Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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000 FXUS62 KTBW 140012 AFDTBW Area Forecast Discussion National Weather Service Tampa Bay Ruskin FL 712 PM EST Thu Dec 13 2018 .UPDATE (Overnight and Friday)... 00Z water vapor and H4 RAP analysis is showing a vigorous upper level low closing off over the southern Plains this evening. It will be this system and associated surface low/cold front that evolve eastward through the day on Friday and bring increasing rain/storm chances to west- central Florida. The system to impact us on Friday/Friday night is a rather complex one with an atypical frontal structure and evolution. The front is not going to be very progressive the next 24-30 hours, especially as it reaches the eastern Gulf of Mexico/Florida west coast. Several mid/upper level disturbances moving over this lower level boundary in conjunction with a highly diffluent upper level southern stream jet pattern suggest several rounds of storms may form and pivot eastward into Florida. This setup makes detailed timing for highest rain chances difficult. Other than a few showers after midnight for the northern Nature Coast, most areas should remain dry for the overnight hours. It appears likely the first round of significant showers/storm coverage will impact areas north of the I-4 corridor during the morning hours, followed by another round or two impacting the Tampa Bay Area during the mid/late afternoon or early evening hours. Yet another round, or the lingering earlier round from Tampa Bay, will then settle south and east into our southern and interior southern zones later in the evening and overnight. These initial rounds of storm (especially in the afternoon/evening) look to be the ones with the greatest potential for embedded severe and rotating storm structures. The initial round into the Nature Coast may very well have organized convection, but we will have to see if any of this activity is rooted at the surface or if its an entirely elevated convective situation. Forecasters will be closely monitoring the lower level environment to determine the potential for concerning elevated radar signals to actually impact at ground level. Greater potential for surface based convection exists later in the day across the region due to a combination of moisture/thermal advection from the south and diurnal heating. Even after the severe threat diminishes, the slow frontal movement means we are unlikely to see a defined clearing of the rain Friday night, but rather lingering elevated chances for showers/weaker storms for all areas. Of note: Many of the higher resolution NWP guidance members are showing a squall-line coming into the I-4 corridor/Tampa Bay Area region during the evening hours Friday, even late evening in a few outlier solutions. While the slow progress of the cold front could certainly make this timing a reality, the overwhelming bias associated with these model solutions is to bring MCS/squall-lines into our region late, sometimes by several hours, compared to what actually ends up occurring. The reasoning for this lies in the difficulty all NWP models have dealing with cold pool generation/evolution and propagation under and ahead of the convection. This is especially true over the open waters of the Gulf of Mexico. These cold pools of air very often move quicker in reality compared to the model simulations, and therefore we see the rain/storms arrive and exit quicker than the model ensembles will forecast. this bias has been applied to the current forecast related to timing of higher rain chances / severe weather threat. && .AVIATION (14/00Z through 15/00Z)... VFR conditions as BKN-OVC mid deck slowly lowers through the night. E and ESE winds under 10 knots overnight begin to increase between 12-15knots with higher gusts and become SE to SSE AFT 14-15Z. Approaching cold front bring increasing chances for showers/storms to KTPA/KPIE/KLAL/KSRQ by early to mid afternoon and then continue into the evening hours. A few storms later afternoon/evening may be strong to severe. Rain/storm chances look to wait until after 21Z for KPGD/KFMY/KRSW but then linger through the evening and overnight hours. && .Prev Discussion... /issued 246 PM EST Thu Dec 13 2018/ ..SLIGHT RISK OF SEVERE THUNDERSTORMS FRIDAY... ..HAZARDOUS CONDITIONS ARE EXPECTED TO IMPACT MARINE WATERS AND BEACHES... DISCUSSION... Robust mid level low over TX this afternoon takes an easterly track...as ridging east of FL moves away...along the western Gulf coast Fri then over the southeastern states during Sat. At the surface - ridging over FL and the east Gulf slides to the east as the surface reflection of the TX mid level low drags a cold front across the Gulf. The front reaches the west-central and southwest FL coastal waters Fri then slips across the state to the Atlantic Sat. Moisture and wind fields begin to increase over the Gulf later tonight ahead of the approaching system...shift into the coastal waters early Fri...then cross west-central FL Fri morning into the afternoon and southwest FL afternoon and evening. These times are first guess and will be adjusted over the next couple of forecast cycle. A squall line or two is expected to form ahead of the front with strong to severe thunderstorms that can produce damaging wind gusts...tornadoes or waterspouts...and locally heavy rainfall with flooding. The most active weather will be moving east across the state toward the Atlantic late Fri night. Large waves and swells will be generated in the western Gulf that translate onto area breaches for the weekend with a high risk of rip currents expected. For Sun - Thu: The mid level low exits the southeastern coast early in the period as ridging builds into the nation/s mid-section from Mexico then slides east through Wed. Surface high pressure settles into the western Gulf Sun then tracks to the eastern seaboard by midweek. On Thu a mid level short wave trough/low moves into the southern plains with a surface low/cold front forming on the western Gulf coast. The upper and surface ridging dominate for much of the period with dry seasonable temperatures. Moisture increases enough by Thu to support some showers and a few thunderstorms. MARINE... Increasing winds ahead of an approaching storm system will require small craft advisories on some of the waters tonight...and spread to the rest of the waters as the system moves into the coastal waters early Fri with thunderstorms and possible waterspouts. The front crosses the state Sat then moves out over the Atlantic. This system is generating large waves and swells in the western Gulf that will translate into the local waters for perhaps much of the weekend. High pressure begins to build into the western Gulf Sun then slides to the eastern seaboard by midweek with winds veering and diminishing. FIRE WEATHER... Atmosphere continues very moist with no low humidity concerns. Storm system coming in from the west will bring showers...thunderstorms... and locally heavy rainfall mainly Fri with some showers and a few storms into Sat. && .Preliminary Point Temps/PoPs... TPA 65 76 65 72 / 10 90 90 30 FMY 66 81 69 74 / 0 40 70 40 GIF 64 80 65 74 / 10 80 70 40 SRQ 66 76 67 72 / 10 90 90 30 BKV 63 78 65 72 / 30 100 90 30 SPG 66 76 65 72 / 10 90 90 30 && .TBW WATCHES/WARNINGS/ADVISORIES... FL...None. Gulf waters...Small Craft Advisory until 7 PM EST Friday for Waters from Englewood to Tarpon Springs FL out 20 to 60 NM-Waters from Tarpon Springs to Suwannee River FL out 20 to 60 NM. Small Craft Advisory from 1 AM to 7 PM EST Friday for Waters from Bonita Beach to Englewood FL out 20 to 60 NM. Small Craft Advisory from 4 AM to 7 PM EST Friday for Coastal waters from Bonita Beach to Englewood FL out 20 NM-Coastal waters from Englewood to Tarpon Springs FL out 20 NM-Coastal waters from Tarpon Springs to Suwannee River FL out 20 NM. && $$ UPDATE/AVIATION...Mroczka Previous Discussion...Rude

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