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Nick F
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Posts posted by Nick F
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Waters of 30C+ west of Bahamas may strengthen Irma again after its slight weakening during eye wall replacement - which has now completed.
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11 minutes ago, FetchCB said:
I have a question which hopefully someone can answer
In the UK it is often said that the more severe the depression/Storm then the more it curves poleward. is it the same with Hurricanes? If so then maybe if GFS is overdoing the depth of pressure within Irma then it would want to curve it poleward ?
This may partly answer your question http://www.aoml.noaa.gov/hrd/tcfaq/G5.html
In a nutshell, the semi-permanent sub-tropical ridge steers the hurricanes, easterlies on the equator side steer storms that form mid-Atlantic westwards towards Caribbean, as we've seen with Irma and Jose. But when they reach the western edge of the sub-trop ridge the flow swings southeasterly and southerly, the western periphery of the ridge currently over SE U.S. and Bahamas, so when a hurricane reaches here - they tend to head NW then N.
Now, differences between model tracks may lie over how much influence or weakness there is in the sub-top ridge on its western edge, as a slacker flow or trough digging SE over eastern U.S. will tend to pull a hurricane poleward, GFS perhaps doing this too soon with slight miscalculations in the steering flow on western edge of sub-trop ridge?
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25 minutes ago, Singularity said:
Typical... the moment Irma is taken a whisker below category 4 strength, deep convection wraps neatly right around the eye... as if in angry retort!
NHC forecast discussion mentioning that Irma is undergoing an eyewall replacement. With it continuing to move over warm waters of 30C and weak shear, likely it only slightly weaken and will maintain as cat 4 before Florida landfall, risk it could strengthen again, hopefully not for the sake of northern Cuba, SE Bahamas and ultimately Florida in its path.
Some interesting facts about Irma tweeted yesterday:
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Jose now a major hurricane, upgraded to cat. 3 with 120mph winds, unfortunately it looks like it could affect Antigua and Barbuda on Saturday, before swinging away NW then N towards Bermuda, by which time it may have weakened a fair bit.
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12z ECMWF ensembles suggest there's no dodging a bullet for Florida
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Storm & Convective Forecast
Issued 2017-09-07 22:09:53
Valid: Fri 8th Sept 6am to Sat 9th Sept 6amDay 1 Convective / Storm Forecast
Synopsis
An upper low and collocated surface low will drop southeastward across northern Britain on Friday. A strong SWly jet aloft will parallel a slow-moving waving cold front across southern England, to the north of the front, a cool and unstable showerly NWly flow will cover much of the UK.
... S ENGLAND ...
Frontal wave moving east and formation of triple point, as wave occludes, will likely combine with increasingly divergent cyclonic vorticity advection aloft ahead of trough moving SE to force ascent of warm moist air to south of frontal boundary. This may support, with ample vertical shear of 30-40 knots and strong SWly flow aloft, organised convection, perhaps some bowing line segments, capable of producing localised flooding, isolated strong wind gusts, hail, thunder and lighhtning. Have issued a MARGINAL risk for potential for isolated flash-flooding, given signal from GFS and ECM for 300-600 j/g CAPE and fairly high preciptable water (PWAT) content of warm sector airmass across S England.
... ELSEWHERE ...
Steep lapse rates generated by cool mid-level temps of upper low drifting SEwards will, with surface heating and warm SSTs, support scattered showers and weakly-electrified thunderstorms. Small hail, isolated localised flooding and gusty winds are possible with stronger showers or thunderstorms.
Issued by: Nick Finnis
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UK Severe Convective & Storm Forecast - Issued 2017-09-07 22:09:53Valid: Fri 8th Sept 6am to Sat 9th Sept 6amClick here for the full forecast
Day 1 Convective / Storm Forecast -
8 minutes ago, karyo said:
And moving northwestwards from what I can see.
Eye heading NW just north of Puerto Rico
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Still strengthening as it heads toward Puerto Rico. 914/915mb, just wow, what a monster
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Florida landfall looking more certain, just a question of where and how strong, two major hurricanes hitting mainland U.S. within a few weeks pretty jaw dropping ... oh then Jose could take a direct aim at Barbuda, St Martin and Puerto Rica ... so cruel
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Looking back at a warmer but wetter than average summer for the UK, and also looking across the Atlantic as more hurricane trouble brews.
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Looking back at a warmer but wetter than average summer for the UK, and also looking across the Atlantic as more hurricane trouble brews.
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Storm & Convective Forecast
Issued 2017-09-01 05:59:17
Valid: Fri 01 Sept 6am to Sat 02 Sept 6amDay 1 Convective / Storm Forecast
Synopsis
Upper trough axis will continue to move east of the UK on Friday, slack N to NWly flow will be unstable across E England closer to upper trough while ridge builds in across the W.
