textproduct: Greenville-Spartanburg

SYNOPSIS

Drier conditions briefly return to the area on Monday ahead of the next low-pressure system, which will bring widespread precipitation to the area Monday night into Tuesday. Behind the system, high pressure will result in chilly and dry conditions Wednesday and Thursday. Unseasonably cold and unsettled weather is expected Friday as another system brings more precipitation to the area.

NEAR TERM /THROUGH MONDAY/

As of 204 PM EST Sunday...

Key Messages:

1) Precipitation and cool temps continue through the late afternoon ahead of a cold front.

2) Next system will approach from the southwest late Monday afternoon as a cold air damming wedge sets up.

A wide swath of cloudiness will prevail across the region through the rest of the afternoon ahead of a cold front analyzed from north AL to the Cumberland Plateau region. Radar shows two swaths of light precip moving east...one basically along/east of the I-85 corridor and the other moving into the mtns at this time, with a break in-between. Fortunately, temps are all safely above 32F, so just a dreary light rain is expected. The front appears to be on track to cross the region this evening.

With the front to our east, a quick transition to high pressure will take place overnight, but with only varying degrees of clearing. Depending on how that plays out, we could have a fog problem around daybreak Monday, but more likely not the way it looks at this moment. Stay tuned. Expect a gradient of low temps from N to S, with locations north of I-40 dropping below freezing but locations across the lower Piedmont staying closer to normal (upper 30s).

By daybreak Monday, the sfc high will have already bridged the mtns and the familiar cold air damming wedge-like configuration will be in the process of developing across the western Carolinas. The sfc high will be weakening on Monday as the center moves from the Midwest to the Hudson River Valley through the day, but moisture return will be underway by afternoon from the southwest ahead of the wave lifting out of the upper trof well to the west. Warm advection and isentropic upglide will develop and spread northeastward late in the day, helping to establish the wedge. The increasing clouds and northeast flow will keep temps below normal and in the 40s. The main forecast problem looks like the timing of precip onset. The fcst was based on the blend, but the trend in the guidance is toward holding off until after sunset with light precip reaching the ground. This seems very reasonable. In fact, the 12Z CAMs support this scenario. Expect the precip probs for Monday to drop accordingly in the next package.

SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/

As of 100 PM EST Sunday:

Overall, there is not much change in the pattern that drove previous forecast package, and as such, confidence continues to increase that accumulating freezing rain for the northern mountains and eastern Blue Ridge Escarpment will result in at least some brief travel impacts for the area Monday night into Tuesday morning. Dry high pressure will set the stage Monday with dewpoints in the 20s as cyclogenesis occurs over the western Gulf. Expect increasing clouds from this system Monday afternoon as moisture advects overhead, with some rain showers likely across the western zones beginning around sunset. The timing of the increasing cloud cover across the northern zones may allow for some radiational cooling at onset, before saturation of the column can occur and precip begins closer to midnight Tuesday. Since the northern mountains and highest elevations of the central and northern Escarpment will struggle to reach the upper 30s Monday afternoon, this combination of brief radiational cooling and wet-bulbing will allow areas to drop below freezing before or around precip onset. Therefore, a period of freezing rain is likely, primarily between 11pm to 6am Tuesday morning, along the Escarpment from central McDowell County to Caldwell County and across much of eastern Mitchell and all of Avery counties. As always, the forecast struggles with the challenges of latent-heat release from rainfall freezing on surfaces, cold-air advection (which there is very little of due to the transient nature of the source high pressure), and precipitation rate (more intense rainfall can actually limit ice accretion). While we are pretty confident in the occurrence of freezing rain in the aforementioned locations, details regarding amounts and the timing of transition to all rain will need to be fine tuned overnight tonight. As such, in close collaboration with our neighbors, we will wait to issue any hazard products until Monday morning, if necessary. No matter what, precipitation will transition to all rain across the impacted zones before mid-morning Tuesday.

Forecast sounding profiles suggest that the higher elevations of the central and southern mountains will generally be too warm for frozen precip. It cannot be ruled out that a very brief period of sleet or freezing rain may occur closer to onset, but given that many of these higher elevations will be firmly embedded within the warm nose itself and initial surface cooling will be inhibited by cloud cover, any meaningful impacts appear unlikely.

