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CRH SSD MAY 1992

CENTRAL REGION TECHNICAL ATTACHMENT 92-11

USING NEW TECHNOLOGY TO LOCATE AND FORECAST THE MOVEMENT OF A FRONT IN THE MESOSCALE

MICHAEL K. HOLZINGER NATIONAL WEATHER SERVICE FORECAST OFFICE DENVER, COLORADO

1. INTRODUCTION The National Weather Service (NWS) is testing much of the future technology in an operational environment at the Weather Service Forecast Office (WSFO) in Denver, Colorado. The Denver AWIPS Risk Reduction and Requirements Evaluation (DARRRE II) workstat­ ion at the WSFO gives forecasters the ability to analyze mesosc- ale features in much more detail and make local forecasts with more precision than was possible a few years ago. WSFO Denver conducted an experimental project, called the En­ hanced Terminal Forecast (EFT) program, between January and November, 1991 (NWS 1990; NWS 1991). In place of standard avia­ tion terminal forecasts, EFT forecasts were issued for the three terminals in the Denver metropolitan area. These are Stapleton International Airport (DEN), Centennial Airport (APA), and Jefferson County Airport (BJC) (Fig. 1A). Compared to conven­ tional terminal forecasts, the EFT had a slightly different format, slightly different amendment criteria, disallowed the use of terms "chance" and "slight chance" during the first three hours, and attempted to nail down weather changes to the nearest 15 minutes during the first three hours. Perhaps, the best opportunity to attempt forecasts to the nearest 15 minutes along the Front Range is provided by warm season wind shifts, either gust fronts or frontal. Warm season wind shifts can often be spotted on Doppler radar an hour or more before they reach DEN. The forecaster can then track and time the wind shift, while providing accurate warning to the airport well in advance of the event. Clear air wind shifts can be tracked in the summer, due to the high sensitivity of the Doppler radar and because of the presence of airborne scatterers (usually flying insects). This paper provides an example of how the EFT forecaster used the new technology to provide updated EFT's for DEN during a time period characterized by significant wind shifts. 'V 7932

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2 . DATA SOURCES Mile High Radar (MHR) is the Doppler radar being used by WSFO Denver. It is similar to the WSR-88D radars currently being deployed by the NWS. Other technologies used in this case (available to Denver forecasters via the DARRRE II workstation) were the experimental mesonet and surface analyses from the Local Analysis and Prediction System (LAPS) model (McGinley 1989; McGinley et al. 1991). The mesonet consists of 22 automated surface observation stations that report data every five minutes. LAPS utilizes all available surface observations, satellite data, profiler data, and radar data to produce surface analyses of several variables every hour. These analyses have recently been made available on the workstation on an experimental basis. The LAPS domain consists of all of central and eastern Colorado plus a small part of Wyoming, Nebraska, and Kansas. MHR is located about 10 miles (16 km) northeast of DEN. Figures 1 through 6 are pictures of workstation displays from MHR (eleva­ tion angle 1.2 degrees) and other data for a radius of approxi­ mately 75 miles (121 km) around MHR. One of the mesonet stations (AUR) is located at DEN. Mountains just west of Denver show up as ground clutter. Each figure is accompanied by a schematic (Figs. 1A-6A) which depicts the front as a dashed line.

3. ANALYSIS AND FORECAST On 26 June 1991, a front moved through DEN three times, twice as a cold front, and once as a warm front. There were no signifi­ cant clouds associated with the front all day. Winds were the only concern to the EFT forecaster. The first frontal passage (cold front) occurred at 1000 UTC (0400 MDT), and had only a weak wind shift associated with it. MHR was not operating until near midday (1800 UTC). At midday, the mesonet and MHR showed that the front had become stationary about 15 to 20 (24 to 32 km) miles southeast of DEN. Figure 1 (MHR reflectivity and mesonet) at 2130 UTC shows the front had not moved much from its midday location. The front could be clearly identified, both on reflectivity (fine line) and on velocity (sharp line defining winds toward and winds away from the radar). The front extended northeast to southwest, east and south of Denver. The new EFT issued at 2137 UTC 26 June 1991 (forecast A in Fig. 7) reflected the forecaster's belief that the front would move north again as a warm front. An estimate of the arrival time of the warm front at DEN was 0300 UTC 27 June 1991.

