Archives For extra-tropical storm

Even in her hey-day Hurricane Bertha was not a powerful or well organised storm and she disappeared as a discrete tropical storm a few days ago. What is left crossing the Atlantic is a significant “blob” of heat and moisture that she dragged into the dog-days of the mid-latitudes.  Unfortunately for meteorologists the energy injected “intravenously” into the mid-latitudes upsets super-computer weather model forecasts. Models cannot handle the excitement!  The result is that various weather forecasts have struggled to agree and have been producing significantly differing outcomes as to the strength of Bertha and to when, where and IF she “makes landfall” in the UK.  Usually, the differences in model forecasts gradually reduce and forecasts increasingly agree, but in volatile atmospheric conditions it often goes down to the wire, like on this occasion!  This is why meteorologists have been unable to confidently pin down the exact track and strength and impact of Bertha on the UK.

Today there remains uncertainty, despite being only 48 hours out from her arrival.  The main cause of uncertainty is how she eventually interacts with an unseasonably strong and southerly jetstream.  Where a low pressure like Bertha arrives and moves under a jetstream makes a big difference to where she goes and whether she strengthens or weakens. The so-called “left-exit” region of a jet exerts most influence on deepening low pressures into significant storms.  Where the jet is slowing down or speeding up or curving can create extra lift, dragging air off the surface causing pressure to drop.

Here are three of the possible forecast outcomes from the GFS (including WRF/NMM which are similar), UKMET and ECMWF models. (Thanks to @BigJoeBastardi for the ECM model output from @WBAnalytics)

#1 Bertha could travel more or less down the eye through the English Channel from the SW.  This would take most of her highest winds and gales through France but would clip the south and SE of England with some heavy rain as the low crosses directly over this area.  The highest winds in these tight LOW pressure systems usually occur to the south of the LOW centre, associated with the warm and cold fronts.  The heaviest rainfall is modelled to fall to the north of the low associated with the occluded fronts and low pressure centre, hence the heavy rain clipping South and SE England. This outcome is currently favoured by the BBC and UKMET office models.

#2 Various other models, the GFS and WRF-NMM amongst them, favour a more northerly route and take the LOW from the SW approaches, through St George’s Channel, across Wales and into the heart of England before exiting into the North Sea.  Broadly speaking, this would be a worst-case scenario because the UK would take the brunt of both heaviest rain (to the north) and the highest winds (to the south).  Nevertheless, for @ridelondon and #Reigate and the SE this might be the preferred route because any fronts and poor weather would pass over our area relatively quickly on Sunday, the worst of it probably from around 9am through to 2pm.  It would also leave only trailing fronts depositing up to 10mm or so of rain across our region and some gusty conditions, wet for a while but nothing too drastic to speak of really.

#3 A third model, the ECMWF has, until recent runs been quite the outlier amongst all competing tracks, sending Bertha further south through France in earlier runs.  More recent ECM runs have played catch-up with GFS and now sends the LOW NE through the UK.  Significantly, ECM drops pressure to 987mb in the Irish Sea.

The outcome could, of course, be somewhere in between these two or something totally unexpected!  It is worth also pointing out that extra-tropical storms arriving in the UK are not uncommon and this one is likely to underwhelm in many places inland where gusts are unlikely to exceed 40mph and, in the SE maybe 30mph gusts will be the widespread maximum.  Coasts, hills and exposed areas are likely to see the worst of it with gusts up to 60mph or more.  Even here it will not be anything like as potent as our storms last winter where winds exceeded 90mph on occasions.  Nevertheless, with leaves on the trees there may be some blow-downs and loose branches and rain in some places might disrupt travel but it is doubtful that wide-spread chaos will ensue as compared with the morning after St Jude last October which was a lot more potent. See below.

 

The best advice is to watch weather warnings and updates carefully on Sunday for any changes.  RGSweather will be posting NOWCAST updates on twitter for Reigate and environs.  Here is the latest from NWS/NCEP showing storm force winds in the sea around the UK, note the unseasonably LOW central pressure on 985mb. In comparison, St Jude storm had a lowest overland central pressure of 976mb back in October 2013.

