Sunday 3/13/16 Forecast

031316 Day 2 Forecast Map

Tough decision to make on whether or not to chase tomorrow with some detail discrepancies between the NAM and GFS. Generally speaking the models are in good agreement. Both have a 65kt mid level jet rotating around the base of a shortwave and nosing into central Arkansas tomorrow afternoon. The NAM is a tad faster with a tad more negative tilt to it, but otherwise they are very similar. Moisture is a little sketchy, but it is looking like dewpoints near 60 wrapping up into central Arakansas tomorrow afternoon. I still have a lot of forecasting to do, but I did check a few soundings from the Gulf Coast this morning. The Lake Charles sounding has low 60 dewpoints and the moisture is fairly deep, but when you go SW to the Corpus Christi and Brownsville soundings it isn’t pretty. I still need to take a closer look tonight, but the NAM is a little more aggressive with quality moisture advection into central Arkansas tomorrow afternoon. The difference seems to be how far the current easterly low level winds over the NW gulf push decent moisture offshore and how good of a fetch you get back off the western gulf tonight into tomorrow. Like I said I need to look closer tonight, but going off the 12Z runs and current buoy data over the gulf it seems like the NAM may have a little better handle on where surface dewpoints currently are. I hope so, because the subtle moisture return difference between the two models could have a big impact on the results tomorrow. Anyway, I’ll come back to that later tonight.

The other big difference between the two models is low level winds and how they impact hodographs. The NAM is putting out some pretty impressive low level shear near Little Rock for tomorrow afternoon. Below is an image of the forecast hodograph for Little Rock tomorrow at 00Z.

031216 Day 2 Hodograph

Note the kink in the hodograph just below 1km and how the critical angle is near 90. That along with decent speed shear in the low level (straight portion of the hodograph) can really help storms to go tornadic. There are plenty of papers on it that will explain it much better than I can, but basically the low level inflow is perpendicular to the low level straight portion of the hodograph shown above, helping storms to ingest the low level vorticity created by the low level speed shear. The technical term would be that the low level vorticity is streamwise . That’s what that critical angle number is geared towards in case you didn’t know. It’s the angle of 10-500m shear vector relative to 10m SR inflow for a right moving storm and it’s right in the sweet spot near 90 degrees with the NAM forecast hodographs across central Arkansas for tomorrow afternoon. That along with the decent moisture return the NAM is showing and the likelihood for discrete supercells across central Arkansas in the 6-8pm timeframe tomorrow is what’s making me want to chase. HOWEVER, the god damn GFS is showing subtly different low level wind fields which throws that critical angle off. That’s because in the 6-8pm timeframe the GFS veers low level winds more relative to the NAM, making a shallower loop in the low level hodograph and puts the critical angle at more like 60 degrees. It may not seem like a big deal, but the devil is in the details when it comes to tornadoes and small difference like that often times mean the difference between supercells being tornadic or nontornadic. BTW I tried to grab pics of both models forecast hodographs, but for some reason COD wouldn’t cooperate and let me grab pics so you’ll have to look for yourself.

Anyway, the short of it is that I think there will definitely be supercells tomorrow in Arkansas since there is good deep layer shear and dewpoints should at least make it into the upper 50’s with good surface heating during the afternoon, but whether or not we get tornadoes or the extent of any tornado threat will likely be guided by low level shear profiles and whether or not dewpoints in the low 60’s can wrap back up into central Arkansas. That is a tough call to make at the moment with the discrepancy between the models and trajectories off the gulf still transitioning through this afternoon. Hopefully tonight the picture will be a little more clear. I’ll try to get an update posted again around 10pm if I don’t go out with friends. I already made arrangements for Duck to have a babysitter and I’m loading my car up as soon as I get done typing this, so I’m good to go on chasing tomorrow. I won’t make the call until late tonight though. If the GFS looks like it’s verifying, I’m going to sit this one out. If the NAM looks like it’s verifying, I’m in. I’ve actually never chased Arkansas before, but I’ve heard it’s not bad around Little Rock and some portions in the SE part of the state. The western half looks like a shit show for chasing with hills and forest everywhere lol. It feels like a really big gamble going that far since the timing is going to have to be just right for us to get tornadoes while it’s still daylight in chaseable terrain. It does look like that could very well happen with the NAM though with storms tracking through the I40/530/Little Rock corridor in the 6-8pm window right as hodographs really start to enlarge. I have an appointment to take my car in to get the dent I just put in it fixed Monday morning at 8am, so it’s going to be a rough one for Mikey if I go lol. I hate making these iffy setup long distance chase calls early in the season. I hate missing tornadoes too though and I always think about that poster that says “do more of what makes you awesome” when debating these decisions. The alternative to chasing tomorrow is getting drunk with my friends tonight and sleeping in tomorrow. Getting hammered and sleeping isn’t what makes me awesome lol. Alright enough rambling. I’ll update at some point tonight if I can and if I chase I’m hitting the road at 8am tomorrow and will post on my way to Arkansas.

 

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