Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 141710 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 110 PM EDT Tue Aug 14 2018 .SYNOPSIS... A weak front will prevail inland today, then dissipate on Wednesday. Inland troughing will then become more prominent during the second half of the week, then persist into early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 1 PM: Sfc analysis indicated a wavy sfc cold front across the middle Carolinas into GA. Latest SPC mesoanalysis indicated 1500-2000 J/kg of CAPE with little to no inhibition. Visible satellite showed a band of building cumulus and showers a little east of the Fall Line. Near term models indicate that this area of cumulus will likely develop into the early evening hours. A few multi-cell storms should form and will likely drift east, possibly with a cool pool give DCAPE values around 1300 J/kg. Another region of cumulus field has developed over SE GA, showers and storms should form in this region between mid afternoon into this evening. I will update the forecast to increase PoPs and weather. Smoke may have limited the cumulus formation earlier today, however, I think the impact of the smoke will be minimal this afternoon. As of 1030 AM: Satellite images indicate a thin plume of wildfire smoke from the Great Lakes Region to Georgia and Carolinas. View out the window indicates that the sky appears a little hazy. Regional VWP shows a few returns around 10 kft. At 10 kft, 12Z KCHS sounding indicate a small inversion with RH around 10%, It appears that this is the level of the smoke. In addition, water vapor images indicate deep dry air across the forecast area. Observed KCHS sounding observed PW around 1 inch, well below the normal of 1.7 inches. Based on the depth of dry air and recent runs of near term guidance, I remove morning PoPs across the region. It appears that isolated to 1-2 thunderstorm clusters could develop across inland GA and along the beaches late this afternoon. I will update the PoPs, weather, and temps to latest thinking. Previous Discussion: Today: The mid level cut-off low to our north was slowly lifting out toward southern New England with mid and upper troughing persisting over the southeast states. A weak stalled surface front will remain inland from our region and weaken through tonight. Model cross-sections and water vapor satellite imagery show very dry air was anchored over the forecast area to begin the day and this dry air aloft will likely result in factors that impact convective coverage and perhaps intensity this afternoon and evening. This morning, we think it will begin rather capped with sunshine for most area and temps warming to around 90 not long after midday. Highs will reach the low to mid 90s, warmest inland today, few changes to forecast readings. Convection: Moisture profiles suggest southeast GA stands to see the best coverage of diurnal convection this afternoon and early evening with sea breeze showers and tstms becoming more and more isolated further north up the SC sea breeze corridor as surface dew points more likely to mix lower than points south this afternoon. Several of the more recent CAMs suggest late afternoon development over inland southeast GA and far southern SC. This may be partially in response to both a substantial deep layered moisture gradient and a region of locally strong upper difluence near the left front quadrant of a 45-50 kt upper jet arcing n-s through the SC upstate into the CSRA. If this feature can enhance updrafts despite the drier air aloft, we can envision some locally strong/severe tstms along and south of the Savannah River region later today. Forecast DCAPEs are very elevated for mid August with over 1300 J/KG progged over much of the forecast area. Damaging wet microbursts would be the main risk, however the lower deep layered moisture profiles suggest multicell development may be difficult to realize. Tonight: We should see diurnal inland fade during the post- sunset hours over the region. Models suggest that there may be isolated convection over some coastal and southern zones overnight, but solutions continue to differ at this juncture. Late tonight, most of the forecast areas will likely be dry but cannot rule out some convergence showers and tstms along coastal areas, especially north of the Savannah River. Lows will range from the lower 70s in cooler well inland spots to mid 70s most other areas. && .SHORT TERM /WEDNESDAY THROUGH FRIDAY/... Wednesday and Thursday: Conditions will trend drier through mid-week as a parent mid/upper lvl low centered north of the region shifts off the Northeast coast, making way for a ridge of Atlantic high pressure to expand over the Southeast United States. A prevailing west/northwest flow aloft should help keep most inland locations dry, especially on Wednesday, but a southerly sfc wind will support PWATs near 2.0 inches across Southeast Georgia. Given the amount of moisture and sfc heating, chances of showers and thunderstorms will remain in the forecast on Wednesday across Georgia. Chances of showers and thunderstorms should expand further north in Southeast South Carolina on Thursday, especially near a seabreeze circulation. During overnight periods, precip activity should be primarily more focused offshore. 