textproduct: Columbia

WHAT HAS CHANGED

A Heat Advisory has been issued for Lancaster County for Thursday afternoon and evening. A few showers or thunderstorms in the Upstate and Charlotte Metro may clip far northern and western parts of the CWA Thursday afternoon. Aviation discussion updated for the 06z TAFs.

KEY MESSAGES

- 1. Above normal temperatures continue, peaking during the Independence Day weekend.

- 2. Increasing chances for showers and thunderstorms late this weekend into next week.

DISCUSSION

Key Message 1: Above normal temperatures continue, peaking during the Independence Day weekend.

Latest guidance shows the potential for higher heat index values today. Currently it appears that the finger of Lancaster County is most likely to approach the 108 degree criteria, therefore a Heat Advisory is being issued for the county with this forecast package. Heat Index values north and west of I-20 will be near advisory criteria meaning extra caution should be taken there during peak heating as well. Will monitor trends to see if the Heat Advisory needs to be expanded further.

Strong upper ridge affecting much of the eastern US will continue to be the dominant feature into the holiday weekend. The anomalously strong ridging will combine with highly anomalous 850mb temperatures (above the 90th percentile) at or above 20C to provide hot conditions across the forecast area. Ensemble guidance suggests lowering atmospheric moisture tomorrow with PWATs falling to around 1.5 inches around 50th percentile. Forecast soundings indicate deep mixing tapping into the drier air aloft which should mix out dewpoints into the 60s resulting in heat indices below advisory criteria, although they are expected to continue to be at or above 100 degrees.

PWATs are forecast to lower even further on Friday to around 30th percentile, but the upper ridge axis will be building into the Carolinas and Mid-Atlantic and temperatures are expected to be a bit warmer, yielding heat indices in the 100-105 range. Continued high temperatures in the upper 90s to around 100 degrees over the weekend may necessitate the need for heat related products for the holiday weekend as humidity rises further due to increasingly southerly low- level flow. While temperatures should be slightly lower early next week, the potential for heat products continues due to increased humidity.

Continue to exercise caution when spending an extended period of time outdoors and make sure to take appropriate heat safety measures during the next 7 days. The elderly, young children, and those without indoor cooling will be at particular risk.

Key Message 2: Increasing chances for showers and thunderstorms late this weekend into next week.

Not much change in the overall forecast. Latest CAMs suggest convection will fire Thursday afternoon in the Upstate and Charlotte Metro area. While not shown in the deterministic forecast, this activity could clip the far northern and western counties. Otherwise, a dry forecast is expected through Saturday.

Ensembles continue to indicate the strong upper ridge impacting the eastern states will begin to weaken and break down Sunday into next week as ridging builds over the intermountain west and Plains states. This will allow for some weak height falls and shortwave energy to move into the region. Atmospheric moisture will be much higher with PWATs back up around 2 inches supporting stronger instability and better chances for convection. Some of the ML severe guidance is suggesting increased potential for severe thunderstorms early next week.

AVIATION /06Z THURSDAY THROUGH MONDAY/

VFR conditions likely through the TAF period.

Light and variable winds tonight with typical early morning MIFG possible at AGS/OGB, though confidence is too low for any mention in the TAFs. A 15 to 20 kt low level jet this morning should preclude a more widespread fog threat. Rain-free conditions continue today with FEW-SCT cumulus developing by midday into the evening. Skies then return to SKC tonight into Friday morning. Winds generally out of the E/SE at 5 to 8 kts after 16Z, returning to light and variable this evening.

EXTENDED AVIATION OUTLOOK...Rainfall unlikely through at least Friday as ridging builds overhead. Brief early morning restrictions possible. Rain chances and possible restrictions start to increase Sunday into early next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Heat Advisory from noon today to 8 PM EDT this evening for SCZ115-116. GA...None.