textproduct: Jacksonville

KEY MESSAGES

For the latest NE FL and SE GA Daily Key Messages please visit: https:/www.weather.gov/media/jax/briefings/nws-jax-briefing.pdf

- Moderate Rip Current Risk through the Weekend

- Daily Heat Index near 105 through the Weekend

- Daily Thunderstorms Localized Flood Risk through Monday

- Patchy Inland Late Night & Early Morning Fog

UPDATE

Forecast is basically track. Broad low level cyclonic flow over the area, high moisture content (PWAT 2.42 inches on 12z sounding), and diurnal instability will lead to quick development of showers and storms today. Rain chances were not modified much as we expect numerous showers and storms overall, with probably categorical chances or about 80% for some inland areas near a weak sfc trough. Have inserted some locally heavy rainfall wording in the forecast as well and has previously been advertised in our PDF briefing. Other small change was to push some rain chances forward into the evening hours.

NEAR TERM /THROUGH TONIGHT/

Highlights Today and Tonight:

- Numerous Showers & Thunderstorms, Mainly Across Northeast Florida

Lower level high pressure will weaken and break down a bit further today while also settling increasingly east of our region, returning more of a southerly component to the mean flow and finally starting a change in the persistent pattern. Certainly nowhere near out of the woods with respect to shower and thunderstorm coverage, as PWATs near or above the 90th percentile for this time of year combine with a mid level disturbance moving overhead to result in numerous activity diurnally today. With a more southerly flow on average (a bit more southeasterly near the coast and southwesterly closer to the Gulf), this will shift the corridor of highest coverage as well as chances for stronger t'storms generally between the I-95 and I-75 corridors where the two sea breezes meet. Aside from isolated gusty downdrafts, heavy rain and localized flooding will be the main concern today with the very high PWATs and storm motion only around 5-10 mph. The southerly flow will allow temps to rise a bit higher than the last several days before convection takes over, as readings in the low 90s will be common except for upper 80s near the immediate coast. Some activity will likely hang on through around the 8 to 10 pm time frame with any lingering instability pockets and outflows, with otherwise decreasing debris clouds and a few coastal low clouds as well with forecast low temps in the low to mid 70s common.

SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/

Main Highlights This Period:

- Localized Flooding Rain Threat

- Isolated Daily Strong Storms Late Afternoon/Evening

- Daily Peak Heat Index 102-107 degF

Elevated rain chances with localized flooding rainfall will continue each day this weekend as tropical moisture lingers and light south- southwest steering flow less 10 kts brings a more dominant west coast sea breeze convective pattern. Morning showers and isolated storms will begin to creep inland from the Gulf Coast region through midday, with expanding coverage toward the Atlantic coast and across southeast GA into the afternoons as diurnal instability increases. The sea breeze merger will focus near the I-95 corridor each afternoon. Storm coverage increases Sunday compared to Saturday as a surface front slides southward across the local area, with convection lingering into Sunday night as an upper level trough carves southward across the southeast Atlantic seaboard and the surface front edges southward across north Florida.

The main thunderstorms hazards this weekend will be a localized flooding rainfall threat with the Weather Prediction Center highlighting all areas each day under a Marginal Risk (level 1 out of 4) of experiencing flash flooding rainfall. This threat will be greater across urban areas and secondary dirt roads. Both bulk shear and lapse rates will be fairly week, but cannot rule out a strong storm or two capable of wet downbursts each afternoon especially toward the St. Johns River basin and I-95 corridor where boundaries merge and heavy precipitation loading could exacerbate downdraft momentum.

Temperatures will range near to just above average before cloud cover thickens with daily highs in the lower 90s with muggy overnight lows in the 70s. Daily peak heat index values are expected to peak just below local Heat Advisory Criteria.

