Genesis of Gap Wind Weather Advisory

April 2004 National Weather Service Volume 3, Number 2

Forecast Process: Genesis of Gap Wind Weather Advisory

Colin D. Sells, Meteorologist, Center Weather Service Unit, Anchorage, AK [email protected]

On March 30, 1982, 1,780 U.S. In what looked like a safe situa- In this Issue: Army paratroopers from the 82nd tion, 6 jumpers were killed and 158 Airborne Division jumped into drop injured, dragged by winds gusting up Forecast Process: zones at Ft. Irwin. to an estimated 40 mph. Genesis of Gap One of the drop zones was two An investigation concluded the Wind Weather miles long. Before the training jump, two locations where the winds had been Advisory 1 wind measurements were taken at measured were sheltered by high either end of the drop zone. The terrain. In between these points was a Alaska Aviation Weather wind speeds read 7 mph at one end, gap in the mountains. Winds gusted Unit: Providing and 11.5 mph at the other. the maxi- causing what the Army called a “Mass Aviation Weather mum safe wind speed for peacetime Casualty Incident.” Someone had Products and Services training drops was thought to be blundered. to the Alaskan Aviation 14.9 mph. Continued on Page 2 Community 5

When’s the Next Front? Would you like an email when a new edition of The Front is published? Write [email protected]

Managing Editor: Michael Graf Contributing Editor: Craig Sanders Editor/Layout: Melody Magnus [email protected]

Mission Statement To enhance aviation safety by increasing the pilot’s knowledge of weather systems and processes and National Weather Service products and services. Figure 1. Surface map, January 20, 2000

1 For the paratroop jump master, the variable outcomes resulting from this mix of weather and terrain were of lethal importance. This type of situation should constrain training activi- ties like the paratroop jump. Aviation meteorologists encoun- ter similar forecasting constraints for air traffic over elevated terrain and for pilots flying in the proximity of the terrain. This is a context in which theory meets reality.

Forecast Reality Just as a pilot recognizes certain patterns when flying and acts on the basis of past knowledge, so to does a forecaster. “The guidance stream may present an outcome that is not accounted for in the forecaster’s mental model,” said Figure 2. Relief map of southwest continental Alaska Curtis in 1998. It is still incumbent on the forecaster to recognize the appli- air activity, provide ample opportu- uncommon and was an important el- cable variables for the specific situa- nity for terrain induced weather to ement in the forecast, particularly given tion, weigh their values accordingly adversely affect aviation operations the local terrain. and produce an accurate and timely throughout the state of Alaska, espe- Figure 2 shows a portion of the product. cially in and around Anchorage. local terrain, including Prince William Alaska has more aircraft, per One rapidly developing phenom- Sound, northern Kenai Peninsula, Por- capita, than any other state in the enon involving the confluence of tage Pass and Turnagain Arm, which United States. These aircraft fly through weather and terrain is the gap wind, extends from Portage to Anchorage. an environment of constantly chang- one example of which killed the para- The municipality of Anchorage ing weather patterns, through troopers in 1982. occupies a triangle of land bordered 5,180,000 square kilometers. Gap winds can impact much by the Chugach Mountains to the east, Many of these aircraft operate out larger airborne objects than paratroop- Knik Arm to the northwest and of Anchorage and fly in close prox- ers, making these wind events of para- Turnagain Arm to the southwest. Ted imity to the rugged local mountain mount importance to all aviation near Stevens airport is located on the west- ranges. Anchorage’s major airport, Ted hilly or mountainous terrain. Here is a ern apex of the triangle. Stevens International, has more than more detailed look at a special gap Portage Pass, connecting the com- 240 daily arrivals, encompassing a va- wind pattern that affected Anchorage munities of Whittier and Portage at its riety of domestic and international air traffic. eastern and western ends respectively, passenger and cargo carriers. is a 16 kilometer gap in the Chugach Adjacent Lake Hood is the world’s Pattern Recognition Mountains. The elevation of this neck busiest seaplane base, with as many as The meteorological situation on of land is only 155 meters. The adja- 800 takeoffs and landings on a typical January 26, 2000, as indicated in Fig- cent portion of the Chugach, a range summer day. ure 1, (Page 1) involved an occluded more than 50 kilometers in width, has The local general aviation airport, front moving north over the Kenai an average elevation of 1300+ meters. Merrill Field, is one of the busiest in Peninsula. At the northern edge of Prince the nation, recording more than As the front progressed up the William Sound the wall of mountains 230,000 takeoffs and landings annu- peninsula, the orientation of the fron- is even higher, averaging about 2000 ally. The varied meteorological condi- tal boundary was northwest to south- meters elevation. tions, the local terrain and the varied east. Such an orientation is relatively

