Thursday, August 21, 2008

Response to AJ-NWS

Response to AJ of NWS - see AJ's comment on the previous post.

AJ makes a very good point here and one that serves to emphasize how difficult the forecast challenge is for the coming 84 hours. The three principal models each have different solutions, and we have little in the way of observational data to help evaluate the model initializations, since the key features analyzed by the models are within a great data void.

First, if one looks at the NAM, the GFS, and the ECMWF model runs from this morning – which I hadn’t when I wrote the original blurb – it is clear that each of the three models has somewhat different solutions for the initial conditions.

As per at 500mb:

The GFS analyzes an IT from south of GoC into north central MX where it connects with a trailing trough from the S/W over the central Missouri River Basin.

The ECMWF has this feature analyzed further north, extending from the south central GoC to SE Kansas.

The NAM is similar to the ECMWF except that it has several additional vorticity maxima further south and southeast which neither of the global models have.

We do know that such a feature is present and that at 12Z this morning it extended from Nebraska south-southwestward entering Mexico in the western Big Bend country.

Each model goes it own way in the forecast fields. The most important issue relates to whether or not there is a feature in the actual atmosphere that is going to spinup into a tropical system off Baja. Only the NAM indicates this evolution. The NAM solution would result in a very deep intrusion of subtropical moisture into the Southwest. At this time we mostly know that there has been ongoing convection in the regions where the NAM forecasts tropical development.

Both the GFS and ECMWF strengthen the inverted trough over north central Mexico and forecast this as the main feature that affects southern Arizona on Saturday and Sunday. The GFS stalls this feature over north central Mexico, while the ECMWF is more aggressive and keeps it further north and moving westward. The wind shear profile forecast on Sunday by the GFS is not nearly as favorable for organized storms (assuming that the key ingredients of moisture and CAPE have moved/developed north of the border). The ECMWF is probably in between the two other models wrt wind shear.

Current IR and water vapor satellite imagery indicate that the feature over north central Mexico has a clear circulation now and that it is moving westward. Significant convection is developing ahead of this feature over the Sierra Madre Occidental.

So where does all this lead? The forecast situation appears even more complicated than what I originally wrote. The most likely scenario for the next 84 hours is not at all clear at this time, and the evolving situation merits careful monitoring.

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