Ivory J. Small SOO WFO San Diego CA (SGX) 19 June 2013 High Impact Weather Hazards for Southern California Aviation Southwest Aviation Weather Safety (SAWS V) Virtual Workshop Presentation June 19, 2013

This presentation is meant to be a snapshot of the high impact weather hazards for Southern California Aviation. Included will be… Strong Northerly versus Strong Northeasterly Wind Patterns Cold Sector Thunderstorms Hybrid Elevated/Surface Based Severe Convection Strong to Severe Monsoonal Convection Coastal Dense Fog Episodes INTRODUCTION

Here is a terrain map of southern California The cross section used for the wind cases is the orange line. The location of the model sounding used is the “C” just west of the northern end of the cross section. [You will now see why we are keying in on point C and the area downwind of point C, and in particular Ontario International Airport (KONT)]. TERRAIN MAP OF SOUTHERN CALIFORNIA

Note the northwest-southeast orientation of the mountains, with huge gaps in the north. The thin arrows are where downslope windstorms readily form, the thick arrows are where gap wind flow readily forms, the dashed lines are the mountain ridgelines, and the areas between the mountain ridgelines are the “gaps”. These gaps and ridgelines in the mountains are important as they help determine the locations of the strongest winds based on the wind pattern and stability. TERRAIN MAP OF SOUTHERN CALIFORNIA

The winds at some locations (for example, the Fremont Canyon RAWS observation site in the mountains southwest of KONT), receive additional enhancement. These sites are downstream of strong gap or downslope wind flow that is further enhanced by a ridgeline, which creates additional strengthening of the downslope windstorm at these well exposed sites. This setup helps generate what is often the strongest north to northeast winds in the area. [These locations are usually on a coastal slope that is immediately downwind of another downslope wind region (or gap wind region), resulting in additional downslope enhancement on a wind that has already experienced gap or downslope enhancement]. TERRAIN MAP OF SOUTHERN CALIFORNIA

These NAM based 12z maps are produced at the Storm Prediction Center (SPC) Here is the 925 mb (approximately 2400 feet MSL) sounding data, heights (decameters), temperature (Deg C) and dew point greater than or equal to 12 degrees C. The panel on the left is for 1200 UTC 1 December 2011 (northerly wind event), and the panel on the right is for 1200 UTC 24 February 2013 (northeasterly wind event). COMPARISON OF 2 WIND EVENTS

The key here is the strong height gradient over coastal California (which essentially mirrors the surface pressure gradient), as seen in the left panel. This drives a more widespread, strong north wind, which hits the usual locations, but also some unusual locations (for example, at more major airports on the coastal plain) in southern California than a more “northeasterly” pattern. The map on the right has a more northeasterly gradient, so winds are more northeasterly and develop in the more “typical” locations. COMPARISON OF 2 WIND EVENTS

Here is the 500 mb (around 18,000 feet MSL) sounding data, heights (decameters) and temperature (degrees C). The panel on the left is for 1200 UTC 1 December 2011, and the panel on the right is for 1200 UTC 24 February Notice the best “upper level support” (or region with the strongest height gradient) is over the coastline on 1 December 2011 (left), but it is well inland on 24 February 2013 (right). COMPARISON OF 2 WIND EVENTS

The 1 December 2011 northerly wind case involves strong gap wind elements moving over the major coastal airports in the north (thick arrow), with mountain wave rotor activity possible inland at KONT (thin arrow). COMPARISON OF 2 WIND EVENTS CAJON PASS

During a “typical” strong northeasterly wind condition (usually called a Santa Ana wind when conditions are rather hot and dry), the winds west of the mountains are strongly influenced by the gaps in the mountains. COMPARISON OF 2 WIND EVENTS

Northerly winds create strong, erratic, and even rotor conditions as the “over the mountain” component overpowers the gap flow in the KONT area. Also the northerly winds are more widespread and northerly when surface pressure gradients are strongest in the north-south direction. High pressure stronger to the north rather than over the Plateau (Nevada and Utah), with pressure gradient values of 8-12 mb or more from KSFO to KLAX are common for such events. COMPARISON OF 2 WIND EVENTS

The northerly “mountain wave generated winds” in the KONT area (thin arrow) also must compete with the more easterly winds from the large pass (Cajon Pass) to the northeast (thick arrow on the right), which also makes the winds erratic when these gap winds and mountain wave winds interact near KONT. COMPARISON OF 2 WIND EVENTS CAJON PASS

For comparison, the 3.7 km WRF-EMS surface wind forecasts are shown by the green wind barbs and the shading (in knots) for the 2 cases. The 3 hour forecast for the first case valid at 1500 UTC 1 December 2011 (left) and the 9 hour forecast for the second case valid at 1500 UTC 24 February 2013 (right) are shown above. The forecasts used are from the most recent available run of the WRF-EMS from the 2 cases near the peak of the event. COMPARISON OF 2 WIND EVENTS

