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Area Forecast Discussion
381 FXUS62 KCHS 301031 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 631 AM EDT SAT JUL 30 2016 .SYNOPSIS... Atlantic high pressure along with an inland trough will prevail into early next week. High pressure will then become suppressed further to the south which could allow a weak cold front to approach from the north by the middle part of next week. && .NEAR TERM /THROUGH TONIGHT/... For Today: The large scale pattern is dictated by a TUTT low north of Hispaniola that heads toward the southern Bahamas, while a mid latitude trough extends south from the upper Midwest through the TN valley. In between the local region lies within a deep ridge that still refuses to weaken overhead. 500 mb heights are similar to yesterday around 5910-5920 meters and a 3-4C cap persists under the associated sinking motion. At the surface a lee side trough remains anchored in place, as the sub-tropical Atlantic ridge extends west across FL. It`ll be another hot and humid day with 850 mb temps ~20C and low level thickness values similar to what they were yesterday. However, there is less compressional heating compared to Friday as the synoptic flow is more SW rather than W, and the sea breeze gets an earlier start. Plus we actually have our first chance of convection in about the past 5 days in place for today, albeit only a low end probability. These conditions should hold max temps "only" in the mid or upper 90s, with maybe a couple of spots across interior SE GA hitting 100F. Although there is again a pronounced inverted-V sounding indicating that dew points will mix out, the overall dryness of the air is less than recent days and with the low level trajectories backing to the S and SW, this will limit dew points from dropping as much as recent days. This will result in afternoon heat indices peaking at 105-109F across the vast majority of the CWFA. Moisture will pool in association with the sea breeze along the US-17 corridor from about 1-4 pm, enough so that a few spots from the Charleston metro to the Beaufort and Hilton Head areas, and then toward the Savannah region will experience some 110-112F heat indices. But the duration and spatial coverage is low enough where we don`t require a Heat Advisory. The potential for showers and t-storms is certainly greater than the past several days, but still not expecting anything more than isolated to scattered coverage, since there remains large condensation pressure deficits, warm mid levels and their accompanying poor lapse rates. But with the sea breeze boundary and the nearby proximity to the Piedmont trough and a weak short wave that skirts by to the N-NW late, we feel that 20-30 PoP is in order after 1 or 2 pm. Greatest coverage is across the Charleston quad-county district where the best boundary interaction and higher Theta-E air is found. DCAPE in excess of 1000-1200 J/kg equates to a potential for strong or perhaps marginal severe downbursts if storms in isolated storms. Tonight: The TUTT low rides up the spine of the Bahamas as a broad trough is situated over the eastern U.S. We`re in the region between both of these large scale features, near the southern extent of the trough. An earlier short wave moves through the area during the evening, enough to keep at least slight chances of showers/t-storms going prior to midnight across our N and NW tier. A warm SW synoptic flow that never fully decouples will again produce a warm and muggy night with low temps that will struggle just to dip below 80F. And we might be near record high minimums for July 30th. && .SHORT TERM /SUNDAY THROUGH TUESDAY/... Sunday and Sunday night: Aloft, the upper ridge will continue to stretch across south Georgia and the Gulf coast through the overnight, with no significant embedded shortwaves poised to impact the area. At the surface, the forecast area will remain under the western periphery of the subtropical high to the east and in between the inland lee trough. Models indicate the potential for more coverage of diurnal showers and storms, but a perusal of soundings doesn`t reveal much significant change from previous days. 500 mb temperatures are slightly cooler, down to around -7 C at times, with modestly steeper mid-level lapse rates. This uptick in coverage will likely have the best chance of occurring further north where the better instability and moisture will be positioned. The forecast still features 20-40 percent PoP`s, with the highest situated over the tri-county region and mainly southeast South Carolina. Lingering activity should diminish in the evening and the overnight will be dry for most areas. We could see an isolated shower or storm along the coast late though. Temperatures are expected to come down a few degrees, with highs mainly in the mid to upper 90s. Lows will be in the