Hook-Echo Storm Chasers

Issued:  04/12/2012.

-  Severe thunderstorms remain possible this afternoon in western Oklahoma, with large hail and a few tornadoes the primary threats.

-  On Friday, severe thunderstorms are likely in western Oklahoma with the primary threats tornadoes and destructive hail.

-  An active severe weather day is still possible on Saturday across much of Oklahoma.  A significant severe weather event may be in the offering if parameters come together.

-  Severe thunderstorms and excessive rainfall are expected on Sunday, primary threats are large hail and damaging winds.

Little to no change from prior forecasts, upper system is evolving as models have progged for several days now.  There are finer details that must be reviewed and considered for the overall forecast and impacts on the same.  Forecast rules still dictate not using enhanced wording this far out for Saturday even though SPC has maxed out the probabilities for Saturday (Day 3).  We’ll see what happens on the Day 2 tomorrow.

SPC has an MD out for parts of western OK and the Day 1 has this well covered, no need to revisit it here.

A slight risk is posted tomorrow for western and parts of central Oklahoma.  The severe weather impacts are greater but storm coverage is in-doubt as models are lower on QPF than prior progs.  However, SPC may go to a moderate risk if it looks like storm coverage will meet the threshold.  18z NAM has lowered CAPE values a little but this is really from 4000 J/Kg to 3000 J/Kg which is probably much more realistic for mid-April.  Either way, favorable instability with expected speed/directional shear will be more than sufficient for supercells.  Any sustained supercell will be capable of destructive hail and a few tornadoes, especially as they move east of HWY 283 and LCL heights lower.  NAM develops some QPF, mainly near I-40 late Friday and this is in keeping with the expected evolution of this event.

The storm coverage on Friday will have major impacts on what happens Saturday and if the full extent of the event can be realized.  While May 24th last year was a significant day and well-known days in advance, that is late May and much greater heating.  This allows the atmosphere to readily reset after prior convection.  So, the main thing to remember, with all the hype about Saturday, is that lots of convection overnight and into Saturday will possibly modulate the event downward.  For now, the continued thinking for me is to forecast the potential and acknowledge the prior caveat.  18z NAM has CAPE values @3000J/Kg Saturday afternoon in the threat area with dewpoints in the mid 60s.  CIN is low and LCL values are near 1000 meters, the latter favorable for tornadoes.  LI values are -8 to -9 across central Oklahoma.  A streak of 850mb dewpoints greater than 15C nose into central Oklahoma by 4pm, which is flat ugly…for lack of better words.  No need to discuss winds, they are beyond favorable.  18z NAM is awfully quiet at 7pm Saturday but it did that for tomorrow and has now trended to convection.  I’d expect to see convection given impressive winds and incoming upper system.  By 10pm, NAM gets one little supercell going near Lawton.  Once again, not unusual for this model and I’d expect that to change.  Concern is that CAPE values remain high even at 10pm, indicating the storms may go well into the night time hours.  This would indicate potential for night time tornadoes and associated hazards.

Still not a clear picture on Sunday and the models aren’t in the best of agreement.  However, there is potential for low-CAPE, high shear thunderstorms, which translates to a severe weather threat.  For now, we’ll have to monitor but it is there.

Risk area for any part of OK/TX (valid:  04/13 Day 1, Day 2, Day 3):

Slight:  100%, 100%, 100%

Moderate:  50%, 100%, 10%

High:  0%, 20%, N/A

Level 2 – looking to chase on Friday, target area is Hobart.

Severe Weather Discussion for April 13-14, 2012.

Issued:  04/10/2012.

Highlights:

-  A few severe thunderstorms are possible today and Wednesday in the TX/OK panhandles.  Primary threat is large hail.

-  Severe thunderstorms are possible on Thursday in the TX/OK panhandle and potentially far western Oklahoma.  Primary threat is damaging hail.

-  Severe thunderstorms remain possible Friday – Sunday across parts of Oklahoma, but the overall forecast is very difficult.

Discussion:

Models continue prior trends of slowing the upper system.  This slow down does several things but the least of which, pushes higher end severe threats beyond Thursday.  As such, today-Thursday aren’t a big focus of this discussion, although I’ll touch on Thursday a little.

SPC has today and Wednesday well handled with going products and see no reason to add to what they have issued.  A watch is possible in the TX/OK panhandle if convection forms.  Similar setup for Wednesday as the ridge will be overhead.  18z NAM indicates the dryline will be in the western Texas panhandle on Thursday with convection near Amarillo late Thursday.  This seems plausible given model slowing trend of the upper system.  Primary threat appears to be damaging hail but given deep layer speed/direction shear, a tornado or two certainly seems possible.  SPC has a slight risk posted and I think they’ll keep that for now, but we’ll see.

