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

[Paul]

Please post your more in depth forecasts and discussion regarding Katia in this thread

[peterf]

thank god for this thread, cheers Paul. they'll firm up as usual as the time nears.

[NO LONGER POSTING]

they'll firm up as usual as the time nears.

I guess so

[Dorsetbred]

Looking at the latest (5PM EST) forecast the swing shows a slight lift towards the north.

http://www.nhc.noaa.gov/refresh/graphics_at2+shtml/210138.shtml?5day?large#contents

[chionomaniac]

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!

[knocker]

I agree with Chio, three days is a long time in meteorology. Latest disussion.

WTNT42 KNHC 082119

TCDAT2

HURRICANE KATIA DISCUSSION NUMBER 43...CORRECTED

NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011

500 PM AST THU SEP 08 2011

CORRECTED 120 HOUR POINT

CONVECTION PERSISTS NEAR THE CENTER OF KATIA AND CLOUD BANDS HAVE

INCREASED IN ORGANIZATION...PARTICULARLY IN THE SOUTHWEST QUADRANT

THAT WAS PREVIOUSLY DEVOID OF CONVECTION. HOWEVER...A RECENT ASCAT

PASS AND TAFB/SAB SATELLITE ESTIMATES SUGGEST THAT THE MAXIMUM

WINDS HAVE DECREASED SLIGHTLY. THE TAFB/SAB ESTIMATES ARE THE BASIS

FOR A SLIGHT DECREASE IN THE INITIAL INTENSITY...TO 75 KT.

KATIA IS MOVING SLIGHTLY EAST OF NORTH...WITH AN INITIAL MOTION OF

005/14. RELIABLE NUMERICAL GUIDANCE SUGGESTS THAT KATIA WILL

CONTINUE TO TURN AND ACCELERATE NORTHEASTWARD OVER THE NEXT SEVERAL

DAYS...AND BECOME EXTRATROPICAL WITHIN TWO TO THREE DAYS. THE

EXTRATROPICAL STORM SHOULD PASS NEAR THE BRITISH ISLES IN ABOUT

FIVE DAYS. GUIDANCE HAS BEEN CONSISTENT AND IN GOOD AGREEMENT ON

THE TRACK OF KATIA...AND THE TRACK FORECAST IS VERY CLOSE TO THE

PREVIOUS ADVISORY.

RELIABLE INTENSITY GUIDANCE SUGGESTS THAT KATIA WILL WEAKEN RATHER

SLOWLY DURING THE NEXT SEVERAL DAYS...AND THE GLOBAL MODELS SUGGEST

THAT THE CYCLONE WILL REMAIN LARGE AND POWERFUL AFTER EXTRATROPICAL

TRANSITION AS IT TRAVERSES THE NORTH ATLANTIC. INITIAL WIND RADII

ARE ADJUSTED OUTWARD IN THE SOUTHWEST QUADRANT BASED ON THE RECENT

ASCAT PASS.

FORECAST POSITIONS AND MAX WINDS

INIT 08/2100Z 34.9N 69.6W 75 KT 85 MPH

12H 09/0600Z 36.9N 68.4W 75 KT 85 MPH

24H 09/1800Z 39.3N 64.6W 75 KT 85 MPH

36H 10/0600Z 41.6N 57.7W 70 KT 80 MPH

48H 10/1800Z 44.2N 47.7W 65 KT 75 MPH...POST-TROP/EXTRATROP

72H 11/1800Z 52.2N 23.8W 60 KT 70 MPH...POST-TROP/EXTRATROP

96H 12/1800Z 59.4N 6.9W 55 KT 65 MPH...POST-TROP/EXTRATROP

120H 13/1800Z...ABSORBED

$$

FORECASTER HOGSETT/FRANKLIN

[chionomaniac]

And again we see from the 18Z GFS that the less that is made of any pre Katia short wave, the more the jet will align favourably for Katia to intensify on the left exit of the positively tilted jet streak. This situation leaves for a deeper re-intensified depression giving stronger winds across Scotland.

Fax chart for Monday 12Z

Certainly looks like a late rather than early Autumnal chart and quite similar to the 18z GFS

[Nick F]

Newly updated t+96 fax chart shows the centre of the extratropical low over the Highlands of Scotland at noon Monday with tightly packed isobars equaly spaced right down to the south coast, so one would assume anywhere away from Nern half of Scotland would be at risk of strong gusts, though western coasts and high ground receiving the highest gusts:

[Nick F]

Surprised how consistently close all of the big three models have been recently up until now with the track of the extratropical depression, not much in it really as it crosses over or just north of Scotland in the greater scheme of things - especially this far out from 96 hour charts.

Some impressive wind strengthens aloft from 18z GFS across northern Britain on Monday

... sustained 90mph winds as low as 850mb (around 1500m asl) and with a very strong upper to mid-level jet stream which tends to descend through the dry air at these levels too (see 500mb RH chart below) - it wouldn't take much for these very strong upper winds to lower to the surface as damaging gusts:

[ZONE 51]

NOAA 96 hour

[nodrog]

beco's prediction... (credit to : beco.org.uk)

[Aaron]

I wonder why the winds would be stronger in the South East than the North East according to beco.

