Virtual Chase Moderate Risk - 27th March 2009

[Paul Sherman]

As I thought on Monday, Tomorrow sees a pretty Strong MODERATE Risk

Have had a peak at some Forecasted Hodographs for Mississippi around the Natchez area for tomorrow and they show very enhanced Sickle shaped Hods, Indicatve of possible Strong Long Tracked Tornadoes across the Area, Would pick a starting point South and East from Stuttgart seeing as we got to know the area last year 1st hand and from there to the MS/AR/LA Border it is surprisingly flat with paddy and cotton fields all the way to the Mississippi River.

Would expect SPC To go with a 15% Hatched Tornado area for tomorrow

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1229 PM CDT THU MAR 26 2009

VALID 271200Z - 281200Z

...THERE IS A MDT RISK OF SVR TSTMS FRI NIGHT ACROSS PARTS OF SE

ARKANSAS...NE LOUISIANA....W CNTRL AND NW MISSISSIPPI...AND WRN

TN....

...THERE IS A SLGT RISK OF SVR TSTMS FRI/FRI NIGHT...FROM PARTS OF

THE SERN PLAINS THROUGH THE LWR MS/MID SOUTH AND GULF STATES....

...SYNOPSIS...

WITHIN A LINGERING LARGE-SCALE TROUGH THAT ENCOMPASSES MUCH OF THE

INTERIOR U.S...A STRONG...COLD SHORT WAVE TROUGH IS NOW DIGGING

THROUGH THE GREAT BASIN AND CENTRAL/SOUTHERN ROCKIES REGION. MODELS

CONTINUE TO INDICATE THAT...AS THIS SYSTEM PROGRESSES INTO AND

THROUGH THE BASE OF THE LARGER SCALE TROUGH LATER TONIGHT INTO

FRIDAY...IT WILL COME INCREASINGLY IN PHASE WITH A BRANCH OF

STRONGER UPPER WESTERLIES EMANATING FROM THE SUBTROPICAL PACIFIC.

AS THIS OCCURS...MODELS GENERALLY INDICATE THAT AN EMBEDDED LOWER/

MID TROPOSPHERIC CYCLONE WILL DEEPEN SUBSTANTIALLY...PARTICULARLY

FRIDAY NIGHT...PROBABLY ACCOMPANIED BY SIGNIFICANT SECONDARY SURFACE

CYCLOGENESIS FROM PARTS OF THE ARKLATEX THROUGH UPPER PORTIONS OF

THE LOWER MISSISSIPPI VALLEY.

THIS EVOLVING SYSTEM WILL BE PRECEDED BY COUPLED POLAR AND

SUBTROPICAL IMPULSES THROUGH PARTS OF THE MID SOUTH AND

SOUTHEASTERN STATES EARLY IN THE PERIOD. WHILE THE INFLUENCE OF

THESE FEATURES ON CONVECTIVE POTENTIAL IS STILL A SOURCE OF FORECAST

UNCERTAINTY...THE PRESENCE OF CONSIDERABLE POTENTIAL INSTABILITY

ACROSS MUCH OF THE GULF OF MEXICO AND GULF STATES SUPPORTS AN

ELEVATED RISK FOR SEVERE STORMS. AND...POTENTIAL EXISTS FOR AN

OUTBREAK OF SEVERE STORMS...INCLUDING THE RISK FOR A FEW STRONG

TORNADOES AND DAMAGING WINDS...IN ADDITION TO LARGE HAIL.

...EASTERN GULF STATES...

A STRONG/SEVERE CLUSTER OF STORMS MAY BE ONGOING ACROSS PARTS OF

ALABAMA/WESTERN FLORIDA AND GEORGIA EARLY FRIDAY. BEFORE LARGE-

SCALE FORCING ASSOCIATED WITH SUPPORTING LEAD UPPER IMPULSES

WEAKENS/SHIFTS NORTHEASTWARD AND EASTWARD INTO A PROGRESSIVELY MORE

STABLE ENVIRONMENT...A RISK FOR DAMAGING WINDS...PERHAPS ISOLATED

TORNADOES...MAY LINGER INTO MIDDAY.

