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(T. F. “Storm” Walsh III)
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Good day everyone!
The Storm Prediction Center (SPC) has designated the following risk areas regarding the probability for Severe Thunderstorms in the Day 3 Outlook for Tuesday, Feb. 28, 2017:
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER THE OZARK PLATEAU INTO SOUTHEASTERN MISSOURI…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST TEXAS NORTHEASTWARD INTO THE MIDDLE MISSISSIPPI VALLEY AND OHIO VALLEY…
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE SLIGHT RISK…
CORRECTED FOR THE MENTION OF POSSIBLE SEVERE IN THE OHIO VALLEY IN THE SUMMARY SECTION
Severe thunderstorms will be possible on Tuesday and Tuesday night from eastern portions of Oklahoma and Texas eastward into parts of the lower and middle Mississippi Valley and Ohio Valley.
A large-scale mid-level trough over the western U.S. will move into the central states on Tuesday as a belt of strong southwesterly 500-mb flow intensifies further during the period as a 100-kt speed max moves to the lower MO Valley by early Wednesday morning. In the low levels, a broad warm sector will become established from the central Gulf Coast states northward into the OH Valley and bounded on the north with an advancing warm front. The west edge of the low-level moisture will likely be located over the eastern portions of TX/OK as an elongated area of low pressure extends from the middle MS Valley/Great Lakes southwestward into OK/KS and developing eastward into the Ozarks during the overnight.
…eastern OK/northeast TX northeastward into the MS Valley and lower OH Valley… Complicated forecast scenario with significant uncertainty is evident in both the coverage and intensity of severe thunderstorms across a large warm/moist sector. Steep 700-500 mb lapse rates (7-8 degrees C per km) atop lower to middle 60s dewpoints over AR and dewpoints near 60 degrees F in the OH Valley will contribute to weak to moderate buoyancy. Strong shear profiles (60-70 kt effective shear) will result in a supercell-wind profile. A wide array of possibilities are plausible at this time, including a risk for warm-air-advection storms developing during the late afternoon/evening over portions of AR and the MS Valley and yielding a tornado and hail risk. Complicating this scenario includes a cirrus canopy implied by model data as well as the degree of heating.
A cold front is forecast to move into the western portions of the broad outlook area during the evening and overnight and push east-southeastward. Thunderstorms are likely to develop along the front and severe is possible with this activity as well. Damaging winds and hail will be the predominate risks early on with this activity before upscale growth into one or more larger convective lines and bowing segments yields a continued wind risk and perhaps a lingering tornado threat.
Based on my analysis of current forecast soundings through F5 DATA, it was noted that large discrepancies between the NAM-WRF and GFS models existed. Given these discrepancies, it was my determination to use the GFS forecast sounding information, as it correlated better to the SPC outlook maps, based on aerial coverage. The NAM-WRF model in no way depicted the severe thunderstorm categorical areas in analysis.
THE FOLLOWING IS TO BE CONSIDERED PRELIMINARY, GIVEN THAT VALUES CAN CHANGE BETWEEN NOW AND TUESDAY, AND BASED ON DISCREPANCIES IN THE MODELS AT THE MOMENT
Based on some chosen severe thunderstorm and tornado parameters, the current forecast Lifted Index and CAPE values indicate the probable threat of significant hail associated with any strong supercells that may develop. The forecast Lifted Index was noted at -4 to -8 within the outlined areas.
Based on the SWEAT (Severe Weather Threat) values, EHI (Energy Helicity Index), TT Index (Totals Totals Index), combined with CAPE and Lifted Indices, the following outlined areas should have the greatest probability of experiencing supercell / severe thunderstorms, and isolated tornadoes. The greater probability for tornadic activity will most likely be seen where the higher SWEAT VALUES (400+) are located.
Again, this is PRELIMINARY and I will most likely update everything on Tuesday.
GFS 3:00 P.M. CST FEB. 28, 2017
GFS 6:00 P.M. CST FEB. 28, 2017
GFS 9:00 P.M. CST FEB. 28, 2017
The SPC has indicated the potential for continued severe weather for Wed., with a 30% probability area in the mix. The following is the current discussion from SPC:
A potentially widespread damaging-wind event is possible on Wednesday (Day 4) from portions of the lower MS Valley northeastward into the TN and OH Valleys, central and southern Appalachians, and Mid-Atlantic states. A mid-level trough is forecast to move eastward across the MS Valley and towards the Appalachians by early Thursday while a cold front sweeps eastward. An appreciably moist/unstable warm sector and very strong southwesterly flow fields aloft will conditionally support an extensive squall line potentially capable of swaths of damaging winds.
