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Area Forecast Discussion
910 FXUS64 KHUN 261057 AAA AFDHUN Area Forecast Discussion...UPDATED National Weather Service Huntsville AL 557 AM CDT Mon Sep 26 2016 .UPDATE... For 12Z TAFs. && .SYNOPSIS... (Issued 321 AM CDT Mon Sep 26 2016) Dry and cool autumn weather is on the way this week after record breaking heat this weekend. This cooler weather will occur as a mid- upper level vortex pushes a surface cold front southeast today and tonight! The front is currently located from central IN southwest over the MS river near KMEM to SE TX. Scattered shower/thunderstorm activity is expected to accompany this front which will be the highest potential for precipitation (though only 30-45% chance) this week. Further to the south (over the gulf coast) a weak upper low continues to churn with some moisture making it northward on the northern flank of this low. A building ridge over the west along with amplifying ridge over the western Atlantic Ocean may cause the large vortex over the Great Lakes/OH River valley regions to become cutoff from the polar jet stream. If this occurs, anticipating another cold front later this week. SL.77 && .DISCUSSION... (Issued 321 AM CDT Mon Sep 26 2016) As mentioned above isolated to scattered showers and thunderstorms associated with a cold front. This activity looks to be more anafrontal with showers/storms likely to be of higher potential just after or along the frontal passage. One of the ingredients that has been so far slow to respond, due to the far removed upper support pushing this front, has been moisture advection. Right now despite the upper low over the gulf coast, the low and mid-levels dewpoint depressions have been high. However, as the front approaches, some saturation in the low to mid-levels should be enough to at least support isolated scattered showers and thunderstorms later this morning. An increase in areal coverage of storms should occur later this afternoon, especially over NE AL. On timing of the front and convective activity: The front should arrive over NW AL by 15Z-16Z, north central AL/S Middle TN by 18Z-20Z, and finally NE AL by 21Z- 23Z. Some of the convective activity will accompany the front while another area of thunderstorms is possible behind the front. Due to the scattered nature of the thunderstorm activity and overall lack of moisture, made some timing adjustments to the POPs and weather forecast. Mainly reducing POPs in the morning and incorporating better timing for this afternoon. Thunderstorms should generally be ordinary in nature due to low instability and wind shear profiles. However, if any storms do have sustained updrafts, anticipating them to be short- lived with gusty winds up to 30 MPH. Meanwhile, the temperature forecast will be challenging especially with already warm overnight temperatures this morning in the 70s. For now, have left mid to upper 80s, but would not be surprised to see slightly higher temps if the front slows its propagation. Post-frontal showers and thunderstorms should gradually end from NW to SE this evening. All lingering activity should end by midnight. The best news for all of those that love fall weather is that overnight lows will drop into the upper 50s, and possibly lower 50s in higher elevations. Sunny conditions should prevail on Tuesday as the upper flow pattern over the Southeast shifts to a prevailing trough with NW/W flow. Daytime highs on Tuesday should be in the mid to upper 70s. Then, one of the coolest nights of the month should occur Tuesday night as the cool cP airmass settles over the region with overnight lows in the low 50s! The evolution of the upper vortex mentioned previously will be the primary focus for the rest of the forecast. It appears that this upper vortex may wobble south on Wednesday/Wednesday night acting to send a reinforcing cold front southward. Moisture convergence along the front and around the trough axis is fairly limited. However, did introduce a slight chance pop mainly over NE AL/Southern Middle TN for Wednesday night as the front moves across the region. This will also mean continued cool daytime highs on Thursday in the 70s and overnight lows near 50 on Thursday night. The weekend`s forecast is a little more uncertain due to the progression of this upper vortex. The ECMWF/Canadian portray a stronger western Atlantic ridge acting to keep this vortex in place across the eastern half of the country. However, the GFS shows a opening of the cutoff low and merging back with the polar jet about 24 hours sooner. So, for now will keep the forecast dry and seasonably cool. SL.77 && .AVIATION... For 12Z TAFs: A cold front will arrive by 19Z at KMSL and 21Z at KHSV. Isolated to scattered showers and thunderstorms will be possible along and just behind the cold front through this evening. Winds will shift from west-southwest to northwest behind the front with speeds of 8-11kt. Eventually, winds will veer to the north overnight. VFR cigs aoa bkn035 are forecast this evening, although lower MVFR cigs blo 020agl are psbl in some areas. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ For more information please visit our website at weather.gov/huntsville.