LONG TERM /MONDAY THROUGH THURSDAY/

Main Highlights This Period:

- Breezy Easterly Winds Return

- Afternoon Thunderstorm Changes Inland

- Elevated Tides with Approach of Sept 11 New Moon

Numerous to widespread rainfall coverage continues Monday with the surface front across north Florida. The front is shunted southward into Monday night as high pressure bridges the building as it builds across the mid-Atlantic region, creating a classic 'fall like' high pressure wedge scenario down the lee side of the Appalachians Tuesday into Wednesday with a coastal trough over the waters just offshore. This pattern will bring breezy ENE winds, coastal showers and more isolated storms during the overnight and morning hours, with rain chances shifting inland each afternoon with diurnal heating.

Late Thu into Fri the coastal trough breaks down as surface high pressure builds across south FL and another front begins to approach from the north late Friday. This pattern shift will transition low winds back to the west to southwest with breezy pre-frontal flow and a more dominant west coast sea breeze convective regime toward the end of the week. There are model discrepancies in this time-frame as the GFS keeps a strong mid level ridge in place across the deep south which will bring above normal temperatures and lower rain chances, while the ECMWF quickly transitions to a long wave and progressive east coast trough and local cold front passage late week. NBM consensus for now appears to hold more onto the GFS solution with above normal highs in the low to mid 90s and more scattered diurnal west coast sea breeze convection Friday.

AVIATION /12Z FRIDAY THROUGH 12Z SATURDAY/

Main operational impacts in the very near term is some brief IFR possible for VQQ and GNV over the next 1-2 hours or so. Otherwise, a few showers and lower clouds will stream onshore into the mid morning hours. A good chance of showers and storms today above 50 percent so have TEMPO groups for all terminals midday. The chances of convection expected to increase fairly quickly from about 15z onward. Given high moisture content IFR vsby are likely some of these storms. Shower and storm activity should wind down by 00z but still some VCSH/ VCTS around 00z-03z. Began some MVFR vsby later tonight for VQQ. Sfc winds are expected to be light and variable early on and then become east-southeast by 15z at 7-11kt, but more southerly wind at GNV expected.

MARINE

Winds shift more southeasterly today over area waters as high pressure becomes situated increasingly east of our area. Prevailing westerly winds are then expected from Saturday through at least Monday morning as a frontal boundary sinks southward towards our region, with this feature maintaining a stormy weather pattern. Stronger high pressure will build down the southeastern seaboard early next week in the wake of this boundary, likely pushing the front over the northeast Florida waters.

Rip Currents: Rip current risk will be moderate at all area beaches today as flow shifts from more directly onshore to southeast to southerly. Risk will tend to drop for the weekend with more offshore winds prevailing.

FIRE WEATHER

Atlantic coastal showers and isolated storms will move inland under southeast winds through midday, with expanding showers and thunderstorms across inland areas through the afternoon and into the early evening. Winds become south to southwest across the Suwannee River Valley this afternoon as the west coast sea breeze moves inland. Saturday, southwest winds prevail through the afternoon which will bring morning showers and isolated storms inland from the Gulf coast region, with expanding rainfall coverage toward the Atlantic coast and across southeast Georgia into the afternoon. Winds along the Atlantic coast become onshore Saturday afternoon as the east coast sea breeze develops and moves inland into the afternoon. Dispersion will be fair to good.

Fog Potential: Gusty and erratic winds will occur with thunderstorm activity. Patchy inland morning fog each day.

HYDROLOGY

Coastal water levels begin to increase mid week next week with the approach of the New Moon on Sept 11th combined with onshore easterly flow. At this time, extended probabilistic guidance keeps water levels below action stage through Wednesday both along the local Atlantic coast and within the St. Johns River basin.

PRELIMINARY POINT TEMPS/POPS

AMG 91 73 92 74 / 70 20 20 10 SSI 86 77 91 76 / 30 10 10 10 JAX 89 74 92 75 / 70 20 60 10 SGJ 89 75 91 75 / 70 30 50 10 GNV 90 73 91 73 / 70 20 50 10 OCF 91 74 91 74 / 70 20 50 10

JAX WATCHES/WARNINGS/ADVISORIES

FL...None. GA...None. MARINE...None.