Sugar Mountain

Forecast Discussion

Summary

SYNOPSIS

... A moist warm front will continue to move north from Georgia tonight as low pressure crosses Tennessee. Expect Canadian high pressure ridge down into our area from North Dakota by late Monday giving us a short dry period. Rain returns on Tuesday as a series of low pressure systems will track from southwest to northeast across the region, resulting in a persistent moist pattern through most of the week.

NEAR TERM /THROUGH MONDAY/... As of 945 PM EST Sunday: Regional radars late this evening depict a fairly solid moderate to heavy rain shield extending to the southwest across northern Alabama and Georgia in advance of H5 vorticity lobes embedded in the WSW flow aloft, and ahead of a weak surface cold front. Isentropic lift and moisture are maximizing across the forecast area at present, but upglide should diminish from the west after 06Z and the upper dpva should pass by 09Z, so the duration of the heaviest part of the event will be fairly short- lived. Some 1 to 1.5 inch rainfall totals will be possible along the TN border and in southwest mountain upslope flow areas through daybreak, but stream and river rises have been fairly modest so far. The current thinking is that any overnight hydro issues in the southwest mountains will be isolated. Thus, no Flood Watches will be posted for the remainder of this event. This will certainly produce moist conditions, however, for subsequent rainfall through the week.

Meanwhile, cold air damming will continue to weaken overnight as the parent high pressure slips farther offshore. Developing westerly flow by daybreak will lead to downsloping and scouring of the surface wedge layer. This will cause temperatures to rise slightly overnight and then sharply through tomorrow as the low clouds finally scatter out and insolation improves without any significant thickness falls from the passing front. The downslope warming and improving sunshine should permit a nice warmup to 60s high temperatures in most areas east of the mountains Monday afternoon. This break in the rainfall for most of Monday and into Monday night could well be the last significant dry weather of the upcoming week.

SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/... As of 255 PM EST Sunday: The fcst will start off dry across the area as a broad Canadian high builds south across the Glakes region and begins to ridge down the east coast. Temps will drop to arnd freezing across the NC mtns while a developing warm front begins to approach the area arnd daybreak. There is some uncertainty with the level of moisture with this front as the NAM continues to have very little qpf response by 12z Tue. Likely a null chance of any wintry precip Tue morning as any light precip shud not arrive until temps warm abv freezing. Temps wont rise much during the day...probably remaining 40s all locales as sfc-based CAA continues and cloud cover increases.

The fcst becomes quite tricky Tue night as a parent 1040 mb high sets up across the ern Glakes. The high config looks to be classical in nature and bad for wintry precip across the colder NC mtn areas as a stg llvl sw/ly jet brings in a sigfnt warm nose aloft. Model temp guidance has trended down overall so mixed in lower values with the previous fcst numbers. This gave more areas for accum fzra over the NC mtns and even a -ra/fzra mix across the nrn fthills and nrn NC piedmont. Temps look to hover just north of freezing generally outside the nrn mtns...but a degree or two difference in sfc Tw/s could create wintry/icy precip.

The amt of precip is not totally certain as there could be robbing convec south and the overall depth of saturation will likely be limited to arnd h7. Therefore...ice nuclei may not be activated esp as the morning progresses...per the latest GFS/NAM soundings. Thus...will anticipate an onset of freezing precip arnd 02z with likely a switch to -fzdz...before a changeover to all rain occurs arnd 14z. Expect a light glaze of ice at most across the non/mtns...and the best ice accum potential of a couple tenths over the nrn NC mtns and high elev nrn fthills...and perhaps even further south across the Black Mtns. Right now...this event is certainty adv level and as the fcst changes warning level conds could develop. A light cold rain will continue thru period with temps only reaching the L40s north and L50s over the far sw/rn zones as the wedge is slow to lift out of the area.

Another issue Tue night into Wed will be the potential for stream flooding across the sw/rn NC mtns. WPC has the area in a Day 3 slight risk of flooding the favored upslope sw-facing regions will likely recieve arnd 2 inches to near 3 inches by then in continued high saturated soils.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 200 PM Sun: An active pattern is still depicted across the CWFA throughout the medium range, still with a strong ridge centered over the eastern Caribbean and deep troughing over the southern Rockies and vicinity. As low pressure traverses the Great Lakes region early Thursday, a cold front will move thru the lower Mississippi Valley, then pivot across the Appalachians as the sfc low moves thru New England. That will promote erosion of the midweek CAD affecting our area, though with the boundary lingering, unsettled weather will persist. Furthermore, global models depict yet another seasonably strong sfc high moving eastward north of the boundary, suggesting CAD will return by Friday. Temperatures within the weekend CAD event currently do not look cold enough for wintry precip, so the main impact initially may just be to prolong the already cloudy/damp conditions.

Model consensus shows pattern change finally occurring next weekend as a shortwave ejects from the Rockies, cyclogenesis occurs somewhere to our northwest, and a cold front swings thru the Southeast. This front is most likely to reach our area on Sunday. The European and Canadian solutions depict a stronger low and better defined cold front than does the GFS, though all of them show at least modest instability in the warm sector preceding the front, while CAD persists for some of our area. These trends are worth watching as they imply some severe weather threat may result for those parts of our area along/south of the wedge boundary.

All in all it is difficult to identify any meaningful break in precip chances between Wednesday evening and Saturday morning, so we will advertise at least likely PoPs for nearly the whole area during that timeframe. A limited diurnal range is expected due to cloud cover and/or wedging, but overall temps will remain near climo. Chances were allowed to drop a bit by Saturday afternoon as warm front shifts northward ahead of the developing low.