الطقس اليوم جميل، لكن هناك مطر في المساء.

الـ is the Arabic definite article, roughly the.
الطقس means the weather (as a general concept, usually definite in Arabic). So الطقس اليوم جميل is literally the weather today is beautiful.

اليوم (today) commonly comes right after the topic to set the time: الطقس اليوم... = Today, the weather...
You can move it for emphasis, e.g. اليوم الطقس جميل (more like Today the weather is nice, emphasizing today), but الطقس اليوم جميل is very natural.

In present-tense Arabic, the verb “to be” is usually not written or said. This is called a verbless (nominal) sentence.
So الطقس اليوم جميل literally reads the weather today beautiful, meaning The weather is nice today.

جميل is masculine because الطقس is grammatically masculine. Adjectives agree with the noun in gender (and also definiteness/case/number).
If the noun were feminine, you’d use جميلة. For example:

• الأجواء اليوم جميلة = The atmosphere/weather today is nice (since الأجواء is treated as feminine plural).

Arabic uses its own comma ، (called فاصلة) which looks different from the English comma. It marks a pause just like in English.
So الطقس اليوم جميل، لكن... is punctuated naturally: The weather is nice today, but...

لكن means but/however. It’s a very common connector.
Other options include:

• ولكن (same meaning; often written with و = and/but)
• غير أنَّ / إلا أنَّ (more formal: however/except that)
  In this sentence, لكن is the straightforward choice.

هناك means there is/there are (literally there). The structure هناك + noun is a common way to introduce existence:

• هناك مطر = There is rain / There will be rain (context decides the time).
  المطر هناك would mean The rain is there, which sounds like you’re pointing to a location where the rain is, not simply stating that rain exists/occurs.

Without الـ, مطر is indefinite: rain (some rain), which is natural when you’re announcing a weather condition.
هناك المطر is generally not used. If you want the rain, you’d usually restructure the sentence, e.g. المطر سيكون في المساء = The rain will be in the evening.

الطقس is pronounced roughly at-taqs (with a “q” sound = ق).
Writing notes:

• It’s spelled ا ل ط ق س.
• The cluster قس at the end makes a consonant-heavy ending: -qs.
• In careful pronunciation: aṭ-ṭaqs (the ط is an emphatic “t”).

في المساء means in the evening. It’s a prepositional phrase:

• في = in
• المساء = the evening

You can also say مساءً (in the evening, adverbially), but في المساء is more straightforward and very common for learners.

Arabic often omits an explicit future marker when the context is clear. Here, في المساء (“in the evening”) naturally suggests a future forecast: there will be rain in the evening.
If you want to be explicit, you can add:

• سيكون هناك مطر في المساء = There will be rain in the evening
• سَتُمْطِرُ مساءً = It will rain in the evening (using a verb)