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Severe Weather 2021

Shakespeare 2016 said:
I thought dewpoints in the low to upper 60s was more than sufficient for this time of year. I could be wrong though.
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They are, but if dewpoints over the open Gulf aren't in the widespread upper 60s or lower 70s, your adequate enough low to mid 60 dewpoints usually won't extend too far onshore, and what moisture that does come inland will be shallow and prone to mixing.
 
Fred Gossage said:
The 12z GFS shows how we would be able to have an appreciable severe weather risk later next week... with the Wednesday/Thursday shortwave in the longwave acting as a lead primer wave ahead of a second trailing shortwave around the Friday timeframe. Lapse rates would be steep enough that low to mid 60 dewpoints would be sufficient for more than a low-end risk (12z GFS develops 1500-2000 CAPE over the warm sector with 62-65 dewpoints, for example). The Canadian agrees on the idea of a significant secondary wave, but it is way too south for a severe risk other than coastal areas. Last night's Euro had a secondary trailing shortwave, but wasn't trying to do anything significant with it. Ensembles aren't offering much support for a second wave yet, but that can often happen when trying to pick out individual waves within an overall low amplitude longwave troughs. That's why I cautioned yesterday not to try to pick out individual shortwave evolution and lock on to any one idea just yet. The GFS has consistently had a trailing system that would go on to develop into a significant cyclone, but it's always been much farther southeast. Today's 12z run has been the first that has been northwest enough to pose a severe weather threat. If the Wednesday/Thursday system does indeed cut off over the Upper Midwest, it would make sense that any trailing system would have the opportunity to amplify farther west and/or northwest. This is the route we would have toward an appreciable severe weather risk, and it's not a completely out-of-left-field type of idea either, but there are a lot of moving parts that would have to make it work, and we are VERY FAR from having a consistent signal for the idea.
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I would think dewpoints in the 64F to 68F range would be more than adequate this time of the year.
 
Euro is in support of the idea of an amplifying second wave Friday into Saturday. However, the lead wave Wednesday is kind of disjointed and that messes with the placement of the surface front and low-level winds ahead of the second system. That helps the surface response to the second wave be pushed more south and east, similar to the previous GFS runs. If the lead wave were cleaner, today's Euro would probably look closer to today's GFS. There seems to be increasing support for an amplifying second wave after a lead Wednesday/Thursday wave that primes the Gulf and then lifts out northward leaving the recovered low-level moisture behind for whatever a second wave does. If that second wave is able to dig and spin up a surface response farther to the west, the door is open for a severe weather threat over the bulk of Dixie Alley. If it's farther east like the new Euro and previous GFS runs have shown, severe weather would be restricted to the Carolinas, southeast Georgia, and down into Florida.
 
Shakespeare 2016 said:
OK, that makes sense.
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Yeah, adequate moisture return for severe weather takes days to build. And until you get to May through August or September when the moisture is somewhat just sitting around, you pretty much need a low-level flow that originates from the Caribbean. And you're not going to get adequate enough boundary layer moisture in place, that is deep enough to not be mixed out, if it's not in place over the Gulf of Mexico ahead of time. A lead wave on Wednesday would facilitate all that ahead of a possible second wave at the end of the week, but that's the only way we'd be able to pull it off. It takes the lead wave to initiate the sustained low-level flow out of the northwest Caribbean and through the Gulf of Mexico into the Southeast.
 
Saw where the 00 GFS parks a snow storm over NE Alabama, and the mountains of Tennessee and North GA on the back end of this storm. Only run I’ve seen that happen but wouldn’t be unheard of this time of year. 89 Huntsville had snow showers on and off Thursday after the Wednesday Tornado.
 
Unless something significant changes (and it still could, but ehh...), the second shortwave late next week doesn't dig in far enough west to pull northward the better dewpoints that get into the Gulf after the lead shortwave zips by to the northwest. That would significantly restrict any risk outside of the immediate coastal areas (if even there). Y'all go back to sleep...
 
