SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0630 AM CST Thu Jan 21 2021 Valid 211300Z - 221200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Some weak thunderstorm activity is possible today across parts of southern Arizona, and mainly this evening in a corridor across parts of eastern Texas into the lower Mississippi Valley. ...Synopsis... Within amplified, branching flow across the eastern Pacific, models indicate that mid/upper ridging may build further, along a positively tilted axis from the mid latitudes, near/west of 150 W, north-northeastward into the Arctic latitudes. Strongest height gradients, and wind fields, are forecast to continue to develop in a sharply anticyclonic regime across the northeastern Pacific, with one significant short wave perturbation digging on its leading edge (to the east of the ridge axis). At mid-levels it appears that this will include an embedded mid-level low reaching southern Oregon and northern California coastal areas by late tonight. Downstream, the westerlies converge into strong west-northwesterly flow east of the Rockies into the western Atlantic. Within the branch emanating from the Arctic latitudes, a mid-level low may gradually progress southeastward and eastward, to the northeast of the Great Lakes region. While an initially broad and deep associated surface cyclone begins to weaken, as secondary cyclogenesis commences along a frontal zone off the Mid Atlantic coast, it appears that a significant reinforcing intrusion of cold air will overspread much of the upper Midwest/Great Lakes region. Meanwhile, within the branch emanating from the northeastern Pacific, an initially closed mid-level low, now turning toward the Southwestern international border area, is forecast to transition to an open wave and accelerate northeastward in response to the approach of the upstream closed low. Models indicate considerable shearing/weakening of this impulse by the time it reaches the southern Great Plains by late tonight, as it begins to round the broad crest of subtropical ridging centered over the southwestern Gulf of Mexico into Caribbean vicinity. Beneath this regime, a narrow plume of low-level moistening is already well underway within southerly return flow across the northwestern Gulf coast vicinity. However, this is mostly elevated above a residually cool and stable boundary layer (even across the northwestern Gulf of Mexico). It is also generally beneath a broad plume of mid/high-level moisture return emanating from the lower latitude eastern Pacific. ...East Texas into lower Mississippi Valley... Thermodynamic profiles are already in the process of becoming saturated, and generally weak lapse rates are likely to prevail through much of the period, beneath relatively warm mid-levels. However, strengthening lift focused along a mid-level baroclinic zone (roughly around 700 mb), perhaps aided by forcing in the right entrance region of a high level jet around the crest of the subtropical ridge, is forecast by this evening. And various model output (perhaps most notably the SPC High Resolution Ensemble Forecast) suggest that this might become supportive of at least some weak thunderstorm activity. ...Southern Arizona... To this point, thunderstorm development has remained focused in relatively close proximity to the northern Gulf of California, aided by the inflow of more moist and potentially unstable air. Weak destabilization does appear underway east-northeastward along the international border area, as the relatively compact mid-level cold core of the low crosses the northern Gulf of California. However, the extent to which convection capable of producing lightning will be maintained, as it accelerates east-northeastward through the day, remains unclear. ...Southern Oregon/Northern Oregon coastal areas... Convection capable of producing lightning may accompany the relatively compact mid-level cold core (perhaps to -28 to -30C at 500 mb) of the digging mid-level low. However, while this could approach coastal areas late tonight, models are suggestive that any such activity should remain offshore through at least 12Z Friday. ..Kerr.. 01/21/2021
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