2018

Mesoscale Discussion #0143


Concerning...Outlook upgrade 


Valid 191452Z - 191615Z


SUMMARY...An upgrade to a Moderate Risk categorical outlook (driven by a 15% significant tornado risk) is planned for the 1630Z outlook.


DISCUSSION...It appears increasingly probable the risk for several tornadic supercells will traverse across northern portions of AL late this afternoon and through the early evening.  Thunderstorm activity will be largely void across a large part of this area (north-central AL) prior to peak heating and a moderately unstable airmass is expected to develop by mid afternoon.  The arrival of a speed max in the mid-levels will correspondingly lead to a marked strengthening in low-level flow and increasing hodograph size, which will be more favorable for supercells and low-level mesocyclones.  Given these factors, confidence has increased such that a 15% significant tornado risk and the potential for longer lived supercells and damaging tornadoes will focus across north-central and northern AL, far southern middle TN beginning during the late afternoon/early evening, then moving east into northeast AL and far northwest GA later this evening.

Mesoscale Discussion #0146


Probability of Watch Issuance...95 percent


SUMMARY...Tornado watch will be issued by 21z across portions of the Mid-South. A few strong tornadoes are possible in addition to large hail.


DISCUSSION...Leading edge of mid-level drying/cooling is progressing across the MS Delta region at roughly 35kt. Over the next few hours lapse rates should adjust across western TN/northeast MS and deep convection is expected to expand in response to this feature. Conditions are becoming increasingly favorable for strong supercell development as boundary-layer moisture is advancing into northern MS/AL with mid 60s surface dew points now evident at TUP into Winston county AL.


Early-day thunderstorm cluster that spread across northern MS is now into northern AL and this convection may impede deeper moisture surge into the Middle TN region. However, a well-defined and increasingly buoyant east-west boundary draped across northern MS/AL should provide a focus for potentially tornadic supercells. Shear profiles are becoming quite strong across this region and it appears discrete storms will evolve over the next few hours in response to approaching short-wave trough. Environmental parameters appear supportive of potentially long-track tornadoes and this will be addressed within the next few hours with a tornado watch.

Mesoscale Discussion #0151


The severe weather threat for Tornado Watch 15 continues.


SUMMARY...Threat for significant tornadoes with supercell cluster near the AL/GA border will continue for another 1-2 hours.


DISCUSSION...Recent radar imagery confirms additional supercell development and rapid intensification of rotation with storms on th south flank of the long-lived supercell cluster that has crossed AL during the evening.  This newer development initially suffered less destructive interference compared to earlier storms, and low-level hodographs have become more favorable for low-level rotation/strong tornadoes per regional VWPs.  The more moist environment near the AL/GA border may offset the storm interactions and maintain a significant tornado threat for another 1-2 hours as the storms spread into GA.

Mesoscale Discussion #0222


Probability of Watch Issuance...80 percent


SUMMARY...A tornado watch will likely be issued for portions of LA and MS. The potential for tornadoes is greatest with warm-sector development that will gradually intensify and focus primarily from central LA northeast into southwestern MS.


DISCUSSION...Radar mosaic shows deepening showers and a few thunderstorms developing in a zone of warm air advection near a remnant maritime front advancing northeast across the lower MS Valley.  To the south of the boundary, middle to upper 60s boundary layer dewpoints are contributing to moderate buoyancy.  A veering wind profile is resulting in a looping hodograph structure and as the low-level flow strengthens from the west with time, the hodograph will enlarge and result in 150-200 m2/s2 0-1 km SRH.  Given the increasing low-level moisture and strengthening low-level shear within a supercell-supportive environment, several supercells will probably develop and pose a risk for a weak tornado or two.


The development of a convective line later this evening will move southeast through the area.  Damaging winds and perhaps a brief/weak mesovortex tornado will be possible with this later activity.

Mesoscale Discussion #0238


Probability of Watch Issuance...95 percent


SUMMARY...Elevated thunderstorm activity over southeast OK and northeast TX may become rooted in the boundary layer by mid afternoon.  This will increase the risk of intense storms capable of tornadoes, large hail, and damaging winds.  A tornado watch will be needed later today.


