2PM Day 1 Convective Outlook CORR 1 for Tuesday, February 17. THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS COASTAL CALIFORNIA AND PORTIONS OF NEBRASKA AND IOWA
CORRECTED FOR NDFD COLOR FILL ISSUE
SUMMARY
Isolated strong thunderstorms will be possible across parts of the West today through tonight. Isolated thunderstorms are also anticipated across the Upper Midwest late this afternoon and tonight.
Eastern Nebraska and southwestern Iowa
Near the base of a negative-tilt shortwave trough and a pronounced 100+ kt mid-level jet, strong ascent is expected to develop late this afternoon amid a mixed and modestly moist air mass. Cool mid-level temperatures and surface dewpoints in the 40s and 50s F may allow a few hundred J/kg of buoyancy to develop. This destabilization and strong ascent may support a fast-moving arc of high-based showers/thunderstorms, particularly where surface temperatures reach into the 60s and dewpoints hold above 40 F.
While buoyancy will be weak, dry low-levels and strong dynamics/background flow could support efficient downward mixing of momentum to the surface. Isolated severe/damaging gusts are possible this afternoon and evening from parts of eastern NE into west-central IA. Convection should weaken rapidly with eastward extent
Elsewhere, isolated strong storms remain possible across portions of the CA coast ahead of a strong secondary upper trough. This is most likely overnight with a few damaging gusts or a brief tornado the primary risks owing to very strong flow and modest inland moisture advection. No changes were made to the outlook here, see the prior discussion for more info.
Synopsis
Recent satellite imagery shows several disturbances across the western CONUS, including a pair of low-amplitude shortwave troughs within the belt of strong southwesterly flow extending from the central Pacific into the central Plains and a deepening upper low off the Pacific Northwest coast. Intense surface cyclogenesis is anticipated ahead of the lead shortwave as it moves quickly northeastward, with the resulting surface low forecast to move across SD/NE, ending the period over southern MN. Evolution of this surface low and parent shortwave, which will become increasingly negatively tilted throughout the day and overnight, will result in strong forcing for ascent from the Mid MO Valley into the Upper Midwest this evening and overnight.
Farther west, the previously mentioned upper low is expected to drop slowly southward as a strong jet (i.e. 100 kt at 500 mb) pivots around its western and southern periphery. Very cold mid-level temperatures and strong/persistent forcing for ascent will support isolated thunderstorms along across much of the West coast throughout the day and tonight.
Coastal California
Limited buoyancy is expected to develop along the near coastal areas of CA as very cold mid-level temperatures (i.e. -25 to -30 deg C at 500 mb) associated with the upper low continue to advect into the region. This buoyancy coupled with the strong forcing for ascent could result in some deeper, more persistent updrafts across the region today, both within the primary frontal band and any more bowing line segments that may follow it. Given the robust deep layer flow across the region, any deeper, more persistent updrafts could result in some stronger, convectively augmented gusts. Additionally, there appears to be enough low-level curvature to support a brief tornado or two within the immediate coastal regions. Highest potential for either hazards currently appears to be after 00Z in the San Luis Obispo/Santa Barbara county areas.
Upper Midwest
Some low-level moisture return is anticipated across the region ahead of the previously mentioned shortwave trough and associated forcing for ascent. Even so, low-level moisture will still be limited, with dewpoints likely remaining in the upper 40s to perhaps low 50s. This is not expected to be enough moisture for surface destabilization across most of the region, despite strong surface heating/boundary-layer mixing and steepening low-level lapse rates in vicinity of the surface low/front. The only exception is across far southeast SD and far northeast NE where a high-based storm or two cannot be completely ruled out. Any that does develop could produce an strong gust or two, although coverage will likely remain less than 10%.
Across the Upper Midwest, capping may tend to largely prevail during the day, with elevated thunderstorms becoming more probable east-northeastward of IA into the MN/WI throughout the evening and overnight. Instability does not appear strong enough to support severe hail with this elevated activity.