Amounts and nature of future capital expenditures;

Expansion and growth of our business and operations;

i

Forward-looking statements are based on numerous assumptions, uncertainties, and risks that could cause future events or results to be materially different from those stated or implied in this report. Many of the factors that will determine these results are beyond our ability to control or predict. Specific factors that could cause actual results to differ from results contemplated by the forward-looking statements include, among others, the following:

Given the uncertainties and risk factors that could cause our actual results to differ materially from those contained in any forward-looking statement, we caution investors not to unduly rely on our forward-looking statements. We disclaim any obligations to and do not intend to update the above list or to announce publicly the result of any revisions to any of the forward-looking statements to reflect future events or developments.

ii

In addition to causing our actual results to differ, the factors listed above and referred to below may cause our intentions to change from those statements of intention set forth in this report. Such changes in our intentions may also cause our results to differ. We may change our intentions, at any time and without notice, based upon changes in such factors, our assumptions, or otherwise.

Because forward-looking statements involve risks and uncertainties, we caution that there are important factors, in addition to those listed above, that may cause actual results to differ materially from those contained in the forward-looking statements. For a detailed discussion of those factors, see Part I, Item 1A. Risk Factors in our Annual Report on Form 10-K for the year ended December 31, 2011.2012.

iiiii

PART I – FINANCIAL INFORMATION

ITEM 1. Financial Statements.

NORTHWEST PIPELINE GP

STATEMENT OF COMPREHENSIVE INCOME

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

1

NORTHWEST PIPELINE GP

BALANCE SHEET

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

2

NORTHWEST PIPELINE GP

BALANCE SHEET

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

3

NORTHWEST PIPELINE GP

STATEMENT OF CASH FLOWS

(Thousands of Dollars)

(Unaudited)

See accompanying notes.

4

NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

1. BASIS OF PRESENTATION

In this report, Northwest Pipeline GP (Northwest) is at times referred to in the first person as “we”, “us” or “our.”

Northwest is indirectly owned by Williams Partners L.P. (WPZ), a publicly traded Delaware limited partnership, which is consolidated by The Williams Companies, Inc. (Williams). As of September 30, 2012,At March 31, 2013, Williams holds an approximate 6668 percent interest in WPZ, comprised of an approximate 6466 percent limited partner interest and all of WPZ’s 2 percent general partner interest.

General

The accompanying interim financial statements do not include all the notes in our annual financial statements, and therefore, should be read in conjunction with the financial statements and notes thereto in our 20112012 Annual Report on Form 10-K. The accompanying unaudited financial statements include all adjustments both normal recurring and others which, in the opinion of our management, are necessary to present fairly our interim financial statements.

The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.

Certain reclassifications fromprior period amounts reported withinGeneral and administrativeTotal operating expenses toOperation and maintenance,related to certain employee related expensesin the Statement of $0.8 million and $2.3 million for the three and nine months ended September 30, 2011, respectively,Comprehensive Income have been madereclassified to conform to the presentation utilized incurrent presentation. The effect of the correction increasedOperation and maintenancecosts $1.3 million, decreasedGeneral and administrativeexpenses $0.5 million, and decreasedTaxes, other than income taxes$0.8 million, with no net impact onTotal operating expenses,Operating Income, orNet Incomefor 2012 Statement of Comprehensive Income..

2. RATE AND REGULATORY MATTERS

Rate Case Settlement Filing

On April 26, 2012, the Federal Energy Regulatory Commission (FERC) unconditionally approved Northwest’s Stipulation and Settlement Agreement (Settlement) filed on March 15, 2012. The supporting or non-opposing customers named in the Settlement represent approximately 99.5 percent of our long-term firm transportation and storage capacity. The Settlement specified an annual cost of service of $466.5 million and established a new general system firm transportation rate of $0.44 per dekatherm, a 7.4 percent increase over the current rate. New rates will become effective January 1, 2013, and will remain in effect for a minimum of 3 years and a maximum of 5 years.

3. CONTINGENT LIABILITIES AND COMMITMENTS

Legal Proceedings

We are a party to legal, administrative, and regulatory proceedings arising in the ordinary course of business.

