In the past 24 hours, the GFS has changed significantly in its depiction of the upper pattern beginning on Tuesday of next week. The Euro (ECMWF) model has also been trending in a similar direction. Both models now agree on some form of a dominant western US trough that will begin spreading strong (greater than 40 knots) southwest midlevel flow in a jet somewhere over the Plains (at this point, central to northern). In response, southerly winds 'switching on' the low level moisture conveyor are shown beginning across the Plains on Tuesday and continuing through the week. I'm posting this update a little prematurely, since the 00z Euro won't be out for a couple more hours - but the information available right now points to chaseable setups returning to the Plains beginning on Wednesday of next week, and continuing possibly for many days afterward.

GFS 500mb forecast for next Thursday afternoon

It's a little early to get nitpicky about the details (like the GFS showing somewhat shallow moisture depth even into Thursday, capping issues and veered 850mb winds), but it's safe to say some Plains and/or Midwest chasing is becoming more likely next week. Given the flip-flopping leading up to this, it's possible the models could be playing one of their Lucy-vs-Charlie Brown football tricks on us all. So, I won't really start giving high chasing probabilities to this system until we get a few more days of model output.

The following table plots the chance of a Plains chase expedition happening in a particular date range (Midwestern chase days are not factored into this table):

2012 Plains Chase Expeditions - Probabilities as of May 18 May 18-21 2% May 22-25 50% May 26-31 30%

Midwest Chase Probabilities: The following table outlines the probabilities of a Lower Midwest chase (within 5 hours of St. Louis) happening within a particular time frame: 2012 Midwest Chases - Probabilities beginning May 18 May 18-19 2% May 20-21 85% May 22-25 50% Midwestern chase forecast discussion (Update based on 5/18 00z data): A local low-key lightning chase still looks likely on Sunday afternoon/night in and close to St. Louis as a cold front moves through. Next week, a Midwest chase or two may be in the cards thanks to the potential upper system moving into the central US - depending on whether or not I'm able to make it out west for one of the Plains setups, if they materialize.

Regarding the Plains, there are a couple of chase forecasting items to talk about today. The first is an anemic trough that is slated to affect the central and northern Plains Friday and Saturday:

GFS 500mb forecast for Saturday afternoon

As you can see there, although we'll have some southwest flow for a time where we need it, it's just too weak to do us much good after the long-term ridging preceding it. We need a stronger trough to help create a good southerly fetch from the Gulf to transport deep moisture, then that same trough needs to provide strong mid and upper level winds above that moisture to create directional shear needed for supercells.

While the above configuration can produce chaseable tornadic storms given a few difficult-to-predict pulses of stronger flow, it needs the moisture component at the surface to be robust enough to a.) break the cap, b.) prevent too much mixing out, and c.) provide lower-based storms. I don't believe we'll have enough moisture to make for a great chase event on Saturday, but I'll concede that there *could* be a tornadic supercell somewhere in this setup. Is it worth a long trip? I don't think so. We're looking at one, maybe two if we're lucky, conditional chase days over Kansas and Nebraska. For me, I'm still not impressed enough to take a couple of vacation days to go for it.

Now let's look beyond the weekend. After many days of agreement on a chaser's worst possible forecast of a persistent 'death ridge', the models have been thrown into some confusion about what the last half of May has in store during the past 36 hours. This can be either good news or bad news, as eventually we'll see some pattern emerge from the haze. Will it be a good one, or a continuation of the doldrums of late? Take a look at this forecast that the GFS draws for next Wednesday:

GFS 500mb forecast for next Wednesday

I like the massive westerly jet over the Pacific. That's the type of energy that can do some damage to the current boring pattern entrenched over the Plains, providing those winds can amplify into a nice trough as they come on shore and plow eastward. The fantasyland GFS (past 180 hours) has it doing just that, but the Euro model has it staying rather flat and skipping over the ridge to the north like a flat stone on a lake surface. Unfortunately, the Euro model has traditionally been much more reliable in the long ranges, so I'm inclined to lean in its direction, as painful as that may be. The Euro's solution shows the broken record continuing, with the good flow staying too far north and deep moisture too far south to do any good.

What's worse, the Euro's depiction of temperatures in the southwestern US becoming a summer-like furnace does not bode well for the rest of the season. That region is where our EML (cap) comes from, and a 'nuclear' cap plus weak upper flow and struggling surface moisture simply means no storms when and where you'd want them.

