Sugar Mountain

Forecast Discussion

Summary

SYNOPSIS

... Canadian high pressure will ridge down into our area from the north Tuesday and Tuesday night as a series of low pressures begin to move up from the Gulf. The combination of quite cold air and abundant moisture will result in an area of wintry precipitation for the mountains and foothills. The moist pattern continues until later this weekend when a cold front ends the rain for at least a couple of days.

NEAR TERM /THROUGH TUESDAY/... As of 240 PM: Dry northwest flow across the region has led to mostly sunny skies outside of some mid to high clouds streaming in from the southwest. Despite increasing cold air advection, the downsloping flow has allowed temperatures to rebound nicely into the low to mid 60s this afternoon east of the mountains. Meanwhile it is much cooler and breezy in the mountains with temps largely in the 40s.

A strong (1040 mb) surface high pressure over the northern plains will quickly transition into a classic cold air damming (CAD) configuration tonight into Tuesday. This will bring much colder temps, especially east of the mountains where highs of 40-45 will be 20 or so degrees cooler than today. Meanwhile, a mid-level disturbance originating from an amplifying long-wave trough over the southern Plains will move over the region late tonight. The main impact of the disturbance will be increasing mid and high clouds. Cold temps are on tap tonight despite the insulating effects of increasing clouds as cold air advecting will drive temps down into the 30s with 20s possible across the mountains.

The forecast becomes complicated in a hurry on Tuesday as increasingly moist mid-levels interacts with dpva crossing the region in the morning. This will likely produce some precipitation aloft, but the big question is will it be heavy enough to overcome the dry low levels initiated and sustained by CAD in place. The current forecast does give a slight chance this will occur mainly across the mountains Tuesday morning. Thermal profiles indicate that a rain/snow mix at the onset is possible across the mountains, especially higher elevations Tuesday morning. Increasing moist ascent at low levels should support increasing PoPs, especially later Tuesday afternoon through the end of the short term period early in the evening. Current forecast warms temps just enough that most of this should fall in the form of rain except across the northern mountains/ foothills where a wintry mix is likely at times. Temps Tuesday are expected to be well below normal, hovering in the mid 30s to low 40s for most of the day with highs generally 38-45. QPF is expected to be generally less than one tenth of an inch during the near term with little to no snow/ice accumulations across the mountains/foothills that do see a wintry mix.

SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 215 pm EST Monday: A prominent trough moving east of the Rockies Tuesday night will permit heights to rise modestly over the southeast through Wed. However, lower levels will remain action- packed across the southeast as sprawling, cold, and dry surface high pressure centered over PA/NY spills southward into a very favorable cold air damming regime Tue night through Wed. During this coldest part of the short-range forecast, the deeper upper jetlet and mid- level DPVA forcing should wrap around the southeast ridge to the north and west of the area. However, low-level isentropic lift should become robust Tuesday night as an 850 mb baroclinic zone lifts northward over the forecast area. The precipitation type and amount forecasts are greatly complicated by two factors: occasional pockets of mid-level drying, and pinning down the southward extent of the freezing surface temps during the heart of the wedge. At least it appears as if the deepest mid-level moisture occurs near the onset of the event when ice nucleation matters most for snow/sleet occurrence. Frozen types should be fairly short-lived along the northern Blue Ridge mountains Tuesday evening before transitioning quickly to freezing rain as a strong +5 to +6 C warm nose develops. Regarding surface temperatures, the forecast favors a consensus blend of raw model temperatures, with more NAM input that usual given the expected favorable NAM handling in the surface wedge layer.

Even with QPF slightly on the conservative side of the model consensus (as ice nuclei may limit droplet development aloft), still get around Warning criteria ice for the northern Blue Ridge area and some Advisory level accumulations farther south along the eastern escarpment toward north Georgia. Plan upgrading the current Winter Storm Watch area to a Winter Storm Warning (in collaboration with RNK given the frozen onset types) and posting a tier of advisory as far east as Alexander, west to Madison, and south to Jackson Co. This could need expanding if the cooler temps pan out on the NAM. These hazards will run from Tuesday evening through the daytime on Wednesday. High elevation southern mountain winds could reach Advisory levels as well, but will hold off on that hazard for now with mainly elevations about 4000 feet affected.

Deeper moisture and forcing will settle across the area Wednesday night through Thursday. Unidirectional and strong southwest flow will provide for excellent moisture transport through the day on Thursday as a western Gulf of Mexico tap sets up. Heavier precipitation will continue to get focused along the southern mountains as waves of surface low pressure ride up the front into the region. In addition, steeper mid-level lapse rates will arrive on Thursday to enhance precip rates in some areas. Finally, surface- based instability should be uncovered along and southeast of I-85 on Thursday afternoon. The GFS features 800 to 900 J/kg of sbCAPE over the lower piedmont by 21Z Thursday. This region will have a relative minimum in deep layer shear, but still 40+ kt in the sfc to 6 km layer, so thunderstorms will be mentioned in the southeast part. Will need to continue to monitor QPF values going forward for a possible Flood Watch, at some point, this week.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 225 PM EST Monday: The active and wet pattern continues through much of the medium range. Guidance in good agreement that isentropic lift and low level moisture keep precip over the area as a frontal boundary stalls over or near the area Friday into Saturday as another cold air damming high, although a weaker hybrid type this time, sets up across the area. This will keep likely to categorical PoP across the area each period. Do not expect any heavy precip during this time, but several rounds of moderate QPF are possible. The good news is that even going on the cold side of guidance due to the damming, temps remain warm enough for all the precip to be liquid. Highs will be near normal across the mountains both days and up to 5 degrees below normal elsewhere. Lows will be well above normal both mornings.

A cold front moves across the area Sunday. Looks like timing will be favorable for instability to develop, especially locations along and south of the I-85 corridor where the damming high can erode given the weaker hybrid type. There may even be enough forcing and weak instability for thunder across the entire CWFA. As usual, there will be quite a bit of shear with this system, but not much helicity. Still, will have to see how much surface based instability can develop and how much the damming can erode or a wedge boundary develop, before any severe potential can be assessed. Again, QPF doesn't look heavy but rounds of moderate are likely. Lows will be well above normal and highs may be as well depending on damming erosion.

Dry high pressure finally moves in Sunday night through Monday but a mild air mass remains in place. Lows will be 10 to 15 degrees above normal with highs around 10 degrees above normal.