Friday Severe Weather and Tornado Threat

I’ve been watching Friday’s severe weather potential with a bit of interest since Sunday. It hasn’t been clear whether ingredients would come together for severe weather with different computer models offering up very different outcomes. This afternoon, both the 18z NAM and GFS show the potential for a rather sizable severe weather event including tornadoes.


The synoptic setup is pretty straightforward. A deep closed-low near the southern tip of Hudson Bay continues spinning with a number of shortwaves advancing east across the region. At the same time, mid level wind fields strengthen during the day tomorrow approaching 40 knots out of the west at 500mb while a weak wave of low pressure forms along a front to the west strengthening the southerly flow in the boundary layer.


Now the GFS (the NAM has been showing this for a while) develops a strong low level jet over the region Friday. This creates a highly sheared environment in the 0-3km layer. Take a look at these hodographs from the 18z GFS and NAM for 18z tomorrow valid at 18z Friday.


The long and curved hodographs are very impressive. In addition the models are developing some surface based instability tomorrow. The extent of the destabilization is very unclear, however.

18z GFS sounding for Hartford valid 18z Friday

18z GFS sounding for Hartford valid 18z Friday

So what will happen? This setup does seem like it could morph into a classic low CAPE/high shear severe weather day if the current depiction by the GFS and NAM comes to pass. Hail is quite unlikely but damaging winds and tornadoes are possible. The threat will depend on the amount of destabilization which is unclear and dependent on cloud cover and convection earlier in the day. That said, lifted condensation levels (LCLs) are quite low so any storm that begins rotating can produce a tornado.

While the NAM is likely overdone at 18z Friday – many of the parameters, especially the 70 j/kg of 0-3km CAPE and storm relative helicity >300 m2/s2, is sufficient for tornadoes and even significant tornadoes per Davies, 2006.

WVIT Severe Threat

We’ll have to watch tomorrow’s setup closely – I’ll keep you advised. If some of the model solutions turn out to be correct tomorrow may be a very busy last day of work before I leave for vacation!


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