The good news, again, is that the Euro hasn't been 100% consistent with this pattern (and the GFS disagrees), so things could still change. However, I'm simply not optimistic yet about a Plains chase for as far out as I can reasonably see.

The following table plots the chance of a Plains chase expedition happening in a particular date range (Midwestern chase days are not factored into this table):

2012 Plains Chase Expeditions - Probabilities as of May 16 May 16-22 5% May 23-31 10% June 1-7 25%

Midwest Chase Probabilities: The following table outlines the probabilities of a Lower Midwest chase (within 5 hours of St. Louis) happening within a particular time frame: 2012 Midwest Chases - Probabilities beginning May 16 May 16 2% May 17-19 2% May 20-21 60% Midwestern chase forecast discussion (Update based on 5/16 00z data): The Midwest should stay fairly quiet through the end of the week. On Sunday and Monday, an approaching cold front underneath a little bit of upper support from the (weak) trough traversing the country should give our region a good shot at storms. As models depict it now, a lightning chase or two looks likely during this time.

The long-range models continue to show mostly bad news for chasers through at least the next 10 days, as a strong upper ridge continues to build over the central US. Strong mid and upper level winds should remain confined to the extreme northern US, near and mostly north of the Canadian border. Both GFS and Euro models show a small shortwave trough developing and trying to move in from the Pacific by late next week (visible on the GFS prog below), but the Euro shows it getting decimated as it runs into the ridge, long before it can affect the Plains.

GFS 500mb forecast for next Friday afternoon

Despite the stubborn ridge, moisture is shown as slowly recovering across the center of the country day by day, but it is questionable that a sufficiently deep moisture layer (robust enough to avoid detrimental afternoon mixing) will reach northward all the way to the good upper support. The situation is similar to late May of 2006, when shallow surface moisture layers 'mixed out' by late afternoon, becoming too dry to support anything more than high-based storms.

If deep moisture can somehow make the long trek in the weak flow to North Dakota/northern Minnesota, wind profiles are supportive of severe weather around Friday/Saturday of next week if mature storms can get established. It may be a worthy chase day or two for those stuck on fixed chase vacations or tours, but doubtful it will be enough to get 'on-call' chasers like me to pull the trigger on such a long trip.

My take on the current pattern is just to wait and make the most of the downtime (saving money and vacation days; taking care of day-to-day tasks usually neglected in the spring). Even if it ends up being late June or not at all, waiting for good patterns to return is always the better choice (in my opinion).

Despite the tornado/supercell threat across the Plains/Midwest being close to zero for the next 2 weeks, some non-severe thunderstorm days should be in the cards here and there, providing that moisture slowly recovers enough to re-introduce daily instability. These conditions can provide some nice low-key chase opportunities for lightning, cloud and storm structure close to home, which of course I will be out for should they occur at dates/times I'm off of work (it's doubtful I will use any vacation days for these).

To sum things up, I don't see any sign of another Plains trip on the horizon. Beyond the 'reasonable model horizon' of May 22, the probabilities will remain at the standard 30%.

The following table plots the chance of a Plains chase expedition happening in a particular date range (Midwestern chase days are not factored into this table):

2012 Plains Chase Expeditions - Probabilities as of May 12 May 12-16 0% May 17-22 5% May 23-31 30%

Unfortunately, long-range models continue to show no sign of good chasing patterns over the Plains/Midwest for as far out as they can reasonably be useful (10 days in the case of the ECMWF). A western trough showing up at the far end of the range appears to be too far north to do much for the Plains in terms of upper flow, except for possibly Montana and North Dakota. At the same time, a weak upper low/trough shown in the east blocks deep moisture return from the Gulf of Mexico, which would keep moisture from flowing that *long* distance north it would need to interact with that upper support (if it materializes).

Worse, the pattern shows no sign of breaking down for what could end up being the bulk of the month of May. Which is to say, there are no signs of progressiveness, but rather a indefinite period of ridging and stagnation of the upper pattern. As a result of this, deep moisture should remain too meager across the entire continental US for even weak-shear thunderstorm days.

All told, storm chasing prospects in the Plains and Midwest look grim for at least the next 2 weeks. The good news is, that rarely lasts, and we should start seeing some improvement in the pattern (or at least some hope on model forecasts) by the last week of the month. In the meantime, we're looking at some downtime for a while.

The following table plots the chance of a Plains chase expedition happening in a particular date range (Midwestern chase days are not factored into this table):

2012 Plains Chase Expeditions - Probabilities as of May 9 May 9-15 0% May 16-21 10% May 22-31 30%