03/21/05 FCST: OK/TX/MO/AR/LA

[Gabe Garfield]

This may be a bit premature, but the GFS has had some run-to-run consistency of late.

A low-amplitude shortwave trough associated with the southern jet stream is forecast to impact the southeastern United States early next week. The GFS forecasts a negatively tilted trough with an associated 100 kt WSW speed maximum at 250 mb over the southeast by 00z 3/23. In addition to this, a very potent low-level jet (on the order of 40 kts) is forecast to develop and draw relatively rich boundary layer moisture inland from the Gulf of Mexico. GFS forecasts dewpoints over 60 F in the northern parts of MS/AL/LA, and Tds close to 70 F nearer to the Gulf coast. The surface low is not forecast to be that deep (on the order of 1000 mb), but the strong pressure gradient should more than make up for it.

Currently, dewpoints in the C/N Gulf AOA 70 F, so moisture should not be an issue.

SUN NIGHT INTO MON...MOST INTRIGUING PERIOD OF THE EXTENDED. THE OVERALL PATTERN IS AGREED UPON...HOWEVER THE DEVIL IS IN THE DETAILS. THE 12Z/15 GFS IS QUITE AGGRESSIVE...MUCH MORE SO THAN ITS 00Z/15 VERSION...HAVING A NEGATIVELY TILTED DISTURBANCE EJECTING OUT OF THE SOUTHERN ROCKIES THROUGH THE SOUTHERN OZARKS. AS PREVIOUSLY MENTIONED...THE UKMET AND ECMWF AGREE WITH THE OVERALL PATTERN... HOWEVER THEY ARE NOT NEARLY AS AGGRESSIVE AS THE 12Z/15 GFS. NEVERTHELESS...THE PATTERN OFFERED BY THE OPS MED RANGE GUIDANCE AND ENSEMBLES IS CONDUCIVE TO SUPPORTING SEVERE WEATHER IN THE LOWER MS RIVER VALLEY. HAVE MENTIONED THIS IN THE LATEST HWOJAN.

Given the time of year, this one bears watching.

Gabe

 

[Gabe Garfield]

The 06z and 12z ETA forecast a much less potent vorticity maximum, so the trend is somewhat downward. However, it still appears that there should be enough moisture/instability/shear to warrant at least some risk of severe weather for Monday.

Right now, the s/w trough that is expected to bring the severe weather together, is currently cutoff from the mean flow several hundred miles west of California. Another s/w trough to its north (near the Aleutians) is keeping the southern s/w trough cut off, but as this system propagates eastward, cold air advection on the backside of this trough should cause the heights to fall on the western side of the southern trough causing it to open up and become progressive.

How this will evolve is still in question. The latest GFS suggests that the southern s/w trough will "shear out" and move into the central U.S. Pay close attention to what happens with the northern impusle, because this will have an impact on what happens with the southern impulse.

Regardless of what happens with this system, the GFS forecasts a rather persistent surface high to develop just south of Florida, which should keep moisture return at a maximum across the Southern Plains and the Southeast. It appears that this will be the true beginning of the severe weather season.

Gabe

 

[Jeff Snyder]

The 6z NAM actually is pretty interesting, with a strong low across central OK/KS, resulting in very strong southerly low-level flow (30-40kts at 850mb) beneath southwesterly flow at higher altitudes. SBCAPE from the NAM is >1000 across central OK, though is limited by meager surface moisture (sfc Tds in the 55-60 range). The main 500mb-250mb flow is farther south into TX, but flow is still sufficient (imo) in OK. Forcing looks pretty strong attm, but we'll see... It appears that the trend is to slow this cyclone down with time, which, if the trend continues, would bring the stronger flow more westward into central OK, better juxtaposed with the higher (albeit still pretty meager) instability. The only real instability is between the backside of a deep cloud deck across eastern OK/TX, and the surface front near I35 by 18z (per 6z NAM again)... We'll see... It's at least something that could provide a SVR warning or a few for the area...

 

[Gabe Garfield]

Both models have slowed down the progress of the vorticity maximum...so, as Jeff said, Monday's activity could be a lot further west (including C. OK and N. Texas). However, I really think that what happens in the deep south could be a newsmaker (provided adequate moisture return).

