A tale of two severe weather surprises – The isolated event of 16 July 2010 and the severe weather outbreak of 17 July 2010

Neil A. StuartNOAA/NWS Albany, NY

NROW 2010

Contrasting forecasts for 16 July (left) and 17 July (right)– Extremely difficult forecast challenges each day

00Z 17 July 500 hPa trough axis relatively

far west

00Z 17 July 850 hPa winds show best low-level wind core from Northern

Plains to western Great Lakes

00Z 17 July 700 hPa RH and

Omega show narrow axis of moisture and

isolated centers of vertical motion over

our region – but look further west -clues for tomorrow

00Z 17 July 925 hPa Θe gradient roughly 10K-15K over our region suggests forcing may be too weak to overcome weak upper dynamics – another Θe gradient developing in the western Great Lakes

00Z 17 July Model QPF shows fairly solids axis of >.01 with many areas expecting >.25 - were model instability/moisture dominating the model precipitation processes in a weak forcing and upper dynamics scenario?

Area soundings at 12Z 16 July showed modest instability at KBUF but upstream (KDTX) and downstream (KALB) deep drying and relatively stable

KDTX was chosen as the upstream sounding because of the mean flow seen in the wind profiles

700-500 hPa lapse rates AKA midlevel lapse rates were not predicted to reach the ≥ 6.5°C/Km threshold around 00Z 17 July

00Z 18 July 500 hPa trough

axis at much closer proximity

to our region

00Z 18 July 850 hPa winds show best low-level wind core over our region

00Z 18 July 700 hPa RH and Omega show center of moisture and Omega that was over the western Great Lakes yesterday was predicted to be over our region

00Z 18 July 925 hPa Θe gradient roughly 10K over our region suggests forcing relatively weak but upper dynamics and low-level jet energy stronger

NAM and GFS MOS showed increasing chances in each run for the evening of 17 July to the early morning hours of 18 July

00Z 18 July Model QPF a bit more scattered but defined band of precipitation in all models- again what aspects of instability and moisture were contributing to model precipitation processes in this weak forcing but stronger upper dynamics scenario?

18Z 17 July through 06Z 18 July – SREF loop shows a broken band of high probabilities for measurable precipitation during the period

18Z 17 July through 06Z 18 July – GEFS loop shows a more solid band of high probabilities for measurable precipitation during the period

Area soundings at 12Z 17 July showed more widespread instability potentially greater CAPE across the region

700-500 hPa lapse rates AKA midlevel lapse rates were not predicted to reach the ≥ -6.5°C/Km threshold around 00Z 18 July either

Observed midlevel lapse rates were modest around 00z 17 July, but much steeper, ≥ 6.5⁰C/Km, around 00Z 18 July- Models did not predict this parameter well

Radar loop from 16 July

Radar loop from 17 July

214 CG lightning strikes in 5 minutes, which averages to > 40 CG strikes per minute! – this does not include IC or CA

75 CG lightning strikes in 5 minutes around the Capital District, which averages to 15 CG strikes per minute! – this does not include IC or CA

Conclusions•Upper trough axis position (500 hPa)– better upper dynamics across our region on 7/17•Boundary layer wind core (850 hPa) much more pronounced on 7/17•Model midlevel moisture and Omega (700 hPa) resolved a well-defined feature on 7/16 and 7/17•Theta-e gradients of 10K-15K at 925 hPa associated with both low-level boundaries• Previous studies - 20K+ gradients with widespread or significant

severe weather outbreaks without strong upper dynamics• These events have shown that 10K-15K gradients can provide a

focus for convection with strong upper dynamics and instability•MOS guidance showed increasing chances for thunderstorms on 7/17 as the event became nearer to real-time

Conclusions•Model/ensemble QPF forecasts suggested much better chances for measurable precipitation on 7/16, but showed well-defined chance for precipitation on 7/17•Area 12Z soundings on 7/16 did not show widespread instability and moisture, suggesting limited chances for convection, severe or otherwise•Area 12Z soundings on 7/17 showed more widespread instability and moisture•Models did not predict midlevel (700hPa-500hPa) lapse rates well• Tracking real-time midlevel lapse rates and extrapolating downstream implied

more instability than forecasted• Midlevel lapse rates ≥ 6.5° C/Km associated with widespread/significant severe

weather• These lapse rates correlate with Warm Season Upper Low research by Matt

Scolara•Bottom line – • Look for intersections between upper dynamics to low-level boundaries defined

by theta-e gradients and wind cores• Look at upstream features in data and guidance for clues beyond 24 hours, and

more research into predicting midlevel lapse rates needs to be done

Thank you for your time and attention!

Questions?

More detailed analyses of severe weather and winter events affecting NWS Albany, NY area at

http://cstar.cestm.albany.edu/PostMortems/CSTARPostMortems/alypostmortemindex.htm