General Weather Discussion - April 2015

[venture]

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[venture]

Okay time to get the April thread underway (a little late, sorry) and we'll be starting off with severe weather. A lot of numbers are back so if you are new, here is a rough refresher of what some of them mean from SEVERE WEATHER INDICES PAGE. Keep in mind you need multiple factors to be in place for anything to happen, so do NOT get worked up over one single value.

Parameter Review

Relation to Severe Potential	CAPE j/kg	Lifted Index (LI)	Storm Relative Helicity	BRN	EHI	CAP*	Shear 0-6km	K-Index	Total Totals (TT)
Low/Weak	< 1000	-1 to -3	150 - 300	< 10	> 1	2+	< 25 kts	15 to 25	40 to 52
Moderate/Favorable	1000-2500	-4 to -7	300 - 400	20 to 45	1 to 4	< 2	20 to 45 kts	26 to 39	53 to 56
High/Extreme	2500 +	-8 or less	400 +	10 to 20	4 +	0.5 to 2	35 to 100 kts	40 +	56 +

* CAP is always a very tricky parameter to mess with. If the cap is too weak, you get storm development easily and usually end up with a grungy mess or "crapvection" as I like to call it. If it is too strong, you don't get anything. So for higher end severe events you want a cap just strong enough to hold off development until maximum heating and instability is achieved.

Now that we are done with that review, here is the outlook for this week...

Monday

This is one of those days that could be big...if it wasn't for a cap. From the graphics below you can see that we are going to be highly unstable on Monday, but we are also going to have a CAP with a strength around 5.0...nothing is going to break that.

CAPE with SWEAT & 3km Helicity	OUN Forecast Sounding

Tuesday

Much the same story for Tuesday, but the dryline does shove east past the Metro area in the afternoon. CAP still forecast to be over 2 at this point, but instability will be high and can't rule out the chance for an isolated severe storm.



Weds/Thurs in the next post...

[venture]

Wednesday

SPC already has this outlooked for much of Oklahoma at 15% severe, or or typically a Slight Risk. Dryline will be setup back to the west and approach the area just west of US 81 by early evening. Instability is expected to be quite high around 3000-3500 j/kg from Central OK back to the North. Right now GFS, which goes out to this time frame, doesn't have a ton of precip developing due to a cap (3.4) still being in place. Forcing should be a bit better with a sharper dryline though.

Main risks will be all modes. Winds will be backed over Central OK during the late afternoon and evening which enhances the potential for rotating storms. Some of the parameters a pretty impressive for what we've seen so far. LI near -7, SWEAT 528, SRH around 267, CAPE 3518, EHI at 5.6, 0-1km Shear at 21 kts (this is at the lowest level), and SRH at 204.

So what do we have working against storm development? CIN at -70 at 7PM isn't really great for storm development, but this could all change depending on what happens that day. Tornado risk also somewhat mitigated by higher LCLs (lowest cooling level) around 859 mb - or about 2500 meters off the surface.



Thursday

SPC already has Eastern OK outlined in a 30% risk area, 15% for Central. Timing wise still in question a bit on how fast the dryline/front will push through. Right now GFS has it coming through the Metro by late afternoon. Won't dive into this along since we still have the other days to get through. Instability will be lower, by overall dynamics appear to be more favorable for severe storms with the main system pushing through. Right now I would expect storms to quickly fire along the boundary and race east - like we normally see this time of year. We'll just have to wait and see as we get closer.

[venture]

Forecast update will be posted tonight on the 00Z models. NAM is in...GFS is in through Tuesday at this point. The quick peak at these...Monday looks pretty well capped. Tuesday...NAM keeps cap, GFS initiates storms. More details in a few.

[venture]

Potential exists for a multi-day severe weather week with at least one day possibly being significant. Stay informed, don't panic or lose sleep, and just have your plan in place. Make sure your plan includes where to go should it be required for your family, including the furry four-legged members, to seek shelter. Also, something I do as well, I have a backup external drive for my PCs that I put in a "go bag" just incase we have to move. With so much of our lives on our computers now, it never hurts to have back ups.

One last comment before I really get into the forecast. Please...everyone...remain respectful. This section of the forums attracts both weather enthusiasts and also those who would prefer to have Sunny and 70 year round. Some people get really excited for extreme weather, others get terrified. Respect both positions and keep the snark and otherwise rude comments to a minimum. Also if you are suffering from severe anxiety when handling severe weather - stop reading. Just have your plan in place and be ready should you need to. Don't torture yourself. Alright, moving on to the weather...

Sunday / Near Term

Area of showers and thunderstorms moving up from North Texas. Right now it looks like most will be in Southern OK, but could see a few move up into the Metro area. No big deal, just some decent rain for a few folks - hopefully for those of us that have grass seed down.