2 CR TA 92-11 MAY 1992

The main thing that concerned the forecaster at this point was the possibility of low level wind shear (LLWS), especially if the front advanced northward as expected (Badner 1979) . The mesonet showed strong south to southwest winds at 15-20 knots (8 to 10 m/s) with gusts up to 33 knots (17 m/s) south of the front, while winds north of the front were from the north at 12-15 knots (6 to 8 m/s) gusting to 17-20 knots (9 to 10 m/s). Horizontal sus­ tained wind shear across the front was about 25-30 knots (13 to 15 m/s)—stronger using wind gusts. Doppler velocities were similar. The forecaster then used an application of the workstation called "Read Cursor" to determine the depth of the frontal boundary. The "Read Cursor" application, among other things, enables the forecaster to find the height above ground of the center of the radar beam at any spot on the MHR display. In this case the forecaster found a spot on the velocity display north of MHR where the radar beam penetrated the frontal boundary aloft, shown by a sudden shift in wind direction. By interpolating between that point and the front at the surface, the forecaster concluded that the frontal boundary over DEN was about 1000 feet AGL. This meant that there was a potential for strong LLWS below 2000 feet, and the forecaster added the remark "LLWS" to the terminal forecast. There were no reports of significant LLWS at DEN from pilots. However, at 2220 UTC there was a pilot report 14 miles south southeast of DEN of +/- 10 knots (5 m/s) of wind shear at 500 feet AGL (Fig. 7). Several LAPS parameters were compared to mesonet, surface obser­ vations, and MHR data. For instance, LAPS winds did a consis­ tently good job of identifying the location of the front, while other parameters, such as LAPS dew points had varying degrees of success. Figure 2 at 2200 UTC shows that there were some prob­ lems at this particular time with attempting to use LAPS dew poi­ nts (overlaid on MHR velocity and the mesonet). For example, mesonet stations at BYE and LTN did not agree with the LAPS analysis. However, LAPS winds at 2200 UTC did a very good job of locating the front, especially in the Denver area. This wind analysis is shown in Fig. 3 along with the Doppler velocity and mesonet data. At 2235 UTC, the front could be seen on MHR as a sharp boundary advancing northward toward DEN. At this time, the forecaster used the "Distance/Speed of Motion" application (a simple linear extrapolation routine) on the workstation to time the front, and updated the forecast at 2237 UTC (forecast B - Fig. 7) to indi­ cate a warm frontal passage (WFP) at 0000 UTC 27 June 1991.

3 CR TA 92-11 MAY 1992

The observations (Fig. 7) show the front going through DEN at 2348 UTC, as indicated by a strong wind shift. The terminal forecast issued over an hour earlier was within 15 minutes of being correct. The forecaster used MHR, the mesonet, and obser­ vations from Centennial Airport to forecast the expected winds behind the front. He then kept the Stapleton tower informed periodically by phone of the progress of the front. A written terminal forecast amendment isn't always sufficient! With the warm air finally over DEN, the high temperature came late — 91°F at 0100 UTC (1900 MDT). This was a day that challenged the public forecaster as well as the aviation forecaster. Another attempt to correlate LAPS dew points with the frontal boundary was done at 0000 UTC. Figure 4 depicts the Doppler velocity, LAPS dew points, and mesonet data. It can be seen here that the dew point analysis at this time correlated better with observations. Figure 5 shows Doppler velocity and mesonet data at 0035 UTC, at which time the front had stalled again just north of DEN. The eastern end of the front was no longer easily located by MHR or other data. A regular EFT forecast update was issued at 0037 UTC (Forecast C — Fig. 7) in which the forecaster anticipated that the front would weaken by 0300 UTC and no longer affect DEN. However, at 0120 UTC the front was moving south again as a cold front. The forecaster again used the "Distance/Speed of Motion" application to time the front southward through DEN (this time as a cold front). An amended forecast (forecast D — Fig. 7), placed the cold frontal passage (CFP) through DEN at 0145 UTC. As with the previous frontal passage, the forecaster called the tower to warn them of the expected wind shift. The phone call was even more important due to the short warning time. As shown by the observations (Fig. 7), the front moved through DEN at 0130 UTC, 15 minutes earlier than expected. The timing could have been better, but it was estimated from a short history of the front's southward movement. Without MHR, there would have been no warning. Figure 6 depicts the Doppler velocity and mesonet observations at 0145 UTC. The front had moved south of DEN once again. By 0245 UTC winds were weakening and the front was harder to find. Winds were not significant at DEN the rest of the night.

