... Dry high pressure will slide east on Thursday as a weak cold front approaches from the west. The front will cross the area Thursday night and Friday, bringing a chance of showers and a few thunderstorms, particularly in the North Carolina mountains. Expect dry and unseasonably warm weather to return for the weekend and persist well into next week.
NEAR TERM /THROUGH THURSDAY/... As of 10pm EDT Wednesday: Quiet evening continues with little change needed for this update.
A few high clouds aloft will drift eastward overnight. Generally light and variable to calm winds tonight that will increase during the day Thursday. Warming trend continues with lows expected to be in the 50s in piedmont areas Thursday morning, and in the upper 40s in mountain valleys.
Embedded shortwave energy will push in overnight and allow for an increase in mid and upper level clouds. This should keep the fog potential at bay over most locations. Can't rule out fog over the mountain valleys and locations near bodies of water by daybreak if there is enough breaks in the clouds.
An upper low over the Midwest will gradually swing a cold front towards the CFWA from the west during the day Thursday. With the return flow well established by this time tomorrow, PWAT values should jump up above 1.00" and provide enough moisture and lift to produce showers ahead of the cold front. CAMs have shown some form of pre-frontal activity that could escape east of the mountains by early afternoon, but the main brunt of the activity will occur during the evening and overnight hours Thursday. Some indications of instability and modest shear will reside over the High Country, which could allow for some embedded thunderstorms along with the showers that do develop during peak heating on Thursday. The cold front will be knocking on our door by the very end of the period, but the actually fropa will occur during the short-term. Temperatures will decrease slightly tomorrow as clouds increase throughout the day, but values will remain ~5+ degrees above normal due to the warm, moist southwesterly flow and very gradual decrease in thicknesses.
SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 230 PM Wednesday: The forecast is still generally on track for the weak cold frontal passage expected Thursday night into Friday, as a tandem of shortwaves propagate through the mean longwave trough centered over the eastern CONUS. Due to the one-two punch of shortwave energy, it looks like the main front will pass through the mountains overnight Thursday night and through the Piedmont early Friday afternoon while a secondary front associated with the second shortwave pushes through the region Friday night. The best upper-level dynamics will be well north of the region and return flow will be significantly limited by the subtropical ridge axis which is cutting off direct moisture flux from the Gulf. Therefore, the opportunity for meaningful rainfall will be confined to the western mountains Thursday evening, where mechanical lift and earlier daytime heating might be able to overcome a weak 700-600mb inversion. This is supported by various model guidance which suggest showers and may be even a couple thunderstorms will be ongoing across the mountains at the beginning of the period. A few showers cannot be ruled out across the Piedmont, especially across Upstate SC and NE GA Thursday evening as well, but given limited instability, moisture, and upper support, rainfall will be light and isolated. The loss of diurnal instability overnight Thursday suggests that as the front moves across the mountains additional showers east of the Escarpment will be very isolated in response to weak frontal convergence. A few NW flow showers cannot be ruled out along the TN border behind the front overnight Thursday night into Friday morning, but chances will be slight. By late Friday morning, with the front moving east of the I-77 corridor and waning NW flow, we will be completely dry once again.
The primary frontal passage will induce weak CAA across the mountains for Friday, causing a drop in temperatures of a few degrees compared to Thursday, but still a couple of degrees above average. East of the mountains, the predominance of westerly, downsloping winds will balance any weak CAA, indicating there will be little change from Thursday's highs, which are expected to be 5-10 degrees above normal. The secondary frontal passage expected with the second shortwave propagating just to our north later Friday will usher in a 1020mb surface high for the southern Appalachians Friday night. Radiational cooling will support low temperatures about 7-9 degrees cooler than Thursday, but still 3-5 degrees above normal. Under zonal flow aloft and the influence of weak high pressure, the region will see a tranquil Saturday with highs still a few degrees above normal.
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... As of 230 PM Wednesday: The weak surface high over the area for Saturday will gradually shift east Saturday evening into Sunday in response to a shortwave ridge over the Mississippi River Valley. Though it is only 4 days out, model guidance begins to diverge significantly with respect to the upper-level pattern, as shortwave energy surges into the High Plains. The 12Z ECMWF is still the most amplified solution, resolving a potent shortwave trough over the middle Plains for Sunday. Nevertheless, all guidance shows that the shortwave energy, no matter how amplified, induces lee cyclogenesis on Sunday over the High Plains and some degree of warm frontal precip is expected to occur downstream of the low. Guidance is in substantial disagreement with respect to the eastward extent of this precip activation, and though the primary activity should be well west of the region, consensus has resulted in slight chance to chance pops over the region beginning late Monday. Eventually, the low pressure system will propagate east across the CONUS Tuesday into Wednesday, suggesting some form of frontal passage for the region heading into the middle of next week. Again, however, there is significant disagreement amongst the deterministic guidance with respect to system strength, placement, and even frontal evolution. For example, the 12Z GFS is forecasting a backdoor FroPa later Tuesday whereas the ECMWF and the Canadian resolve a traditional FroPa earlier on Tuesday. These differences are wreaking havoc with PoPs in the extended and for now we had to resort to a blend of solutions with deference to the ECMWF/Canadian "consensus". Regardless, expect many adjustments to PoPs and sensible weather in the days to come.
Fortunately, despite all of the discrepancies in the upcoming system for next week, the impacts on our area in terms of rainfall and temperatures are currently unimpressive. Temperatures are expected to stay 5-10 degrees above normal through Tuesday while rainfall accumulations are expected to stay below an inch. Expect some cooling on Wednesday behind any FroPa that occurs.