MESO-ANALYST SEVERE WEATHER GUIDE
*** This is for guidance purposes only. Specific criteria numbers are provided as guidance. ***
Pete Wolf, SOO
National Weather Service, Wichita, Kansas
Given that thunderstorm development, mode, and resultant weather are a function of the thunderstorm=s immediate environment, knowledge of near-storm environmental conditions should allow one to correctly anticipate convective mode and resultant weather. The purpose of this guide is to provide forecasters with useful fields for evaluating near-storm environment conditions.
The guide provides Aguidance@ information. Forecasters should not take the exact numbers literally. They also should not focus on any one factor in a given category. For example, having one factor in the large hail category suggest unfavorable conditions for hail does not mean hail will not occur. Finally, forecasters should focus on near-storm environment information when possible, as opposed to larger scale environmental conditions.
Category |
FACTOR (linked to notes at bottom of page) |
VERY FAVORABLE |
FAVORABLE |
SO - SO |
UNFAVORABLE |
(1) Low Level Boundary (convergence, frontogns) |
- Strong - > 125kts - < 15 - Strong Upglide - > 100 |
- Moderate - 100 - 125 - 15-30 - Weak/Mdt Upglide - 75-100 |
- Weak - 80 - 99 - 30-60 - Little/No Upglide - 50-75 |
- Lack of boundary - < 80 - > 60 - Downglide - < 50 |
|
|
(7) Environ. or Updraft Freezing Level Height (AGL) |
- > 30 - < 11,000 ft - Strong, deep meso - > 125 kts |
- 20-29 - 11,500 - 13,500 ft - Mdt, deep meso - 100-125 |
- 12-19 - 14,000 - 16,000 ft - Weak/shallow meso - 80-99 |
- < 12 - 17,000+ ft - No meso - < 80 |
|
(9) 500-700mb Dewpt Depression (C) (10) 850-500mb Lapse Rate (C/km) (11) Sfc thetaE-lowest thetaE in 400-700mb layer (K) (12) 3-hour Surface Pressure Change (mb) (13) Lowest 50- to 100-mb Dewpt Depression (C) |
- > 20 - > 7.5 - > 25 - > 6 - > 15 - moist- to dry- adiab. - parallel |
- 10-19 - 6.5-7.5 - 18-25 - 4 - 6 - 10 - 15 - near moist adiab. - < 45 degree angle |
- 5-9 - 5.5-6.5 - 12-18 - 3 - 4 - 5 - 10 - wk or shlw invrsn - 45-60 degree angle |
- < 5 - < 5.5 - <12 - < 3 - < 5 - stg or deep invrsn - > 60 degree angle |
Tornado
|
(16) 2 x Sqrt (Approx LFC-500mb CAPE) + SRI = (17) 0-2km SR-Helicity (near storm inflow region) (18) Lowest 50- to 100-mb Dew Point Depression
(19) Mesocyclone movement relative to pre-existing or storm-induced boundary |
- > 100kts - > 400 - > 30 kts - < 11F (6C)
- On cool side of shallow boundary, or directly on deeper boundary (if inflow air sufficiently unstable) |
- 80-99 - 250 - 400 - 20-29kts - 12-18F (6-10C)- Moving at small angle over cooler air behind a shallow boundary (if inflow air sufficiently unstable) |
- 60 -79 - 150 - 250 - 10-19kts - 19-27F (10 - 15C)
- No boundary detected, deviant motion observed. |
- < 60 - < 150 - < 10 kts (5) - >27F (15C)
- Moving at sharp angle ovr cool, stable air, or on warm side of bndry movingaway from bndry |
Flash Flood
|
(21) Precipitable Water (% of normal) ACorfidi Vectors@ suggest:(23) Precip Efficiency (look for warm rain process) ATraining@ Pattern
|
- < 5 - >200% - Bckbldg/Stnry storms - High - Well below normal
- Classic (storm line stationary relative to training location) |
- 5 - 10 - 150-200% - Slow storm mvmt - Moderate - Below normal
- Near-classic (storm line mvmt < 10kts relative to trng loc.) |
- 10-20 - 100-150% - Moderate mvmt - Low - Near normal
- Semi-training (Line mvmt 10-15kts relative to trng loc.) |
- > 20 - < 100% - Fast storm mvmt - Very Low - Above normal
- No training pattern observed |
(1) Look at strength of convergence and frontogenetic/frontolytic nature of boundary (frontal circulation).
(2) Take 2 times the square root of CAPE, and add the low-level SR-Inflow in knots. You are incorporating both SR-inflow and CAPE to come up with a theoretical updraft intensity. (Note: Take square root of CAPE rather than square root of 2 x CAPE to account for factors like entrainment). Volume browser could be set up to help with this.
(3) Some CIN is a favorable condition for severe convection. Here, CIN values provided are given for anticipating storm formation, not severe nature of storms.
(4) At times, convection can initiate when isentropic upglide allows parcels aloft to reach their LFC.
