SPC Mar 20, 2017 1300 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 20 Mar 2017

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0739 AM CDT Mon Mar 20 2017 Valid 201300Z - 211200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM NORTHERN INDIANA AND NORTHEASTERN ILLINOIS TO EASTERN INDIANA AND WESTERN OHIO... ...SUMMARY... Scattered thunderstorms are possible through evening across parts of the Midwest eastward to the Appalachians. Isolated storms are also forecast across parts of the Intermountain West to north-central Plains, and parts of West Texas and southeastern New Mexico. ...Synopsis... In mid/upper levels, the synoptic pattern will be dominated by ridging crossing the East Coast, a progressive trough from Baffin Island across Hudson Bay to the upper Great lakes, ridging moving eastward across the Rockies, and a large trough approaching the West Coast. A series of shortwaves will traverse associated cyclonic flow. The strongest of those perturbations -- now evident in moisture-channel imagery over north-central SK -- will move southeastward across Lake Superior late tonight. While the strongest DCVA will remain well north of our general-thunder area, associated strengthening cyclonic flow and height falls aloft are expected to spread southeastward from MB and the Dakotas across the upper Mississippi Valley, upper Great Lakes, and parts of the Ohio Valley. At the surface, 11Z analysis showed a wavy warm-frontal zone extending from a weak low over the MO/IA border to central portions of IL/IN near ongoing convection, then southward across central KY and middle TN. A weak cold front extended from the low westward across southern NE, then northwestward to another weak low near BFF. A separate surface trough and weak cold front extended from the low southwestward over central/southwestern KS. By 00Z, what's left of the MO/IA surface low should reach the EVV/MWA area, along a cold front extending to near the southeastern corner of KS, across northern OK, to eastern CO. Deep mixing and lift accompanying a weak/collocated surface trough, confluence line and dryline, extending SW-NE across west TX and southeastern NM, will support isolated thunderstorm potential mainly late afternoon. By 12Z, the weak low may be located over western/central VA, with cold front to southeastern MO, then quasistationary across northern OK, through a weak frontal-wave low over northern or western OK, then cold front again over parts of the TX Panhandle. ...Midwest, Ohio Valley... An ongoing area of elevated convection and precip with embedded thunderstorms from eastern IL to western OH is expected to decline slowly in coverage and intensity through the remainder of the morning, as it moves southeastward away from optimally supportive buoyancy and lift above the relatively low-theta-e boundary layer. Isolated hail approaching severe levels cannot be ruled out, especially along the west end of the complex where inflow will be relatively unmodified by other convection and moisture is greatest. Another round of convection should develop around midday into mid afternoon, over the northeast IL to Michiana/northwestern OH region, within a regime of gradual height falls and gently difluent flow aloft. This activity further will be supported by a low-level warm-advection regime that replenishes instability/moisture aloft behind the morning activity, and related isentropic lift to LFC. The upstream reservoir for that air-mass recovery appears to be well-represented by the 12Z ILX sounding, which yielded 1.2-inch PW, 7.5-8 deg C/km midlevel lapse rates, and 1600 J/kg elevated MUCAPE. By afternoon, over the western parts of the newer convective regime, elevated buoyancy should be greatest in magnitude and vertical extent, with MUCAPE up to the 1000-1500 J/kg range, amidst 30-40-kt effective-shear magnitudes. This should support the potential for isolated severe hail from a few organized multicells. Lapse rates, moisture and buoyancy each should decrease with eastward extent, as LFCs get higher in elevation, acting as limitations on the eastern extent of severe potential. ...Southeast MO/northeastern AR/western KY/western TN... A frontally initiated convective signal, but of varying extent and intensity, has appeared in some synoptic models and many of the convection-allowing progs for this afternoon. Strong thunderstorm winds and isolated hail conditionally are possible under a regime of difluent flow aloft, and along the eastern rim of an EML that will supply steep midlevel lapse rates, atop increasing boundary-layer moisture. However, deep shear will be relatively modest, as will near-surface flow, with forward propagation/acceleration being the only viable mechanism to yield strong storm-relative inflow. Given the weak prefrontal surface winds, convective-scale forcing is in question. Still, some portion of this area may need unconditional severe probabilities today as mesoscale uncertainties decrease. ..Edwards/Gleason.. 03/20/2017 Read more



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