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Severe Weather Threat May 17-19, 2025

Ebaad Jaffery said:
So the run just previous to the ones you are referencing now, I believe, quite literally had it swapped. NAM was going for an outbreak and the HRRR was calling for little to no convection even occurring in North TX. So I wouldn’t rely on the models for a telltale picture on today.
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As of right now it looks like FWD is more concerned with the NE TX location later on this evening. In regards to the tornado threat.




By mid-afternoon, the surface dryline boundary will be pulled
eastward, advancing into western North Texas during peak heating.
Strong to severe thunderstorms are expected to develop near/east
of this boundary with aid from strong synoptic ascent from a deep
upper trough pivoting into the Central Plains. With significant
instability characterized by SBCAPE exceeding 4000 J/kg and strong
deep-layer shear exceeding 50 kts, a supercellular storm mode
should dominate initially, before perhaps eventual upscale growth
into clusters heading into the evening. This will support all
severe hazards, including large hail and a few tornadoes,
especially as 0-1 km shear/helicity increase heading towards
sunset with the onset of a strengthening low-level jet over parts
of Northeast Texas. Another concern is the anticipated fast storm
motions later today, even for established supercells with deviant
motion. Mean steering flow will be in excess of 55 mph, left
splitting supercells could race northeast in excess of 60 mph, and
even right-movers could see ENE motions as high as 40-50 mph.
This will be of even greater concern if convection is still
ongoing after nightfall which is possible in parts of Northeast
Texas.




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Atlantic said:
2025 has incredibly violent so far... What has been happening and why?
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2024 has been similar in activity between march and May.

Tornado outbreak sequences are caused by PNA patterns, which is based on geopotential height and sea level pressure anomalies in the general area between the north west pacific and west coast of North America.

-PNA patterns are identified when there are height falls (measured in decameters, and is caused by troughing), which promotes sea level pressure to drop over a large area. This eventually leads to the development of surface lows which then move east and are responsible for severe weather outbreaks.

PNA patterns can be influenced by ENSO state, but for the most part the teleconnection is highly variable, and usually doesn’t stay in a specific phase for more than a week.

However, how often the PNA does go negative is more effected by ENSO state, and during neutral or La Niña, (particularly during the spring) North Pacific jet extensions tend to occur more often. (The key phrase is “more often” as these can happen regardless of ENSO state) These jet extensions cause Rosby wave breaking (which is just the breakdown of the jetstream into troughs/ridges) which are then westerly injected into North America.

Depending on the amplitude of the wave guide, which can go from low, neutral, or high amplitude, are responsible for most of our weather.

The trough present right now is a high amplitude longwave which is the trough type that produces the most substantial tornado outbreaks.

This year, including last year, we have had prevalent plains and Dixie tornado outbreaks, 2024 was during a La Niña and this year is a cold neutral, recall that 2011 was during a La Niña.

I wouldn’t be surprised if we have a more active summer than usual, 2004 was a neutral-Modoki ENSO state, and a -PNA pattern was present throughout the summer, which helped 2004 become the most tornado active season on record, with the help of the active hurricane season as well.

Obviously, this is a “VERY GENERAL” explanation and skips a laughable amount of critical details, but I don’t feel like writing a bible.
 
Update:
As of 530 AM, high-based showers and isolated storms are beginning
to develop within a mid-level moisture plume extending
northeastward from the Texas Hill Country. This trend should
continue this morning, with perhaps a couple of more robust cells
capable of small hail and gusty winds becoming established after
sunrise. The main severe weather risk is still from mid afternoon
into the evening, and will primarily be in North Texas where an
environment supportive of supercells will be in place with all
severe hazards possible. Overnight CAM guidance remains fairly
subdued in terms of convective coverage within the forecast area,
while coarser global guidance continues to support widespread
convection ahead of the dryline. Current thinking is still that
CAMs are underdone, and have gone more bullish with PoPs than most
short-term guidance would indicate given there will be strong
synoptic forcing available and a very unstable buoyant sector
ahead of the dryline. The remainder of the trends outlined below
remain valid for today`s event.

Out of the FWD office
 
To put into perspective how unusual (but not unprecedented) the 10% SIG TOR hatch is for North MS/Mid-South during the month of May, the last time we had a 10% SIG TOR hatch or greater equivalent was on May 25th, 2011 and again on May 2nd, 2010.
 
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