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Area Forecast Discussion
345 FXUS64 KHUN 311549 AAB AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 1049 AM CDT WED AUG 31 2016 .UPDATE... Morning update. && .DISCUSSION... The mid-level cloud deck that stretched from NE AL down through Cullman county overnight is gone, allowing temps to warm up into the mid 80s as of the 15Z sfc obs. The warming trend thus far today is a few degrees ahead of yesterday, however additional cloud cover along with afternoon convection is expected. Afternoon temps will reach into the low/mid 90s with heat index values around 100 degrees. The combination of a prefrontal trough and the convergence axis that produced the previously mentioned cloud cover, could produce isolated to scattered and thunderstorms this afternoon. Locations in NE AL and southern middle TN along the terrain have the highest coverage. Slightly adjusted the POPs heading into this afternoon, otherwise no major changes were made to today`s forecast. JMS && .AVIATION... (Issued 618 AM CDT WED AUG 31 2016) For 12Z TAFs: VFR conditions are forecast through the period. An isolated shower or thunderstorm is possible this afternoon and evening, but the probability is too low to include in the TAFs at this time. Surface flow will become northwest to north today and turn northeast tonight. && .PREV DISCUSSION... (Issued 306 AM CDT WED AUG 31 2016) As of 08Z latest surface obs, IR satellite and SPoRT Nighttime RGB product indicate some mid- level clouds developing across the eastern half of the area. These are forming along a convergence axis that extends from NE TN W/SW towards central MS and is a result of a low pressure over east- central GA/western SC and high pressure to our west. Adjusted sky grids to account for these but believe that these clouds should dissipate towards sunrise as forecast soundings suggest. For today, as the low pressure system south of Hudson Bay drops S/SE through the day it will push a weak pre-frontal trough that is currently over the OH River Valley south through the region. The drier air in place yesterday continues to linger over the area but should be pushed back to the east as the trough moves in. The lift associated with this boundary may interact with the weak mid-level convergence axis to spark isolated to scattered showers with a few embedded thunderstorms. The best chances for convection today will be across the higher terrain of NE AL and southern middle TN. Activity should quickly taper off towards sunset today leaving mostly clear skies tonight. The actual "cold" front is not forecast to push through the area until Thursday afternoon. Models continue to be underwhelming when it comes to precip chances associated with the front. The best upper level support remains to our NE and while we do see an increase in total moisture through the column, lapse rates aloft remain virtually unchanged as the front rolls through. Cloud cover is expected to increase throughout the day, which will limit daytime heating and keep temps from reaching much of the 90 degree mark. There is an increase in 0-6km shear Thursday afternoon that may allow for an isolated strong storm but given the factors mentioned above believe that this will be few and far between. Made a few minor tweaks to the PoPs for Thursday with the best chances again forecast to be across the higher terrain of NE AL and southern middle TN. Outside of precip chances, Thursday is expected to be a bit breezy. There will be two factors that should lead to a tightening of the pressure gradient over the area. The first being the front moving through and the other is what is currently Tropical Depression 9. All guidance is indicating TD9 lifting north out of the central Gulf of Mexico Wednesday into Thursday and moving across northern FL/S GA Thursday into Friday. While any tropical related increase in winds will be well to our SE, the pressure differences between this system and high pressure building in behind the front will lead to an increase in the gradient over the area. Nudged up wind speeds to 15-20mph with gusts up to 25 mph both Thursday and Friday. Drier air will quickly rush in behind the front with dewpoints dropping 10-12 degrees from Thursday afternoon to Friday morning. This will provide some relief from the heat and humidity as we head into the Labor Day weekend. One interesting thing to note about Fri into Saturday that continues to show up in the guidance is as that as the trough moves through the East coast and begins to interact with TD9 a piece of energy splits from the trough. This drops into the southern Appalachians Friday evening into Saturday morning. Some of the guidance wants to try and develop a shower or two on Saturday and Sunday over the higher terrain to our E/NE as the piece of energy moves through. Think this is a bit overdone due the amount of dry air advecting into the area and if anything does form it will be well to our east. Temperatures through the Labor Day weekend will be pleasant with afternoon highs in the mid to upper 80s and overnight lows in the low to mid 60s. A few of the cooler spots should see lows drop into the upper 50s Saturday morning. Unfortunately, the heat and humidity quickly return early next week as an upper level ridge builds over the area. Stumpf && .HUN Watches/Warnings/Advisories... AL...NONE. TN...NONE. && $$ For more information please visit our website at weather.gov/huntsville.