...SOUTHERN PLAINS THROUGH THE LWR MS VALLEY AND MID SOUTH...

A CONTINUING RETURN FLOW OFF THE GULF OF MEXICO...BENEATH STEEP

MID-LEVEL LAPSE RATES ASSOCIATED WITH ELEVATED MIXED LAYER AIR

ADVECTING EAST NORTHEASTWARD OFF THE MEXICAN PLATEAU...IS EXPECTED

TO MAINTAIN MODERATE INSTABILITY INTO THIS PERIOD. CAPE OF

1000-2000 J/KG IS EXPECTED OVER A FAIRLY BROAD AREA FROM THE

SOUTHEASTERN PLAINS THROUGH THE LOWER MISSISSIPPI VALLEY/CENTRAL

GULF COAST REGION. HOWEVER...MUCH OF THIS INSTABILITY ALONG AND

EAST OF THE LOWER MISSISSIPPI VALLEY MAY BE BASED ABOVE A SHALLOW

STABLE NEAR SURFACE LAYER...NORTH OF A CONVECTIVELY ENHANCED SURFACE

BOUNDARY LINGERING NEAR THE CENTRAL/EASTERN GULF COAST.

WITH THE MAIN UPPER FORCING LAGGING TO THE WEST...CONVECTIVE

POTENTIAL THROUGH MUCH OF THE DAY MAY BE FAIRLY LOW.

AN EXCEPTION MAY BE ALONG A DRY LINE OR PRE-FRONTAL WIND SHIFT

ACROSS EASTERN TEXAS INTO SOUTHEAST OKLAHOMA...WHERE GUIDANCE

SUGGESTS STORMS MAY INITIATE AS EARLY AS MIDDAY...IN AN ENVIRONMENT

FAVORABLE FOR SUPERCELLS. DESPITE SOMEWHAT MODEST SOUTHERLY

LOW-LEVEL FLOW...STEEP LAPSE RATES ARE EXPECTED TO BE FAVORABLE FOR

THE PRODUCTION OF LARGE HAIL AND DAMAGING WIND GUSTS. AS

LARGE-SCALE FORCING INCREASES WITHIN THE EXIT REGION OF A STRONG

MID/UPPER POLAR JET STREAK...AND AHEAD OF AN EASTWARD SURGING COLD

FRONT...ONE OR MORE BANDS AND CLUSTERS OF STORMS ARE EXPECTED TO

FORM AND SPREAD NORTHEASTWARD THROUGH THE OZARK PLATEAU. ACTIVITY

ALSO SHOULD DEVELOP SOUTHWARD THROUGH THE LOWER MISSISSIPPI VALLEY

DURING THE LATE AFTERNOON AND EVENING HOURS.

OF PRIMARY CONCERN IS THE DEVELOPMENT AND INTENSIFICATION OF A

SOUTHERLY LOW-LEVEL JET NEAR THE SECONDARY CYCLONE DURING THE MID/

LATE EVENING HOURS...WHICH WILL YIELD ENLARGING CLOCKWISE CURVED

LOW-LEVEL HODOGRAPHS. THIS IS EXPECTED TO CONTRIBUTE TO INCREASING

TORNADIC POTENTIAL...INCLUDING THE RISK FOR A FEW STRONG

TORNADOES...ACROSS PARTS OF SOUTHEASTERN ARKANSAS/NORTHEAST

LOUISIANA/WEST CENTRAL AND NORTHWEST MISSISSIPPI AND WESTERN

TENNESSEE BY THE 28/03-06Z TIME FRAME.

RAPID NOCTURNAL WARM SECTOR BOUNDARY LAYER DESTABILIZATION APPEARS

POSSIBLE...AS STRONG UPPER FORCING...AHEAD OF AN INCREASINGLY

NEGATIVELY TILTED UPPER TROUGH PIVOTING AROUND THE MID-LEVEL

CYCLONE. AND...SEVERAL LINES/CLUSTER OF STRONG/SEVERE STORMS

CAPABLE OF PRODUCING DAMAGING WINDS/HAIL AND ISOLATED TORNADOES

APPEAR LIKELY TO SPREAD ACROSS THE LOWER MISSISSIPPI VALLEY/CENTRAL

GULF COAST AND MID SOUTH FRIDAY NIGHT INTO EARLY SATURDAY.