All models continue to show a vigorous, negatively-tilted shortwave trough that is forecast to impact the southeast on Monday night into Tuesday. Wind fields at all levels are forecast to be very strong, with southerly low-level flow of 40-50 kts and southwesterly 500 mb flow of 50-75 kts. Helicity values should be more than sufficient for tornadic supercells on Monday, and especially by Monday night (values on the order of 300-500 m2/s2). If sufficient moisture can be realized (Tds of >60 F), strong tornadic activity would be possible, especially during the overnight hours.

From the Jackson, MS HWO:

THE SOUTHERN BRANCH OF THE JET STREAM WILL REMAIN STRONG OVER THE SOUTHERN UNITED STATES. A SERIES OF UPPER-LEVEL DISTURBANCES ARE EXPECTED TO MOVE THROUGH THIS FAST FLOW LATE THIS WEEKEND AND EARLY NEXT WEEK. THE STRONGEST OF THESE DISTURBANCES NOW APPEARS MOST LIKELY FOR LATE MONDAY INTO EARLY TUESDAY AND THIS UPPER-LEVEL PATTERN HISTORICALLY PRODUCES SEVERE WEATHER IN THE LOWER MISSISSIPPI RIVER VALLEY REGION. AS A RESULT...THE BEST RISK OF SEEING SEVERE STORMS...INCLUDING TORNADOES...APPEARS TO BE MONDAY NIGHT INTO EARLY TUESDAY.

Gabe

 

[Jeff Snyder]

Looking at the 12z NAM, it appears that the southern plains will see its first signficant severe weather event this year on Monday. The NAM is showing CAPEs >2000 in a narrow corridor ahead of the dryline and behind the pre-frontal precip/cloud shield (NAM indicating a dry slot crossing the dryline, clearing out the area immediately ahead of the front). This will increase if we can get some higher quality moisture into eastern TX and OK, though the NAM is showing surface Tds >60 across far southeastern OK and eastern TX.

It appears that the very strong low-level flow will result in very strong low-level shear (0-3km SRH >250 m2/s2 widespread ahead of the front). Mid-level flow isn't that strong, which means the storms will likely be chaseable in terms of storm motion. The linear orientation of the forcing would tell me that a severe squall line is most likely, but the orientation of the mean shear vector across the front would favor some cellular activity, with the degree of low-level shear yielding a tornado risk.

 

[Michael P. Morris]

66 hour GFS still showing limited instability, but the dynamics look good at this time, with the left exit region of the 250 mb jet positioned over SW OK/TX Panhandle colocated with a 500 vortmax. The lower level moisture still looks a bit sketchy, as Jeff mentioned, leading to the limited CAPEs. However, given the amount of forcing it could be enough to maybe get a line or maybe some discrete cells. The dryline will likely be the initiator in the event that something happens, as it appears fairly well-defined on the 66/72 hour ETAs. I am excited, may have to skip the Numerical Methods test...

 

[Glen Romine]

66 hour GFS still showing limited instability, but the dynamics look good at this time, with the left exit region of the 250 mb jet positioned over SW OK/TX Panhandle colocated with a 500 vortmax. ...

Michael, this thread is discussing the forecast for Monday the 21st (so evening would be 00Z the 22nd), so you'd want be looking at the 78-84 hr forecast time frame from the 12Z run this morning.

To stay on topic - the ETA does show improved moisture moving into the southern plains on Monday - but also indicates a large region of precipitation around for much of the early part of the day in the warm sector. This moves out with the large scale forcing sliding to the northeast. Could see enough clearing and convergence at the triple point to get some low-topped convection in the dry slot, but such possibilities are difficult to foretell this far out. Both the 12Z ETA and GFS are pretty consistent with the larger scale, particularly the timing of the mid-level shortwave for this far out - but the GFS builds more ridge in behind the system and doesn't dig the surface low nearly as far to the south, and you'll want the low as far south as possible to improve convergence in the better moisture. South of the surface low along the dryline convection could be more questionable. Neither model generates convection there, and soil conditions in west Texas are rather damp which may take some of the gusto out of the dryline.

