It appears that, it all of the sudden started getting packets while I was running the tests. You can see that weewx started up 5.5 hours before it started getting data from interceptor.
Aug 31 07:59:45 raspberrypiFB weewx[456]: engine: Locale is 'en_GB.UTF-8'
Aug 31 07:59:45 raspberrypiFB weewx[456]: engine: pid file is /var/run/weewx.pid
Aug 31 07:59:45 raspberrypiFB weewx[362]: Starting weewx weather system: weewx.
Aug 31 07:59:45 raspberrypiFB systemd[1]: Started LSB: weewx weather system.
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: Using configuration file /etc/weewx/weewx.conf
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: Loading station type Interceptor (user.interceptor)
Aug 31 07:59:45 raspberrypiFB weewx[559]: interceptor: MainThread: driver version is 0.30
Aug 31 07:59:45 raspberrypiFB weewx[559]: interceptor: MainThread: device type: acurite-bridge
Aug 31 07:59:45 raspberrypiFB weewx[559]: interceptor: MainThread: sensor map: None
Aug 31 07:59:45 raspberrypiFB weewx[559]: interceptor: MainThread: mode is sniff
Aug 31 07:59:45 raspberrypiFB weewx[559]: interceptor: MainThread: sniff iface=eth0 promiscuous=0
Aug 31 07:59:45 raspberrypiFB weewx[559]: interceptor: MainThread: sniff filter 'src 192.168.5.230 and dst port 80'
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: StdConvert target unit is 0x1
Aug 31 07:59:45 raspberrypiFB weewx[559]: wxcalculate: The following values will be calculated: barometer=prefer_hardware, windchill=prefer_hardware, dewpoint=prefer_hardware, appTemp=prefer_hardware, rainRate=prefer_hardware, windrun=prefer_hardware, heatindex=prefer_hardware, maxSolarRad=prefer_hardware, humidex=prefer_hardware, pressure=prefer_hardware, inDewpoint=prefer_hardware, ET=prefer_hardware, altimeter=prefer_hardware, cloudbase=prefer_hardware
Aug 31 07:59:45 raspberrypiFB weewx[559]: wxcalculate: The following algorithms will be used for calculations: altimeter=aaNOAA, maxSolarRad=RS
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: Archive will use data binding wx_binding
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: Record generation will be attempted in 'hardware'
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: Using archive interval of 300 seconds (specified in weewx configuration)
Aug 31 07:59:45 raspberrypiFB weewx[559]: engine: Using binding 'wx_binding' to database 'weewx.sdb'
Aug 31 07:59:45 raspberrypiFB weewx[559]: manager: Starting backfill of daily summaries
Aug 31 07:59:46 raspberrypiFB weewx[559]: restx: StationRegistry: Station will not be registered: no station_url specified.
Aug 31 07:59:46 raspberrypiFB weewx[559]: restx: Wunderground: Posting not enabled.
Aug 31 07:59:46 raspberrypiFB weewx[559]: restx: PWSweather: Posting not enabled.
Aug 31 07:59:46 raspberrypiFB weewx[559]: restx: CWOP: Data for station FW1491 will be posted
Aug 31 07:59:46 raspberrypiFB weewx[559]: restx: WOW: Posting not enabled.
Aug 31 07:59:46 raspberrypiFB weewx[559]: restx: AWEKAS: Posting not enabled.
Aug 31 07:59:46 raspberrypiFB weewx[559]: engine: Starting up weewx version 3.7.1
Aug 31 07:59:46 raspberrypiFB weewx[559]: engine: Starting main packet loop.
Aug 31 13:21:41 raspberrypiFB weewx[559]: interceptor: MainThread: skipping rain measurement of 0.3: no last rain
Aug 31 13:21:41 raspberrypiFB weewx[559]: engine: garbage collected 0 objects
Aug 31 13:21:41 raspberrypiFB weewx[559]: engine: Launch of report thread aborted: existing report thread still running
Aug 31 13:21:59 raspberrypiFB weewx[559]: engine: Launch of report thread aborted: existing report thread still running
Aug 31 13:22:01 raspberrypiFB weewx[559]: cheetahgenerator: Generated 14 files for report StandardReport in 18.81 seconds
Aug 31 13:22:04 raspberrypiFB weewx[559]: imagegenerator: Generated 12 images for StandardReport in 2.66 seconds
Aug 31 13:22:04 raspberrypiFB weewx[559]: copygenerator: copied 9 files to /var/www/html/weewx
Aug 31 13:22:18 raspberrypiFB weewx[559]: engine: Launch of report thread aborted: existing report thread still running
Aug 31 13:22:24 raspberrypiFB weewx[559]: cheetahgenerator: Generated 14 files for report StandardReport in 6.47 seconds
Aug 31 13:22:27 raspberrypiFB weewx[559]: imagegenerator: Generated 12 images for StandardReport in 2.56 seconds
Aug 31 13:22:27 raspberrypiFB weewx[559]: copygenerator: copied 0 files to /var/www/html/weewx
Aug 31 13:22:42 raspberrypiFB weewx[559]: cheetahgenerator: Generated 14 files for report StandardReport in 6.42 seconds
Aug 31 13:22:45 raspberrypiFB weewx[559]: imagegenerator: Generated 12 images for StandardReport in 2.56 seconds
Aug 31 13:22:45 raspberrypiFB weewx[559]: copygenerator: copied 0 files to /var/www/html/weewx
Aug 31 13:22:54 raspberrypiFB weewx[559]: engine: Launch of report thread aborted: existing report thread still running
Aug 31 13:23:01 raspberrypiFB weewx[559]: cheetahgenerator: Generated 14 files for report StandardReport in 6.48 seconds
Aug 31 13:23:03 raspberrypiFB weewx[559]: imagegenerator: Generated 12 images for StandardReport in 2.63 seconds
Aug 31 13:23:03 raspberrypiFB weewx[559]: copygenerator: copied 0 files to /var/www/html/weewx
Aug 31 13:23:19 raspberrypiFB weewx[559]: cheetahgenerator: Generated 14 files for report StandardReport in 6.42 seconds
Aug 31 13:23:21 raspberrypiFB weewx[559]: imagegenerator: Generated 12 images for StandardReport in 2.63 seconds