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

- Hot Weather Continues Today. Heat Advisories from 11 AM 7 PM for Locations South & East of Waycross, GA. Heat Advisories are Possible for Portions of Our Area on Friday

- Strong to Isolated Severe TStorms Possible this Afternoon & Evening. Main Impact Area: Northeast FL. Primary Hazards: Wind Gusts of 40-60 mph, Frequent Lightning Strikes, and

- Torrential, Slow Moving Downpours that Could Create Localized Flooding

- Numerous Afternoon and Evening TStorms on Friday & Saturday

- Isolated to Scattered Afternoon & Evening TStorms Sun - Wed

NEAR TERM /THROUGH TONIGHT/

Highlights through Tonight:

- Near Record High Temperatures Continue Today with Dangerous Heat Indices for Portions of Our Area. Heat Advisories in Effect from 11 AM through 7 PM Today for Locations South and East of Waycross, GA. - Strong to Isolated Severe Thunderstorms are Possible this Afternoon and Evening Across Northeast and North Central FL. Wet Microburst Potential Will Exist within Stronger Storms, Creating Wind Gusts of 40-60 mph, along with Frequent Lightning Strikes and Localized Flooding.

A cold front north of the area extending from the SC coast westward into central GA while weak surface high pressure is over the north half of the Gulf. Weak west to northwest winds 4-8 mph are observed across the area between the two surface features. Satellite imagery this morning shows a swirl of mid to high level clouds over north central and central FL denoting the center of the mid/upper level low across the FL peninsula. GOES Satellite estimates of Total Precipitable Water level (PWATs) indicates some of the drier air over SE GA this morning has begun to erode south of Waycross to near 2 inches, but drops off into with PWATS in the 1.40 to 1.60 inch range along and north of US-82 to the Altamaha river and northward to near the surface front while over 2.0 inches over NE FL.

This morning's 12Z upper air sounding revealing high moisture level of 2.08 inches of Precipitable Water (very close to the 90th percentile based on climatology) and fairly moist low to mid levels with some drier air above 500 mb as well as slightly cooler 500mb temps -7 to 8C. The cooler mid level temperatures and high moisture levels will support stronger mid level lapse rates with as seen by 800 to 600 mb level lapse rates around 7.0 C/km. While the sounding also convective temperature is 94 degrees, abundant sunshine today will allow temperatures to quickly reach the mid 90s and allow the onset of scattered showers and isolated thunderstorms by 12PM-2PM period as the Gulf seabreeze pushes towards I-75, then 2-4PM along the Atlantic seabreeze as it moves onshore towards I-95.

As the Atlantic seabreezes continues inland this afternoon strong heating, shortwave energy aloft, and interactions from shower and thunderstorm outflows with the Atlantic/Gulf seabreezes will spur scattered to numerous thunderstorms. Isolated strong to marginally severe thunderstorms will develop along storm mergers and outflow collisions focused from highway 301 to to US-17 with main impacts being wet downbursts producing wind gusts 40-60 mph, frequent lightning, and locally heavy downpours. Slow storm motions will create slow moving, torrential downpours with heavy rainfall rates of 2-3 inches per hours leading to locally 1-3 inch quick rainfall totals that could lead to minor flooding over urban, low-lying, and poor drainage areas. Westerly winds will turn west southwesterly 5 to 10 mph, switching to east to southeasterly behind the Atlantic seabreeze 8-12 mph with gust 15 to 20 mph at the beaches.

Hot west to southwest flow in the low levels over the area will push high temperatures into the mid to upper 90s over NE FL while drier air and lack of thunderstorm coverage across much of SE GA will allow highs to soar into the upper 90s to around 100s. Highs will approach within a couple of degrees of daily record highs today, through likely stay just below. While dewpoints will mix to the mid to upper 60s over most of SE GA along and north of US-82, higher humidity from dewpoints in the mid to upper 70s will make feels like temperatures soar into the 108 to 112 degrees. These dangerous heat index values will also lead to Major heat risk affecting anyone without cooling/hydration. Therefore, a heat advisory is in effect today through 7PM south of US-82 over inland SE GA, the SE GA coast, and all of NE FL.

Tonight, thunderstorms will continue to persist just past sunset through 9-10PM before waning in coverage and intensity inland. Scattered showers will continue underneath remaining mid to high level storm clouds before dissipating by midnight. Skies will become mostly clear after midnight with shallow ground fog forming over localized areas that received heavy rainfall. Lows will fall to the low/mid 70s west of I-95, mid 70s along I-95, upper 70s along the coast and St Johns river.

SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/

Main Highlights This Period (Friday and Saturday):

- Heat advisories posted for coastal areas of NE FL Friday

- Strong to possibly severe thunderstorms possible Friday and Saturday

A weak surface low will remain centered over the Carolinas, with an associated frontal boundary extending across Georgia. An upper trough will move into the region Friday and remain overhead Saturday, increasing instability and supporting above-normal chances for afternoon and evening thunderstorms. Temperatures will remain near to slightly above average.

Friday: The heat wave will persist, with Heat Advisory is posted for coastal areas of NE FL as heat index values reach 105109F ahead of convection. Increasing instability ahead of the upper trough, combined with precipitable water values of 2.102.40 inches, will support numerous afternoon and evening showers and thunderstorms. This environment will favor locally strong to severe storms, with strong instability and weak shear supporting microbursts, gusty outflows, frequent lightning, and locally heavy rainfall. Scattered to widespread convection is expected Friday afternoon and evening. Lows will be in the mid 70s.

Saturday: The upper trough will remain overhead, resulting in increased coverage of afternoon and evening thunderstorms, with rain chances generally 6070% south of Waycross. Greater storm coverage should provide some relief from the most extreme heat, with heat index values of 102106F and highs in the lower to mid 90s. Stronger storms will remain possible, with frequent lightning, gusty winds, and locally heavy downpours. Lows will be in the lower to mid 70s.