Glen

 

[George Tincher]

The latest hazardous weather outlook from Jackson, MS NWS sounds rather ominous for Monday. I'll just add this small snip from it and leave it at that:

THE MAIN TIME PERIOD FOR SEVERE STORMS WILL BE MONDAY AFTERNOON THROUGH MONDAY NIGHT. AT THIS TIME...CERTAIN PARAMETERS ARE COMING TOGETHER TO SUPPORT A SIGNIFICANT SEVERE WEATHER EVENT. THE MAIN RISK DURING THIS EVENT WILL BE FROM SUPERCELL THUNDERSTORMS PRODUCING VERY LARGE HAIL...STRONG TORNADOES AND DAMAGING WINDS.

[/quote]

 

[Amos Magliocco]

I'm as ready to chase as anybody, but these stacked lows are a tough sell. I don't like what they do with the low-level wind fields for a few reasons: (1) southwesterly low-level flow can erode boundary layer moisture out ahead of the dryline [the "CAPE-robber" scenarios they talked about years ago] so that the dryline mixes east rapidly, as it seems to do on the 12z NAM, and (2) surface flow is nearly parallel to the boundary resulting in weaker convergence. All this on top of what we all know about the effect of veered surface winds on the hodograph.

I thought this system was starting to show some negative tilt and expected it would slow on this morning's runs, but instead its a few hours faster and further east.

Nearer the triple point, around TUL, directional shear looks more appealing and there's still ~1000 j/kg MLCAPE by 0Z co-located with ~200 m2/s2 0-1k SRH. That fact alone means Oklhahoma chasers should be watching carefully. But things slacken off upstairs, and *if* the system should dig south even slightly more than depicted, SR anvil level winds will be lousy.

I hate bashing setups, so I'll qualify this by saying if I still lived in the southern plains, I'd be more excited. There's definitely a small probablitily of supercells and tornadoes in the cards and I would be preparing to chase. But my nine hour drive to TUL and the small fortune in gas costs demand better odds.

 

[Jeff Snyder]

Just for a look at the thoughts from various NWSFOs in the area...

1pm Tulsa HWO mentioning signficant severe weather event with tornado threat; 4am AFD mentioning "potentially very active weather day"...

4am OUN AFD: "BETTER CHANCES FOR SEVERE STORMS MUCH MORE LIKELY ON MONDAY OVER CENTRAL/EASTERN ZONES AND POINTS EAST. "

10am SHV AFD: "SIGNIFICANT SEVERE WEATHER AS VERY STRONG NEGATIVELY TILTED SHORTWAVE TROUGH
EJECTS FROM THE CENTRAL ROCKIES INTO THE EASTERN KANSAS. THE WARM SECTOR WILL BE PRIMED AND JUICED WITH VERY STRONG SHEARING FAVORABLE FOR TORNADIC ACTIVITY." ; Noon HWO: "A SIGNIFICANT TORNADO POSSIBLE FROM MONDAY INTO MONDAY NIGHT. "

7am JAN HWO: "AT THIS TIME...CERTAIN PARAMETERS ARE COMING TOGETHER TO SUPPORT A SIGNIFICANT SEVERE WEATHER EVENT. THE MAIN RISK DURING THIS EVENT WILL BE FROM SUPERCELL THUNDERSTORMS PRODUCING VERY LARGE HAIL...STRONG TORNADOES AND DAMAGING WINDS."

Well, I'm sure you're as up to the situation as I am... I wouldn't have quoted the above if it weren't for the fact that this could end up as a doozy of an event, perhaps more subjective than objective given that it'll be the first tornadic/severe event in a long time for the area...