4. SUMMARY The experimental EFT program and format encouraged forecasters to be more precise with the terminal forecasts. Without the new technology, it would be extremely difficult to try to forecast weather changes to the nearest 15 minutes. With the new technol-

4 CR TA 92-11 MAY 1992 ogy, there are some instances, as in this case, for which avia­ tion forecasts can be made in the short term to the nearest 15 minutes. LAPS, while new to the operational environment at Denver, has already become a useful analysis tool. Accurate and timely aviation forecasts require that the airport being served will support the forecaster by issuing special observations promptly as criteria require. Communications shortcomings can also be a hindrance to effective use of the EFT, especially in short-fused situations. This points to the need to investigate improved dissemination techniques to go along with the improved forecast capabilities. Forecasters at WSFO Denver frequently encounter situations similar to this case, and are able to provide accurate advance warning, written and by phone, to aviation users. Additional case studies could help reveal further utility of the EFT con­ cept . 5. REFERENCES Badner, J., 1979: Low-level Wind Shear: A Critical Review. NOAA Technical Memorandum NWS FCST-23.

McGinley, J., 1989: The Local Analysis and Prediction System. Preprints, 12th Conference on Weather Analysis and Forecast­ ing, Monterey, American Meteorology Society, 15-20.

McGinley, J., S.C. Albers, and P. A. Stamus, 1991: Validation of a Composite Convective Index as Defined by a Real-Time Local Analysis System. Weather and Forecasting, 6, 337-356. National Weather Service, 1991: Enhanced Terminal Forecast (EFT) Operational Test. Weather Service Operations Manual Letter 1-91.

National Weather Service, 1990: WSFO Denver Enhanced Terminal Forecast Risk Reduction Plan.

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OBSERVATIONS

DEtl SA 0253 E260 BKN 40 024/76/49/O9O3/903/FEW AC/ 114 1078 DEII SA 0153 150 SCX E270 OVC 40 0 2 3 / 8 1 / 4 8 / O 10 8 / 9 8 2 / WS1I FT 30 CB DSNT E DEN SP 0132 270 -OVC 45 0203G14/981/WSIIFT 30 CB E-SE MOVG HE FEW AC DEN SA 0053 270 -OVC 45 007/91/25/2215G2B/980/CB E-SE MOVG NE FEW AC DEN RS 2353 E270 OVC 50 010/86/43/2414G21/978/WSMFT 48 CB DSNT E-8 FEW AC/ 500 1377 89 RADAT 30161 DEN SA 2250 E250 OVC 40 007/86/51/0310/977/FEW CU AC DEN SA 2152 E250 OVC 40 005/87/49/3612/978/BIHOVC ALQDS FEW AC DEN SA 2053 E280 OVC 40 000/88/45/3412G17/979/B1NOVC ALQDS FEW AC/ 730 1077 DEN SA 1951 E280 OVC 30 013/06/46/3413/981/GRDL WSIIFT FEW AC DEN SA 1852 E20O OVC 40 02 1 / 0 4 / 4 1 / 1 104/90 4/FEW AC 1IOCTV DEN SA 1751 E280 OVC 40 034/00/41/0605/9B9/FEW AC W IIOCTY/ 825 1077 52 DEN SA 1650 250 -BKN 40 050/7 5/4 3 / 2 7 03 / 9 9 4 / FEW AC OMTIIS SW-HW IIOCTY DEN SA 1550 250 -BKN 60 056/70/45/0503/996/FEW AC OMTNS SW-HW DEN SA 1451 E250 BKN 60 069/67/45/3504/998/FEW AC SW-NW/ 015 1076 DEN SA 1350 160 SCT 250 SCT 60 077/5B/41/1703/999 DEN SA 1254 160 SCT 250 SCT 60 070/57/36/0103/998 DEN SA 1151 160 SCT 250 SCT 50 051/54/30/3004/993/ 310 1052 52 RADAT 38158 DEN SA 1050 250 SCT 30 041/53/31/1 305/990/FEW AC ALQDS DEN SA 0951 250 SCT 20 034/59/24/3504/990 DEII SA 0852 CLR 20 027/63/22/1904/989/FEW Cl SW-W/ 303 1001 DEN SA 0753 300 SCT 30 027/60/31/1104/988/ 98659 DEN SA 0653 E220 BKN 300 BKN 30 024/66/27/1804/988 DEN SA 0551 E230 BKN 25 019/74/17/1905/988/ 110 1002 00 DEII SA 0453 E230 BKN 25 015/80/12/2008/987 DEN SA 0352 E230 BKN 30 008/B7/21/201OG17/986 DEN SA 0253 160 SCT E230 BKN 50 009/82/18/2008/984/ 115 1072 DEN SA 0153 E220 BKN 60 00 3 / 8 7 / 2 2 / 2 4 07 / 9 8 3/CU DSNT E-SE IIRZN FEW AC ALQDS DEN SA 0052 E210 BKN 60 002/91/26/2315G21/981/FEW CU SE FEW AC ALQDS DEN SA 2352 180 SCT E220 BKN 60 999/95/30/2112G25/979/FEW CU ACSL ALQDS/ 305 1142 00 RADAT 20163 DEN SA 2250 170 SCT E220 BKN 60 993/98/32/2315G30/970/FEW CU ACSL ALQDS DEN SA 2152 E250 BKN 60 982/100/34/2010G25/976/FEW CU DEN SA 2052 250 -BKN 60 986/99/33/2121G26/977/FEW CU SW-NW/ 719 1108 DEN SA 1952 E250 BKN 60 994/99/32/2215G20/970/FEW CU S-NW DEN SA 1851 250 -BKN 60 001/98/31/1707/981/CU W

