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Good evening everyone!
This is a preliminary assessment, regarding the Severe Weather probability for Tuesday, Apr. 26, 2016.
The Storm Prediction Center (SPC) has designated a 30% probability of severe weather occurring over portions of the SOUTHERN/CENTRAL PLAINS REGION. The following is the most recent discussion from SPC:
VALID 261200Z – 011200Z …
SEVERE WEATHER OUTBREAK POSSIBLE ON D4/TUE…
CONCERN FOR A MORE ACTIVE SPRINGTIME SEVERE WEATHER PATTERN CONTINUES WITH THIS OUTLOOK ISSUANCE FOR THE CENTRAL AND SOUTHERN PLAINS ON DAY 4/TUESDAY AND INTO THE MID MS VALLEY TO NORTHEAST TX REGION ON DAY 5/WEDNESDAY.
…DAY 4/TUESDAY… THE GREATEST THREAT FOR SEVERE WEATHER APPEARS TO BE ON DAY 4/TUE. DETERMINISTIC AND ENSEMBLE GUIDANCE CONTINUES TO BE IN FAIRLY GOOD AGREEMENT…INDICATING INGREDIENTS IN PLACE FOR A POTENTIAL SEVERE WEATHER OUTBREAK ACROSS PORTIONS OF THE SOUTHERN/CENTRAL PLAINS TUESDAY AFTERNOON/EVENING. A STRONG UPPER TROUGH OVER THE WESTERN STATES WILL BECOME NEGATIVELY TILTED WHILE PIVOTING EAST TOWARD THE PLAINS. A 50-60+ KT SOUTHWESTERLY MIDLEVEL JET WILL OVERSPREAD NM INTO WRN TX/OK AND KS BY TUESDAY AFTERNOON. AT THE SURFACE…A LEE CYCLONE WILL DEVELOP OVER EASTERN CO AND SHIFT TO THE EAST/NORTHEAST INTO CENTRAL AND OR NORTHWEST KS. A SHARP DRYLINE WILL EXTEND SOUTH THROUGH SOUTHWEST/SOUTH-CENTRAL KS THROUGH WESTERN OK TO WEST-CENTRAL TX. MEANWHILE…A WARM FRONT SHOULD BE DRAPED ACROSS NRN KS INTO THE MID-MS VALLEY. MODEL VARIABILITY WITH THE PLACEMENT OF THE SURFACE LOW…WARM FRONT AND DRY LINE HAS RESULTED IN WEST AND NORTHWEST EXPANSIONS OF THE 15% AND 30% SEVERE-WEATHER THREAT AREAS.
SOUTH/SOUTHEASTERLY LOW LEVEL FLOW WILL TRANSPORT MID 60S TO NEAR 70 DEW POINTS NORTHWARD ACROSS THE WARM SECTOR…WITH MODERATE TO STRONG INSTABILITY IN PLACE BY AFTERNOON. STEEP MIDLEVEL LAPSE RATES AND STRONG…VERTICALLY VEERING SHEAR PROFILES WILL FAVOR SUPERCELLS CAPABLE OF ALL SEVERE HAZARDS INCLUDING VERY LARGE HAIL AND TORNADOES. THESE THREATS ARE EXPECTED TO PERSIST TUESDAY EVENING ACROSS THE CENTRAL AND SOUTHERN PLAINS AS STRONGER FORCING/HEIGHT FALLS SPREAD ACROSS THESE AREAS.
The following are maps from my F5 DATA Severe Weather forecasting software. These maps are output of actual forecast soundings. Some are from the GFS model, and others from the NAM-WRF model. Had to utilize both, as the GFS does not provide solutions for some of the indices and parameters. I will post the graphics, then explain after the graphics, what the various values indicate. There is one discrepancy I noted in my analysis however…the BRN (Bulk Richardson Number) values from the model solutions do not correspond with the information contained in the SPC outlook, regarding strong, vertically veering shear. The BRN number at the time of analysis (which may be erroneous that far out in the forecast period), indicates instability is much greater than shear, which would indicate more multi cell, or squall line type storms. The SPC outlook states the opposite, and the environment should be pretty favorable for supercell development. I believe we will see more agreement in the modeling on Mon-Tue, in regard to both sounding values, and area coverage and location. Between the outlook information, and what indices are showing, this may have the potential of becoming a PDS (Particularly Dangerous Situation), depending upon IF all the forecast soundings remain constant, and consolidate better.
CAPE (from Wikipedia): In meteorology, convective available potential energy (CAPE), sometimes, simply, available potential energy (APE), is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability, which makes it very valuable in predicting severe weather. It is a form of fluid instability found in thermally stratified atmospheres in which a colder fluid overlies a warmer one.
The current forecast CAPE indices over the risk area for Tuesday, approach 3000-4000 j/kg by late afternoon. Based on current standing values, CAPE indices of 2500+ indicate EXTREME CAPE, indicating the atmosphere to be very unstable and very buoyant.
The forecast Lifted Index (L.I.) indicates values of -8 to -12. This index is another measure of instability. Lifted Indices of -8 or less, indicate extreme instability. Combined with the buoyancy of the extreme CAPE values, this index provides “lift” in the atmosphere, which in this case is extreme. This is where the updrafts “basically”, so to say, come from which keep hail circulating upward within a supercell. These values are extreme enough to cause large, damaging hail during this outbreak.
L.I. (from Wikipedia):
The lifted index (LI) is the temperature difference between an air parcel lifted adiabatically Tp(p) and the environment Te(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.
The lifted index can be used in thunderstorm forecasting, however, convective available potential energy (CAPE) is considered by most as a superior measurement of instability and is preferred by many meteorologists for convection forecasting. However, LI is easier and faster to determine without using a computer, as determining CAPE requires integration from one level to another.
LI is generally scaled as follows:
- LI 6 or Greater, Very Stable Conditions
- LI Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely
- LI Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating, …)
- LI Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
- LI Less Than -6: Very Unstable, Severe Thunderstorms Likely With Lifting MechanismCRAVEN BROOKS SIGNIFICANT SEVERE PARAMETER INDEX
The Craven Brooks Index is another parameter which indicates pretty much the type of severe weather an area may experience. As you can see, values in the risk area are from 60,000-80,000. Based on the scale, values of 45,000 or greater indicates widespread severe weather is likely. Multiple significant hail, wind, and/or tornado events may also occur.
STP or Significant Tornado Parameter is just that. It pretty much indicates where significant tornado activity may occur. The values in the map, for the most part are averaging 2 t0 7. Values of 1 or greater indicates the potential for significant tornadoes (EF2-EF5) increase dramatically.
From Haby Hints: The Energy Helicity Index (EHI) is a combination of two indexes. By itself, it is the best index available for tornado prediction since it combines both CAPE and Helicity. The CAPE is the amount of pure instability present from a parcel of air that rises from the lower PBL. Helicity is the product of low level shearing (known as streamwise vorticity) and storm inflow directly into the streamwise vorticity. The Helicity is storm relative which means the Helicity is calculated from the storm’s frame of reference.
The EHI in the maps are running from 2-6. In the areas of 4 or greater, the likelihood of strong/long track tornadoes is very high.
So, the bottom line at the moment seems to point to an active severe weather outbreak, with a decent probability of large, damaging hail, damaging thunderstorm downdrafts, and a few strong, long tracked tornadoes in some areas of the probability area.
I intend to be updating on this again on Tuesday, when the outlook becomes the Day 1 Convective Outlook.
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS