[HM]

Well, the pattern lately has been very similar to 1967 and this doesn't seem to want to change as we press into the future. During mid May of 1967, a building western ridge began as a trough developed across the Great Lakes/Ontario. Ultimately, on the back side of the trough, there was the development of a subtropical jet upper low which positioned across the Southeast, much like the forecast coming up. Now, the exact scenario in 1967 will not play out as far as how the situation in the Gulf/Southeast plays out but the pattern is quite similar, anomaly-wise.

Interestingly enough, the same type of trough is expected to pass across the Great Lakes and Northeast this Saturday. Back in 1967, there was severe weather with this feature and it produced an F3 in Maryland-->

Garrett County
MAY 19, 1967 002 1915 1 0 F3 023

Here was the 500mb mean geopotential height from 5/18/67 through 5/21/67 (to get an idea of the similarities):

http://i10.photobucket.com/albums/a127/HMisdead/May67.jpg

Now the winds that day at 500mb:
http://i10.photobucket.com/albums/a127/HMisdead/May67500mb.jpg

Finally, here is the 850mb winds that day:
http://i10.photobucket.com/albums/a127/HMisdead/May67850mb.jpg

Clearly, the situation coming up doesn't seem to have the same type of winds in the forecast at these levels. Also, 1967 was less meridional with the flow than what is predicted this Saturday. The LLJ and MLJ were more zonally-orientated and allowed for the strongest winds to come further south.

I think NE WV, N VA, MD and PA have a risk for isolated supercells given the current situation right now. The models have been trending stronger with the wind fields with each run and there is a nice Theta-E ridge pumping up across the interior portions of the Mid Atlantic. Also, the models are developing a surface wave along the front which helps to back the flow, especially across PA/MD/NJ. The areas that can intercept this instability/moisture advection with the best winds will have a tornado threat.

Expect a sharp gradient between the coastal areas and inland areas where temperatures could jump 15-20° in 50 miles and there will be a noticeable warm front that gets stuck. The areas in the moist/unstable air mass nearest this front will have maximized low level veering and would support supercells. I think the target state on Saturday is Pennsylvania given everything I am seeing, so far, and it seems to make sense given the placement of the jets and where the expected warm sector may be.

[Rtd208]

HM,

What are your thoughts as far as severe weather goes on Saturday/Saturday night in NJ? We may not be in most prime area for tornadic activity but I think severe weather is a good bet which was highlighted by Mt.Holly in their afternoon AFD. Also I think heavy rain will be an issue as well.

Ron-NJ

[HM]

Rtd208, on May 15 2009, 12:43 AM, said:

HM,

What are your thoughts as far as severe weather goes on Saturday/Saturday night in NJ? We may not be in most prime area for tornadic activity but I think severe weather is a good bet which was highlighted by Mt.Holly in their afternoon AFD. Also I think heavy rain will be an issue as well.

Ron-NJ

I think it is all a matter of cloud cover and where the warm front is. If we can effectively destabilize, there will certainly be a severe weather risk later Saturday through the night. A good southwesterly low level surge will come out ahead of the front, either way, and so there should be thunderstorms. The profiles are moist throughout most of the troposphere with LI between -3 and -6 before the FROPA. The most impressive feature for heavy rain are the precipitable water values pushing 2 inches across most of the Delaware Valley but average 1.5"-2.0"!! So, no doubt, heavy rainfall seems like a good bet.

[HM]

I'll tell you what, it will be close for NJ. I am really liking the +RN and supercell threat across E PA. Seems like MLCAPES ~ 1000 j/kg for eastern PA on the new 00z GFS with low LCL heights well below 1000m. The winds will be locally backed from the close proximity to the warm front/coastal boundary and there is a small maximum in the LLJ over E PA which exceeds 30kts.

If we don't get the severe aspect of this, it certainly is going to be a loud and wet evening across much of the Delaware Valley! I want to wait for tomorrow's data before I include parts of W NJ/DE in the tornado risk, considering how sensitive this area is...

[weatherwiz]

Excellent post, HM! Didn't look at 0z data yet (had to go out and walk around and cool my mind after the Bruins game) but I had been interested in this for the past few days...all that was needed was some instability. I briefly looked at the 12z data today and the NAM had some juicy cape values. Shear is definitely there as is helicity...lapse rates not that bad either. If moderate instability can develop watch out! Could definitely see some isolated discrete supercells and some right movers.

I really like where we are heading pattern wise for severe wx potential out this way. Once we get into the stronger heating things could become quite fun.

