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

- Isolated Strong Storms Possible this Afternoon and Evening. Hazards: Gusty Winds (40-60 mph), localized flooding, & frequent lightning.

- Isolated to Widely Scattered Afternoon Thunderstorms Sunday and Monday. Increasing Thunderstorm Coverage Beginning Tuesday.

NEAR TERM /THROUGH TONIGHT/

Highlights through Tonight:

- Potential for strong thunderstorms this afternoon and evening

Lingering upper shortwaves ahead of the weakening frontal boundary in central GA will help trigger morning showers and isolated storms moving inland from the Gulf. Convection will then spread east- northeastward through the day in the general southwest flow. Slightly drier air (PWATS 1.6-1.8 in) may advect in from the northeastern Gulf keeping coverage to isolated to widely scattered across NE FL this afternoon. Best chances for storms will be along the sea breeze merger near I-95 and SE GA closer to the diffusing frontal boundary. An isolated strong storm may be possible along boundary mergers producing gusty winds and heavy rainfall. Convection will shift off the Atlantic coast later this evening.

Amplifying mid-upper trough digging down the eastern US will shift the upper "heat dome" ridge away from the region bringing a break from Heat Advisory conditions. Highs today will be in the low-mid 90s with max heat indices in the 100-106 F range. Overnight lows will be in the 70s.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/

Main Highlights This Period (Sunday and Monday):

- Limited thunderstorm activity for SE GA Sunday and all areas Sunday

- Isolated strong to severe thunderstorm possible across NE FL and north-central FL on Sunday.

- Hotter and drier; heat index in the 100-105 range each day.

The leftover stalled frontal zone will remain stretched across southeast GA as it decays on Sunday. Though the driest airmass will remain to the north, precipitable water will fall to around 1.5" or lower across southeast GA while HiRes ensemble guidance sends a moist tongue of PWAT > 2" across northeast and north-central FL through the afternoon hours amid a south-southwesterly steering flow. This should encourage scattered thunderstorm development on the progressive Gulf breeze during the afternoon hours. PoPs were kept at 30-40% range for now but that may need to be increased, particularly along the St Johns River corridor and points east where the late day sea breeze collision is likely.

There will be an upper shortwave gliding across the boundary but timing will be critical in whether it will support ascent or hinder convection with wake subsidence. If it lines up timing with supporting ascent the potential for stronger or possibly severe isolated storms will increase. Main concerns will be downburst or gusty microburst winds up to 60 mph. Enough storm motion, around 10 mph should minimize flooding risk.

For Monday, the local airmass will be on the dry side to start but there are signals that moisture will be advected west-to-east and may bring sufficiently moist air into southeast GA by the late afternoon, though confidence is low. With that uncertainty and possible lingering dry air, thunderstorm chances are in the "isolated to widely scattered" coverage range (15-25%) for Monday afternoon. One consideration will be the potential for high downdraft energy both Sunday and Monday with the infiltration of dry layers aloft.

As drier air pushes in this weekend, it will help temper heat index readings as dry aloft helps mix out and lower surface humidity levels. While ambient air temperature highs will push up toward the mid/upper 90s each day, expect heat index to be in the 100-105 degree territory.

LONG TERM /TUESDAY THROUGH FRIDAY/

Main Highlights This Period (Tuesday through Friday):

- Daily Thunderstorm Chances Returns on Tuesday

Deeper moisture returns by Tuesday as broad upper troughing remains established across the eastern US. Several mid level shortwaves will be directed toward the southeast from Tuesday onward. With little migration in the low-level ridge axis, southwesterly flow, a dominant Gulf breeze, and pinned Atlantic sea breeze will be predominant through the work week. This will likely lead to late afternoon and evening storms favoring the I-95 corridor each day. If timing holds, a surface front is suggested to nose southward across GA which may result in an increased potential for more robust convection next weekend as it stalls.

Above normal temperatures are expected to continue through the week with highs generally in the low 90s Tuesday onward with heat index values generally below local heat advisory criteria at this time.

AVIATION /06Z SATURDAY THROUGH 06Z SUNDAY/

Light rain over SSI will linger over the next hour or so. Due to recent rainfall, patchy fog may develop near VQQ and GNV in the pre- dawn hours. Light winds early this morning become SW at 5-10 kts by 15Z. Another round of showers and storms will move in the vicinity of the TAF sites from southwest to northeast. Convection may pass near GNV beginning around 15Z. Have PROB30s in for potential thunderstorms impacting the TAF sites between 19-24Z. Confidence isn't high enough to put as prevailing or TEMPO.

MARINE

A frontal zone will extend across the Carolinas and into Georgia with a high pressure ridge to the southeast. Rounds of mainly afternoon and evening showers and thunderstorms can be expected each day. Brief southerly wind surges are expected during the late afternoon and evening hours as the Atlantic sea breeze shifts onshore. But, winds are expected to remain below Small Craft Exercise Caution levels. The cold front will push into the local waters Sunday night into Monday and then lift back northward as a warm front by mid-week.

Rip Currents: Offshore winds will prevail throughout the morning hours, along with low surf heights, during the next several days. With the outgoing tide occurring later in the afternoon, the overall risk will be low to low end moderate this weekend as an afternoon seabreeze brings south to southeast winds to the beaches. Surf/breakers will remain 1-2 feet or less.

FIRE WEATHER

Unsettled weather continues today with showers and storms possible along a stalling front and the sea breeze this afternoon. A few storms will be capable of strong wind gusts, locally heavy rainfall and frequent lightning.

West to southwesterly breezes continue today and for the next several days with the exception of the coastal areas where the afternoon Atlantic sea breeze will shift winds east to east- southeast.

A dry northwest flow will bring lower humidity and mostly dry weather to southeast Georgia Sunday with minimum humidity between 30- 40% while scattered to isolated afternoon showers and storms favor northeast FL. Hotter temperatures on Sunday and Monday will increase mixing heights across southeast GA, leading to locally high dispersion each afternoon.

CLIMATE

Near record high temperatures are possible at some of our designated climate sites during the next few days:

Friday, August 21: KJAX: 101/1900 KGNV: 99/1983 KAMG: 100/1980 KCRG: 99/2016

PRELIMINARY POINT TEMPS/POPS

AMG 94 73 96 72 / 60 20 0 0 SSI 93 75 95 76 / 50 30 10 0 JAX 94 73 94 74 / 40 20 20 0 SGJ 94 75 93 75 / 40 20 20 0 GNV 92 73 93 73 / 30 10 30 0 OCF 92 74 91 73 / 30 20 50 0

JAX WATCHES/WARNINGS/ADVISORIES

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