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 situ- ation 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 af- ter 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.