2 Time Site Wind Peak Wind was generating a gap wind event. The forecaster then determined that the 15z PANC N/A phenomena would intensify as the PATO N/A PAWR 07020G30KT N/A front moved north, and end only after the front itself had pushed past 16z PANC 00005KT Portage Pass. PATO 1133G48KT 71 PAWR 07020G30KT N/A Despite the modest 16Z winds at Anchorage, the forecaster issued a 17Z PANC 100009G17KT 34 PATO 1230G49KT 57 Center Weather Advisory (CWA) for PAWR 07020KT N/A low level wind shear at 1615Z (Table 2). As he was composing it, the first 18Z PANC 28010KT PATO N/A urgent pilot reports (PIREP), Table PAWR 06020KT N/A 3, started coming in. Shortly after- wards wind speeds jumped sharply 19Z PANC 31005KT PATO 06027G44KT 53 at Anchorage, wind sheer increased PAWR 06015G25KT N/A and a B747 reported Mdt-Svr turbu-

20Z PANC 14009KT lence. PATO 11013G18KT 38 Figure 3, Page 4, is a 0.5 veloc- PAWR 06015G25KT N/A ity scan taken at 1749Z from the Kenai 21Z PANC 21003KT 26 WSR-88D, clearly showing a distinct PATO 05017KT 31 finger of higher velocity winds tra- PAWR 06020G25KT N/A versing Portage. On the basis of this Table 1. Wind observations from PANC (Anchorage, AK), PATO (Portage, AK), and other observational data available, and PAWR (Whittier, AK) staff renewed the CWA at 1815Z. CWAs are warnings issued by the Table 1 shows the observations The density differential between NWS CWSU Meteorologist, in accor- for the morning of January 26. Note the frontal mass and that of the air dance with NWS Directive 10-803 that at 15Z winds at Whittier (PAWR) mass it was displacing had trapped a Support to Air Traffic Control Fa- were out of the east at 20 knots with large pool of air in Prince William cilities. They are designed as a short gusts up to 30 knots, actually not all Sound. Unable to escape through the range forecast “primarily for use by that unusual for that particular loca- northern end of the sound because of air crews to anticipate and avoid ad- tion; however, an hour later, Portage, the higher mountains there, the trapped verse weather conditions in the en at the other end of the pass, was re- mass of air was squeezing route and terminal environment.” porting a peak wind of 71 knots. through the only exit avail- Meanwhile, 65 kilometers away at able, the much lower Por- ZAN1 CWA the other end of Turnagain Arm, the tage Pass. The air mass was ZAN1 CWA 261815 ZAN CWA 102 VALID UNTIL 262015 winds at Ted Stevens were variable at picking up velocity, as it VCNTY ANC 5 knots. Computer generated model did so, in a classic example STRONG LLWS OF +/-15 TO 30 KTS WITHIN guidance products were unable to re- of the Bernoulli’s Theo- 25 NM RAD OF PANC...AND WITHIN 020 AGL OBS AND solve such a local incongruity. rem at work. FCST THE NEXT TWO HOURS. The duty forecaster at the Anchor- The comparatively age Center Weather Service Unit lower pressure at the Por- CDS JAN 00+ (CWSU) believed that the front would tage end of Portage pass, ZAN1 CWA continue to progress to the north. The combined with the fron- ZAN1 CWA 261615 ZAN CWA 102 VALID UNTIL 261815 forecaster was certain the front’s angle tal pressure on the pool of VCNTY ANC of orientation, parallel to Portage Pass air trapped in Prince Will- STRONG LLWS OF +/-15 TO 30 KTS WITHIN and relatively perpendicular to the iam Sound, resulted in an 25 NM RAD OF PANC...AND WITHIN 020 AGL OBS AND mountains at the north end of Prince increased velocity of flow FCST THE NEXT TWO HOURS. William Sound, would be a primary from Whittier through the contributor to the winds coursing pass to Portage. In effect, CDS JAN 00+ through the pass. the approach of the front Table 2. Center Weather Advisories (CWAs)

3 ANC UUA/OV ANC/TM 1533/FL005/TP B737/RM -15 KTS IAS DURD ANC UUA/OV ANC/TM 1554/FL006/TP MD80/R +/-15 KTS IAS DEP 24L ANC UUA/OV ANC/TM 1555/FLDURC/TP B747/RM +/-15 KTS IAS AT 005 DEP 24L ANC UUA/OV ANC/TM 1600/FLDURD/TP MD11/RM +/-10KT LLWS 005-SFC FINAL APCH RY14 ANC UUA/OV ANC/TM 1629/FLDURC/TP B747/TBCONT MOD/RM +15 KIAS AT MIDFIELD DEP RY6R ANC UUA/OV ANC/TM RY 6R/TM 1643/FLDURC/TP MD11/TB MOD-SEV 012-016 ANC UUA/OV ANC/TM RY 6R/TM 1636/FLDURC/TP DC10/TB LGT/RM +/- 20KT LLWS ANC UUA/OV ANC/TM 1710/FLDURC/TPB747/TB MDT-SVR SFC-060/RM LLWS +40/25KT Table 3. Urgent PIREPS.