DISCUSSION...Radar/satellite imagery shows a band of elevated convection extending from Pushmataha/Choctaw counties OK southward into Kaufman/Navarro counties TX.  Some increase in intensity has been noted in the past hour, although activity remains sub-severe.  A substantial capping inversion was in place over this region at 12z, but the 18z FWD/LZK soundings indicate rapid cooling at the base of the EML and a diminishing cap.  Given current trends, the thunderstorm activity may become surface-based in the next couple of hours.


Strong low-level wind fields will remain in place ahead of the convection, and may increase through early evening.  Local VAD profiles already show 0-3 km SRH values of 300-400 m2/s2 with forecast soundings suggesting values over 500 m2/s2 this evening. Assuming activity can remain semi-discrete, a few intense supercells are likely to evolve.  Parameters appear favorable for the risk of strong tornadoes, along with large hail and damaging winds.  PDS wording is being considered in the tornado watch for this region.

Mesoscale Discussion #0244


The severe weather threat for Tornado Watch 40 continues.


SUMMARY...Tornado Watch 40 continues across the discussion area.  Some uptick of convection on the southeastern periphery of the watch has been noted and may potentially necessitate local spatial extensions.


DISCUSSION...The severe threat continues across Tornado Watch 40.  Throughout the day, storms have generally struggled to intensify above severe levels - probably due to lingering mid-level inhibition and less steep lapse rates compared to areas west (i.e., far western Arkansas).  Latest radar observations/satellite suggests that this scenario may be gradually changing as insolation/surface heating has resulted in gradually deepening convection in central/northern Louisiana an adjacent areas of northeastern Texas.  A few of the cells in northern Louisiana have exhibited weak rotation in the past 30 minutes.  Surface temperatures have approached 80F in this area, supporting moderate instability especially across the southern half of the WW area.  With time, the combination of storms migrating into the watch area from the west, in addition to storms gradually increasing in the southern half of the WW, a gradually increasing low-level jet, and cooling mid-level temperatures will likely result in an increased threat for all severe hazards into the evening hours.  This includes the potential for a couple of strong tornadoes.  Furthermore, local spatial extensions of WW 40 may be needed if convection organizes further across central/north-central Louisiana.

Mesoscale Discussion #0246


The severe weather threat for Tornado Watch 40 continues.


SUMMARY...A significant tornado threat may increase and areas downstream from the Sevier County tornadic supercell ---including Montgomery and Yell counties--- are under an increasing tornado risk.


DISCUSSION...A supercell with a history of tornadic activity over southwestern AR and northeast TX will continue to the northeast over the next 1-2 hours ahead of a north-northeast to south-southwest oriented band of thunderstorms.  The mesoscale environment immediately on the inflow side of the supercell is characterized as very moist with surface dewpoints around 70 degrees F.  Based on the wind profile from the Little Rock (LZK) VAD, around 450 m2/s2 0-1 km SRH is present---supportive of low-level mesocyclonegenesis and a tornado risk.  Similar thermodynamic profiles were sampled by the 00Z SHV and LZK raobs (very moist through the troposphere with moderate buoyancy, 1600-1900 J/kg MLCAPE).  Given the strong low-level shear and ample buoyancy, the tornado risk may continue to increase over the next 30 minutes to 1-2 hours before the supercell becomes embedded within the eastward-moving convective band from the west.

Mesoscale Discussion #0247


The severe weather threat for Tornado Watch 40 continues.


SUMMARY...Greatest risk for a tornado will seemingly focus from Howard County northeast to the Hot Springs vicinity during the next 1-2 hours.  A significant tornado is possible.


DISCUSSION...Latest surface analysis places a maximum pressure fall at Arkadelphia (1.5-2.0 mb per hour) within a tongue of rich low-level moisture with dewpoints in the 68-70 degree F range.  Strong and strengthening low-level shear has been observed from the LZK VAD (550 m2/s2 0-1 km SRH with observed storm motion).  Another favorable factor for tornado potential is the expectation for the Sevier/Howard County supercell to remain quasi-discrete for at least the next hour as the outflow from the QLCS from the west will probably not negatively impact the storm inflow/updraft in the near term.

