Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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236 FXUS62 KCHS 261947 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 347 PM EDT Sun May 26 2024 .SYNOPSIS... Hot conditions are expected through Memorial Day. A cold front will move into the region Monday night and stall into mid week. High pressure will return for the second half of the week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... This afternoon through this evening: At the surface high pressure is located to the east while to the west there are two features of note. Hand analysis from 12Z this morning depicts a shortwave trough on the lee side of the Appalachian Mountains as well as a much weaker shortwave across MS/AL. The trough to the lee of the mountains is the focus of possible precipitation this afternoon, as the shortwave across MS/AL is quite weak and likely will not be enough forcing to spur showers/thunderstorms. Isolated showers and thunderstorms are possible this afternoon across the SC Midlands, with some of those showers/tstorms impacting far inland counties of the local CWA. However, coverage is not expected to be great and PoPs have been lowered from this morning`s forecast. The thunderstorm risk this afternoon is very conditional, if storms are able to form mesoanalysis suggests that DCAPE values are upwards of 1200 J/kg, along with 40 knots of effective shear. This would lead to a threat of damaging wind gusts. Low level lapse rates are forecast to be around 7 to 7.5 C/km, which would also lead to a marginal hail threat. It will be another seasonably hot and humid day with highs warming into the lower 90s with a few mid 90s possible in a few areas. These temperatures combined with moderate levels of humidity will support heat indices in the 98-101 range. Overnight: Dry conditions will prevail into early Monday morning. It will be warm/humid night with lows only dropping into the lower 70s with mid 70s at the coast and beaches. The record high minimums could be challenged, mainly at the Charleston AFB/International Airport (KCHS) and Downtown Charleston (KCXM), but current data suggest values will fall just short of record levels. && .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... Monday: A broad trough axis will exist across the central United States Monday morning with a parade of embedded shortwaves ejecting northeast. The embedded waves will advect across the Mid-Atlantic states Monday afternoon with the core of the DCVA remaining well north of the region. As this occurs, the wave axis will begin to take on a negative tilt with SC and GA just on the cusp of a split jet streak (RRQ and LFQ). Ahead of this parade of shortwaves and accompanying PVA, weak pressure falls will occur on the lee side of the Appalachians. Several runs of the NAMnest have been fairly consistent on showing an approaching MCS in the morning hours of Monday quickly falling apart only to have fresh convective initiation to occur across the Midlands of SC and the Piedmont of NC. Bulk shear values at this time are around 30 kt with net storm motion to the east 30 to 40 kt. Hodographs are rather straight which would support initially discrete cells with some upscale growth possible into clusters as multiple cold pool amalgamations occur. The main uncertainty for Monday is for how long will these updrafts be able to maintain themselves. NAM and GFS forecast soundings still show impressive capping Monday afternoon across coastal GA and SC. In fact, Monday morning and early afternoon looks dry. Highs will likely reach into the lower 90s as 850 MB temperature surge towards 19 C. Both the NAM and GFS then quickly erode the CAP by the evening hours (less so towards Long and McIntosh counties). For the inland SC counties, enough modification to the thermodynamic profiles appears likely with storms advecting east from the Midlands. Storms will have the potential to produce damaging wind gusts and even large hail. As the storms move across the area, they will likely start to weaken. Expect low temperature Sunday in the upper 60s to lower 70s. Tuesday: A double- barreled shortwave will dive south out Ontario/ Quebec with positive amplification taking place. This will drive a surface cold front to the coast where it will come to an abrupt halt. The true 850 MB cooling will be held back though as winds veer around from the northwest and downslope flow commences. This type of setup usually produces some of the warmer days across the area (especially at the coast) where a sea breeze gets pinned at the beaches. Given this, have blended in the the NBM 75th percentile for high temperatures on Tuesday with widespread lower 90s forecast. The only chance of precipitation will be along the coastal strip in proximity to the surface cold front. Low temperatures Tuesday night will be mostly in the mid 60s (inland) to lower 70s at the beaches. