Ex Katia - UK Bound. Forecasts and in-depth discussion

[toggerob]

The last Buoy on the State side is 44140, and i'd imagine with the current track Buoy 62095 offshore Ireland will be the one after in getting any data from this Hurricane/Extratropical Storm. I haven't heard of any recon aircraft being operated by either the US or UK that will be following Katia into the Atlantic.

http://www.ndbc.noaa.gov/

[kold weather]

Katia is holding its own very well, I have a sneaky feeling extra-tropical forcing is already giving this system a boost, though to be fair it is certainly still tropical for the time being and has a rather impressive inner core, something that Katia has managed to keep intact for the last few days despite shear being fairly high at times.

As iceberg said, a stronger system to start with may well have some interesting results, a deeper system and stronger system then what some of the models are expecting is quite possible.

[Paul]

Nick has updated our daily Katia article:

http://www.netweather.tv/index.cgi?action=news;storyid=798;sess=

This is the latest forecast track for Katia (GFS)

And finally, we have a UK wide weather watch online as of earlier this morning:

http://www.netweather.tv/index.cgi?action=forecast;type=alerthome

[Iceberg]

Kafka is looking increasingly well organised and is developing quite a strong eye wAll on ir. This is probably a boarderline cat 2 one with press down to 960mb

Sorry done on phone quickly

[knightrider]

Fairly consistent directionally from all the models bar the two BAMM ones (which would be a major concern if one of them came off). Now it's down to how far North/South she blows and how much she interacts with the other low and the jet.

Some very windy conditions across the whole of the UK according to GFS

Not relenting until late on Monday:

London seems to be in one of these lee areas looking at the above two surface wind charts

Judging by that looks like most of the UK is going to get battered to some extent, gale-force winds at the very minimum.

[bobbydog]

the BAM models are increasingly favouring a track across the southern half of the country. does anyone know how reliable they are?

Edited September 9, 2011 by bobbydog

[Iceberg]

BAMM is really a tropics only model, for shallow none developed systems, so interesting but to be ignored really.

[Coast]

the BAM models are increasingly favouring a track across the southern half of the country. does anyone know how reliable they are?

The BAMM model (Beta and advection model, medium layer) is included on Wundermap. The BAMM is a simple trajectory model that is very fast to run, and did the best of any individual model at 3-5 day track forecasts in 2005. Since this model is always available, we have included it along with the "big four". In general, one should not trust the BAMM model for the 1-2 day time period when output from "the big four" are available. "The big four" are generally not available for tropical disturbances, and for these situations we post plots of a number of other non-global models such as the LBAR, A98E, etc. All of these models are described in detail on NHC's web site.

BAM (Beta and Advection Model)...

This model follows a trajectory from the Aviation run of the MRF model to provide a track forecast. This model incorporates a correction known as the "Beta Effect". This is used to account for the fact that the Coriolis force resulting from the rotation of the earth is greater toward the poles, so the winds on the northern side of the storm in the Northern Hemisphere tropical cyclone are turned more than those on its southern side. If no other winds were steering a tropical cyclone the "Beta Effect" would cause a westward-headed storm to drift toward the north in the Northern Hemisphere, and toward the south in the Southern Hemisphere. There are three (3) versions of the BAM...

1.. BAMS - the BAM Shallow, this version averages winds from 5,000 to 10,000 feet (850 - 700 mb)

2.. BAMM - the BAM Medium, this version averages winds from 10,000 to 24,500 feet (850 - 400 mb)

3.. BAMD - the BAM Deep, this version averages winds from 24,500 to up to 47,000 feet (850 - 200 mb)

For a weak hurricane without a well-developed eye wall extending deep into the atmosphere, or for a tropical storm, the shallow version of the model may work well, because storms of this nature tend to be steered by low-level winds. As the storm grows stronger and the eye wall gets deeper the deeper versions become more accurate, for these types of storms are steered more by the winds in the upper-level. If the forecast from the three versions is similar the forecaster can then assume that the storm may go as predicted, but, if the version vary by a great deal, then the forecaster has less confidence in the track predicted. The large differences can also point to wind shear in the atmosphere, which could affect the intensity forecast as well.

http://www.hurricane...net/hurmdls.htm

[Robbie Garrett]

Am I correct in saying Coast, that the BAMM could be the likely path after reading that or am I reading it wrong?

[Coast]

Am I correct in saying Coast, that the BAMM could be the likely path after reading that or am I reading it wrong?

