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Area Forecast Discussion
684 FXUS64 KHUN 280822 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 322 AM CDT THU JUL 28 2016 .DISCUSSION... Latest water vapor imagery shows a subtropical high off the southeast Atlantic Coast and a stronger high over the Desert Southwest. Meanwhile, a weak trough axis extended through the Mississippi Valley region. At the surface, a weak area of low pressure extended across the southern MS Valley, with a weak boundary trailing eastward and southward from this low. An area of showers continues to slowly push to the northeast across western TN and northern MS in response to a vorticity max in the base of the longwave trough. Much of the forecast area remains under the influence of the subtropical high, with slightly drier air noted across the TN Valley. This will change as we move into the morning hours, as the axis of the trough shifts east, as does the upper ridge. Though for the next few hours, a few light showers may develop, with cloud cover will keep temps from dropping too much through sunrise. The ongoing showers to our west are expected to enter west Alabama toward the mid to late morning hours, as the trough axis shifts northeastward. Although cloud cover will limit the daytime heating, strong southerly flow will continue to push warm and moist Gulf air into the area. Forecast soundings show PW values rising from 1.8 to 2.2 inches by the afternoon. Meanwhile, forecast CAPE values will rise to near 1000 j/kg. These factors, combined with the enhanced lift from the upper trough, will lead to a greater coverage of thunderstorms than we have previously seen. However, confidence is still somewhat low and therefore will keep PoPs to the likely category. Locally heavy rainfall will be possible, though the dry soil moisture will limit the likelihood of flash flooding, unless precip rates are high. Max temperatures will greatly depend on the coverage of thunderstorms, though highs will be a few degrees below those observed yesterday due to the cloud cover expected. The upper trough will weaken by sunset, with the main piece of energy shifting to the east. The coverage and intensity of lingering showers and thunderstorms will become limited and will likely diminish entirely by midnight. The forecast area will remain under a broad and weak upper level trough through much of the weekend, as the subtropical ridge moves well south of the area. Several disturbances are expected to develop and move through the longwave trough, which will provide for scattered thunderstorms, generally in the afternoon through early evenings. Much of the forecast will depend on mesoscale features, as the synoptic lift will generally remain weak. Have maintained the 50 PoP in for Friday through Sunday, with lower rain chances during the overnight periods. The upper level high over the desert SW will strengthen over the weekend, and push eastward during the day Monday. The broad upper trough over much of the eastern CONUS will strengthen slightly in response to the upper ridge. This will put the forecast area in weak northwest flow aloft Sunday night and Monday, with guidance showing a vort max developing over the upper midwest and moving southeast into the area. However, given the uncertainty on day 5, will maintain our 40 PoPs in for this period. The remainder of the forecast will be dominated by the upper level ridge, with rain chances depending on mesoscale interactions. 73 && .AVIATION... (Issued 1157 PM CDT WED JUL 27 2016) For 06Z TAFs: Ceilings are expected to oscillate btwn VFR and MVFR over the next 24 hours as a disturbance over the AR/MS/TN region moves northeast affecting N AL/S Middle TN later tonight into tomorrow. Low MVFR cigs will be the first to move over the TN valley especially close to KMSL. Have included SCT prevailing for the next 8 hours but will likely see BKN025 at KMSL. Also could see this drop in cloud cover at KHSV but not as sure on timing. Then, -shra should begin sometime btwn 28/14Z-18Z and increase in coverage throughout the day with drops to IFR possible. Have included prevailing MVFR starting after 28/17Z through the rest of the forecast period for the reduced VIS likely with any shra. In addition, could also see -TSRA btwn 28/19Z-28/24Z which would act to reduce the flight category to IFR. SL.77 && .PRELIMINARY POINT TEMPS/POPS... Huntsville 86 71 89 71 / 60 50 50 40 Shoals 83 71 89 71 / 60 50 50 40 Vinemont 84 70 87 70 / 60 50 50 40 Fayetteville 83 70 86 70 / 60 50 50 40 Albertville 86 70 88 70 / 50 50 50 40 Fort Payne 86 70 87 70 / 50 50 50 40 && .HUN Watches/Warnings/Advisories... AL...NONE. TN...NONE. && $$ For more information please visit our website at weather.gov/huntsville.