345 FXUS62 KJAX 210757 AFDJAX Area Forecast Discussion National Weather Service Jacksonville FL 357 AM EDT Wed Jul 21 2021 ...STRONG TO ISOLATED SEVERE THUNDERSTORMS POSSIBLE EACH AFTERNOON THROUGH FRIDAY... ...HEAT INDEX VALUES APPROACH 105 DEGREES ON THURSDAY AFTERNOON... .CURRENTLY... Overnight surface analysis depicts a stalled and wavy frontal boundary extending from coastal Texas east-northeastward across the lower Mississippi Valley to coastal portions of the Carolinas. This persistent boundary was keeping the axis of Atlantic ridging displaced southward across the FL peninsula. Aloft...cutoff troughing over the lower Mississippi Valley continues to retrogradeslowly westward as it gradually fills, which was allowing deep- layered ridging to build westward from the Bahamas towards the FL peninsula. Otherwise, a potent shortwave trough progressing across the St. Lawrence Valley along the U.S./Canadian border was digging a longwave trough over the eastern Great Lakes, Ohio Valley and New England. Latest GOES-East derived Total Precipitable Water imagery indicates that a slightly drier air mass was advecting northeastward from the Gulf of Mexico towards the southern Suwannee Valley and north central FL, where latest RAP/LAPS analyses were indicating PWATs falling to around 1.8 inches, while a deep tropical air mass featuring PWAT values around or above 2 inches persists from the I-10 corridor northward throughout southeast GA. Low stratus ceilings and patches of fog were slowly expanding in coverage across inland portions of southeast GA, mainly from Waycross westward. High altitude debris cloudiness was persisting elsewhere, with temperatures and dewpoints in the low to mid 70s area-wide as of 08Z. .NEAR TERM [Through Tonight]... Low level flow will continue to slowly veer to west-southwesterly today as deep-layered ridging begins to build northward from the FL peninsula towards north central FL. A slightly drier air mass will prevail across north central FL beneath this building ridge, while deep west-southwesterly flow continues for locations along and north of the Interstate 10 corridor. A similar weather pattern will ultimately continue for northeast FL today, as scattered convection develops towards noon in the western Suwannee Valley and moves briskly eastward, with activity then increasing in coverage and intensity as it approaches the I-95 corridor, where it will meet a pinned Atlantic sea breeze boundary. Mesoscale boundary interactions, such as convective outflows intersecting the Atlantic sea breeze boundary, will likely result in storms pulsing along and east of I-95 late this afternoon. Model soundings depict Downdraft CAPE values approaching 1,000 j/kg over northeast and north central FL this afternoon, and stronger storms pulsing near the coast will be capable of producing downburst winds of 40-60 mph, frequent lightning strikes, and heavy downpours. Meanwhile, shortwave energy embedded within the deep west- southwest flow will progress eastward across southeast GA this afternoon and early this evening, where multiple rounds of convection will be possible through midnight. Likely to categorical POPs were placed in the forecast grids along the I-10 corridor and across most of inland southeast GA, with scattered coverage expected over north central FL. Locally heavy rainfall totals will be possible this afternoon through early this evening in the northern Suwannee Valley and inland southeast GA, mainly for locations south of Alma and Douglas. The first round of convection developing around noon in these locations should keep highs in the upper 80s, while plenty of morning sunshine further eastward should boost highs to the lower 90s for locations along and east of U.S. Highway 301. Maximum heat index values should climb to around 100 in these locations before convection and cloud cover increases by the mid to late afternoon hours. Shortwave energy will push off the Georgia and South Carolina coast tonight, with deep and convergent low level westerly flow possibly keeping scattered convection re-developing over the western Suwannee Valley and portions of inland southeast GA well into the evening hours. Otherwise, debris cloud cover will gradually thin overnight, with lows falling to the low and mid 70s, except upper 70s at coastal locations. .SHORT TERM [Thursday Through Friday]... Prevailing westerly flow will bring in increased levels of moist air over the region which will result more widespread thunderstorms for the end of the week, with PWAT values reaching upwards of about 2.25 inches as the effects of ridging weakens to the south over the Florida peninsula and troughing broadens over Georgia and northeast Florida. The main threat from storms during this period is excessive localized rainfall. High temperatures on Thursday are expected to be above the seasonal average, with max temps reaching up into the lower to mid 90s for some inland areas. Overnight low temperatures will be in the lower 70s for inland areas and in the mid to upper 70s nearer to the coastline. .LONG TERM [Saturday Through Wednesday]... Flow will shift about to become more out of the northeast as drier air advects in from out of the north and a possible developing low pressure system moves across central Florida from east to west on Sunday and Monday. Potential for scattered to numerous diurnal showers and storms during this period with heavy localized rainfall being the most likely threat. Placement of the low pressure system will determine which areas receive the most rainfall during the weekend and into next week, with the most likely area currently forecasted as being over north central Florida. Temperatures for this period are expected to be close to the seasonal climate average with cooler max temps occuring in areas with abundant rainfall and cloud cover. && .MARINE... A frontal boundary stretching across the southeastern states will remain stalled through Thursday evening as high pressure remains centered over the Florida peninsula. Offshore winds will prevail through Friday, and strong thunderstorms developing over the Suwannee Valley will push eastward over the near shore waters during the late afternoon hours, with gusty winds and frequent lightning strikes possible. This pattern of stronger late afternoon and evening thunderstorms may continue on Thursday and Friday. High pressure building southward from the Great Lakes region into the Ohio Valley will push the frontal boundary southward over the Georgia waters on Friday, with this boundary then stalling over the northeast Florida waters this weekend. Weak low pressure is then expected to develop offshore of the Georgia waters along this boundary this weekend. Showers and thunderstorms will increase in coverage over our local waters from Friday through the weekend as this weak low pressure center moves westward towards the east central or northeast FL coast. Seas of 2-4 feet will prevail over our local waters during the next several days. Rip Currents: Prevailing offshore winds through the morning hours and low wave heights will keep a low risk in place at all area beaches during at least the next few days. && .HYDROLOGY... Rivers continue to recede across our region, with mostly minor flooding ongoing along the lower Santa Fe River. River levels should fall below flood stage by the weekend, except around the Three Rivers Estates gauge. && .PRELIMINARY POINT TEMPS/POPS... AMG 89 73 91 73 90 / 60 20 70 20 70 SSI 90 78 91 77 89 / 50 20 60 30 50 JAX 92 74 93 75 89 / 50 20 60 30 70 SGJ 92 76 91 75 89 / 40 20 50 30 60 GNV 92 74 91 75 90 / 50 10 60 20 80 OCF 91 74 91 75 90 / 40 20 50 10 70 && .JAX WATCHES/WARNINGS/ADVISORIES... FL...None. GA...None. AM...None. &&