Although these advisories can be is- land and sea breezes, diurnal moun- local wind flow with entirely different sued for the same phenomena de- tain and valley winds, thunderstorm consequences. scribed by advisories from WFOs, outflow winds, glacier induced The CWA, in addition to other they also regularly address meteoro- katabatic winds, down slope wind aviation warnings and advisories, can logical conditions that do not meet events or mountain wave turbulence; help the pilot make informed deci- national In-Flight Advisory criteria; however, this is exactly the kind of sions; however, the most important conditions that, in the opinion of the event pilots and meteorologists need concept addressed here is not to take CWSU Meteorologist, adversely im- to watch out for. weather conditions for granted. Just pact aviation safety. The Alaskan gap wind event cited as a good meteorologist must learn here was not common wind event and to account for patterns not in his “da- Summary therein lies the danger. A subtle change tabase,” so too must a pilot. Over the course of a career, an in the orientation of the front, by just aviator may not see many frontal in- a few degrees so that it was not paral- References duced gap winds as compared with lel to the pass, would have altered the Curtis, Joel C., 1998: The Forecast Process: One Forecaster’s Perspective. Reprint from First Conference on Artificial Intelligence January 1998, Phoenix Arizona by the Ameri- can Meteorological So- ciety, Boston, MA. Sells, Colin D. 1992: Some Statistics with Respect to Alas- kan Aviation in which Weather is a Cause/ Factor. Alaska Region Technical Attachment T-92-27. National Weather Service Weather Ser- vice Procedural Direc- tive 10-803, Support to the Air traffic Control Facilities and NWS Pro- cedural Statement WR15-2003 Support tp Air Trafic Facilities.

Figure 3. Velocity scan of the Kenai Peninsula

4 The Alaska Aviation Weather Unit: diverse weather conditions and high Providing Aviation Weather Products and Services dependence on aviation required spe- to the Alaskan Aviation Community cialized aviation weather services. Who would be better qualified and experi- enced to provide these services than Jeff Osiensky, Meteorologist In Charge, Alaska Aviation Weather Unit, Anchorage the meteorologists who live in Alaska? [email protected] Before the mid 1990s, the Weather Service Forecast Offices Weather is very complex in essentials: food, clothing, building ma- (WSFOs) in Anchorage, Juneau and Alaska. Weather conditions can terials and mail. Aviation is essential to Fairbanks provided services by issu- vary dramatically within only a the Alaskan way of life. ing Area Forecasts (FAs), SIGMETs few miles. Take a high latitude lo- The vast areas of Alaska, with its (WSs), AIRMETs (WAs) and Termi- nal Forecasts (FTs). These NWS text products were available to pilots mainly through the FAA Flight Service Stations. Major changes began to appear in the 1990s as use of the Internet ex- ploded. Maturing PC and communi- cation technology allowed people to access weather information in their homes. The aviation community asked for more direct, real time access to weather over the Web, and NWS answered with new and expansive Web pages. Traditional aviation text products provided good information, but the focus shifted toward the need for aviation weather graphics. Figure 1. Size of Alaska as compared with the 48 contiguous states. cation, combine it with extreme terrain over short distances, and add proximity to a large ocean body. It all adds up to produce some of the most extreme weather conditions observed on earth. The size of Alaska is remark- able, covering nearly 20 percent of the land area in the lower 48 contiguous states, see Figure 1. Add the fact that Alaska averages 16 times more aircraft per capita than any other state in the nation. Most of Alaska is reachable only by aircraft. The lifeline to these “bush” communities comes from pilots, who provide the Figure 2. Map depicting the 25 area forecast zones within Alaska.

5 products. The areas of responsibility of forecasters are divided into two sec- tions with the Alaska Range serv- ing as the dividing line. One forecaster is responsible for southern Alaska and the Volcanic Ash Advisory Center (VAAC). This forecaster deals with all products and services associated with volcanic ash. The second fore- Figure 3. Example of an AAWU Surface Weather graphic. caster is responsible for all of interior and northern Aviators quickly began to request cal aviation weather charts for Alaska. Alaska, north of the Alaska Range. even more graphical products. In 1995, The AAWU is staffed by 12 Alaska is divided into 25 forecast with the help of aviation organizations meteorologists. Each shift has two areas or zones. The size and shape of WSFOs and political efforts, the idea forecasters, responsible for issuing these zones were determined by simi- of a specialized aviation weather unit and updating the Area Forecasts, lar climatological characteristics, in Alaska became a reality. AIRMETs, SIGMETs and graphical see Figure 2, Page 5. The new Alaska Aviation Weather Unit (AAWU), staffed 24/7 with highly trained NWS forecasters, focuses on providing aviation weather text and graphics exclu- sively for Alaska. A great deal of customer interaction and outreach oc- curred to insure that the aviation community was fully involved in the process. One of the first significant changes in services was the rou- tine production and issuance of graphi- Figure 4. Example of an AAWU 24 hour Significant Weather graphic.

