April 6th-7th Severe Thread

[andyhb]

Well this one seems to have snuck up on us a bit, the strong trough moving across the Four Corners tomorrow will become increasing neutral/negatively tilted and spread strong shear profiles across a large section of the Gulf Coast region by tomorrow evening associated with a rapid developing surface low. In addition to that, strong moisture advection south of a retreating warm front (speed of the movement will be crucial with the expectation of a large amount of precip along it) should provide at least modest instability despite average mid level lapse rates. The trough orientation/low level wind fields appear highly favorable for supercells and potentially a few significant tornadoes across LA and S MS into S AL later into the evening (somewhat reminiscent of Christmas 2012 in terms of threat area and the concern about warm frontal movement). This thread can be moved at the mods' discretion since this will be a likely be a multi-region event, perhaps continuing further into the SE/EC on Monday. In any case, this D1 is a borderline moderate risk for tornadoes and is an excellent discussion from Peters and Dr. Marsh.

 

DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK
1257 AM CDT SUN APR 06 2014

VALID 061200Z - 071200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE MIDDLE AND UPPER TX
COAST AREAS INTO THE CENTRAL GULF COAST STATES...

...SYNOPSIS...
A POSITIVELY-TILTED SHORTWAVE TROUGH LOCATED OVER NM INTO NWRN
MEXICO AT 12Z THIS MORNING SHOULD BECOME NEUTRALLY ORIENTED TODAY AS
IT TRACKS EWD THROUGH TX/OK. STRONGEST HEIGHT FALLS /60-120 METERS
AT 500 MB/ WILL OCCUR DURING THE SECOND HALF OF THE PERIOD AS THE
SHORTWAVE TROUGH MOVES ENEWD REACHING THE ARKLATEX TO THE OZARKS
REGION BY 12Z MONDAY. A DEVELOPING SURFACE LOW WILL TRACK FROM DEEP
S TX TO SE/E TX BY EARLY EVENING...AND THEN UNDERGO GREATER
DEEPENING INTO THE OVERNIGHT PERIOD REACHING WRN TN AT 12Z MONDAY AS
A 1000 MB LOW. A SURFACE BOUNDARY SHOULD EXTEND NEWD FROM THE DEEP
S TX LOW AT 12Z TODAY ALONG THE TX COAST AND THEN EWD ALONG THE
CENTRAL/NERN GULF COAST. STRENGTHENING SLY LOW LEVEL WINDS THROUGH
THE FORECAST PERIOD INTO THE LOWER MS VALLEY/CENTRAL GULF STATES
WILL SUPPORT THE MORNING COASTAL FRONT TO ADVANCE INLAND...AS THE
SURFACE LOW TRACKS TO THE MID-UPPER TX COASTAL PLAIN THIS AFTERNOON.
ONGOING SHOWERS/EMBEDDED TSTMS FROM E TX INTO THE CENTRAL GULF
STATES AT 12Z TODAY SUPPORTED BY LOW LEVEL WAA/ISENTROPIC ASCENT
SHOULD PERSIST INTO THE AFTERNOON...AND TEND TO LIMIT THE NWD
ADVANCEMENT OF THE WARM SECTOR THROUGH SRN PARTS OF LA/MS AND SRN AL
THROUGH 07/00Z. FURTHER MOISTENING IS EXPECTED FARTHER INLAND
TONIGHT AS THE LLJ STRENGTHENS.

