I nearly fell of my chair upon seeing the 12z GFS op run, and for a very different reason to the usual!
Focusing on the areas that look to recieve the highest values - the S-Central and SW regions:
First up, we have Tuesday's fairly active frontal system being followed by rapid and intense convective showers on Wednesday, driving well inland on very strong winds and supported by a fair bit of CAPE - conditions could become atrocious in the downpours. Going by what EURO-4 and WRF come up with, GFS looks to be killing off the convection too quickly as it runs inland. WRF delivers close to 30mm widely from the event as a whole (accounting for 2-5mm from the rain from today into tomorrow):
http://modeles.meteociel.fr/modeles/wrfnmm/runs/2014020312/nmm-25-48-0.png?03-17
...and EURO-4 is close to that, but with the 30mm values not extending quite so far inland.
The Met Office warnings speak of 20-30mm in 24 hours, with as much as 40mm over SW'rn hills, so it looks like their data supports WRF and to some extent EURO-4 over GFS.
Then, we have that 'swooping' system for Thursday into Friday, which is now being moved along more slowly by GFS, so engaging with the jet stream more effectively, so enabling it to track further north and cross the south, bringing 20-25mm widely (I actually think that's underdone given how much moisture would be wrapped up in that system).
WRF is bringing that system in too based on where it's at by +72 hours - both models have made major adjustments since their 00z runs.
After that one comes the weekend rain - mostly on Saturday from another active frontal system, delivering 14-15mm widely. By the end of that, the accumulations from 12z today look like this on the GFS 12z op run:
http://modeles2.meteociel.fr/modeles_gfs/runs/2014020312/132-777UK.GIF?03-12
...which is potentially a little on the low side, depending on the convective element discussed earlier. Notice how little the NE regions of England and Scotland recieve by comparison - they might as well be on another continent! At this rate everyone will be forced to move there
Now then - what about the phenominal setup progged by GFS for 7-8 days time which has persisted from the 06z to the 12z...? Well, that's about as bad as it gets really - moisture is scooped up from very far south, but instead of getting wrapped up into one dominant low, there's a whole load of smaller features competing for the goods - which leads to huge masses of rainfall with embedded waves and considerable convective potential at times too.
By 192 hours, before that event is even finished with us, the totals look like this:
http://modeles2.meteociel.fr/modeles_gfs/runs/2014020312/192-777UK.GIF?03-12
...which is just shocking, really. Factoring in convective underestimation and I reckon you're looking at over 100mm in that 8 day period across most of Wales, the SW and the Solent, with 70-80mm extending as far inland as Oxford and as far east as Dover. Based on past experience - chaining convection and the like - local totals on windward hill slopes could be in the order of 150mm or so.
ECM has less of a ridge acros Europe from day 7, which means that the troughing moves across the UK with greater ease and doesn't scoop up anywhere near as much of that airmass of relatively high 850's and moisture content - we stay on the cool side of things, but still have multiple disturbances to contend with, each bringing a period of moderate rain, with convevtion possible in between.
By way of, ahem, 'balance with GFS', the model then shows more of a disrupted trough for day 10 than GFS, which would mean a slower moving frontal system and hence a larger amount of precipitation.
So yeah... wetter than an otters pocket doesn't even do justice to the current model output
Originally Posted by: Stormchaser