Leading edge of the upper system moves into the central U.S. as the center approaches the coast.  Friday continues to look interesting as wind speeds aloft will be good and strong directional shear will be in-place across the region.  12z/18z NAM continues to develop convection along I-35 and I-44 during the day and still not sure why as this is well removed from the dryline, which should be over the eastern Texas panhandle.  Despite the convection over central OK, decent CAPE does develop with 2000-2500 J/Kg in a small zone near the dryline.  The limited areal coverage may be due to prior convection near CDS-LTS in the morning.  There aren’t obvious forcing mechanisms, so who knows as to what is really happening.  The 12z GFS is similar with rainfall in the previously mentioned area but is also farther east with the dryline.  Models did over-convect last night and much of the convection appears to occur in areas that the models remove CIN.  This far out, really tough to call, so will monitor model trends.

Saturday and Sunday still have severe weather potential but luckily are outside the scope of this forecast.

Probabilities:

Risk area for any part of OK/TX (valid:  04/11 Day 3):

-  Slight:  100%

-  Moderate:  25%

-  High:  N/A

Chase Status:

Level 4 – watching Friday

Severe Weather Discussion for April 12 – 14, 2012.

Issued:  04/09/2012.

Highlights:

-  A few severe thunderstorms are possible in the Oklahoma panhandle and western Oklahoma on Tuesday and again on Wednesday.  Primary threats are large hail and damaging winds.

-  Severe thunderstorms are possible Thursday-Saturday.  At least one medium-end event appears likely within this time period, but still a little far out to know which day(s) present said threat.

Discussion:

Complex weather setup for later in the week as a longwave trof develops across the western U.S.  Model outputs are in decent agreement but there are important differences.  The focus of this forecast is really on Thursday, since that is within the short range models (NAM).  On Thursday, a shortwave trof ejects northeast into Colorado with strengthening flow aloft across the threat area.  Models agree well here and even the overall setup.  There is disagreement on the dewpoints that will be in-place across the region.  NAM has widespread upper 50s while the GFS has low 60s and some mid 60s across the region.  CAPE values for both models are in the 2000 J/kg range but the upper 50s dewpoints results LCL heights much greater on the NAM while the GFS would be more favorable for tornadoes.  This is a situational awareness issue that won’t be resolved until Thursday.  The NAM has previously handled dewpoints issues decently but SREF 15z indicates 60F dewpoints are likely in the threat area with LCL values below 1500 meters.  Despite the ultimate moisture evolution of this event, sufficient deep layer moisture will be available for severe thunderstorms along  the drying in the Texas panhandle.  Storms will not move all that fast, so they’ll likely only get a little bit into Oklahoma before dissipating late Thursday.  GFS appears to be over-convecting into the night time hours and this would be supported by the current over convecting in Oklahoma.

On Friday, upper system continues to evolve across the western U.S. and flow across Oklahoma increases.  A dryline will remain near the TX/OK border.  Deep layer moisture return is expected to continue and despite CAPE values 2000-2500 J/Kg, this model does not develop convection until after dark.  At this time, there appears little reason for storms to not develop on Friday, but we’ll have to see.

ECWMF and GFS want to eject out a rather large portion of the western U.S. longwave trof on Saturday.  GFS is quite muddied and likely having extensive trouble with convection in the prior days and its own over convecting.  Either way, the available moisture and expected strong lift, will need to be monitored.

Probabilities:

Risk area for any part of OK/TX (valid:  04/10 Day 3):

-  Slight:   100%

-  Moderate:  20%

-  High:  N/A

Chase Status:

Severe Weather Discussion for April 9 – 15, 2012.

Issued:  04/08/2012.

Highlights:

-  Severe thunderstorms are likely in northern Oklahoma on Monday.  Destructive hail and a few tornadoes are possible.

-  A few severe thunderstorms are possible in western/central Oklahoma on Tuesday.

-  Active severe weather period is possible Thursday – Sunday with multiple episodes of severe weather.  At least one medium-end event may be in the offering this time period.

-  Temperatures will be in the upper 70s Monday, mid 70s Tuesday, low 70s Wednesday, upper 70s Thursday-Sunday.

Probabilities:

-  Severe Thunderstorms in Oklahoma:  > 75%

-  Storm Chase:  60%

Discussion:

Active mid-April pattern looks to be setting up for Oklahoma with multiple rounds of thunderstorms likely, along with attendant threat of severe thunderstorms.  SPC has risks areas posted for Monday, Tuesday, Thursday, and Saturday.  The latter two are in their Day 4-8 outlook, indicating a 30% area.  Monday’s risk also has a 30% areas and a 10% hatched area.

