our wintry weather is now coming to an end! The upper stratosphere is going through what is called the 'Final Warming', which is what the name implies- the last warming of the stratosphere before spring effectively begins.

During the winter months, large swaths of above normal temperatures in the Northern Hemisphere will push north and occasionally break into the Arctic and perform the sudden stratospheric warming. During a sudden stratospheric warming, this warm air overpowers the semi-permanent polar vortex in the Arctic Circle and weakens it. If the warming event is strong enough, the polar vortex may be broken up into pieces, pushed to lower latitudes or even be completely obliterated. If you followed this blog in the last winter season, you know what a complete collapse of the polar vortex implies- brutal, pure Arctic cold.

We've been in the spring season for a while now, but as you've experienced, we've been in a more wintry pattern than a spring pattern. A good reason for this could very well be that the stratosphere has not experienced its final warming until the last few days. Up until now, we've been seeing batches of anomalous warm air across the Northern Hemisphere, but they haven't been able to become forced into the Arctic. Now that we are experiencing the Final Warming scenario (which happens every late winter/early spring), I expect that we will be rid of persistent cold air shots in about 2 weeks time. You may see that as a long deadline, but remember that the animation shown above comes from 10 millibars- that's in the upper stratosphere. You can't push air from the stratosphere down to the troposphere in just a few days- the process of a stratospheric warming requires a couple of weeks to feel its full effects.

Andrew