Environmental Matters

We are subject to the National Environmental Policy Act and other federal and state legislation regulating the environmental aspects of our business. Except as discussed below, our management believes that we are in substantial compliance with existing environmental requirements. Environmental

5

NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

expenditures are expensed or capitalized depending on their future economic benefit and potential for rate recovery. We believe that, with respect to any expenditures required to meet applicable standards and regulations, the FERCFederal Energy Regulatory Commission (FERC) would grant the requisite rate relief so that substantially all of such expenditures would be permitted to be recovered through rates. We believe that compliance with applicable environmental requirements is not likely to have a material adverse effect upon our financial position or results of operations.

Beginning in the mid-1980s, we evaluated many of our facilities for the presence of toxic and hazardous substances to determine to what extent, if any, remediation might be necessary. We identified polychlorinated biphenyl (PCB) contamination in air compressor systems, soils and related properties at certain compressor station sites. Similarly, we identified hydrocarbon impacts at these facilities due to the former use of earthen pits and mercury contamination at certain natural gas metering sites. The PCBs were remediated pursuant to a Consent Decree with the U.S. Environmental Protection Agency (EPA) in the late 1980s, and we conducted a voluntary clean-up of the hydrocarbon and mercury impacts in the early 1990s. In 2005, the Washington Department of Ecology required us to re-evaluate our previous mercury clean-ups in Washington. Currently, we are conducting assessment and remediation activities for

NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

mercury and other constituents to bring the sites up to Washington’s current environmental standards. At September 30, 2012,March 31, 2013, we had accrued liabilities totaling approximately $6.8$6.3 million for these costs which are expected to be incurred through 2017. We are conducting environmental assessments and implementing a variety of remedial measures that may result in increases or decreases in the total estimated costs.

In March 2008, the EPA promulgated a new, lower National Ambient Air Quality Standard (NAAQS) for ground-level ozone. However, in September 2009, the EPA announced it would reconsider the 2008 NAAQS for ground-level ozone to ensure that the standards were clearly grounded in science, and were protective of both public health and the environment. As a result, the EPA delayed designation of new eight-hour ozone non-attainment areas under the 2008 standards until the reconsideration iswas complete. In January 2010, the EPA proposed to further reduce the ground-level ozone NAAQS from the March 2008 levels. In September 2011, the EPA announced that it was proceeding with required actions to implement the 2008 ozone standard and area designations. In May 2012, the EPA completed designation of new eight-hour ozone non-attainment areas. Based on the published designations, no Northwest facilities are located within the non-attainment areas. At this time, it is unknown whether future state regulatory actions associated with implementation of the 2008 ozone standard will impact our operations and increase the cost of additions to property, plant and equipment. Until any additional state regulatory actions are proposed, we are unable to estimate the cost of additions that may be required to meet thisany such new regulation.

Additionally, in August 2010, the EPA promulgated National Emission Standards for hazardous air pollutants (NESHAP) regulations that will impact our operations. The emission control additions required to comply with the hazardous air pollutant regulations are estimated to include capital costs in the range of $3 million$500 thousand to $4$1 million through 2013, the compliance date.

In FebruaryOn January 22, 2010, the EPA promulgated a final rule establishingset a new one-hour nitrogen dioxide (NO₂) NAAQS. In FebruaryThe effective date of the new NO₂ standard was April 12, 2010. This standard is subject to challenges in federal court. On January 20, 2012, the EPA determined pursuant to available information that no area in the country is violating the 2010 NO₂ NAAQS, and thus, designated all areas of the country as “unclassifiable/attainment,attainment.” meaning that information available at that time did not indicate that air quality in these areas exceeded the NAAQS. Also, at that time, the EPA noted its plan to deploy an expanded NO₂monitoring network beginning in 2013. However, on October 5, 2012, the EPA proposed a graduated implementation of the monitoring network between January 1, 2014 and January 1, 2017. Once three years of data is collected from the new monitoring network, the EPA will reassess attainment status with the one-hour NO₂ NAAQS. Until that time, the EPA or states may require ambient air quality modeling on a case by case basis to demonstrate compliance with the NO₂ standard. Because we are unable to predict the outcome of the EPA’s or states’ future assessment using the new monitoring network, we are unable to estimate the cost of additions that may be required.

