textproduct: Columbia

WHAT HAS CHANGED

Aviation discussion updated for 12z TAF issuance.

KEY MESSAGES

- 1. Rain chances increase today into early next week.

- 2. Uncertainty in temperatures Friday and this weekend.

DISCUSSION

Key Message 1: Rain chances increase today into early next week.

The upper ridge which has been in place all week will begin to shift slightly eastward on Friday, which should work to strengthen southwest flow aloft over the area. At the same time, high pressure over the Great Lakes will shift toward New England and begin to ridge down the east coast. The result is a cold air damming scenario over the Mid-Atlantic as atmospheric moisture is on the increase. The wedge boundary will move through the Carolinas, though there remains some uncertainty in how far south it will go, as is quite common in these setups. PW values are progged to near 2" into the weekend, and periodic bursts of shortwave energy will enhance shower and thunderstorm activity over the area. Friday through Saturday may be particularly active, with the potential for convection to continue past the diurnally favored timeframe and into the overnight period. Weak instability in a weakly sheared environment should limit severe potential Friday, but dynamics become a bit more favorable for storm organization by Saturday.

The deepest moisture moves out of the region on Sunday with upper ridging building back in over the Southeast. However, PWAT values remain above normal and diurnal convection is favored, albeit with less upper support as the area is in between shortwaves. The ridge axis moves nearer to the forecast area for Memorial Day into Tuesday which may further limit coverage, but there will still be plenty of moisture and instability to support isolated to scattered convection.

Rainfall amounts will be quite varied across the region depending on the wedge boundary and any training that occurs. NBM QPF amounts have increased slightly, with anywhere from 0.25" in the eastern midlands to over 1.5" possible in the upper CSRA and western Midlands. Locally higher and lower amounts will of course be possible based on where the wedge boundary and stationary front set up.

Key Message 2: Uncertainty in temperatures Friday and this weekend.

Highs temperatures today will remain above normal in the low 90s, but heat indices will fall short of any heat headline thresholds.

Given the previously discussed CAD setup, there should be a large temperature gradient over the forecast area for at least Friday into Saturday, which is not always handled well by the NBM. Conceptually, it wouldn't be out of the question to see the northern portion of the forecast area struggle to rise out of the mid 70s Friday and possibly Saturday, and slight adjustments were made to the inherited forecast high temps to account for these biases. Confidence in the wedge holding longer than depicted by model guidance is increased by the expectation of widespread showers and thunderstorms. And while NBM temperatures weren't adjusted much if at all for Sunday or Monday, with a frontal boundary stalled near the area and the expected continuation of widespread cloud cover and precipitation, wouldn't be surprised if temperatures continued to hold close to late May normals into early next week.

AVIATION /12Z THURSDAY THROUGH MONDAY/

VFR conditions continue with brief restrictions possible again early this morning. Isolated showers and thunderstorms possible later today.

Early morning restrictions at AGS and OGB should improve after 12-14Z as light southerly to southwesterly winds around 3-5 kts develop.

SCT cumulus is expected to redevelop through the day ahead of an approaching front, bringing greater coverage of afternoon/evening showers and thunderstorms. While the peak activity is expected to remain north and west of our CWA, isolated coverage cannot be ruled out. Based on the latest HREF guidance, isolated storms may develop near all terminals after 20-22Z and continue sporadically until the end of the current TAF update. Therefore, PROB30s have been added in all but the OGB sites. The front is expected to linger around, possibly triggering continued showers and producing MVFR to IFR cigs into Friday morning. Winds will remain light, mainly out of the SW, but could be variable in and around convection.

EXTENDED AVIATION OUTLOOK...High low level moisture may bring more rounds of morning restrictions from fog/stratus into the weekend. Scattered afternoon showers and storms become possible today with more chances this weekend.

CAE WATCHES/WARNINGS/ADVISORIES

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