You are looking at an outlook that is part of the medium range forecast (the outlook for days 4-8). The most important thing to note is that lack of a risk does not mean zero risk. Generally speaking, confidence has to be pretty high for the Storm Prediction Center to have an outlook area this far into the future.
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ZCZC SPCSWOD48 ALL ACUS48 KWNS 180907 SPC AC 180907
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0407 AM CDT Tue Sep 18 2018
Valid 211200Z - 261200Z
Medium-range models appear to be in fairly close agreement through Day 5 (Saturday 9-22), after which agreement begins to break down substantially, as an upper trough moves inland from the eastern Pacific across the Intermountain West. Substantial differences with respect to the progression/speed of this trough result in similarly substantial differences in surface pattern evolution, resulting in low confidence with respect to any convective forecast beyond Day 5.
On Day 4 (Friday 9-21) however, an initial upper trough is forecast to be advancing quickly eastward across the Great Lakes and later the Northeast U.S., and adjacent eastern Canada. This trough will be accompanied by a seasonably strong cold front, progged to shift across the Northeast and Ohio Valley and shift off the New England coast by the beginning of Day 5. While instability along the length of the front – particularly across New England – will likely remain limited, strong westerly/west-southwesterly flow through a deep tropospheric layer may support potential for gusty/damaging winds, with a frontal line of convection. Thus, a severe risk area is being included Day 4, from parts of the Midwest/Ohio Valley northeastward into New England.
By Day 5, with the progressing portion of the front shifting into the western Atlantic, severe weather threat will diminish, and then with increasing discrepancies amongst the models the rest of the period, no additional outlook areas will be included at this time.
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