Severe Risk - Negligible

Monday

Dryline starts to sharpen up over Western OK and will slowly move eastward a bit. NAM and GFS are pretty much in agreement (maybe 20 miles off) on the placement of the dryline by 7PM on Monday - roughly around US 183. Ahead of the dryline over the Central 1/3rd of OK instability will be very high to extreme. Wind profiles, instability, and the various severe indices on the forecast soundings would all point to severe weather and some on the higher end of the scale. The big fly in the ointment on this setup for Monday is the strong cape. If you followed the chart I posed earlier, cap strength is around 3.3 to 3.7 depending on the model you look at. This is going to be tough to break without any significant lift or trigger in the atmosphere. If a storm does happen to pop, and bust the cap, it'll be severe with giant hail. Is there a chance? GFS would say no. NAM...might be more flexible as it reduces CIN (convective inhibition) quite a bit just along the dryline. My expectation is we'll see a warm, mugger day with cumulous start to bubble along and ahead of the dryline. We might see one or two elevated showers try to go up and then get killed off by the cap. I can't say the severe risk is zero though, JUST in case one makes it through.

Severe Risk - Conditional between I-35 and US 183. Main risk, if storms fire, very large hail.

Tuesday

Tuesday's setup is starting to evolve as GFS has now started to show storms firing along/near/just southeast of I-44. Instability will again be very high to extreme ahead of the dryline. The key to this day producing is 1) will the cap break and 2) will storms fire early enough before the cap begins to restrengthen going into the evening hours. Of course we also need to watch for the actual placement of the dryline as well. Right now it has it right on top of OKC by 4PM and stalls it there before retreating west. Based on forecast soundings, main threats appear to be large hail and damaging winds with some pretty high LCLs (lifted condensation level, where the base of the clouds typically are...when a parcel reaches its saturation point - http://en.wikipedia.org/wiki/Lifted_condensation_level) that will keep the tornado threat down. As I'm typing this the SPC outlook hasn't gone out yet, but I expect a Marginal day at least.

Severe Risk - Conditional to Marginal along and east of I-44.

Weds & Thurs coming up...

[venture]

Wednesday Discussion

Depending on what happens Tuesday, we could have some early morning storms in Central OK on Wednesday. Nothing major, just enjoy to make the forecast even more complicated. We'll get more of an idea of how things could evolve when the NAM is available tomorrow morning for this time period, but for now we are relying on the GFS. At this time it has the dryline parked in far Western OK by 1PM. Below is the imagine showing surface-based CAPE and CIN.



You can see how CIN is eroded ahead/along of most of the dryline will up into Kansas and all the way south to TX. CAPE values at or above 3000 j/kg all over this area. By 4PM CIN erodes even further to the east, but starts to build back around sundown (as usual). Storm initiation around 2-4PM over the NW quarter of OK...



GFS doesn't build storms into the rest of the state until after dark, but I'm not completely on board with that - so we'll see what NAM the next couple of GFS runs show. Supercell Composite Index is pretty high for what we've seen so far this year (and really all of last year)...



I would expect Wednesday to at least be an Enhanced Risk day, but I would not be shocked to see a Moderate Risk over South Central Kansas through North Central and parts of Central OK.

[venture]

Thursday Outlook

More questions, at least in my mind, on how Thursday will evolve. A lot will determine on how Wednesday turns out. Dryline movement appears to be pretty quick. It'll be in the Metro in the late morning and pushing through the Eastern 1/3rd of the state by evening. We could have on going storms through the entire morning on thursday, so that will impact how things play out. Right now, most likely area for severe weather would be in SE OK, or areas further east outside of Oklahoma.

[John1744]

The NWS really seems to be for lack of a better word kind of hyping up this weeks storms. Way more warnings about staying observant and keeping safe than I've seen from them in the past.

[venture]

The NWS really seems to be for lack of a better word kind of hyping up this weeks storms. Way more warnings about staying observant and keeping safe than I've seen from them in the past.

By all accounts, this is shaping up to be a high level event and I would not be shocked if somewhere in the Plains gets the first High Risk of the year.

[LocoAko]

The NWS really seems to be for lack of a better word kind of hyping up this weeks storms. Way more warnings about staying observant and keeping safe than I've seen from them in the past.

Where are you seeing this? I barely see any mention of severe weather on Norman's NWS page, forecast discussion, Facebook, or Twitter. And I agree with venture that this could be fairly high end... where perhaps alerting the public days in advance to keep an eye on this week's weather wouldn't be uncalled for.

[John1744]

Where are you seeing this? I barely see any mention of severe weather on Norman's NWS page, forecast discussion, Facebook, or Twitter. And I agree with venture that this could be fairly high end... where perhaps alerting the public days in advance to keep an eye on this week's weather wouldn't be uncalled for.

