SPC Sep 14, 2017 0600 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 14 Sep 2017

SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1238 AM CDT Thu Sep 14 2017 Valid 141200Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHERN ARIZONA...SOUTHERN UT...SOUTHWEST COLORADO AND NORTHWEST NEW MEXICO... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF THE GREAT BASING INTO THE FOUR CORNERS REGION... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A PORTION OF THE EASTERN DAKOTAS INTO NORTHERN MINNESOTA... ...SUMMARY... Strong to severe storms may occur this afternoon to early evening from the Great Basin into the Four Corners region with locally damaging wind and large hail the main threats. A few strong to severe storms with hail and strong wind gusts are also possible from the eastern Dakotas into northern Minnesota. ...Synopsis... Positive-tilt northern-stream upper trough extending from the Pacific Northwest into the southern Canadian prairies will advance slowly east today. The cutoff upper low now over southern CA will transition to an open wave and move through the Great Basin as it begins to interact with southern branch of the northern stream trough. Closed upper low over the OH Valley is forecast to deamplify as it transitions to an open wave and continues through the Northeast States. ...Great Basin and Four Corners Region... Despite somewhat limited low-level moisture with near-surface dewpoints in the 40s F, modest (7-7.5 C/km) mid-level lapse rates and diabatic warming will contribute to marginal instability with 500-1000 J/kg MLCAPE expected. Deeper forcing for ascent with primary vorticity maximum attending the southern-stream shortwave trough will be favorably timed with maximum boundary-layer destabilization from northern AZ into southern UT, contributing to development of numerous thunderstorms by early afternoon. Mid-level jet within base of this impulse will contribute to sufficient vertical shear (35-40 kt) for a few supercells, especially across northern AZ into extreme southern UT. Otherwise, multicells should remain the dominant mode. A few instances of damaging wind and large hail will be the main threats with this activity. ...Eastern Dakotas into northern Minnesota... Frontal zone will persist from SD through Northern MN, possibly shifting slowly south during the day. Heating in the warm sector will occur, boosting MLCAPE to 1500 J/kg, but convective inhibition and weak forcing should limit warm-sector thunderstorm development. Zonal flow aloft with embedded weak impulses will reside above the frontal zone. Current indications are that stronger storms will probably initiate within corridor of deeper forcing along and just north of the cold front from southeast ND or northern SD into northern MN. Much of this activity will probably remain elevated/undercut by the front. However, a few storms developing near the surface front may become surface based. Generally weak vertical shear will promote multicells with a few instances of hail and locally strong wind gusts possible into early evening. ..Dial/Cohen.. 09/14/2017 Read more



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