832 FXUS64 KMOB 181146 AFDMOB Area Forecast Discussion National Weather Service Mobile AL 646 AM CDT Wed Aug 18 2021 .AVIATION... 12Z issuance...VFR is generally prevalent across our forecast area early this morning, with the exception of a few small patches of MVFR to IFR ceilings and visibility reductions across portions of south central/southwest AL and southeast MS. VFR continues through the period, except for localized reduced ceilings/visibility values near isolated to scattered SHRA/TSRA that will develop northward across the area late this morning into this afternoon/ early evening. A few MVFR ceilings and/or MVFR to IFR visibility reductions will also be possible again late tonight into early Thursday morning across parts of interior southwest/south central AL and southeast MS. Southeasterly to southerly winds will increase to 5-10 knots today before becoming light by early this evening. /21 && .PREV DISCUSSION... /issued 434 AM CDT Wed Aug 18 2021/ NEAR TERM UPDATE /Now Through Thursday/...A typical summer time pattern is in store throughout the near term as upper level ridging begins to build in over the region from the western Atlantic. At the same time, Fred continues to move northeast over the Appalachians as it slowly transitions into a non-tropical low and a broad, positively tilted trough over the central CONUS moves northeast into the Great Lakes Region. However, the these features will stay well outside of the local area as the aforementioned upper level ridging sets up over the Southeast. Isolated to scattered showers and storms are expected to develop offshore and along the coast this morning before developing inland this afternoon. Essentially, convection will follow the typical diurnal pattern. PWAT's this afternoon are forecast to range from 1.8 to 2.2 inches so convective coverage will be slightly higher than it was today. We opted to cap coverage at high end scattered this afternoon due to the building heights and somewhat limited coverage on CAMs. Decent instability will be present this afternoon with guidance indicating MLCAPE between 1000-2500 J/kg and DCAPE over 1000 J/kg. Model soundings also indicate the presence of dry air in the midlevels so a few strong storms cannot be ruled out. By Thursday the upper level ridge will encompass much of the northern Gulf Coast, stretching all the way from Texas to the southeast coast of the US. Meanwhile, a surface high pressure begins tracking westward across the northern Gulf on Thursday evening, causing PWAT's to decrease to around 1.5-1.8 inches offshore. Moisture will remain abundant inland, however, due to southwesterly flow, leaving PWAT's around 1.8 to 2.1 inches. Scattered showers and storms can therefore be expected once again on Thursday with convection developing along the seabreeze. Highs today and Thursday afternoons will be hot as they reach into the low to mid 90s. These temperatures in combination with the increased moisture will cause heat indices to soar into the 102-107 degree range. We will continue to monitor heat indices as these values are just below advisory criteria. Lows tonight are forecast to dip down into the lower to mid 70s inland with readings in the mid 70s to around 80 degrees along the immediate coast. Swell approaching the surf zone has decreased early this morning and the HIGH rip current risk along local beaches is no longer in effect. 1-2 ft/5-6 s period swells, high tidal ranges, and outgoing tide in the afternoon will still support a MODERATE rip current risk today. Long period swells associated with tropical cyclone Grace moving westward from the Yucatan Peninsula into the southwestern Gulf of Mexico are forecast to arrive along the northern Gulf Coast by early Thursday evening. The rip current risk will become HIGH again along local beaches Thursday evening/night into Friday and Friday night. 14/21 SHORT TERM /Thursday night Through Friday night/...Summertime continues on through Friday. The upper level trough that brought slightly higher rainfall chances on Thursday will be slowly lifting northeast out of the Mid-Ohio valley. In response to the upper trough, the large ridge over the eastern seaboard will slide south and west across the northern Gulf Coast. By Friday, the ridge axis will likely be directly over head. Moisture across the area will be rather high with PWATS around 2.1 to 2.2 inches across the area. The best moisture will likely be over the interior counties and decreasing towards the coast as you move towards the center of the upper ridge. This will likely result in scattered to potentially numerous storms across the area with rain chances highest along and north of highway 84 becoming more scattered as you move south towards the coast. Stronger storms may be tougher to come by as deep moist profiles will keep MLCape values around 1000 J/KG and Dcapes around 500 J/KG. The bigger issue will be the potential for training storms and localized flooding. Steep surface lapse rates will support storm development, and with weak synoptic flow only around 10 knots throughout the layer storms will be likely to remain in the same location or possibly train over similar areas. Given PWATS in excess of 2 inches leading to warm tropical rain processes, rain rates could near 2 to 4 inch per hour rates in the stronger storms leading to rate driven flooding. The main caveat will be the presence of the upper ridge providing some subsidence across the area. Storms will quickly subside Friday night as the upper ridge takes hold. Temperatures will be seasonable with highs in the low 90s and lows in the mid 70s. Heat indices will range in the 104 to 107 range given the higher low level moisture values. A HIGH risk of rip currents will likely exist during this period as swell from Tropical Storm Grace makes its way northward through the Gulf. BB/03 EXTENDED TERM /Saturday Through Tuesday/...A broad upper ridge over the southern plains and deep south shifts northward and expands significantly across much of the southern CONUS through Monday as an upper trof over the interior eastern states shifts eastward to be oriented just off the east coast. A combination of a surface ridge over the north central Gulf coast states shifting westward followed by a surface trof becoming better defined over the Carolinas into the area will result in a primarily northwest flow over the forecast area through the period. Precipitable water values remain around 2.0 inches through the period, but model soundings show progressively drier air flowing into the area through at least Monday roughly beneath 750-800 mb. Have stayed with chance pops each day for now, although it's possible these may be somewhat overdone. Highs in the lower to mid 90s on Saturday trend higher by Sunday to the mid to upper 90s. This could result in heat index values approaching heat advisory criteria (108), but this could be offset by somewhat lower surface dewpoint values. Will continue to monitor. /22 MARINE...Light to occasionally moderate southerly flow will generally persist through the period, with the exception of light west to northwest flow during the late night and early morning hours late this week and this weekend. Tropical Storm Grace is forecast to strengthen to a hurricane and track westward from the Yucatan Peninsula into the southwestern Gulf of Mexico Thursday night into Friday. Long period swells associated with this system will bring building seas late this week. /21 && .MOB WATCHES/WARNINGS/ADVISORIES... AL...None. FL...None. MS...None. GM...None. && $$ This product is also available on the web at: http://weather.gov/mob