Monday will see the formation of a mid-level ridge to the west of Oklahoma, putting this area in northwest flow aloft.  Not always a wet flow this time of year, but certainly transitions to such by late May.  However, the unusual amount of Gulf moisture will make this setup more favorable than is typical in mid-April.  Dewpoints of at least the low 60s is likely to be located just south of a synoptic front over northern OK/southern KS.  The ultimate location of this front remains in doubt as models disagree.  CAPE values are progged to be in the 2500 J/Kg range.  Models show storms developing during the afternoon hours and progressing southeast.  Directional shear will be maximized due to northwest flow aloft and speed shear will be decent.  As such, supercells are the expected storm type.

Upper ridge continues just west of the region and dryline retreats to the western Texas panhandle.  Cold front looks to hang up in northern Oklahoma or maybe into central.  NAM develops QPF during the afternoon hours and CAPE values are in the 2500 J/Kg range.  Winds aloft are slightly weaker than Monday but directional shear will still be maximized under northwest flow aloft.  As such, a few severe thunderstorms seem possible on Tuesday.  SPC’s slight risk is confined to the panhandle and far western Oklahoma, so we’ll see how that changes in the next day or so.

Upper ridge then shifts eastward and severe threat decrease on Wednesday.  GFS still has some QPF over Oklahoma but not going to add POPs for now.  By Thursday, the upper ridge has shifted eastward and a western U.S. shortwave trof takes aim on the area.  GFS and ECWMF disagree on the evolution of this system with the EC farther south and stronger.  Despite which model verifies, deep layer moisture return will continue across the region and wind speeds aloft will strengthen.  GFS is lower on instability than would be expected on Thursday afternoon, so this will be monitored.  The dryline is epxected to be in the eastern TX/OK panhandle.

Upper system shifts northeast of the region on Friday but the western U.S. longwave trof remains in-place.  As such, moisture will remain across the region and another round of severe thunderstorms is possible on Friday.  Speed and directional shear both look favorable for supercells along with CAPE values.  Prior convection on Thursday will likely drive what happens on Friday.  There is no obvious upper support this far out, but the potential for something seems plausible.  GFS weakens CIN values and develops QPF Friday afternoon.

Powerful western U.S. trof looks to develop by Saturday and translate eastward on Sunday.  This system will tropical moisture ahead of it with a surface pattern that resembles the prior few days.  Given the presence of deep layer shear and favorable thermodynamics, another round of severe thunderstorms seems possible in western/central Oklahoma.  By Sunday, the trof ejects northeast and should drive the dryline well into Oklahoma by Monday.

The forecast this week is certainly complex and the convective setup will change day to day.  However, the ovverall synoptic setup favors multiple rounds of severe thunderstorms.  Chasing potential seems decent this week, just have to find the day(s) that look best.  Special discussions will be needed starting tomorrow, focusing on the latter half of the week.  Not ignoring tomorrow, as SPC has medium-end probabilities already posted.

Did not get fancy with temperatures, but in-general went warmer than the going rate much of the week.  Next Sunday could be too warm, but that will be driven by convective processes.

Severe Weather Discussion for March 16 – 22, 2012.

Issued:  03/15/2012.

Highlights:

-  A few severe thunderstorms are expected Friday afternoon and evening across western Oklahoma.  Primary threat is large hail.

-  Severe thunderstorms are possible Sunday afternoon and evening in western Oklahoma.

-  Severe thunderstorms are possible Monday afternoon and evening across much of Oklahoma.

-  Heavy rain threat develops Monday into Wednesday across central/eastern Oklahoma.

Discussion:

12z models continue prior trends of developing a medium-end event across Oklahoma by Sunday.  Opportunities for medium-end impacts will continue into early next week, but are currently outside the scope of this discussion.

Intense western U.S. longwave trof is expected to take shape by Sunday and combine with unseasonable moist conditions downstream to produce two severe weather episodes, followed by an excessive rainfall event.  There is a threat of severe thunderstorms on Friday, but this is expected to remain in the low-end of the scale.  SPC has it well covered in the Day 2 product and I really can’t add that much to what they have typed.  Saturday is also well covered in the Day 3 product and while moist conditions will remain east of the drylne, lack of defined shortwave trof is expected to keep thunderstorm coverage limited.  As always, can’t rule out one or two boomers.

