Severe Weather Discussion for April 6 – 12, 2015.

Severe Weather Discussion for April 6 – 12, 2015.
Special Weather Discussion for April 8 – 9, 2015.
Issued:  04/05/2015.
Highlights:
–  Active severe weather days are likely on Wednesday and Thursday with the potential for significant severe weather.  Finer details are still lacking (i.e. morning convection, Wednesday’s storms, dryline position, etc), however the potential is there per model progs.  This is a good time to review severe weather safety plans and clean out storm shelters, which honestly should be done by now anyway.
–  Slight chance of severe thunderstorms late Tuesday.  Unknown if storms will develop, but if they do severe weather will occur.
–  Showers and thunderstorms are possible on Sunday.
–  Temperatures will be in the low 80s Monday, mid 80s Tuesday – Thursday, upper 60s Friday, mid 70s Saturday and upper 60s Sunday.
Probabilities:
–  Severe Thunderstorms:  > 75%
–  Storm Chase:  > 75%
Risk area for any part of Oklahoma (valid:  04/06 Day 3):
Slight:  100%
Enhanced:  100%
Moderate:  35%
High:  N/A
Discussion:
Models indicate that a medium wavelength trof/closed low will translate into the western U.S. Tuesday and move eastward towards the region.  Maritime tropical air will move northward ahead of this system as a strong cap develops across Oklahoma.  These ingredients will set the stage for one or more episodes of severe weather across Oklahoma.  I have opted to combine the weekly discussion with a special weather discussion re Wednesday – Thursday.
Evening surface analysis shows a warm front across southern Texas characterized by low 60s dewpoints to the north and upper 60s to the south.  Models prog this boundary to move rapidly north tonight and be located in northern Oklahoma on Monday.  Dewpoints will increase across the state on Monday with mid/upper 60s across the state late Monday.  Winds aloft will be marginal although CAPE will be near 4000 J/Kg.  Models show weak shortwave ridging across Oklahoma and at this point, believe storms will not form.  This is supported by the 18z 4km NAM.
Upper system moves onshore Tuesday as moisture continues to increase across the state.  A weak cold front will nudge into northern Oklahoma, potentially reaching HWY 51.  4km NAM shows extreme surface CAPE by late afternoon in the 5000 J/Kg range with low 70s dewpoints.  Shortwave ridging may relax enough for storms to develop and NAM shows some along the cold front.  If storms form, they will be severe given favorable shear.
Models had been in good agreement for Wednesday/Thursday time period and this is the greatest potential for high-end severe weather.  12z GFS/NAM trended much faster with the upper system, although the 00z/12z EC are consistent and slower.  Ensemble members are scattered, so it appears there is great uncertainty re Wednesday/Thursday.  The EC does have a bias of moving systems too slow over the western U.S. but not sure if that issue is at play for this setup.  The EC appears to briefly close the low off near the California coast and the up stream kicker sends it east on Tuesday.  The GFS/NAM do not slow the system down and keep it a trof.  This system may not be well sampled by the upper air network until Monday morning, so I have little hope the 00z runs will clear this matter up.  For now, I am sticking with the ECMWF (EC) as it has been consistent.  It is also the most prudent move, since I’ll keep severe chance going for central Oklahoma on Thursday.
Wednesday – all models agree that the dryline will move into far western Oklahoma by late afternoon, characterized by dewpoints in the upper 60s to the east of this feature.  A strong cap will also be in-place but is expected to be breached given ample daytime heating and potentially a while timed shortwave trof.  Surface CAPE will be in the 4000 J/Kg range with adequate shear and impressive mid-level winds.  The EC has weaker flow at 200mb, but not sure if this is a model bias.  Deep layer moisture will also be in-place with 850mb dewponts near 15c and PW near 1.5.  NAM erodes all CIN Wednesday afternoon, which is likely not correct.  PRIND – severe thunderstorms will develop Wednesday afternoon in far western Oklahoma and move east.  Supercells will be favored, resulting in potential for tornadoes, softball hail and winds to 80mph.  Potential for longlived supercells is there, especially if the GFS/NAM are correct.  It is also possible that weakening upper support and increased capping due to night time will reduce storm intensity before reaching central sections.  At this point, tough to see if central Oklahoma will get storms on Wednesday, but I do think it will be after 8pm if it happens.
Thursday – following the EC, upper system will eject east-northeast as a negative tilt.  A dryline will surge through western Oklahoma, reaching HWY 81 by early afternoon.  Winds will increase across the area with impressive 700mb/500mb flow.  Storm development by early afternoon appears likely and CAPE values will support severe thunderstorms.  Again, supercells will be favored given winds and instability.  If the GFS/NAM are correct, the dryline will surge well into eastern Oklahoma by early afternoon and the threat will be there on Thursday.  Plenty of time to trend the models.  Also, early April does not favor multiple rounds of severe weather, so we’ll see.  I do think we may end up with western Oklahoma getting hit Wednesday, skipping central and eastern getting hit Thursday.
Previously mentioned up stream kicker moves into the western U.S. on Saturday and shifts east into the area on Sunday.  EC has it in Texas, which should be showers/thunderstorms for Oklahoma.  I have not spent much time on this part of the forecast.
First guess at numbers needed refinement based on model progs.  I sided with the warmer EC Tuesday – Thursday than going NWS numbers.  Next weekend is iffy given the bust today.  I’d think we’ll be cooler than models prog, due to rain, but went with upper 60s anyway.
Verification:
OKC Temps for 03/30 – 04/05
My Numbers:  mid 70s, low 80s, low 80s, low 80s, mid 60s (midnight high mid 70s), upper 60s and low 70s.
NWS Numbers:  low 70s, upper 70s, upper 70s, low 80s, upper 60s, low 60s and upper 60s.
Actual:  78F, 86F, 82F, 84F, 73F, 66F and 54F.
Chase Status:
Level 4 – Monitoring Wednesday and Thursday.

 

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