Area Forecast Discussion
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666 FXUS62 KTAE 212054 AFDTAE AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 354 PM EST Sun Dec 21 2014 ...Heavy Rain and Severe Weather Possible from Tuesday through Midday Wednesday... .Near Term [Through Tonight]...
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Water vapor imagery this afternoon shows a subtropical plume of mid and upper level moisture streaming across the area. This plume can be traced back to the eastern Pacific in association with the subtropical jet. It appears as though there are weak disturbances embedded in this plume as shown by areas of thunderstorms developing in the Gulf. There is also a weak frontal boundary stretched across the Gulf, separating a cool and moist air mass to the north from a more tropical air mass to the south. Across the local area, we are in the cool and moist air mass with low clouds and patches of rain across the area. This trend will continue through the overnight hours with hi-res models focusing the most activity across the southern and eastern half of the area. Some of the hi-res guidance continues to show the potential for embedded thunderstorms with heavier rain later tonight across the southeast big bend, so heavy rain wording was continued in that area. Temperatures will be rather stagnant tonight with all of the clouds around. .Short Term [Monday Through Tuesday Night]... A prolonged period of isentropic ascent will continue through this period over the frontal boundary as it slowly works its way northward across the region on Monday and Tuesday. By late Monday afternoon, parts of the FL Big Bend could already be in the warm sector with dew points in the mid 60s. With an increasingly sheared environment already in place, a few isolated strong to severe supercells cannot be ruled out. This threat will continue Monday night. Greater destabilization could occur on Tuesday as the warm front moves further inland and dew points reach the upper 60s. Again, we would expect development of discrete convective cells. With low level helicity enhanced near the warm front, isolated supercell tornadoes would be the primary threat through Tuesday. As a 130-150 kt upper level jet streak enters the base of the Great Plains trough, deep layer QC forcing will increase downstream over the eastern Gulf Coast region as a low level jet strengthens. The primary storm mode will evolve toward a pre- frontal squall line Tuesday night into Wednesday with QLCS tornadoes and damaging straight line winds possible with this line. Another concern we have been monitoring is the potential for heavy rain. Confidence is increasing that we will see periods of heavy rain, particularly from Tuesday afternoon into Wednesday as PW approaches 2 inches, near the December record for TLH. See the hydrologic discussion below for details on any flood threats. .Long Term [Wednesday Through Sunday]... As alluded to above, the threat for severe weather and flooding will continue into Wednesday. The timing trend for the cold frontal passage has been slowing down for the most part in the various models and the boundary may not be cleared east of the forecast area until closer to 00Z Thursday. Noticeably cooler and drier air will arrive behind the front. Overnight temps will remain above freezing, but the daytime high on Christmas day will be chilly, generally in the upper 50s. Near normal temperatures are then expected through Saturday night. There a big difference in the models with the timing of the next frontal system. The GFS brings a dry front quickly through the area on Saturday, while the latest Euro develops a wave on the front, pulls much more moisture into the system, and doesn`t clear the boundary east of the area until midday Sunday.
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&& .Aviation...
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[Through 18Z Monday] Poor conditions will continue through the next 24 hours with ceilings generally in the IFR to LIFR range and visibilities dropping into the IFR range along with areas of rain. A stalled frontal boundary south of the area will begin to lift northward on Monday with low ceilings and areas of rain continuing.
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&& .Marine...
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Light to moderate northeast winds will become onshore by Monday, increase to cautionary levels on Tuesday and then reach advisory levels Tuesday night. A shift to westerly winds will occur after a cold frontal passage on Wednesday. Strong to severe storms could precede the front. Winds will then veer to the northwest and drop below headline criteria Thursday morning. Light onshore winds are forecast for later in the week.
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&& .Fire Weather...
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Low level moisture will be abundant over the next few days with drier air moving in on Wednesday. Red flag conditions are not expected over the next week.
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&& .Hydrology...
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While there is still some uncertainty with respect to the exact placement of heaviest rainfall with the storm system on Tuesday and Wednesday, confidence is increasing that there will be a period of significant rainfall occurring across the region. At this point, highest storm total accumulations of 4 to 6 inches are forecast for the FL Big Bend and South Central GA with 2 to 3 inches across the northwestern zones. The bulk of this rainfall will occur from Tuesday through Wednesday morning. Locally higher totals will be possible. Rainfall totals of this magnitude will increase the potential for localized flooding across the region, particularly during times of heaviest rainfall on Tuesday Night into Wednesday Morning. With area rivers and associated tributaries at or below normal flows for December, the currently predicted rainfall amounts are generally expected to produce only rises to action stage on most of the river forecast points. The notable exceptions to this may be within the Ochlockonee and Withlacoochee River Basins where there`s increased potential to reach minor flood stage later this week.
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&& .Preliminary Point Temps/PoPs...
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Tallahassee 55 71 61 72 63 / 80 60 50 100 100 Panama City 56 71 64 72 64 / 70 50 50 100 100 Dothan 50 66 59 71 61 / 50 50 50 100 100 Albany 48 64 56 72 62 / 70 60 50 100 100 Valdosta 53 68 60 73 63 / 100 70 50 90 100 Cross City 60 74 62 76 65 / 100 70 40 70 100 Apalachicola 58 70 64 71 65 / 80 60 50 90 100
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&& .TAE Watches/Warnings/Advisories... FL...None. GA...None. AL...None. GM...None. && $$ NEAR TERM...DVD SHORT TERM...WOOL LONG TERM...WOOL AVIATION...DVD MARINE...WOOL FIRE WEATHER...MCDERMOTT HYDROLOGY...WOOL/GODSEY

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