typhoon_tip
Meteorologist
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I am sure there are straight up linear-statistical arguments and/or analogs out there that may or may not support this reasoning, but I believe there may be some burgeoning potential for a Bertha threat to the SE Seaboard.
Model trend:
The most recent model runs have blipped a tad back N with the D4-D5 position. The 4 day's worth of model runs overall, however, average quite a bit west. Recall, when this was first picked up coming off Africa and processed as an Atlantic Basin entity, it was fairly unanimously forecast to curve N prior to 60W. There was a period of time where there was a growing consensus on a turn nearing 60W. Since, we have seen more than a few model solutions that have actually brought a weak version of Bertha up toward the Bahamas.
The trouble is...that re-curve at 60W scenario was predicated on the assumption that the models were right regarding the weakness/trough supposed to dig into that area. That I believe was an error as far as magnitude was concerned. The reason why is because the PNA is falling more actively now in the family derived index (even though the PNA is proven to be less useful during JJA, still...the current depiction(s) are quite reminiscent of -PNA).
In a sense, the future of Bertha's track may very well be determined by the Pacific relay into western N/A. For example, the ECM 00Z operational model is bringing a significant ridge node into the 160-135W by 35N region of the eastern Pacific. This naturally eases a negative teleconnector into the Pac NW/Rockies states; seeing that actually modeled in the operational version at D7 becomes a nice intuitive fit for teleconnector spread. That said, this all transitively correlates to pulling the western Atlantic core westerlies polar-ward. Subsequently, allowing subtropical ridge expansion, not decay. It would really be interesting if the NAO was signaling one way or the other, but for the time being it is wobbling around neutral.
As for the 12Z run...the eastern Pacific --> western N/A continuity is well enough; but the model is fiddling with the R-wave lengths over the east. This is possible considering that the wave lengths overall seemed a little too long for climatology, in the 00Z run. However, the merging of a mid-latitude cold trough axis closure with Bertha, than running a hybrid monster toward the New England coast(s) is a bit absurd looking.. Very little confidence in such detailed events for glaringly obvious reasons. So there is likely to be some significant model depiction changes - not just in the ECM ...but the , as well. In general though, the ensembles of the offer intriguing suggestions that over the next 5 days (at least) there will indeed be ridge strength increases in the middle-western Atlantic, which as a general background canvas makes one wonder if the models are rushing the polarward turns with Bertha.
So, all and all...
- Ensemble mean appearance of subtropical ridge strengthening in the western-middle Atlantic over the next week
- Operational runs more than less supportive
- The "weakly" correlative summer PNA is falling rather concertedly among the members, and that overall theme is a great fit for the ECM 150-100-->50W generalized ridge-trough-ridge expression; as also more than less supported in recent operational solutions; which conceptually asserts that limiting re-curve conduits should be considered.
- Trough originally supposed to carve into 60W (or so) region and pull Bertha out of the south has recently attenuated in runs, and instead appears now to want to lift out prior to Bertha gaining that longitude. That immediately inserts a question as to how successful these more polarward guidances will be, particularly considering the above three factors.
I'll be thinking about this stuff... I may have more ideas on this, but this is the general idea (for me) right now, and I believe the 'threat' (if you want to call it that) is a bit above the norm for a TC in the 11am advisory's temporal-spatial relationship to the U.S. Definitely will be interesting see how this evolves.
Lastly, though they offer no forecast reasoning, the mid-range folk released the following statement:
"...TS BERTHA FORECAST TO BECOME HURRICANE MAY APPROACH THE BAHAMAS AND SOUTHEAST SEABOARD NEXT WEEKEND UNDER MID LEVEL RIDGING. SEE DISCUSSIONS/ADVISORIES.
CISCO/ROSENSTEIN"
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LoisCane
Veteran Storm Chaser
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Posts: 1237
Loc: South Florida
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All I can add here is that most of the models don't jive well because one part rings true but the other part doesn't.
Ukmet has a weak storm in an area where she should finally thrive due to very warm water. If part of the forecast makes no sense you have to question the forecast despite the fact that the Ukmet has worked well so far with this storm.
I still don't see how this storm barrels into a strong high and I don't see the weakness at 60 being so weak.
And, I keep thinking on this comment "allowing subtropical ridge expansion, not decay. "
Too iffy to say without a large shadow of a doubt that Bertha couldn't get trapped under a strong ridge or shoved west by a building one ala Dora or even like a few other storms that have ended up between the border of the Carolinas before re-curving finally out to sea.
Fronts often go flat just when you need them yet help draw a storm in without carrying it off to sea.
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http://hurricaneharbor.blogspot.com/
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