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Srain

Central/Western Summer Medium/Long Range Discussions

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Yes and Tony's idea I agree with, but it isn't mixing out of moisture like you mentioned yesterday, that would be more of a lacking of quality moisture depth moving that far north to begin with.

 

Fair enough. But if the moisture quality is bad, it won't take much for moisture to mix out -- to a much greater extent than models predict. For example, GFS shows 65F dewpoints but I wouldn't be surprised if the actual dewpoints on the day of the event is closer to 55F.

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Except the moisture will be more difficult to mix out since it is quite deep, there is a large pool of 14˚C+ H85 dewpoints in KS on Wednesday evening on the GFS.

 

ARWjeFD.gif

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Fair enough. But if the moisture quality is bad, it won't take much for moisture to mix out -- to a much greater extent than models predict. For example, GFS shows 65F dewpoints but I wouldn't be surprised if the actual dewpoints on the day of the event is closer to 55F.

Not if we see the type of moisture depth that Andy just posted. If this type of moisture quality comes north then I don't expect widespread mixing out. Furthermore going back to the Euro's depiction of the east coast troughing, it is the only model that has it even remotely to that depth and that is a known issue with the Euro over amplifying east coast troughing especially in the northeast. It has done this since the last update on the model and every time it ends up being too strong.

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Fair enough. But if the moisture quality is bad, it won't take much for moisture to mix out -- to a much greater extent than models predict. For example, GFS shows 65F dewpoints but I wouldn't be surprised if the actual dewpoints on the day of the event is closer to 55F.

The GFS has much more favorable trajectories for moisture return than the Euro.  If the GFS pattern materializes, I'd be extremely shocked if dewpoints were below 60F.

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Well, the GFS & Euro love the idea of maintaining an sfc anticyclone over the NE GOM for several days leading up to the event. Given the drought, that could be a deal breaker WRT moisture.

 

Tony probably has the right idea here. The first system is likely to be a significant severe wx producer but the better widespread tornado chances for chasers will probably wait for subsequent systems.

Also, models have, in my memory, overdone the moisture return in the long range re: several events this year, only to swing back toward lower dews in the short range. But again, I'm with Tony re: the potential for other systems following Wednesday's trough. If for some reason moisture won't work out with Wednesday's trough, it almost certainly will for the next system, given the zonal flow pattern allowing the Gulf to be open for business after Wednesday. And there are hints of an even wider warm sector down the line.

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The second trough is the one that I'm really looking forward to. I probably shouldn't be getting excited about something so far in the future, but the early signs look promising.

 

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The Gulf trajectories aren't that much different on the GFS than the Euro... both are showing northerly or westerly winds along the west coast of FL. I'm not sure why the GFS paints so much moisture over the Plains. Nowhere in the entire GOM are 850 Td's greater than 8 degrees C. And suddenly over the drought-stricken Plains the dewpoints go up to 15 degrees C? A bit questionable.

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The second trough is the one that I'm really looking forward to. I probably shouldn't be getting excited about something so far in the future, but the early signs look promising.

 

That antecedent pattern is synoptically very similar to some really volatile events (and extended ones too).

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The Gulf trajectories aren't that much different on the GFS than the Euro... both are showing northerly or westerly winds along the west coast of FL. I'm not sure why the GFS paints so much moisture over the Plains. Nowhere in the entire GOM are 850 Td's greater than 8 degrees C.

 

The primary 850 mb moisture source is coming from eastern Mexico, where it has been entrenched longer. Northerly/westerly winds in FL don't really have much to do with that.

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xnp4B5S.gif

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Might the GFS be overdoing the 850 mb moisture over Tamaulipas in days 3-5? If so, is overestimation of low-level moisture in the Bay of Campeche a problem when the model analyzes prevailing southerlies/southeasterlies over the mountainous terrain in that area? I am just interested in knowing whether this is a common mesoscale problem in this area (and not just for the GFS).

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Moisture pooling in eastern MX can only go so far when all the air is recycled continental air. When that does get advected over the Plains there will be significant modification.

If the flow is into eastern MX from the Gulf it isn't recycled continental air... That would be like saying air originating from southern and eastern Texas with a flow in from the Gulf is recycled continental air

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If the flow is into eastern MX from the Gulf it isn't recycled continental air... That would be like saying air originating from southern and eastern Texas with a flow in from the Gulf is recycled continental air

 

The flow into eastern MX is circulated around anticyclonic flow in the northeastern GOM, so yes it is recycled.

 

High quality moisture would be flow into eastern MX straight from the Caribbean.

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The flow into eastern MX is circulated around anticyclonic flow in the northeastern GOM, so yes it is recycled.

 

High quality moisture would be flow into eastern MX straight from the Caribbean.

No it is not... Loop the 850mb dew point maps, the higher concentration of moisture is from an air mass in the Gulf right now that gets pulled into MX starting 2 1/2 days from now and it gets reenforced by a flow out of the Bay of Campeche. Beyond that the moist air is then pulled northward preceding the trough out of MX and the far Western Gulf. What happens in the northeastern Gulf is irrelevant to the pattern because that air doesn't get pulled northward until after this moisture surge does. By that point we are talking late Thursday into Friday.