Mesoscale Discussion #0304


Probability of Watch Issuance...95 percent


SUMMARY...One or two tornado watches will be needed by mid-afternoon (20-21Z) across central Kansas into southeast Nebraska.


DISCUSSION...Surface mesoanalyses this afternoon showed an area of low pressure located in northwest KS, while the equatorward-trailing dryline was continuing to shift east through southwest KS.  DDC radar imagery showed this boundary located between KGCK and KDDC at 1740Z and into central portions of the OK and TX Panhandles.  Clearing skies per visible satellite imagery have allowed for stronger surface heating across the warm sector where the environment continues to moisten per southerly low-level winds. These changes in boundary-layer thermodynamics (reduction in inhibition) have allowed cumulus clouds to develop along the KS portion of the dryline into adjacent part of the OK Panhandle, as the environment further destabilizes (mixed-layer CAPE is already up to 2500 J/kg).  


Deeper moist convection is expected by 20-21Z along the KS dryline as stronger ascent within the exit region of southwesterly mid- and upper-level jets shift into the central Plains this afternoon and evening. Trends in the operational HRRR continue to suggest storm development along the KS portion of the dryline by 21Z, with some potential for initiation by around 20Z. Further low-level moistening and increasing vertically veering winds will support supercell development with an attendant threat for all severe hazards.  Initial storms will be capable of producing very large hail, while a tornado threat (some strong) will increase, given a strengthening southerly low-level jet resulting in increased hodograph curvature this evening.

Mesoscale Discussion #0307


The severe weather threat for Tornado Watch 59 continues.


SUMMARY...The tornado threat across southeastern Nebraska and north-central Kansas is expected to increase over the next couple of hours. Large hail and wind damage will be likely with the stronger storms as the activity move eastward across tornado watch 59 through early evening.


DISCUSSION...The latest surface analysis shows a cold front moving southeastward across south-central Nebraska and northwest Kansas.  A dryline extends southward from the front across west-central Kansas. A narrow corridor of maximized low-level moisture is present ahead of the dryline from north-central Oklahoma into north-central Kansas where surface dewpoints are generally in the lower to mid 60s F. This combined with surface heating has resulted in moderate instability with the RAP showing MLCAPE of 2000 to 3000 J/kg.


Radar imagery shows a line of semi-discrete storms ongoing along the western edge of moderate instability from near Hastings, NE to just east of Dodge City, KS. The cells within the line will continue to move eastward into stronger instability and should gradually intensify. Due to the instability, steep mid-level lapse rates of 8.5 to 9.0 C/km and strong deep-layer shear evident on the Hastings WSR-88D VWP, hailstones of greater than 2 inches in diameter will be possible with the stronger supercells. In addition, a 40 to 50 kt low-level jet is forecast to rapidly strengthen across central and eastern Kansas into southeast Nebraska early this evening. For this reason, low-level shear will become increasingly favorable for tornadoes. 0-3 km storm-relative helicities are forecast to increase into the 400 to 500 m2/s2 range suggesting that a strong tornado or two will be possible as well.

Mesoscale Discussion #0317


The severe weather threat for Tornado Watch 62 continues.


SUMMARY...Very large hail and an increasing threat for storms to become tornadic exists across the eastern Texas/Oklahoma Panhandles into northwest Texas and western Oklahoma into the early evening.


DISCUSSION...Mid-afternoon mosaic radar imagery indicated ongoing discrete storm development near and east of a dryline which extended south through far eastern TX Panhandle from an area of low pressure in far northwest OK.  These storms will continue to track to the east-northeast into western OK this afternoon, with additional storm development likely in northwest TX in the southwest part of WW 62.  The CAPE/shear parameter space east of the dryline has become very conducive for storms to mature into supercells with the potential for tornadoes and very large hail.  Special NSSL sounding at Elk City, OK at 1827Z indicated very strong instability (surface-based CAPE of 4000 J/kg and midlevel lapse rates near 8 C/km).  In addition to strong effective bulk shear expected to remain oriented across the initiating boundary for discrete storms, the NSSL sounding showed effective inflow shear of 28 kt, with low-level hodograph curvature to support rotating updrafts.

