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Potential Major Storm 9/18


NEG NAO

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Yeah I would think this would be a case of widespread gusts of 30-40 outside of the squalls and then some severe gusts in the squalls themselves, just because of how strong the winds appear to be just off the surface. An isolated tor can't be ruled out.

Yeah I agree. Instability is so limited on models so it's hard to get too excited about it despite the big time dynamics and talk of widespread wind damage from spc.

I think a low topped line of convection could push through most of the Northern MA, though, and despite the poor timing there should still be a decent amount of strong winds mixing down to the surface.

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I think that this may be the first time that I can remember the models showing LLJ wind speeds

of 50-60 KT around 950-900 mb during a warm season convective event. The models are also

showing some very high helicity values across the region. We may see WAA convection during

the day Tuesday well out ahead of the main line. Any discrete convection will have to be

monitored for rotation and possible isolated tornadoes. The models suggest that the there will be a

forced squall line passage Tuesday evening or at night. With SST's of over 70

degrees, most of the 50-60 KT LLJ could mix down if the line holds together.

There could be an embedded isolated tornado threat along the

strongest part of the line. The model runs should be interesting of the next few days to

help refine this high wind threat across the region.

4 km NAM afternoon WAA convection

powerful LLJ with main line

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The SPC SREF has some pretty impressive parameters just glancing through the wind fields and low level shear...the 0-1km helicity is pretty impressive right ahead of the front.

But the surface instability is very meager...the best instability is south of our area (and coincidentally away from the best kinematics). The "best case" scenario for severe weather/wind enthusiasts will probably be a low-topped squall line along the periphery of the 60kts of 0-6km bulk shear and juxtaposed with some weak instability at the surface.

The image below is valid for 60 hours...timed with the best lift as the front comes through. Timing (as in time of day) is poor for severe wx...but you never know with these numbers.

SREF_1KMHEL_SSB_MEDIAN_MXMN__f060.gif

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I'm sort of meh on any squall line/NCFR potential. The best dynamics begin to lift out as fropa occurs and the timing is overnight.

I think the more interesting possibility is more low topped supercells and a conditional tornado threat if we can generate some surface based instability. Also some damaging synoptic gusts possible if LLJ overperforms and we are able to mix down with steep low level lapse rates.

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I'm sort of meh on any squall line/NCFR potential. The best dynamics begin to lift out as fropa occurs and the timing is overnight.

I think the more interesting possibility is more low topped supercells and a conditional tornado threat if we can generate some surface based instability. Also some damaging synoptic gusts possible if LLJ overperforms and we are able to mix down with steep low level lapse rates.

Certainly a possibility. It's worth noting that we're once again dealing with a situation where the shortwave in association with the 500mb trough and best height falls is well off to the north and west...falling into some of the questions you presented above in respect to the development of a squall line or low topped type deal.

The low topped supercell idea is interesting .. the SPC SREF has increased the surface instability a little bit but it still remains pretty meager (500 j/kg)...We'll see if that will suffice. The 50-60kt LLJ at 900-950mb is impressive.

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Certainly a possibility. It's worth noting that we're once again dealing with a situation where the shortwave in association with the 500mb trough and best height falls is well off to the north and west...falling into some of the questions you presented above in respect to the development of a squall line or low topped type deal.

The low topped supercell idea is interesting .. the SPC SREF has increased the surface instability a little bit but it still remains pretty meager (500 j/kg)...We'll see if that will suffice. The 50-60kt LLJ at 900-950mb is impressive.

Yeah the synoptic forcing does begin to decrease a bit but still sizable given the strengthening jet streak to our NW with us in the right entrance region.

I noticed on the SREF plumes that dew points were struggling out of the mid 60s even for places like ISP. That won't cut it. The op NAM does get some 70+ td air in here. If we're looking at 74/66 stick a fork in any decent convection. 74/73 is a different story.

500-750 j/kg would likely be sufficient for some low topped action if the low level wind fields progged by guidance verify, IMO.

Yeah if we can muster >500 j/kg of surface based CAPE then we certainly could see some low topped stuff. 0-1km shear is more impressive than last Saturday and LCLs are about as low. The issue IMO is the amount of moisture advection in the boundary layer.

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Even without any severe weather were still looking at atleast a low end wind event right? I see upton says they will likely have to issue a wind adv atleast that's what the am disco said.....

Possibly. LLJ magnitude will need to verify as modeled and not shift away from the tri-state or weaken much. In addition the boundary layer will need to remain mixed in order to transport some of that momentum down to the surface. Not a slam dunk.

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I see moumt holly is saying they will prob have wind damage without gettimg any severe weather.. And the latest upton disco they say if the lattest models are right they will have to increase there forcasted wind speeds witch are 25-30 with gust to 40 but like you said thats if the models are right and dont change.....

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The trough is definitely a bit NW of my liking, similar to last Saturday. However, the trough is more amplified than the past one was, so I think we will be closer to the latitude of the best lift this time around than we were last Saturday. It's still a potential problem, though.

Our SB CAPE for last Saturday's event was really high...it was >2000 J/KG in the NYC/LI areas. Our low-level lapse rates were also not too bad. I'm not sure we'll be able to generate those impressive surface parameters that we had last Saturday.

However, the low-level wind fields are even more impressive...perhaps significantly so. But if the lapse rates near the surface are not that high, and the SB CAPE stays too low, then any updraft might get sheared apart.