2014-08-08_19-19-29

Interestingly, the longer term weather impact of Bertha into next week is probably better modeled than her immediate track on Sunday.  She is forecast to move slowly up the North Sea and merge with and deepen her “parent” LOW to the N of Scotland.  This will introduce cooler unstable showery NW winds to the UK for the early part of next week. Showers and some more organised bands of rain are likely to be frequent visitors, especially to the NW and west coast.  It will also feel cooler and more breezy for the whole country.  So, put the balmy warm days of summer on hold next week.

http://www.bbc.co.uk/weather/features/28707422

http://www.bbc.co.uk/weather/features/28707422

 http://metofficenews.wordpress.com/2014/08/06/ex-bertha-more-likely-to-miss-uk/

Update: Fri pm

More model agreement now on a northerly track.  Potential for UK worst-case scenario. Wind and rain crossing the country.  Inland 30-40mph, coastal and hills poss 50-60mph.

2014-08-08_20-37-10

Reigate and UK weather looks increasingly unsettled through the next week or so as the jetstream goes up a gear and becomes much more active and autumnal, possibly culminating early next week with the delivery of our first extra-tropical storm of the season, Bertha, on a fast jet from across the Atlantic. More on her later.

 

This week a couple of significant secondary LOW pressure systems are forecast to spin up rapidly out of the SW bringing wet breezy weather in the South.  Wednesday early morning looks especially wet but with a possible rapid clearance behind the active front.  Rain totals could be significant and warrant warnings nearer the time, watch the UKMO for these.

Friday looks a similar performance, even wetter for some, with another secondary depression developing rapidly from a parent LOW over the mid-Atlantic near to S Iceland.  This secondary low emerges quickly from the south bringing potentially wet weather but it is further off so things could change…

Hurricane season in the Tropical Atlantic basin starts in June and ends in November.  The National Hurricane center in Miami keeps an expert eye on every development in the tropical Atlantic but forecasters in the UK also take a great interest in them too.  This is because hurricanes can impact the UK as extra-tropical storms.

Several tropical disturbances look like they could potentially affect UK weather in the next few weeks, Bertha being the first.  Of course, the UK has never seen a “true tropical hurricane” (i.e. with an eye, wind sustained no fronts etc etc) but we certainly get the remains of tropical storms (called extra-tropical storms when they leave the tropics and arrive in the mid-latitudes).  Many / most extra-tropical storms never make it to our shores (e.g. Humberto), many also die a death or veer off long before they have any real impact on the UK.  Some arrive as late summer or autumn depressions often characterised by wet and breezy weather: this could be Bertha next week.

potential Bertha tracks

potential Bertha tracks

Whether or not extra-tropical storms arrive on UK shores they can still inject a lot of heat and moisture and energy into the late summer and autumn mid Atlantic at mid-latitudes.  This can perk up UK weather significantly and spoil the dog days of August. This can cause trouble for forecasters as models become unreliable.  Bertha is significant because models seem to be agreeing, at this stage anyhow,  that she will arrive near the UK sometime late next weekend or Monday.  Some models show her deepening into a significant summer storm (GEM), others show her moving south into Biscay and producing a Spanish plume of warm/not thundery weather.  In any case, expect more rain this week and next and do keep an eye on forecasts as the weather gets interesting, I would not be surprised to see a flurry / slurry of weather warnings for heavy rain at times.

Finally, the overall outlook remains rather unsettled for us as the charts above show: slightly below average temps with no heat wave on the cards yet (although heat cannot be ruled out at times as a plume event is possible) and plenty of rain spikes with over 150% more rain than normal shown on the NCEP charts.  As a rough guide, daily rain totals in Reigate exceeding around 10mm would be considered a really wet day… so there are several potential candidates for some wet days in the next week or so.

Some places in Japan have recently seen more than 250mm IN ONE DAY in tropical storm Nakri, so our worst weather is still nothing compared with some global extremes. In addition, think of Japan as Super-Typhoon Halong bares down on the southern islands this week.

http://www.accuweather.com/en/weather-news/halong-becomes-super-typhoon-j/31488164

Update:

Unusually Bertha appears on Atlantic UKMO chart as a tropical storm status and is downgraded about 12 hours later to ex-TS Bertha.  The charts for late Sunday / Monday look astonishingly autumnal with a perky jetstream invigorating the storm under a trough. Nothing certain yet, as exact track and intensity is variable still but this is a turn up for the books.  Last year no ET-hurricanes made it to the UK, Humberto drifted off harmlessly in mid September and actually built us a nice HIGH pressure as it made “landfall” over Iceland.  Perhaps Bertha might do the same thing?