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General Severe Weather Discussion 2018

weatherwiz

By weatherwiz,
in New England

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WxBlue

WxBlue

Posted
Posted

While still a long shot, especially around here, it wouldn't surprise me if we get a tornado or two in the 5% area (NY/VT). NAM is actually showing a impressive curved hodograph up to 3 km level to yield helicity up to 400 m2 s-2. It's especially impressive at the lowest 1 km (up to 250 m2 s-2). Critical angle is also near 90, which is exactly what you want to see out west and south if you're looking for tornadoes. SPC pointed out that thermodynamics might be a limiting factor to today's threat, with mid-level lapse rate being weak and models likely overdoing the dew points (due to lack of vegetation). Usually, I also want to see wind backing up more in low-level region, but you got to work with what you get in New England. Finally, it's the NAM so yeah I'd ignore the over-dramatic PDS tornado hazard on this plot. Sometimes this model overdo severe weather parameters and severe event did under-perform yesterday somewhat. 

Conclusion? Small chance of a tornado or two in NY/VT given impressive hodograph and shear despite limiting thermodynamics. Could be a classic HSLC (high-shear, low CAPE) event if dew point are right on point.

nam_newyork.png

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Hoth

Hoth

Posted
Posted
32 minutes ago, WxBlue said:

While still a long shot, especially around here, it wouldn't surprise me if we get a tornado or two in the 5% area (NY/VT). NAM is actually showing a impressive curved hodograph up to 3 km level to yield helicity up to 400 m2 s-2. It's especially impressive at the lowest 1 km (up to 250 m2 s-2). Critical angle is also near 90, which is exactly what you want to see out west and south if you're looking for tornadoes. SPC pointed out that thermodynamics might be a limiting factor to today's threat, with mid-level lapse rate being weak and models likely overdoing the dew points (due to lack of vegetation). Usually, I also want to see wind backing up more in low-level region, but you got to work with what you get in New England. Finally, it's the NAM so yeah I'd ignore the over-dramatic PDS tornado hazard on this plot. Sometimes this model overdo severe weather parameters and severe event did under-perform yesterday somewhat. 

Conclusion? Small chance of a tornado or two in NY/VT given impressive hodograph and shear despite limiting thermodynamics. Could be a classic HSLC (high-shear, low CAPE) event if dew point are right on point.

nam_newyork.png

Meh lapse rates are often what screw our severe threats, even when the other parameters line up. We tend to need those rare EML days to get siggy, but we do get some fun on fat CAPE days. Still, 6.5 to 7 C/km is certainly not awful by our standards.

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yoda

yoda

Posted
Posted

Could have been a MOD risk day if better CAPE :o

@weatherwiz

Quote 

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1112 AM CDT Fri May 04 2018

   Valid 041630Z - 051200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NEW YORK...VERMONT...NEW HAMPSHIRE...AND MAINE....

   ...SUMMARY...
   Severe thunderstorms are expected today east of the lower Great
   Lakes into the upper Ohio Valley and western portions of New
   England.  This includes a threat for widespread damaging winds, as
   well as tornadoes across parts of New York and Vermont.

   ...NY/PA into New England...
   Morning water vapor loop shows a very strong, compact, and
   progressive shortwave trough moving eastward across MI.  This
   feature will track rapidly eastward across NY/PA this afternoon. 
   Impressive mid-level height falls will occur across this region this
   afternoon and evening, promoting thunderstorm development along and
   ahead of the approaching cold front.  Wind fields will rapidly
   strengthen through the day, indicating the potential for a rather
   widespread damaging wind event, including a few tornadoes across the
   ENH risk area.  This threat appears to be in two distinct waves.

   The first area of risk today will be as isolated/discrete cells form
   this afternoon in vicinity of the surface warm front over northeast
   NY near the US/Canadian border, and with less confidence farther
   south across eastern NY/ and much of VT/NH.  Forecast soundings show
   only marginally favorable thermodynamic parameters.  However,
   low-level winds will be very strong with large, looping hodographs. 
   If convection can remain discrete for awhile, an enhanced tornado
   threat will develop in this area.  These storms will track eastward
   into western ME before weakening this evening.

