textproduct: Columbia

WHAT HAS CHANGED

Rainfall amounts have increased for this weekend, however those higher amounts will be localized. Aviation updated for 18Z.

KEY MESSAGES

- 1. Showers and thunderstorms expected through next week

- 2. Uncertainty in temperatures north of I-20 on Fri and Sat

DISCUSSION

Key Message 1: Showers and thunderstorms expected through next week

Mean PWAT values from the GEFS and ECMWF ensembles rise to above 1.5 inches Friday and remain above normal through much of next week. As an upper level ridge shifts slightly east tonight and Friday, moisture advection strengthens over the Southeast. 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 States and into the Carolinas. A variety of triggers at the surface including the sea breeze aided by SE low level flow will support convection each day through the extended. The highest rainfall chances appear to be Friday night into Saturday as the deeper moisture moves across the FA and several shortwaves support convective activity. The deepest moisture moves out of the area on Sunday with upper ridging building back over the Southeast. With PWAT values still above normal and onshore flow still in place we can expect at least scattered showers and thunderstorms each day into early next week.

The severe threat each day will be low. Isolated downburst winds will be the main threat. Saturday may be the greatest chance of organized multi-cell clusters with deep layer shear around 20 kts. However with widespread cloud cover and rain anticipated, lapse rates are not expected to support a widespread severe threat.

As CAMs begin to come in for late this week and the weekend we are beginning to see the convective nature of the QPF. In general over the next three days, we may see 0.5 to 1 inch of rainfall but it will be varied widely with localized higher and lower amounts.

Key Message 2: Uncertainty in temperatures north of I-20 on Fri and Sat

As discussed in KM1 the edge of the CAD will push into the Carolinas on Friday. The exact timing and location of the boundary by Friday morning is uncertain with a large spread in temperature guidance for the Catawba region. The NBM is favoring the warmer side of the model spectrum however we know that this setup typically leads to temperatures cooler than most models show. Despite lowering forecast high temps, areas such as northern Lancaster counties may still be too warm. Another common issue with model guidance in CAD setups is eroding the wedge too quickly the following day. So we have favored cooler temperatures again for the northern FA on Saturday.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Generally VFR conditions expected this afternoon and evening, with a few showers-storms around. MVFR-IFR stratus likely early Friday morning.

A typical summer day continues this afternoon with some strato- cu and a few showers and storms across the area. Winds will continue to remain fairly light and variable from the southwest to southeast, gusts up to 10 knots. Shower and storm coverage will increase this afternoon but still only included a prob30 for all TAF sites except OGB. Light winds expected overnight before stratus moves in early Friday. At least MVFR cigs likely for all sites after 0800z with IFR likely at OGB. With weak winds in the morning, stratus will linger through roughly 1500z.

EXTENDED AVIATION OUTLOOK...Abundant low level moisture will bring more rounds of morning restrictions from fog/stratus into the weekend along with scattered afternoon showers and storms possible each day through the weekend.

CAE WATCHES/WARNINGS/ADVISORIES

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