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April 2004 National Weather Service Volume 3, Number 2

Mission StatementTo enhance aviation safety by

increasing the pilot’s knowledge ofweather systems and processes andNational Weather Service productsand services.

Managing Editor: Michael GrafContributing Editor : Craig SandersEditor/Layout: Melody Magnus Melody.Magnus@noaa.gov

In this Issue:

Forecast Process:Genesis of GapWind Weather Advisory

Alaska Aviation WeatherUnit: ProvidingAviation WeatherProducts and Servicesto the Alaskan AviationCommunity

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On March 30, 1982, 1,780 U.S.Army paratroopers from the 82nd

Airborne Division jumped into dropzones at Ft. Irwin.

One of the drop zones was twomiles long. Before the training jump,wind measurements were taken ateither end of the drop zone. Thewind speeds read 7 mph at one end,and 11.5 mph at the other. the maxi-mum safe wind speed for peacetimetraining drops was thought to be14.9 mph.

In what looked like a safe situa-tion, 6 jumpers were killed and 158injured, dragged by winds gusting upto an estimated 40 mph.

An investigation concluded thetwo locations where the winds had beenmeasured were sheltered by highterrain. In between these points was agap in the mountains. Winds gustedcausing what the Army called a “MassCasualty Incident.” Someone hadblundered.

Forecast Process: Genesis of Gap Wind Weather AdvisoryColin D. Sells, Meteorologist, Center Weather Service Unit, Anchorage, AKColin.Sells@noaa.gov

Figure 1. Surface map, January 20, 2000

Continued on Page 2

When’s the Next Front?Would you like an email when a

new edition of The Front is published?Write melody.magnus@noaa.gov

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For the paratroop jump master,the variable outcomes resulting fromthis mix of weather and terrain wereof lethal importance. This type of situ-ation should constrain training activi-ties like the paratroop jump.

Aviation meteorologists encoun-ter similar forecasting constraints forair traffic over elevated terrain and forpilots flying in the proximity of theterrain. This is a context in which theorymeets reality.

Forecast RealityJust as a pilot recognizes certain

patterns when flying and acts on thebasis of past knowledge, so to does aforecaster.

“The guidance stream may presentan outcome that is not accounted forin the forecaster’s mental model,” saidCurtis in 1998. It is still incumbent onthe forecaster to recognize the appli-cable variables for the specific situa-tion, weigh their values accordinglyand produce an accurate and timelyproduct.

Alaska has more aircraft, percapita, than any other state in theUnited States. These aircraft fly throughan environment of constantly chang-ing weather patterns, through5,180,000 square kilometers.

Many of these aircraft operate outof Anchorage and fly in close prox-imity to the rugged local mountainranges. Anchorage’s major airport, TedStevens International, has more than240 daily arrivals, encompassing a va-riety of domestic and internationalpassenger and cargo carriers.

Adjacent Lake Hood is the world’sbusiest seaplane base, with as many as800 takeoffs and landings on a typicalsummer day.

The local general aviation airport,Merrill Field, is one of the busiest inthe nation, recording more than230,000 takeoffs and landings annu-ally. The varied meteorological condi-tions, the local terrain and the varied

air activity, provide ample opportu-nity for terrain induced weather toadversely affect aviation operationsthroughout the state of Alaska, espe-cially in and around Anchorage.

One rapidly developing phenom-enon involving the confluence ofweather and terrain is the gap wind,one example of which killed the para-troopers in 1982.

Gap winds can impact muchlarger airborne objects than paratroop-ers, making these wind events of para-mount importance to all aviation nearhilly or mountainous terrain. Here is amore detailed look at a special gapwind pattern that affected Anchorageair traffic.

Pattern RecognitionThe meteorological situation on

January 26, 2000, as indicated in Fig-ure 1, (Page 1) involved an occludedfront moving north over the KenaiPeninsula.

As the front progressed up thepeninsula, the orientation of the fron-tal boundary was northwest to south-east. Such an orientation is relatively

uncommon and was an important el-ement in the forecast, particularly giventhe local terrain.