The dark green shaded areas indicate the strongest winds. Under the northerly flow regime, the models show the mountain wave as it attempts to push downslope and into the valley locations near the mountains such as KONT. The strongest core is actually near KONT, so the dominant flow is due to a mountain wave at KONT (left). COMPARISON OF 2 WIND EVENTS UNDER THE NORTHERLY FLOW REGIME, THE MODEL SHOWS THE MOUNTAIN WAVE AS IT ATTEMPTS TO PUSH DOWNSLOPE AND INTO THE VALLEY LOCATIONS NEAR THE MOUNTAINS SUCH AS KONT. THE STRONGEST CORE IS NEAR KONT

Under the northeasterly flow regime, the model shows the mountain wave region as “thin and more isolated” covering only a strip just southwest of the mountain crest, and gap winds from the large pass northeast of KONT is the dominant flow at KONT (right). COMPARISON OF 2 WIND EVENTS UNDER THE NORTHEASTERLY FLOW REGIME, THE MODEL SHOWS THE MOUNTAIN WAVE REGION AS “THIN AND MORE ISOLATED” COVERING A STRIP JUST SOUTHWEST OF THE CREST, AND GAP WINDS FROM THE NORTHEAST ARE MORE DOMINANT AT KONT

The above shows the dominant mechanism for strong winds at KONT for the 2 cases. It should also be mentioned that just before the winds arrive at the surface and just after the winds lift off the surface, the low level wind shear can be rather strong. These conditions must be carefully watched, especially at the beginning and at the end of an event. COMPARISON OF 2 WIND EVENTS IN THIS 1 DECEMBER CASE, ERRATIC MOUNTAIN WAVE CONDITIONS ARE MORE DOMINANT AT KONT IN THIS 24 FEBRUARY CASE, GAP WINDS FROM THE NORTHEAST ARE MORE DOMINANT AT KONT

In summary… The left panel shows northerly winds reaching the coastal plain, with strong crosswind components for airfields with east-west runways. Also a downslope windstorm with erratic rotor winds at times was occurring at KONT. The right panel shows northeasterly winds, with some key coastal airports escaping the wind, and the predominant wind at KONT is gap wind from the large gap to the northeast (the Cajon Pass). COMPARISON OF 2 WIND EVENTS AREA OF STRONG NORTHERLY WINDS FLOW THROUGH THE PASS REACHING KEY COASTAL AIRPORTS AND FLOW OUT OVER THE COASTAL WATERS AREA OF STRONG NORTHEAST WINDS REMAIN NORTHWEST OF THE KEY COASTAL AIRPORTS ERRATIC MOUNTAIN WAVE WINDS REACH DOWNSLOPE TO KONT AREA OF STRONG NORTHEASTERLY WINDS FLOW THROUGH THE PASS AND FLOW OUT OVER THE COASTAL WATERS

For comparison, the WRF-EMS wind forecasts are shown by the green wind barbs and the shading (in knots) for the 2 cases. The 3 hour forecast for the first case valid at 1500 UTC 1 December 2011 (left) and the 9 hour forecast for the second case valid at 1500 UTC 24 February 2013 (right) are shown above for comparison. The forecasts used are from the most recent run of the WRF-EMS from the 2 cases near the peak of the event. COMPARISON OF 2 WIND EVENTS

For the northerly wind case on the left, the winds associated with the mountain wave and hydraulic jump can be seen as a 60 knot core expending down the mountain slope into the valley, and is more likely to reach KONT with erratic mountain wave/rotor winds. For the northeasterly flow case on the right the downslope winds associated with the wave/hydraulic jump were forecast to remain up near the mountain crest. COMPARISON OF 2 WIND EVENTS STRONG CORE EXTENDING DOWNSLOPE TO NEAR KONT STRONG CORE REMAINING NEAR THE MOUNTAIN CREST STRONG CORE EXTENDING DOWNSLOPE TO NEAR KONT STRONG CORE REMAINING NEAR THE MOUNTAIN CREST

This indicates that the strong gusty winds, which occurred in both cases, were likely due to a more “pure” mountain wave feature in the 1 December case (left), and more of a gap wind from the large pass (the Cajon Pass) to the northeast of KONT in the 24 February 2013 case (right). COMPARISON OF 2 WIND EVENTS STRONG CORE EXTENDING DOWNSLOPE TO NEAR KONT STRONG CORE REMAINING NEAR THE MOUNTAIN CREST STRONG CORE EXTENDING DOWNSLOPE TO NEAR KONT STRONG CORE REMAINING NEAR THE MOUNTAIN CREST

