The severe threat for this week does not seem significant to me, cutoff lows usually do not do much in the way of severe. I do not think there is any major event regarding the cutoff.
A lot of things look up for major events (perhaps four), coming up in the next two-three weeks. First of all is the one that I sorta called off twice. Way back about 1-2 weeks ago, I had predicted an event around this time period (did not write it on the blog). Then I "cancelled" the event after models had consistently predicted no such event would happen for several days. Shortly after that, the models caught on (goes to show that it's perhaps better to go with observations and extrapolation than pure models in the long range). But I had doubts on whether it would cut off, and believing that it would, in my last post I had written that it might.
That idea looks to be wrong. A strong Pacific jet will prevent that from happening this time. The 500mb hemispheric pattern looks pretty ideal, in fact. In many outbreaks, there was a cutoff or a closed low over the SW that gave way to a strong jet stream which drove a larger, more mobile longwave trough that ended up being a big event. The reloading period had already been done a few days ago. Right now, the key continues to be the ridging over the CONUS.
Revision: the posts I had...
http://www.easternuswx.com/bb/index.php?s=&showtopic=128581&view=findpost&p=1831884
http://www.easternuswx.com/bb/index.php?s=&showtopic=128581&view=findpost&p=1832127
http://www.easternuswx.com/bb/index.php?s=&showtopic=128581&view=findpost&p=1833459
There are two disturbances that the renouned polar vortex over Alaska (key forsynoptic severe outbreaks) are going to spit out. One initially will amplify pretty far west according to models. The problem with this one is because it amplifys over the EPAC instead of the Intermountain West, it will likely become very high amplitude once it approaches the Plains, and thus, shear vectors will be unidirectional. Another potential problem is regarding moisture return, since models do predict a closed low over the Bermuda area which could lead to a high pressure cell too far north to tap into a Caribbean moisture source. Although I believe the models are underdoing the moisture return, most of the biggest outbreaks do not have that feature and instead have a semizonal, maybe slightly purturbed, flow.
The second disturbance is the question mark. If this trough materializes, it has a lot of potential. This one should have a source region closer to Alaska as the polar vortex splits, and one piece flies into Siberia. The question mark for this is indeed how the first trough moves away, and how strong a ridge is left behind over the southern CONUS. Will it amplify? If the first trough does not move away quickly enough, will there be enough return flow and instability? All those questions should be resolved within the coming days. FWIW, the Euro has been questioning the validity of the possibility of this, being incredibly pessimistic. So obviously, I'd trust the better model for now. Analysis, extrapolation, and pattern recognition suggests that in a case like this, the more widespread severe wx the first produces, the less the second produces. So really, I don't know which is bigger, except that I'd expect a least one significant severe wx outbreak with these two threats.
As the other piece of the PV moves into Siberia, the pattern will reload. A ridge will redevelop over the Aleutians, but it will be short-lived as the reformed polar vortex over Siberia reinvigorates the Pacific jet, which will most likely undercut the Aleutians ridge yet again. This new piece could lead to another big event in early April.
Stay tuned.
Thursday, March 22, 2007
Sunday, March 18, 2007
Major update - Multiple threats coming at the turn of the month possible
OK yeah I wrote two days ago that I did not expect any major outbreaks in March. I will change that because the timing of the "big tamale" may be earlier than expected.
Note that the latest European has a significant system around the 240 hr range, and so did the 18Z GFS. I am divided on the threat will not materialize like what is depicted on those runs, because the fundamental pattern really is sorta tricky. Sure, we just had the reloading with the sleet storm over the NE, and that piece will reinforce the Hudson Bay PV, but the big problem is the flat ridge over the CONUS. When does this go away is the question? We've had the Aleutian blocking high now eroded. As the next system dives down into the CONUS, the energy over the EPAC will split, sending the next system into Mexico where it will cut off (still a severe threat around the 21st but not too big given that cutoffs usually don't produce a lot of severe). The other piece will deflect northward and form a very strong and deep polar vortex (PV) over Alaska. This is where it gets interesting.
