week in review 8/28/13 to 9/4/13 john cassano. weather situation – strong upper level ridge over...
TRANSCRIPT
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 / 1000-500 mb thickness– BOS: Slightly smaller 1000-500 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
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 EvaluationObs John NWS NAM GFS
Thurs Tmin 65 65 (0) 64 64 64
Thurs Tmax 95 94 (-1) 93 95 95
Thurs Precip 0 0 (0) 0 0 0
Fri Tmin 70 67 (-3) 64 64 64
Fri Tmax 90 95 (-5) 96 97 99
Fri Precip Trace 0 (0) 0 0 0
Error points 9 15 14 16