6

NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

Safety Matters

Pipeline Integrity RegulationsWe have developed an Integrity Management Program that we believe meets the United States Department of Transportation Pipeline and Hazardous Materials Safety Administration final rule that was issued pursuantrequired to the requirements of the Pipeline Safety Improvement Act of 2002. The rule requires gas pipeline operators to develop an integrity management program for transmission pipelines that could affect high consequence areas in the event of pipeline failure. The Integrity Management Program includes a baseline assessment plan to be completed in 2012 along with periodic reassessments to be completed within required timeframes. In meeting the integrity regulations, we have identified high consequence areas and developed our baseline assessment plan. We are on schedule to complete the required assessments within the required timeframes.meet this regulation.

Currently, we estimate the cost to complete the required initial assessments and associated remediation through 2012 will be primarily capital in nature and range between $8 million and $10 million. Ongoing periodic reassessments and initial assessments of any new high consequence areas will be completed within the timeframes required by the rule. Management considers the costs associated with compliance with the rule to be prudent costs incurred in the ordinary course of business and, therefore, recoverable through our rates.

Other Matters

Various other proceedings are pending against us and are considered incidental to our operations.

Summary

We estimate that for all matters for which we are able to reasonably estimate a range of loss, including those noted above and others that are not individually significant, our aggregate reasonably possible losses beyond amounts accrued for all of our contingent liabilities are immaterial to our expected future annual results of operations, liquidity and financial position. These calculations have been made without consideration of any potential recovery from third-parties. We have disclosed all significant matters for which we are unable to reasonably estimate a range of possible loss.

4.NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

Other Commitments

We have commitments for construction and acquisition of property, plant, and equipment of approximately $33.3 million at March 31, 2013.

3. DEBT AND FINANCING ARRANGEMENT

Credit Facility

In September 2012, WPZ amended its existing $2 billion senior unsecured revolving credit facility agreement to increase its aggregate commitments by $400 million. This facility was also amended to provide an additional $400 million increase to be available under certain conditions in the future.

Total letter of credit capacity available to WPZ under the $2.4 billion credit facility is $1.3 billion. At September 30, 2012,March 31, 2013, WPZ had a total of $250 million outstanding under the credit facility and no letters of credit have been issued and no loans are outstanding under the credit facility.issued. We may borrow up to $400 million under the credit facility to the extent not otherwise utilized by WPZ and Transcontinental Gas Pipe Line Company, LLC. At September 30, 2012,March 31, 2013, the full $400 million under the credit facility was available to us.

5.4. FINANCIAL INSTRUMENTS

Fair Value of Financial Instruments

The following methods and assumptions were used to estimate the fair value of each class of financial instruments for which it is practicable to estimate that value:

Cash and advances to affiliate— – The carrying amounts of these items approximates their fair value.

7

NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

Long-term debt— – The disclosed fair value of our long-term debt, which we consider as a level 2 measurement, is determined by a market approach using broker quoted indicative period-end bond prices. The quoted prices are based on observable transactions in less active markets for our debt or similar instruments. The carrying amount and estimated fair value of our long-term debt, including current maturities, were $694.1 million and $834.1 million, respectively, at March 31, 2013, and $694.0 million and $835.6 million, respectively, at September 30, 2012, and $693.8 million and $826.3$840.2 million, respectively, at December 31, 2011.2012.

6.5. TRANSACTIONS WITH AFFILIATES

We are a participant in WPZ’s cash management program, and we make advances to and receive advances from WPZ.program. At September 30, 2012March 31, 2013 and December 31, 2011, the2012, our advances due to us by WPZ totaled approximately $53.3$98.7 million and $52.0$29.3 million, respectively. These advances are represented by demand notes.notes and are classified as Current Assets in the accompanying Balance Sheet. The interest rate on these intercompany demand notes is based upon the daily overnight investment rate paid on WPZ’s excess cash at the end of each month, which was approximately 0.01 percent at September 30, 2012.March 31, 2013. The interest income from these advances was minimal during the ninethree months ended September 30, 2012March 31, 2013 and September 30, 2011.March 31, 2012. Such interest income is included in “Other (Income) and Other DeductionsExpenses – Miscellaneous other (income) deductions,expenses, net” on the accompanying Statement of Comprehensive Income.

