Tonight in Chutz Laaretz ותן טל ומטר לברכה

Tonight Tuesday evening December 4th 2018  Yidden in Chutz Laaretz begin to say ותן טל ומטר לברכה in Shmoneh esrei.

Halachos if one forgets to say during the winter:

If one forgot to say VeTen Tal UMatar in Barech Alenu,
if one remembers before saying Baruch Atta Hashem at the end of the bracha, one should go back to the VeTen Tal Umatar and continue from there.

if one remembers after saying Hashem's name in Barech Alenu, one should finish the bracha Mivarech HaShanim and not conclude with Lamdeni Chukecha.

if one remembers after saying the bracha of Mevarech HaShanim, one shouldn't say VeTen Tal UMatar Libracha right there unless one is likely to forget to say it in Shama Kolenu in which case one should say it right after Mevarech HaShanim. if one remembers during Shema Kolenu, one should say it before the words Ki Atta Shomea

if one remembers after saying Hashem's name of Shomea Tefillah, one should say Lamdeni Chukecha and return to Veten Tal prior to Ki Atta.
If one remembered after Shoma Tefillah before Retzeh one may say VeTen Tal UMatar Libracha right there.

if one remembered after beginning Retzay prior to saying Yeheyu LeRatzon after Elokay Netsor one should return to Brach Alaynu.

if one finished Yeheyu LeRatzon one should repeat Shmoneh Esrei.
Unsure if one forgot Ten Tal
If one is unsure whether one said VeTen Tal Umatar or not,
 if it's within 30 days, we assume one didn't say VeTen tal and so one has to repeat, however, after 30 days, one doesn't repeat.
If one said the line "VeEt Kol Minei Tevuata Litova VeTen Tal UMatar Livbracha" 90 times one is assumed to have said VeTen Tal if one is unsure whether one said it or not. Some say that one should say continue "Al Peni HaAdama". Some say one should repeat the line 101 times, but after the fact 90 time is sufficient.
Some say it's not proper to say this line 90 times because it puts oneself into a dispute, however, if one regularly is unsure whether one say Veten tal or not, one should say this line 90 times.
If one is sure that one had in mind to say the addition or change to Shmoneh Esrei before starting Shmoneh Esrei and also during Shmoneh Esrei, and then after one completed Shmoneh Esrei one is uncertain whether one said it, one shouldn't repeat Shmoneh Esrei. However, if one became uncertain during Shmoneh Esrei, one should assume one didn't say it and return.
If one is sure that one had in mind to say the addition or change to Shmoneh Esrei before starting Shmoneh Esrei but isn't certain that one remembered during Shmoneh Esrei, if after completing Shmoneh Esrei (or before) one becomes uncertain, one must repeat. However, if one only becomes uncertain a while after Shmoneh Esrei (after starting other prayers), one doesn't have to repeat.
However, if a person knows that he losses his thoughts in Shmoneh Esrei and could forget to say it even if one remembered a moment earlier, some authorities hold one should always return (in cases of doubt) unless one is nearly certain that one said it.