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SmartD

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  1. Ex-Helene: Good consistency from global models with ex-Helene tracking NNE over Ireland into the Iceland gap as a deepening system. This is consistent with the limited climatology for these types of transition in the NE Atlantic - Debbie(1961), Gordon (2006) and Ophelia (2017). Nb that all three of these systems produced damaging winds over Ireland/W/NW UK. Study of the three previous storms suggests that transition produced a hybrid Shapiro-Keyser like structure similar to transitions in zonal flows and somewhat similar to wintertime rapid deepeners. However, this may not be reflected in Met Office analyses (ASXX) and in measures of rapid deepening (Sanders&Gyakum 'bomb' criterion). The hybrid structure typically consists of a deep warm core ('warm seclusion') around which exrtratropical-like conveyor belt features form. Analysis of simulations and satellite imagery ( Gordon & Ophelia) for the cases suggests they all produced sting jets soon after transition. By the time of 'landfall' over Ireland the surface wind field in the frontal fracture region was dominated by the cold conveyor belt jet with SJ(s) in close proximity above. A possible exception was Debbie where a SJ may have been reaching the surface directly in western Ireland. Typically this scenario produces extreme and damaging wind gusts over land.
  2. The difference between the GFS OP and GFS FV3 para is striking. Likely it's due to the different handling of the trough to the west, but treatment of convection with ex-Helene may be an issue as well. Here is the pressure of the PV (potential vorticity) =2 surface T120 VT 12 UTC 18Z September. The filament of PV extending from the large scale trough west and north of the UK and curving back to the Azores is key to the evolution of ex-Helene and is handled differently by FV3 along with the trough-ridge itself which appears sharper and less progressive in the OP run consistent with the weaker development. Unfortunately I couldn't retrieve MSLP from the Para file but it produces a much weaker system farther east. All the global models can struggle with trough extension- typically producing less sharp features than those verified, esp at T120. Therefore one should treat individual model solutions with a great deal of caution. However, on climatology grounds (comparing with Debbie (1961), Gordon (2006) and Ophelia (2017)) I would favour the western OP solution and this may not properly depict the intensity and trough interaction yet. David
  3. Did you mean right entrance?. This solution for Helene resembles ex-Debbie (1961), ex-Gordon (2006) and ex-Ophelia (2017) all of which produced damaging winds over Ireland/western UK. At the estimated time of transition the storms were near the right entrance of a meridional jet streak on the flank of a trough. Composite anomalies for the 3 cases from the NNRP.
  4. Blended WV & IR, GFS T+6 850hPa wind vectors (coloured by speed) and windspeed isotachs at 250 hPa valid 06 UTC.
  5. Greetings- nice thread,hope you don't mind me butting in...David As well as the tropical systems I think it's worth pointing out the conveyor belt of moisture affecting the NW UK. The pattern is just mobile enough to avoid very high accumulations over the Lakes and SW Scotland and the max in IVT (Integrated [water] Vapor Transport) is moving into the North Sea this morning. Radar accumulations suggest only ~50mm in 24hr in parts of the NW rather than the 100mm+ totals associated with 'Atmospheric River' flooding events. IR image https://aviationweather.gov/satellite MIMIC Total Precipitable Water http://tropic.ssec.wisc.edu/real-time/mtpw2m/product.php?color_type=tpw_nrl_colors&layer=tpw&prod=natl&timespan=24hrs&anim=html5 IVT product courtesy Ben Moore, http://www.atmos.albany.edu/student/bmoore/natlantic_ivt.html
  6. Yes, this is a very interesting storm which appears to be in the extreme category (on landfall) as October '87. David
  7. A reminder... Results from a reanalysis simulation of the October 87 storm: Out-going Long-wave Radiation (cloud top brightness temperature) at 18 UTC 15 October 1987, windspeed at 850 hPa. OLR and wspd 850 hPa at 02 UTC 16 October. The area of winds > 50 m/s over SE England is the Sting Jet. Diagnosed 10m gusts 02 UTC 16 October. David
  8. http://www.see.leeds.ac.uk/research/yorkshire-centre-of-the-royal-meteorological-society/ An open talk at School of Earth and Environment, University of Leeds. Wherein Prof Browning will discuss the role of the 'Sting Jet' in the Oct'87 storm and report on the latest research into the role of mesoscale jets in the damaging 3 Jan 2012 windstorm. Well worth a few hours of your time if you're in the area, David
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