Elsewhere, including across most of the mountains and all of the foothills and Piedmont, temperatures will remain well-above freezing with highs east of the mountains in the lower 50s. Liquid precipitation amounts are expected to be quite healthy as the system traverses the region Tuesday, with widespread accumulations of 0.75" to 1.25" likely and isolated higher amounts possible before the system departs early Tuesday evening. These amounts will be beneficial to the region and pose no flooding threat.

The timing of the system departure may impact higher elevations, especially along the TN border, which will drop below freezing fairly quickly later Tuesday afternoon into the early evening hours as moisture departs the region. Right now, the expectation is that moisture will depart before temps become critical; however, any slowing of this timing may necessitate a chance of light snow in these locations in next forecast package. Fortunately, dry conditions return on Wednesday behind this system with only slightly cooler high temperatures, as return flow to our west inhibits any meaningful cold-air advection.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

As of 100 PM Sunday:

The extended begins on Thursday with what is likely our warmest day east of the mountains, as dry high pressure, higher thicknesses, and weak downslope flow support high temperatures near normal. The next shortwave trough digs into the southern Plains Thursday night as high pressure quickly traverses the Ohio Valley and Mid-Atlantic. Once again, cyclogenesis occurs near the western Gulf Coast as moisture rapidly advects toward the region overnight Thursday into Friday morning. Compared to the system earlier in the week, the strong, 1033mb high pressure may briefly anchor itself over the Mid-Atlantic states during this time, a backdoor cold front would initiate CAA across the region overnight, and CAD would develop as precip begins to fall across the region early Friday morning. Temperatures at this time appear cold enough that the initial precipitation may fall in a frozen form across a significant portion of the area, but modest diurnal heating will transition much of this precipitation to all rain, especially across the mountain valleys and east of the mountains. Once again, the highest mountain elevations and favored areas of the northern mountain region may not recover significantly and frozen p-types may dominate through Saturday morning. Nevertheless, right now, Friday looks to be a cold and wet day across the area, with temperatures potentially 15-17 degrees below normal area wide. At this time, there is decent agreement among the deterministic guidance on the timing of this next system, but ensemble guidance shows greater spread, with many members showing onset later on Friday. Obviously, this forecast has plenty of time to evolve, but the evolution of this system will need to be closely monitored.

Given the cold high temperatures, it will take comparatively less effort to cool to critical temperatures Friday night. Guidance is suggesting that a secondary shortwave may ride along the southern stream and bring a second round of precipitation to the forecast area, with the precipitation gradient increasing from northwest to southeast. At this time, temperatures may approach freezing, especially across the western NC mountains and the central and northern mountains. With a strong inversion in place, if temperatures drop below freezing, the primary threat may be a brief period of freezing rain Friday night into Saturday morning before temperatures warm, with highs on Saturday reaching the lower to mid-50s east of the mountains.

There is enough spread among the ensemble guidance that the NBM maintains precipitation chances into Sunday. However, confidence is extremely low on timing and potential winter weather impacts and as such, the resultant weather grids are low confidence as well.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: Widespread restrictions, mainly an MVFR ceiling, will prevail across the western Carolinas until the frontal passage this evening. Meanwhile, patches of IFR/LIFR ceiling were developing from north Georgia into the Upstate of SC. Ahead of the front, wind was light and variable, favoring S to SW, and that should continue until frontal passage. Precip will be all rain. The frontal passage will sweep the lowest cloud layer off to the east but wind will remain light and slow to veer. By late evening, all terminals should be VFR with a light NW to N wind, but with some stronger flow in the valley around KAVL. The VFR break will be short-lived. By late Monday afternoon, the next system will approach from the SW, but restrictions won't arrive until after the end of the period.

Outlook: Moisture will move up from the southwest Monday evening, with widespread MVFR to IFR conditions developing Monday night. There will be a small chance of freezing rain early Tuesday morning at KAVL and KHKY. Restrictions will persist through Tuesday, before dry and mainly VFR conditions return Wednesday. Another system may affect the region Friday into the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.