[earthlight]

Classic warm frontal jamming situation where this thing actually will get stuck and won't move. This is depicted well on the SREF mean with the severe thunderstorm probability dropping off extremely sharply in Western NJ and eastward.

What's most interesting about the situation is the bust potential either way as well as the dynamics...with the warm front in the area there is going to be plenty of support for some low topped supercells if the degree of potential instability can be realized. Keep in mind that with the LLJ migrating the way it is we often see ongoing convection contaminate the warm sector. Also the wind fields trend unidirectional shortly after the convection gets going on most models, so that could also put a lid on tornadic potential and swing things more towards wind damage and some marginally severe hail given the plume of modestly steep lapse rates. The moisture is definitely there..we need the rest to fall into place, we will see.

[HM]

earthlight, on May 15 2009, 02:21 AM, said:

Classic warm frontal jamming situation where this thing actually will get stuck and won't move. This is depicted well on the SREF mean with the severe thunderstorm probability dropping off extremely sharply in Western NJ and eastward.

What's most interesting about the situation is the bust potential either way as well as the dynamics...with the warm front in the area there is going to be plenty of support for some low topped supercells if the degree of potential instability can be realized. Keep in mind that with the LLJ migrating the way it is we often see ongoing convection contaminate the warm sector. Also the wind fields trend unidirectional shortly after the convection gets going on most models, so that could also put a lid on tornadic potential and swing things more towards wind damage and some marginally severe hail given the plume of modestly steep lapse rates. The moisture is definitely there..we need the rest to fall into place, we will see.

Your two concerns are exactly right and could put a lid on any supercell potential. I would imagine as surface pressure falls occur across N WV/PA/MD, the winds may locally back along favorable terrain slopes and could be enough to allow some discrete type storms. However, will the dynamics lag the front during this period where backing is possible? If the thunderstorms decide to develop (all instability aside and assuming it is favorable) later because of a slightly slower solution with the dynamics, then the profiles will be largely unidirectional. The only source for low level veering left, at that point, would be the warm front stuck across the coastal plain.

You are also right about the potential contamination from prior convection/debris and lack of appreciable cap once that moves out and things begin to warm up. The sun poking through will probably be more self destructive than usual, in this case, and favor a widespread development. We would then have to hope that they congeal along the front and that gives folks downstream more time to heat up.

[HM]

weatherwiz, on May 15 2009, 01:47 AM, said:

Excellent post, HM! Didn't look at 0z data yet (had to go out and walk around and cool my mind after the Bruins game) but I had been interested in this for the past few days...all that was needed was some instability. I briefly looked at the 12z data today and the NAM had some juicy cape values. Shear is definitely there as is helicity...lapse rates not that bad either. If moderate instability can develop watch out! Could definitely see some isolated discrete supercells and some right movers.

I really like where we are heading pattern wise for severe wx potential out this way. Once we get into the stronger heating things could become quite fun.

Hey Paul and thanks! There are certainly the usual concerns out there that would be choking any potential Northeast/Mid Atlantic potential but at least there is something on the table in the short range. At the very least, the synoptic setup and profiles favor a lot of thunderstorms and rainfall for many in the Mid Atlantic!

I see two opportunities for enhanced SRH.

1. Localized backing winds as pressure falls pass across favorably sloped terrains in WV/MD/PA. There could be a classic SE/SW wind convergent line, for a time, across PA while this process is occurring.

2. As thunderstorms press eastward towards the coastal plain "jammed warm front" and assuming there is good instability, they could encounter enhanced low level shear.

Other than that, it appears the main mode of severe will likely be severe wind gusts along with the heavy rainfall threat.

[earthlight]

HM, on May 15 2009, 02:36 AM, said:

Your two concerns are exactly right and could put a lid on any supercell potential. I would imagine as surface pressure falls occur across N WV/PA/MD, the winds may locally back along favorable terrain slopes and could be enough to allow some discrete type storms. However, will the dynamics lag the front during this period where backing is possible? If the thunderstorms decide to develop (all instability aside and assuming it is favorable) later because of a slightly slower solution with the dynamics, then the profiles will be largely unidirectional. The only source for low level veering left, at that point, would be the warm front stuck across the coastal plain.

You are also right about the potential contamination from prior convection/debris and lack of appreciable cap once that moves out and things begin to warm up. The sun poking through will probably be more self destructive than usual, in this case, and favor a widespread development. We would then have to hope that they congeal along the front and that gives folks downstream more time to heat up.

Ah, severe weather forecasting in the northeast. What a pain it can be, especially once you get excited seeing the nose of the LLJ really making headway and finally supplying some rich moisture. Agreed on all fronts, especially the first point. It's often a ticking clock in this part of the CONUS in regards to the potential for discrete development--the window closes quickly.