It's way out on it's own and a bit 'out of depth' on this side of the Atlantic, so not really to be considered. The bulk of the models are going for the tip of Ireland and then NW Scotland. If the BAMM came off I would be amazed and probably under a lot of rubble......

Interestingly I have noted that a lot of the US models have shifted further South this afternoon, I wonder how they will handle the set-up this far from home?

Here's a close up of the US late model tracks:

Typical info coming through at the end of the runs is:

Model: AP08 Hours from late cycle initialization: 108 Coordinates: 57.2N 4.5W This is either a late cycle model or the previous run of a model that was not available this run. Wind Speed: 34 kts (39.1 mph, 63 km/h, 17.5 m/s) MSLP: 980 mb (28.94inHg, 980 hPa)

Model: AP01 Hours from late cycle initialization: 90 Coordinates: 58N 5.8W This is either a late cycle model or the previous run of a model that was not available this run. Wind Speed: 40 kts (46 mph, 74.1 km/h, 20.6 m/s) MSLP: 968 mb (28.59inHg, 968 hPa)

Model: AP16 Hours from late cycle initialization: 114 Coordinates: 56.3N 6W This is either a late cycle model or the previous run of a model that was not available this run. Wind Speed: 36 kts (41.4 mph, 66.7 km/h, 18.5 m/s) MSLP: 985 mb (29.09inHg, 985 hPa)

http://www.tropicala...strun=1&large=1

[bobbydog]

It's way out on it's own and a bit 'out of depth' on this side of the Atlantic, so not really to be considered. The bulk of the models are going for the tip of Ireland and then NW Scotland. If the BAMM came off I would be amazed and probably under a lot of rubble......

Interestingly I have noted that a lot of the US models have shifted further South this afternoon, I wonder how they will handle the set-up this far from home?

interestingly though, the BAM models have taken a more southerly track from the start, where they are much nearer to home. however, time will tell as to which models get this right

[Harry]

Quite a few are shifting South as well

[Liam J]

I don't think anyone in South East England need to worry about Katia making much of an impact down there, ECM, GFS and UKMO are generally in agreement with the storm making landfall over Western Ireland then proceeding to track NE/NNE towrds Scotland.

Further minor changes in track can be expected over future runs.

I still think N England, Scotland and Ireland shouild be prepared for a powerful depression.

[Robbie Garrett]

Interestingly I have noted that a lot of the US models have shifted further South this afternoon, I wonder how they will handle the set-up this far from home?

Would ROFL, if they are better at doing it than us.

[Aaron]

I don't think anyone in South East England need to worry about Katia making much of an impact down there, ECM, GFS and UKMO are generally in agreement with the storm making landfall over Western Ireland then proceeding to track NE/NNE towrds Scotland.

Further minor changes in track can be expected over future runs.

I still think N England, Scotland and Ireland shouild be prepared for a powerful depression.

We are now under a weather warning. Yellow warning could be expanded, and a orange one introduced. I guess the Met were just waiting until the picture was clearer to release any warnings!

[AndyLincs]

Reading the met office forecast for Monday for our area (Spalding) and incidently not in the warning area, the wind speeds / gusts forecasted are superior to the vast majority of locations within the warning area. Being in the "sheltered east", 15 miles from the shores of the Wash and at sea level, I often thought these were ingredients for reduced wind speed and gusting. What theory therefore would the met office be basing their projections on?

[If Wishes made Weather]

I think the thing to note from the latest model tracks is that they are rather more dispersed than previously indicating less certainty. Also, not a few show the track taking a rather more south-east direction to the end of the run. This would be hard as it would expose the already exposed west to wind that will move round to the WNW quarter for longer as well as moving the general effect more to the south.

[knocker]

WTNT42 KNHC 091436

TCDAT2

HURRICANE KATIA DISCUSSION NUMBER 46

NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011

1100 AM AST FRI SEP 09 2011

KATIA HAS MAINTAINED A RAGGED EYE FEATURE IN BOTH CONVENTIONAL AND

MICROWAVE SATELLITE IMAGERY DURING THE PAST 6 HOURS...SO THE

INTENSITY WILL REMAIN AT 75 KT...WHICH IS CONSISTENT WITH NHC AODT

AND SAB INTENSITY ESTIMATES. DESPITE THE HIGH LATITUDE OF THE

CYCLONE...KATIA CONTINUES TO HAVE A WELL-DEFINED UPPER-LEVEL

OUTFLOW PATTERN.

