The massive heat wave in Europe is pretty alarming. Chiefly, because there have been a huge spike in heat related casualties; but as it pertains to this forum I am pretty curious about the vines there.Rioja and Rivera Del Duero for example are already seeing days over 40c (104f) and looks like more to come. Anyone in the area that can comment on the grapes/vineyards?
Unfortunately I'm not in Spain at the moment, so I can't comment on the specific situation there, but I can speak generally to the risks of early season heat events.
I'm guessing that the vines in Rioja and Ribera have probably gone through flowering but are not yet at veraison. While the high temperature is certainly concerning, as Geoff mentioned, early season heat spikes are lower risk than those that happen closer to harvest from a fruit quality perspective. The fruit is not susceptible to dehydration at this point or some of the chemical reactions that can compromise the flavor and color of almost-ripe grapes. There is some indication that temperatures over 35C can reduce fruit set, so for regions still going through flowering, yields may be a bit lower.
The main effect at this point in the season is that temperatures over 35C will halt all of the vine's metabolic processes. Photosynthesis and respiration stop, effectively freezing the vine's development until the weather cools off, so veraison and harvest timing may be delayed. Sunburn on the fruit can be an issue, but bush vines should be less susceptible to this. The biggest concern is if leaves get scorched, development could be slowed down further.
Grapevines (unlike grapes) are hardy. Let's just hope those conditions don't persist later in the growing season!
I understand that phenolic development is happening in this phase of growth, and hot weather can lead grapes to produce more of almost all of the phenol compounds. Should we expect bigger tannins and darker colors in the [red] wines from a vintage with heat spikes this early?
The short answer is, there's a good chance that this could be the case, but it's too early to tell.
As you mentioned, pre-veraison is an important time for phenolic development. The vine is making tannin and some of the precursors that will be be converted into anthocyanin (color) beginning at veraison. While tannin production is stimulated by higher temperatures, production of these precursors is more closely tied to sunlight exposure than heat (since these compounds act like "sunscreen" for the the vine).
Ultimately, the concentration of these compounds in wine depends largely on berry size which depends on the water status of the vine. High temperatures can induce water stress, limiting berry size, but this is not always the case, for example, if the spring was very wet or if irrigation is used generously.
The weather after veraison is really important too. Color is degraded by heat, so a warm end to the summer could ultimately reduce color. An important concept that's been getting more attention recently is the extractability of tannin. There can be a high concentration of tannins in the grapes, but low extractability, resulting in lower concentrations of tannin in the wines. A few researchers are looking at this, it seems that the weather after veraison is important, but there is still a lot that we don't understand.