Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
580 FXUS62 KCHS 160958 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 558 AM EDT Tue Jul 16 2024 .SYNOPSIS... High pressure will persist offshore, while a weak trough resides inland through mid week. A cold front will approach the region late week and likely stall nearby. && .NEAR TERM /THROUGH TONIGHT/... No changes were needed for the sunrise update. The forecast is on track. Today: Same pattern, different day. Upper ridging centered off the Southeast U.S coast remains in place with the Bermuda High nudging in from the east. There some subtle changes noted in the various moisture fields this morning, likely due to modest convective overturning induced by scattered strong to locally severe tstms Monday afternoon. Surface dewpoints are tad lower compared to 24-hours ago as are both PWATs and 850 hPa theta-e values. This will have an impact on both the level of heat/humidity and convection this afternoon and evening. Temperatures: RAP soundings show quite a bit of warm air in place atop the nocturnal inversion early this morning. Once the inversion mixes out, temperatures will rise quickly under a solid amount of insolation. Low-level thickness schemes and 850 hPa temperatures support highs in the upper 90s well inland, lower-mid 90s across the coastal corridor with upper 80s/near 90 at the beaches. Dewpoints will mix out across the interior again this morning into the early afternoon before surging with the approach of the afternoon sea breeze circulation. Dewpoints well into the 70s will hold across the coastal counties with readings expected to surge back into the upper 70s during peak heating. Heat indices look to peak in the 105-108 range well inland with 108-112 roughly along/east of the I-95 corridor. Monday`s Excessive Heat Warning appears to have verified nicely with a post analysis of observed heat indices from reliable observations showing a broad swatch of heat indices 113-117 running from coastal portions of Chatham County through central Beaufort County and along/east of US-17 in Charleston County/far eastern Berkeley County. Given the overall lower net moisture values expected today, any instances of heat indices 113 or greater look to be much more isolated and generally below warning thresholds. Per coordination with neighboring offices, a Heat Advisory was posted for the zones along and east of the I-95 corridor from noon until 7 PM. This included both the Savannah and Charleston Metro Areas. Thunderstorms: Another round of isolated to scattered airmass driven showers/tstms is expected this afternoon/evening. 850 hPa theta-e values are several Kelvin lower than yesterday which will persist through peak heating. Expect isolated showers/tstms to initiate early afternoon along parts of the inland propagating sea breeze with convection becoming more scattered inland as the late afternoon/evening progresses. There is a modest signal that a weak impulse will slide northeast across Southeast Georgia this evening as the upper ridge buckles slightly. This may help push a more concentrated cluster of showers/tstms east/northeast across the area south of the I-16 corridor, possibly reaching as far north as Hilton Head and Beaufort. Pops this afternoon will range from 20% at the coast with 30-40% inland with 20-30% along/north of I-16 and 40-50% south of I-16 this evening. An isolated pulse severe tstm or two will be possible, especially where updrafts are enhanced in the vicinity of mesoscale boundary collisions. The main risk looks to be localized damaging winds with frequent cloud-to-ground lightning. Tonight: Convection will gradually wind down with rain-free conditions prevailing after midnight. It will remain warm and humid with lows from the mid 70s well inland to the lower 80s at the beaches and Downtown Charleston. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... Ridging aloft will begin to break down on Wednesday as a broad trough amplifies into the southeastern states. At the surface a cold front will begin to approach the region, likely stalling in the vicinity of the Carolinas late in the week. PWATs ahead of the cold front will increase to over 2 inches along with moderate to strong instability. With some synoptic forcing from the approaching front combined with the local sea breeze, scattered to numerous showers and thunderstorms are forecast to develop each afternoon, with PoPs generally increasing as the week progresses. While the threat for organized severe thunderstorms is low, with DCAPE values forecast around 800 J/kg a strong to marginally severe thunderstorm cannot be ruled out. Conditions Wednesday and Thursday are expected to be hot and humid, with slightly cooler high temperatures on Friday as the cold front approaches. Heat Index values on Wednesday and Thursday are forecast to reach 105-107, with some locations reaching 108F. Heat Advisories may be required for portions of the forecast area. However, afternoon convection may hinder temperatures from reaching their full potential. Friday high temperatures are forecast to reach into the low to mid 90s, with Heat Index values of 102-105, below Heat Advisory criteria. Overnight low temperatures will provide little relief, only dipping into the mid 70s inland and upper 70s to around 80 along the coast. The record high minimum temperatures could be challenged at all three of the climate sites, see the Climate section for more details. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... Broad troughing aloft will prevail this weekend, with high pressure extending into the forecast area from the Atlantic early next week. At the surface, a front is forecast to linger in the vicinity of the Carolinas through the weekend, yielding elevated PWATs. Between lingering forcing from the nearby front and the afternoon sea breeze circulation scattered to numerous showers are forecast each afternoon. Guidance continues to be rather bullish with PoPs through the long term period, so the current forecast maintains chance to likely PoPs, still above climatology. With above normal precipitation chances and increased cloud cover through the period temperatures are forecast to return to near normal, or even slightly below normal through the weekend. && .AVIATION /10Z TUESDAY THROUGH SATURDAY/... 16/12z TAF Discussion: KCHS/KJZI/KSAV: VFR for much of the period through 17/12z. Risk for showers/tstms will increase late afternoon into this evening hours for KSAV as a cluster of convection moves in from the west and southwest. A TEMPO group for MVFR conditions was maintained, but delayed slightly to 23-01z. The main convection axis looks to remain west of KCHS and especially KJZI, so mention of TSRA will be included for the 06z TAF cycle. Extended Aviation Outlook: Scattered afternoon showers/tstorms could produce brief flight restrictions at all terminals this week. && .MARINE... Today: Modest nocturnal surging should end prior to daybreak. South to southwest winds will prevail today with sea breeze enhancements likely near the land/sea interface and Charleston Harbor as the sea breeze moves inland. Speeds will average 10-15 kt, but may be closer to 15 kt with gusts to 20 kt at the beaches and Charleston Harbor. Seas will average 2-4 ft. Tonight: Another modest noctural surge is expected, especially for the South Carolina waters north of the Savannah River Entrance. Winds will range from 15-20 kt in the South Santee- Edisto nearshore leg to 10-15 kt with gusts to 20 kt over both Georgia marine zones. Winds will remains somewhat tempered in the Charleston Harbor, but should still hold in the 10-15 kt range. Seas will average 2-4 ft. Wednesday through Saturday: High pressure will prevail across the marine zones while a cold front approaches the inland zones. This will yield a slightly pinched pressure gradient between the approaching cold front and the high pressure over the marine waters. Overall, conditions are expected to remain below Small Craft Advisory criteria. However, there is a chance that the Charleston nearshore waters could see wind gusts to 25 knots on Wednesday. Otherwise, generally S winds are forecast, 10-15 knots. Slightly stronger wind speeds are possible along the direct coastline in association with the afternoon sea breeze. Seas should average 3 to 4 ft. && .CLIMATE... Record High Minimum Temperatures: July 16: KCHS: 78/2023 KCXM: 82/2021 KSAV: 82/1883 July 17: KCHS: 78/2023 KCXM: 82/2021 KSAV: 80/1883 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory from noon today to 7 PM EDT this evening for GAZ116>119-138>141. SC...Heat Advisory from noon today to 7 PM EDT this evening for SCZ043>045-047>052. MARINE...None. && $$