[ZONE 51]

I wonder why the winds would be stronger in the South East than the North East according to beco.

red-orange-yellow-green it yellow for the north east and southeast on there

[ZONE 51]

taking GFS 18z and zoning the first hit late sunday into monday morning,

then these winds moving east-see on 2nd chart.

Orkney and Shetland Islands affected severely by monday afternoon

with widespread gales and storm force in the North with possible severe gusts across many areas, and long drawn isobars indicating prolonged gales with long pushes on trees and buildings, no break from the wind so incredibly dangerous for some areas.

[Aaron]

Should be quite a spell of wind for everyone then.

[ZONE 51]

moderators please move my posts to the chat one meant to post in there!

[Roger J Smith]

Now that we have a "serious" thread as well as a chat thread, I will repost the forecast map that I posted over there earlier with some discussion.

Based mainly on 12z model guidance but confirmed largely by trends on 18z guidance, the following main features can be expected with Katia's onslaught on Sunday night and Monday.

-- rapid increase in SSW to WSW gales and storm force winds in Ireland and southwest England overnight Sunday-Monday.

-- peak winds in northwest Ireland and adjacent NI around 0600-0900 from WSW, potential for gusts to 90 knots or 100-105 mph.

-- peak winds in southwest Scotland, western Isles, northwest England around 0900-1500h with potential for gusts to 80 knots (90-95 mph).

-- peak winds further south and east will depend more on frontal dynamics as a rapidly moving occlusion develops, this feature will produce local gusts to 70 knots (80 mph) even close to the Channel coast and in exposed locations in eastern England.

-- as usual, there will be lee effects that partially shelter some regions and localities, as the map attempts to illustrate, but on a more local scale, most hills facing west will have more severe gusts than other locations around them.

The map showing my predictions as of 0300h follows. An update will be issued if the 00z guidance shows a markedly different outcome.

[Coast]

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:

-- as usual, there will be lee effects that partially shelter some regions and localities, as the map attempts to illustrate, but on a more local scale, most hills facing west will have more severe gusts than other locations around them.

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

[Wivenswold]

00z GFS still has centre of the low brushing the NW tip of Scotland which is further north than the average track shown on the above maps. UK Meto further South too. Latest NAE seems to downplay chances of shortwaves interferring with main system. I'm looking forward to pouring over each new run with a jewellers' eyepiece.

[Coast]

Quite a dramatic change in the jet as Katia interacts over Sunday into Monday:

[If Wishes made Weather]

Noted Wivenswold - excluding the two very southerly tracks, the mean of the bunch is just a tad north of the Clyde/Forth valley.

[Coast]

Swell chart for Monday lunchtime, Western Irish coasts seem to be at some risk:

[Iceberg]

Katia is strenghtening and quite rapidly IMO, NHC have confirmed that the Shear has lessened considerably, and unfortunately is also passing over the GS for the next 24 hrs.

A smaller tighter eye is forming as a convective ring encircles it.

Dvorak numbers certainly support the 75 kts rating.

The offshot of this is that when Katia starts extra-tropical transition it will likely be 10-20mb lower than thought 12 hrs ago.

However All the models, global and tropical initiated Katia at 90-100kts for the 00Z suite, not very realistic and so the wind swave will be smaller than shown by the models.

Again a point to note is that pressures for the globals models vary massively for Katia at initialisation, GFS at 983mb and ECM at 953mb, NHC have Katia at 973mb, Dvorak at 970. Given this somewhere around 965 to 970 seems right.

Extra tropical transistion looks to start in Approx T30.

Pressure is set to fall or maintain during extra tropical transition due to the JetStream interaction.

Full phase change to Baroclinic Low looks to take place just prior to ex-Katia approaching western Ireland.

Given it's lower pressure at initialisation ECM then goes on to be the most aggressive with Pressure (at 950-960) and winds, where it's possible for N.Ireland and western Scotland to have sustained winds of 75-80kts according to the ECM.

GFS and Meto are almost certaintly too high for the pressure and so are underplaying the winds post transistion, this is backed up by the T48 chart from the NAE which has pressure 4mb lower than either model for the first pre-katia low.

I am deliberately not going into the details of Katia and how it effects the UK as this is still to uncertain IMO.

For a guess at path and intensity on approach I['d go for a straight blend of ECM and GFS, GFS ENS will have initialised Katia at the same pressure and so will all be underplaying the intensity.

Finally I would make the point again that due to the lack of obs from Katia there are alot of unknowns.

Latest Picture of Katia below, which id due to be of the Irish coast in only 72 hrs........

[Coast]

Latest indications of the position from the NHC are:

[Iceberg]

Latest update from NHC confirms that the pressure is at least 968mb if not lower and likely is a little lower. The knock on impact is that Katia will have a head start...

"THE CLOUD PATTERN CONTINUES WELL ORGANIZED AND IN FACT A DRIFTINGBUOY NEAR THE CENTER OF THE HURRICANE RECENTLY REPORTED A MINIMUMPRESSURE OF 968 MB. THE INITIAL INTENSITY IS KEPT AT 75 KNOTS.HOWEVER WEAKENING IS INDICATED SINCE THE HURRICANE IS ALREADYREACHING COOLER WATERS AND KATIA IS FORECAST TO BECOMEPOST-TROPICAL IN ABOUT 36 HOURS"