Friday, Saturday and perhaps even Sunday are likely to be sunny, unseasonably warm days with highs in the 80s in the Roanoke Valley and points south and east and mid 70s to near 80 in the New River Valley and points west. Monday appears to be the likely timing for the bulk of rain from the combination of Tropical Storm Karen and an approaching cold front. This timing will have to be refined, so just pencil in Sunday as a dry day (maybe not the evening) for now.
Tropical Storm Karen has an uphill battle to reach hurricane status.Wind shear aloft is keeping Karen from developing that nice whirlpool-type look with distinct spiral arms and an eye that you expect to see from a mature tropical cyclone on a satellite picture, despite maximum winds of 65 mph, only 9 mph below hurricane status. The idea of shear retarding tropical systems can be confusing if you’re used to the idea that shear — winds blowing in different directions and/or speed at different levels of the atmosphere — actually helps intensify thunderstorms and makes tornadoes more likely. But with a large system born out of the the evaporational energy of warm sea surfaces, shear can rip the tops off the developing system, spreading out heat and moisture and de-centralizing the inner vortex of storms and rain bands needed for growth of the tropical system. Karen is likely to run into more shear as it moves northward, especially with an approaching cold front and upper-level trough, but has enough warm water to build on and perhaps just enough periods of fairly moderate shear that a strong tropical storm or minimal hurricane is possible by landfall, likely somewhere between New Orleans and Appalachicola, Fla., on Saturday.
Whether Karen becomes a hurricane or not, it will be a big blob of tropical moisture that will slide northeastward ahead of an approaching cold front late Sunday into Monday and possibly Tuesday. Forecast guidance has been very consistent in taking the remnant circulation center of Karen near or right smack on top of Southwest Virginia. Keep in mind though that Karen won’t be a point or a thin line, but more of a blob, so where exactly the center of whatever circulation is left goes isn’t necessarily a good indicator of where the heaviest rain will occur. Many inland tropical systems spin the heavier rain bands and potentially severe storms to the east of their tracks. Two factors, however, may raise the rain levels for Southwest Virginia, even if the center tracks to our east: (1) mountainous terrain and (2) an approaching cold front. Each of these will provide a focus for moisture circulating westward around the north side of Karen’s circulation to run up against, get lifted, and possibly squeezed out in torrents. Forecast models are continuing to shift back and forth with the rainfall amounts and how far east or west the heaviest band will go, but this total precipitation output map from Thursday’s 12Z GFS presents a general idea of broad expectations that a heavy swath of rainfall will develop somewhere over or near us come Monday.
National weather over the next 48 hours is truly a 3-ring circus, with the potential for severe thunderstorms and tornadoes in the Upper Midwest/Northern Plains and just a couple hundred miles to the west, a blizzard in the High Plains.