Figure 2 shows a portion of thelocal terrain, including Prince WilliamSound, northern Kenai Peninsula, Por-tage Pass and Turnagain Arm, whichextends from Portage to Anchorage.

The municipality of Anchorageoccupies a triangle of land borderedby the Chugach Mountains to the east,Knik Arm to the northwest andTurnagain Arm to the southwest. TedStevens airport is located on the west-ern apex of the triangle.

Portage Pass, connecting the com-munities of Whittier and Portage at itseastern and western ends respectively,is a 16 kilometer gap in the ChugachMountains. The elevation of this neckof land is only 155 meters. The adja-cent portion of the Chugach, a rangemore than 50 kilometers in width, hasan average elevation of 1300+ meters.

At the northern edge of PrinceWilliam Sound the wall of mountainsis even higher, averaging about 2000meters elevation.

Figure 2. Relief map of southwest continental Alaska

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was generating a gap wind event. Theforecaster then determined that thephenomena would intensify as thefront moved north, and end only af-ter the front itself had pushed pastPortage Pass.

Despite the modest 16Z windsat Anchorage, the forecaster issued aCenter Weather Advisory (CWA) forlow level wind shear at 1615Z (Table2). As he was composing it, the firsturgent pilot reports (PIREP), Table3, started coming in. Shortly after-wards wind speeds jumped sharplyat Anchorage, wind sheer increasedand a B747 reported Mdt-Svr turbu-lence.

Figure 3, Page 4, is a 0.5 veloc-ity scan taken at 1749Z from the KenaiWSR-88D, clearly showing a distinctfinger of higher velocity winds tra-versing Portage. On the basis of thisand other observational data available,staff renewed the CWA at 1815Z.

CWAs are warnings issued by theNWS CWSU Meteorologist, in accor-dance with NWS Directive 10-803Support to Air Traffic Control Fa-cilities. They are designed as a shortrange forecast “primarily for use byair crews to anticipate and avoid ad-verse weather conditions in the enroute and terminal environment.”

Table 1 shows the observationsfor the morning of January 26. Notethat at 15Z winds at Whittier (PAWR)were out of the east at 20 knots withgusts up to 30 knots, actually not allthat unusual for that particular loca-tion; however, an hour later, Portage,at the other end of the pass, was re-porting a peak wind of 71 knots.

Meanwhile, 65 kilometers away atthe other end of Turnagain Arm, thewinds at Ted Stevens were variable at5 knots. Computer generated modelguidance products were unable to re-solve such a local incongruity.

The duty forecaster at the Anchor-age Center Weather Service Unit(CWSU) believed that the front wouldcontinue to progress to the north. Theforecaster was certain the front’s angleof orientation, parallel to Portage Passand relatively perpendicular to themountains at the north end of PrinceWilliam Sound, would be a primarycontributor to the winds coursingthrough the pass.

The density differential betweenthe frontal mass and that of the airmass it was displacing had trapped alarge pool of air in Prince WilliamSound. Unable to escape through thenorthern end of the sound because ofthe higher mountains there, the trappedmass of air was squeezingthrough the only exit avail-able, the much lower Por-tage Pass. The air mass waspicking up velocity, as itdid so, in a classic exampleof the Bernoulli’s Theo-rem at work.

The comparativelylower pressure at the Por-tage end of Portage pass,combined with the fron-tal pressure on the pool ofair trapped in Prince Will-iam Sound, resulted in anincreased velocity of flowfrom Whittier through thepass to Portage. In effect,the approach of the front

Time Site Wind Peak Wind

15z PANC N/APATO N/APAWR 07020G30KT N/A

16z PANC 00005KTPATO 1133G48KT 71PAWR 07020G30KT N/A

17Z PANC 100009G17KT 34PATO 1230G49KT 57PAWR 07020KT N/A

18Z PANC 28010KTPATO N/APAWR 06020KT N/A

19Z PANC 31005KTPATO 06027G44KT 53PAWR 06015G25KT N/A

20Z PANC 14009KTPATO 11013G18KT 38PAWR 06015G25KT N/A

21Z PANC 21003KT 26PATO 05017KT 31PAWR 06020G25KT N/A

ZAN1 CWAZAN1 CWA 261815ZAN CWA 102 VALID UNTIL 262015VCNTY ANCSTRONG LLWS OF +/-15 TO 30 KTS WITHIN25 NM RADOF PANC...AND WITHIN 020 AGL OBS ANDFCST THE NEXT TWO HOURS.