Mesoscale Discussion #0427


The severe weather threat for Tornado Watch 94, 96 continues.


SUMMARY...The severe threat is increasing across the tornado watch area, with large, damaging hail, severe wind gusts and a threat of a few tornadoes.


DISCUSSION...Storms oriented along the old outflow boundary have increased markedly in the last hour, with a few hail cores now exceeding 2.00". Cells were initially splitting per 18Z observed ALB straight line hodograph, but the southern cells are becoming dominant. Southwesterly low-level winds will continue feeding these cells with unstable air, and they may increasingly take on supercell characteristics over the next few hours. Any deviant motion of these cells to the south will likely result in strengthening mesocyclones, and perhaps a few tornadoes. Wind driven, damaging hail is also likely.

Mesoscale Discussion #0923


Probability of Watch Issuance...95 percent


SUMMARY...Supercell thunderstorms are expected to form later this afternoon and into the evening. Very large hail, significant damaging winds, and a few tornadoes are possible this evening. A tornado watch will likely be issued in the next couple of hours.


DISCUSSION...Latest HRRR guidance and timing of the upper-wave on 6.2 micron water vapor imagery suggest convective initiation is likely between 22Z and 23Z this evening in eastern Montana. Strong surface heating and low-level moisture advection has increased surface temperatures into the mid to upper 80s and dewpoints into the low 70s. This warm, moist boundary layer combined with very steep mid-level lapse rates (over 9 C/km per UNR 19Z sounding and RAP mesoanalysis) has led to extreme instability across the area (Over 4000 J/kg MLCAPE). The GGW 19Z sounding showed a 50 knot wind speed at 500 mb which is above the 90th percentile for late June. This leads to 50 to 60 knots of effective shear, which combined with the extreme instability will support supercells as the primary storm mode initially. The extreme values of instability, deep-layer shear, and mid-level lapse rates will support very large hail, especially for the first few hours after storm development.


Low-level east-southeasterly flow, veering to west-southwesterly flow at 3 km will also favor rotating low-level mesocyclones. While the speed shear in the lowest 1 km will initially be a limiting factor for tornadic production, 850 mb winds will strengthen significantly after 00Z. Current thinking is that storm mode may become less discrete around the time of the improving low-level shear profile which could limit the tornado threat. However, the last few HRRR runs have hinted that a discrete storm mode may persist long enough into the evening to support the tornado threat.

Mesoscale Discussion #0929


The severe weather threat for Tornado Watch 234, 235 continues.


SUMMARY...Threat for very large hail, strong wind gusts, and tornadoes continues from eastern MT across much of ND.


DISCUSSION...Thunderstorms developed quickly across eastern MT as the forcing for ascent attendant to the approaching shortwave trough encountered the warm, moist, and uncapped airmass across the area. Several of these storms have recently exhibited more eastward motion(versus northeastward), suggesting well-developed and organized updrafts exist. This more eastward motion will also take the storms into airmass even more supportive of growth and persistence, with surface dewpoints commonly in the 70s. Current mesoanalysis estimates MLCAPE is over 4000 J/kg (locally over 5000 J/kg) across southeast MT. Shear is also quite strong, with effective bulk shear over 50 kt. All of these factors suggest a continued severe threat with very large hail and strong wind gusts as the primary severe hazards. A tornado or two is also possible.


Farther east, discrete supercell over Dunn County has recently shown a strengthening trend, likely a result of entering the uncapped and strongly unstable airmass across southwest ND. Southeasterly low-level winds beneath increasing westerly flow aloft is supporting strong deep vertical shear as well as enhanced low-level shear and helicity. Given the strong instability and favorable shear, the ongoing discrete storm will pose a risk for all severe hazards, including very large hail and tornadoes.

Mesoscale Discussion #0933


The severe weather threat for Tornado Watch 234, 235 continues.


