... Dry high pressure builds in through Wednesday with a slow cooling trend. Meanwhile, Tropical Cyclone Ian will move north into the eastern Gulf of Mexico and approach our area late in the week, affecting our weather this weekend.
NEAR TERM /THROUGH TONIGHT/... As of 130 AM: Mountain valley fog mainly in the Little TN basin will dissipate quickly after daybreak. Cool and dry high pressure builds in from the north today. N wind this morning turns more westerly for the afternoon, with gusty conditions across the mountains. There may be some clouds but skies should be mostly clear. Highs will be up to 10 degrees below normal across the mountains and up to 5 degrees below normal elsewhere.
Gusts diminish this evening with winds turning back more northerly. Mostly clear conditions continue. Lows will be around 10 degrees below normal.
SHORT TERM /WEDNESDAY THROUGH THURSDAY/... As of 200 PM Mon: Base of upper trough will remain near the southern Appalachians through Wednesday, then shift east allowing heights to rise Thursday. With confluent flow aloft, Canadian high pressure will continue to build into the area during this time. Min temps will fall into the 30s at high elevations Wed and Thu mornings, but otherwise lower 40s. Winds veer to easterly Wednesday, and with the loss of downslope component max temps should trend a bit cooler. As the stage sets for CAD, things will become increasingly breezy late Wed through Thursday. Nocturnal cooling will be offset by cirrus invof jet streak in the exit region of the trough, and as high altitude moisture ejects from TC Ian. Morning frost looks less likely than once thought.
LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 130 PM Monday: TC Ian remains the dominant feature for the medium range forecast. The synoptic expectations have not changed. Namely, landfall appears likely in the vicinity of the Florida Big Bend. Ian will subsequently track northward into quasi-zonal upper flow behind a departing trough. Sfc high will be in position behind that trough to foster hybrid CAD as Ian spreads moisture over a coastal front. Cloud cover and the CAD look to make Friday an especially chilly day, with max temps only in the 60s across most populated areas; a limited diurnal range likely will last through the weekend with mins above and maxes below normal.
Our global deterministic models remain in decent agreement on the above features. As noted yesterday, the spread is much greater when considering the ensemble members, some of which show Ian's progress being substantially slowed or stopped by the damming high. Slower trend in some of the recent deterministic runs may reflect similar scenarios, although the idea of Ian getting completely held off by the high still would seem unusual. We are delaying the onset of impactful QPF compared to previous forecast, but have increased chance to the "likely" range circa Saturday. Consensus remains for the best short-term precip rates to occur more or less between midnight and sunset Saturday. Hard to get any more confident on timing than that, as the GFS/Euro/Canadian solutions begin to diverge in the finer details in that time. Models continue to enhance QPF along the wedge boundary, generally depicted early Saturday across the southern half of the CWA, and where upslope forcing is strongest, later Saturday into early Sunday as the circulation makes its nearest pass to the Blue Ridge Escarpment.
QPF event totals of 5" or more look likely for the Escarpment areas, which is a figure associated with increased risk of landslides. We are messaging the heavy rain as a significant threat based on current forecast depiction. With strong pressure gradient developing across the area as high builds in prior to Ian's arrival, the winds associated with the system's circulation won't be that much stronger than we'd have with any CAD event. Extratropical transition should be underway while Ian (or remnant) crosses the area, and that could lead to stronger winds than what models currently depict. Coupled with saturating soils, we will continue to message the possibility of at least isolated power outages as a result of falling trees. TC tornado threat looks likely to remain low. The more easterly track favored by the GFS keeps us stable enough that the threat would be minimal; this is more representative of the official track at this point. The westerly track of the EC on the other hand brings temps back into the mid-upper 70s Saturday afternoon, along with sizable SBCAPE and low-level shear parameters into a portion of the CWA.