CDS JAN 00+

ZAN1 CWAZAN1 CWA 261615ZAN CWA 102 VALID UNTIL 261815VCNTY ANCSTRONG LLWS OF +/-15 TO 30 KTS WITHIN25 NM RADOF PANC...AND WITHIN 020 AGL OBS ANDFCST THE NEXT TWO HOURS.

CDS JAN 00+

Table 2. Center Weather Advisories (CWAs)

Table 1. Wind observations from PANC (Anchorage, AK), PATO (Portage, AK),and PAWR (Whittier, AK)

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Although these advisories can be is-sued for the same phenomena de-scribed by advisories from WFOs,they also regularly address meteoro-logical conditions that do not meetnational In-Flight Advisory criteria;conditions that, in the opinion of theCWSU Meteorologist, adversely im-pact aviation safety.

SummaryOver the course of a career, an

aviator may not see many frontal in-duced gap winds as compared with

land and sea breezes, diurnal moun-tain and valley winds, thunderstormoutflow winds, glacier inducedkatabatic winds, down slope windevents or mountain wave turbulence;however, this is exactly the kind ofevent pilots and meteorologists needto watch out for.

The Alaskan gap wind event citedhere was not common wind event andtherein lies the danger. A subtle changein the orientation of the front, by justa few degrees so that it was not paral-lel to the pass, would have altered the

ANC UUA/OV ANC/TM 1533/FL005/TP B737/RM -15 KTS IAS DURDANC UUA/OV ANC/TM 1554/FL006/TP MD80/R +/-15 KTS IAS DEP 24LANC UUA/OV ANC/TM 1555/FLDURC/TP B747/RM +/-15 KTS IAS AT 005 DEP 24LANC UUA/OV ANC/TM 1600/FLDURD/TP MD11/RM +/-10KT LLWS 005-SFC FINAL APCHRY14ANC UUA/OV ANC/TM 1629/FLDURC/TP B747/TBCONT MOD/RM +15 KIAS AT MIDFIELDDEP RY6RANC UUA/OV ANC/TM RY 6R/TM 1643/FLDURC/TP MD11/TB MOD-SEV 012-016ANC UUA/OV ANC/TM RY 6R/TM 1636/FLDURC/TP DC10/TB LGT/RM +/- 20KT LLWSANC UUA/OV ANC/TM 1710/FLDURC/TPB747/TB MDT-SVR SFC-060/RM LLWS +40/25KT

local wind flow with entirely differentconsequences.

The CWA, in addition to otheraviation warnings and advisories, canhelp the pilot make informed deci-sions; however, the most importantconcept addressed here is not to takeweather conditions for granted. Justas a good meteorologist must learnto account for patterns not in his “da-tabase,” so too must a pilot.

ReferencesCurtis, Joel C., 1998: The

Forecast Process: One Forecaster’sPerspective. Reprintfrom First Conferenceon Artificial IntelligenceJanuary 1998, PhoenixArizona by the Ameri-can Meteorological So-ciety, Boston, MA.

Sells, Colin D.1992: Some Statisticswith Respect to Alas-kan Aviation in whichWeather is a Cause/Factor. Alaska RegionTechnical AttachmentT-92-27.

National WeatherService Weather Ser-vice Procedural Direc-tive 10-803, Support tothe Air traffic ControlFacilities and NWS Pro-cedural StatementWR15-2003 Supporttp Air Trafic Facilities.