SUMMARY...Maturing convective line will continue to pose a threat for all severe hazards as it moves eastward across eastern MT and into western ND. Discrete storms ahead of the line also pose a threat for all severe hazards.


DISCUSSION...Thunderstorm coverage across eastern MT has increased over the past hour as the shortwave trough and attendant forcing for ascent continues to progress northeastward. Overall radar presentation indicates cold pool amalgamation has likely occurred with these storms with the resulting convective line extending from about 20 mi E of OLF southwestward to the storm 50 mi NNE of SHR. Additionally, rotation within the reflectivity at the northern end of the line suggests a MCV/book-end vortex has developed, validating the overall organization of the convective system. Expectation is for the line to progress eastward, maintained by moist low-level inflow from a strengthening low-level jet and steep mid-level lapse rates. Primary threat with this line is strong wind gusts, some of which could be over 65 kt. Hail is also possible as are tornadoes, particularly in any areas of the convective line where the outflow surges forward.


A few discrete supercells also exist ahead of the convective line. Supercell in Carter County MT continues to pose a tornado threat (verified by storm-spotter video) as does the nearly stationary storm over Dunn County ND. Very large hail and strong wind gusts are also a threat from these discrete storms.

Mesoscale Discussion #1086


Probability of Watch Issuance...60 percent


SUMMARY...Instability is increasing across the discussion area. Strong shear and low-level hodograph turning near a surface warm front will lead to storms capable of all severe hazards. A WW may be needed in the next hour or two.


DISCUSSION...Destabilization has begun to increase in the wake of earlier convection across south-central Iowa as evidenced by a broader area of agitated cumulus, from near Des Moines, IA south/southwestward into north-central Missouri, appearing on visible satellite imagery over the past hour. RAP analysis shows approximately 1500-2500 J/kg of MLCAPE. With the approach of another lobe of mid-level vorticity, increasing synoptic ascent should lead to initiation of scattered severe thunderstorms along and near the surface warm front. Strong effective bulk shear values of 40-50 kts will support organized storms capable of large hail and damaging wind gusts. Furthermore, RAP analyzed effective SRH values of 100-200 m2/s2 -- supported by KDMX and KDVN VAD data -- coupled with a very moist boundary layer and backed surface flow near the boundary, suggests a couple of tornadoes will be possible. A WW is possible in the next hour or two.


**This MCD preceded the Pella, Iowa and Marshalltown, Iowa EF3 tornadoes

Mesoscale Discussion #1482


Probability of Watch Issuance...80 percent


SUMMARY...All severe hazards, including tornadoes, are possible are the region this afternoon and evening. A Tornado Watch will be likely be issue over portions of the region by 21Z.


DISCUSSION...Recent surface analysis reveals a fractured frontal structure with two effective warm fronts draped across the region. One is more aligned with the temperature gradient and extends from a low near OFK eastward across northern IA and into northern IL while the other is better aligned with the moisture gradient, extending from a low near HON across southern MN and into central WI. Strong pressure falls across MN suggest a northeasterly motion of the surface lows, particularly the stronger low across northeast NE, will continue and perhaps accelerate. Cumulus fields along the front in NE have become more agitated over the past hour while some clearing has been noted across northern IA between the two warm fronts. Overall, trends continue to suggest a quick northward motion of the southern warm front across northern IA and into southern MN while the low tracks northeastward and the cold front sweep in from the east.


Recent mesoanalysis suggests a corridor of strong effective storm-relative helicity exists along the MN/IA border where surface winds are still southeasterly. Bulk shear values are also quite strong as both the low and mid-level flow strengthen ahead of the approaching shortwave trough. Consequently, kinematic field support a severe risk, including the potential for tornadoes. 


Thermodynamic fields are a bit more questionable, particularly with northern extent where the cloud cover remains. Even so, the quickly modifying airmass will likely be free of convective inhibition within the next hour or so. Once this capping erodes, a narrow window appears to exist for discrete development within the warm sector before the approaching cold front leads to a predominately linear mode. All severe hazards are possible and a Tornado Watch will likely be issued over portions of the region by 21Z.

