2018 Mid-Atlantic General Severe Discussion

[BlizzardNole]

1 hour ago, losetoa6 said:

Don't worry ...yours is coming

6z gfs


Sent from my SM-N950U using Tapatalk
 

I'll be shocked if I get one third that amount.  I am sitting at 0.9 for the whole month of May.

 

[Eskimo Joe]

lol at the 6z GFS precip...we won't get a quarter of that.  Meso guidance is a real snoozer today locally, even looks north of here.

[mappy]

7 minutes ago, Eskimo Joe said:

lol at the 6z GFS precip...we won't get a quarter of that.  Meso guidance is a real snoozer today locally, even looks north of here.

i mean i guess if you are only looking at the HRRR, sure. 

[smokeybandit]

I feel for my son though.  Rain or not, soccer is likely to be canceled all week just on the persistent threat of thunderstorms every day.

[H2O]

at least things are starting off with more sun today than same time yesterday.

[mappy]

updated day 1

15% wind 15% hail 2% tornado runs along the PA line

 

One source of uncertainty in this scenario is an area of outflow to
   the south, across parts of VA/MD/Delmarva and eastern PA,
   originating from yesterday's MCS, and sampled peripherally by the
   12Z IAD sounding.  Airmass recovery is expected from the southwest,
   around the northwest rim of that outflow pool and south of the
   morning convective/frontal baroclinic zone.  Expect midday to
   afternoon preconvective destabilization arising from both theta-e
   advection and diabatic surface heating.  68-70 F surface dew points,
   such as forecast by the NAM, may be overdone considering the
   available recovery trajectories, and the nearest dew points that
   large are 300-400 nm away over NC, on the other side of the outflow
   pool.

   Regardless, a plume of EML air advecting over this region will
   foster steep midlevel lapse rates, overlying strengthening
   boundary-layer lapse rates and low/mid-60s F surface dew points. 
   That combination still supports peak MLCAPE in the 1500-2500 J/kg
   range, amidst strong west-southwesterly mean-wind and deep-shear
   vectors.  Forecast soundings suggest that, despite a nearly
   unidirectional vertical wind profile, effective-shear magnitudes of
   45-55 kt may be realized.  Downward momentum transfer from strong
   flow above 700 mb, into a well-mixed preconvective boundary layer,
   should offer favorable conditions for severe thunderstorm winds. 
   Given the strong westerly component of the near-surface flow,
   more-unstable inland air may be shunted eastward to very near the
   coast across much of the region, extending the severe threat
   accordingly, before the MCS encounters too much stable marine-layer
   air and weakens.

[mappy]

Combo of high EML, and CAPE, should provide enough for storms dropping out of PA to gain strength. Just need them to come through earlier in the evening rather than later.

HM on twitter earlier

[87storms]

6 hours ago, Round Hill WX said:

Hell of a light show for the past 2 hours here in Round Hill. 

that thing came through poppin' at around 3am last night.  woke me up.  nice fireworks display for about an hour.

[Cobalt]

Not sure if this was posted, but it seems as if the storm was a confirmed derecho

 

[mappy]

Id wait until NWS comes out saying so, but perhaps it was. 

[mappy]

lot of fun discussion about it this morning. 

[SomeguyfromTakomaPark]

1 hour ago, mappy said:

updated day 1

15% wind 15% hail 2% tornado runs along the PA line

 

One source of uncertainty in this scenario is an area of outflow to
   the south, across parts of VA/MD/Delmarva and eastern PA,
   originating from yesterday's MCS, and sampled peripherally by the
   12Z IAD sounding.  Airmass recovery is expected from the southwest,
   around the northwest rim of that outflow pool and south of the
   morning convective/frontal baroclinic zone.  Expect midday to
   afternoon preconvective destabilization arising from both theta-e
   advection and diabatic surface heating.  68-70 F surface dew points,
   such as forecast by the NAM, may be overdone considering the
   available recovery trajectories, and the nearest dew points that
   large are 300-400 nm away over NC, on the other side of the outflow
   pool.

   Regardless, a plume of EML air advecting over this region will
   foster steep midlevel lapse rates, overlying strengthening
   boundary-layer lapse rates and low/mid-60s F surface dew points. 
   That combination still supports peak MLCAPE in the 1500-2500 J/kg
   range, amidst strong west-southwesterly mean-wind and deep-shear
   vectors.  Forecast soundings suggest that, despite a nearly
   unidirectional vertical wind profile, effective-shear magnitudes of
   45-55 kt may be realized.  Downward momentum transfer from strong
   flow above 700 mb, into a well-mixed preconvective boundary layer,
   should offer favorable conditions for severe thunderstorm winds. 
   Given the strong westerly component of the near-surface flow,
   more-unstable inland air may be shunted eastward to very near the
   coast across much of the region, extending the severe threat
   accordingly, before the MCS encounters too much stable marine-layer
   air and weakens.