Hmm maybe not directly from NWS but the individuals on Twitter/Facebook that work at NWS themselves!

[ultimatesooner]

I'm headed to Wichita in the morning to work all week and I haven't been up there since the late 1990s - anyone with experience up there know the best local radio/television resource for weather coverage?

[ou48A]

I'm headed to Wichita in the morning to work all week and I haven't been up there since the late 1990s - anyone with experience up there know the best local radio/television resource for weather coverage?

A few years ago KAKE 10 ABC was probably the best in Wichita KS.
They will give you good warnings.... but beyond that they are not anywhere near as good as OKC.
Asked to see your hotels storm shelter. If you don’t feel comfortable with it considered switching hotels.
Good luck.

PS:KFDI - FM 101.3 Might be your best radio bet…. But I’m not sure anymore?

[venture]

00Z Model Discussion includes the continuation of the headlines we've had now regarding severe weather this week. There will be severe weather this week that may impact some population centers, so it will be important that people revisit their severe weather precautions involving both large hail and tornadoes. With that all said, I want to remind people that high end events like the one expected for this week any parameter that doesn't line up perfectly can throw the whole thing off. Whether it is the cap or moisture quality - any little thing can throw things off. Some things we need to watch for...

1) Cap. This is going to be a major player in keeping Monday and Tuesday under control, but also Wednesday could be impacted. Too weak of a cap can allow things to go early and we get a sloppy setup. Too strong and it limits things completely.

2) Upper energy speed. The key for high end events is having everything in place, that includes upper air support. The main storm system won't be on shore until late tomorrow, models won't really have a great hold on it until Tuesday morning. There is always a chance that when it is fully sampled that everything could get slowed down which throws most forecasts off.

3) Moisture return. High end events you need deep low level moisture. We already have dewpoints in the upper 60s and low 70s throughout South Texas and across most of the Gulf Coast. If that gets transported up as expected, it will go a long way in securing storm development.

With all that said, please keep in mind no one is rooting for a storm to hit your house. Any phrasing used when saying "great for storms" or something similar, is referring to the forecast itself - not that we want it happen over your head. So don't take offense.

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MondayMonday's severe weather outlook for Oklahoma is pretty quiet. Dryline moving into place with dewpoints well into the mid 60s ahead of oit. Instability will be very high to extreme across the area ahead of the dryline, but a stout cap should keep storms from forming. However, the chance is not zero and if one does make a run at busting the cap and succeeds then it will have a lot of instability to play with. Very large hail the main threat, but tornado threat is not zero with any storm that can go.

Images below - top row left to right: Forecast Sounding around Clinton, Moisture Convergence, CAPE and CIN forecast; bottom row: Supercell Composite, Sig Tornado Index, and Dewpoint/Lifted Index.



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TuesdayTuesday's severe weather outlook for Oklahoma shows the chance for storms a bit higher than Monday. Both NAM and GFS weaken the amount of CIN in the atmosphere at peak heating, but still maintain a strong cap. GFS initiate storms along the dryline however by late afternoon from North Central to Central to Southwest OK along every storm's favorite interstate (I-44). Should storms develop, very large hail is the main threat with a tornado threat that would increase with any storms that survive into the evening as the LLJ kicks up. However, the threat isn't all that great - as it appears now - and the dryline will start retreating at that time which can usually remove needed support for the storms to survive.



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WednesdayWednesday remains the most significant day this week for Oklahoma, as it appears now, for severe weather. Images below are for both the NAM and GFS to compare the two. Both are a little iffy on convection firing still, but the overall setup is still one that will likely produce and warrants very close watch. Timing on Wednesday is still in question for the upper storm system and whether or not it will be slower than currently projected. If it comes in slower, it throws this all off. So if we take it all at face value for now we see an environment that will be extreme unstable with CAPE values north of 4000 j/kg. We are going to have a very sharp dryline with dewpoints in the mid 60s ahead of it and into the 20s behind it in Western Oklahoma. CIN is projected to be removed by mid to late afternoon ahead of the dryline, so this will be the window for initiation.


Cap strength appears to be moderate right now, so this could easily hold convection back until peak heating kicks in when all the ingredients are perfect for the storms. Storms that form will have the risk of very large hail initially and reasonable LCLs and winds will allow for a tornado threat to increase as storms mature and the LLJ kicks in. As you can see by the forecast sounding and the Sig TOR numbers, they will be extreme for much of the time period when storms will be most active. I would like to point out that after sunset, with a strong LLJ, models always tend to go a bit crazy on the Sig TOR numbers - so keep that in mind.


One feature I pointed out on the model, and really no way for the NAM to know this ahead of time (in most cases) is the appearance of a slight bulge to the dryline on I-40 around the Clinton/Weatherford areas. This is something we need to keep an eye on because it could highlight a location where storms could pop first.