Figure 3. Velocity scan of the Kenai Peninsula

Table 3. Urgent PIREPS.

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Weather is very complex inAlaska. Weather conditions canvary dramatically within only afew miles. Take a high latitude lo-

cation, combine it with extremeterrain over short distances, andadd proximity to a large oceanbody. It all adds up to producesome of the most extremeweather conditions observed onearth.

The size of Alaska is remark-able, covering nearly 20 percentof the land area in the lower 48contiguous states, see Figure 1.Add the fact that Alaska averages16 times more aircraft per capitathan any other state in the nation.

Most of Alaska is reachableonly by aircraft. The lifeline tothese “bush” communities comesfrom pilots, who provide the

essentials: food, clothing, building ma-terials and mail. Aviation is essential tothe Alaskan way of life.

The vast areas of Alaska, with its

diverse weather conditions and highdependence on aviation required spe-cialized aviation weather services. Whowould be better qualified and experi-enced to provide these services thanthe meteorologists who live in Alaska?

Before the mid 1990s, theWeather Service Forecast Offices(WSFOs) in Anchorage, Juneau andFairbanks provided services by issu-ing Area Forecasts (FAs), SIGMETs(WSs), AIRMETs (WAs) and Termi-nal Forecasts (FTs). These NWS textproducts were available to pilotsmainly through the FAA FlightService Stations.

Major changes began to appearin the 1990s as use of the Internet ex-ploded. Maturing PC and communi-cation technology allowed people toaccess weather information in theirhomes.

The aviation community askedfor more direct, real time access toweather over the Web, and NWSanswered with new and expansiveWeb pages.

Traditional aviation text productsprovided good information, but thefocus shifted toward the need foraviation weather graphics.

The Alaska Aviation Weather Unit:Providing Aviation Weather Products and Servicesto the Alaskan Aviation Community

Figure 1. Size of Alaska as compared with the 48 contiguous states.

Jeff Osiensky, Meteorologist In Charge, Alaska Aviation Weather Unit, AnchorageJeffrey.Osiensky@noaa.gov

Figure 2. Map depicting the 25 area forecast zones within Alaska.

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Aviators quickly began to requesteven more graphical products. In 1995,with the help of aviation organizationsWSFOs and political efforts, the ideaof a specialized aviation weather unitin Alaska became a reality.

The new AlaskaAviation WeatherUnit (AAWU),staffed 24/7 withhighly trained NWSforecasters, focuseson providing avia-tion weather textand graphics exclu-sively for Alaska.

A great deal ofcustomer interactionand outreach oc-curred to insure thatthe aviation commu-nity was fully in-volved in the pro-cess. One of the firstsignificant changes inservices was the rou-tine production andissuance of graphi-

cal aviation weather charts for Alaska.The AAWU is staffed by 12

meteorologists. Each shift has twoforecasters, responsible for issuingand updating the Area Forecasts,AIRMETs, SIGMETs and graphical

products.The areas of

responsibility offorecasters are di-vided into two sec-tions with theAlaska Range serv-ing as the dividingline.

One forecasteris responsible forsouthern Alaska andthe Volcanic AshAdvisory Center(VAAC). This fore-caster deals with allproducts and ser-vices associatedwith volcanic ash.The second fore-caster is responsiblefor all of interiorand northern

Alaska, north of the Alaska Range.Alaska is divided into 25 forecast

areas or zones. The size and shape ofthese zones were determined by simi-lar climatological characteristics,see Figure 2, Page 5.

Figure 3. Example of an AAWU Surface Weather graphic.

Figure 4. Example of an AAWU 24 hour Significant Weather graphic.

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In other words, each zone shouldexperience common weather condi-tions across its area on any given day;however, it is not quite that simple.Because of the diverse topographythere are many sub-climatic zoneswithin the state.

Each forecast zone may consist ofseveral sub-zones, with even more di-verse weather conditions. AAWUforecasters have a difficult task ofconveying this variability while keep-ing the product concise. A pilot whomay be flying from Northway toTanana, for example, might f lythrough as many as three sub-regimesof weather even though the flight istraversing a single forecast zone.