As this happens, a massive block develops over Scandanavia (yes, another block, as I said blocks don't go away easily). Some ensemble members are progging closed 576 contours at 500mb over northern Scandanavia (keep in mind, this is March and that's near the Arctic circle!). So this pattern becomes pretty stagnant, with the PV on the Alaska and Russia side, and the block in northern Europe, and flat zonal flow pretty much everywhere except invof the block. So now the question is, how powerful does this block become? If it becomes so strong that there's cross polar flow, the PV over Alaska becomes essentially trapped. OTOH, if the block slowly breaks down, this will allow the PV energy to branch off. The timing of the PV branching off is indeed the timing of what I think could be an extremely active period. This, according to the GFS Ensemble mean, should occur ~228 hr.
However, as the PV branches off and leaves a small portion over the GOA to amplify into the next longwave over the western CONUS, what happens to the flat zonal ridge over the CONUS? This is why I am divided on the initial threat. If that ridge is too powerful, the low cuts off again. BUT, many of the biggest outbreaks occur after a cutoff over the SW is ejected out. The reason is that following pieces of energy start out more progressive and less amplified in the EPAC and thus don't have as much tendency to cut off after amplifying over the West (in fact, just take a look at Palm Sunday 1965 for an example of this, the pattern was very much in support of cutoffs, the the shortwave that spawned the outbreak just barely missed being cut off over the SW, even though the part that it split from did indeed cut off).
The next few disturbances after that are uncertain, but let me just say this: ANY disturbance which manages to stay progressive (the chances will be enhanced if we have semizonal Pacific jet with small kinks in it) will have the potential to be significant severe producers. This is going well after 240 hr, and will most likely feature big events from March 29-April 10. The caveat for multiple threats is if the first disturbance manages to really amplify and form into a new PV, which will shut down the pipeline.
Note that an alternative option would be for the big event to occur at ~240 hr, but although the mainstream models seem to show it, it seems like the less likely option right now.
All of this is subject to much change, of course, as this pattern's pretty dynamic and unpredictable.
Note that the latest European has a significant system around the 240 hr range, and so did the 18Z GFS. I am divided on the threat will not materialize like what is depicted on those runs, because the fundamental pattern really is sorta tricky. Sure, we just had the reloading with the sleet storm over the NE, and that piece will reinforce the Hudson Bay PV, but the big problem is the flat ridge over the CONUS. When does this go away is the question? We've had the Aleutian blocking high now eroded. As the next system dives down into the CONUS, the energy over the EPAC will split, sending the next system into Mexico where it will cut off (still a severe threat around the 21st but not too big given that cutoffs usually don't produce a lot of severe). The other piece will deflect northward and form a very strong and deep polar vortex (PV) over Alaska. This is where it gets interesting.
As this happens, a massive block develops over Scandanavia (yes, another block, as I said blocks don't go away easily). Some ensemble members are progging closed 576 contours at 500mb over northern Scandanavia (keep in mind, this is March and that's near the Arctic circle!). So this pattern becomes pretty stagnant, with the PV on the Alaska and Russia side, and the block in northern Europe, and flat zonal flow pretty much everywhere except invof the block. So now the question is, how powerful does this block become? If it becomes so strong that there's cross polar flow, the PV over Alaska becomes essentially trapped. OTOH, if the block slowly breaks down, this will allow the PV energy to branch off. The timing of the PV branching off is indeed the timing of what I think could be an extremely active period. This, according to the GFS Ensemble mean, should occur ~228 hr.
However, as the PV branches off and leaves a small portion over the GOA to amplify into the next longwave over the western CONUS, what happens to the flat zonal ridge over the CONUS? This is why I am divided on the initial threat. If that ridge is too powerful, the low cuts off again. BUT, many of the biggest outbreaks occur after a cutoff over the SW is ejected out. The reason is that following pieces of energy start out more progressive and less amplified in the EPAC and thus don't have as much tendency to cut off after amplifying over the West (in fact, just take a look at Palm Sunday 1965 for an example of this, the pattern was very much in support of cutoffs, the the shortwave that spawned the outbreak just barely missed being cut off over the SW, even though the part that it split from did indeed cut off).
The next few disturbances after that are uncertain, but let me just say this: ANY disturbance which manages to stay progressive (the chances will be enhanced if we have semizonal Pacific jet with small kinks in it) will have the potential to be significant severe producers. This is going well after 240 hr, and will most likely feature big events from March 29-April 10. The caveat for multiple threats is if the first disturbance manages to really amplify and form into a new PV, which will shut down the pipeline.