There was extensive damage reported from winds up to 70 mph for the 1 December case at the foot of the mountains (between KONT and the mountains in the Rancho Cucamonga Area) that was much stronger than the winds reported at KONT, which supports a downslope windstorm in the area. Effects on aviation in the Los Angeles Basin were much worse with the 1 December 2011 case than the 24 February 2013 case since the crosswind and erratic nature of the winds at the major airports was worse with north winds than with northeasterly winds. COMPARISON OF 2 WIND EVENTS STRONG CORE EXTENDING DOWNSLOPE TO NEAR KONT STRONG CORE REMAINING NEAR THE MOUNTAIN CREST STRONG CORE EXTENDING DOWNSLOPE TO NEAR KONT STRONG CORE REMAINING NEAR THE MOUNTAIN CREST

Here are some important features in a sequence at KONT (for the 1 December 2011 event only). METAR KONT Z 35015G31KT 10SM FEW015 13/M07 A2973 RMK AO2 PK WND 36031/0846 WSHFT 0838 SLP060 T METAR KONT Z VRB04KT 10SM CLR 13/M03 A2975 RMK AO2 SLP066 T METAR KONT Z 32011G26KT 290V360 10SM CLR 13/M08 A2976 RMK AO2 PK WND 03029/1031 SLP069 T SPECI KONT Z 03024G48KT 3SM HZ SQ SCT008 12/M07 A2973 RMK AO2 PK WND 03048/1125 COMPARISON OF 2 WIND EVENTS

Another sequence shows a wild variation of wind speeds and directions, with a rotor clouds seen and reported by the observer, (for the 1 December 2011 event only). SPECI KONT Z 03009G16KT 10SM SCT090 BKN100 13/M07 A2976 RMK AO2 METAR KONT Z VRB04KT 10SM SCT100 BKN220 13/M06 A2978 RMK AO2 SLP082 APRNT ROTOR CLDS VC NE T METAR KONT Z 25008KT 10SM SCT100 BKN210 13/M04 A2978 RMK AO2 SLP082 ACSL VC NE T METAR KONT Z 01010G15KT 330V050 10SM SCT110 BKN210 17/M08 A2977 RMK AO2 SLP076 T METAR KONT Z 02010KT 10SM SCT095 BKN210 16/M09 A2979 METAR KONT Z 03022G38KT 10SM VCBLDU FEW110 BKN150 17/M11 A2978 COMPARISON OF 2 WIND EVENTS

2145 UTC 8 February 2013 shows open cell post-frontal convection Fueling operations can be disrupted by lightning and small, occasionally accumulating hail can occur. Usually cells have to be wide and slow moving, or be along a nearly stationary convergence line to generate significant accumulating hail. Occasionally the thunderstorms are along the cold front, but most often thunder occurs in the cold convective region behind the front. COLD SECTOR CONVECTION

Microburst winds can be an issue when there are strong 850 mb winds (around 5000 feet MSL) behind the stronger convective elements. Large hail is even less of a problem, and when it occurs, it will probably be associated with a mini-supercell. Lightning can affect fueling operations under these cold post-frontal conditions. COLD SECTOR CONVECTION

Here is the 500 mb (around 18,000 feet MSL) sounding data, heights (decameters) and temperature (deg C). The panel on the left is for 0000 UTC 8 February 2013, and the panel on the right is for 0000 UTC 9 February The pool of cold air is basically enclosed by the contours. COOL SECTOR CONVECTION COLD AIR

Here is the 850 mb (around 5000 feet MSL) sounding data, heights (decameters), temperature (Deg C) and dew point greater than or equal to 8 degrees C. The panel on the left is for 0000 UTC 8 February 2013, and the panel on the right is for 0000 UTC 9 February The tight temperature gradient (seen as the red dashed lines in the blue boxes) can be seen sweeping through the area, leaving Southern California vulnerable to cold air convection. COLD SECTOR CONVECTION

This event began as an elevated convective event in the morning over the coastal waters off southern California, and by afternoon surface based convection developed as well. There were 5 reports of quarter-sized hail within about 30 miles of KSNA. The computer generated forecast indicated 1000 J/kg of mid- upper level instability to sweep through on 10 September 2011 (basically strong enough updrafts for severe thunderstorms). Also the computer generated forecasts indicated a mid to upper level lifted index of nearly -4 degrees C to develop (also an indicator of updrafts strong enough for severe thunderstorms). HYBRID ELEVATED/SURFACE BASED CONVECTION EVENT WITH LARGE HAIL ON 10 SEPTEMBER 2011

