Allergic Rhinitis: Given the symptoms of nasal congestion, postnasal drip, sore throat, and sneezing, especially in a region like Seattle, WA, which has high pollen counts during certain times of the year, allergic rhinitis is the most likely diagnosis. The presence of sneezing, a common symptom of allergies, further supports this diagnosis.

Viral Upper Respiratory Tract Infection (URTI): Common colds are prevalent and can cause similar symptoms. The sore throat could be indicative of a viral infection.
Sinusitis: Either acute or chronic, sinusitis can present with nasal congestion, postnasal drip, and sore throat. The presence of facial pain or pressure could help differentiate this from allergic rhinitis.
Non-Allergic Rhinitis: This includes a variety of conditions that cause chronic nasal symptoms without an allergic basis, such as vasomotor rhinitis, which could be considered if allergic testing is negative.

Invasive Fungal Sinusitis: Although rare, this condition can present with similar symptoms and is critical to diagnose early, especially in immunocompromised patients, as it can be life-threatening.
Bacterial Pharyngitis (Strep Throat): While the primary symptom here is a sore throat, it's essential to consider bacterial causes, especially if the sore throat is severe or accompanied by fever, as untreated strep throat can lead to complications like rheumatic fever.
Anaphylaxis: If the symptoms are part of a more severe, systemic allergic reaction, anaphylaxis must be considered, especially if there's a history of severe allergies.

Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can cause a variety of symptoms including sinusitis, but would typically be accompanied by other systemic symptoms.
Sarcoidosis: A condition that can affect multiple parts of the body, including the sinuses, and could present with similar symptoms, though it would likely be accompanied by other systemic findings.
Tumor (Nasal or Sinus): Although extremely rare, a tumor in the nasal passages or sinuses could cause obstructive symptoms similar to those described, and would be an important diagnosis not to miss.