I'm in the Hudson Valley and the only thing farther south than me is NYC and Jersey, yet I still don't understand what people mean when they say "Southern Tier".
(yes, that STNY post about AK's made me wonder)
The Southern Tier is a geographical term that refers to the counties of New York State west of the Catskill Mountains along the northern border of Pennsylvania. It is a loosely defined term that generally includes the counties on that border Pennsylvania west of Delaware County inclusive.
The region is bordered to the south by the Northern Tier of Pennsylvania, and together these regions are known as the Twin Tiers.
The counties (and major cities and towns) generally included in the Southern Tier are:
Delaware County – Village of Delhi & Village of Sidney & Village of Hancock
Broome County – Binghamton, Endicott, Johnson City & Vestal
Tioga County – Owego, Barton & Waverly
Chemung County – Elmira & Horseheads
Steuben County – Corning, Bath & Hornell
Allegany County – Wellsville, Belmont & Alfred
Cattaraugus County – Olean, Salamanca, Ellicottville, Randolph & Little Valley
Also occasionally included in the "Southern Tier" designation less frequently are Schuyler County, Chautauqua County (primarily the region in the Conewango Creek watershed, other areas are part of the Niagara Frontier), Yates County, Cortland County, Tompkins County, and (far more rarely) Schoharie County, Chenango County, and Otsego County (the last three of these are more commonly considered part of the "Central Leatherstocking Region"). In 2009 the National Weather Service added Wyoming County to its definition.
The actual definition varies significantly. The Encyclopedia of New York State lists only Chautauqua, Cattaraugus, Allegany, and Steuben Counties as part of the Southern Tier, with anything east of that being considered Central New York. Other definitions define it as the Elmira, NY MSA, the Corning, NY μSA and the Binghamton, NY MSA, which includes Steuben, Chemung, Tioga and Broome Counties but not Chautauqua, Cattaraugus or Allegany, which are considered Western New York.
In virtually all contexts, the Southern Tier is considered a part of the broader Upstate New York region.
Much of the Southern Tier, except for Allegany and Cattaraugus Counties, is in area code 607. The westernmost portion of the Southern Tier is also located in New York's 29th congressional district as of 2007; the rest is divided among New York's 24th congressional district and New York's 22nd congressional district, the last of these containing a curious gerrymander ribbon that runs through Binghamton, connecting the heavily liberal enclave of Ithaca to other like-minded areas in upstate.
I have always assumed it starts at Binghamton and heads West from there. Basically where the Southern Tier Expressway starts.