|Wind||5.01mph from the E|
... Dry high pressure will lead to warmer temperatures through the weekend. Dry conditions are expected through the day on Saturday then rain is expected Saturday night and Sunday as a cold front crosses our region from the west. Thunderstorms may also be possible on Sunday and the rain changes over to light snow in the North Carolina Mountains Sunday night as the precipitation ends.
NEAR TERM /THROUGH THURSDAY/... As of 630 PM EST Wednesday: Stratocu hanging on across portions of the mountains in the NW flow. These clouds should slowly dissipate overnight as patches of cirrus move overhead through the night. Winds have tapered off across most of the area, but gusty winds continue across portions of the mountains. These should taper off overnight as well as high pressure moves into the area. Expect good radiational cooling conditions tonight as temperatures drop 4-8 degrees below normal for lows.
Surface high will be in control of the forecast for Thursday as the center of it gets suppressed south and east of the CFWA, while higher heights move in from the west thanks to the upper ridging traversing across the east-central CONUS. With the position of the high, 850 mb winds will pickup a west-northwesterly direction, which will allow for a downslope component to filter in during the daytime period Thursday. As a result, expect temperatures to be a few ticks warmer compared to today with mostly sunny skies.
SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 210 PM EST Wednesday: Shortwave ridging will slide across the Appalachians tomorrow night through Friday while surface high pressure shifts offshore. This will allow for light return flow and a moderating airmass as heights rise and low-level temperatures gradually warm. Afternoon highs on Friday will climb into the upper 50s to low 60s with cool overnight lows in the upper 30s to mid 40s. The synoptic pattern will become highly amplified heading into Saturday as an intense Pacific jet dives across the Great Basin and helps carve out a deep trough across the Great Plains. Farther downstream, southerly return flow will strengthen in response with dewpoints recovering into the 50s and warm afternoon highs in the low to upper 60s. A few warm advection showers will be possible across the western Upstate and southwest mountains on the leading edge of deeper moisture return, but most locations east of the mountains should remain dry during the day.
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 223 PM EST Wednesday: Attention quickly shifts to Saturday night through Sunday as confidence continues to increase in a potentially impactful weather event across the region. An intense trough will become highly amplified as it swings through the Mississippi Valley and into the Appalachians late Saturday night through Sunday. The trough will take on a negative tilt as it rapidly lifts through the region on Sunday with secondary surface cyclogenesis occurring in the lee of the mountains. Tremendous height falls aloft (~20dam in 12hrs at 500mb) in concert with robust wind fields will support intense forcing for ascent as the wave translates across the area. Very strong and deep meridional flow will transport anomalous moisture into the area (PWATs in the 90th percentile) with rain chances quickly ramping up from west to east Saturday night through Sunday.
Warm advection showers will increase in coverage across the southern escarpment with shower potential spreading across the entire area by Sunday morning. The main focus will be the potential for a severe weather episode in the form of a strongly forced line of storms within a prefrontal surface trough. The 12z suite of global guidance has trended several hours slower regarding the timing of convection with a potential line entering western zones during the late morning hours and pushing across much of the area during the afternoon. Continued moisture return will help dewpoints climb into the low 60s, which should be enough to realize 200-500 J/kg of surface-based instability. While these values are not exceptionally high by any means, the magnitude of forcing should be sufficient to augment the lack of more robust thermodynamic fields. Shear profiles will be more than favorable for severe weather as a 55-65kt low-level jet translates across the area. This will support 50-60kts of deep-layer vertical wind shear along with 30-40kts of 0-1km shear. The main limiting factor precluding higher confidence in severe weather right now is the deep meridional flow through much of the column. As a result, hodographs do not depict as much turning in the low-levels without more backing of the 0-1km flow. Despite this, guidance still depicts 150-300 m2/s2 of 0-1km SRH which would still be sufficient for line embedded mesovortex development should a bowing segment become favorably aligned with the low-level shear vectors. Will keep the HWO clear of severe weather mention for now, but should current trends continue a mention will need to be added.
A secondary threat for isolated flash flooding will also need to be monitored. Anomalous moisture combined with the deep meridional flow will promote training linear segments that could drop a quick couple inches of rain. Storm total rainfall is maximized across the southern escarpment where 2-4" will be possible with generally 1-2" elsewhere. Streamflows are currently quite low owing to ongoing drought conditions and ensemble plumes generally keep gauges at or below advisory level. Should heavy enough rainfall rates persist in a favorable location an isolated flooding threat cannot be discounted along the escarpment.
Convection will quickly push east of the area by late afternoon to early evening on Sunday. A very sharp temperature gradient will exist along the cold front will cold advection quickly ramping up. Cold air may catch up to the back edge of the precipitation shield across the mountains during the late afternoon to early evening hours with a change over to snow followed by a brief period of northwest flow snow showers. Current forecast snow totals are generally 1-3" across the high terrain above 3500ft. Cold and dry air will filter across the region thereafter with all precipitation coming to an end by Monday morning. Early next week will start off cold and dry with gradual modification of the airmass into mid week as a dry pattern continues.