Convective Addiction

FCST: 07/17/10 – Upper Midwest

Posted on 15. Jul, 2010 by Jesse in Forecasts, Uncategorized

Saturday afternoon and evening could yield the next in a series of active severe weather episodes across the Upper Midwest.  A stalwart H25 flow of 80 kts will persist over the Northern Plains, with an attendant H25 streak ~100 kts evident on the 00z NAM and GFS runs between 00z and 03z Sunday.  This will place parts of the Northern Plains in the right entrance region of an upper level jet maxima, lending to better upward motion and a more favorable environment for severe thunderstorms.  Associated with this seasonably staunch H25 flow will be an attendant shortwave with H5 flow AOA 40 kts across the region, coupled with a respectable SW H85 flow ~ 20-30 kts, increasing to 50 kts after dark.  A 1004 mb surface low should move out of the Dakotas into WC MN by 00z Sunday, with an attendant warm front draped across central Minnesota and a cold front reaching the I-29 corridor between 21z Saturday and 00z Sunday.

NAM MSLP/SFC at 21z – 20100717

250 mb UA profile at 00z – 20100718 per NAM

500 mb Profile at 00z – 20100718 per NAM

850 mb Height/Temp/RH at 00z – 20100718 per WRF

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Ahead of this system, warm and unstable air will advect northward ahead of the advancing surface low, with CAPE values across far SE SD, central and southern MN and northern IA AOA 3,000 J/KG, coupled with surface Theta-E values AOA 350K and strong SFC-500 mb bulk shear values on the order of 30-50 kts across much of the region, with the highest values largely concentrated in MN.  While LI indices across the region will be AOA -10C, of particular concern for tornadic potential will be the rather strong 700 mb temps on the order of 12-13 C, though both the NAM and GFS are showing some erosion of the CIN in portions of southern and western Minnesota.  Additionally, the advancing cold front should be sufficient to overcome CIN in the western portion of the severe risk area.  However, the availability of widespread strong surface winds will be of a less common occurrence across the region, as the strength of the warm front has been rather nebulous on the past few model runs.

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SB CAPE at 21z – 20100717 per NAM

700 mb Temp at 00z – 20100718 per NAM

SB Lifted Index at 00z – 20100718 per NAM

EHI (via MLCAPE) at 21z – 20100717 per RUC Guidance

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While damaging winds and large hail look to be the main severe weather threats, a narrow window will exist for tornadoes with discrete supercells that may be able to take advantage of the WF boundary draped across central Minnesota, particularly INVO the triple point, an area which will feature a more austere CAP, or further east near or north of MPX.  Current thinking is that convection may initiate in western Minnesota ahead of the surface low by 19z, with discrete activity persisting for a short while (~ 1-2 hours at most) before quickly congealing into an MCS that moves ESE across the region.

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Potential Hail Size per RUC Guidance at 21z – 20100717

SFC – 3 km Lapse Rates at 21z – 20100717 per RUC Guidance

Sig. TOR Parameter at 00z – 20100718 per NAM

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How might’ the convective scenario unfold?  A few composite radar reflectivity simulations from some of the major model simulations are shown below.  If correct, there are some indications that several substantial supercells could precede a large MCS event later this afternoon and into this evening.

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Simulated Reflectivity at 19z – 20101717 per 4km WRF

Simulated Reflectivity 19z – 20100717 per HRRR

Simulated Reflectivity at 19z – 20100717 per WRF

Forecast written by Convective Addiction’s Jesse L. Risley at 12:00 AM July 17th