THE INITIAL MOTION ESTIMATE IS 050/25 KT. KATIA IS WELL NORTH OF THE

SUBTROPICAL RIDGE AXIS ORIENTED ALONG 33N LATITUDE...AND WATER

VAPOR IMAGERY INDICATES THAT THE CYCLONE IS NOW ENTERING THE FAST

WESTERLY FLOW REGIME THAT LIES NORTH OF 40N LATITUDE. AS A

RESULT...KATIA IS EXPECTED TO ACCELERATE NORTHEASTWARD THROUGH THE

REMAINDER OF THE FORECAST PERIOD. THE HURRICANE SHOULD PASS OVER

MUCH COLDER WATER BY 24 HOURS AND BECOME A POWERFUL EXTRATROPICAL

CYCLONE. THE FORECAST TRACK IS DOWN THE MIDDLE OF THE TIGHTLY

PACKED MODEL GUIDANCE...AND LIES CLOSE TO THE NHC CONSENSUS MODEL

TVCN.

KATIA WILL BE MOVING OVER WATER TEMPERATURES LESS THAN 26C BY 12

HOURS...AND OVER SSTS LESS THAN 20C BY 24 HOURS. THEREFORE...A

FAIRLY QUICK TRANSITION TO AN EXTRATROPICAL CYCLONE IS EXPECTED BY

24 HOURS WHEN THE SYSTEM WILL ALSO BE MERGING WITH AN APPROACHING

FRONTAL ZONE. ALTHOUGH KATIA IS FORECAST TO WEAKEN BELOW HURRICANE

FORCE BY 36 HOURS...THE LARGE EXTRATROPICAL LOW PRESSURE SYSTEM IS

EXPECTED TO CONTAIN STORM-FORCE WINDS WHEN IT APPROACHES THE

BRITISH ISLES IN 3 DAYS.

NO 96-HOUR FORECAST POINT HAS BEEN PROVIDED DUE TO COMPUTATIONAL

ISSUES WHEN SYSTEMS MOVE EAST OF ZERO DEGREES LONGITUDE.

FORECAST POSITIONS AND MAX WINDS

INIT 09/1500Z 39.3N 65.8W 75 KT 85 MPH

12H 10/0000Z 41.1N 60.4W 70 KT 80 MPH

24H 10/1200Z 44.1N 49.9W 65 KT 75 MPH...POST-TROP/EXTRATROP

36H 11/0000Z 47.8N 37.8W 60 KT 70 MPH...POST-TROP/EXTRATROP

48H 11/1200Z 51.5N 26.5W 60 KT 70 MPH...POST-TROP/EXTRATROP

72H 12/1200Z 58.5N 7.5W 55 KT 65 MPH...POST-TROP/EXTRATROP

96H 13/1200Z...EAST OF ZERO DEGREES LONGITUDE

$$

FORECASTER STEWART

[Harry]

I don't think anyone in South East England need to worry about Katia making much of an impact down there, ECM, GFS and UKMO are generally in agreement with the storm making landfall over Western Ireland then proceeding to track NE/NNE towrds Scotland.

Further minor changes in track can be expected over future runs.

I still think N England, Scotland and Ireland shouild be prepared for a powerful depression.

I really don't understand how people can be so complacent re where the storm will hit. It's over 3,000 miles away, is going to undergo some serious internal re-configuration in terms of dynamics, is going to be riding the jet stream, interacting with another powerful low and be under the influence of ridging to the South...just because the three models are in pretty rough agreement with regards the track, they're not in terms of it's evolution, deepness, orientation and frontal evolution.

It's Friday, she is not expected to arrive for another 60 - 72 hours...I really wouldn't count any eggs yet!

[chionomaniac]

I have just been catching up with today's latest tracking of Katia. Yesterday, I suggested that by this evening we would have a better idea of the track and intensity of ex Katia and that certainly looks to be the case.

The main difference that we are seeing is that the track is trending south with slightly more intensity. And why has this occurred? Well it now looks that the models are getting a grip on how Katia crosses the jet stream and subsequently gets empowered by it. For the last couple of days we have seen the models hinting at a pre Katia shortwave. But this option appears now to be reducing. This means that the disruption to the jet stream that this short wave was producing is also reducing. So now we have a slightly flatter and stronger jet streak with Katia sitting at it's left exit zone. A perfect position for reintensification!