LONG TERM /SUNDAY THROUGH THURSDAY/

Main Highlights This Period (Sunday through Wednesday):

- Strong to possibly severe thunderstorms possible each afternoon and evening

The weak surface low and associated frontal boundary will persist across the Southeast. Low-level flow, while generally southwesterly, will be light enough to allow the Atlantic sea breeze to push inland each afternoon. Meanwhile, a persistent upper ridge over the Desert Southwest, Southern Plains, and ArkLaTex region will facilitate a drier mid- to upper-level air mass, leading to a gradual reduction in precipitation chances early next week.

Sunday: Scattered afternoon and evening showers and thunderstorms will remain possible as the upper trough axis shifts offshore. Temperatures will remain in the lower to mid 90s, with heat index values around 105F.

Monday and Tuesday: Precipitation chances will decrease as the sprawling ridge strengthens across the southern Plains and ArkLaTex region, resulting in a drier mid to upper level atmosphere. On Monday, PoPs will range from 1020% across Southeast Georgia to 3050% across Northeast Florida. By Tuesday, isolated to widely scattered showers and thunderstorms are expected area wide, primarily during the afternoon and evening.

Wednesday: Another upper-level trough will move into the eastern CONUS, providing an increase in moisture and forcing. This will support scattered afternoon and evening showers and thunderstorms, with rain chances increasing to 3545%. Storms will be driven primarily by diurnal instability and sea breeze interactions. While widespread severe weather is not expected, isolated strong storms will remain possible, with gusty winds, frequent lightning, and locally heavy rainfall as the primary hazards.

Highs through the long term period will generally be in the lower to mid 90s, with afternoon heat index values around 105F. Overnight lows will remain in the mid 70s. Overall, hot and humid conditions will continue, although heat impacts should be less extreme than during the current heat streak.

AVIATION /18Z THURSDAY THROUGH 18Z FRIDAY/

VFR conditions to begin the afternoon with west to southwesterly winds 5-8 knots will switch to southeasterly behind the developing Atlantic seabreeze as it moves inland from near SSI and SGJ through CRG and then JAX and near VQQ now through 20Z. Thus far, storms have not yet developed over the immediate area, as mid to high level clouds lift north over the NE FL TAF sites, but building cumulus clouds bases 3.5 to 4.5 kft will increase through 19Z with VCTS after by 19Z at all sites this afternoon. Have tempo groups for stronger thunderstorms 20Z-24Z for IFR VSBY and gusty winds up to 30 knots which may need to be raised for more intense thunderstorms that merge near the Northeast FL terminals.

Coverage of thunderstorms will decrease after 00Z with VCSH until 03- 04Z as light showers continue after sunset under thinning mid to high level leftover thunderstorm clouds and light. Have added MVFR fog prevailing at VQQ and GNV after 07-09Z with light southerly to southwest winds around 5 knots trending calm inland after 04Z. Southwest winds 5-8 knots will resume late on Friday morning with cumulus clouds developing in strong diurnal heating with VCSH at GNV and VQQ by the end of the period as the Gulf seabreeze pushes inland.

MARINE

Weak high pressure to the west over the northern Gulf waters will remain through the weekend into early next week and continue a west to southwesterly flow over the waters. The Atlantic seabreeze will develop each afternoon and move slowly inland, shifting winds to south southeasterly and then southerly overnight as wind speeds approach caution levels in the early evening and diminishing late evening. A couple of cold front will move into the southeast states and stall north of the area waters through early next week. Scattered showers and thunderstorms will develop and impact the near shore waters adjacent to northeast Florida this afternoon int early evening. Coverage of thunderstorms will increase in the afternoon and early evening hours over the local waters on Friday and Saturday. Coverage of afternoon and evening thunderstorms will decrease Sunday into Monday with more isolated or widely scattered thunderstorms.

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 today and low to low end moderate this weekend as an afternoon seabreeze brings light south to southeast winds to the beaches.

FIRE WEATHER

- Patchy high dispersions over SE GA this weekend through Monday

The frontal boundary across Georgia will sustain daily afternoon and evening thunderstorm activity, with the highest coverage anticipated Friday and Saturday. Potential exists for strong to locally severe storms, capable of producing gusty, erratic winds, frequent lightning, and locally heavy rainfall. Outside of these storms, no significant fire-weather concerns are expected.

Fog Potential: Fog formation is unlikely, and any erratic wind patterns will be isolated to thunderstorm activity.

CLIMATE

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

Thursday, August 20th: KJAX: 101/1900 KGNV: 99/1900 KAMG: 104/1995 KCRG: 98/2016

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

PRELIMINARY POINT TEMPS/POPS

AMG 75 97 72 93 / 0 60 50 50 SSI 78 94 77 92 / 10 40 30 60 JAX 75 94 74 92 / 20 40 20 70 SGJ 76 93 75 92 / 20 40 20 60 GNV 73 93 73 90 / 30 60 30 70 OCF 74 93 74 91 / 20 50 40 50

JAX WATCHES/WARNINGS/ADVISORIES

FL...Heat Advisory until 7 PM EDT this evening for FLZ021-023-024- 030-031-035-038-120-124-125-132-136>138-140-220-225-232- 233-236-237-240-322-325-333-340-422-425-433-522-533-633. Heat Advisory from 11 AM to 7 PM EDT Friday for FLZ038-125-132- 137-138-232-233-237-325-333-433-533-633. GA...Heat Advisory until 7 PM EDT this evening for GAZ153-154-162- 163-165-166-264-350-364. MARINE...None.