Area Forecast Discussion
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112 FXUS62 KTAE 220955 AFDTAE AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 455 AM EST Sat Nov 22 2014 ...Severe Thunderstorms Expected on Sunday... .Near Term [Through Today]...
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The water vapor imagery early this morning depicts a potent shortwave digging through the southwest states. Quite a few CG lightning strikes have been detected with this feature in northern Mexico, which is another sign that it`s a potent disturbance. Closer to home, a warm front will start to develop over the Gulf this afternoon with gradually increasing moisture and lift across the area. We expect light rain to gradually develop from south to north through the afternoon hours, primarily affecting the Florida big bend, but most of it is not expected until after 4 pm EST. Farther north and west into Alabama and Georgia, PoPs are lower with most of the rain expected to hold off until tonight. Amounts today will generally be on the order of a tenth of an inch or less. Highs are expected to range from the mid 60s across the far north to the lower 70s across the southeast big bend. .Short Term [Tonight Through Sunday]... Confidence has increased in a severe weather event for our forecast area on Sunday - particularly over the western half of our forecast area (southeast Alabama and the western Florida Panhandle). The Storm Prediction Center has highlighted those areas in an "Enhanced Risk" of severe weather on their latest Day 2 Convective Outlook. Prior to the severe weather risk, we should have a round of rain (and some embedded, elevated storms). This will occur primarily from late this afternoon, as discussed above, into tonight and will be forced by low-level WAA and isentropic ascent to the north of the surface warm front over the Gulf. There is not much instability, even for elevated parcels, so the sensible weather was worded more for a steady rain with the possibility of isolated storms. We are forecasting an areal average of around 1" of QPF overnight, although some locally higher totals are possible. There is fairly good model consensus on clearing much of this rain north of our area quickly between 12Z and 15Z Sunday as the surface warm front surges inland in response to rapid deepening of the surface low in the Mid Mississippi River Valley. Some of the global models generate some convection near the coast in the morning, but we are leaning towards a brief clearing at this point given that the strongest low-level forcing will be displaced north, with mid-upper level forcing yet to arrive from the west. The potent shortwave / PV anomaly referenced in the near term discussion will eject quickly east-northeast from coastal Texas tonight, to central and northern Georgia by Sunday evening. Both global models and convection-allowing models indicate vigorous convection along this lobe of strong forcing, and the timing is relatively similar across the board. Therefore, we expect that there will be some sort of QLCS in progress near coastal Louisiana and into the NW Gulf of Mexico by daybreak Sunday, and that should quickly translate east into our area by early afternoon. While some CAMs show a continuous line of storms, others indicate mixed modes of line segments and supercells. Given the strong forcing associated with the ejecting wave, we anticipate primarily linear modes, but some fore-running supercells, line mesovortices, or broken line segments will also be possible. Given the model consensus timing, the primary severe weather risk should be between about 18Z Sunday and 03Z Monday. Probabilistic guidance, as well as the SPC forecast, indicate the greatest threat of severe weather will be over the western half of our forecast area. The environment will be favorable for both damaging winds and tornadoes. Strong low-mid level wind fields are expected with 850mb winds around 50 knots and 700mb winds around 60 knots. These should help facilitate fast storm motions, and downward momentum transfer of some of the stronger winds supports the damaging wind threat. Meanwhile, by 18Z Sunday an average of the NAM, GFS, and ECMWF show MLCAPE of 600 j/kg, 0-1km SRH of 190, and 0-6km shear of 55 knots - certainly a favorable environment for supercell thunderstorms and a tornado risk. The threat of tornadoes would be higher if discrete cells can form. However, there will be a significant component of the 0-3km (low-mid level) shear vector normal to the expected line of storms - likely around 40 knots or so. Such values tend to correspond to an increased risk of tornadoes and enhanced wind damage from QLCS mesovortices. To summarize: the primary threats will be damaging winds and tornadoes, and we think the severe weather could be a little more widespread and/or significant than a typical wintertime event. .Long Term [Sunday Night Through Friday Night]... Although the storms are likely to exit quickly Sunday evening, the synoptic cold front (and upper level longwave trough axis) will still be positioned well west of the area. This should allow for continued showers in the east/southeast parts of our forecast area through Tuesday. As the trough begins to push east on Tuesday Night and Wednesday, a shortwave rounding the base of the trough could lead to coastal low development along the cold front off the southeast Atlantic coast. This may lead to one final round of rain over the area on Tuesday Night and Wednesday before we finally dry out on Wednesday Night. Cooler temperatures will arrive on Tuesday and should linger through the end of the work week.