PILOT REPORT DENPIRCO UBUS90 KWBC 262233 APA UUA /OV DEN 161014/TH 2220/FL005/TP LR35/R14 17R LLWS +/- 10 KT 500 AGL

STAPLETON FORECASTS DENFTADEH TTAAOO KDEN 270123 AMD DEN FT AMD 1 270122 0122 Z C250 OVC 2313 OCHL G23. 0145Z CFP 250 -BKN. 17Z 120 SCT 250 -BKN 0108.. DENFTADEN TTAAOO KDEN 270037 DEN FT 270122 C250 OVC 2 313 OCNL G23. 03 Z 250 -BKN. 17 Z 120 SCT 250 -BKN 010B.. DENFTADEN TTAAOO KDEN 262238 DEN FT AMD 1 262322 2237 Z C250 OVC 3612 LLWS WND 2115 AT 500 FT. OOZ WFP 150 SCT C250 OVC 2113 OCNL G23. 06Z 150 SCT 250 -BKN 2008 . 17 Z 120 SCT 250 -BKN 0708.. DENFTADEN TTAAOO KDEN 262137 DEN FT 262222 C250 OVC 3 412 OCNL G20 PSBL LLWS WND 2220 AT 1000 FT. 03 Z 150 SCT C250 OVC 2012 OCNL G20. 09Z 150 SCT 250 -BKN 2008 . 17 Z 120 SCT 250 -BKN 0708..

Fig. 7

A CR TA 92-11 MAY 1992

OBSERVATIONS

E DEH SA 0253 iso SCT E270 OVC 40 023/Bl/J / ^ E_£E MOVG HE FEW AC DEII BA 0153 270 -OVC 45 22^5028^/980/CB E-SE MOVG HE FEW AC DEH SP 0132 ^?o"ovc °oVo 8 o^”«a"/?”Jw.«rT 48 cb dsht e-s DEH SA 0053 DEH RS 2353 5; 1577 09 padat 30161 E2 50 OVC 40 °?Z^n7/49/3S12/978/BlHOVC ALQDS FEW AC DEH SA 2250 730 1077 DEH SA 2152 DEH SA 2053 till SE JS 0? AC/ DEH SA 1951 DEH SA 1852 52 DEH SA 1751 IS S3 lj 1 Hi HI HS-.UMSJ DEH SA 1650 DEH SA 1550 DEH SA 1451 SS.:s S HpHliliJ ass,-sir,.,. SA 1350 DEH 3 B 158 DEH SA 1254 Hi Hi SSCT IS S5^1

PILOT REPORT

DEHPIRCO 3 D8US90 KWBC 26223 2220/FL005/TP LR35/RH 17R LLWS + /- 10 KT APA UUA /OV DEH 161014/TH 500 AGL

STAPLETON EORECASTS dehftaden DEH^FT AHd'i22?0132 01^2 C250 OVC 2313 OCHL G23. °1^5120FSCT5250B-BKN 0108..

DEHFTADEN TTAAOODEH FT KDEN270122 270037 C250 OVC 2313 nrHOCHL. r7G23.-t 03 Z 250 -BKN. 172 120 SCT 250 -BKN 0108.. DEHFTADEH TTAAOO KDEN 2237Z C250 OVC 3612 LLWS WHO 2115 AT 500 FT. °EN ^TM50”“ 02 5 O^OVC^ 2 113 OCHL G23. 06Z 150 SCT 250 -BKN 2008. 17 Z 120 SCT 250 -BKN 0708.. DEHFTADEN __ — ",,D J”°" r h is; IS IIS :SK 88:. Fig. 7