Wednesday, May 06, 2015

Here And There On May 6th

Thunderstorm activity did not develop anywhere yesterday over southern Arizona, not even over higher mountains to east of Tucson area - as indicated below on a CG flash density plot from Weather.Graphics for 24-hours ending at 1430 UTC this morning. Although lower-level moisture and PW remained fairly constant (the 00 UTC sounding yesterday afternoon for TWC above) shows that subsidence warming between about 300 mb and 600 mb effectively eliminated the CAPE. So a dramatic difference across the state between May 4th and 5th.

Back on May 1st I contrasted the 168-h forecasts from the GFS and the ECMWF models. The GFS showed a strong 500 mb, closed low digging southward near the west coast, while the ECMWF did not. The current NAM 500 mb forecast from this morning (above, valid at 00 UTC on the 8th - which was the valid time of those long-range forecasts) shows that the GFS forecast the correct trend for this next system. The very next forecast cycle, back when I did the original post, had both models in close agreement, as they've continued since. All the models continue to forecast this to be a moisture-starved system when it comes across Arizona, producing mostly gusty winds.

Finally, there is early tropical activity near Florida in the Atlantic. Special outlook from the NHC (above) this morning indicates a 60% likelihood that a tropical or subtropical storm will develop during the next few days. What's a subtropical cyclone? Definition below from NHC Glossary:

Subtropical Cyclone:
A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources, and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (usually greater than 60 n mi), and generally have a less symmetric wind field and distribution of convection.

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