You should expect the average temperature in Harlingen right-through September is 24, with highs being around 30 with low daily temperautes being 21 celsius. The table below includes more detail:
| Month | Degrees Celsius |
|---|---|
| Temperature | 24 |
| High Temperature | 30 |
| Low Temperature | 21 |
| Sunshine Per Day | 10 |
Useful Links: Harlingen Weather: Average Weather & Yearly Climate