Area Forecast Discussion
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309 FXUS62 KTAE 210916 AFDTAE AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE TALLAHASSEE FL 416 AM EST Fri Nov 21 2014 .Near Term [Through Today]...
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Surface high pressure will begin to move off the Atlantic coast with easterly surface winds developing. It will remain dry today with mostly sunny skies expected and afternoon highs just a couple of degrees below average in the mid 60s to near 70 across the area. .Short Term [Tonight Through Sunday]... The catalyst for our changes in the weather along the Gulf coast in the short term period is a potent mid-upper level shortwave trough evident in early morning water vapor loops over southern California. Model analyses indicate the dynamic tropopause (PV1.5) is down around 500mb associated with the PV anomaly, and there was even a few CG lightning strikes noted near the San Francisco Bay area. All of these are indicators of a strong wave, and models are in good agreement with digging it ESE to central Texas by Saturday afternoon. The consensus then has the wave ejecting ENE with a negative tilt into the Deep South on Sunday. The strength of the ejecting shortwave (500mb height anomalies as low as 2 to 2.5 std deviations below normal LA-AL) will lead to a strengthening low-mid level wind field on Sunday, with 850-700mb winds above the 95th percentile for this time of year. Those increasing wind fields will lead to strong northward moisture flux. When combined with overall strong QG forcing and above normal precipitable water values (+2 std deviations), the rain chances are quite high from Saturday Night into Sunday. It appears as though an area of rain, and perhaps embedded elevated thunderstorms, will take shape over the northern Gulf on Saturday Night in the low-level WAA / isentropic ascent regime north of a surface warm front. Model forecast soundings indicate very little instability with a moist adiabatic profile through much of the troposphere and a shallow inversion near the surface. This should limit coverage and intensity of thunderstorms initially, but moderate to occasionally heavy rain appears likely. As the surface warm front surges north on Sunday morning in response to strong surface pressure falls from the Ozarks into the Deep South, our forecast area should enter the warm sector. Most model guidance builds MLCAPE over 500 j/kg over at least part of the area (some close to 1000 j/kg) coincident with about 50-60 knots of deep layer shear and 30-35 knots of 0-1km shear. This is a favorable environment for severe thunderstorms, and a second round of convection is expected to develop. The latest SPC Day 3 Convective Outlook places a Slight Risk across our entire area. .Long Term [Sunday Night Through Friday]... Most models agree that a lagging mid-upper level trough axis over the Plains will keep at least moderate QG forcing in place over our forecast area through Tuesday. Therefore, rain chances were maintained in the forecast through that time, although we removed the mention of thunder as instability should be lower by that time. Cooler temperatures are expected on Tuesday and Wednesday before slow moderation toward the end of the week.
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&& .Aviation...
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[Through 12z Saturday] VFR conditions are expected to prevail through the period with ENE winds just under 10 knots developing during the mid-morning hours.
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&& .Marine...
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We have issued a Small Craft Advisory, except for the far eastern nearshore legs, valid through the weekend. Winds should gradually increase today and could hit 20 knots offshore by this afternoon. For that reason the advisory was started at 18Z. Easterly winds of 20-25 knots will persist through Saturday Night before veering to the south on Sunday. It will be around that time of the wind shift that winds should finally begin to increase in the far eastern legs. So SCEC headlines and then an advisory are likely at some point for those areas. Model wind gust guidance indicates some gusts to near gale force will be possible over the coastal waters from late Saturday Night into Sunday, but as of right now it seems too limited in scope to warrant any gale headlines. Related to the increasingly strong onshore flow on Sunday, the current forecast also calls for surf heights at area beaches to peak in the 6-7 foot range which would warrant a High Surf Advisory. The extratropical surge guidance, based on the GFS model, does indicate an increase in water levels in Apalachee Bay on Sunday morning to action stage (just shy of flood levels). However, (1) the GFS model may be overestimating surface winds at that time -AND- (2) any surge appears likely to happen after high tide which would reduce the possible water levels. This will be monitored as the event approaches as specific timing and wind speeds will make a big difference.
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&& .Fire Weather...
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Low level moisture will gradually increase starting today with widespread rain overspreading the region Saturday night through Sunday.
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&& .Hydrology...
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Moderate to heavy rainfall looks likely from Saturday Night into Sunday, with some rain possibly persisting into Monday and Tuesday. Rainfall totals from Saturday into early next week look to be mainly between 2.5 and 3.5 inches, on average. This should not be sufficient to create widespread flooding given current low flows on area rivers. Still, some minor flooding on a few of the rivers cannot be ruled out if a more concentrated area of heavier rainfall were to fall in a large part of a drainage basin. In urbanized areas, minor flooding problems will also be possible.
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&& .Preliminary Point Temps/PoPs...
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Tallahassee 69 44 70 63 73 / 0 0 20 90 90 Panama City 66 50 69 65 72 / 0 0 30 90 80 Dothan 67 43 67 60 73 / 0 0 20 90 80 Albany 66 41 67 60 72 / 0 0 10 90 90 Valdosta 66 45 70 62 73 / 0 10 10 90 90 Cross City 70 46 75 65 74 / 0 10 20 90 80 Apalachicola 68 51 69 66 73 / 0 0 30 90 80
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&& .TAE Watches/Warnings/Advisories...
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FL...None. GA...None. AL...None. GM...Small Craft Advisory from 1 PM this afternoon to 7 PM EST Sunday for Coastal waters from Ochlockonee River to Destin FL out 60 NM.
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&& $$ NEAR TERM...DVD SHORT TERM...LAMERS LONG TERM...LAMERS AVIATION...DVD MARINE...LAMERS FIRE WEATHER...DVD HYDROLOGY...LAMERS/GODSEY

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