...MID-UPPER TX COASTAL AREA TO CENTRAL GULF COAST STATES...
A FEW ONGOING...STRONGER ELEVATED STORMS WILL BE POSSIBLE AT 12Z
TODAY ACROSS PORTIONS OF SOUTH CENTRAL AND SOUTHEAST TX ALONG THE
ERN EXTENT OF STEEPER MIDLEVEL LAPSE RATES WHERE MUCAPE SHOULD BE
AROUND 1000 J/KG AND CLOUD-BEARING SHEAR WILL BE SUFFICIENT FOR
MARGINALLY SEVERE HAIL. ALTHOUGH SURFACE HEATING MAY BE LIMITED
TODAY...MID-UPPER 60S SURFACE DEW POINTS MOVING INLAND ACROSS THE
MID-UPPER TX COASTAL PLAINS /WITHIN 60-90 MILES OF THE COAST/
COMBINED WITH STEEP MIDLEVEL LAPSE RATES WILL RESULT IN MODERATE
INSTABILITY. THIS COMBINED WITH WEAK HEIGHT FALLS SUGGESTS SURFACE
BASED STORMS SHOULD DEVELOP BY EARLY AFTERNOON ACROSS THE TX PORTION
OF THE SLIGHT...WITH STRENGTHENING SWLY MIDLEVEL WINDS ATOP A 30-35
KT SLY LLJ RESULTING IN STRONG DEEP SHEAR FOR ORGANIZED
STORMS...INCLUDING SUPERCELLS. ALL SEVERE HAZARDS WILL BE POSSIBLE
WITH THIS ACTIVITY THROUGH THE AFTERNOON.

MEANWHILE...MODERATE INSTABILITY SHOULD ALSO DEVELOP INLAND ACROSS
SRN LA...SERN MS AND SWRN AL THIS AFTERNOON. ALTHOUGH FORCING ALOFT
SHOULD BE WEAK PRIOR TO 07/00Z...MUCH OF THE CONVECTION ACROSS THIS
AREA WILL BE SUPPORTED BY LOW LEVEL WAA ALONG THE STRENGTHENING SLY
LLJ. THE AVAILABLE INSTABILITY AND INCREASING LOW LEVEL SHEAR /0-1
KM SRH AROUND 200 M2 PER S2/ AND DEEP LAYER SHEAR SUGGEST ORGANIZED
STORMS WILL BE POSSIBLE. THE SEVERE HAZARDS SHOULD TEND TO BE
DAMAGING WINDS AND/OR A TORNADO THREAT DURING THE AFTERNOON...WITH
HAIL A LOWER PROBABILITY GIVEN WEAKER LAPSE RATES.

PRECIPITABLE WATER VALUES OF AT LEAST 1.5 INCH ARE EXPECTED TO
SPREAD NNEWD ACROSS THE LOWER MS VALLEY REACHING SERN AR AND NRN
MS/AL BY EARLY EVENING. STRENGTHENING SSWLY LOW LEVEL WINDS WILL
SUSTAIN THIS NWD RETURN OF MOISTURE ACROSS THE LOWER MS
VALLEY/CENTRAL GULF COAST STATES TONIGHT...WITH SURFACE DEW POINTS
IN THE MIDDLE 60S REACHING NORTH CENTRAL-NERN MS TO WRN/SRN AL AFTER
07/06Z. DESPITE INSTABILITY TENDING TO REMAIN MARGINAL FARTHER FROM
THE GULF COAST...THE CONTINUED INFLUX OF HIGHER THETAE AIR WELL
INLAND INTO THE OVERNIGHT PERIOD WILL SUPPORT AN AFTER-DARK TORNADO
THREAT ACROSS THE LOWER MS VALLEY/CENTRAL GULF COAST STATES. AS THE
LLJ STRENGTHENS...A FEW STRONG/SIGNIFICANT TORNADOES WILL BE
POSSIBLE FROM SERN LA...SRN MS AND SWRN AL FROM AROUND 07/00Z
THROUGH 12Z MONDAY. GIVEN THE TRACK OF THE SURFACE LOW AND INFLUX
OF RICH MOISTURE FARTHER INLAND AS THE NIGHT PROGRESSES...INCREASING
HODOGRAPH CURVATURE WILL SUPPORT A TORNADO THREAT INTO COLUMBUS AND
TUPELO MS AFTER 06Z AND THROUGH THE EVENING/TONIGHT INTO AL AND THE
WRN FL PANHANDLE.

..PETERS/MARSH.. 04/06/2014

[1900hurricane]

Already a Severe Thunderstorm Watch up for me, although it looks like it's for elevated hailers. Still north of the warm front here.

[1900hurricane]

As for the main threat further east into Louisiana, I'm particularly concerned with the period just after dark, roughly from 00Z to 06Z. The setup does look pretty impressive. It's probably not be the most likely course of action, but I wouldn't be terribly surprised to see the SPC upgrade to a Moderate Risk within the hatched area for a later outlook.