(5) Incorporate both speed and directional shear below 400mb level by adding the amount of veering, in degrees, to the amount of speed shear, in knots. The greater either directional or speed shear, or both, the larger the result will be. Note: Do not utilize this factor when average wind speeds aloft are less than 20 kts.
(6) Compare average flow in the 400-700mb layer to individual storm motion (not average motion of all storms). Stronger SR-flow may exist in the presence of strong mesocyclones.
(7) Environment Freezing Level (where environmental temp. curve crosses 0C) or Updraft Freezing Level (where updraft parcel crosses 0C): use appropriate one given situation (e.g. strong SR-flow aloft, use environment level...weak SR-flow aloft, use updraft level). This may be important for correctly accounting for hail melting potential.
(8) Look for mesocyclone to be at least 10,000 feet deep, persisting for at least several volume scans. Mesocyclonic flow alters storm-relative flow pattern such as to distribute precipitation away from the warm, saturated updraft core.
(9) Use the average dew point depression value in the 400-700mb layer. Volume browser can be set up to help with this.
(10) Volume browser can be used to determine this lapse rate.
(11) Take the difference between the surface theta-e and
lowest theta-e value in the 400-700mb layer. Can do this with ADIFF@ button on volume browser. This is helpful for determining the potential for damaging winds and wet-microbursts. The steeper the lapse rate, and/or the drier the air aloft, the greater this Adifference@ will be.(12) Can get values on volume browser or LAPS/MSAS menus. This can be helpful for detecting the initiation of wake low damaging winds.
(13) Difference between surface temperature and dew point values...helpful for determining microburst potential.
(14) Subjective view of wet-bulb temperature (Tw) curve below 700mb. How deep of a Tw inversion is there, and how strong is it? Is the Tw curve closer to moist-adiabatic or dry-adiabatic? Damaging winds can reach the ground with a temperature inversion (e.g. during the evening hours), but this is more difficult when you have a wet-bulb temperature inversion (and thus little dry air for evaporative cooling to counter effects of low-level inversion). Note: Incorporate Tw inversion strength as well. The closer the Tw curve is to moist-adiabatic, the deeper it can be and still allow damaging winds to reach the ground.
(15) Damaging winds are often less likely to result from a line of storms moving at a sharp angle to the flow aloft compared to a line of storms moving along the mean flow (where momentum mixdown can result). Also look at environmental ingredients for damaging winds (dry air aloft, steep lapse rate, etc).
(16) Similar to #2 above, except utilize the amount of CAPE below 500 or 600 mb. Recent research has found that CAPE in the lowest few hundred mb may be an important factor for determining tornado potential. The strength of SR-inflow should also be considered.
(17) 0-3 km layer may be too deep. Try to determine how much of calculated 0-3km SRH is within 0-2km layer from hodographs. Focus on small-scale wind changes near storm inflow region, not general environmental SRH values. Small-scale changes in near-surface wind can drive SRH up or down substantially.
(18) LCL heights can be approximated using lowest 50-mb or 100-mb average dew point depressions. Every 5F (~3C) of dew point depression roughly correlates to 1,000 feet of cloud base height AGL. Research suggests low LCL heights (below 1000 meters) is favorable for tornado development. Note: Influx of saturated air from FFD area into updraft can cause lowering of LCL height to favorable level.
(19) Determine movement of mesocyclone relative to pre-existing or storm-induced boundary. How deep is the cool air behind the boundary? Is the cooler air unstable enough to allow convection to survive, or will it be too stable and cause rapid storm weakening?
(20) Slower storm motion yields greater potential for excessive rainfall.
(21) Greater precipitable water values (relative to normal) increase potential for excessive rainfall.
(22) Backbuilding or slow moving convection has greater potential for producing excessive rainfall.
(23) Greater precipitation efficiency yields greater precipitation rates. Look for conditions favoring warm rain process (e.g. depth of layer from LCL to freezing level at least 3-4 km, low cloud base heights, etc). Also look for weak SR-flow at storm top (don
=t lose as much precip through anvil), K-index values of 36+, etc.(24) Lower flash flood guidance (FFG) values favor a greater flood threat.
(25) Are storms training over the exact area? If the overall line motion is near 0 (relative to the area where training is occurring), you have ideal training. The more a line moves, relative to its orientation (e.g. east/west mvmt of north-south oriented line), the less
Aideal@ the training setup is for excessive rainfall.
For severe storm initiation, 5 key ingredients were included in the guide. These ingredients focus on instability, shear, and lift.
1) Low-level boundary: For this factor, focus on the nature of low-level convergence and frontogenetic forcing. The boundary does not necessarily need to be surface based.
2) 2 x Sqrt (CAPE) + SR-Inflow: This ingredient focuses on potential updraft intensity. The theoretical maximum updraft velocity is described by the square root of 2 x CAPE. However, factors such as entrainment should be considered. Thus, only the square root of CAPE is used. It is multiplied by 2 to convert result to knots. Finally, SR-Inflow (in knots) should be considered when determining potential updraft strength. If low-level SR-inflow is 30 knots, for example, is the initial parcel velocity at the bottom of the updraft zero?