6 (FIR). The AAWU area of responsibility is the Anchorage FIR which encompasses 2.4 mil- lion square miles, see Figure 5, Page 7. The AAWU is also designated as the VAAC. The Anchorage VAAC is one of nine international centers that monitor and forecast the dispersion of volcanic ash. The gritty and chemical nature of volcanic ash poses significant dangers to aviation because of the damage it causes to aircraft engines. VAAC forecasters are trained to use a suite of cutting edge remote sensing tools, Figure 5. Map depicting the Anchorage Flight Information Region (FIR) and major North Pacific Jet including GOES, Routes. AVHRR and MODIS satellite imagery In other words, each zone should The AAWU also produces a vari- to detect volcanic experience common weather condi- ety of graphics products. Every 6 ash plumes. tions across its area on any given day; hours a series of four graphics is pro- Certain bands on the satellite’s however, it is not quite that simple. duced, see Figure 3. The MVFR/ spectrum of information are more Because of the diverse topography IFR chart, Turbulence, Icing and sensitive to volcanic ash and are used there are many sub-climatic zones Surface Weather charts are posted to to identify the plume. within the state. the AAWU Web site at http://aawu. Forecasters use meteorological Each forecast zone may consist of arh.noaa.gov. model wind field data and volcanic several sub-zones, with even more di- Every 12 hours, a Significant ash plume dispersion models to pre- verse weather conditions. AAWU Weather Chart is produced and posted pare a forecast of ash plume move- forecasters have a difficult task of on the Web for 24, 36, 48 and 60 ment. conveying this variability while keep- hours, see Figure 4, Page 6. These The Anchorage VAAC area of ing the product concise. A pilot who charts include surface weather, responsibility encompasses the entire may be flying from Northway to MVFR/IFR, moderate or greater tur- Anchorage FIR in addition to far east- Tanana, for example, might fly bulence and freezing levels. ern Russia. The VAAC works closely through as many as three sub-regimes The AAWU is also one of four with the Federal Aviation Administra- of weather even though the flight is Meteorological Watch Offices tion (FAA) Air Route Traffic Control traversing a single forecast zone. (MWOs) along with the Aviation Center’s Center Weather Service Unit AIRMETs are significant meteo- Weather Center in Kansas City, MO, meteorologists and air traffic manag- rological messages located in areas WFOs Honolulu and Guam. ers to warn aircraft of possible ash where occasional moderate or greater Each MWO has warning or encounters. turbulence and icing would exist. SIG- SIGMET responsibility for one or The VAAC also works very closely METs are issued for severe turbulence several Flight Information Regions with volcanologists from the Alaska and icing.

7 Figure 6. Map depicting the 9 Volcanic Ash Advisory Centers and their respective areas of responsibility.

Volcano Observatory in Anchorage This product also provides an ini- access site specific weather informa- and Fairbanks. tial, 6, 12 and 18 hour forecast of the tion across Alaska. The VAAC provides guidance to ash plume including plume height. The In the meantime, the AAWU will the MWOs that have SIGMET re- VAA is posted on the Web as well and continue to improve upon its current sponsibility on the projected move- is used quite extensively by the airlines. product suite in concert with the ment of the ash. Its products are avail- changing requirements of its aviation able on the Web at: http://aawu. Looking Toward the customers. arh.noaa.gov/vaac.php. Future... In Alaska, the AAWU is the Me- The AAWU is committed to serv- Author Biography teorological Watch Office and the ing its customer customers and part- Jeff Osiensky is the Meteorolo- VAAC. The principle product of the ners with excellence as we move for- gist In Charge of the Alaska Aviation VAAC is the Volcanic Ash Advisory ward in the development of new and Weather Unit in Anchorage. Prior to (VAA). improved forecast products and this position, Jeff served for 2 ½ years This text product gives specific services. as Meteorologist In Charge of the information about the volcano includ- The future of the AAWU is very Center Weather Service Unit located ing elevation, latitude and longitude, exciting as it embraces the digital fore- at the Anchorage Air Route Traffic see Figure 6. cast era. The gridded aviation database Control Center. Jeff has spent more of the future will allow customers to than 5 years in aviation weather forecastinin Alaska.