Learn Hardcore Turkish: Bugün hava durumu yağmur diyor, ona göre plan yapalım. - The weather forecast says rain today, so let’s plan accordingly.

Diyor is the present continuous of demek “to say,” so it literally means “is saying.”
Turkish often personifies sources of information: news, reports, and forecasts can “say” things. So Hava durumu yağmur diyor = “The weather forecast says ‘rain’.” This is completely natural and very common.

Why is there no verb after yağmur? Shouldn’t it be yağacak or yağmur yağacak?

In this construction, the forecast is “saying” a label or content word: yağmur (“rain”). It’s shorthand for fuller statements like:
- Bugün yağmur yağacak. (It will rain today.)
- Bugün yağmur bekleniyor. (Rain is expected today.)
So … yağmur diyor is an economical, idiomatic way to report what the forecast says.

What exactly does ona göre mean, and what does ona refer to?

-a/-e göre means “according to” or “in line with,” “accordingly.”
Ona is the dative form of o (“that/it/he/she”) and here refers back to the information just mentioned (the forecast).
You could also say:
- Buna göre plan yapalım. (“According to this …”) Slightly tighter reference to what was just said.
- Hava durumuna göre plan yapalım. (“According to the forecast …”) Most explicit.

Why is it ona göre, not some other case? What case does göre require?

Göre is a postposition that requires the dative case.
Pronouns and nouns take -a/-e (with buffer -y- if needed):
- bana göre, sana göre, ona göre
- Ali’ye göre
- hava durumuna göre

What form is yapalım? Is it a command?

Yapalım is the 1st person plural optative/volitional: “let’s do (it).”
It’s used to make suggestions: plan yapalım = “let’s make a plan.”
If you add the question particle, yapalım mı?, it becomes a softer, explicitly consultative suggestion (“shall we…?”).

Why plan yapalım and not planlayalım or plan edelim?

The most common everyday collocation is plan yapmak (“to make a plan”).
Planlayalım (from planlamak) is correct and a bit more formal/technical (“let’s plan”).
Plan etmek exists but is less idiomatic in casual speech; plan yapmak or planlamak are preferred.

What’s going on morphologically in hava durumu? Why the final -u?

Hava durumu is a standard noun–noun compound (belirtisiz isim tamlaması): “weather’s condition,” i.e., “weather conditions/the weather (forecast).”
In such compounds, the second noun takes a 3rd person possessive suffix (durum-u), while the first stays bare (hava).
As a set phrase, hava durumu commonly refers to the weather forecast in media and everyday talk.

Can I change the word order? For example: Hava durumu bugün yağmur diyor?

Yes. Time adverbials like bugün are flexible. All of these are acceptable, with slight differences in emphasis:
- Bugün hava durumu yağmur diyor. (Neutral; time-first is very common.)
- Hava durumu bugün yağmur diyor. (More focus on the forecast as the source.)
More marked orders are possible but less natural in everyday speech.

Is the comma before ona göre plan yapalım necessary?

It’s optional. You can write:
- Bugün hava durumu yağmur diyor, ona göre plan yapalım.
- Or split into two sentences: … diyor. Ona göre plan yapalım.
The comma simply signals a slight pause between “reason” and “suggestion.”

Could I use der instead of diyor? What about diye?

Der is the simple present and sounds generic/gnomic or literary. In modern speech for reporting current information, diyor is natural.
Diye is a quotative/subordinator meaning “saying/that/so that,” and it attaches to the quoted content before another verb:
- Hava durumu ‘yağmur’ diye uyarıyor. (“… warns, saying ‘rain’.”)
- It’s not a direct substitute for diyor in your sentence.

What are some other natural ways to express the same idea?

Bugün hava durumu yağmur gösteriyor. (“… shows rain.”)
Hava durumuna göre bugün yağmur yağacak.
Bugün yağmur bekleniyor.
Meteoroloji bugün yağmur diyor. All are idiomatic; pick based on the tone and how explicitly you want to reference the forecast.

What’s the difference between yağmur, yağmurlu, and yağmur var?

Yağmur = “rain” (noun). Used in labels and with existence/future: yağmur var/yağmur olacak.
Yağmurlu = “rainy” (adjective). Used to describe hava: Bugün hava yağmurlu.
Yağmur var = “there is rain” (often for current conditions). With forecasts, you’ll also hear Bugün yağmur olacak or yağmur bekleniyor.

Any pronunciation tips for words in this sentence?

Bugün: front rounded ü (lip rounding), stress usually on the second syllable: bu-GÜN.
Yağmur: the letter ğ lengthens the preceding vowel; it’s not a hard “g.” So roughly “yaa-mur.”
Diyor: written with one y (not “diyiyor”); pronounce as di-yor.
Göre: front rounded ö; “GÖ-re.”

Is the tone/register casual or formal? How would I make it more formal?

The given sentence is natural and neutral-casual—fine for everyday conversation.
More formal/explicit options:
- Hava durumuna göre bugün yağmur bekleniyor; buna göre plan yapalım.
- Meteoroloji Genel Müdürlüğü’nün verilerine göre bugün yağmur bekleniyor. (Very formal.)

Can ona göre also mean “in his/her opinion”? Could that be confusing?

Yes. X’e göre can mean “according to X (in X’s opinion).”
- Ali’ye göre bu plan iyi. (“According to Ali, this plan is good.”)
In your sentence, context makes it clear ona refers back to the forecast, not a person. If you want to avoid any ambiguity, use hava durumuna göre.