UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): June 13, 2016

Northern Tier Energy LP

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (602) 302-5450

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On June 13, 2016, Northern Tier Energy LP (“NTI”) issued a press release announcing the declaration of a cash distribution of $0.18 per NTI common unit for the prorated period of April 1, 2016, through June 13, 2016. The prorated distribution is conditioned upon the closing of the proposed merger between NTI and a wholly-owned subsidiary of Western Refining, Inc. (“WNR”), and is expected to be paid together with the merger consideration on the closing date (the “Closing Date”) of the proposed merger to NTI common unitholders of record immediately prior to the Closing Date.

The press release is furnished as Exhibit 99.1 to this Current Report on Form 8-K and incorporated by reference herein.

Information about the Proposed Merger and Where to Find It

This Current Report on Form 8-K may be deemed to be solicitation material in respect of the proposed merger of NTI and a subsidiary of WNR. In connection with the proposed merger, WNR filed with the Securities and Exchange Commission (the “SEC”) a Registration Statement on Form S-4 that includes a proxy statement of NTI that also constitutes a prospectus of WNR. The Registration Statement was declared effective by the SEC on May 23, 2016. NTI commenced mailing to its security holders a definitive proxy statement/prospectus on or about May 23, 2016. WNR and NTI also plan to file other documents with the SEC regarding the proposed transaction. INVESTORS AND SECURITY HOLDERS ARE URGED TO CAREFULLY READ THE REGISTRATION STATEMENT, PROXY STATEMENT/PROSPECTUS AND ANY OTHER DOCUMENTS THAT HAVE BEEN FILED OR WILL BE FILED WITH THE SEC IN CONNECTION WITH THE PROPOSED MERGER OR INCORPORATED BY REFERENCE IN THE PROXY STATEMENT/PROSPECTUS. Investors and security holders may obtain free copies of the proxy statement/prospectus and other documents containing important information about WNR and NTI once such documents are filed with the SEC through the website maintained by the SEC at www.sec.gov. Copies of the documents filed with the SEC by WNR will be available free of charge on WNR’s website at www.wnr.com under the “Investor Relations” section or by contacting WNR’s Investor Relations Department at (602) 286-1530. Copies of the documents filed with the SEC by NTI will be available free of charge on NTI’s website at www.northerntier.com under the “Investors” section or by contacting NTI’s Investor Relations Department at (602) 302-5450.

This communication shall not constitute an offer to sell or the solicitation of an offer to sell or the solicitation of an offer to buy any securities, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction. No offer of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

Participants in Solicitation Relating to the Merger

NTI, WNR and certain of their respective directors and executive officers may be deemed to be participants in the solicitation of proxies from the common unitholders of NTI in connection with the proposed merger. Information about the directors and executive officers of the general partner of NTI is set forth in the 2015 Annual Report on Form 10-K for NTI, which was filed with the SEC on February 26, 2016. Information about the directors and executive officers of WNR is set forth in the Proxy Statement on Schedule 14A for WNR’s 2016 annual meeting of shareholders, which was filed with the SEC on April 22, 2016. These documents can be obtained free of charge from the sources indicated above. Other information regarding the participants in the proxy solicitation and a description of their direct and indirect interests, by security holdings or otherwise, is also contained in the proxy statement/prospectus and may be contained in other relevant materials to be filed with the SEC when they become available.

Forward-Looking Statements

This Current Report on Form 8-K contains certain “forward-looking statements” which reflect NTI’s views and assumptions on the date of this press release regarding future events. These forward-looking statements include statements about, among other things, future: payment of distributions including the amount and timing thereof, total distribution net of customary reserves; expenses, reserves and estimates relating thereto; operating performance, cash flows, fluctuations in crude oil and feedstock prices, finished product prices, capital expenditures, working

capital requirements, inventory fluctuations and other cash reserves deemed necessary or appropriate by the board of directors of its general partner, approval of the merger at a special meeting of common unitholders and the expected close of the merger of NTI with WNR on or about June 24, 2016, or at all. They involve known and unknown risks, uncertainties and other factors, many of which may be beyond its control, that may cause actual results to differ materially from any future results, performance or achievements expressed or implied by the forward-looking statements. All forward-looking statements speak only as of the date hereof, NTI undertakes no obligation to update or revise publicly any such forward-looking statements, and NTI cautions you not to place undue reliance on these forward-looking statements. Please refer to NTI’s filings with the SEC for more detailed information regarding these risks, uncertainties and assumptions.

This communication serves as a qualified notice to nominees and brokers as provided for under Treasury Regulation Section 1.1446-4(b). Please note that 100 percent of NTI’s distributions to foreign investors are attributable to income that is effectively connected with a United States trade or business. Accordingly, NTI’s distributions to foreign investors are subject to federal income tax withholding at the highest effective tax rate.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Date: June 13, 2016

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