If this is maintained then Katia will pack a hefty punch for this time of year.

[Interitus]

For me we have seen a multitude of variations on the same theme in the last two days due to the difficulty in the models handling how ex Katia breaks through the jet stream. This type of thing happens so infrequently that it is no surprise that the computer programs cannot consistently keep to the same forecast. I think that by tomorrow evening we will have a far better idea of how Katia will amalgamate with the long wave trough and whether there is likely to any significant shortwave further south. What we do have is a significant amount of warmth of moisture and heat associated with Katia but all model outputs that I have seen suggest that the worst of the rain will be out in the Atlantic - not a bad thing at all!

Sorry I don't buy this, the models are exhibiting no more variation than normal and will have progged the passage of Katia somewhere between Scotland and Iceland for over week by the time it arrives.

[chionomaniac]

Sorry I don't buy this, the models are exhibiting no more variation than normal and will have progged the passage of Katia somewhere between Scotland and Iceland for over week by the time it arrives.

Yes, the models have programmed Katia's arrival towards the NW of Scotland for a while now. I have not suggested that the models have exhibited more variation than normal. However, I have been studying Katia's forecast interaction with the jet stream during this time and whether you buy it or not there has been inter-run and inter-model variation regarding how Katia arrives. This is no less than one would expect.

Edited September 9, 2011 by chionomaniac

[Interitus]

NO 96-HOUR FORECAST POINT HAS BEEN PROVIDED DUE TO COMPUTATIONAL

ISSUES WHEN SYSTEMS MOVE EAST OF ZERO DEGREES LONGITUDE.

FORECAST POSITIONS AND MAX WINDS

INIT 09/1500Z 39.3N 65.8W 75 KT 85 MPH

12H 10/0000Z 41.1N 60.4W 70 KT 80 MPH

24H 10/1200Z 44.1N 49.9W 65 KT 75 MPH...POST-TROP/EXTRATROP

36H 11/0000Z 47.8N 37.8W 60 KT 70 MPH...POST-TROP/EXTRATROP

48H 11/1200Z 51.5N 26.5W 60 KT 70 MPH...POST-TROP/EXTRATROP

72H 12/1200Z 58.5N 7.5W 55 KT 65 MPH...POST-TROP/EXTRATROP

96H 13/1200Z...EAST OF ZERO DEGREES LONGITUDE

$$

FORECASTER STEWART

The computational issues as systems cross the meridian is interesting, though I preferred the explanation from earlier this morning -

NO 96-HOUR POINT IS BEING GIVEN BECAUSE FORECAST POINTS IN THE

EASTERN HEMISPHERE BREAK A LOT OF SOFTWARE.

Love it

Yes, the models have programmed Katia's arrival towards the NW of Scotland for a while now. I have not suggested that the models have exhibited more variation than normal. However, I have been studying Katia's forecast interaction with the jet stream during this time and whether you buy it or not there has been inter-run and inter-model variation regarding how Katia arrives. This is no less than one would expect.

Yes, but the rarity of the event doesn't enter the equation, the models purely run with the data input not likelihood. I remember the same argument cropped up with the unusual cold of last December which the models all handled admirably.

Edited September 9, 2011 by Interitus

[Nick F]

Katia south of Nova Scotia now:

Not long until is becomes extratropical, it is begining to pass over SSTs nearing 20C and is heading towards the baroclinic zone/frontal boundary which marks the boundary between colder polar air to the north and warmer tropical air to the south:

Still a little bit further to go toward the NE before is gets entrained into the strongest upper westerlies of the jet stream coming out of NE Canada, though by noon Saturday it looks pretty much under the cosh of the jet by then - and then will likely accelerate in motion as it is steered across the Atlantic towards Britain over the weekend.

- 9pm tonight

- noon Saturday.

Be interesting to see whether and how long ex-Katia retains an eye after it's 'officially' deemed extratropical ...

[ZONE 51]

GFS 00z latest run appears to have shifted the track slightly south, and a tighter center, the southern flank of strongest belt of damaging winds looks to be a little further south creating a wider belt of severe winds, this also shows on the wind flow surface charts.

need an expert on this and how significant it would be.

first chart monday 0000hrs second is 0900hrs

purple lines indicate track.

-on each image-18z left - 00z right.

Edited September 10, 2011 by ElectricSnowStorm