Subarachnoid Hemorrhage (SAH): This is often considered the most likely diagnosis for a thunderclap headache due to its severity and sudden onset, which can mimic the description of a "thunderclap." SAH is a medical emergency that requires immediate attention.

Migraine: Although typically not as severe as a thunderclap headache, some migraines can present with sudden, severe pain. However, they usually have other accompanying symptoms such as aura, nausea, or sensitivity to light and sound.
Tension Headache: While less likely to be described as a thunderclap, severe tension headaches can have a rapid onset and may be considered in the differential, especially if other symptoms like neck pain or scalp tenderness are present.
Cluster Headache: Known for their severe, unilateral pain, cluster headaches can start suddenly and reach maximum intensity within minutes, potentially fitting the thunderclap description.

Subarachnoid Hemorrhage (SAH): Already mentioned, but crucial enough to reiterate due to its high morbidity and mortality if missed.
Cervical Artery Dissection: This condition involves a tear in the wall of one of the arteries in the neck, which can lead to a stroke. It often presents with sudden, severe headache and neck pain.
Venous Sinus Thrombosis: A blood clot in the venous sinuses of the brain can cause a sudden, severe headache, among other symptoms. It's a rare but potentially life-threatening condition.
Pituitary Apoplexy: A condition where there is sudden hemorrhage or infarction of the pituitary gland, leading to severe headache, visual disturbances, and potentially life-threatening hormonal imbalances.

Reversible Cerebral Vasoconstriction Syndrome (RCVS): Characterized by sudden, severe headaches and reversible vasoconstriction of the cerebral arteries, often without an identifiable cause.
Primary Thunderclap Headache: A rare condition where the headache is the only symptom, and no underlying cause can be found despite extensive investigation.
Colloid Cyst: A rare, benign brain tumor that can cause obstructive hydrocephalus, leading to sudden increases in intracranial pressure and severe headache.