Daily Planning for thermal soaring
Much of my recent experience in forecasting for "today" has been competition forecasting.  I certainly don't claim to have 100% success, but in the 18 days I did in 2006 for example, only one really went wrong plus another when we should have been a little more patient and not called it a day quite so early.  (But the decision was influenced by the fact that it was the last day of the comp). 

My two comps in 2007 coincided with disastrous weather - I correctly predicted rain!
The 2006 'failed' day in question (8 Aug) had been expected it to be superb, and I had advised the task setter accordingly - he set an area task.  Sadly for me, an unexpected large area of wave induced medium cloud persisted over the Midlands and caused irritation but not total disaster.  The longest distance flown that day was over 500 kms, but it certainly wasn't the day of the year. Incidentally, when a remarkably similar scenario to that 'failure' happened two weeks later a the next comp, I got it right - I had learnt.
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Info is available via the links on the weather page
Reproduced here is the table of major links but there are many others on the weather page.
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Phil Layton's analysis and forecast chartsUK Met Office charts, temps, winds, etcRain forecast from GFS + pressurerain radar half hourly
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winds at 900 + 850 mbsRASP / blipmapsmeteosat hi-ressatpic hourly vis or i/r
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Winds+ aviation weatherTry, Tdew, cloud, etcSoundings measuredTAFs
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weathercommercewebcamsx/c weatherTafs & Metars back-up
Synoptic situation
This probably needs less attention than might be imagined.  If you are planning to fly, in all probability you will already have had a look at the charts. 
Remember that anticylonic curvature is good, cyclonic might mean showers.   Phil Layton especially recommended.
Plain Language forecasts
Again, you will doubtless have looked at TV or listened to the radio.  These are good up to a point. 
One of my favourite internet plain language forecasts is from weathercommerce.   They don't claim the impossible - forecasting isn't ever like that.  It is totally realistic with "confidence levels".  Take note of  forecast maximum temperatures
Satpics
Look at the position of approaching fronts - the top cover might stretch hundreds of kilometres ahead     Do confirm correct date
Study movement of cloud, and don't ignore high level cloud - this might be of major significance.
Radar (use in conjunction with satpics)
Frontal rain, particularly the movement animation, shows very clearly.  Met Office is as good as any. 
Showers are much more difficult to handle early in the day (but easier from mid-morning onwards), but for example, can give very useful points if say fringing the North Sea or coming in through the Cheshire Gap
Cloud & Rain forecasts
Many sources GFS being as quick and useful as any
Winds - flying and high level (cirrus drift)
Many sources.  My favourites are from weatheronline, Meteoblue (Basel),  Met Office F 214 and the various soundings forecasts
Webcams often are not optimised for the sky.  Nevertheless, used with prudence, they can be very useful
Airfield Forecasts (TAFs)
Surprisingly limited value.  Do remember that cloudbase forecasts are for the lowest during the period covered by the TAF 
Airfield Reports (METARs)
Very useful especially dewpoints.  I happen to prefer them in code.  But many will prefer the plain language X/C weather
Temperature maxima
You will probably have noted from the plain language sources, but weatheronline gives a good clue.  Add 2 degrees for the local hot spots
Dewpoints (Tdew)
Knowledge of Tdew is vital.  Remember that it might change during the morning - if could reduce as the ground dries or rise as overnight fog clears. 
An excellent forecast can be had from weatheronline subject to updating from latest airfield reports 
Soundings
Familiarity via the tutorial will give ideas as to trigger temperatures, potential shower development, etc
RASP / BLIP maps 
These are a little complicated but well worth understanding.  An abridged version is available and in due course, there are hopes for the DigiRating add on
Wave
Don't ignore possibility that wave might influence (enhance, kill) thermal activity even over flat lands. 
Experience has shown that widespread wave often occurs when the 5,000 ft (850 mbs) wind direction is between 290 and 320 degrees.
On occasions, large areas of wave-induced medium cloud that hardly moves can cut off the sun for extended periods in those wind directions.  
Sea Air Penetration and Sea Breeze Convergence
On shore winds can result in very poor convection for considerable distance inland.  See the sea-breeze tutorial
Summary check list with points that  might have been missed
  • General synoptic situation
  • Fronts and top cover
  • Rain - clearance or arrival or showers
  • Cumulus or blue?
  • Start of convection (any abrupt trigger or gradual)
  • Cu bases / blue depth.  Check on Tdew might point to areas more likely to be blue
  • Thermal strengths - rule of thumb is that climb rates approximate to cloud base, ie 4 knots with 4,000 base
  • Spreadout - look for "lids" on vertical growth
  • Shower potential
  • Cirrus top cover (satpics helpful)
  • Winds speed and direction.  Consider sea air penetration or sea breeze convergence
  • End of convection - later if tall clouds, earlier if blue/small cumulus.   Or of course, with approaching front
  • Wave influence - do consider in lee of hills and more generally in NW winds
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