AIRMETs are significant meteo-rological messages located in areaswhere occasional moderate or greaterturbulence and icing would exist. SIG-METs are issued for severe turbulenceand icing.

The AAWU also produces a vari-ety of graphics products. Every 6hours a series of four graphics is pro-duced, see Figure 3. The MVFR/IFR chart, Turbulence, Icing andSurface Weather charts are posted tothe AAWU Web site at http://aawu.arh.noaa.gov.

Every 12 hours, a SignificantWeather Chart is produced and postedon the Web for 24, 36, 48 and 60hours, see Figure 4, Page 6. Thesecharts include surface weather,MVFR/IFR, moderate or greater tur-bulence and freezing levels.

The AAWU is also one of fourMeteorological Watch Offices(MWOs) along with the AviationWeather Center in Kansas City, MO,WFOs Honolulu and Guam.

Each MWO has warning orSIGMET responsibility for one orseveral Flight Information Regions

Figure 5. Map depicting the Anchorage Flight Information Region (FIR) and major North Pacific JetRoutes.

(FIR). The AAWU areaof responsibility is theAnchorage FIR whichencompasses 2.4 mil-lion square miles, seeFigure 5, Page 7.

The AAWU is alsodesignated as theVAAC. The AnchorageVAAC is one of nineinternational centersthat monitor and fore-cast the dispersion ofvolcanic ash.

The gritty andchemical nature of vol-canic ash poses signifi-cant dangers to avia-tion because of thedamage it causes to air-craft engines.

VAAC forecastersare trained to use a suiteof cutting edgeremote sensing tools,including GOES,AVHRR and MODISsatellite imagery

to detect volcanicash plumes.

Certain bands on the satellite’sspectrum of information are moresensitive to volcanic ash and are usedto identify the plume.

Forecasters use meteorologicalmodel wind field data and volcanicash plume dispersion models to pre-pare a forecast of ash plume move-ment.

The Anchorage VAAC area ofresponsibility encompasses the entireAnchorage FIR in addition to far east-ern Russia. The VAAC works closelywith the Federal Aviation Administra-tion (FAA) Air Route Traffic ControlCenter’s Center Weather Service Unitmeteorologists and air traffic manag-ers to warn aircraft of possible ashencounters.

The VAAC also works very closelywith volcanologists from the Alaska

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

Volcano Observatory in Anchorageand Fairbanks.

The VAAC provides guidance tothe MWOs that have SIGMET re-sponsibility on the projected move-ment of the ash. Its products are avail-able on the Web at: http://aawu.arh.noaa.gov/vaac.php.

In Alaska, the AAWU is the Me-teorological Watch Office and theVAAC. The principle product of theVAAC is the Volcanic Ash Advisory(VAA).

This text product gives specificinformation about the volcano includ-ing elevation, latitude and longitude,see Figure 6.

This product also provides an ini-tial, 6, 12 and 18 hour forecast of theash plume including plume height. TheVAA is posted on the Web as well andis used quite extensively by the airlines.

Looking Toward theFuture...

The AAWU is committed to serv-ing its customer customers and part-ners with excellence as we move for-ward in the development of new andimproved forecast products andservices.

The future of the AAWU is veryexciting as it embraces the digital fore-cast era. The gridded aviation databaseof the future will allow customers to

access site specific weather informa-tion across Alaska.

In the meantime, the AAWU willcontinue to improve upon its currentproduct suite in concert with thechanging requirements of its aviationcustomers.

Author BiographyJeff Osiensky is the Meteorolo-

gist In Charge of the Alaska AviationWeather Unit in Anchorage. Prior tothis position, Jeff served for 2 ½ yearsas Meteorologist In Charge of theCenter Weather Service Unit locatedat the Anchorage Air Route TrafficControl Center. Jeff has spent morethan 5 years in aviation weatherforecastinin Alaska.