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No it is not... Loop the 850mb dew point maps, the higher concentration of moisture is from an air mass in the Gulf right now that gets pulled into MX starting 2 1/2 days from now and it gets reenforced by a flow out of the Bay of Campeche. Beyond that the moist air is then pulled northward preceding the trough out of MX and the far Western Gulf. What happens in the northeastern Gulf is irrelevant to the pattern because that air doesn't get pulled northward until after this moisture surge does. By that point we are talking late Thursday into Friday.

I think that he is referring to the wind vectors. If you look at the 850 mb flow over the SW Gulf / BOC, you will note that as the surface low forms in the Gulf after 00Z 18 April, the winds in the entire SW Gulf / BOC shift from SSE to N or NWly, from TX south to Tehuantepec. Thus the initially mT air mass gets modified / mixed out by the infusion of continental air mass from the Southern Plains. Maybe your definition of a high-quality air mass differs from other people's in some regards. But I could be wrong.

 

gfs_atlantic_024_850_rh_ht.gif

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Except the low level flow in behind that low isn't very strong and there already is moisture in southern TX currently via mesoanalysis, so there wouldn't be a ton of modification occurring before flow out of the southern BOC resumes.

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I think that he is referring to the wind vectors. If you look at the 850 mb flow over the SW Gulf / BOC, you will note that as the surface low forms in the Gulf after 00Z 18 April, the winds in the entire SW Gulf / BOC shift from SSE to N or NWly, from TX south to Tehuantepec. Thus the initially mT air mass gets modified / mixed out by the infusion of continental air mass from the Southern Plains. Maybe your definition of a high-quality air mass differs from other people's in some regards. But I could be wrong.

gfs_atlantic_024_850_rh_ht.gif

I'd consider 70-90% RH with pockets over 90% pretty high quality when it comes to moisture and as Andy noted there isn't a strong push behind that low infusing dry air.

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The 00Z GFS cuts off the moisture source days 3-5 with significant shortwave ridging (anticyclonic flow) over the South-Central Plains, which would do much to kill the severe prospects on Wednesday. But the run is slower overall with the ejection of the low, making Thursday a better bet for severe weather. The slower run does compensate at least somewhat for the initial low-level moisture mixing.

 

gfs_namer_108_850_temp_ht.gif

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Yeah it is strong across the board, with an upper level jet >150 kts across the full suite, which is exceedingly rare this time of year. The Euro is on another level though, 200 kts on both the most recent 00z and 12z runs is basically unheard of in late April.

 

Edit: 00z Euro maintains in the 180-200 kt range.

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SPC is more concerned about timing than moisture with the latest day 4-8 outlook:

LATER IN THE WEEK MODEL SOLUTIONS BEGIN TO DIVERGE A BIT AS LARGE-SCALE PATTERN CHANGES. ECMWF IS CONSIDERABLY FASTER THAN THE GFS ALLOWING A STRONG SHORT-WAVE TROUGH TO TRANSLATE INTO THE ROCKIES BY WEDNESDAY. DIFLUENT FLOW ALOFT AND STRENGTHENING SHEAR PROFILES WOULD PROVE POTENTIALLY FAVORABLE FOR A PLAINS SEVERE EVENT. HOWEVER...GFS DOES NOT EJECT THIS FEATURE UNTIL LATER THURSDAY. EITHER OF THESE SOLUTIONS COULD GENERATE A POTENTIALLY SIGNIFICANT SEVERE EVENT BUT TIMING DIFFERENCES PREVENT DELINEATING AN ORGANIZED SEVERE THREAT THIS PERIOD.

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The GFS has taken steps towards the ECMWF the past three runs.

 

Which would be more intimidating when facing whatever comes after this thing.

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The GFS has taken steps towards the ECMWF the past three runs.

 

Yep. So much for "the ECMWF is way overdone with eastern troughing, that'll never happen." Eastern troughing to an extent that probably mitigates anything significant next week is basically the model consensus now.

 

992 mb nor'easter hugging Cape Cod on the 12z GFS for Wednesday. :axe:

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Yep. So much for "the ECMWF is way overdone with eastern troughing, that'll never happen." Eastern troughing to an extent that probably mitigates anything significant next week is basically the model consensus now.

 

992 mb nor'easter hugging Cape Cod on the 12z GFS for Wednesday. :axe:

Still 4-6 days out...But looking better and better for the ECMWF to go up 3-0 for this month.

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Still 4-6 days out...But looking better and better for the ECMWF to go up 3-0 for this month.

Yeah and if it continues to win the long-range battle, look out ~29 Apr onward.

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The 00z EPS mean from last night is ridiculous, you would think it is some wacked-out solution from the post-truncation GFS with the size of the trough it has building in the west.

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Still an absolute monster of a trough setting up in the west on the 12z Euro. It covers quite a large section of the NE Pacific.

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