Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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944 FXUS62 KCHS 260834 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 434 AM EDT Wed Jun 26 2024 .SYNOPSIS... High pressure will remain over the western Atlantic while weak cold fronts linger near our area through the end of the week. && .NEAR TERM /THROUGH TONIGHT/... Upper level ridging encompasses much of the western CONUS with more or less zonal flow through the eastern U.S. save for a couple of convectively induced short-wave troughs (and decaying convection) moving through the mid Mississippi River Valley. A wavy stationary boundary/trough cuts through the Carolinas into southern Georgia separating some drier air well inland from the very humid air mass that remains in place across our region. Some semblance of a weak surface low (thermally induced) lingers over southeast Georgia. Quiet conditions prevail for the moment with temperatures in the middle 70s to around 80. Stronger northern stream short-wave energy will dig into the Great Lakes region later today and begin to drive upper level troughing across the Appalachians with upper level height falls beginning to edge into the southeast region later this afternoon through tonight. Today: Coastal southeast will remain within the broader scale surface low pressure pattern lingering across the region with another day of very warm temperatures coupled with a very humid air mass wedged along the eastern Carolinas and southeast Georgia coast. Ample instability (MLCAPE values are running over 2K J/Kg early this morning although capped at the moment. But with heating, high-res guidance suggest instability will tick up into the 2500-3500 J/Kg range this afternoon. But with the lack of any larger scale forcing mechanism, afternoon convection will be driven by the inland sea breeze push and any old boundaries. But showers and thunderstorms are a good bet to develop inland off the coast this afternoon and forecast will continue to advertise isolated to scattered pops. With ample instability along with fairly decent DCAPE values to work with, a few strong to severe storms are a possibility. Heat index values: Similar to the last few days, surface dewpoints well inland (west of the I-95 corridor) will tend to mix/lower this afternoon, keeping heat index readings in check despite temperatures approaching the hundred degree mark. Along and east of the I-95 corridor and especially along the coast, middle 70s to near 80 dewpoints will lead to heat index values at or above 108F once again this afternoon. A Heat Advisory has been issued for area along and east of the I-95 corridor. Tonight: Diurnally driven convection will run its course through the evening hours. As mentioned, upper level troughiness will edge into the southeast region through the course of the night with enhanced QG-forcing for ascent working across the Appalachians and into the western Carolinas. It does appear at this juncture that any associated shower activity will remain well upstream through the night and plan to maintain a dry forecast. Low temperatures once again will only dip into the middle 70s to around 80. && .SHORT TERM /THURSDAY THROUGH SATURDAY/... Thursday: Mid-level longwave troughing initially over the East Coast will gradually shift offshore as time progresses. More notably, a shortwave embedded in the overall flow is expected to move over our region during the late afternoon or evening, then stall overhead overnight. A cold front located to our northwest at daybreak should very slowly move into our area late in the day, likely stalling across our area overnight. There will be a plume of deep moisture ahead of the front. PWATs could exceed 2.25", which is above the 90% mark for CHS per SPC Sounding Climatology. The heat will be the main concern due to 850 mb temperatures, low-level thickness values, and compression near the front. Highs are expected to reach the mid to upper 90s across most of our area, except cooler at the beaches. Dew points well into the 70s near the coast will cause heat indices to rise to 108-110 degrees. So Heat Advisories may be needed, most likely for our coastal counties. Forcing from the front, the afternoon sea breeze, and the shortwave will generate convection in the afternoon. The synoptic models and long range CAMs are in good agreement, indicating scattered to numerous coverage. MLCAPEs should approach 1,500 J/kg across portions of our area with some shear. With DCAPEs exceeding 1,000 J/kg, a few marginally severe storms with damaging winds are possible just about anywhere. Hence, the SPC has our area under a Marginal Risk for severe thunderstorms, which seems reasonable. Additionally, there will be the potential for locally heavy rainfall underneath the thunderstorms due to fairly weak steering flow and the potential for training. Convection should gradually decrease during the evening. But isolated remnant convection should persist everywhere overnight. Lows will be in the 70s. Friday: The mid-levels should consist of a shortwave hovering