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.

gfsNE_500_avort_021

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.

gfsNE_850_spd_024

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.

HFDNAMandGFS

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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Thursday’s Bust – What Went Wrong?

All the ingredients seemed to be lining up for a significant severe weather event, particularly in western Connecticut and adjacent parts of New York. What did we see? Sporadic damage reports and a dying squall line that moved out of the Poconos.

There was certainly a  lot of potential on Thursday. Models had been showing moderate amounts of instability and very strong shear. Forecast and observed hodographs indicated the potential for tornadoes in any discrete cells that developed or along squall lines.

18z NAM 6 hour HVN 0-3km hodograph forecast

While deep layer (and low level) shear was sufficient for supercells a strong cap near 850mb prevented convection from developing ahead of the main line. Here’s a 23z RAP sounding (initialization) for HVN and you can see right off the bat the problem.

Yikes!!! That’s a lot of CIN. The LFC is nearly 775mb with a large negative area on that sounding. While the squall line looked exceptionally impressive over Pennsylvania it began to fizzle as it worked east. The large amount of convective inhibition became problematic as the squall line’s cold pool was unable to force surface based parcels to the LFC.

Our models had shown that the CIN present during the day would weaken but apparently that was not the case. While low level moistening helped eliminate some of the negative area it was not enough. The cap was also quite low. Typically we see caps around 700 mb… not 850mb! The exceptional warm 850mb temperatures (over +22c) effectively served to cap convection.

With a convective temperature of 95F we just weren’t going to get rid of that CIN. The warm layer at 850mb (NAM did do well with this) was just too much. With stronger surface heating and more low level moistening things may have been different.

The storms were most certainly a bust. The Storm Prediction Center issued the first ever (that I can remember) day 2 moderate risk for Connecticut. The probabilities for severe wind and significant severe were some of the highest you’ll ever see in Connecticut.  In fact while the event was ongoing the SPC sent out a discussion saying they contemplated an upgrade to “high risk” which would have been the first time since May 31, 1998 parts of the state have been in a high risk! As the squall line was leaving the Poconos the SPC expected the storm to strengthen and called it a derecho.

In the end a lack of synoptic forecasting (500mb heights were neutral from 18z-00z) and strong CIN thanks to a stout low level cap did our severe chances in. Given the potential it was important to mention the possibilities. Severe weather is a challenge to forecast – certainly much more challenging than a winter snowstorm!

Significant Severe Weather Threat Later Today

1630z SWODY1

After two bouts of severe weather last night with a spectacular lightning display, gusty winds, and large hail in Litchfield County the real show may not be until later today.

The Storm Prediction Center has upgraded portions of southern New England to a “moderate risk” which means a 45% chance or greater likelihood of severe weather within 25 miles of any given point.

Much like last Wednesday (during the Hampden County tornado) the instability in the atmosphere is extreme. When CAPE exceeds 3000 j/kg instability is high when it exceeds 5000 j/kg it’s extreme and that’s what is in place just to our west thanks to high dew points and a remnant elevated mixed layer aloft.

16z SBCAPE (Courtesy: SPC Mesoanalysis)

Though there will be moderate to strong shear in the atmosphere capable of producing rotating updrafts (and subsequently large hail) the tornado threat is low. The wind shear today is mostly speed shear as opposed to directional shear. Last Wednesday winds turned rapidly with height (directional shear) which is favorable for tornadoes particularly when that change happens in the lowest mile or two of the atmosphere.

Today’s severe weather event will likely bring an organized squall line capable of producing destructive winds. Out ahead of the squall line any storms that organize will be capable of producing large hail and potential a brief/isolated tornado but nothing like last Wednesday.

12z NAM 6 Hour Forecast 500mb Heights and Vorticity

The atmosphere will remain capped until 2 or 3 p.m. and then we’ll see things explode to our northwest. The cap will be weakened as a disturbance moves in from the west resulting in large-scale ascent. As 500 mb heights fall you generally see the removal convective inhibition (known as CIN and pronounced “sin”)  and thunderstorms start to fire.

I expect the significant severe weather threat to exist down to the Connecticut shoreline today as well. These are the kinds of events that pose a severe weather risk for the entire state.