… E/SE ENGLAND and E MIDLANDS …
Steep lapse rates towards E England with proximity to colder mid-levels of upper trough, surface heating and local breeze convergence will support development of scattered heavy showers and isolated thunderstorms today. Weak vertical shear will mean severe weather is unlikely, showers/storms will be slow-moving – so may produce localised flooding, particularly where they cluster along convergence zones. Small hail is possible and conditions along convergence zones will be ideal for a few funnel clouds.
Issued by: Nick Finnis
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UK Severe Convective & Storm Forecast - Issued 2017-09-01 05:59:17Valid: Fri 01 Sept 6am to Sat 02 Sept 6amClick here for the full forecast
Day 1 Convective / Storm Forecast-
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Quite unusual for a major hurricane to form so far east over the tropical Atlantic too.
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Storm & Convective Forecast
Issued 2017-08-30 21:58:02
Valid: Thurs 31 Aug 2017 - 6am to Fri 01 Sept 2017 - 6amDay 1 Convective / Storm Forecast
Synopsis
Long wave upper trough axis will slide east across the UK during Thursday. A slack cyclonic and unstable westerly flow, veering NWly across the far west, covers the UK, supporting widespread showers and scattered thunderstorms.
... IRELAND, ENGLAND, WALES and S SCOTLAND ...
Upper trough and associated cold mid-levels will create steep lapse rates across much of the UK as it slides eastwards on Thursday ... which will support the development of heavy showers and scattered thunderstorms which, with light winds aloft, will drift slowly eastwards.
Troughs in the flow may focus showers/storms into slow-moving clusters capable of producing localised minor flooding, otherwise weak vertical shear / unidirectional wind profiles should discourage an organised severe risk, though there maybe isolated instances of hail to 1 - 1.5cm given steep lapse rates and potential for strong updrafts with surface heating in late August sunshine. Also, a few funnel clouds, which may even reach the ground as a brief/weak tornado, can't be ruled out - given favourable conditions of several wind convergence zones, light winds aloft and fairly low cloud bases.
Issued by: Nick Finnis
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UK Severe Convective & Storm Forecast - Issued 2017-08-30 21:58:02Valid: Thurs 31 Aug 2017 - 6am to Fri 01 Sept 2017 - 6amClick here for the full forecast
Day 1 Convective / Storm Forecast -
Storm & Convective Forecast
Issued 2017-08-29 09:20:33
Valid: Tues 29th Aug 6am to Weds 30th Aug 6amDay 1 Convective / Storm Forecast
Synopsis
An upper trough axis lies SW to NE across the far NE Atlantic and a cut-off upper low sits over Iberia to the west of an upper ridge over central, southern and eastern Europe. At the surface, a weak cold front will slide slowly SE across England and Wales, with a warm and huimid airmass across SE UK and an unstable showery flow across NW UK. A shallow area of low pressure covers the near continent which will deepen slightly and bring an area of heavy and perhaps thundery rain across SE UK tonight.
... SE UK ...
The NW edge of a plume of very warm and humid airmass residing on the near continent will sit across SE England today and overnight. Isolated elevated heavy showers, perhaps with sporadic lightning, may occur where Ac cas becomes deep enough with subtle forcing along NW edge of plume. Otherwise, overnight there is an increasing risk of embedded elevated thunderstorms within pulses of heavy rain developing and moving NE from N France / East English Channel across SE England ... as baroclinic zone and surface convergence developing over northern France in response to falling heights/temps from the NW create increased ascent of plume over near continent. There is a risk of minor localised flooding from these storms, mainly Sussex and Kent overnight.
... N. IRELAND and W SCOTLAND ...
With steepening lapse rates from the NW, as upper trough / cold pool slides SE, convection may become deep enough amongst the showers expected here to produce sporadic lightning and small hail.
Issued by: Nick Finnis
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UK Severe Convective & Storm Forecast - Issued 2017-08-29 09:20:33Valid: Tues 29th Aug 6am to Weds 30th Aug 6amClick here for the full forecast
Day 1 Convective / Storm Forecast-
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Brief heavy shower passing over here near Croydon, blue skies to the south, better shot of the London convective skies (earlier) in the tweet below
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A few heavy showers popping up across the SE, though no sferics, tower to my SE near Croydon
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06z AROME breaks out heavy showers (maybe thundery?) across SE corner early Tuesday morning
00z ECMWF had some rainfall across SE England between 06z and 12z Tuesday too.
Some weak CAPE spreading NE across the SE on Tuesday morning on both AROME and EC, but GFS shows no CAPE or rainfall what so ever.
Be interesting who's right in the end, AROME is normally quite good a picking up these elevated destabilisation convective situations.
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Becoming information overload on twitter now for Harvey, thanks to all the pro mets and U.S weather websites I decided to follow - mainly for storm chasing I do over there.
Convective/Storm Discussion Thread - 1st August onwards
in Storms & Severe Weather
Posted
Nice shelf cloud over Southend, Essex this evening