Main thing for Wednesday is don't panic or be afraid. Just get informed and have a plan. Also make sure you have at least 3 methods of getting weather warnings. Keep in mind that even with the most significant tornadoes in history that we've had here in Central OK, the overwhelming majority of people escape without injury. Just be smart and aware and you'll be okay.

[venture]

Run down of the SPC outlooks...



I expect Tuesday to go to at least marginal if not slight since some of the models actually produce precip tomorrow along the dryline versus today.

[Anonymous.]

Looks like the systems are really lining up out there, going to be a busy next few weeks. Hopefully all of OK gets significant moisture. Western OK needs a few good soakings to give them a fighting chance this summer.

[venture]

Here are some quick snapshots of the Euro 00Z run from last night. It has precip along the dryline all 3 days with very high to extreme instability.

[bchris02]

So is this looking like a potential PDS watch kind of day? So far the TV meteorologists haven't jumped on the hype bandwagon.

Is this type of setup normal for early April? It seems to me that the triple-point setup with the dryline and stalled out cold front that takes a few days to push through is more typical of mid-late May.

[venture]

So is this looking like a potential PDS watch kind of day? So far the TV meteorologists haven't jumped on the hype bandwagon.

Is this type of setup normal for early April? It seems to me that the triple-point setup with the dryline and stalled out cold front that takes a few days to push through is more typical of mid-late May.

Coverage is the main mitigating factor, more then likely, with the local media not going totally overboard yet. Though they also may finally be learning that inciting panic isn't the best thing. It's hard to say if any watches would be PDS at this point - all comes down to coverage and such. Ingredients will be in place though for a tornado to be strong (EF2+).

We'll have to see how the hi-res models handle things when we start getting into tomorrow and Wednesday.

For Today, the latest HRRR is now popping the dryline north of Lawton with an isolated supercell (and other convection) around 8PM. Movement would be ENE and enter the Metro around 10PM if things would hold together.

[Bobby821]

Hmm The SPC just issued the day 2 outlook but it just has a sliver of Northern Oklahoma in the Marginal risk area this differs quite a bit from what NWS Norman has posted for us tomorrow they have a good chunk of central Oklahoma in Marginal risk. I wonder why there is so much of a difference in thinking between the 2 is for?

[jn1780]

Hmm The SPC just issued the day 2 outlook but it just has a sliver of Northern Oklahoma in the Marginal risk area this differs quite a bit from what NWS Norman has posted for us tomorrow they have a good chunk of central Oklahoma in Marginal risk. I wonder why there is so much of a difference in thinking between the 2 is for?

The NWS Norman graphic was posted before the latest two day outlook came out. I'm sure there is a SPC team that has a meeting to discuss what the outlook is going to say and then they release it. The local forecasting team then follows suit.

[zachj7]

Wednesday looks pretty crazy. I wonder how far south storms will fire on the dryline. The recent NAM leaves some question on whether the cap will break. My initial chase target is Northern OK around 35 to near Wichita at the triple point. I am sure that will change. I suspect storms will probably fire off however down the dryline, just how far south? We will see... I expect a moderate risk at least by tomorrow.

[Anonymous.]

Dryline on the move today. Some weak CU field trying to backbuild westward around I-44 towards Lawton. Cap will probably win today, though.

[venture]

Hmm The SPC just issued the day 2 outlook but it just has a sliver of Northern Oklahoma in the Marginal risk area this differs quite a bit from what NWS Norman has posted for us tomorrow they have a good chunk of central Oklahoma in Marginal risk. I wonder why there is so much of a difference in thinking between the 2 is for?

SPC seems to be a bit off with Tuesday overall. Models have been showing Tuesday as the more likely day (compared to today) to break the cap...yet today they actually had a risk area outlined.

Wednesday looks pretty crazy. I wonder how far south storms will fire on the dryline. The recent NAM leaves some question on whether the cap will break. My initial chase target is Northern OK around 35 to near Wichita at the triple point. I am sure that will change. I suspect storms will probably fire off however down the dryline, just how far south? We will see... I expect a moderate risk at least by tomorrow.

NAM has issues with convective precip, which is why it has been pretty blank when you see other models show precip forming. If I were chasing, I would play the dryline. Triple Point is easy pickings up north, and it will show with chaser convergence. Down here it'll probably be more ideal isolated photogenic stuff.

Dryline on the move today. Some weak CU field trying to backbuild westward around I-44 towards Lawton. Cap will probably win today, though.

HRRR really wants to break it, but seeing the high level clouds over that area just makes me thing it is locked in to not firing today. We'll see...only takes one.

[venture]

The short range models really want to pop SW OK.

RAP



HRRR