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Good Friday Low Pressure System
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31-03-2010 9:23pm#1Our next weather comes early Friday morning as a low pressure system arrives from the west, bringing some snow on hills on its leading edge in the southern half of the country, in a moderate southerly wind.
The system is currently deepening south of Greenland, as upper divergence increases in the left exit region of a strengthening jet streak.
By 7am Friday morning the system becomes vertically stacked and reaches its minimum pressure of around 988hPa just off the Kerry coast. Precipitation rates will have reached their peak by then and will start to decrease as it pushes eastwards across the country. Still, it will initiall give spells of snow on hills above ~250m, especially in the southwest, but will turn more and more to rain as the warm sector crosses the south.
By midday Saturday, the backbending occlusion should still be giving some spells of precipitation, rain for most, but with snow lowering to around 200m later in the day behind it. By that stage precipitation should be dying out as a transient ridge approaches Saturday night, giving a cold night and a bright start to Sunday.9
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I am wondering if there could be some convective thundery showers developing on Friday afternoon as a slack trough develops on the back edge of the main frontal zone?
No sure of how unstable the airmass will be on the southern flank of the frontal mass, but at the moment it does look that southern areas esp could be at risk, but all depends on how north the main frontal zone will lodge, and at this very early stage, it looks like it may lodge/pivot over the northern half of the country, which could help lift temps in the southern half of the country if the sun breaks though from any considerable length of time, triggering off an few big mammas!
Maybe not though.
Edit: Latest DMI HIRLAM I think may be suggesting this as well:
although hard to read too much into yet.0 -
The Dublin Airport GFS forecast sounding does hint at a narrow window in the late afternoon where a few hundred joules of CAPE may be available, especially with some diabatic heating. Probably most likely in the southeastern third of the country. One to watch
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Tomorrow afternoon is indeed looking good for some of the biggest showers yet this year. The ingredients are there for an interesting afternoon for the southern half of the country.
With good boundary level moisture content, where the post-frontal cloud breaks, allowing decent insolation, several hundred Joules of CAPE are likely, and with equilibrium temperatures of possibly -45°C, electrification is highly likely. Heavy downpours of rain and hail could give localised flash flooding.
The key will be how much insolation we get - if the cloud breaks for long enough, and there is no cirrus shield, surface temperatures of 8°C or more should kick off the show.
I'm sure ESTOFEX will issue us with at least a thunderstorm area, if not a Level 1, in tonight's forecast.
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Certainly a notable bank of instability behind that approaching front alright:
Source: http://en.vedur.is/weather/observations/satellites/#teg=iceland0 -
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wild handlin wrote: »Anyone got anything more to add about the possability of thunderstorms tomorrow??
What parts of the country could we see a potential? Is it just the south, or could the West bear something??
Still a hard one to call Will. If heavy showers or thunderstorms do occur certainly the southern half of the country would be most at risk, but as stated yesterday it depends on how far north the frontal zone will move, which even at this stage seems a little uncertain.
Latest models have the mid western region within the highest threat zone but really could be a nowcast situation, should this situation evolve that is 0 -
snow ghost wrote: »Owen, they are the sacred hearts of Jesus - tis easter weekend you know, when Jesus turned rocks into Easter eggs. Everyone with a heart next to their profile when signed in is going to Heaven, anyone without one is doomed!!! :eek:
There's none beside mine Snow, I am doomed!
:( 0 -
Deep Easterly wrote: »There's none beside mine Snow, I am doomed!:(
Get thee behind me satan!
Ay least it's warm in Hell Deep.;)
Anyway you'll be well prepared for it... living so close to Heavenly Mayo, yet just over the border, must be like purgatory.0 -
snow ghost wrote: »Get thee behind me satan!
