Area Forecast Discussion
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692 FXUS62 KTAE 222047 AFDTAE AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 347 PM EST Thu Jan 22 2015 ...Heavy rain and a marginal chance for severe weather expected on Friday... .Near Term [Through Tonight]...
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Surface frontogenesis appeared to be occurring from a developing cyclone near the TX coast, eastward across the northern Gulf of Mexico. This was happening ahead of a potent upper level trough digging over NM; with moist, southwest flow streaming out ahead of this trough over the Gulf Coast. The latest numerical and statistical guidance remain in excellent agreement with regard to tonight`s rainy forecast. Light rain will develop west of Tallahassee and Albany this evening, then increase to moderate levels late tonight as the rain goes on to cover our entire forecast area. Lows will be above average, in the lower to mid 50s in GA & AL, and upper 50s in FL. Any thunderstorms that develop late tonight will be over the Gulf coastal waters. .Short Term [Friday Through Saturday Night]... A strong +PV anomaly diving through the Southwest can be seen clearly on WV imagery this afternoon. This anomaly is being advected into the base of a pre-existing shortwave trough whose axis lies from the Central Plains, through the Southwest. As the aforementioned anomaly rounds the base of the trough, it will strengthen the shortwave and associated dynamics as the entire system moves east through the Southern Plains tomorrow, exiting the Mid-Atlantic coast Saturday night. At the surface, Gulf cyclogenesis has already begun along the Texas Gulf coast associated with the existing shortwave. Deep WAA caused by southwesterly flow ahead of the shortwave has established highly sloped isentropic surfaces along the entire Gulf coast. A cross sectional frontogenesis analysis depicts the warm front extending up through 500 mb. National radar imagery shows a large rain shield, associated with this warm front, extending east from west Texas through Mississippi. It is likely that the surface low will become linked with the resultant diabatically forced, mid-level PV anomaly. From there, these two features will only strengthen each other as the mid-level anomaly gets advected east-northeast through the Southeast over the next 24-36 hours. The strengthening system means two things for our region (1) heavy rain starting late tonight and lasting through Friday and (2) a strengthening low-level jet, increasing the potential for isolated strong to severe storms along the warm front. Expanding on the above, the current forecast advertises widespread 2-3 inch rainfall totals over the duration of the event. Isolated higher rainfall amounts in the 3 to 5 inch range will be possible where training of cells occurs. The heaviest rain will likely be confined within the 7am-7pm time period tomorrow. The severe threat is a bit more tricky and will rely heavily on the placement of the heaviest rain within the expected rain shield. The further north that the heaviest rain occurs, the further north the surface low and attendant surface based warm front will penetrate into the Tri-State region. Due to the expected steady antecedent rainfall, lapse rates look to be very weak and would not suggest a potential for deep rotating updrafts. However, with low-layer shear values in the 30-40 knot range, there will be a threat will be for a few tornadoes to develop immediately along the front. This is why the exact placement of the warm front becomes so important. The large rain shield will lift north out of the Tri-State region on Friday evening, with a cold front expected to traverse the local area. There will remain the potential for a few strong to severe storms along the cold front as it reaches the eastern Big Bend of Florida where lapse rates steepen a bit in the wake of the large rain shield and 50-60 kts of deep layer shear align with the tail of the departing LLJ. In this area on Friday evening, the primary threats will still be for an isolated tornado, with damaging straight line winds also possible. Uncertainty remains high with regards to both severe weather scenarios. A little bit of light rain will trail the frontal system as the dynamic shortwave passes through the Southeast, though all rain should be coming to an end around sunrise on Saturday. .Long Term [Sunday Through Thursday]... There is good model consensus that an eastern U.S. trough will remain in place for the duration of the extended range forecast, though there is uncertainty as to how deep the trough will penetrate into the south. The current temperature forecast reflects highs slightly below seasonal norms, with overnight lows near seasonal levels. No rain is expected through the end of next week.
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&& .Aviation...
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[Through 18Z Friday] VFR cigs will gradually descend to MVFR levels after midnight, as light rain begins. As the rain increases to moderate levels early Friday morning, cigs will reach IFR levels, and remain at IFR through much of the day.
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&& .Marine...
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The strong frontal system referenced above will likely result in frequent gale force gust tomorrow over our Gulf waters. Sustained winds will likely be solidly in advisory levels. After a brief break in the gusts on Friday night another round of solid advisory conditions are expected on Saturday in the wake of a cold front. Thereafter, headline conditions are expected to continue through mid-week with occasional spikes back to advisory levels possible.
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&& .Fire Weather...
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Even though the atmosphere will gradually dry out after a total washout and Wet Flag conditions on Friday, conditions will remain quite unfavorable for Red Flag conditions for the next several days.
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&& .Hydrology...
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The upcoming system for tonight and Friday is likely to produce a large area of 1.5-3" of rainfall with locally higher amounts. This will cause rivers across the area to rise, but most will likely remain below flood stage. Currently, the most vulnerable areas to minor flooding appear to be the Kinchafoonee near Dawson and the upper Withlacoochee near Valdosta. Ensemble forecasts show the potential for these sites to rise to minor flood stage. The highest rainfall is currently forecast along the Ochlockonee, so sites there may also be more vulnerable to minor flooding.
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&& .Preliminary Point Temps/PoPs...
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Tallahassee 57 67 52 56 36 / 80 100 80 20 0 Panama City 58 68 50 55 42 / 90 100 50 10 0 Dothan 51 65 45 53 35 / 90 100 50 10 0 Albany 51 64 49 54 34 / 90 100 80 20 0 Valdosta 55 68 55 57 36 / 80 100 80 20 0 Cross City 59 72 60 61 37 / 60 100 80 30 0 Apalachicola 61 68 54 58 42 / 70 100 70 10 0
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&& .TAE Watches/Warnings/Advisories...
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FL...None. GA...None. AL...None. GM...Small Craft Advisory from 7 PM Friday to 7 PM EST Saturday for Apalachee Bay-Coastal waters From Ochlockonee River to Apalachicola FL out to 20 NM-Coastal waters from Suwannee River to Keaton Beach FL out 20 NM-Coastal waters from Apalachicola to Destin FL out 20 NM-Waters from Suwannee River to Apalachicola FL from 20 to 60 NM-Waters from Apalachicola to Destin FL from 20 to 60 NM. Gale Warning until 7 PM EST Friday for Apalachee Bay-Coastal waters From Ochlockonee River to Apalachicola FL out to 20 NM-Coastal waters from Suwannee River to Keaton Beach FL out 20 NM-Coastal waters from Apalachicola to Destin FL out 20 NM-Waters from Suwannee River to Apalachicola FL from 20 to 60 NM-Waters from Apalachicola to Destin FL from 20 to 60 NM.
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&& $$ NEAR TERM...FOURNIER SHORT TERM...HARRIGAN LONG TERM...HARRIGAN AVIATION...FOURNIER MARINE...HARRIGAN FIRE WEATHER...FOURNIER HYDROLOGY...MOORE

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