Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
940 FXUS64 KHUN 141458 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 958 AM CDT Sat Jul 14 2018 .NEAR TERM...(Rest of Today) Issued at 958 AM CDT Sat Jul 14 2018 An upper-ridge will remain almost directly overhead across North Central Alabama this afternoon. Morning fog has lifted, but there's plenty of moisture to go around underneath this ridge as seen by the lingering low to mid cloud cover on visible/IR satellite. These clouds make for a slightly more complicated forecast as they may impact how quickly temperatures "jump" into the lower 90s by midday. The very moist air mass is still in place and models have continued to underestimate dewpoints as readings in the mid 70s to upper 70s are common across much of the region. With little wind/flow to speak of, do not think there will be much air mass modification. Therefore, even if we "mix out" to dewpoints of 73-76 degrees this afternoon, it will only take low 90s highs to reach Heat Advisory criteria in many locations -- especially in Northwest Alabama and the HSV-DCU Metro. Where as yesterday an expansion of the Heat Advisory was essentually a sure thing, today is very much forecast dependent on what clouds do and how high temperatures can climb. However, do feel an expansion is warranted for two reasons. First, the very oppressive tropical- like air mass in place will make it easier for HIs of 105 or greater to be reached once heating gets going. Secondly, while there should be slightly better coverage of showers/storms, the impact of subsidence underneath the high pressure ridge will limit their coverage and thus keep the local boundary layer undisturbed. Thus, will expand the Advisory to the same area as yesterday and include Lawrence, Cullman, Limestone, Morgan, and Madison Counties. Given the latest temperature trends will start the Advisory an hour earlier at 11 AM CDT. Speaking of storms, once some breaks in the clouds develop, the latest HiRes guidance continues to show widely scattered pulse convection developing this afternoon. Latest HRRR indicates around 2500 J/Kg of SBCAPE and 1500 J/Kg of MLCAPE to work with, but virtually no shear. A strong storm is possible, but this will be the exception. Lightning, however, will continue to be a danger with any storm that develops. If you hear thunder or see lightning -- move indoors! .SHORT TERM...(Tonight through Monday) Issued at 322 AM CDT Sat Jul 14 2018 A loss of daytime heating/instability should help dissipate coverage of showers/storms tonight. Locations that receive a "good" wetting rain would be more apt to be affected by fog late Saturday night into Sunday morning. Muggy conditions otherwise are expected, with low temperatures cooling only into the lower/mid 70s. An almost repeat performance as far as warmth and rain chances to start a new week during Sunday. Say almost, as high temperatures should trend down another degree or two, which effectively lessens a risk of heat advisory conditions. That being said, heat indices will still range from the upper 80s to 104 degrees. Somewhat higher rain chances are expected on Sunday, and especially during Monday, as lift preceding an approaching frontal boundary is realized. Model precip water forecasts ranging in the 1.7" range today, will increase into the 1.9" on Sunday and over 2" by Monday. Model soundings, especially on Sun/Mon both show an inverted-V look; which could be indicative of a microburst potential (especially on Mon with higher instability and moisture values). Thunderstorms in these high moisture regimes can become prolific lightning producers as well. .LONG TERM...(Monday night through Friday) Issued at 322 AM CDT Sat Jul 14 2018 An upper trough well into the Canadian provinces will start to drag a cold front through the area Tuesday morning with the front through most of the area by the late afternoon. The mid level trough is more pronounced than the 500mb trough and will provide the synoptic scale lift for the front. Even so, there is very little support for this front outside the convergence along the surface feature. This will still be able to support numerous thunderstorms all day. Looking at GFS forecast soundings, nothing is catching my eye to support severe storms. The CAPE is high as usual, above 2500 J/KG but the DCAPE, Lapse Rates, WBZ/FZL heights are all marginal or less and there is little to no shear or helicity. PW values near 2 inches combined with the instability and lift could produce a strong storm or two but at this point, severe looks unlikely. Clouds and rain will cool highs a bit; in the mid to upper 80s. Dry air will be quick to move in Tuesday night with surface high pressure building in along with northerly winds. Storms should end from north to south shortly after midnight. With dew points dropping and skies clearing, lows will be in the upper 60s to around 70. Should see mostly sunny days Wed and Thu with high pressure firmly in control with dew points falling into the upper 60s. This seems high but when you compare it with the mid 70s of late, the upper 60s dew points will feel pleasant! But given that it is mid July and heights don't lower much, it's still going to be hot with highs around 88-91 degrees both Wed and Thu. Honestly, left the slight chance pops in the south for Wed, the east for Thu and most of the area Thu Night but think this is too high and we won't have much of anything. Lows will be the coolest Wed night, in the mid to upper 60s and warming to around 70 for Thu night. A trough starts to deepens aloft on Friday between two strong high pressure ridges, one over the Atlantic and one over the southwestern states. With dry air remaining at the mid levels, looks like the feature won't give us much precip till later Friday and mostly in the east. Both GFS/ECMWF are showing something forming along the Gulf Coast in response to an upper jet streak that could cut off our moisture as the winds turn southerly on Saturday. There are still some differences this far out so we'll wait and see as we get closer. With the clouds increasing at least, highs will be in the upper 80s both Fri/Sat. && .AVIATION...(For the 12Z TAFS through 12Z Sunday morning) Issued at 648 AM CDT Sat Jul 14 2018 Under high pressure at the surface and aloft, variable cloud coverage prevailed across the region. Patchy light fog that was affecting a few terminals should dissipate shortly after 12Z. Modest atmospheric moisture and strong daytime heating will result in the development of isolated to scattered showers and thunderstorms. Did not include precipitation given uncertain model placement and timing. Showers/t-storms if they occur, should be in the afternoon hours. Any showers that form will move slowly, and capable of producing locally heavy rainfall; gusty winds and frequent lightning. Shower activity should wind down this evening. More late night fog development is possible, especially in/near areas that are impacted by precipitation on Saturday. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...Heat Advisory until 6 PM CDT this evening for ALZ001>003. TN...NONE. && $$ NEAR TERM...AMP.24 SHORT TERM...RSB LONG TERM...LN AVIATION...RSB For more information please visit our website at weather.gov/huntsville.