Ay least it's warm in Hell Deep.;)
Anyway you'll be well prepared for it... living so close to Heavenly Mayo, yet just over the border, must be like purgatory. 0 -
snow ghost wrote: »Owen, they are the sacred hearts of Jesus - tis easter weekend you know, when Jesus turned rocks into Easter eggs. Everyone with a heart next to their profile when signed in is going to Heaven, anyone without one is doomed!!! :eek::D:D 0
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The GFS, along with the UKMO, seem to be hinting at a band of potent showers striking the South and Southeast coasts tomorrow night in their last couple of runs also:
this is probably from the main area of instability that is forming on the the back end of the front as we speak as it swings around the forecast slack area of low pressure as it moves to or over the south of Ireland. Saturday could be another day for shower potential too. We'll see.0 -
Deep Easterly wrote: »Still a hard one to call Will. If heavy showers or thunderstorms do occur certainly the southern half of the country would be most at risk, but as stated yesterday it depends on how far north the frontal zone will move, which even at this stage seems a little uncertain.
Latest models have the mid western region within the highest threat zone but really could be a nowcast situation, should this situation evolve that is
Many thanks for that Deep Easterly. Hopefully something will develop tomorrow.0 -
The postfrontal instability comma system is associated with an upper vort max, which is progged to weaken as it passes to the south early Friday, before reintensifying over northern France later. It should therefore not directly be an issue for us.
ESTOFEX have gone with thunderstorms for the southern half of the country tomorrow afternoon, basically in line with the ideas posted above.
A cold front crosses UK and NW/N-France during the daytime hours from west to east with a classic postfrontal shower/thunderstorm set-up. A plume of well mixed maritime air and temporarily diabatic heating support adequate updraft strength during peak time heating for showers/thunderstorms. Marginal hail and strong wind gusts accompany those storms with the first risk probably being the more dominant one, given the lowering WBZ level. An isolated waterspout/tornado may develop mainly along the S/SW coast of UK, as LL lapse rates increase somewhat, but there are no indications for a more elvated tornado risk. Onshore activity diminishes after sunset with ongoing activity offshore (e.g. the English Channel). A large thunderstorm area may be the best choice with such a pattern and during a season, where insolation may assist in initiation over various places. Isolated thunderstorms may occur well to the east of the highlighted area over central France, but coverage will be negligible.0 -
The postfrontal instability comma system is associated with an upper vort max
Bet ya can't say that as gaeilge Su! :P
BTW, did anyone else see Jean Byrne's forecast after the 9 O'Clock news? Did you see on the graphics "ForeNoon"? Where did that come from? Might just be me, but I've never heard of that before ever - since when did the 'morning' become known as 'forenoon'???
New one on me.
Bring on the rain tomorrow forenoon, can't believe I'm missing heavy rain.0 -
One thing that is not favourable for intense shower set up tomorrow is the actual surface temps.
Friday's max temp forecast:
Forecast maxes of 8c to 10c across the country, with perhaps a degree or 2 leverage either side. With cold uppers forecast, this may not be a huge problem but ideally a 12c to 14c range at least at surface level would be more encouraging in creating imbalance and volatility.
Really hard one to call for tomorrow to be honest, but I really hope some of us see something. It is about time some big fecker showers started bubbling up over the country.0 -
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Tá ceart agat, níl clue ar bith agam!
I plotted a GFS forecast sounding for roughly the centre of the area of interest (52.5N 8W, or just west of Cashel) for 4pm tomorrow afternoon. A surface temperature/dewpoint of 8.3/5.6°C is giving 400J/kg of CAPE. With the GFS usually underdoing surface temperatures during solar heating by a degree or two, this CAPE is probably a conservative value, and in reality we could be talking 500-600J/kg. We need to keep an eye on how temperatures are reacting tomorrow afternoon, if we get above 8 then we're in business! But with wet ground, we will need prolonged solar heating to get this to happen, so it is indeed a close call.