SPC DAY 4 SEVERE WEATHER OUTLOOK
Based on discrepancies in the models, and limited accuracy out to day 4, I am not posting any forecast soundings with this map. I will most likely have an update Wednesday, but won’t be able to have one out until mid to late afternoon.
This is long, but the following copied document will provide you with severe weather parameters. It should help you to understand some of the weather jargon I speak of in my synopsis:
Severe Weather Parameters:
100mb Mixed Layer Convective Avail. Potential Energy (100mb MLCAPE): MAJOR INDICATOR
1-1000 Marginally Stable
1001-2500 Moderately Unstable
2501-3500 Very Unstable
>3500 Extremely Unstable
Convective Inhibition (CIN):
Lower values indicate higher chance for convection
0 – 25 Joules per kilogram are small
25 – 50 Joules per kilogram are moderate
> 50 Joules per kilogram as large
Mid-Level Lapse Rates:
Higher is more unstable.
6 is conditionally unstable for moist ascent
Counterclockwise trace is better for severe weather with rotation in storms
0-6km Shear Vector: MAJOR INDICATOR
35-40 Supercells typically form
Bulk Richardson Number Shear(BRN Shear or sometimes BRNSHR):
35-40 Supercells typically form
>100 Supercells probable
Thompson/Edwards/Mead Effective Bulk Shear:
25-40+ Supercells become more probable
0-2km SR Winds (low-level storm relative winds):
15-20+ Favorable for long-lived supercells.
0-1km Shear Vector:
15-20 Translate to enough spin to favor supercells.
20-25 Good chance for tornadoes
0-1km SR Helicity: MAJOR INDICATOR
100 m2s-2 suggest “an increased threat of tornadoes with supercells”.
0-3km SR Helicity:
> 250 m2s-2 suggest “an increased threat of tornadoes with supercells”.
4-6km SR Winds:
15-40 kts. favors supercell tornadogenesis
Energy Helicity Index (EHI):
>2 translates to a high probability of supercells
>1 Supercells possible
Significant Tornado Parameter:
1 Tornadoes Possible
Significant Hail Parameter:
>1 Significant (=> 2” diameter) Hail Possible
1.5-2 Significant (=> 2” diameter) Hail Likely
2-4 Significant (=> 2” diameter) Hail Probable
>4 Significant (=> 2” diameter) Hail Extremely Likely
(hail suspended in an updraft routinely appears on cross-sections of radar reflectivity; indeed, hail is likely present whenever reflectivity exceeds 55 dBZ )
Past studies of hail-producing thunderstorms over the Middle West (hail observed at the ground) found that the Wet Bulb Zero (WBZ on SPC Skew-T Sounding) was located at altitudes between 5,000 and 12,000 feet over 90% of the time, with a clustering of observations near 9,000 feet.
SHOWALTER INDEX/ MODIFIED SHOWALTER INDEX:
3 to 1 Low Instability, thunderstorms are possible but strong lift needed
0 to -3 Moderate Instability, thunderstorms are probable
-4 to -6 Strong Instability, thunderstorms are likely
< -6 Extreme Instability, High potential for severe storms
< 15 no probability for air mass thunderstorms
15-20 20% probability for air mass thunderstorms
21-25 20-40% probability for air mass thunderstorms
26-30 40-60% probability for air mass thunderstorms
31-35 60-80% probability for air mass thunderstorms
36-40 80-90% probability for air mass thunderstorms
>40 near 100% probability for air mass thunderstorms
Values over +30 indicate potential MCC’s.
The lower the value is (i.e. the greater the negative number), the better the chance for thunderstorms and the greater the threat for severe weather.
TOTAL TOTALS INDEX:
<44 Thunderstorms unlikely
44-48Scattered thunderstorms, severe weather unlikely
48-52 A few severe thunderstorms possible
52 Severe thunderstorms are likely
SWEAT INDEX (SEVERE WEATHER THREAT):
<272 Thunderstorms unlikely
273-299 Non-severe thunderstorms are possible
300-400 Thunderstorms will approach severe limits
401-600 Increased risk of severe storms or isolated tornadoes
601-800 Tornadoes almost always occur
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS