Wednesday, May 07, 2008

Forecast soundings

Jason Simpson of ABC 33/40 was kind enough to share some forecast sounding data with me.

The NAM model is showing the following EHI values tomorrow at 4 p.m. CT:

Gadsden 2.5
Birmingham 3.9
Tuscaloosa 4.9

EHI is a combination of CAPE (an indicator of instability) and helicity (an indicator of a storm's potential to rotate) values.

Now I need to use a disclaimer here, but sometimes EHI values of 3-4 indicate the potential for strong tornadoes and EHI values over 4 indicate the potential for violent tornadoes. I am not saying this will happen, nor is anyone else, to my knowledge.

What this tells me is that models continue to persist with the idea that necessary parameters for a significant severe weather threat (including hail, wind, and tornado potential) will be in place tomorrow afternoon across North and Central Alabama.

Please remain weather aware. My sincere hope is that no matter what the weather does tomorrow, there are no injuries or fatalities in the State of Alabama. OR anywhere, as Dewdrop so appropriately pointed out in her comment!

---

Excerpt from Birmingham NWS morning discussion:

LET THE COMPLICATIONS OF THE FORECAST BEGIN...HAVE 
REALLY STRUGGLED WITH THURSDAYS SYSTEM AS THE LACK
OF A CLEAR SURFACE BOUNDARY IS STILL ELUSIVE.
HOWEVER...WITH PLENTY OF DIVERGENCE ALOFT WITH THE
WEAKENING UPPER SYSTEM AND WITH WHAT I WILL CALL A
STRONG MIDLEVEL CONVERGENCE BOUNDARY...THERE WILL BE
ENOUGH LIFT FOR WIDESPREAD THUNDER TO DEVELOP. ON
THE SEVERE SIDE...THERE IS PLENTY OF EVIDENCE AT THIS
POINT TO SUGGEST SUPERCELLS WILL THREATEN BEGINNING
MID MORNING THROUGH THE AFTERNOON. MOST OF THE SEVERE
INDICIES ARE ON THE HIGH SIDE ACROSS THE NORTHERN TWO
THIRDS OF THE AREA...BUT ARE TENDING TO LOSE THEIR
LUSTER AS THE EVENING WEARS ON. THIS IS
MAINLY DUE TO THE BEST DYNAMICS PULLING OUT TO THE
NORTH AND GENERALLY CONTINUING TO WEAKEN OVER TIME.
DON'T WANT TO BE LULLED INTO A FALSE SENSE OF SECURITY
HOWEVER...WITH PLENTY OF INSTABILITY AND DEEP LAYER
SHEAR REMAINING THROUGH MUCH OF THE AFTERNOON. THIS
WILL ESPECIALLY BE THE CASE IF THE CENTRAL AND EASTERN
AREAS STAY CAPPED THROUGH THE MORNING HOURS AND WE STAY
RAIN FREE EARLY. SUPERCELL DEVELOPMENT ALMOST APPEARS
TO BE A GIVEN AT THIS POINT AND MOST APPEAR TO HAVE THE
FEEL OF BEING LONG LIVED...ESPECIALLY FURTHER NORTH.
QUESTIONS REMAIN ABOUT THE TORNADIC POTENTIAL...AS I
PUT A LOT OF STOCK IN THE 0 TO 1 KM SHEAR VECTORS.
WITH THE SURFACE LOW LAGGING THERE WILL BE A SMALL
WINDOW FOR THE SURFACE WINDS TO BEGIN BACKING WITH TIME.
FEEL LIKE THIS WILL BE THE CASE ACROSS MAINLY THE
NORTHERN HALF OF THE AREA AND ONLY THROUGH THE MORNING
AND POSSIBLY INTO THE EARLY AFTERNOON HOURS. THIS WILL
CAUSE THE LOW LEVEL SHEAR VALUES TO INCREASE SIGNIFICANTLY
AND TORNADOES WILL BE A THREAT. HOWEVER...LOOKS AS IF
THE WINDS WILL VEER QUICKLY IN THE AFTERNOON...THEREBY
DECREASING THIS THREAT AS THE AFTERNOON AND
EVENING CONTINUES. IF THE VEERING IS AS SIGNIFICANT
AS SOME MODEL OUTPUT SUGGESTS...WE MAY TRANSITION FROM
MAINLY SUPERCELLS INTO A MORE LINEAR LINE OF CONVECTION
BY EVENT END.

---

Check out the latest Short Range Ensemble products from
the SPC, valid for 4 p.m. Thursday:





2 comments:

Dewdrop said...

Or anywhere else for that matter... scary numbers, Mike.

Mike said...

Amen x 2, Dew!