File start time : 10 4 1 18 0
File ending time: 10 4 9 6 0
Chosen date in meteorological file: 10 4 2 15
YR: 2010 MON: 04 DAY: 02 HOUR: 15 AT POSITION: 353.0 143.5 LAT.: 52.50 LON.: -8.00
PRSS: 0.9770E+03
MSLP: 0.9891E+03
TPP6: 0.9766E-03
UMOF: -0.9656E-01
VMOF: -0.3564E+00
SHTF: 0.7500E+02
DSWF: 0.6240E+03
RH2M: 0.8320E+02
U10M: 0.4700E+00
V10M: 0.5325E+01
T02M: 0.2814E+03
TCLD: 0.6100E+02
SHGT: 0.9791E+02
CAPE: 0.4000E+03
CINH: 0.0000E+00
LISD: 0.2710E+03
LIB4: -0.1237E+01
PBLH: 0.1199E+04
PRESS HGT(MSL) TEMP DEW PT WND DIR WND SPD
HPA M C C DEG M/S
E = Estimated Surface Height
977. 98. 8.3 5.6 185.0 5.3
975. 120. 7.5 3.6 187.4 6.2
950. 333. 5.4 1.8 190.2 7.9
925. 550. 3.3 0.7 191.8 8.2
900. 772. 1.3 -0.3 193.9 8.5
850. 1229. -2.4 -4.3 199.0 8.6
800. 1707. -5.4 -12.0 204.1 8.4
750. 2210. -8.8 -15.4 204.0 7.8
700. 2740. -13.1 -17.4 205.8 7.8
650. 3300. -17.7 -21.4 209.8 7.4
600. 3893. -22.6 -25.0 214.5 7.2
550. 4524. -27.7 -29.4 215.1 7.8
500. 5202. -33.0 -35.5 209.9 8.1
450. 5934. -37.9 -46.3 187.7 8.5
400. 6738. -42.3 -59.0 159.3 8.8
350. 7632. -46.5 -63.1 148.8 8.8
300. 8647. -49.3 -65.8 172.7 6.7
250. 9839. -49.9 -66.0 217.3 6.8
200. 11298. -49.8 -273.1 242.0 10.6
150. 13167. -52.0 -273.1 255.4 10.7
100. 15787. -53.0 -273.1 266.4 10.0
50. 20203. -57.3 -132.8 273.7 12.1
20. 25946. -59.2 -273.1 269.2 22.90 -
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The Valentia 06Z sounding shows some marked mid/upper level cooling behind the front, with a convective temperature of around 7-8°C. With further upper cooling taking place throughout the day and a good clearance already making its way through Kerry and Cork, we look like we may be in business later!
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Despite the benign looking cloudscapes here there seems to be heavy showers breaking out at random in my locality. Still raining here but much lighter. 1.2mm so far since it started around 2.25pm. Temp back down to 6.5c at the mo.
Edit: Rain Rate back up to 18.0mm/ph
Edit Edit: Signs of intensification as it moves to my north:
Sharp clearance just to my south. Still stuck under heavy spurts here. Total from shower so far at 3.0mm.0 -
Showers are just starting to pop up on the radar now, and should intensify over the next couple of hours. The 12Z sounding shows a cool surface (6.2°C), but further inland temperatures are now up to 10°C and the latest satellite shows the convection nicely. Note the small circulation forming to the southeast of the Wexford coast, it's almost like a pseudo polar low system, but without the weather!
We should see maximum activity up to around 6pm before things start to diminish.0 -
That little circulation off the southeast coast seems to be in an area of increased upward motion (cyan) and cold temperature advection (dark blue) at 700hPa. The very steep low level lapse rates cause strong upward motion, which leads to the formation of the small meso low.
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Skies are moderately convective here at the mo but sunny spells are scant and very brief while temp remains stubbornly low. 8.1c currently. All the darker stuff is to my south and west.
Seems to be a few sharp showers bubbling up at the moment over parts of the midwestern region in particular:
Edit: Nice if small looking shower moving over Galway Bay/North Clare at the mo. Current Docks cam grab:
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