Where do you get these zoomed in convective outlooks?

[yoda]

2 hours ago, mappy said:

9z HRRR is a real snoozer for today. 06z NAM has a line dropping south around 23z-00z

13z HRRR is better... not sure about severe... but storms are around us

[mappy]

2 minutes ago, DCTeacherman said:

Where do you get these zoomed in convective outlooks?

http://www.spc.noaa.gov/public/state/images/

 

[vastateofmind]

17 minutes ago, mappy said:

Id wait until NWS comes out saying so, but perhaps it was. 

Really interesting to hear the derecho talk this morning...in looking at radar around 6:30 p.m. last night (and I could kick myself for not grabbing a screencap), there appeared to be a prominent bow echo, with the leading edge bearing down on the Alexandria/Belle Haven area, then bending back over the District to the north and PWCo to the south. Some excellent pics of shelf clouds associated with last evening's derecho or near-derecho, too, on Twitter today.

[mappy]

2 minutes ago, yoda said:

13z HRRR is better... not sure about severe... but storms are around us

yeah looks like it doesn't come together until its south of me (boo). but at least it has a decent line forming. 

1 minute ago, vastateofmind said:

Really interesting to hear the derecho talk this morning...in looking at radar around 6:30 p.m. last night (and I could kick myself for not grabbing a screencap), there appeared to be a prominent bow echo, with the leading edge bearing down on the Alexandria/Belle Haven area, then bending back over the District to the north and PWCo to the south. Some excellent pics of shelf clouds associated with last evening's derecho or near-derecho, too, on Twitter today.

I don't know the clear definition, as there seems to be a new proposed one going around too. Discussion seems to be based on that. 

[vastateofmind]

Not the best shot of the last night's approaching shelf cloud (snapped just before 6:30 p.m. IMBY), as it was almost ready to move directly overhead...but you can see some of its movement at mid-screen. At that point, I think FfxCo was also under a tornado warning, so those cheesy-looking, grey/green clouds just above the treeline gave me pause.

 

[smokeybandit]

Quote

According to the National Weather Service criterion, a derecho is classified as a band of storms that have winds of at least 50 knots (90 km/h; 60 mph) along the entire span of the storm front, maintained over a time span of at least six hours. Some studies add a requirement that no more than two or three hours separate any two successive wind reports.

 

[mappy]

6 minutes ago, vastateofmind said:

Not the best shot of the last night's approaching shelf cloud (snapped just before 6:30 p.m. IMBY), as it was almost ready to move directly overhead...but you can see some of its movement at mid-screen. At that point, I think FfxCo was also under a tornado warning, so those cheesy-looking, grey/green clouds just above the treeline gave me pause.

 

great video! Yup, green is a tell tale sign of updrafts, in this case, lots of hail. rotation was very broad. 

[yoda]

14z HRRR decent for most... even mappy gets in on some of the action... but its after 22z

[mappy]

14z HRRR looking better.

12Z NAM seems to slowed down by an hour compared to 06z... 6-7pm the line drops out of PA. 

 

[yoda]

3 minutes ago, mappy said:

14z HRRR looking better.

12Z NAM seems to slowed down by an hour compared to 06z... 6-7pm the line drops out of PA. 

 

12z NAM NEST looks decent as well IMO for most of us

[Eskimo Joe]

Meso models do look better for the mason-dixon folks.  PA is cooking too...SBCAPE pushing 3,000 near Pittsburgh.  

[mappy]

3 minutes ago, Eskimo Joe said:

Meso models do look better for the mason-dixon folks.  PA is cooking too...SBCAPE pushing 3,000 near Pittsburgh.  

EML looking good too. 

[Eskimo Joe]

Just now, mappy said:

EML looking good too. 

There is a great amount of DCAPE...my rule is 1,000j/kg or greater = widespread wind threat and we're pushing 1,100 north of I-70.  I think you and I might get some action on the southern flank.

[yoda]

1 minute ago, Eskimo Joe said:

There is a great amount of DCAPE...my rule is 1,000j/kg or greater = widespread wind threat and we're pushing 1,100 north of I-70.  I think you and I might get some action on the southern flank.

The 1000 J/KG DCAPE line is into VA... almost looks wedge-like in its delineation lol

[mappy]

5 minutes ago, Eskimo Joe said:

There is a great amount of DCAPE...my rule is 1,000j/kg or greater = widespread wind threat and we're pushing 1,100 north of I-70.  I think you and I might get some action on the southern flank.

Yup. As I said earlier, if it can drop out of PA earlier rather than later, it will feed off our CAPE and should give us a nice little event. 

[mappy]

84/70 IMBY. Swampy. 

[yoda]

Doubt it works well for storms... but I like the 12z HRDPS presentation from 21z and on

ETA:  15z HRRR has said line... but its 01z-02z time period

[mappy]

SPC going Mod risk up in New England at next update.