Personally, I think there's enough reason to pay attention this this threat as there is any severe thunderstorm threat in the northeast. The thermodynamics are potentially there, the kinematics are there, we just need things to fall into place.

About the contamination--yes, that gets very frustrating. I can't count the amount of times i've seen the sun pop up well ahead of the cold front, forcing the development of widespread convection/debris. Such is life with the llj and warm front positioned like that.

[weatherwiz]

HM, on May 15 2009, 02:43 AM, said:

Hey Paul and thanks! There are certainly the usual concerns out there that would be choking any potential Northeast/Mid Atlantic potential but at least there is something on the table in the short range. At the very least, the synoptic setup and profiles favor a lot of thunderstorms and rainfall for many in the Mid Atlantic!

I see two opportunities for enhanced SRH.

1. Localized backing winds as pressure falls pass across favorably sloped terrains in WV/MD/PA. There could be a classic SE/SW wind convergent line, for a time, across PA while this process is occurring.

2. As thunderstorms press eastward towards the coastal plain "jammed warm front" and assuming there is good instability, they could encounter enhanced low level shear.

Other than that, it appears the main mode of severe will likely be severe wind gusts along with the heavy rainfall threat.

I actually even like Central NY as well for potential severe...SPC also has this area in a slight risk. With sfc flow/LL flow out of the S/SW/SE across the region this could help for areas in the Valley to mix out better and allow for better heating potential. Many times in the Valley they clear out much better than anyone else. The Valley also helps to further enhance the flow/helicity. I think here too may have a better shot at seeing some early morning sun mixing in.

One thing I have noticed up here is when you have a LLJ of at least 30-35 knots with a warm front in the area and a pre-frontal trough coming through LL winds actually back more than models show and b/c of this models underdo the LL helicity. Now the models already have the best helicity values go right through NYS and it really wouldn't surprise me if they are again underdone.

The big thing that appears to be lacking is the sfc warmth/instability. This was one major limitation this past Saturday and will probably be so again. I also think it's possible (and like you mentioned) the best jet dynamics lag too far behind the front but with front parallel to the upper level flow maybe this can slow down the eastward progression of the front enough for the best dynamics to catch up??? There have been plenty of events though where many thngs are in place but the front and the storms outrun the best dynamics.

Lapse rates don't look as great as earlier either although these may be underdone if sfc warmth and BL heating are underdone.

Too bad the sfc low isn't tracking about 100-150 mi further SE though, this would place the area in a much better position to the ML/UL jets and allow for better forcing/lift to overcome lack of instability.

All in all I could see some sort of low topped line with winds the main threat and with some embedded cells acquiring rotation and if decent instability can be realized a few isolated brief tornadoes would certainly be possible.

[HM]

It is amazing how much the upper air patterns resemble 1967 but it it too bad the results will probably NOT be quite the same. :rolleyes:

As you guys pointed out, it is too bad that all features are not further south. I think that is the key difference between this upcoming event and the event back in 1967. Things then had a more zonal orientation to the jet configuration and that helped bring the jets further south to enhance deep layer shear/SRH. But a very similar pattern and even surface pressure pattern to what is modeled for Saturday.

Many things have to come together but at least it is something to watch. And Paul I agree with you and the SPC that S NY is certainly in the potential.

[Eskimo Joe]

If places along I-95 can get some nice sun, how warm do you think it could get? I was thinking closer to low, maybe mid 80s?

[rainshadow]

Eskimo Joe, on May 15 2009, 09:46 AM, said:

If places along I-95 can get some nice sun, how warm do you think it could get? I was thinking closer to low, maybe mid 80s?

Matt,

This looks like a dirty warm sector, without much forecast capping. Location of LLJ could make weaker convection going really anytime in the afternoon. But if the sun would come out tomorrow for a few hours during the afternoon, I could see PHL reaching something like 86. I forgot to throw this in the afd this morning, but most of the forecast helicity is concentrated in the lowest 1km which on paper is favorable at the least for supercells. Overall I did agree with the way SPC forecast the slight risk this morning, the forecast helicity/cape combination was much better in places to our west and with the best convection slated to arrive in the evening you know how that goes. Nevertheless I think this is the first legitimate needs to be followed severe weather threat for us since all that hail in late March.

[ezweather]

Tony,

Looks like I'll have to give my Deputy Skwywarn coordinator an update on the threat for severe thunderstorms for Camden County. I'll be at the Philadelphia Eagles Academy tomorrow. I wonder if we'll be doing drills inside the bubble or outside. Maybe I should bring along a weather radio that has the siren built in it. I guess the activity is coming late in the day, especially after 4pm to something like 11pm.

