textproduct: Columbia

WHAT HAS CHANGED

Updated both key messages but the overall pattern remains the same. Aviation discussion updated for 12z TAFs.

KEY MESSAGES

- 1. Scattered afternoon showers and thunderstorms possible on today and tomorrow. A few strong thunderstorms are possible later today.

- 2. Dangerous heat possible today and tomorrow, with a signal for a more impactful and intense heat event during the opening days of July.

DISCUSSION

Key Message 1: Scattered afternoon showers and thunderstorms possible on today and tomorrow. A few strong thunderstorms are possible later today.

The upper ridge centered over the Mississippi River will strengthen significantly today. In response to the building ridging, upper level winds will be out of the northwest today while a shortwave caught in the flow passes through the region. First a decaying shortwave and associated MCS will pass into the mountains, generally weakening throughout the morning. This may yield some more widespread cloud cover than guidance allows for and complicates the rest of the forecast a bit. The CAMs have a decent handle of the morning convection and continue to indicate that a second complex of thunderstorms will move into the CWA from the Upstate this afternoon and will mainly impact areas along the I-26 corridor and points to the north and east. These same locations are outlooked in a Marginal (1/5) risk for severe weather from the SPC. Modeled soundings continue to show an inverted-V profile and adequate DCAPE between 750 and 1000 J/kg indicating the potential for strong downburst winds. Long, skinny CAPE profiles and high PWATs also show the potential for heavy rainfall. While everyone has the chance of seeing a shower or thunderstorm today, chances drop off quickly towards the CSRA.

Lingering moisture is expected to bring another chance for scattered showers and thunderstorms on Monday, though confidence and coverage is lower. The threat for severe weather is also lower on Monday as the aforementioned upper ridge continues to build and slide east towards the region.

Key Message 2: Dangerous heat possible today and tomorrow, with a signal for a more impactful and intense heat event during the opening days of July.

Another hot and humid day is in the forecast today, though the timing of today's thunderstorm complex will impact how hot locations across the northern half of the forecast area get. The current forecast has highs in the upper 80s and lower 90s in the north and lower to mid 90s in the south. The heat will combine with dewpoints in the lower to mid 70s to produce peak heat index values as high as 107 in a few spots this afternoon. While we are not expecting to meet Heat Advisory criteria today, these values are high enough that caution needs to be used if you plan to be outdoors for an extended period of time. A Heat Advisory may be needed for a portion of the CSRA on Monday when heat index values may approach 110 degrees in the afternoon.

It appears that drier air will filter into the region during the mid-week period, decreasing heat index values, though it will remain hot as the strong upper ridge anchors itself over the eastern CONUS. By late week, and into the holiday weekend, guidance continues to maintain a strong upper ridge, with 850-mb temperatures approaching or exceeding 20C. This pattern favors temperatures increasing again, with several days of highs at or above 100 degrees possible during the Thursday to Sunday period. If trends continue, heat products may be needed several days in a row. Be sure to make heat safety preparations now and have a plan to monitor children, the elderly, pets, and anyone without adequate indoor cooling.

One potential caveat to this extended period of heat could be development in the tropics. The NHC maintains a 20% chance for cyclone formation over the next 7 days off the SC coast. Guidance indicates that thunderstorms that move through the Carolinas today will drift over the Gulf Stream and could acquire tropical characteristics before heading back west towards the United States. It is important to note that confidence in any organized tropical development remains low, as ensemble guidance continues to show considerable spread on development and track. Having said that any clouds and rain, tropical or otherwise, would limit the impact of the heat so the potential bears watching in the coming days.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

VFR conditions likely outside of afternoon convection Sunday.

The active summer pattern continues Sunday with west-southwest winds developing, gusts to 20-22 knots and typical summer cu around 5-6k feet; some morning MIFG is lingering mainly at AIK and CAE through 12z, with some periodic drops to MVFR possible. Some strong thunderstorms will move into the region by late morning into the afternoon- evening, but coverage will be fairly scattered. Prob30 groups included for this potential throughout much of the afternoon for TSRA and wind gust potential. Confidence in coverage is not terribly high given some morning cloud cover from upstream convection however. Thunderstorm coverage should clear out around 00-01z Monday.

EXTENDED AVIATION OUTLOOK...Afternoon convection will develop again Monday but is less likely for the remainder of the week as ridging builds overhead. Brief early morning restrictions also possible early next week.

CAE WATCHES/WARNINGS/ADVISORIES

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