1000-850mb thickness values along with the downsloping wind component aloft should support afternoon temps in the lower 90s each day, with a few inland locations potentially reaching the middle 90s. Overnight lows should range in the low/mid 70s away from the coast Wednesday night and Thursday night. Friday: A typical summertime pattern will be in place for much of the day with an Atlantic ridge aloft and the area between weak low pressure inland and Atlantic high pressure at the sfc. However, the ridge should begin to weaken aloft and retreat toward the western Atlantic late as a mid/upper lvl trough of low pressure advances over the Midwest toward the Mid-Atlantic states. Given the trend aloft along with a persistent southerly sfc flow supporting PWATs near 2.0 inches, showers and thunderstorms are possible for all areas. Highest chances of precip are anticipated in northern areas during the afternoon/evening closer to the approaching trough. Overall high temps should peak in the lower 90s away from the coast. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... Greater chances of precip should return this weekend and then persist into early next week as a mid/upper lvl trough of low pressure quickly advances from the Midwest to the Northeast and Mid- Atlantic states, forcing the Atlantic high to weaken over the area and retreat further offshore. Greatest chances of precip should occur inland and north, closer to a sfc trough axis just west of the area and near the base of the mid/upper trough of low pressure passing to the north. Deep moisture characterized by PWATs around 2.0 inches, SBCAPE values near 2000-2500 J/kg and marginally stronger low-lvl wind fields suggest more active afternoon/evening convection days with potentially stronger pulse/outflow boundary driven thunderstorms Saturday into Monday. By Tuesday, the mid/upper lvl trough axis should be positioned well offshore with a ridge of high pressure building over the Southeast United States. High temps should trend a degree of two warmer as the ridge builds over the area Tuesday with most locations peaking in the lower 90s away from the coast. A few locations could warm into the middle 90s across Southeast Georgia. Lows should range in the mid/upper 70s this weekend and early next week, warmest near the coast. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... Both KSAV and KCHS will remain VFR through 12Z Wednesday with any risk of SHRA/TSRA along the sea breeze this afternoon too low to mention at KCHS but we cannot totally rule out isolated tstms in the general area very late in the day or this evening. At KSAV, the potential for late day, early evening storms is in the 30 percent range at this point with model timing still a bit uncertain. Extended Aviation Outlook: VFR conditions should prevail at CHS and SAV terminals Wednesday into early Friday. Brief flight restrictions are possible with afternoon/evening showers and thunderstorms Friday and Saturday. && .MARINE... Southwest synoptic flow will prevail with conditions similar to recent days with speeds ebbing mid morning til early afternoon then increase again during the afternoon and peak once again in the evening hours near shore and somewhat later on further offshore. Speeds will average 15 kt with gusts to 20 kt at times during favorable surge times. Seas will average 2 to 4 ft, highest offshore. Wednesday through Saturday: Conditions are expected to remain below Small Craft Advisory levels through the weekend. High pressure building over the coastal waters will support a more relaxed gradient and south/southwest winds topping out near 15 kt Wednesday and Thursday, before the pressure gradient tightens this weekend between low pressure inland and high pressure over the western Atlantic, resulting in wind speeds up to 15-20 kt. Strongest winds should occur late weekend as a mid/upper lvl trough of low pressure approaches to the north. Seas should generally range between 1-3 ft Wednesday into Thursday, before slowly building back to 2-4 ft this weekend, highest in northern waters late weekend. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...NED SHORT TERM...DPB LONG TERM...DPB AVIATION... MARINE...

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