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Area Forecast Discussion
Issued by NWS Huntsville, AL
Issued by NWS Huntsville, AL
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899 FXUS64 KHUN 171125 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 625 AM CDT Wed Jul 17 2024 ...New AVIATION... .NEAR TERM... (Today) Issued at 329 AM CDT Wed Jul 17 2024 A warm and moist air mass resides across the TN Valley early this morning, featuring temps ranging from the u60s-l70s (E) to u70s-l80s (W). With dew points in the lower 70s and only a few high-level clouds overhead, locally dense fog has developed in the valleys of northeast AL, and an SPS for visibility reductions may be needed prior to the end of the shift. On the synoptic scale, a pattern change is currently underway as WSW winds aloft are beginning to strengthen in response to a strong northern stream trough digging southeastward into the northern Great Lakes. This trend will persist today as the trough advances eastward across southern Ontario, resulting in consolidation of a downstream surface cyclone across northern New England/southern Quebec. Clusters of showers and thunderstorms continue early this morning both in the vicinity of the cyclones trailing cold front (to our NW) and convective outflow boundaries, and we will need to monitor for development of thunderstorms to our north prior to sunrise as one such boundary drifts southeastward into Middle TN. Additional showers and thunderstorms may develop within a conditionally unstable air mass (featuring MUCAPE up to 1000-1500 J/kg) and region of enhanced low-level warm advection and moisture transport across central AL, and this activity may clip our southeastern zones prior to sunrise as well. Over the course of the late morning, the combination of lower- tropospheric warm advection and a rapidly destabilizing boundary layer will result in a development of scattered thunderstorms across northern AL. However, through mid-day, the greatest concentration of convection will likely remain to our north. By this afternoon, the outflow boundary will slip southward into AL, enhancing the coverage of storms region-wide between 18-00Z. With temps in the lower 90s supporting CAPE in the 1500-2500 J/kg range and a gradual increase in flow aloft/deep-layer shear, a few strong multicell clusters will be possible, capable of producing locally damaging wind gusts (up to 50-60 MPH), small hail and frequent lightning. && .SHORT TERM... (Tonight through Saturday) Issued at 329 AM CDT Wed Jul 17 2024 Storm coverage will remain high across the CWFA this evening and throughout the morning hours on Thursday as the actual cold front makes its way southward into the region. However, we expect instability to diminish with the loss of insolation this evening and CAPE values will not recover much during the day tomorrow due to widespread convective overturning and debris cloudiness. Thus, our primary focus will shift from strong storms to locally heavy rainfall, as many locations will receive multiple rounds of heavy precipitation. Present indications are that the front will stall near or immediately southeast of the forecast area from Thursday night-Friday night, and our highest POPs have been reserved for the southeastern zones with this in mind. Fortunately, highs will only reach the m-u 80s, with no heat-related concerns on either Thursday or Friday. && .LONG TERM... (Saturday night through Tuesday) Issued at 329 AM CDT Wed Jul 17 2024 The local forecast area will remain to the east of a mid-level longwave trough (extending from the Great Lakes into the northwestern Gulf of Mexico) for much of the extended period, with initially light SSW flow aloft expected to strengthen by Monday/Tuesday as the height gradient contracts between a subtropical ridge retrograding across FL and a developing upper low over the MO Valley. As this occurs, a weak area of surface low pressure is expected to evolve across west-central TX Friday night, which will force the stalled frontal boundary to lift northwestward through the region once again on Saturday. Precisely how far north and west the front makes it is highly uncertain at this time, but regardless of position, we will be either sufficiently close to the boundary or within the moist warm sector, and this will promote several rounds of rain and thunderstorms from Sunday-Tuesday (both during the day and at night). Although shear may strengthen a bit by Monday/Tuesday, deep moisture and weak lapse rates will limit storm intensity, with lightning and locally heavy rainfall the main threats. Heat Risk will not be a concern through the end of the extended period. && .AVIATION... (12Z TAFS) Issued at 625 AM CDT Wed Jul 17 2024 Conditions remain VFR at the HSV/MSL terminals this morning, with just a few high clouds spreading across the region from the NW. Scattered TSRA will gradually develop over the course of the morning and become fairly widespread in coverage this aftn as an outflow boundary drifts southward from extensive convection to our north. Additional SHRA/TSRA are expected this evening and through the overnight period, as a cold front shifts southeastward into the region. We have included TEMPO and PROB30 groups to address the periods of most concern for storm impacts, including MVFR cigs and vsby reductions. Prevailing sfc winds will be from WSW today, but may shift to N late in the period. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...None. TN...None. && $$ NEAR TERM...70/DD SHORT TERM...70/DD LONG TERM...70/DD AVIATION...70/DD