textproduct: Columbia

WHAT HAS CHANGED

Flash flood threat continues through tonight with periods of heavy rain expected. Flood Watch continues for the Columbia Metro Area and counties to the south and east. Key message updated to reflect latest model guidance. Aviation discussion updated for 18Z TAF Update.

KEY MESSAGES

- 1. Scattered to numerous showers and thunderstorms expected today, resulting in heavy rainfall and localized flash flooding. Rain persists into the Saturday, mainly across the eastern forecast area.

- 2. Near to below normal temperatures through Sunday then a more summer like pattern returns next week with possible severe threat Tuesday and Tuesday night.

DISCUSSION

- 1. Scattered to numerous showers and thunderstorms expected today, resulting in heavy rainfall and possible localized flash flooding. Rain persists into the Saturday, mainly across the eastern forecast area.

Radar imagery early this afternoon showing a area of lighter rain moving into the Midlands and extending back into northeast GA, mainly north, on the cool side of the stalled frontal boundary. More convective elements are noticeable on the most recent scans as the atmosphere destabilizes. PWAT axis of 2.2-2.3 inches extends across the forecast area along and just north of the stalled boundary which combined with a very deep warm cloud layer will support efficient, high rainfall rates. A somewhat anomalous CAD has developed and is evident in the instability fields of the SPC mesoanalysis showing stable air with existing CIN across the SC Upstate into northeast GA with more instability noted across northwest through central GA into central and eastern SC to coastal NC, along and south of the stalled frontal boundary. HRRR has been struggling with existing precipitation and has been generally poor the past few days with the handling of convection. HREF guidance continues to suggest mainly scattered showers and thunderstorms developing through the afternoon then convection clustering over the central and eastern Midlands this evening with some possible sea breeze interactions. HREF LPMM rainfall totals have some areas of 2 to 4 inches with locally higher amounts. Flood watch remains in effect through 06z tonight. Chance of rain continue tomorrow as shortwave energy and upper trough axis swing through the region but expect the axis of heavier rainfall to shift to the Coastal Plain. After collaboration with neighbors, decided to not extend flood watch at this time with expectation of higher rainfall expected east of our area and would like to see what materializes today and tonight, but later shifts can re- evaluate with new guidance and observations.

- 2. Near to below normal temperatures through Sunday then a more summer like pattern returns next week with possible severe threat Tuesday and Tuesday night.

The combination of the stalled frontal boundary with extensive clouds and intermittent rainfall will keep max temperatures near to below normal today through Sunday. The cooler temperatures are expected across the northern portion of the forecast area further north of the boundary. Ensemble guidance suggests the upper ridge over the intermountain west will expand and strengthen into the Plains states by Sunday resulting in downstream upper troughing along the East coast. This should result in some drier air working into the region Sunday into early next week, aided by downsloping northwesterly flow. Therefore chances of rain should be decreasing with the best chances focused along the eastern Midlands and CSRA Sunday and then very low chances of rain Monday. Temperatures will gradually warm back to near and above normal through early next week with highs back in the mid to possibly upper 90s by mid week.

As the upper troughing deepens across New England into the Mid Atlantic region by Tuesday strengthening cyclonic flow around the base and increasing atmospheric moisture will bring an increased threat for strong to severe storms preceding a southward moving cold front. SPC has outlooked much of the Mid Atlantic and southern Appalachians into central NC near our northern counties in a 15 percent outlook on Day 5 and this is supported by increased probabilities among the ML guidance as well.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Mix of restrictions expected throughout the TAF period with periodic lowering cigs and lowered vis due to rainfall.

Multiple rounds of showers and embedded thunderstorms are moving into the area from the west. Expect this activity to continue into tonight, periodically affecting the terminals. When a shower or storm passes over a terminal, expect lowered cigs and visibility. Outside of activity this afternoon and evening, conditions are likely to be VFR, but on the lower side of the flight category. Cigs are forecast to be predominantly IFR after about 02z for several hours with varying vis due to showers overnight. The activity is forecast to diminish after about 12z-13z and conditions gradually improve through the end of the TAF period. West to northwest winds expected into tonight before becoming light and variable to nearly calm late. Winds increase to 5-8 kts out of the west after about 13z once again.

EXTENDED AVIATION OUTLOOK...Periodic restrictions from showers and thunderstorms likely into the weekend for all TAF sites.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Flood Watch through late tonight for SCZ027-028-031-037-038- 041-135>137. GA...None.