Friday, March 4, 2016

Severe Weather Discussion for March 7th, Posted March 4th

This is a discussion for the potential severe weather event on March 7th, 2016 in Oklahoma.

Models Used: 18z GFS, 12z ECMWF, 15z SREF

    Energy currently elongated near the Gulf of Alaska, beginning to round the base of a strong upper level low in the Gulf, will continue to be pushed east as the strongest portion of the upper level low will retreat north and west over the Aleutian Islands by Sunday. System of interest will make landfall in California around 15z Sunday, digging as it does so to form a ridge across the Northern Plains. Small piece of energy will be sheared off into the Aleutian Islands upper level low as this landfalling occurs, and this will weaken the primary energy enough until it begins to re-organize while progressing into the Four Corners region. By 00z Tuesday, this strong shortwave will be located over western Kansas, with a much stronger upper level trough digging well south into Baja California, and eventually into Mexico. This stronger upper level feature will force a slight ridge over the Four Corners region, and also act to pump a large ridge over the Midwest and Eastern U.S., in response to the general long wave trough pattern in the Western U.S. into the Plains. As the shortwave rides the western fringe of this ridge, weakening will occur before the energy is ingested by another vorticity maxima riding the Canada / United States border.

    Potential severe weather event will become set up with the strong shortwave moving into western Kansas, attaining what will pass as a negative tilt in the process. 500-millibar jet streak AOA 60 knots will form by 18z 3/07 over the Texas and Oklahoma Panhandles into western Kansas as the shortwave moves into the Plains. Jet streak will move east by 00z, with speeds weakening slightly to around 55 knots out of the southwest over much of Kansas and Oklahoma. Best wind features on the 700-millibar level at 00z 3/08 will be in the terms of a jet streak positioned in central / eastern Kansas, Oklahoma (outside of the panhandle and southwest portion of the state), northwestern Arkansas and western Missouri, with speeds up to 55 knots out of the west-southwest. 850-millibar wind field finds southerly winds across the Plains, with speeds of 40 to 50 knots in the Oklahoma area. Also will make note of sustained lower-level warm air advection, most intense from about 00z 3/07 until the time of the severe weather event. This advection will see 850-millibar dewpoint temperatures rise to the 10-12 degree Celsius mark, enabling moistening of the PBL. Most intriguing feature is 925-millibar wind direction at 00z 3/08 out of the south-southeast at speeds AOA 40 knots for much of Oklahoma, and slightly higher into Kansas. This veering wind profile is confirmed when observing projected surface winds at the same time to be almost out of the southeast, if not south-southeasterly.
    Surface low with strength of approximately 992 millibars in south-central Nebraska by 00z 3/08 will enhance moist airmass fetch from the Gulf of Mexico, tracing surface and 850-millibar wind fields back to the Galveston, Texas to Mobile, Louisiana region. Surface dewpoint projection from the 18z GFS show values nearing 60 degrees Fahrenheit by 12z Tuesday, although 00z Tuesday values see a lower, but still impressive swath of dewpoint numbers AOA 55 degrees.

    Storm Prediction Center currently outlooks western Oklahoma into central Kansas and eastern Nebraska, as well as much of central Texas for a 15% chance of severe weather on the long range Day 4-8 outlook for this event. Latest SPC SREF run shows a maxima of over 50% probability of supercell composite values AOA 1.0 occurring simultaneously with over 0.01” of precipitation falling - basically the probability of strong thunderstorms occurring - along the Red River south into the region immediately northeast of Abilene, Texas at 03z, with values gradually increasing and pushing almost due north towards Woodward, Oklahoma by 06z 3/08. The presence of this 50% to 70% maxima advecting northward during the night conveys uncertainty exhibited by the SPC SREF members, but also the confirmation that strong thunderstorms are possible in this environment. Projected 0-6 kilometer shear will increase past 00z Tuesday, as the nocturnal lower-level jet kicks in and the aforementioned veering wind profile continues to take shape across Oklahoma.
    GFS suite has taken a more aggressive tone in this event since a particularly bullish 00z run on 3/04, which triggered a potentially tornadic environment signal for a good portion of Oklahoma for this Monday night event. Analysis of forecasted soundings using the 18z GFS continue to indicate a potentially tornadic environment near Lawton, Oklahoma with SBCAPE exceeding 1800 j/kg amidst close to 60 j/kg CIN. Surface to 6km shear forecasted at 50 knots, combined with 0-3km storm-relative helicity values near 300 m2/s2 only acts to confirm this strong thunderstorm environment for the southwest Oklahoma area, the same region highlighted by the latest run of the SPC SREF suite for potential strong thunderstorms. Examination of forecasted soundings over Norman shows lower instability on the order of 800 to 900 j/kg SBCAPE, although values slightly higher for MLCAPE on the order of AOB 1000 j/kg. Storm-relative helicity and shear values generally consistent with Lawton numbers, although weaker instability and still-present PBL temperature inversion may pose problems for convection in central Oklahoma. Despite this, veering wind profile affirms potential for strong thunderstorms in the general state of Oklahoma, particularly if the inversion can be broken. Environment in Norman becomes far more favorable for convection by 06z 3/08, with SBCAPE and MLCAPE both taking a significant jump to AOA 1200 j/kg, although the presence of a now-weakened surface temperature inversion on the magnitude of 10-20 j/kg CIN still presents some concern. Additionally, substantial lowering of bulk shear and storm-relative helicity values, despite maintaining a veering wind profile on this forecasted sounding, highlights how factors may line up for strong to severe thunderstorms, but not necessarily an explosive severe weather event according to current projections.
    In terms of tornado potential- lowering LCLs as convection approaches, combined with sufficient instability and SRH / shear lead me to believe this event will pose the first notable threat for rotating thunderstorms this spring season. Not incredibly impressed with the potential for a large-scale - nor a large-number - tornado event, but the environment should be supportive of potentially tornadic thunderstorms. This will be further addressed as the timeframe of this event approaches.

    Overall, this event is showing signs of supporting strong to severe thunderstorms over Oklahoma. Highest concern from most recent model runs rests with western Oklahoma, particularly along a corridor of Eldorado to Comanche, to Buffalo to Medford. It is this corridor that should see the best forcing for thunderstorms, and will likely have the best threat for tornadic thunderstorms. Further east, in the Oklahoma City / Norman region, severe thunderstorms will still be possible, albeit higher uncertainty is present with potential capping and forcing concerns. Will re-evaluate this portion of the state in the next discussion to try and clear up this uncertainty, should successive model runs follow suit.