Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
780 FXUS64 KHUN 181110 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 610 AM CDT Mon Jun 18 2018 .UPDATE... For 12Z TAFS. && .NEAR TERM...(Today) Issued at 217 AM CDT Mon Jun 18 2018 Latest water vapor imagery shows an upper low over the Pacific Northwest and a ridge encompassing much of the eastern US, with a weak upper low spinning from northwest Alabama and into northeast Mississippi. At the surface, winds have become light across most of the area, with temperatures in the upper 60s to lower 70s. As temperatures continue to fall by another few degrees, dewpoint depression will approach 0 and patchy fog is expected to develop. In fact, GOES 16 Nighttime Microphysics composite shows some fog beginning to develop in valley locations across northeast Alabama and northwest Georgia. So, we might see a bit more prevalence of fog this morning than previous mornings, but most of the area will see visibilities remaining above dense fog advisory criteria. The aforementioned upper low currently over northwest Alabama will continue to move to the northwest over the next few hours before it weakens altogether. Meanwhile, the subtropical ridge will begin to build westward across the area. Although surface flow will remain relatively light through the day, we will see a more divergent flow pattern across the area. All of this will combine to create a less favorable environment for thunderstorm development this afternoon. However, still cannot rule out widely scattered thunderstorms, as ample daytime heating may combine with lingering outflow boundaries. We may see two separate areas of storms, one over the west close to the upper low and the other over the east, where terrain effects may enhance the lift. Even still, did not go higher than a 30 percent coverage given the uncertainty in the mesoscale details. Given steep low level lapse rates and ample instability, combined with PWs around 1.75, strong winds up to 50 mph and locally heavy rainfall are possible with any storm that develops. Outside of thunderstorm areas, expecting another hot day across the TN Valley, with highs rising into the upper 80s to lower 90s and heat indices in approaching 100 degrees for some locations. .SHORT TERM...(Tonight through Wednesday night) Issued at 217 AM CDT Mon Jun 18 2018 The subtropical ridge will continue to build northwestward through the overnight hours and become established across the TN Valley by Tuesday. Another mild night is expected tonight, with overnight lows only falling into the lower 70s. Patchy fog is possible once again, though the coverage of fog will depend on if and where we see thunderstorms today. Tuesday is looking to be mostly dry across the region, as subsidence becomes more pronounced. For consistencies sake, will maintain a slight chance for thunderstorms Tuesday afternoon, as the ridge isn't overly strong and any mesoscale boundaries could provide the lift needed to generate an isolated thunderstorm. In addition to it being the driest day of the period, Tuesday will also be the warmest given the upper level high rises, with temperatures warming into the lower 90s. The ridge will begin to break down across the area Tuesday night as a trough ejects east of the Rockies and into the Plains. Meanwhile, models are showing the potential for tropical waves moving around the northwest periphery of the ridge on Wednesday, clipping the TN Valley to the northwest. These weaknesses in the ridge will help to enhance the diurnal convection across the area, with the highest coverage expected across northwest Alabama. We will also see thunderstorm chances continuing into the overnight hours as models bring a wave through the Lower/Mid Mississippi Valley. Temperatures will be a degree or two cooler on Wednesday, with highs in the upper 80s to lower 90s. .LONG TERM...(Thursday through Sunday) Issued at 217 AM CDT Mon Jun 18 2018 The medium range models are in fair agreement on the pattern evolution late this week (days 4-5), then differ on amplification next weekend (days 6-7). The amplification of the Plains upper low/trough will bring shortwaves northeast from the previous Gulf upper low and through lower OH/TN valleys on Thursday into Friday. This should bring a good chance of one or more MCS clusters to the region. Will side with the less amplified ECMWF vs the GFS next weekend, although the blended PoP guidance doesn't appear too unreasonable at this point. With this in mind, the upper ridge broadens west to east across the Gulf Coastal region. This will likely bring an uptick to the heat with suggested blends yielding lower 90s for highs both days 6-7. Thunderstorms will become more diurnally heat driven, and corresponding PoPs both days will be in the lower chance range. && .AVIATION...(For the 12Z TAFS through 12Z Tuesday morning) Issued at 610 AM CDT Mon Jun 18 2018 VFR conditions will prevail at the KMSL and KHSV terminals through the next 24 hours. Winds will shift to the west later this morning and increase to around 6 kts. Thunderstorms are possible again this afternoon, however the coverage and confidence is too low to add thunder to the forecast. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...73 SHORT TERM...73 LONG TERM...17 AVIATION...73 For more information please visit our website at weather.gov/huntsville.