[Jon]

CIPS Top 8 Analogs Prob of Svr report from 4/2 12z run of the GFS nailed it, just happened to notice a 60% probability mark and saved the image last Wednesday.

 

[wxmeddler]

I was watching this since Friday. NAM at that time had very strong Staff and Flag Hodo's at 00z tonight in the swamps of LA under strong H5 forcing. The only thing that lacked was CAPE in that it was less than 500 j/kg b/c of weak ML lapse rates.

[SmokeEater]

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHEAST
LOUISIANA...SOUTH MISSISSIPPI AND THE ADJACENT COASTAL WATERS.

.DAY ONE...TODAY AND TONIGHT

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE ENTIRE
OUTLOOK AREA THROUGH TONIGHT. THE PRIMARY THREAT WILL BE LARGE
HAIL AND STRONG WIND GUSTS FROM THE STRONGEST STORMS. AN ISOLATED
STRONG TORNADO OR TWO WILL BE POSSIBLE AS A WARM FRONT MOVES NORTH
FROM THE GULF COAST DURING THE AFTERNOON. SEVERE THUNDERSTORM OR
TORNADO WATCHES MAY BE POSTED BY THE STORM PREDICTION CENTER LATER
TODAY.
 

[wxmeddler]

Mods: Merge topic with SE Subforum topic on same event?

http://www.americanwx.com/bb/index.php/topic/43405-april-6-7-severe-obsdiscussion/

[1900hurricane]

A small piece of the warm sector has made it over southeast Texas. With surface cyclogenesis continuing to progress, it shouldn't be too long before the warm front starts to come ashore across Louisiana too.

Sent from my iPhone

[wxmeddler]

77/74 in Bay City,TX (KBYY) (SW of Galveston)

[Stebo]

77/74 in Bay City,TX (KBYY) (SW of Galveston)

Soupy to say the least.

[andyhb]

Yeah 70s dews present across the northern Gulf of Mexico, I'm interested to see how far those get pulled once the surface low develops and moves northward this evening, the radar across LA, S MS and S AL has cleared pretty nicely from WF precip.

[crownweather]

Significant Tornado Parameter numbers gradually are increasing along the Gulf Coast.  Could be a long night!!  

 

[1900hurricane]

KLIX put up an 18Z sounding, and while it could use some more/fatter CAPE (surprise, surprise) I'd say that's a pretty good environment once the warm front passes through.

 

[1900hurricane]

Tornado watch now out:

 

 

DISCUSSION...THUNDERSTORMS DEVELOPING ALONG THE SABINE RIVER VALLEY
ARE EXPECTED TO BECOME PROGRESSIVELY SURFACE BASED AS THEY MOVE EAST
AND ENCOUNTER A WARM FRONT LIFTING NORTHWARD. THE AIR MASS TO THE
SOUTH OF THIS FRONT IS MOIST AND UNSTABLE...AND GIVEN THE STRONG
VERTICAL SHEAR ALONG THE FRONT...SUPERCELLS ARE POSSIBLE WITH A RISK
FOR TORNADOES AND DAMAGING WINDS.

 

WWUS40 KWNS 062026
WWP4

TORNADO WATCH PROBABILITIES FOR WT 0064
NWS STORM PREDICTION CENTER NORMAN OK
0323 PM CDT SUN APR 06 2014

WT 0064
PROBABILITY TABLE:
PROB OF 2 OR MORE TORNADOES : 40%
PROB OF 1 OR MORE STRONG /F2-F5/ TORNADOES : 20%
PROB OF 10 OR MORE SEVERE WIND EVENTS : 30%
PROB OF 1 OR MORE WIND EVENTS >= 65 KNOTS : 10%
PROB OF 10 OR MORE SEVERE HAIL EVENTS : 20%
PROB OF 1 OR MORE HAIL EVENTS >= 2 INCHES : <05%
PROB OF 6 OR MORE COMBINED SEVERE HAIL/WIND EVENTS : 60%

&&
ATTRIBUTE TABLE:
MAX HAIL /INCHES/ : 1.0
MAX WIND GUSTS SURFACE /KNOTS/ : 60
MAX TOPS /X 100 FEET/ : 450
MEAN STORM MOTION VECTOR /DEGREES AND KNOTS/ : 24035
PARTICULARLY DANGEROUS SITUATION : NO

&&
FOR A COMPLETE GEOGRAPHICAL DEPICTION OF THE WATCH AND
WATCH EXPIRATION INFORMATION SEE WOUS64 FOR WOU4.

