Why a slight risk for hurricanes?

[Mike Peregrine]

Was thinking back over the years of Stormtrack online - and can't recall anyone ever asking this (may have been covered and I'm just forgetting) -

Why is it that typically we only see a slight risk issued on days with hurricanes and tropical systems making landfall? Hazards are spelled out in the day one language ... including large areas affected by widespread wind damage, flooding rainfall and a 5% probablistic for tornadoes that may exist within embedded mini-supercells. Just curious why all of this doesn't warrant a moderate or high risk - particularly since there is no question that a lot of people over a widespread area will be affected?

Also - there is a 15% high wind probablistic outlined for AZ, NM but none for Florida? - I understand that these are CONVECTIVE outlooks and designed to outline the risk areas for severe thunderstorms (and that the NHC is the center assigned to issue warnings in connection with hurricanes) ... but I wonder why this would preclude the SPC from outlining risk areas of its own ... what am I not getting?

 

[Jeff Snyder]

The SPC issues for convective risks. Yes, there is plenty of convection in a hurricane (well, convection makes up hurricanes), but the risks are better handled by the NHC, since it's a tropical system. Winds would be severe courtesy of the pressure gradient force to begin with, not necessarily because of intense thunderstorms creating severe downdrafts. In fact, CAPE is often very low (no surface heating and lots of precip makes the surface layer quite stable), further evidence that the "severe" winds (>58mph) aren't really being driven by convective processes normally associated with thunderstorms (e.g. downdrafts in supercells, bow echos, etc.). Yes, convection in the hurricane helps bring winds from above the surface to the surface, but I think you know what I mean. Hail is typically no problem at all, as it is only rarely observed. Tornadoes can be problematic, but they are usually quite brief and weak (weak instability tends to make sustained, strong tornadoes difficult to produce). OK, so what I'm saying is that the winds are caused moreso by the pressure gradient force than by convective processes.

 

[Mike Peregrine]

But if you ultimately can't differentiate between damage caused by the wind gradient of a tropical system from the wind damage caused by imbedded convection, why even issue a slight risk at all? - Why not just have a 'see text' that directs attention to the NHC? Guess I don't see the point -

 

[Jeff Snyder]

I think the SLT risk is issue almost entirely because of the tornado threat. 5% torn probs would support a Slight risk... Speaking of tornado probs, I'm somewhat surprised that we havne't seen a tornado watch out yet. SPCMesoanalysis is shows strong low-level shear over eastern FL, with 250-300 0-1km helicity underneath ~1000 CAPE. One tornado warning just expired for rotation embedded in one of the rainbands...