Note that an alternative option would be for the big event to occur at ~240 hr, but although the mainstream models seem to show it, it seems like the less likely option right now.
All of this is subject to much change, of course, as this pattern's pretty dynamic and unpredictable.
Friday, March 16, 2007
ok yeah I do apologize for not updating this
There was a big tornado outbreak on Mar. 1. Moisture was good and the dynamics were more than sufficient with the only thing preventing a bigger tornado outbreak was the convection to the south which resulted in a more concentrated tornado outbreak rather than a larger more spread out one, and also, the separation of the better low-level kinematic fields from the better instability in the north.
The pattern since then has been docile. Large flat zonal ridge over the U.S. Because of that, I do not believe there will be any MAJOR severe wx outbreaks for the rest of March. REVISION 11:49 AM PDT Mar 16... the 12Z GFS is particularly ominous for an outbreak in about a week. This one is a tough one to forecast since there could be a potential for the disturbance to cut off at a very early stage of the game, which will cut down the severe chances. Right now am leaning on a minor severe wx event, and go more bullish as time goes by if it warrants. The overall synoptic scale looks pretty good actually with an ejection of a trough over the NE giving way to the Pacific jet. So of course it deserves watch...I may have overlooked it yesterday night when I posted this entry.
I think the next big severe wx event(s) will come when we get the next major "reloading period", the time when we get some time of sufficiently long lasting cold intrusion. The current pattern is downright terrible for any type of severe wx, if this was in May or June, and could in fact be one of those severe season enders since cool snaps are pretty difficult to get in June. So definitely, better now than later. Research from some renouned long-range experts do suggest that the NAO would switch to negative sometime in April. The long range progs also suggest that the Pacific jet should be somewhat above normal in terms of strength. In this case, the PJ is NOT GOOD if it keeps things zonal. We're gonna need some type of minor amplification, outside of the major amplification, the block over the Aleutians that's causing the backup and resultant stale pattern.
I still think a big outbreak is very possible during April, especially early April (in fact, with the polar vortex over the Pole and dropping energies down, along with a fairly zonal Pacific, prelim indications are this could be significant -- if it materializes). Looking at the really long range, right now it seems the Aleutian block could remain for reappear frequently given the tropical influences (the Nina). Since wavelengths do shorten in the summer, what effects that could have is still uncertain to me right now.
The pattern since then has been docile. Large flat zonal ridge over the U.S. Because of that, I do not believe there will be any MAJOR severe wx outbreaks for the rest of March. REVISION 11:49 AM PDT Mar 16... the 12Z GFS is particularly ominous for an outbreak in about a week. This one is a tough one to forecast since there could be a potential for the disturbance to cut off at a very early stage of the game, which will cut down the severe chances. Right now am leaning on a minor severe wx event, and go more bullish as time goes by if it warrants. The overall synoptic scale looks pretty good actually with an ejection of a trough over the NE giving way to the Pacific jet. So of course it deserves watch...I may have overlooked it yesterday night when I posted this entry.
I think the next big severe wx event(s) will come when we get the next major "reloading period", the time when we get some time of sufficiently long lasting cold intrusion. The current pattern is downright terrible for any type of severe wx, if this was in May or June, and could in fact be one of those severe season enders since cool snaps are pretty difficult to get in June. So definitely, better now than later. Research from some renouned long-range experts do suggest that the NAO would switch to negative sometime in April. The long range progs also suggest that the Pacific jet should be somewhat above normal in terms of strength. In this case, the PJ is NOT GOOD if it keeps things zonal. We're gonna need some type of minor amplification, outside of the major amplification, the block over the Aleutians that's causing the backup and resultant stale pattern.
I still think a big outbreak is very possible during April, especially early April (in fact, with the polar vortex over the Pole and dropping energies down, along with a fairly zonal Pacific, prelim indications are this could be significant -- if it materializes). Looking at the really long range, right now it seems the Aleutian block could remain for reappear frequently given the tropical influences (the Nina). Since wavelengths do shorten in the summer, what effects that could have is still uncertain to me right now.
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