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Global ensembles continue to advertise a seasonably strong amplified wave (1 to 2 SD anomaly) over Quebec and New England on Wednesday which moves offshore next weekend. The latest run of the deterministic GFS is now again advertising wave breaking occurring next weekend, but not nearly as impressive as the 25.12z model run. Taking a look at the WPC Cluster Analysis for this time period shows the rapid wave breaking remaining in the minority of solutions. Only one cluster shows wave breaking occurring on Saturday which accounts for only about ~25% of all GFS, ECMWF, and CMC ensemble solutions. On Sunday, there is more broad ensemble support for wave breaking to occur southeast of the Canadian Maritimes. This working with an overall slower progression of the wave would favor slightly below normal temperatures starting Thursday and continuing into Saturday. A warming trend will then commence starting Sunday (as well as a return of afternoon showers and thunderstorms). Wednesday: The primary wave axis will be located near Ontario and Quebec Wednesday with a secondary shortwave on the southern flank of the primary wave axis. During the day Wednesday, the surface cold front will likely be located near the SC/ GA coast with the 850 MB front lagging behind. Temperatures will still be warm on Wednesday with highs in the lower 90s. No precipitation is expected. Thursday and Friday: By Thursday, the surface cold front will be offshore with surface high pressure nudging in from the Ohio River Valley. Surface high pressure will slowly head southeast through the day with the potential for the cold front to buckle across our southern zones as southern stream energy ejects east out of Texas. The confidence and predictability of this is extremely low at this point, but given the time of year this is certainly possible. For now, have kept the forecast mostly dry for Thursday and Friday, but future forecast packages will need to keep an eye on this to see if PoPs need to be introduced. Saturday and Sunday: The mid-level wave will be amplifying off of the Canadian Maritimes with surface high pressure exiting the Mid- Atlantic coastline. Warmer and more humid conditions will return by Sunday. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... Prevailing VFR through the 18Z TAF period at KCHS/KJZI/KSAV. Showers and thunderstorms are possible this afternoon, however the best chances remain inland from the terminals. KCHS/KJZI have the best chance at seeing a shower/tstorm, however confidence in direct impacts to the terminals remain low so there is no mention in the 18Z TAFs. Extended Aviation Outlook: Monday: VFR expected in the morning hours ahead of an approaching mid-level wave. Late in the afternoon, showers and thunderstorms across the Midlands of SC will advect southeast and approach the terminals by the evening hours. If these storms hold together all the way to the GA/ SC coast is still very uncertain at this point given a stout 700/ 850 MB cap in place. If the storms do hold together, restrictions is visibilities and ceilings are possible with gusty winds. Tuesday: A surface cold front will be pinned along the SC/ GA coast. Winds will be out of the northwest around 10 kt. There is a slight chance of showers and thunderstorms at the terminals due to their proximity to surface cold front. VFR conditions are expected, except with any showers and thunderstorms that directly impact the terminal. Wednesday through Friday: VFR as a surface cold front remains south of the region. There could be some precipitation late Thursday/ early Friday which might produce some restrictions in visibilities and ceilings. Otherwise, VFR. && .MARINE... Rest of Today and Tonight: Southerly winds this afternoon around 10 knots will prevail, with 10 to 15 knots and some gusts to around 20 knots in the Charleston Harbor and the coastline as the sea breeze circulation sets up. Conditions along the coastline will diminish this evening, and southerly winds around 10 kt will persist through the night. Monday through Friday: Marine conditions remain below Small Craft Advisory levels through the week. South to southwest will largely persist through midweek until a cold front pushes east of the waters on Wednesday. High pressure will be the dominant feature for late week. Winds will average 15 knots or less and seas 2-3 feet, with occasional 4 feet early in the week. && .CLIMATE... Record High Minimum Temperatures: May 27: KCHS: 76/1991 KCXM: 79/2019 KSAV: 77/1878 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...CPM SHORT TERM...Haines LONG TERM...Haines AVIATION...CPM/Haines MARINE...CPM/Haines