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I dont see any inversion in this event during the time of max LLJ. Why would warm SST's bring an inversion like we see with similar systems in the winter/fall/spring? That needs to be monitored, because the way things look on soundings I have seen, the mixing will occur when the strongest winds are just above the deck. In fact, its dry adiabatic to the height of the max LLJ at 925 mb.

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I dont see any inversion in this event during the time of max LLJ. Why would warm SST's bring an inversion like we see with similar systems in the winter/fall/spring? That needs to be monitored, because the way things look on soundings I have seen, the mixing will occur when the strongest winds are just above the deck. In fact, its dry adiabatic to the height of the max LLJ at 925 mb.

Yeah that's just the thing. There's a short window around here climatologically when we can develop surface based instability/maintain steep low level lapse rates during strong southerly LLJs.

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spcday2_091712.png

...NY/WRN NEW ENGLAND TO NERN GULF COAST...

NARROW LINE OF CONVECTION SHOULD DEVELOP EARLY IN PERIOD AND CROSS

MID-ATLC AND CENTRAL PIEDMONT REGIONS...WITH DAMAGING WIND BEING

PRIMARY THREAT. A FEW TORNADOES ALSO ARE POSSIBLE.

AT START OF PERIOD...MORE THAN ONE BAND OR CLUSTER OF TSTMS MAY BE

ONGOING...MOST LIKELY OVER SRN APPALACHIANS REGION AND OVER ERN

OH/WRN PA AREA. AS EACH MOVES INTO PROGRESSIVELY HIGHER-THETAE AIR

MASS...SVR POTENTIAL WILL INCREASE...WHILE EVOLUTION OF MORE UNIFIED

SQUALL LINE IS EXPECTED. FLOW ALOFT WILL BE MOSTLY PARALLEL TO

CONVECTIVE FORCING...INDICATING RAPID DEVELOPMENT OF LINEAR MODE

THAT WOULD BE MAINTAINED THROUGH PERIOD. OVER MID-ATLC REGION...SFC

LOW SHOULD LIFT NEWD FAST ENOUGH TO OUTPACE NWD EXTENT OF FAVORABLE

DESTABILIZATION. BEFORE THAT HAPPENS...HOWEVER...RELATED

ISALLOBARIC FORCING AND BACKING OF LOW-LEVEL FLOW TO ITS E SHOULD

AFFECT NRN FRINGE OF AT LEAST MRGLLY UNSTABLE AIR DURING

DAY...BENEATH 50-65 KT LLJ. THIS WOULD ENLARGE LOW-LEVEL HODOGRAPHS

FAVORABLY FOR SUPERCELL/TORNADO POTENTIAL WITH ANY

DISCRETE/SUSTAINED CELLS THAT CAN FORM AHEAD OF MAIN LINE...AND QLCS

TORNADO THREAT WHERE SQUALL LINE ENCOUNTERS UNSTABLE AND SFC-BASED

EFFECTIVE INFLOW PARCELS.

BUOYANCY WILL DIMINISH WITH NWD EXTENT AMIDST WEAK MIDLEVEL LAPSE

RATES. FCST SOUNDINGS SUGGEST MLCAPE RANGING FROM AROUND 1000 J/KG

OVER ATLC COASTAL DELMARVA TO LESS THAN 100 J/KG OVER ERN NY.

STILL...NEAR-NEUTRAL TO VERY SLIGHTLY UNSTABLE THERMODYNAMIC

PROFILES MAY BE SUFFICIENT TO SUPPORT CONDITIONAL WIND POTENTIAL

OVER PARTS OF ERN NY AND WRN NEW ENGLAND...GIVEN STRENGTH OF

AMBIENT/DEEP-LAYER FLOW.

HODOGRAPHS SHOULD DIMINISH IN SIZE WITH SWWD EXTENT AS LLJ GETS

WEAKER AND SFC WINDS ARE RELATIVELY VEERED. HOWEVER...GREATER

BUOYANCY IS EXPECTED OVER VA/CAROLINAS...GIVEN RICHER LOW-LEVEL

MOISTURE AND MORE TIME FOR CLOUD-MUTED SFC DIABATIC HEATING IN

ADVANCE OF MAIN TSTM BAND. MEANWHILE...SERN RIM OF FAVORABLE

DEEP-LAYER FLOW/SHEAR WILL OVERSPREAD WRN FRINGES OF WARM

SECTOR...CONTRIBUTING TO SVR POTENTIAL. HOWEVER...WITH SWWD

EXTENT...THREAT WILL BECOME MORE TRANSIENT AND CONDITIONAL...AS

STRONGEST DEEP-LAYER ASCENT AND ISALLOBARIC FORCING PULL AWAY FROM

THOSE PORTIONS MS/AL/FL PANHANDLE ALONG/AHEAD OF FRONT.

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NAM still brings dews into the lower 70s which seems too agressive....if we dont get dews into the 70s its going to be harder to get the good winds to mix down.

sfc_con_dewp.gif

Currently 70 dews are confined to the gulf coast, we are going to need some serious WAA to get those this far north.

The moisture advection will occur with the strong LLJ progged with this system, you should have no problem getting to 70 degree dew points. Plus you have mid to upper 60 degree dews all the way to Southern IL.

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