AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 1015 PM EST Tue Jan 24 2012 .UPDATE...A tricky FOG forecast is on tap once again tonight. It is being complicated by an assortment of CLOUD cover at different levels across the forecast area. Starting with a surface analysis, it seems AS though a stalled FRONT continues to linger from near KPAM-KMAI-KTMA, or roughly bisecting our forecast area from SW-NE. To the north of this front, dewpoints drop off sharply into the upper 40s and lower 50s, while the LOW-level AIR mass south of the front is still quite moist (dewpoints 60-65F). The area south of the front is where we would have the best CHANCE of seeing some fog development tonight. Indeed by 02z, Apalachicola (KAAF) had already reported a 1/4SM VISIBILITY and Perry (K40J) had reported visibility as low as 1/2SM. HI-resolution model guidance including the 4km WRF and NAM runs, as well as the HRRR, show the fog initially developing near the coast and then slowly expanding inland. This seems quite logical, although it will probably not be a uniform south-to-north evolution. The aforementioned cloud cover will play a role in keeping some areas devoid of fog much longer. The latest IR satellite shows MID-upper level clouds eroding quickly over the last several hours east of the Apalachicola River, and this is where the initial Dense Fog ADVISORY was placed for our Florida zones (02-13z valid time). To the north in Georgia, we coordinated the advisory area with WFO JAX. BKN-OVC low clouds situated west of Thomasville and Moultrie should limit the westward extend of fog in our SW GA zones. Additionally, the front is expected to slowly drift south overnight, and drier air will slowly work towards the FL-GA border. Thus, the temporal extent of the fog in SW GA may be limited to pre-SUNRISE, and the initial advisory for our GA zones was limited to 02-10z. Looking ahead to the THUNDERSTORM potential Thursday NIGHT, things still seem generally on TRACK with previous thinking. SPC SWODY3 places areas west of a DHN-MAI-AAF line in a Slight Risk for severe thunderstorms. The 25.00z NAM run continues to be a blend of the 25.12z runs of the faster ECMWF and slower GFS. It brings some convective PRECIPITATION into our western areas around 03z Friday (10pm EST Thursday Night). Forecast soundings indicate about 500 j/kg of near-surface CAPE closer to the Gulf coast, with a lot of the CAPE profile concentrated in the lowest 3-4km AGL" onClick="return popup(this, 'notes')">AGL. Therefore, while the forecast CAPE values are not overwhelming, the more compact CAPE region would favor greater low-level accelerations. SHEAR profiles are, as is typical of the cool season, quite favorable for organized thunderstorms. In fact, a lot of the shear in the 0-3km layer will LIKELY be concentrated in the lowest 500m AGL. There is some concern about southeasterly FLOW ahead of the approaching front not being the most favorable pattern for MOISTURE return. However, this may be negated by fairly warm water temperatures with both of our offshore buoys at 71F this evening. Bottom line, there will be a lot of low-level shear and a lot will ultimately depend on the lapse rates and CAPE profiles in the lowest 3-4km. The HWO wording will be modified this evening to indicate a bit more concern. && .AVIATION [through 00z Thursday]...With front lingering along the I-10 corridor overnight, low cigs/VSBY may be an issue for KECP, KTLH, and KVLD. To the north, enough dry air is expected to keep conditions VFR through the forecast period. Best chance for conditions near airport mins will be KVLD by morning. Slightly better conditions expected to the west. Clouds will gradually lift through the morning with VFR conditions everywhere by the afternoon. && .MARINE...A Marine Dense Fog Advisory has been issued for all of our coastal waters within 20 NM of shore until 16z Wednesday. This was the result of a FEW surface observations near the coast reporting visibilities below 1 mile, as well as NUMEROUS HIGH-res models showing strong indications of fog. The fog is expected to lift around mid-morning with enough mixing in the marine boundary layer to dissipate the dense fog. With offshore buoys SSTs running at 71F, fog is not a concern for the offshore zones.