3) CIN: A cap is favorable for severe convection: This term focuses on sufficient weakening of the cap for convective development.
4) 700-850mb isentropic flow: This ingredient indirectly accounts for lift due to jet streaks, short-waves, fronts aloft, etc. Isentropic flow above the boundary layer should reveal lift where convection initiates not in association with a near-surface boundary.
5) B.Lyr-400mb Wind Speed/Direction Difference: This ingredient looks for speed and directional shear in the 0-6 km layer, approximately. While you could substitute actual 0-6 km shear (hodograph length) here, the term prevented is one easy to use when overlaying plan-view winds on satellite imagery, for example. In this term, simply add the number of degrees of veering to the number of knots of speed shear. The greater the directional shear, and/or the greater the amount of speed shear, the greater the end result will be.
There are 4 terms for this category...with the focus on melting potential.
1) 400-700mb SR-Flow (kts): This term is used to determine whether large hailstones will fall through the warm, saturated updraft core, or outside of the updraft. The stronger the SR-Flow, the more realistic it is to use environmental freezing level (or wet-bulb zero) level to determine potential melting. For weak SR-Flow aloft (<15 kts), the higher updraft freezing level (or between the updraft and environmental levels) may be better for assessing melting potential.
2) Environment or Updraft Freezing Level: Based on strength of SR-flow aloft. Updraft freezing level is determined by finding where updraft parcel (moist adiabat) crosses 0-Celsius line.
3) Mesocyclone Intensity/Strength: Strong mesocyclones promote strong SR-flow...and are often associated with large hail.
4) 2 x Sqrt (CAPE) + SR-Inflow: As described earlier, this focuses on potential updraft intensity. The stronger the updraft, the larger the hailstones can grow.
This is the most difficult category, because many unknowns exist regarding why one storm produces damaging winds and another one does not. This category includes 7 factors.
1) 500-700mb Dew point Depression (C): Drier mid-level air supports stronger downdrafts...by promoting larger DAPE (Downdraft Available Potential Energy).
2) 850-500mb Lapse Rate (C/km): Similar to mid-level dry air, large lapse rates (> 6.5C/km) promote larger DAPE (Downdraft Available Potential Energy).
3) Sfc Theta-e - Lowest Theta-e (in 400-700mb layer): Large differential theta-e is favorable for wet microbursts.
4) 3-Hour Surface Pressure Change: Can be valuable for anticipating wake low wind events.
5) Lowest 50- to 100-mb Dew point Depression: A favorable microburst characteristic.
6) Wet-Bulb Temperature Profile below 700mb: May be better than simply the thermal profile for determining if a low-level inversion will prevent damaging winds from reaching the ground. Take evaporation potential into account. During the evening, when damaging winds may still be occurring, there may be a thermal inversion, without a significant wet-bulb temperature inversion.
7) Movement of Storm Line Relative To 500-700mb Flow: Observations have shown that severe winds are more likely from storms moving with the flow aloft than at a significant angle.
There are 5 factors listed for this category.
1) 2 x Sqrt (LFC-500mb CAPE) + SR-Inflow: Similar to what has been described earlier, except the focus is below 500mb, as suggested by recent research.
2) 0-2km SR-Helicity (near storm inflow region): Recent research suggests 0-3 km layer is too deep. 0-1km layer may be best, but 0-1.5 or 0.2 km layer (below 850mb) may be easier to determine. Look for pockets of higher helicity where winds back locally (this can be determined from a mesonet, profilers, or even WSR-88D velocity imagery at times).
3) 400-700mb SR-Flow: Based on recent research suggesting the importance of SR-flow aloft for tornadic storms.
4) Lowest 50- to 100-mb Dew Point Depression: This can be related to the height of the LCL (roughly 1000 feet per 5 degrees of near-surface dew point depression). Lower LCL heights are favorable for tornadogenesis, according to recent research.
5) Mesocyclone movement relative to boundary: Recent VORTEX research suggests tornadogenesis is most favored from a mesocyclone moving on the cool-side of an existing boundary...provided the air on the cool side is sufficiently buoyant. SR-helicity tends to be maximized on the cool side of boundaries.
There are 6 factors for this category.
1) Storm Motion (kts): Simply put, a slower storm produces greater rainfall amounts.
2) Precipitable Water (% of normal): relates to precipitation efficiency of storms. Note: Might be better to look at theta-e values (relative to normal), and look for where theta-e maxima are feeding the storms of interest, as opposed to theta-e minima. Climatological normal values of theta-e are not as well known, or easy to determine, as for precipitable water.
3) ACorfidi Vectors@: Suggestion of nearly stationary or back-building convection would indicate a much greater flood threat.
4) Flash Flood Guidance: Lower values of flood guidance relates to a higher flood threat.
5) ATraining@ Pattern: The more a line of storms moves relative to the axis of training, the lower the flood threat. For example, an east-west oriented line moving east will produce much greater rainfall amounts than an east-west oriented line moving southeast or south.