Week in Review 8/28/13 to 9/4/13 John Cassano

Weather Situation – Strong upper level ridge over central US – Jet stream well north of US – Weak frontal boundary from mid-Atlantic to northern Plains – Southwest monsoon activity over Rocky Mtns

Denver Key Weather Elements – Strong upper level ridge over central US persists through period (500 mb) Ridge builds westward over CO during forecast period – No jet dynamics (300 mb – not shown) – Surface high over mtns with low over plains (SLP) – Airmass warms slightly through period (700 mb T) – Increasing monsoon moisture to west (500 mb RH)

Impact of Key Weather Elements – Expect slight warming of airmass in response to: Building upper level ridge Generally clear skies and strong daytime heating – Large diurnal range in T with clear skies and dry airmass – Winds will be mainly diurnal – downslope at night, weak, upslope during the day with no significant impact on T or precip – Generally clear skies, although increasing monsoon moisture may lead to high clouds late in the day – No precipitation expected

Boston: Key Weather Elements – 500 mb trough east of Boston Wednesday through Friday AM, then flow becomes zonal – Weak 500 mb shortwave passes north of Boston on Thursday – No jet dynamics – H in eastern Canada early in period forces NE flow and CAA – Sfc ridge over Boston on Thursday – light winds – Ridge shifts east of Boston on Friday with return flow and WAA – Lots of low level (850 mb) moisture – clouds and fog

Impact of Key Weather Elements – Cooling trend through Thursday in response to CAA from Canadian H – Warming on Friday as surface ridge shifts east of Boston – Fog and clouds will reduce diurnal temperature range – Winds will be NE’ly on Thursday, become light Thursday night / Friday morning with ridge axis over Boston, and then switch to SW’ly as ridge shifts east These changes in wind direction will drive change of CAA to WAA – Ample low level moisture will result in mostly cloudy skies with fog likely overnight – Little dynamic forcing for precipitation despite moisture availability – expect no measurable precipitation

Model Verification 500 mb – DEN: 500 mb ridge slightly stronger in obs than forecast SLP / mb thickness – BOS: Slightly smaller mb thickness over Boston on Thursday, otherwise good agreement 700 mb T – DEN: Obs slightly warmer than forecast on Wednesday and Thursday and cooler on Friday 500 mb RH – DEN: Higher RH in obs Thursday afternoon and Friday

GFS 24 h forecast Thursday AM

GFS Analysis Thursday AM

GFS 48h forecast Friday AM

GFS analysis Friday AM

GFS 24 h forecast Thursday AM

GFS analysis Thursday AM

GFS 12 h forecast Wednesday PM

GFS analysis Wednesday PM

GFS 36 h forecast Thursday PM

GFS analysis: Thursday PM

GFS 60 h forecast: Friday PM

GFS analysis: Friday PM

GFS 36 h forecast: Thursday PM

GFS analysis: Thursday PM

GFS 60 h forecast: Friday PM

GFS analysis: Friday PM

Denver Verification: Impact of Key Weather Elements – Expect slight warming of airmass in response to: Building upper level ridge Generally clear skies and strong daytime heating Large diurnal range in T with clear skies, dry airmass Both min and max T will increase through forecast period All of the above was true on Thursday but not true on Friday – Winds will be mainly diurnal – downslope at night, weak, upslope during the day – Generally clear skies, although increasing monsoon moisture may lead to high clouds late in the day – True on Thursday, not true Thursday night and Friday – No precipitation expected

Observed Weather Denver – Wednesday night Mostly cloudy until 8PM then scattered clouds / clear Wind shifts from NW to W to SW overnight T min: 65 F (5AM) – Thursday Clear to scattered clouds early then mostly cloudy Light / variable winds T max: 95 F (3PM)

Observed Weather Denver – Thursday night Mostly cloudy Southwesterly downslope winds T min: 70 F (8AM) – Friday Mostly cloudy Winds light and stayed mainly westerly until very late in afternoon T max: 90 F (3PM)

Boston Verification: Impact of Key Weather Elements – Cooling trend through Thursday in response to CAA from Canadian H – Warming on Friday as surface ridge shifts east of Boston – Fog and clouds will reduce diurnal temperature range – Winds will be NE’ly on Thursday, become light Thursday night / Friday morning with ridge axis over Boston, and then switch to SW’ly as ridge shifts east These changes in wind direction will drive change of CAA to WAA – Ample low level moisture will result in mainly cloudy skies with fog likely overnight – Little dynamic forcing for precipitation despite moisture availability – expect no measurable precipitation

Observed Weather Boston – Wednesday night Fog E to NE winds T min: 62 F (7PM) – Thursday OVC, drizzle (trace of precipitation) NE winds, CAA, falling T through day T max: 67 F (10AM)

Observed Weather Boston – Thursday night Overcast Light northwest to southwest winds T min: 63 F (7AM) – Friday Overcast to mostly cloudy until mid-afternoon then scattered clouds West to southwest winds T max: 84 F (5PM)

Denver Forecast Evaluation ObsJohnNWSNAMGFS Thurs Tmin6565 (0)64 Thurs Tmax9594 (-1)9395 Thurs Precip00 (0)000 Fri Tmin7067 (-3)64 Fri Tmax9095 (-5) Fri PrecipTrace0 (0)000 Error points

Boston Forecast Evaluation ObsJohnNWSNAMGFS Thurs Tmin6263 (-1) Thurs Tmax7270 (-2)70 73 Thurs PrecipTrace0 (0)010 Fri Tmin6361 (-2) Fri Tmax8478 (-6) Fri Precip00 (0)000 Error points11106