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

1. Week in Review8/28/13 to 9/4/13 John Cassano

2. 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

8. 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)

9. 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

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30. 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

31. 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

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49. Model Verification • 500 mb • DEN: 500 mb ridge slightly stronger in obs than forecast • SLP / 1000-500 mb thickness • BOS: Slightly smaller 1000-500 mb thickness over Boston on Thursday, otherwise good agreement • 700 mb T • DEN: Obsslightly warmer than forecast on Wednesday and Thursday and cooler on Friday • 500 mb RH • DEN: Higher RH in obs Thursday afternoon and Friday

50. GFS 24 h forecast Thursday AM