Please Discuss

Paul Sherman

[kold weather]

It does look like being possibly a very good set-up, small chance of it going high risk but for now moderate seems reasonable because there has to be an awful lot of uncertainty because of the risk of overnight MCS and the cloud shield that could be left over behind them. Will have to see in about 12-18hrs time.

[NL]

Overnight MCS have put the SPC in the position of caution as they backtrack from moderate to 'strong' slight. Although this has occured SPC state that the downgrade should not be focused on as moderate severe probabilities will exsist.

SPC should have a better idea on the effects of the MCS have on todays developments, an upgrade seems likely.

In contrast blizzard conditions have started to affect w/Kansas s/Colorado NW/Oklahoma into the Texas panhandle...Thundersnow is to be expected with 1-2" of snow p/hr....Sno-nado anyone!!!

[Paul Sherman]

Upgraded again to MODERATE Risk

SPC Have a 10% hatched area but todays risk is a very messy affair with a Surging Cold Front and embedded Supercells within the Line, more of a Threat is going to be some Large Hail and Wind Damage as the Line progresses Eastwards, would expect some Bow Echo like Features along the line (Which in itself can produce low end EF2 Damage)

If I was out there I would not even attempt a Chase today, as along with nightmare Storm Motions, Crap Visibility, you also have dreadful Chase Terrain. One to Watch but NOT Chase IMO

Paul S

[Paul Sherman]

MD Issued

MESOSCALE DISCUSSION 0294

NWS STORM PREDICTION CENTER NORMAN OK

1024 AM CDT FRI MAR 27 2009

AREAS AFFECTED...ERN TX

CONCERNING...SEVERE POTENTIAL...WATCH LIKELY

VALID 271524Z - 271700Z

A THREAT OF MAINLY HAIL AND DAMAGING WIND GUSTS IS EXPECTED TO

DEVELOP OVER THE NEXT FEW HOURS...AND A WW MAY BECOME NECESSARY.

A SLIGHTLY ELEVATED CLUSTER OF THUNDERSTORMS HAS ALREADY FORMED JUST

N OF A WEAK SURFACE BOUNDARY...AND ON THE SRN END OF MID/UPPER LVL

HEIGHT FALLS AHEAD OF THE APPROACHING TROUGH. MEANWHILE...A SFC COLD

FRONT WAS DEVELOPING AND SURGING SEWD OUT OF NW/W-CNTRL TX.

MORNING SOUNDINGS SHOW VERY STEEP MID/UPPER LAPSE RATES IN PLACE

WITH AMPLE DEEP LAYER SHEAR/LONG HODOGRAPHS FOR ORGANIZED SEVERE

CONVECTION. AS SOME HEATING AND DESTABILIZATION TAKES PLACE ACROSS

ERN TX...THE SEVERE THREAT WILL INCREASE. ALTHOUGH TEMPERATURES IN

THE LOWER 70S F WILL BE SUFFICIENT FOR SURFACE BASED PARCELS...THE

LOW LEVEL SHEAR AND OVERALL WIND SPEEDS WILL BE LOW INITIALLY.

SURFACE WINDS WERE AT OR BELOW 5 KT IN GENERAL OVER ERN/SERN TX AS

OF 15Z...WITH RELATIVELY WEAK LOW LEVEL HODOGRAPHS FORECAST BY THE

NAM AND RUC MODELS THROUGH THE AFTERNOON. THE GFS SHOWS STRONGER LOW

LEVEL SHEAR WITH A THREAT OF TORNADOES.

WITH EXTENSIVE LOW LEVEL CLOUDINESS IN PLACE...AND WEAK LOW LEVEL

FLOW...THE INITIAL THREAT WILL BE LARGE HAIL AND DAMAGING WIND GUSTS

AS STORMS BECOME MORE ORGANIZED. THE TORNADO THREAT WILL INCREASE

WITH TIME AND EWD EXTENT...MAINLY OVER FAR ERN TX INTO LATER

TODAY...WHEN LOW LEVEL FLOW INCREASES.