Mesoscale Discussion #1514


The severe weather threat for Tornado Watch 392 continues.


SUMMARY...Several rotating storms are ongoing across Pennsylvania. An increase in low-level rotation is expected as the low-level jet increases this evening. Downstream tornado watch issuance may be necessary for southeastern NY and northern NJ.


DISCUSSION...Several discrete, rotating storms are ongoing across portions of Pennsylvania, per radar mosaic, with much of the strongest rotation near the PA/NY border where a diffuse surface boundary is draped. As the evening progresses, an increase in the 850 mb low-level jet is forecast across northern/central PA through about 03Z. Any storms that remain discrete will likely see an increase in low-level rotation as the jet enlarged low-level hodographs. Given the expected increase in storm rotation over the next few hours, downstream tornado watch issuance may be necessary in far southeastern NY and northern NJ.

Mesoscale Discussion #1622


The severe weather threat for Tornado Watch 422 continues.


SUMMARY...A favorable environment and supercellular storm type will support a continued tornado risk with experimental guidance suggesting damage intensities 80-135 mph (upper EF0-EF2).


DISCUSSION...KPOE and KSHV radar imagery show a cluster of supercells in a southwest-northeast broken band and a few discrete supercells located over north-central LA.  The storms are located in a very moist boundary layer with surface dewpoints around 70 degrees F and lowest 100mb mean mixing ratios 15-16 g/kg.  The KPOE VAD shows strong speed shear in the lowest 1km (30 kt) with a veering wind profile through the mid levels.  Given the favorable mesoscale environment and supercellular storm type, the tornado risk will continue downstream to the east-northeast across north-central LA during the next hour.  Experimental probability guidance is suggestive of damage intensities primarily within a 80-135 mph range (upper EF0-EF2) with any ongoing or subsequently developing tornado.

Mesoscale Discussion #1669


Probability of Watch Issuance...95 percent


SUMMARY...Threat for severe storms capable of tornadoes continues to gradually increase. A Tornado Watch will be needed soon.


DISCUSSION...Warm advection continues to be the primary forcing for ascent, evidenced by linear/arching reflectivity structure and fairly transient updrafts. However, stronger forcing for ascent is quickly approaching as a potent shortwave trough moves into the southern Plains. Storm in Franklin county AR has recently shown better organization, shedding a well-developed left-split and developing a strong updraft during the past half hour. With the increased forcing for ascent, storm coverage is expected to increase. The thermodynamic environment is fairly marginal as shown by the deep but only weakly unstable layer from the surface up to 700 mb on the 00Z SHV sounding. Even so, instability is expected to gradually increase as mid-level temperatures cool within the approaching shortwave. In contrast to the thermodynamics, the kinematics are already favorable for rotating storms with an increasing low-level jet expected to result in even stronger low-level shear profiles. Primary threat will be tornadoes, one or two of which could be strong. Damaging wind gusts are also possible. Small hail cannot be completely ruled out.

Mesoscale Discussion #1687


The severe weather threat for Tornado Watch 434 continues.


SUMMARY...The risk for severe storms, including supercells with potential for tornadoes, may continue around the Peoria and Springfield IL areas through 6-7 PM CST, before diminishing.  The need for an additional watch is not anticipated.


DISCUSSION...Supporting mid-level forcing for ascent (as inferred from water vapor imagery) continues to gradually pivot northeastward through northern/central Illinois.  The southern periphery of this forcing appears likely to remain roughly along/north of the I-70 corridor, and provide the southern limit to ongoing and subsequent thunderstorm initiation.  


The warm front, ahead of the mid-level forcing, appears likely to remain the northern limit to any appreciable further severe weather potential.  It appears that this boundary will continue to slowly advance north of the I-74 corridor.  However, the onset of warm sector boundary layer cooling by the 01-02Z time frame is expected to provide the main impetus for diminishing convective trends.


Until then, for another hour or two, the environment around the Springfield/Peoria areas appears to remain conducive to discrete supercells accompanied by a continuing risk for tornadoes, in addition to severe hail and localized strong surface gusts.