$

[1900hurricane]

I don't think we need to worry about the Jackson area just yet, as they are still well north of the warm front and all the storms around there are elevated rather than surface based. If/when the warm front clears the area though, that's when things might get interesting.

[andyhb]

Echo tops from the svr warned cell in SW AL are hitting 45k ft from BMX, that tells me along with the hail spike that mid level lapse rates are at least half decent over the area, which is supported by mesoanalysis with 6.5-7+ C/km mid level lapse rates over basically all of the risk area.

[andyhb]

Tornado watch likely coming for the Gulf Coast...

 

MESOSCALE DISCUSSION 0298
NWS STORM PREDICTION CENTER NORMAN OK
0624 PM CDT SUN APR 06 2014

AREAS AFFECTED...SOUTHEAST LOUISIANA...SOUTHERN
MISSISSIPPI....SOUTHERN ALABAMA...AND FAR WESTERN FLORIDA PANHANDLE

CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY

VALID 062324Z - 070130Z

PROBABILITY OF WATCH ISSUANCE...80 PERCENT

SUMMARY...THUNDERSTORMS ARE EXPECTED TO INITIATE OVER THE NEXT FEW
HOURS...INCREASING IN COVERAGE THROUGH THE EVENING. TORNADOES...A
FEW DAMAGING WIND GUSTS...AND ISOLATED LARGE HAIL WILL BE POSSIBLE.

DISCUSSION...LATEST OBSERVATIONS SUGGEST THAT THE SURFACE WARM FRONT
/LOCATED ALONG THE NORTHERN GULF COAST AT 23 UTC/ IS BEGINNING TO
LIFT NORTHWARD. SOUTH OF THIS BOUNDARY...THE AIRMASS IS
CHARACTERIZED BY SURFACE BASED INSTABILITY OF UP TO 2000 J/KG...DEEP
LAYER SHEAR VALUES AT OR ABOVE 45 KTS...AND EFFECTIVE STORM-RELATIVE
HELICITY VALUES BETWEEN 200-300 M2/S2. GIVEN THIS FAVORABLE
ENVIRONMENT...WITH THE INITIATION OF SURFACE BASED CONVECTION
BECOMING INCREASINGLY LIKELY DURING THE NEXT FEW HOURS /INCLUDING
SUPERCELLS/...A TORNADO WATCH MAY BE NECESSARY WITHIN THE NEXT HOUR.

..MARSH/THOMPSON.. 04/06/2014

[andyhb]

This recent VWP from Jackson is a good indicator of the type of wind fields we will be dealing with.

 

[1900hurricane]

Yeah, it's getting closer to go time. This one really excites me for some reason. Maybe it's just because it's happening on a lazy Sunday evening for me with nothing else to do but follow how things unfold. Whatever the reason, I'm not complaining!

[1900hurricane]

Also, nice profiler; it really does give a nice picture of what we might be dealing with dynamically. I'm also very interested in seeing what the impending 00Z soundings look like.

[SmokeEater]

And the first TW goes to West Texas? lol

[Hvward]

[SouthernNJ]

Looks like a developing supercell SW of Montgomery AL.

That is more of a SE forum storm though.

[Chinook]

supercell developing near Madden Mississippi, 45 mi northeast of Jackson radar site. This looks like it has high reflectivity and large hail.

[andyhb]

The main story is likely going to be the cells developing on confluence bands in LA currently.

[tornadotony]

The main story is likely going to be the cells developing on confluence bands in LA currently.

This.

[SouthernNJ]

Pretty good rotation near Philadelphia MS.

[andyhb]

Pretty good rotation near Philadelphia MS.

 

Elevated from the looks of it.

[wxmeddler]

All the forces are just getting aligned. Prime time was always supposed to be 2z to 8z.

[Chinook]

For those of you who like helicity-- here is the RAP analysis of about 1500 m2/s2 of helicity, most of which can't be used by stable surface air parcels