Weather Diary Tue Feb 25 8am: skies clear except stratus over PG bowl perhaps 1/8 St at UNBC; cold, v. light winds Noon: St in bowl turned to haze, otherwise clear, v. light wind 4pm clear, light NNE wind – beaufort 0-1 Evening: clear, cold, stars visible Tuesday summary: Day was cold and cloudless except morning St in valley, and with very light wind. Wed Feb 26 8am: skies clear except St in bowl; v light wind from N-NW Noon: skies clear, haze in bowl; winds calm to gusting beaufort 2 4pm: temps warmed to -2 or so, wind beau 0-1 from NE, Ci clouds coming in from SW about 1/8 at 4pm increasing to 4/8 by 6pm Evening: skies becoming thin overcast probably in Ci. Wednesday summary: Day was similar to tues, except warmer with increasing clouds in late afternoon and overnight probably related to a weak weather system Thu Feb 27 8am: remnant clouds from night with As/St to SE, some Ci after 8 am. Winds light from N. Noon: light N wind, trace Ci inc to 4/8 As to east, temp near 0 4pm: clear, winds have increased to Beaufort 3-4 from North Evening: clear and windy Thursday summary: High clouds from prev night kept temps warm and prevented St in bowl overnight, winds increased significantly from the N during the afternoon. Fri Feb 28 8am: windy from ENE beaufort 3-7. clear. Noon: windy from ENE beaufort 3-7. clear. 4pm: windy from E – N beaufort 3-7. clear. Haze over mtns to east. Friday summary: clear and cold with strong winds from N-E. Weekly summary: Started out cold, calm and clear except for morning St in valley. On Wed temperatures increased, in evening clouds came in, cleared on Thursday followed by increasing northerly winds Thursday afternoon and Friday. Marking guidelines for weather diary Each day: 5 marks (20 marks total) More than 3 observations per day, most not at roof times, at least one in the evening Observations should report qualitative clouds, wind, weather (none this week), temps. Etc. Must be a useful daily summary each day that ties everything together Deduct marks for no daily summary (-2 or -3), unclear or incomplete observations, observations that overlap roof times or not in evening Weekly summary: 5 marks A paragraph that summarizes the observations and their trend over the week Marks based on how well they do this Note that summaries could be implicit in the observations. Total marks = 25 scaled to a mark out of 1

TWRF February Characterized by cold, mostly clear conditions with no strong mid- latitude cyclones (MLCs) passing through, temperatures warmed from Tuesday to Wednesday and then cooled with strong north winds Thursday and Friday Mornings T, W characterized by St in bowl; poor air quality due to stagnation conditions Clear skies indicated by both Kdown and Ldown, except during period W night to R when some clouds and warmer T were present associated with weak L (c) and proximity of arctic front (b) Region was under the influence of a 500 hPa ridge or “omega block” at upper levels for the week – see 500 hPa maps next page, that effectively blocked or weakened MLCs from passing through This 500 hPa ridge is associated with anticyclonic conditions (H) at the surface, and the prevalence of cA (continental arctic) air over the region (see MSLP maps each day) The 500 hPa ridge moved from along the BC coast on Tue to the center of BC on Wed, associated with the warming temperatures The 500 hPa ridge moved back westward from center of BC to offshore from Thursday to Friday  this results in northerly flow of air aloft over BC Thursday and Friday  associated with the outbreak of fresh arctic air and a new anticyclone (d) moving southward with renewed cooling temperatures and N surface wind The surface H over BC Tuesday (a) weakened and moved SE of BC from Tuesday to Thursday Behind the H there was a weak L (c) over northern AB that also tracked south, some cloud associated with the L extended over BC and PG on Wed night Also, as the initial surface H moved away, the arctic front (b) oriented along the coast mountains moved to near PG – associated with increased T and also with cloud over PG on Wed night A new H from the arctic (d) moved southward to northern AB on Thurs and Fri bringing in fresh cA air and pushing the arctic front to the west back over the coast mountains The strong pressure gradient to the SW of this new H (d) resulted in strong north winds developing on Thurs and continuing on Fri, and colder temperatures ac/bd

Tue 4am Wed 4am Thu 4am Fri 4am Temps warm as ridge moves east over BC Tues  Wed Temps cool as ridge moves back west over coast placing BC in strong wind from N Thur  Fri

* * Tuesday, Feb 25 Anticyclone (H - a) over BC associated with upper ridge seen at 500 hPa Arctic front (b) against Coast Mtns with PG in cA air Offshore systems prevented from crossing BC by upper ridge / sfc anticyclone Slack pressure gradient and subsiding air due to anticyclone over PG resulted in clear conditions with poor air quality a b Tue 10 am

* * * a a b b Wednesday Feb 26 Anticyclone (a) over central BC in morning is pushed southeastward over BC/AB border by 10pm Arctic front (b) at the same time progresses eastward to lie nearly through PG by 10 pm Increase in clouds during evening associated with the movement of the arctic front closer to PG, as well as with weak L system east of ridge (c) moving from north to south (some sfc analyses connect the arctic front to this L) Wed 10 pm Wed 10 am Wed 10 pm c c c

* * * c a c b Thursday Feb 27 Anticyclone (a) has moved SW of BC Arctic front (b) begins to move back westward in response to fresh arctic anticyclone (d) moving southward Weak L system (c) continues to move southward east of region Strong pressure gradient SW of (d) results in strong northerly winds Thu 10 pm Thu 10 am c b d d

* * c c b Friday February 28 Arctic front (b) continues to move back westward in response to fresh arctic anticyclone (d) moving southward to lie over northern AB Strong pressure gradient SW of (d) results in strong northerly winds over the central interior of BC and decreasing temperatures as the fresh cA air intensifies Weak L system (c) continues to move southward east of region and is no longer relevant Fri 10 am d