[NL]

Latest visible satellite image shows cirrus clouds ahead of the MCS affecting east Texas at the moment, certainly looks that dinurnal heating will play ball today and this could turn out to be quite a severe episode.

[Tony Gilbert]

Yes agreed!

When the jet stream dips this far south it will utilise the best possible warm moist air feed from the Gulf. Even though early season, this has all the hallmarks of being an interesting set up (virtual chase). SPC priorities Triple point for the most potent risk of tornado development. Current forecast synopsis delivers the TP into Louisiana by early evening where very strong CAPE values are being predicted by the RUC , which is well beyond what the NAM model had suggested earlier. Contrary to common belief, it is not always the front left exit region of the jet stream that can develop the best upper lift. On this occasion the 'RIGHT' exit region fans outward creating lift within the diffluent zone. This is timed in nicely with the northward 70 dew point plume and strong CAPE.

Whilst wind veer looks rather weak at surface the dynamics from mid levels look to more than make up for this discretion. Given the level of deep layer shear we have the potential for supercell development where tornado genesis relies less on the environmental shear and more on the vorticity produced by the downdraft of the storm itself. Certainly one to watch for if you can manage to keep awake till the early hrs tonight. :lol:

[NL]

New MD for severe watch possible for that very area..

MESOSCALE DISCUSSION 0298

NWS STORM PREDICTION CENTER NORMAN OK

0234 PM CDT FRI MAR 27 2009

AREAS AFFECTED...SRN LA

CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE

VALID 271934Z - 272030Z

REMNANT CONVECTIVE OUTFLOW BOUNDARY REMAINS DRAPED ACROSS SRN LA

FROM VERNON/BEAUREGARD PARISH COUNTY LINE...SEWD TO ST LANDRY

PARISH. THIS WIND SHIFT HAS YET TO SURGE VERY FAR INLAND AND WILL

LIKELY FOCUS EWD-MOVING THUNDERSTORM CLUSTER AS IT PROGRESSES

DOWNSTREAM FROM JASPER COUNTY TX. VERY MOIST BOUNDARY LAYER AND

REASONABLE SHEAR MAY SUPPORT A LOCALIZED TORNADO THREAT WITH ANY

SUPERCELLS THAT EVOLVE ALONG THIS BOUNDARY.

[Tony Gilbert]

Yes, some big changes in the pattern is occurring right now!

[Gorky]

Dangerous cell North of Houston with a very nice couplet.

[sammie]

Dangerous cell North of Houston with a very nice couplet.

I'm about to ask two very stupid questions. What is a couplet and what am I looking on the radar cap at to see it?

[Gorky]

A couplet is an area on the radar where winds are close together and blowing in opposing directions, indicating circulation.

On my radar, red colours indicate winds moving towards the radar site, green represent colours moving away from the radar site. Where you have bright colours (intense winds) of both colours next to each other, it is a good indication of rotation with the possibilty of a tornado.

Note how on the radar shot I posted, the bright green next to the bright red is located in the hook echo of the storm (the clear V shaped area at the southern end of the storm which the rain wraps around). This is an indication of a strong rotating updraft (mesocyclone). Most tornadoes form from such mesocyclones...

[sammie]

A couplet is an area on the radar where winds are close together and blowing in opposing directions, indicating circulation.

On my radar, red colours indicate winds moving towards the radar site, green represent colours moving away from the radar site. Where you have bright colours (intense winds) of both colours next to each other, it is a good indication of rotation with the possibilty of a tornado.

Note how on the radar shot I posted, the bright green next to the bright red is located in the hook echo of the storm (the clear V shaped area at the southern end of the storm which the rain wraps around). This is an indication of a strong rotating updraft (mesocyclone). Most tornadoes form from such mesocyclones...

Ahh I see it! Thank you very very much! <3

[dave48]

Poor Oklahoma! Blizzard warning on one side, tornado warning on the other!

[Tony Gilbert]

20 tornadoes reported through this period. Thankfully no injuries!

An interesting cluster noted to the far east coast of the USA which was well off the earlier SPC risk box. At least 3 tornadoes within the suggested risk zone on this thread.

Ps. Sorry I was unable to virtual chase late.