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&& .Aviation...
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[Through 12z Sunday] VFR conditions will prevail through the early afternoon hours, but as low level moisture continues to increase and a warm front begins to lift northward from the Gulf, ceilings are expected to lower to MVFR levels this afternoon from south to north with some showers beginning to stream northward from the Gulf, possibly affecting ECP, TLH, and VLD before sunset. After sunset, rain is expected to overspread the area with ceilings continuing to lower to IFR.
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&& .Marine...
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Buoy and offshore tower observations support the current Small Craft Advisory, and easterly winds of 20-25 knots are expected to persist through this evening over most of the coastal waters. The winds will begin to veer to the south late tonight and into Sunday, with 20-25 knots spreading all the way to the Apalachee Bay and Big Bend coastline. The majority of model guidance is indicating gale force gusts later tonight and on Sunday, with some models as high as 40 knots or so. Therefore, we have issued a Gale Warning for all but the far eastern legs beginning 06Z tonight. The latest extratropical surge guidance, based on the 00Z GFS winds (which seem reasonable), has reduced surge forecasts on Sunday morning in Apalachee Bay. Flooding is not expected, but water levels may be higher than usual following the early morning high tide. High surf is expected on Sunday, though, with our current forecast calling for 7 foot surf. A High Surf Advisory will likely be issued later today for our beaches.
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&& .Fire Weather...
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Rain will overspread the area from south to north starting late in the day today through Sunday with wet flags likely on Sunday.
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&& .Hydrology...
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Storm total rainfall amounts Saturday through Tuesday will be on the order of 2 to 3 inches, with most of the rainfall coming over a 24 hour period from Saturday Night into Sunday. Low river levels will likely prevent any river flooding even with these high totals. Some localized flooding will be possible in the more urbanized locations that receive higher rain totals over a short period of time, but the threat of flash flooding does not appear to be sufficiently high to warrant a Flash Flood Watch.
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&& .Preliminary Point Temps/PoPs...
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Tallahassee 70 61 74 63 77 / 60 90 70 40 40 Panama City 67 64 74 66 73 / 40 90 70 30 20 Dothan 67 58 73 61 74 / 30 90 90 20 20 Albany 67 58 72 62 75 / 30 90 90 40 30 Valdosta 69 60 75 63 76 / 50 90 90 40 50 Cross City 72 64 77 64 76 / 70 90 80 40 50 Apalachicola 70 65 74 67 75 / 60 90 60 30 30
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&& .TAE Watches/Warnings/Advisories...
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FL...High Risk of Rip Currents for coastal parts of Bay, Franklin, Gulf, and Walton Counties from 1 PM EST this afternoon through late Sunday Night. GA...None. AL...None. GM...Small Craft Advisory from 1 AM to 7 PM EST Sunday for Apalachee Bay-Coastal waters from Suwannee River to Keaton Beach FL out 20 NM. Small Craft Advisory until 1 AM EST Sunday for Coastal waters From Ochlockonee River to Apalachicola FL out to 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 from 1 AM to 7 PM EST Sunday for Coastal waters From Ochlockonee River to Apalachicola FL out to 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...DVD SHORT TERM...LAMERS LONG TERM...LAMERS AVIATION...DVD MARINE...LAMERS FIRE WEATHER...DVD HYDROLOGY...LAMERS

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