SPC Mar 19, 2017 1300 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 19 Mar 2017

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0746 AM CDT Sun Mar 19 2017 Valid 191300Z - 201200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM CENTRAL/SOUTHERN IOWA ACROSS NORTHERN/CENTRAL ILLINOIS TO NORTHWESTERN INDIANA... ...SUMMARY... Marginally severe hail may occur tonight over parts of the Corn Belt from southern or central Iowa to northwestern Indiana. ...Synopsis... In mid/upper levels, large-scale ridging over central North America will be dampened on the north side by a series of closely spaced shortwave troughs and accompanying vorticity lobes, currently evident in moisture-channel imagery from northern MB westward to northeastern BC and southwestward to the Canadian Rockies. As these perturbations become better-phased in the latter half of the period, and the synoptic ridge moves eastward over the upper Great Lakes, a field of height falls will spread southeastward over the northern Plains and upper Mississippi Valley. Within that field, a weak shortwave trough -- currently apparent over the MT/WY Rockies -- will pivot eastward over the northern Plains. The southern part of that perturbation should reach the upper Mississippi Valley by 12Z. At the surface, a cyclone was analyzed at 11Z well offshore from the East Coast, south of Cape Cod and east of DE, with cold front southwestward across northern FL to the northeastern Gulf. This boundary became diffuse and quasistationary over LA and the Arklatex region, then a warm front across eastern OK and southern/western KS. As the cold front proceeds down the FL Peninsula through the period, the warm front will move northeastward across KS and the Ozarks. Meanwhile, a cold front associated with the northern-stream perturbations will develop over the northern Plains, connecting to a weak surface low over southern NE by 00Z and extending northeastward across MN. By 12Z, the cold front should extend from eastern Lake Superior over eastern IA, northwestern MO and eastern KS, to a weak low in the END-ICT region. A dryline over the central/southern High Plains, south of that cold front, should remain convectively quiescent through the period. ...IA/IL/IN... Diurnal convective potential should be suppressed by a stout EML capping the boundary layer in the warm sector and along the surface warm front, where despite surface diabatic heating, convective-scale lift will be kept rather weak and shallow beneath the passing mid/upper ridge. However, we expect increased thunder potential through the evening, along with the risk for isolated severe hail from the most intense cells, especially early in the convective cycle before storm modes get interactive and messy. Initial activity in this regime should develop within 2-3 hours after dark, along the left side of a southwesterly low-level jet intensifying to 45-55 kt. In that regime, moisture advection/ transport between 750-900 mb (straddling the base of the EML) will saturate that layer, beneath 8-8.5 deg C/km midlevel lapse rates. This should reduce MUCINH effectively to nil as parcels with increasing moisture content start being isentropically lifted to LFC. Elevated MUCAPE of 1500-2500 J/kg is possible, based on upstream moisture and lapse rates imparted to assorted model forecast soundings. The aforementioned height falls and related tightening of the mid/upper-level height gradient will strengthen flow aloft enough to contribute to effective-shear magnitudes around 35-45 kt -- mostly from speed shear, given the unidirectional nature of winds forecast in the CAPE-bearing layer. Thunderstorms forming over the western/middle parts of the outlook area should shift generally eastward with time overnight, crossing portions of IL and IN. The hail threat generally should diminish toward daybreak as the supportive LLJ and related storm-relative low-level winds weaken, and convection moves into a less-unstable environment. ...lower MO Valley region... The final stages of an ongoing LLJ/warm-advection regime, in the leading fringes of elevated moisture return, are supporting isolated mainly shallow convection at this time over parts of northwestern MO. Although modified 12Z soundings from TOP/SGF suggest deep MUCAPE and ELs above the 200-mb level, CAPE density is low with absolute CAPE values roughly in the 1000-1500 J/kg range. Those soundings and moisture-channel imagery each indicated substantial mid/upper drying as well. Related entrainment issues and lack of stronger low-level forcing on the elevated warm front appear to have restricted convective coverage/strength so far. While a pulsing of intensity with isolated/brief hail near severe limits cannot be ruled out across parts of MO this morning, the threat appears too isolated and marginal for an unconditional outlook area. ..Edwards/Gleason.. 03/19/2017 Read more



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