Area Forecast Discussion
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114 FXUS62 KTAE 131939 AFDTAE AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 339 PM EDT Sun Apr 13 2014 .Near Term [through Tonight]...
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As the influence of the ridge of surface high pressure that has been in place across our area continues to be eroded, boundary layer flow should increasingly veer to a S-SE direction. This should enable greater low-level moisture to be advected N-NWwd around the perimeter of the high pressure. 0-1km mixing ratios are projected to increase from around 9 g/kg at 18z this afternoon, to around 12 g/kg by 12z Monday (based on a consensus of NAM, GFS, and ECMWF). Most model guidance shows a corresponding surge of higher low-level RH from the S-SE overnight tonight. This will lead to an increase in cloud cover - stratus building in mainly after 06z, combined with cirrus. The greater cloud cover should lead to above normal low temperatures tonight in the lower 60s. The cloud layers and winds around 4-5 knots overnight may prevent much fog formation, but we still included a mention of patchy fog. NAM forecast soundings show some moderate vertical motion fields in the low stratus layer as it arrives, so some sprinkles or drizzle can`t be ruled out. However, that is really the only model that indicates that, so we have opted not to include any mention of RW- or DZ at this time.
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&& .Short Term [Monday Through Tuesday Night]...
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The recent stretch of fair and seasonably warm weather that we have experienced during the past several days will be coming to an abrupt end by Monday night and Tuesday, as the next system in a series of shortwaves digs SE towards the NE Gulf Coast. This upper level system is expected to develop a weak Sfc low which will race northeastward towards NY and New England on Tuesday. This low will drag a fairly strong cold front through the CWA on Tuesday and clear the region on Tuesday night, with much cooler and drier air to follow. Out ahead of the cold front, there will be some threat for severe weather, mainly across SE AL and western portions of the FL Panhandle where the Storm Prediction Center has placed its eastward extent of a Slight Risk. However, both wind shear and instability appear to be more limited for this event than the more significant one last week, and this is becoming apparent in many of the Hi-res mesoscale model runs which are now coming into temporal range for our region. Nevertheless, still have a period of Isolated Svr Storms in the grids in and around the Slight Risk area, with potential damaging wind gusts being the primary threat. As for rainfall amounts, which clearly caused the most widespread problems with the previous system, the prognosticated situation continues to gradually improve. While bands of heavy rainfall are still likely with this developing low, it now appears that the models are in good agreement that it will likely be too progressive to be an extreme rainfall producer for our area, with the heavier rain bands propagating very quickly to the east. Nevertheless, parts of SE AL could still see 1.5" to 2.0" of storm total rainfall (with locally higher amounts), but most areas across the CWA should generally see between 0.75" and 1.5" of rain before all is said and done. See the Hydrology section below for potential riverine impacts. Finally, with this upper level shortwave digging so far to the south for this time of year, it will be accompanied by a quick burst of some unseasonably cold air. Temps on Tuesday will likely fall from west to east during the day, and lows on Tuesday night could reach the mid to upper 30s over the interior. .Long Term [Wednesday Through Sunday]... Given the latest more progressive Global model runs where the ECMWF has come into very good agreement with the GFS, the overall fcst confidence has increased substantially across our CWA. This should result in a quick hitting, but quick exiting low pressure system as the very progressive upper level synoptic pattern should have swept any lingering showers or storms well off to our east by the beginning of the period. In fact, this progression now appears to be rapid enough to allow plenty of the cooler and drier air to become quickly established over much of the CWA by Wednesday morning, with the majority of the numerical and raw model guidance now showing low temperatures dipping down into the mid to upper 30s across the western half of the region. A mostly sunny and unseasonably cool day will follow on Wednesday, with a gradual warming trend expected for the end of the week and into next weekend. As for rain chances by the latter part of the period, the models have begun to diverge significantly once again, with the GFS still indicating the potential for a possible two-pronged rainfall event, while the ECMWF shows a split in the jet stream flow, which would allow one shortwave to pass to our north and another to pass to our south. This would leave most of the CWA protected with fair and seasonable conditions, but it is clearly too far in advance to buy solely into one of these solutions. Therefore, will plan on a compromised resultant for the end of the period, using our statistically verified blend of GFS/ECMWF/HPC guidance which has shown to be quite difficult to beat over the long haul.
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&& .Aviation...
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[through 18z Monday] VFR conditions with S-SE winds will prevail today. Lower CIGS may build in overnight from the south or southeast, particularly at TLH and VLD. MVFR CIGS seem most likely, with IFR possible especially at those two terminals. A gradual improvement in ceilings is expected on Monday morning. Some patchy fog is possible.
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&& .Marine...
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Winds and seas will begin to gradually increase out of the south then southwest ahead of the approaching cold front on Monday night into Tuesday reaching cautionary levels. After the cold front passes through later from west to east on Tuesday, winds will quickly increase out of the northwest to Small Craft Advisory levels over the waters, with headline conditions lingering into Wednesday morning. Furthermore, only a brief respite in the elevated winds and seas is expected, as a tight pressure pattern is expected to keep conditions fairly difficult for small craft for much of the week.
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&& .Fire Weather...
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Relative humidity levels will remain above critical thresholds for the next several afternoons. The next wetting rains should arrive Monday Night and Tuesday.
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&& .Hydrology...
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While a few of our area rivers remain in minor flood stage (except for the Choctawhatchee river near Bruce which is at moderate flood levels and will likely remain quite elevated for several days), most have crested and begun to slowly recede. However, yet another batch of rainfall on Monday night into Tuesday may cause some of these rivers to return to minor flood stage. The good news is that the models continue to come into better agreement that the low pressure system which will be responsible for the coming rains will be very progressive. Therefore, with the rain expected to come to an end by late Tuesday afternoon or evening, now expect lower storm total rainfall amounts across the CWA. These should generally range from 0.75" over the SE FL Big Bend to generally 1.0" to 1.5" elsewhere. A few locations over SE AL could still see closer to 2.0" before all is said and done, so expect any concerns caused by this system to be minor. The most up to date, specific river forecast information can be found on the AHPS page at http:/water.weather.gov/ahps2/index.php?wfo=tae.
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&& .Preliminary Point Temps/PoPs...
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Tallahassee 63 82 62 73 39 / 10 20 50 70 10 Panama City 65 77 62 65 44 / 10 30 70 70 10 Dothan 62 82 57 65 36 / 10 40 90 60 10 Albany 63 82 62 66 38 / 10 30 70 70 10 Valdosta 63 85 63 73 40 / 10 20 50 70 20 Cross City 63 85 63 76 42 / 10 20 30 70 30 Apalachicola 66 77 65 69 46 / 10 20 50 70 10
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&& .TAE Watches/Warnings/Advisories... FL...None. GA...None. AL...None. GM...None. && $$ NEAR TERM...LAMERS SHORT TERM...GOULD LONG TERM...GOULD AVIATION...LAMERS MARINE...GOULD FIRE WEATHER...LAMERS HYDROLOGY...GOULD

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