The above is the 30 hour forecast of the NAM mb heights (solid orange contours in intervals of 60 meters) and the High Level Most Unstable CAPE (green contours in intervals of 200, and shaded) valid at 1800 UTC 11 September Notice the HLMUCAPE values of over 700 J/kg in the coastal areas showing a forecast of potentially severe convection. This is one method to determine how conducive the air mass is to strong thunderstorm development (basically a measure of the largest potential updraft strength due to air lifted from somewhere between 700 and 500 mb). HYBRID ELEVATED/SURFACE BASED CONVECTION EVENT WITH LARGE HAIL ON 10 SEPTEMBER 2011 VERY UNSTABLE AIRMASS MOVING NORTH THROUGH THE AREA INDICATED BY THE BLUE SHADING

The above is the 30 hour forecast of the NAM mb heights (solid orange contours in intervals of 60 meters) and the High Level Lifted Index (green contours in intervals of 0.9 C, and shaded) valid at 1800 UTC 11 September Notice the HLLI values of below -1.5 in the coastal areas showing a forecast of potentially severe convection. This is just an alternate method to determine how conducive the air mass is to strong thunderstorm development (basically a measure of the potential updraft strength due to air lifted from 700 mb to 500 mb). HYBRID ELEVATED/SURFACE BASED CONVECTION EVENT WITH LARGE HAIL ON 10 SEPTEMBER 2011 VERY UNSTABLE AIRMASS MOVING NORTH THROUGH THE AREA INDICATED BY THE BLUE SHADING

Based on the actual sounding at KNKX (Miramar), the updraft strength would result in values sufficient for severe convection. HYBRID ELEVATED/SURFACE BASED CONVECTION EVENT WITH LARGE HAIL ON 10 SEPTEMBER 2011

The above 1830 UTC 30 August 2012 satellite imagery shows some key ridgeline positions (dashed black lines) in southern California Also shown are the surface wind barbs (in knots). The location of the convergence between the westward moving monsoonal air mass and the eastward moving flow from the west at the surface is shown as dashed white lines. WIDESPREAD STRONG TO SEVERE MONSOONAL CONVECTION

Typically the most volatile setup for strong convection that impact aircraft getting in and out of the basin is when: Convection begins earlier and further west than usual on convergent “arcs” of easterly winds extending through the canyons and passes and converging with the westerly low level flow. Upper level features organize it into lines. Moisture and instability is unusually high. WIDESPREAD STRONG TO SEVERE MONSOONAL CONVECTION

The 1200 UTC 30 August 2012 KNKX sounding (near San Diego) shows strong instability. A lifted index value of nearly -5 degrees C along with over 1200 joules of positive energy occurs when air is lifted from 771 mb (an estimate of the potential for elevated updraft strength for a parcel lifted from about 7000 feet MSL). This shows the potential for severe high based convective weather is high based on both indices. Numerous thunderstorms did develop, even reaching severe limits. Convection over the mountains, and also convection due to the upper level wave created lines of thunderstorms, which can change air traffic patterns in and out of Southern California on such days. WIDESPREAD STRONG TO SEVERE MONSOONAL CONVECTION

The 06 hour NAM mb heights and vorticity valid at 1800 UTC 30 August 2012 along with the water vapor imagery at 1710 UTC 30 August 2012 shows the upper level wave that will enhance the afternoon convection. WIDESPREAD STRONG TO SEVERE MONSOONAL CONVECTION

The 12 hour NAM mb heights and vorticity valid at 0000 UTC 31 August 2012 along with the water vapor imagery at 2131 UTC shows the upper level wave has enhanced the convection as it moved inland. WIDESPREAD STRONG TO SEVERE MONSOONAL CONVECTION

The 2200 UTC 30 August 2012 infrared satellite imagery, overlaid with some of the popular air routes is shown above. The main threats are; The possibility of deviations due to lines of thunderstorms, microburst winds, large hail (not nearly as common as microburst winds), and lightning. WIDESPREAD STRONG TO SEVERE MONSOONAL CONVECTION

Typical dense fog sounding Above normal boundary layer temperature with a strong (around 10 degrees C or more) surface based temperature inversion. High overhead or nearby with only slow changes in the surface pressure pattern. No eddy circulation in the coastal waters (no Catalina Eddy or upper low being reflected in the coastal wind flow). Pattern can persist for days under these conditions. Sounding found at ( PROLONGED EPISODE OF NIGHT AND MORNING DENSE FOG

The above is an example of nightly dense fog episodes and indicates how variable the fog can be from night to night. Each point is the visibility when an hourly report of 3 miles or less was received. PROLONGED EPISODE OF NIGHT AND MORNING DENSE FOG

It can be seen that like many places, there are challenges in every season for the aviation forecaster to deal with. Forecasters must first be able to determine the threats based on the synoptic setup, then hone in on the problem at hand. It is my hope that this presentation gives at least some idea of what we look for in southern California. CONCLUSION

THE END