Sunday appears to be a little more spicy across western parts of the state back west into the Texas panhandle.  12z EC/NAM/GFS all hint at strengthening wind fields across the threat area during the day.  Additionally, moisture will remain in-place with upper 50s to low 60s common in this area.  Big difference among the models is the location of southwest winds, which typically mark the dryline.  In this case, deep layer moisture is sitting well east of the southwest winds and this eastward location appears to be the convective initiation point.  The GFS is farther east with the conventional dryline while the NAM supports the prior discussion on separation between the deep moisture and southwest winds, on the order of 150 miles.  Wind fields aloft will be quite favorable by late afternoon for severe thunderstorms, with crossover maps showing appreciable difference in wind direction between the surface and 500mb.  GFS has low-balled CAPE values the past several days, so that part of the model is ignored for the somewhat more reliable NAM.  WRT to the NAM CAPE values, there is an area of 2000 J/Kg in the eastern Texas panhandle by 4pm.  Models seem to want to light off storms by early afternoon and this would seemingly occur given presence of a shortwave trof moving into the region.  Time of year would not necessarily favor such an early start but given several warm days and temperatures starting off in the 60s, shouldn’t take much to reach the low/mid 70s.  As such, Sunday has some chase potential and am currently watching Shamrock, Texas to Woodward as a chase area.

Beyond Sunday potential continues for medium-end impacts Monday-Wednesday.  On Monday the impacts appear to be related to severe thunderstorms with a transition to an excessive rainfall event late Monday through Wednesday.  This is currently outside the scope of this discussion, but the threat will be monitored over the coming days.  The GFS wants to slam eastern Oklahoma with 9-10 inches of rain during mentioned time frame, which would certainly have a flood risk associated with it.

Probabilities:

Risk area for any part of OK/TX (valid:  03/16 Day 3):

-  Slight:  100%
-  Moderate:  30%

-  High:  N/A

Chase Status:

Level 4 – Watching Sunday and Monday, both appear to provide decent chase potential.

Severe Weather Discussion for March 12 – 18, 2012.

Issued:  03/11/2012.

Highlights:

- A few thunderstorms are possible on Friday, severe chances appear low.

- Severe thunderstorms are possible Saturday and Sunday across western Oklahoma.  Confidence is low due to the D6/D7 potential but will handle with special discussions should a medium-end or greater threat develop.

-  Temperatures will be near 80F on Monday, low 80s Tuesday, near 80F Wednesday-Thursday, and upper 70s Friday-Sunday

Probabilities:

-  Severe Thunderstorms in Oklahoma:  40%

-  Storm Chase:  25%

-  Winter Precip:  < 5%

Discussion:

Southwest U.S. closed low currently ejecting northeast today and will clear the region tonight.  Upper system will weaken as it shifts northeast with little push on moisture to the south of Oklahoma.  As such, big warm-up is expected this week, lasting much of the weak.  Models do prog temperatures to cool a little as robust and deep layer moisture return commences by the weekend, with subsequent development of low clouds.

GFS and ECMWF are consistent and in decent agreement on a western U.S. longwave trof developing by Friday and shifting slowly eastward into the central U.S.  The timing is likely suspect as models tend to develop these systems a little too fast.  However, even with current timing, storm chances will increase by the weekend with Saturday and Sunday both looking favorable for thunderstorms.  Strong and veered flow aloft appears to indicate potential for organized storms and hence severe potential.  The 40% probability is for at least low-end severe chances, approaching medium-end.

GFS is preferred this week, odd to say the least but the ECMWF looks too deep.  Even if the ECMWF does verify, the overall impact on the Friday-Sunday forecast seems limited.  Currently, Friday has some potential for severe thunderstorms in northwest Oklahoma as deep moisture resides just east of a dryline in the Texas panhandle.  Mid/upper level flow looks weak but certainly enough for some shear.  Additionally, CAPE values are @1500J/Kg across parts of NW OK with the GFS showing near 2000 J/Kg.  Saturday looks to be a better setup as stronger flow aloft moves over the region and the dryline advances eastward.  Sunday has a similar setup and the flow strengthens even more.  Kansas may see the better activity as the strongest flow aloft will move over that region.  Given time of year, tough to say if the setup will truly develop into a medium-end event but the potential certainly exists.

Models lift the system northeast across the area on Tuesday which would seemingly continue severe chances into Monday and Tuesday.  These days are beyond this forecast period but at this juncture it appears storm season will kickoff on-time this year.

Didn’t get fancy with numbers, didn’t even look at MOS.  Temperatures will be very warm this week, due in-part to the impressive deep layer moisture return, which has already begun.  18z GFS shows low 50s surface dewpoints into central Canada by late week.