Week in Review8/28/13 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

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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

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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

GFS 24 h forecastThursday AM

GFS AnalysisThursday AM

GFS 48h forecastFriday AM

GFS analysisFriday AM

GFS 24 h forecastThursday AM

GFS analysisThursday 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 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

Boston Forecast EvaluationObs John NWS NAM GFS

Thurs Tmin 62 63 (-1) 63 65 62

Thurs Tmax 72 70 (-2) 70 70 73

Thurs Precip Trace 0 (0) 0 1 0

Fri Tmin 63 61 (-2) 61 62 61

Fri Tmax 84 78 (-6) 79 84 77

Fri Precip 0 0 (0) 0 0 0

Error points 11 10 6 10