Definitely have to check out the data from the 12z run today. I'll be curious to see what you have to say...

rainshadow, on May 15 2009, 11:17 AM, said:

Matt,

This looks like a dirty warm sector, without much forecast capping. Location of LLJ could make weaker convection going really anytime in the afternoon. But if the sun would come out tomorrow for a few hours during the afternoon, I could see PHL reaching something like 86. I forgot to throw this in the afd this morning, but most of the forecast helicity is concentrated in the lowest 1km which on paper is favorable at the least for supercells. Overall I did agree with the way SPC forecast the slight risk this morning, the forecast helicity/cape combination was much better in places to our west and with the best convection slated to arrive in the evening you know how that goes. Nevertheless I think this is the first legitimate needs to be followed severe weather threat for us since all that hail in late March.

[hudsonvalley21]

SPC's thoughts

DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1210 PM Cdt FRI MAY 15 2009

...LOWER GREAT LAKES INTO UPPER OH VALLEY...

THE PRESENCE OF ONGOING SHOWERS AND TSTMS AND RESULTANT CONSIDERABLE
CLOUD COVER ARE EXPECTED TO LIMIT DAYTIME HEATING AND LAPSE RATES
WITHIN PRE-FRONTAL AIR MASS SATURDAY. NONETHELESS...INCREASING
BOUNDARY LAYER MOISTURE ALONG TRAILING EXTENSION OF STRONG LLJ
DEVELOPING RAPIDLY INTO QUEBEC WILL SUPPORT MLCAPE APPROACHING
500-800 J/KG BY AFTERNOON. AS SUCH...EXPECT TSTMS TO GRADUALLY
INTENSIFY THROUGH THE DAY ALONG FRONT AS STRENGTHENING DEEP-LAYER
ASCENT BEGINS TO ACT ON DESTABILIZING AIR MASS.

THE STRONGER VERTICAL SHEAR...BOTH IN THE LOW LEVELS AND THROUGH A
DEEPER LAYER...WILL REMAIN CONFINED TO NY/PA PORTION OF SLIGHT RISK
WHERE SRN EXTENSION OF 40-50 KT LLJ WILL BE COLOCATED WITH 40-50 KT
WSWLY MIDLEVEL FLOW. THIS SETUP SHOULD YIELD A LINEAR CONVECTIVE
SYSTEM ALONG FRONT WITH THE POTENTIAL FOR EMBEDDED BOWING
STRUCTURES. WHILE DAMAGING WIND GUSTS WILL BE THE PRIMARY SEVERE
WEATHER THREAT...SOME HAIL AND A BRIEF TORNADO OR TWO WILL BE
POSSIBLE.

[PSUHazletonWx]

You had me at "liking the supercell threat across E PA". :thumbsup:

[CoastalWx]

Nice discussion HM. I personally like the Lancaster County area of PA for combo if shear and instability. Of course I mean that region, not just the county...from Reading to Lancaster to York area. Those areas seem to do well in setups like this. Cloud cover may be a problem so realizing the instability may be an issue like some have pointed out. Certainly interesting to watch.

[SnowAlbedo]

I'll be at the Chili Cookoff in DC tomorrow. Maybe I should bring my weather radio?

[Derecho]

Pretty much every tornado threat identified days ahead of time for the Mid-Atlantic or NE busts.

[Roger Smith]

Very strong winds aloft at 18z Saturday across the lower Gt Lakes and most of NY-PA, seems like a very good set-up for severe storms as far north as central Ontario (which is really south-central Ontario, just the name given to it before people realized how far north Ontario actually went).

So for Canadian readers (eh) could be severe gusts to 110 km/hr or even a local F1 tornado in regions north and east of Toronto on Saturday afternoon (and this is a long weekend in Canada, a lot of people will be opening up their summer cottages in the region).

Same forecast for much of NY, PA, NJ and western New England, with the trailing storms through MD, DC and parts of VA. Wind gusts to 70 mph possible and F2 tornadic potential especially (looking at all guidance) in s.e. PA and w NJ.

The system is very strong on the ground already, look at radar across MO and KS at the present time, and then factor in that upper level winds will be almost twice as strong by Saturday. That has to make up for a lot of the reducing effect of moving away from the source of Gulf heat and moisture. Could be a memorable outbreak even, can't rule out an F3 tornado in this set-up. The 500-mb wind field on the 24 and 30h panels from 12z looks ominous.