Wednesday, March 30, 2016

Severe Weather Discussion for March 30, Posted March 30

A severe weather event is forecasted to unfold across the Plains and Mississippi Valley today, March 30th.

           Large upper level low currently stationed across the Western US will provoke a potential severe weather event in the Plains and lower Mississippi Valley today. Situation is complicated as convection later in the day will be heavily influenced by morning convection, residual cloud cover, as well as dryline placement later in the day.

               Stout upper level low centered across Nevada and Utah will begin to flatten as energy currently located in the Dakotas is ushered northeastward in pace with the jet stream, which will be arching north of mid-level ridging currently stationed in the lower Mississippi Valley. This energy in the Dakotas will be drawn towards another system in south-central Canada, resulting in a band of elevated vorticity extending from Canada back to Nebraska at 12z today. Further, the vorticity max currently in southern California, extending in a semicircle offshore the West Coast into Oregon, will keep the upper level low positively tilted and aid in this flattening-out.
    Moisture flow will continue to amplify across the Southern Plains, with water vapor imagery showing the subtropical jet feeding a moisture plume into the region. As such, cloud development occurred across the area earlier Tuesday and continues to persist at the time of posting. To this extent, continued moistening of the planetary boundary layer through isentropic ascent, notable 850-millibar moisture plume advection, and substantial low-level warm air advection will result in areas of drizzle to showers overnight, particularly across Texas and Oklahoma. 3/30 00z KOUN sounding showed strong temperature inversion between 800-millibar and 700-millibar levels, and I suspect this inversion will be strong enough to suppress anything more than broad, generally-weak precipitation. Surface plots at 0700z show areas of mist and light rain across central and eastern Texas, in line with favored model guidance. Model guidance troubles will be discussed more in depth in the following sections. Uncertainty with this particular subject of morning precipitation will feed into the morning and afternoon discussions below.

Early Morning to Noontime…
         Convection-allowing model guidance has been promoting the formation of showers and thunderstorms along the Red River, most notably in southeast Oklahoma in the mid-morning hours today, following expected overnight precipitation and continued moistening of the lower levels. Not completely convinced we see hail threats as advertised by some, including the threat of up to tennis ball-sized hail as per NWS OUN in their hazardous weather outlook posted yesterday. However, as the atmosphere continues to become more conducive to severe weather through continued warm air and moisture advection overnight, will not rule out severe weather tomorrow morning, especially given expected values of elevated instability that may favor stronger storms. I do believe hail is a primary threat with these storms, especially in their elevated nature, but again, I am not completely sold on the idea of severe and/or significant hail.
         Primary point of this timeframe discussion is to what extent precipitation is ongoing by and after 12z. Model guidance has been hinting at potential convection across portions of Oklahoma, especially south and east in the morning hours, but has also left the door open for weaker, more broad precipitation in central and northern Oklahoma. Regardless, expect overcast cloud cover given continued lower-level warm air and moisture advection, as well as taking into account 3/30 00z soundings across Texas and the western Gulf Coast. I am not taking the HRRR model into account for this discussion, as although it is currently handling surface precipitation (or lack thereof) rather well, it brings surface temperatures across Oklahoma and Texas into the 80s and 90s, well above current and more-realistic temperature forecasts introduced by the NWS. Additionally, am not using the NCAR ensemble, as all members have weak, broad, and scattered precipitation at initialization (3/30 00z) across Texas, which was proven to not be the case when compared with 3/30 00z radar. In terms of morning precipitation, am currently favoring 00z 4km WRF-NMM model, as well as SPC SREF system, as both models handle initialization conditions at the surface with regards to precipitation coverage well, and perform well with expected high temperatures later in the day. Given high model uncertainty with this event, however, making these two forecasts ‘favored’ is not exactly as affirmative a word as it is made out to be.
          In all, expectation is for precipitation, possibly convective in nature, to be ongoing in portions of Texas and Oklahoma by daybreak today and in the morning hours in general. Most favored region for this activity, as promoted by model guidance and SPC probabilistic thunderstorm outlooks, is southeast Oklahoma into northeast Texas, east into Arkansas. Weaker and more broad precipitation also possible for central Oklahoma, where uncertainty is greatest.

Afternoon and evening…
        Remarkably complex situation will unfold for the afternoon and evening hours. It has been noted that the upper level low in question has been progressing east more slowly than anticipated yesterday, and this has been feeding into a further-west trend with the dry line positioning later on in the afternoon today. Case in point, 4.0km WRF-NMM guidance shows convective initiation in the afternoon hours along the dry line, which is projected to be planted almost immediately east of the Oklahoma panhandle in due north-south orientation. This is the most westward solution I’ve seen, and while it has handled overnight precipitation well so far as of this posting, and the upper level low has been progressing slower than thought, the far westward positioning of this dry line is something to monitor. To speak more frankly, this solution is suspect, although this model still remains among favored guidance as of posting time. 3/29 21z SPC SREF takes a nice middle ground between this westward solution and further-east guidance, placing the dryline approximately along a north-south line of Lawton, Oklahoma to Burlington, Oklahoma by the afternoon hours. As such, thunderstorm initiation along the dryline would threaten the Oklahoma City metro area, and eventually may threaten the Tulsa region.
        Concerns have been brought up frequently in discussions around the lack of convective initiation on the dry line in some model guidance, despite the presence of an uncapped, supercell-favorable storm environment. This would be the result of a lack of trigger mechanism, it appears, as some guidance shows weak convergence along the dry line. I am personally opposed to this solution, and while it may very well pan out, I am more pressed to believe favored guidance, which does initiate convection along the dry line in the afternoon hours.
In the event convection does initiate along the dry line, it is expected that thunderstorms would quickly become supercellular, with all modes of severe weather possible. The new SPC Day 1 outlook has placed central and southern Oklahoma in the Enhanced Risk of severe weather, primarily for potentially significantly large hail, in addition to damaging winds and a non-zero, yet relatively low tornado threat. Although it still retains unrealistically-high surface temperatures tomorrow afternoon, new runs of the HRRR model continue to gradually increase the coverage and intensity of convective initiation along the dry line in central Oklahoma. Initiation here is along the Lawton-Burlington line previously mentioned, but I am not willing to put stock into the HRRR currently given its unrealistic surface temperatures.
       Outside of Oklahoma, best tornado dynamics outlined by SPC Day 1 projection and SPC SREF guidance will be placed in Louisiana/Arkansas/Texas area, where a 10% hatched delineation has been posted in the Day 1 outlook. Despite this, as mentioned previously, non-zero tornado threats will exist in Texas and Oklahoma in addition to these more favorable dynamics.

        To summarize, potential severe weather event is on the table today for the Southern Plains into the lower Mississippi Valley, with high uncertainty still included in the forecast. Broad, weak precipitation expected to develop and move into Texas and Oklahoma overnight into the morning hours will likely temper a full-on severe weather outbreak for the Plains, but afternoon clearing and favorable dry line positioning may allow the formation for strong to severe storms in the afternoon across central and eastern Oklahoma. Widespread convection is expected for the eastern Texas / northern Louisiana / Arkansas region, where tornado dynamics will be best. Large hail will be a day-long threat for the Oklahoma area, particularly with a round of thunderstorms in the morning that may form in the midst of the weak, broad precipitation to produce hail, and then again in the afternoon when potentially-supercellular development occurs along the dry line.

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.