The 997mb low progged in central OK by 0z Tues certainly does look nice... However, the 12z NAM is punching the dryline way ahead the low, stretching from near PNC to Hugo/Antlers to northeastern TX by 0z... There appears to be a double warm-front structure with this system -- the first warm front from the center of the low eastward to near Nashville and the second warm front stretching southeast from the low. By 0z, it appears that the dryline 'catches' up with the warm front in OK, at least partially. This is kind of a tough analysis given the widespread precip ahead of the second warm front causing relatively cool surface temperatures (upper 50s mainly). Looking at sfc flow, the winds remain very backed to ahead of the southernmost warm front with southeast surface flow (or east on the northside of I40), with slightly backed flow to the east of the dryline. With sbCAPEs >1500 in eastern OK and >2500 in northeastern TX, instability shouldn't be a huge problem. That said, there really isn't much juxtaposition of the low-level helicity and the instability, since helicity is maximized ahead of the dryline, while the instability is maximized ahead of the warm front. In all reality, however, this is relatively typical, so I think the best spot would be near the triple point or immediately east-southeast of the surface low (pretty near I-40 just east of I35).... Unfortunately, sfc Tds are only forecast to be in the upper 50s, yielding T-td deficits in the 15 degrees range, which definately won't help the LCL heights. In addition, the core of the 500mb jet streak is progged to be farther south into TX, leaving pretty weak mid-level flow across most of OK (~30-45kts).

We'll have to see what the warm sector looks like on Monday morning... It could be very contaminated from widespread precip Sunday evening and overnight thanks to very strong forcing ahead of the low (warm-air advection, DPVA, transverse circulation upper-level divergence, etc), which would prevent signficant potential instability from developing.

 

[Gabe Garfield]

As far as instability issues are concerned, I think that the key will be what happens with the WAA precip that forms on Sunday. If this can move out soon enough and good insolation occurs...with an outflow boundary somewhere in the region, it could get downright nasty on Monday.

Concerning moisture, I was shocked to see how quickly the western GoM is recovering...mid 60s already along the immediate Gulf Coast. Also, the NAM has been upping its moisture/instability forecast with each run...from ~1000 j/kg to ~1500 j/kg of CAPE in E. Ok (from last night's run to this morning's run) and CAPE up to 2500 j/kg in the Arklatex region (compared to the 1800 j/kg or so in previous runs). Also bear in mind that the ETA tends to underforecast CAPE, so I wouldn't be a bit surprised by values exceeding 3000 j/kg in the Arklatex region.

Gabe

 

[Gabe Garfield]

00z 21 Mar NAM run shows the surface low over C. OK now...much further west than was forecast in earlier runs. Even so, it appears moisture will be meager (Tds only in the upper 50s) at best near the surface low. However, strong forcing near the sfc low may be able to compensate for the relative lack of instability (CAPEs ~1500 j/kg) and present a tornado threat. We shall see, though.

Another concern of mine is the relatively strong forcing...a squall line will probably form (at least along the primary dryline) relatively soon after initiation. On the other hand, though, the orientation of the shear vector across the dryline may allow storms to remain discrete a bit longer in spite of the strong forcing.

The Arklatex region still appears to be under the gun, with CAPEs approaching 3000 j/kg and helicities over 400 m2/s2. Throw in an outflow boundary and there is the potential for a few significant tornadoes, IMO. I am especially concerned about the overnight hours in the border region of LA/MS. CAPE will still be over 2500 j/kg at 06z and colocated with helicities >500 m2/s2. Definitely could be a few newsmakers there.

Gabe

 

[Jeff Snyder]

My concerns remain on the mode of convection with the strong linear forcing, as well as the relatively high LCLs heights owing to the T-Td deficits. That said, the SPC has gone with a moderate risk across parts of the area, evening mentioning strong tornadoes possible. Of course, this is all contigent upon sufficient insolation and resultant adequate surface-based instability...

It is interesting to note that the 0z GFS shows the surface low just east of ICT at 0z Tues (Mon evening), while the NAM has the low near OKC... I'm hoping the GFS doesn't pan out, since it veers the winds down here in OK pretty early in the afternoon...

 

[beaudodson]

Yes there are defin some differences in the models to the placement of the low. Too early to determine which one will be correct. The GFS is further south and the NAM is further north with the placement of the low track. Should be an interesting few days with more to follow.

 

[Jim Bishop]

Monday looks like the first big tornado day for the southern plains this year. The current SPC forecast has a good handle on it right now. If I could chase Monday, I'd say north-central/northeast Texas through central/eastern Oklahoma would be the best areas to find tornadic supercells. Monday looks HUGE.