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Severe Weather Threat October 7-8


Dsnowx53

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I figure with how good the 12z Euro looked today, as well as the NAM being consistent, it's time to start this thread. We are juxtaposing sufficient low-level CAPE along with very impressive low-level shear, relatively steep mid-level lapse rates, and very impressive moisture advection, along with a potent autumn-like trough.

I'm not so much worried about the the timing given that we will still have plenty of lift and moisture, but the exact location of the strongest forcing and just how strong it is could either limit convection coverage, or give the area plenty of convective debris/crapvection.

Attached is the 00z NAM Helicity, SB CAPE, SB CIN, valid for 9z (5am) Wednesday morning.

post-73-0-82755300-1412657870_thumb.gif

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Slight risk for the coastal areas:

 

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..DELMARVA/NRN MID-ATLANTIC REGION TO SRN NEW ENGLAND...   POTENTIAL EXISTS FOR STRONG TO SEVERE STORMS ACROSS PARTS OF   DELMARVA...ERN PA...NJ TO SRN NEW ENGLAND...INCLUDING THE NYC METRO   MAINLY DURING THE LAST 6 HOURS OF THE FORECAST PERIOD /06-12Z/.   MODELS ARE CONTINUING TO TREND WITH THE SHORTWAVE TROUGH MOVING   THROUGH THE OH VALLEY TODAY BECOMES NEGATIVELY-TILTED FROM THE   MID-ATLANTIC STATES TO SRN NEW ENGLAND TONIGHT WITH 60-90 METER   HEIGHT FALLS.  INCREASING DEEP LAYER FORCING ACCOMPANYING THIS   SHORTWAVE TROUGH LATE IN THE PERIOD COMBINED WITH LOW-LEVEL   MOISTENING/DESTABILIZATION SUGGESTS TSTM DEVELOPMENT WILL OCCUR FROM   DELMARVA...NRN MID-ATLANTIC REGION TO SRN NEW ENGLAND.  12Z ECMWF   INDICATED MUCAPE IN EXCESS OF 500 J/KG ACROSS THE SLIGHT RISK AREA   BY 06Z.  STRENGTHENING DEEP LAYER SHEAR AND INCREASING LOW-LEVEL   SHEAR ALONG A STRENGTHENING /UP TO 50 KT/ SSWLY LLJ WILL PROVE   FAVORABLE FOR ORGANIZED STORMS.  00Z 4 KM WRF NMM/NSSL RUNS EACH   SUGGEST A QLCS WILL STRENGTHEN/SHARPEN OVER DELMARVA/NJ BETWEEN   06-08Z AND TRACK ACROSS LONG ISLAND TO CT/RI AND PARTS OF MA BETWEEN   08-12Z. 
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I can not see the AREA avoiding a tornado watch being issued around 9-10 PM and expiring around 5 AM.  Also afraid of 1-2 isolated F-Zero to F-One Tornado report in the area with tree and minor to moderate property damage.  I do not like the parameters being shown, hope this all goes to waste.  30-40 knot squall like winds look impossible to avoid with this QLCS type convective system.  FOLKS LEAVE THE CELL PHONES AND WEATHER RADIOS IN ALARM READY MODE BEFORE GOING TO BED... PLEASE.... ALSO HAVE FLASHLIGHTS WORKING AND READY. just in case.... Really hope these parameters go to waste....

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The analogs show a cluster of tornado reports right around the NYC metro area. If anyone remembers 10/28/95, that continues to be a slight match. That event produced a tornado on Staten Island.

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It should also be noted that the 4km NAM shows discrete cells firing across the area. Based on the model data, I could see this prompting a 5% tornado risk by SPC and a Tornado Watch. If, as mentioned, the storm mode doesn't get too messy/clustered.

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6z NAM an still has an decent eml/cap until at least 3z over LGA. That would keep alot of convection from firing up earlier and build more instability. The 6z GFS has is not as robust with the cap. But it's starting to pick up on it more. The 500mb vort moving through will likely break it later.

 

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The focal point for severe wx appears to be where the 500mb vort coincides with the instability and strong shear building near the coast. If the timing and amount of destabilization is right on the NAM, strong to severe t-storms with isolated tornadoes possibly early tomorrow am. Especially for the eastern sections.This isn't just MUCAPE, much of it SBCAPE and MLCAPE as well:

 

post-187-0-68875300-1412681816_thumb.gif

 

post-187-0-66632400-1412681845_thumb.gif

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6z NAM an still has an decent eml/cap until at least 3z over LGA. That would keep alot of convection from firing up earlier and build more instability. The 6z GFS has is not as robust with the cap. But it's starting to pick up on it more. The 500mb vort moving through will likely break it later.

 

 

This is the fundamental issue with this setup for me. People have referenced overdone instability, etc on models as potential mitigating factors. But to me it comes down to prefrontal convection which will, in turn, mitigate the instability in the warm sector. If the NAM is correct with its hybrid EML and capping -- we'll likely see this event unfold. But often times in these setups, especially w/ the warmer ocean on a southerly flow with moisture return, the atmosphere becomes too favorable for precipitation development early-on. 

 

I'll be watching today to see if the models are correct in holding off the earlier convection. But I wouldn't be surprised to see that development occur, and essentialyl kill off any chance of getting discrete cells within a moderately unstable environment. If those do develop...I think we could have more than one isolated tornado. The setup is very favorable for spinups. 

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This is the fundamental issue with this setup for me. People have referenced overdone instability, etc on models as potential mitigating factors. But to me it comes down to prefrontal convection which will, in turn, mitigate the instability in the warm sector. If the NAM is correct with its hybrid EML and capping -- we'll likely see this event unfold. But often times in these setups, especially w/ the warmer ocean on a southerly flow with moisture return, the atmosphere becomes too favorable for precipitation development early-on. 

 

I'll be watching today to see if the models are correct in holding off the earlier convection. But I wouldn't be surprised to see that development occur, and essentialyl kill off any chance of getting discrete cells within a moderately unstable environment. If those do develop...I think we could have more than one isolated tornado. The setup is very favorable for spinups. 

 

This being a nocturnal event in October helps form the cap. Surface temps cool more and the forces more warm air to rise aloft. I'd be more concerned with prefrontal convection, if this was during the day. But we'll see.

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This being a nocturnal event in October helps form the cap. Surface temps cool more and the forces more warm air to rise aloft. I'd be more concerned with prefrontal convection, if this was during the day. But we'll see.

Agree 100 % - especially if we get any early clear skies and any radiational cooling since this system is not progged to reach the area until well after midnight in NYC metro -sunset comes early these days at 6:28 PM

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For those of us that are lacking a bit in knowledge can you please explain what this shows in terms of potential?

 

It's showing you the 0-3km storm relative helicity, which basically measures the potential for updraft rotation in right-moving supercells. And so this is the low level helicity between 0 and 3 km layers above the the surface. 

 

The amounts being shown are high and probably overdone. 

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The HRRR is like a somewhat toned down RAP. Still some very impressive parameters across much of NJ. Updraft helicities spike by 7z as more robust storm clusters move through.

 

Yeah, seems more realistic.

 

I'm also noticing a disconnect by a few hours between the best low level helicity and instability. Something to watch moving forward. 

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17z HRRR via WeatherBell shows semi-discrete cells forming near the PA/NJ border at 2 a.m. And racing NE into the NW burbs and lower Hudson Valley by 3 a.m. They're near the upper bounds of the reflectivity scale and decent instability still hangs back across most of NJ by 3/end of run.

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