over our region in the morning. It is expected to dissipate into the afternoon, followed by weak zonal flow during the evening and overnight. At the surface, a stationary front should be spread across our area in the morning. It`s expected quickly lift north as a warm front into the afternoon, becoming located well north of our area by the evening hours. Deep moisture will persist across the region. PWATs should stay ~2.25", which is well above normal. Similar to Thursday, high temperatures are expected to reach well into the 90s for most areas. Dew points well into the 70s near the coast will cause heat indices to rise as high as 108-110 degrees, which would prompt Heat Advisories, most likely for our coastal counties. Forcing from the remnant shortwave, departing front, and the afternoon sea breeze will generate convection. Models point to another round of scattered to numerous coverage, especially in the afternoon. Instability should be similar to Thursday. However, shear and DCAPEs appear to be lower. This would lower the severe risk. But given the atmospheric setup and time of year, a strong or marginally severe storm with damaging winds is possible just about anywhere. There will also be the potential for locally heavy rainfall underneath the thunderstorms due to weak steering flow and the potential for training. Convection should gradually decrease during the evening. But it`s most likely to persist closer to the coast overnight. Lows will be in the 70s. Saturday: Weak mid-level zonal flow will initially be over our region, with broad High pressure building later into the day. Surface High pressure will be in the western Atlantic, while weak troughing develops over the Southeast U.S. later in the day. Deep moisture will continue to prevail across the region. Similar to the previous two days, high temperatures are expected to reach well into the 90s for most areas. Dew points well into the 70s near the coast will cause heat indices to rise as high as 110 degrees along the coast. So more Heat Advisories may be needed. Instability from the afternoon heat and humidity will generate convection along the sea breeze as it moves inland during the afternoon. Strong or marginally severe storms are possible. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... Surface troughing or weak fronts will be impacting the Southeast U.S. during the long term, while High pressure is located in the western Atlantic. Expect the typical diurnal convection with the highest POPs each afternoon and evening, then trending lower overnight. High temperatures will be well into the 90s each day. Additionally, heat indices could approach 108 degrees along the coast through Monday. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... KCHS/KJZI/KSAV: A brief bout of MVFR cloud cover is milling around the Charleston tri-county area, possibly from the isolated evening showers and thunderstorms. Latest forecast will have a brief period of MVFR conditions at KJZI. Otherwise, overall VFR conditions will prevail overnight through Wednesday evening. Showers and thunderstorms will again be possible Wednesday afternoon into Wednesday evening. Recent guidance trends suggest KSAV will have the higher probabilities for showers and storms, although with low confidence in timing/duration. Forecast will have VCSH at KSAV as well as KCHS during the afternoon as a start. Extended Aviation Outlook: Mostly VFR. However, convection could bring brief flight restrictions, mainly each afternoon and evening. && .MARINE... Persistent southerly flow lingers across the coastal waters through tonight, generally 10 to 15 knots with some gusts to around 20 knots. Seas 2 to 3 feet through today building to 2 to 4 feet tonight. Extended Marine: A typical summertime pattern is expected with High pressure in the western Atlantic and occasional surface troughing or fronts over the Southeast. Each day, expect gradually backing winds. They`ll be strongest along the land/sea interface and across the Charleston Harbor with the formation of the afternoon sea breeze. Each night, winds will gradually veer, possibly surging closer to the coast. Seas will be 2-3 ft. Rip Currents: Today, increasing 1-2 foot swell energy to around 9 seconds is expected to begin impacting the South Carolina beaches later today and into the southeast Georgia coast tonight. With that, internal guidance suggests a moderate rip current risk developing along the coast, particularly the South Carolina beaches as we go through the day. Thursday a 1-2 ft SE swell around 7 seconds and another SE swell around 9 seconds will be impacting the beaches. Per internal calculations, this would increase the rip current risk. We opted to maintain a Low risk for our SC beaches, but added a Moderate Risk at our GA beaches, where rip currents are most likely at Tybee Beach. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory from 11 AM this morning to 7 PM EDT this evening for GAZ101-116>119-138>141. SC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening for SCZ043>045-047>052. MARINE...None. && $$