The Williams Companies, Inc. (Williams) charges its subsidiary companies for management services provided by it and other affiliated companies. Such corporate expenses charged by Williams, WPZ, and other affiliated companies, for the ninethree months ended September 30,March 31, 2013 and 2012, and 2011, were $32.1$16.0 million and $26.8$10.3 million, respectively. These expenses are included in “General and administrative expense” on the accompanying Statement of Comprehensive Income. Management considers the cost of these services to be reasonable.

We have no employees. Services necessary to operate our business are provided to us by an affiliate, Northwest Pipeline Services LLC (NPS). Pursuant to an administrative services agreement, NPS provides personnel, facilities, goods,Williams and equipment not otherwise provided by us that are necessary to operate our business.certain affiliates of Williams. In return, we reimburse NPSWilliams for all direct and indirect expenses it incurs or payments it makes (including salary, bonus, incentive compensation, pension and other benefits) in connection with these services. For the ninethree months ended September 30,March 31, 2013 and 2012, and 2011, we were billed $51.8$15.4 million and $46.7$15.9 million, respectively. Such expenses are primarily included in “General and administrative” and “Operation and maintenance” expenses on the accompanying Statement of Comprehensive Income.

During the periods presented, our revenues include transportation transactions and rental of communication facilities with subsidiaries of Williams. Combined revenues for these activities, for the nine months ended September 30, 2012 were minimal. Combined revenues for the nine months ended 2011 were $18.4 million. The reduction in revenues from 2011 is a result of Williams’ spin-off of its former exploration and production business, which was completed on December 31, 2011. These revenues, associated with transportation transactions, are now reflected with the revenues from outside parties.

NORTHWEST PIPELINE GP

NOTES TO FINANCIAL STATEMENTS

(Unaudited)

During the ninethree months ended September 30,March 31, 2012, and 2011, we declared and paid equity distributions to our parent of $107.5 million and $94.0 million, respectively.$28.0 million. No equity distributions were paid to our parent during the three months ended March 31, 2013. During October 2012,May 2013, we declared and paid equity distributions of $30.0$27.0 million to our parent.

During the ninethree months ended September 30,March 31, 2012, and 2011, we received contributions of $3.5$1.2 million and $4.4 million, respectively, from our parent to fund a portion of our expansion related expenditures for additions to property, plant, and equipment. In October 2012,No contributions were received from our parent authorized an additional $5.1 million capital contribution to us to fund a portion of our expenditures for additions to property, plant and equipment.during the three months ended March 31, 2013.

We have entered into various other transactions with certain related parties, the amounts of which were not significant. These transactions and the above-described transactions are made on the basis of commercial relationships and prevailing market prices or general industry practices.

8

Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations

GENERAL

The following discussion should be read in conjunction with the Financial Statements, Notes, and Management’s Discussion and Analysis contained in Items 7 and 8 of our 20112012 Annual Report on Form 10-K and with the Financial Statements and Notes contained in this Form 10-Q.

RESULTS OF OPERATIONS

ANALYSIS OF FINANCIAL RESULTS

This analysis discusses financial results of our operations for the nine-monththree-month periods ended September 30, 2012March 31, 2013 and 2011.2012. Variances due to changes in natural gas prices and transportation volumes have little impact on revenues, because under our rate design methodology, the majority of overall cost of service is recovered through firm capacity reservation charges in our transportation rates.

Our operating revenues increased $1.6 million.$7.4 million, or 7 percent, as compared to the first three months of 2012. This increase is primarily attributed to higher reservation charges and the timing effect of leap year in 2012.new rates, which became effective January 1, 2013.

Transportation service and gas storage service accounted for 98 percent and 2 percent, respectively, of our operating revenues for the period ended March 31, 2013, and 97 percent and 3 percent, respectively, of our operating revenues for both periods.the period ended March 31, 2012.

