Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Thu Jun 01 2017 Valid 011300Z - 021200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER EASTERN NEW ENGLAND... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE NORTHERN ROCKIES AND CENTRAL/NORTHERN MONTANA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM WEST TEXAS TO PORTIONS OF MISSISSIPPI... ...SUMMARY... Isolated strong to marginally severe storms will be possible across parts of coastal New England this afternoon, as well as from southwest Texas to Mississippi, and over parts of the northern Rockies. ...Synopsis... The upper-air pattern will continue to be dominated by three major features: 1. Troughing over the Hudson Bay and Great Lakes, with associated cyclonic flow extending from the Upper Midwest across the central/southern Appalachians to the Mid-Atlantic and New England. The primary cyclone will dig southeastward from the southern James Bay region across southern QC through the period. Meanwhile, the primary shortwave trough -- now evident in moisture-channel imagery over ON just north of Lake Superior -- will pivot southeastward over Lake Huron today then eastward to northern NY by 00Z. This shortwave trough then will cross northern New England. 2. A prominent ridge over the Rockies, northward across SK to the western Canadian Arctic islands. This ridge will shift eastward to the Dakotas, mid/lower Missouri Valley and Ozarks through the period. 3. Troughing from a large cyclone over the Gulf of Alaska southeastward across the Pacific Coast States to northwestern MC. The northern segment of an embedded shortwave trough will eject northeastward to western MT today. The southern part of this regime -- currently manifest as a shortwave trough with a vorticity max near the northern end of the Gulf of California -- will break off the northern-stream flow belt and move slowly eastward over northwestern MX and the southwestern borderlands through the period. A weak lead wave -- currently located over parts of north-central and northwest TX -- will drift northeastward across the Red River Valley today while weakening further. At the surface, A weak, residual, quasistationary frontal zone was analyzed at 11Z from the Hampton Roads area across the Carolinas, northern GA, central portions of MS/AL, and northeast/north-central TX, becoming diffuse farther west. This feature should continue gradual weakening, with little net movement. Another boundary, initially drawn over the KS/NE border region then east-southeastward across northern MO and southern IL, should move slowly northward as a warm front today. ...Eastern New England... Widely scattered to scattered thunderstorms should form over western parts of the outlook area during midday into early afternoon, in a broken, nearly north-south band. This activity will form in a zone of weak lift near a surface trough and ahead of the shortwave perturbation ejecting from Lake Huron. Convection should move eastward across eastern New England with isolated damaging gusts possible. The threat is expected to diminish as activity encounters relatively stable marine air and/or moves out of the area into New Brunswick. After areas of morning stratus clouds erode away, followed by a few hours of diabatic surface heating, forecast soundings reach convective temperature with minimal CINH. Cold mid/upper-level air (i.e. 500-mb temps -18 to -21 deg C) will contribute further to steepened low/middle-level lapse rates. Although low-level moisture will be lower than prior days, with surface dew points generally low-mid 50s F, this still should be sufficient to support preconvective MLCAPE of 500-800 J/kg, locally higher. Deep-layer flow will be nearly unidirectional, with around 30-35 kt effective-shear magnitudes forecast. ...Northern Rockies, MT... Isolated to widely scattered thunderstorms are expected to form this afternoon near the ID/MT border, and over west-central MT. Convection should develop and expand as large-scale DCVA/lift ahead of the ejecting shortwave trough moves over marginal low-level moisture and favorable diabatic heating of higher terrain. This activity should progress northeastward across the outlook area, atop favorably deep and well-mixed subcloud layers for occasional strong-severe gust risk. Forecast soundings show steep low/middle-level lapse rates with MLCAPE to near 500 J/kg, locally higher. Favorable speed shear will exist beneath a belt of southwesterlies aloft, enhanced by the shortwave height falls, contributing to effective-shear magnitudes of 40-50 kt. Lack of more robust low-level theta-e will preclude a better-organized severe risk. ...TX to lower Mississippi Valley... Widely scattered to scattered thunderstorms may produce isolated strong/damaging gusts and marginally severe hail this afternoon, along and south of the remnant surface front. Foci for convection otherwise will be nebulous, mainly in the form of outflow and differential-heating boundaries. Still, given the weakness of MLCINH, in the presence of pockets of strong diabatic heating and rich low-level moisture, 1000-2000 J/kg MLCAPE should develop. Localized areas of marginal deep shear are possible given the kinematic influence of mesoscale boundaries and the presence of increasing upper-level winds with eastward extent over this corridor. ..Edwards/Goss.. 06/01/2017Read more
from SPC Forecast Products http://ift.tt/rATNzD
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