Fred Gossage said:
Unless something significant changes (and it still could, but ehh...), the second shortwave late next week doesn't dig in far enough west to pull northward the better dewpoints that get into the Gulf after the lead shortwave zips by to the northwest. That would significantly restrict any risk outside of the immediate coastal areas (if even there). Y'all go back to sleep...
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What a waste of a nice system too …
 
Weatherphreak said:
Saw where the 00 GFS parks a snow storm over NE Alabama, and the mountains of Tennessee and North GA on the back end of this storm. Only run I’ve seen that happen but wouldn’t be unheard of this time of year. 89 Huntsville had snow showers on and off Thursday after the Wednesday Tornado.
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Check out 11/2/1966

 
So the new PDO reading for October via NCDC came in at a whopping -3.06, which is the strongest negative monthly PDO reading since 1955 per that calculation.

I'm not sure what Nate Mantua's calculation has since the two are different, but it's becoming clear that we've shifted out of the +PDO dominant regime that has been in place since 2013/2014.
 
Can't see the last two post images on this danged mobile. Could it be the Vancouver BC waterspout?



More info

Edit: Okay, can see them now. Still a good video and just so "Canadian." :)
 
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andyhb said:
So the new PDO reading for October via NCDC came in at a whopping -3.06, which is the strongest negative monthly PDO reading since 1955 per that calculation.

I'm not sure what Nate Mantua's calculation has since the two are different, but it's becoming clear that we've shifted out of the +PDO dominant regime that has been in place since 2013/2014.
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What does this mean?
 
andyhb said:
So the new PDO reading for October via NCDC came in at a whopping -3.06, which is the strongest negative monthly PDO reading since 1955 per that calculation.

I'm not sure what Nate Mantua's calculation has since the two are different, but it's becoming clear that we've shifted out of the +PDO dominant regime that has been in place since 2013/2014.
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The CPC Ocean Briefing PDO values seem to follow the more conservative route similar to Nate Mantua's method, and the new October reading there is -2.45. This is definitely a high-end -PDO regime. Like you said, we've essentially reset the North Pacific to pre-2013 conditions.
 
gangstonc said:
What does this mean?
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full-jcli-d-14-00647.1-f1.jpg


Left is a positive (warm phase) PDO, right is a negative (cold phase) PDO. There's a whole lot that can be explained about the PDO than what we have time or room for here, but it is essentially the configuration of sea surface temperatures and atmospheric pressures over the North Pacific. It is one of the main teleconnections that helps drive our weather pattern across North America, but like ENSO, it works on a longer timescale than things like the NAO, PNA, AO, etc. Negative/cold phases are usually more prevalent during a La Nina, and the two kind of work in tandem with each other to help drive what is typical climatology of a La Nina pattern. By looking at how the SST gradients take shape in the image above, you can see how a -PDO would drive a strong Pacific jet that would be aimed at the West Coast of the United States. Having that cooler water then spread down the west coast of Mexico also helps weaken the temperature gradient that can drive a stronger/northerly-placed subtropical jet. Both of these factors kind of tip the coin a little toward polar jet dominant type storm systems that are more favorable for large scale severe weather events if other conditions become favorable.
 
Thursday may be trying to trend toward a sneak attack after all for MS/AL/GA and maybe into TN depending on the northward extent of low-level moisture. 300mb temperatures are running very cold with the system (below -40C), which is helping overcome the lack of better boundary layer thermos, but now the GFS, Euro, and UKMET are pulling low to even mid 60 dewpoints up north through MS/AL and 60-61 dewpoints even into southern TN. Given the cold upper-levels, this is leading to forecast soundings with anywhere from 400 to as much as 1100 j/kg of CAPE, even with sfc temperatures under 65F. With a low-amplitude negative tilt trough swinging through, I think we need to watch this. It may not be a big event by any means, but with such a favorable background state, and with what October already did, we need to watch every system. I'm starting to think, given the lack of a convective blowup over the warm sector or along the coast with the trough extending as far down as it does, the lower dewpoints inland until the last minute might actually have a chance of helping this one. This will be a really fickle setup though, with a good amount of Forecasted Convective Amplification Deficiency potential, and not a high ceiling to begin with.
 
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