Operating expenses increased $10.7 million, or 6 percent,The increase in our net income for the first three months of 2013, as compared to the comparable period in 2012, is primarily due primarily to i) higher allocated overhead from affiliates of $5.3 million; ii) higher depreciation of $2.2 million, attributed to property additions; iii) higher employee incentive compensation expense of $1.4 million; and iv) higher pension costs of $1.3 million.the increase in operating revenues associated with our new rates.

CAPITAL EXPENDITURES

Our capital expenditures were $90.8$8.2 million and $72.5$21.0 million for the ninethree months ended September 30,March 31, 2013 and 2012, and 2011, respectively. We anticipate our total 2012Our capital expenditures will be between $130 million and $150 million. Of this total, $55 million to $65 millionestimate for 2013 is considered nondiscretionary due to legal, regulatory, and/or contractual requirements.discussed in our 2012 Annual Report on Form 10-K. This estimate includes the following new capital projectsproject proposed by us:

North and South Seattle Lateral Delivery ExpansionsExpansion

We have executed agreementsan agreement with Puget Sound Energy to expand the North and South Seattle laterals andLateral to provide an additional lateral capacity of approximately 84 MDth per day and 74 MDth per day respectively. We estimate the expansion of the two lateralscapacity. The project is on schedule for a November 2013 in-service date, and is estimated to cost between $28$23 million and $32$25 million. North Seattle is scheduled for a November 2012 in-service date and South Seattle is scheduled for a fall 2013 in-service date.

9

Item 4. Controls and Procedures

Our management, including our Senior Vice President – West and our Vice President and Treasurer,Chief Accounting Officer, does not expect that our disclosure controls and procedures (as defined in Rules 13a-15(e) and 15d-15(e) of the Securities Exchange Act) (Disclosure Controls) or our internal controls over financial reporting (Internal Controls) will prevent all errors and all fraud. A control system, no matter how well conceived and operated, can provide only reasonable, not absolute, assurance that the objectives of the control system are met. Further, the design of a control system must reflect the fact that there are resource constraints, and the benefits of controls must be considered relative to their costs. Because of the inherent limitations in all control systems, no evaluation of controls can provide absolute assurance that all control issues and instances of fraud, if any, within Northwest have been detected. These inherent limitations include the realities that judgments in decision-making can be faulty and that breakdowns can occur because of simple error or mistake. Additionally, controls can be circumvented by the individual acts of some persons, by collusion of two or more people, or by management override of the control. The design of any system of controls also is based in part upon certain assumptions about the likelihood of future events, and there can be no assurance that any design will succeed in achieving its stated goals under all potential future conditions. Because of the inherent limitations in a cost-effective control system, misstatements due to error or fraud may occur and not be detected. We monitor our Disclosure Controls and Internal Controls and make modifications as necessary; our intent in this regard is that the Disclosure Controls and Internal Controls will be modified as systems change and conditions warrant.

Evaluation of Disclosure Controls and Procedures

An evaluation of the effectiveness of the design and operation of our Disclosure Controls was performed as of the end of the period covered by this report. This evaluation was performed under the supervision and with the participation of our management, including our Senior Vice President – West and our Vice President and Treasurer.Chief Accounting Officer. Based upon that evaluation, our Senior Vice President – West and our Vice President and TreasurerChief Accounting Officer concluded that these Disclosure Controls are effective at a reasonable assurance level.

Third-Quarter 2012First-Quarter 2013 Changes in Internal Controls

There have been no changes during the thirdfirst quarter of 20122013 that have materially affected, or are reasonably likely to materially affect, our Internal Controls.

10

PART II. OTHER INFORMATION

Item 1. LEGAL PROCEEDINGS.

The information called for by this item is provided in Note 3.2. Contingent Liabilities and Commitments, included in the Notes to Financial Statements included under Part 1, Item 1. Financial Statements of this Form 10-Q, which information is incorporated by reference into this item.

11

ITEM 6. EXHIBITS

The following instruments are included as exhibits to this report.

12

13

SIGNATURE

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

Date: October 31, 2012

EXHIBIT INDEX

SIGNATURE

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

Date: May 8, 2013

EXHIBIT INDEX