Scientific Forecaster Discussion
000 FXUS62 KCHS 302014 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 414 PM EDT Sun Apr 30 2017 .SYNOPSIS... Atlantic high pressure will give way to a cold front which will move through Monday night into early Tuesday. High pressure will then bring dry weather through Wednesday night before rain chances return late in the week as a stronger storm system affects the area. Drier high pressure should then return by the end of the weekend. && .NEAR TERM /THROUGH TONIGHT/... Subtropical ridging will hold firm for much of the night before steadily breaking down prior early Monday. A cold front moving across the lower Mississippi Valley will trek east across the Deep South tonight and is on target to be very near the Alabama- Georgia border by daybreak. While some pre-frontal warm air advection/isentropic assent could help generate a few showers across the far western areas during the early morning hours, most of Southeast South Carolina and Southeast Georgia should remain dry. Gridded pops will be held below mentionable thresholds. A strengthening low-level jet will keep the boundary layer fairly well mixed tonight, which should result in yet another warm and humid night for the end of April. Lows will only drop into the lower-mid 70s; warmest at the coast. && .SHORT TERM /MONDAY THROUGH WEDNESDAY/... Monday: Deep layered ridging will slowly retreat east over the Atlantic as a large area of low pressure lifts northeast over the Midwest. A southward extending cold front associated with this low will make a run toward the Southeast United States, likely losing some speed/strength as it becomes further separated from the parent low. Ahead of the front, deep moisture advection within a southerly flow along with increasing isentropic ascent will help spawn some showers inland late morning into early afternoon hours. Sfc temps should peak a degree or two cooler than the previous day (low to mid 80s) due to expanding cloud cover, but sfc heating should still be sufficient to produce SBCAPE near 1000 J/kg well inland. These values in combination with lifted index values around -6 C and mid- lvl lapse rates near 6.5 C/km support afternoon thunderstorms away from the coast. However, the bulk of precip activity should come near or just ahead of the cold front as it progresses through the area early Monday night. Given the timing of the front, instability will be weaker, but 0-6 bulk shear between 35-40 kt could help a few thunderstorms become organized along/near fropa, resulting in strong and/or damaging straight-line winds. At this time, the threat for greater severe weather coverage is low. Most precip will end early Tuesday as the front shifts offshore. Overnight lows will range in the mid/upper 50s inland to lower 60s near the immediate coast. Winds/Lake Winds: The pressure gradient will tighten over the area ahead of an approaching cold front Monday, suggesting gusty wind potential (near 30 mph) when afternoon heating promotes mixing into 35-40 mph low-lvl winds fields. A Lake Wind Advisory could eventually be needed late Monday morning into Monday evening for gusts of 25 kt near the beaches of Lake Moultrie. Tuesday and Wednesday: High pressure will build over the Southeast behind the departing cold front offshore. Expect all areas to be dry and temps to be slightly cooler on Tuesday, followed by gradual warming on Wednesday ahead of the next system progressing over the South Central United States. In general, afternoon temps will peak in the lower 80s on Tuesday, then middle 80s on Wednesday. Tuesday night lows will range in the mid/upper 50s inland to lower 60s along the immediate coast. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... Mainly dry through Wednesday night before rain chances return with the approach of a stronger storm system toward the end of the week. Although there is a bit better agreement between the GFS and ECMWF than 24 hours ago, there are still some timing differences so forecast confidence is lower than normal. Conditions look at least marginally favorable though for some stronger storms Thursday. Rain chances should diminish by Friday night but we kept small rain chances into Saturday given the uncertainty regarding the . timing of the cold front and strength/timing of the upper trough. Temperatures should be near or above normal through at least Thursday night before likely dropping below normal Friday through the weekend. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/... VFR for the rest of the afternoon into this evening. Guidance shows MVFR cigs developing after midnight at KSAV with the main axis of MVFR cigs remaining west of KCHS. Risk for showers/tstms will increase after 18z Monday at both terminals, so will not include any mention at either terminal this cycle. Gusty winds will impact the terminals for much of the period, possibly dropping a bit this evening before increasing again overnight as low-level jetting intensifies. Extended Aviation Outlook: Ceiling and/or visibility restrictions are likely Monday evening into Monday night due to low clouds and showers/thunderstorms along/ahead of a passing cold front. Breezy/gusty conditions are also expected Monday afternoon ahead of the front. Another round of flight restrictions is possible with showers/thunderstorms Thursday/Thursday night, possibly lasting into Friday. Breezy/gusty conditions are also expected Monday, Thursday and Friday afternoons. && .MARINE... Tonight: Southerly winds will steadily increase tonight as pre- frontal low-level jetting takes hold. Speeds are expected to reach 15-20 kt by midnight and continue through the early morning hours Monday. In Charleston Harbor, winds will remain 15 kt, but will be closer to 15-20 kt near the Harbor entrance. Seas will also build, reaching 4-6 ft South Carolina nearshore waters, 4-5 ft Georgia nearshore waters and 5-7 ft Georgia offshore waters by sunrise Monday. Small Craft Advisories have been posted for the South Carolina waters and Georgia offshore waters for tonight and will remain in force. Small Craft Advisory conditions will likely be ongoing for all waters Monday into Monday night when winds gust near 25-30 kt and seas build as high as 6-8 ft. Small Craft Advisories will likely be needed for all waters until fropa nears/occurs Monday night into early Tuesday. However, 6 ft seas could linger in offshore Georgia waters through late Tuesday morning. Otherwise, no significant issues are expected over the coastal waters until Thursday when another cold front approaches. Much uncertainty exists regarding the timing of the frontal passage, but conditions look poor either way through Friday. && .CLIMATE... Observed minimums this morning indicate all three climate sites should break their respective record high minimums for today. Record high minimums for 30 April: KCHS: 69/2014 and previous years. KCXM: 73/1912. KSAV: 72/1953 and previous years. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from 8 AM Monday to midnight EDT Monday night for AMZ330-354. Small Craft Advisory from midnight tonight to 6 AM EDT Tuesday for AMZ352. Small Craft Advisory from midnight tonight to noon EDT Tuesday for AMZ350. Small Craft Advisory until noon EDT Tuesday for AMZ374.