upper 70s. Please see the Climate section below for more details regarding the near record warmth through the month of July. Monday through Tuesday: The upper ridge will break down and become more centered between the desert southwest and the lower Mississippi Valley. Moisture will increase across the area, with precipitable water values around 2.25 inches, and 500 mb temperatures will cool further to around -8 C. This will all happen as a well defined shortwave approaches the area from the northwest Monday afternoon and evening. As such, it appears Monday will likely be the most active day and the increase in convective coverage will extend through Tuesday. The near storm environment still isn`t overly impressive for severe thunderstorms, but there will likely be a few strong to marginally severe storms where boundary interactions occur. As the calendar shifts over the August, temperatures will cool more into the low to mid 90s. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... No significant changes were made to the going forecast for the long term period. A return to more active showers and thunderstorms will continue at least for Wednesday and Thursday as embedded shortwaves approach and push through the area from the northwest. The forecast generally features 30-40 percent chances each afternoon and evening, and these values will certainly be refined as the time period draws closer. Temperatures also continue to look cooler, with highs mainly in the low 90s, which would be right around normal for early August. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... While there is a risk for isolated convection near the terminals from about 18-23z, the probability of any direct impacts is far to remote to include anything other than VFR conditions with the 06z TAF set for KCHS and KSAV. Extended Aviation Outlook: Brief flight restrictions are possible with afternoon/evening showers/thunderstorms Sunday through Wednesday. Otherwise, VFR will prevail. && .MARINE... Today: The overall synoptic pattern remains essentially unchanged from recent days, with an inland trough of low pressure and the oceanic ridge poking west across FL. The local waters will lie on the NW periphery of this area of high pressure, and once sea breeze circulations develop by 16-17Z, W winds of 10-15 kt winds will back to the S and SW at 12-18 kt. Conditions at and near the mouth of Charleston Harbor will likely experience the strongest winds with return flow aloft from inland storms, as gusts peak at times near 20 kt. Seas will hold at or below 3 ft throughout. Tonight: High pressure to the south and an inland trough will dictate the general pattern, and once again another nocturnal low level jet will traverse the marine community. Geostrophic winds are as high as 20 or 25 kt, so we won`t require a Small Craft Advisory, but there is enough mixing to generate S and SW winds as great as 15-20 kt. Seas will build to as much as 4 ft with the favorable trajectories and higher winds. Sunday through Thursday: The persistent pattern of a trough of low pressure inland and high pressure to the east will continue to drive south to southwest flow across the local waters. The periods of strongest winds will likely occur in the afternoon and evening hours along the land/sea interface with the development of the sea breeze circulation. Sunday will likely be the day with the strongest winds, and we could see Small Craft Advisory gusts to 25 knots for a few hours around the Charleston Harbor and the Charleston County coast. Otherwise, winds will generally top out around 15 knots. Seas will primarily range 1-3 feet with the potential for some 4 footers during the time periods with elevated winds. && .CLIMATE... Record high minimums for 30 July... KCHS: 80 set in 2011. KCXM: 82 set in 1999 and previous. KSAV: 81 set in 1878. Record high maximums for 30 July... KCHS: 100 set in 1999. KCXM: 98 set in 1942 and previous. KSAV: 101 set in 1896. ***Top 5 Warmest Months on Record by Average Temperature*** Charleston International Airport - KCHS (Records Back to 1938) 1. 86.1 - July 1986 2. 86.0 - July 2016 (2 days left) 3. 85.5 - July 1993 4. 83.9 - July 1992 5. 83.8 - July 1998 Savannah International Airport - KSAV (Records Back to 1874) 1. 86.7 - July 1993 2. 86.0 - July 2016 (2 days left) 3. 85.7 - July 1986 4. 85.7 - July 1875 5. 84.8 - August 2011 Downtown Charleston - KCXM (Records Back to 1893) 1. 86.0 - July 1998 2. 85.5 - June 1998 3. 84.9 - July 1977 4. 84.8 - July 1986 5. 84.8 - July 2016 (2 days left) ***Top 5 Driest Months on Record for July*** Savannah International Airport - KSAV (Records back to 1871) 1. 0.82 - 1888 2. 1.21 - 2016 (2 days left) 3. 1.25 - 1881 4. 1.33 - 1949 5. 1.35 - 1972 && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ SYNOPSIS... NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE... CLIMATE...