textproduct: Charleston

WHAT HAS CHANGED

All Sections have been updated

KEY MESSAGES

- 1) Showers and thunderstorms through Tuesday could produce locally heavy rainfall and isolated flooding. Some storms could also become strong to severe.

- 2) Hot and humid conditions will continue this afternoon, with slightly cooler temperatures on Monday and Tuesday.

- 3) Minor coastal flooding is possible during the evening high tide cycles today through Wednesday.

DISCUSSION

KEY MESSAGE 1: Showers and thunderstorms through Tuesday could produce locally heavy rainfall and isolated flooding. Some storms could also become strong to severe.

Today and Tonight: Another rather active afternoon and evening is on tap as upper ridging over the western Atlantic weakens and upper troughing over the Ohio River Valley noses into the deep south. Increasing west-southwesterly flow aloft and weakening subsidence should provide an environment a little more suitable for increased storm coverage this afternoon. Taking a look at surface obs this morning and there appears to be a weak outflow boundary draped west to east across the Tri county area and downtown Charleston. Ongoing satellite imagery shows increased cumulus development along this boundary and a couple isolated thunderstorms already. With steady southwesterly flow, the seabreeze will likely stay pinned close to the coast and by late this afternoon I would expect the intersection of the seabreeze and the aforementioned outflow to be the primary focus for storms across the Charleston Metro (rather similar to yesterday). Further to the west, storms will likely be ongoing across central South Carolina into eastern Georgia. With slightly increased mid-level to deep layer flow and rather impressive deep moist profiles, storms will be capable of damaging winds once again, especially if we can get storms to cluster up into more multicellular clusters. Given the increased coverage and potential for damaging winds across the area, a Slight Risk for severe thunderstorms is in place for most of the our South Carolina areas and a Marginal risk further south into eastern Georgia where coverage may be a bit lower until later in the evening when instability will be decreasing.

PWATS will be around 2 inches this afternoon and while deep layer flow will be a little stronger than it has been the past few days, storms will be capable of producing rather intense rain rates around 2 to 3 inches per hour. As long as storms keep moving than we should not have any flooding issues; however, if storms linger around the wrong place too long (looking at you Charleston and Savannah Metros) localized flash flooding would be possible. Just based on this mornings analysis, if there were an area of concern for flooding it would likely be along that aforementioned outflow boundary near the Charleston Metro. Especially given the potential for storms to train along that boundary during high tide this evening leading to rather similar flooding conditions to the previous two days across the metro.

Monday and Tuesday: A weak backdoor boundary/surface trough stalling across the region early in the week should remain the primary focus for shower and thunderstorm activity. Given increased cloud cover and storm coverage over the early part of the week, instability and severe potential should be lower than the past couple of days; however, A few thunderstorms could become strong to marginally severe each afternoon with damaging winds being the primary concern. Given the low level focus for storms and potentially a weak area of low pressure developing along this boundary, a more confined area of heavy rainfall could become of concern if the rainfall totals begin to pile up with multiple days of rain. The latest SPC Day 2 Convective Outlook advertises a Marginal Risk for severe weather across the entire area Monday while WPC highlights the entire area in Marginal Risk for excessive rainfall Monday and Tuesday.

KEY MESSAGE 2: Hot and humid conditions will continue this afternoon, with slightly cooler temperatures on Monday and Tuesday.

As a hot, humid airmass remains across the Southeast, expect temperatures to continue to rise into the mid to upper 90s through the afternoon before showers and thunderstorms initiate off the seabreeze as it pushes inland. Expect dewpoints to be highest along the immediate coastline and up into the Charleston Tri-County as values reach into the upper 70s, resulting in peak heat index values near 110F. Inland areas will have slightly lower heat index values with dewpoints in the lower 70s. Therefore, a Heat Advisory remains in effect across southeast Georgia and southeast South Carolina coastal counties, Tidal Berkeley, and Inland Berkeley until 7PM EDT.

Monday and Tuesday: A weak cold front will push through on Monday, allowing for slightly cooler air to advect into the region. Thereafter, the front will remain just south of the region into Tuesday. This will yield increased cloud coverage and rainfall. Afternoon temperatures will only reach into the upper 80s to lower 90s for Monday and Tuesday. Heat index values will peak into the low to mid 90s across the inland counties, with values peaking into the upper 90s to lower 100s.

These slightly cooler temperatures will be short-lived unfortunately as H500 heights begin to rise again on Wednesday as a ridge strengthens aloft. Expect temperatures to rise back up into the low 90s on Wednesday, then further climbing into the mid to upper 90s on Thursday and Friday.

KEY MESSAGE 3: Minor coastal flooding is possible during the evening high tide cycles today through Wednesday.

Increasing astronomical tides associated with Monday's lunar perigee and Tuesday's New Moon will bring a risk of minor coastal flooding starting this evening. This evening's high tide (~7PM EDT) at the Charleston Harbor (CHTS1) is forecasted to reach 7.0 ft MLLW. Therefore, a Coastal Flood Advisory deems likely this evening for coastal Charleston and coastal Colleton Counties, especially if showers and thunderstorms continue to persist across the Tri-Country into the late evening.

However, the greatest threat is expected Monday and Tuesday evening as astronomical tides continue to increase, mainly along the coastal Charleston and coastal Colleton Counties. Additionally, heavy rainfall occurring near, or during the time of the evening high tide cycles could exacerbate flooding.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

VFR conditions should prevail across the area through early afternoon; however, satellite and radar observations already show thunderstorm activity starting to pickup in and around the CHS/JZI terminals. Shower/thunderstorm chances will increase over the next couple of hours especially near CHS and JZI where the greatest risk for thunderstorms will likely be. Gusty winds and temporary reductions in vsbys/ceilings will be possible within the strongest of storms. Coverage during the afternoon is not as widespread further south towards SAV; however, similar impacts are possible at SAV as we head into the early evening hours around 00Z. Conditions should gradually improve at CHS/JZI by early evening with only some lingering showers through around 04Z and slightly later for SAV given the later increase in storms. VFR conditions should then return at all terminals by around 04Z Monday, then persist through 18Z Monday. Some MVFR ceilings may try to work into the area early 12-18z Monday morning but it does appear to stay north of any of the aviation sites.

Extended Aviation Outlook: Shower and thunderstorm coverage, along with risks of associated restrictions, will remain elevated through Tuesday.

MARINE

This afternoon through Thursday: Ahead of a weak cold front, expect breezy southwesterly winds to persist as the seabreeze pushes ashore this afternoon. Similar to the previous couple days, expect afternoon thunderstorms to initiate off the seabreeze and likely bring a risk of strong winds. As this aforementioned weak cold front gradually settles across the South Carolina waters on Monday night into Tuesday morning, this will yield variable winds over the next couple days. This change of pattern will also yield scattered showers and thunderstorms across the coastal waters through Tuesday. A few storms will be capable of producing strong to severe wind gusts, frequent lightning, and locally higher seas. Otherwise, marine conditions remain under Small Craft Advisory criteria.

Rip Currents: A moderate risk for rip currents will remain from the South Santee River down to Savannah through this evening, due to 1-2 ft swells with wave periods up to 9 seconds along with increasing tidal influences from the upcoming lunar perigee on Monday. In addition, SSW winds will likely generate a moderate longshore current, possibly supporting rip currents near piers and jetties.

CLIMATE

Record High Minimum Temperatures:

July 12: KCHS: 79/2020

CHS WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory until 7 PM EDT this evening for GAZ217>219- 239>241. SC...Heat Advisory until 7 PM EDT this evening for SCZ045-148>152. MARINE...None.