   Farther west, 12z model solutions are consistent in the development
   of a fast moving squall line along the front that will race across
   the ENH area this afternoon and evening.  Given the intense wind
   fields (50-60 knots just above the surface), widespread damaging
   winds are possible including significant wind damage.  If the squall
   line can attain sufficient mesoscale organization, the environment
   also supports QLCS tornadoes.  These storms will affect much of NY
   and parts of northern PA before sweeping into VT/NH and parts of
   western ME/MA this evening.  Marginal thermodynamic parameters are
   the primary mitigating factor for a categorical upgrade.
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wxeyeNH

wxeyeNH

Posted
Posted

Wow,  I'm on the line between slight and enhanced with SPC.  Im not good at record keeping but can't remember the last time.  Does anyone keep records of enhanced or moderate for C/NNE?  Still cloudy and cool but looks like we are about to break out into brighter skies. 

Living on an exposed hilltop always makes me nervous.  Wonder if we will get a tornado watch out of this setup?

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WxBlue

WxBlue

Posted
Posted
1 hour ago, dendrite said:

You’ll get that extra backing out of the south in the Hudson/CT river valleys too. When I see the lowest barb out of 200 I assume the valley will be closer to 180. 

Generally more backing in real life than modeled, too. At least it seems that way from my past chasing experiences.

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WxBlue

WxBlue

Posted
Posted
1 hour ago, Hoth said:

Meh lapse rates are often what screw our severe threats, even when the other parameters line up. We tend to need those rare EML days to get siggy, but we do get some fun on fat CAPE days. Still, 6.5 to 7 C/km is certainly not awful by our standards.

Especially with hodograph as impressive as today. That'd be a big day in Southern Plains if you put our hodograph there with their typical thermodynamics.

I'm surprised at 10% issuance, but at the same time, I get why the probability is pretty high for our standard. 

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yoda

yoda

Posted
Posted
6 minutes ago, weatherwiz said:

Tornado Watch coming soon up north 

mcd0350.gif

  

Mesoscale Discussion 0350
   NWS Storm Prediction Center Norman OK
   0118 PM CDT Fri May 04 2018

   Areas affected...Portions of northern NY...VT...NH...and far western
   ME

   Concerning...Severe potential...Tornado Watch likely 

   Valid 041818Z - 041945Z

   Probability of Watch Issuance...80 percent

   SUMMARY...The threat for a few tornadoes is increasing this
   afternoon, and damaging winds will become likely with a line of
   thunderstorms moving in from the west. Tornado watch issuance is
   likely.

   DISCUSSION...A sub-996 mb surface low over Lake Michigan as of 18Z
   will continue developing northeastward into southern Ontario/Quebec
   this afternoon as a compact mid-level shortwave trough evident on
   water vapor satellite imagery moves over the same general area.
   Recent subjective surface analysis shows a warm front extending
   eastward from the low across northern NY/VT into central NH. A moist
   low-level airmass characterized by dewpoints in the lower to mid 60s
   exists to the south of the front. Current expectations are for the
   warm front to lift northward to the international border through the
   remainder of the afternoon. Even though some mid/high-level clouds
   are present across this region, filtered diurnal heating has allowed
   surface temperatures to warm into the mid to upper 70s across
   central NY. Although mid-level lapse rates remain generally modest
   (around 6-6.5 degrees C/km in the 700-500 mb layer), they are
   sufficient coupled with the diurnal heating to support MLCAPE up to
   1000 J/kg this afternoon and early evening.

   A very strong mid-level jet of 80-100+ kt will overspread this
   region through peak heating, and effective bulk shear values of
   50-70 kt will strongly favor updraft rotation with any thunderstorms
   that can form along the warm front or within the open warm sector.
   The threat for a few tornadoes should be focused along and just
   south of the warm front, namely northern NY into VT and parts of NH,
   where effective SRH values will likely exceed 250 m2/s2.

   Damaging winds, perhaps widespread, will also be a substantial
   concern as a line of thunderstorms will likely move from west to
   east across this region through 02-03Z along/ahead of a cold front.

   ..Gleason/Hart.. 05/04/2018

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...GYX...BTV...ALY...BGM...BUF...
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Drz1111

Drz1111

Posted
Posted

If I were chasing I'd head right near the border where the front is hanging up.  Probably on the north slope of the Adiorondacks.  Plattsburgh would normally be my target but I think an SE wind off of Champlain isn't likely to be buoyant this time of year, which could create a microscale hindrance to tornado formation near Champlain.

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