Area Forecast Discussion
Issued by NWS Huntsville, AL
Issued by NWS Huntsville, AL
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695 FXUS64 KHUN 040918 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 418 AM CDT Sat May 4 2024 ...New NEAR TERM, SHORT TERM, LONG TERM... .NEAR TERM... (Today) Issued at 418 AM CDT Sat May 4 2024 Nocturnal convection will continue to spread eastward to the east of the I-65 corridor early this morning, with this activity occurring within a moist southerly flow regime in the lower troposphere between a ridge of high pressure along the Atlantic Seaboard and an area of low pressure lifting northeastward across KS. Although deeper lift associated with a shortwave trough (currently crossing the TN Valley in prevailing WSW flow aloft) has contributed to a bit more thunderstorm activity than originally anticipated, weak mid-level WSW winds (10-20 knots) will hinder any risk for storm organization, with lightning and brief heavy rainfall the main impacts. Patchy/locally dense fog will also gradually develop through sunrise given the very moist nature of the boundary layer, but observational data does not warrant an SPS at this time. Present indications are that the remnants of the overnight precipitation regime will shift slowly northeastward and out of the region between 12-15Z, warranting a sharp gradient in POPs from SW-to-NE across the CWFA. Beyond this timeframe, weak subsidence is anticipated as the shortwave trough advances further northeastward into the central Appalachians, with only isolated- widely scattered showers and storms possible during the afternoon hours. Partly cloudy-mostly sunny skies will allow temperatures to reach the l-m 80s this afternoon. && .SHORT TERM... (Tonight through Monday night) Issued at 418 AM CDT Sat May 4 2024 The general synoptic pattern across the TN Valley will remain largely unchanged tonight and Sunday, with light/moist southerly flow expected to persist in the low-levels beneath a zonal flow regime aloft. There are some indications that another weak mid- level vort max (perhaps convectively-induced) may track eastward across our region late this evening, and with some agreement from the 00Z CAMs on an increase in convection, we have included a low- medium chance POP between 3-9Z. This activity will in all likelihood exit the region prior to sunrise on Sunday, with a warm/humid but dry start to the day followed by development of isolated afternoon showers and storms as temps rise into the m-u 80s. Over the course of the day tomorrow, another shortwave trough is predicted to lift northeastward from the Red River Valley of OK/TX within the flow around a strong upper low tracking east- northeastward from the Great Basin into the central Rockies. Due to the motion of the upper low, a well-defined lee cyclone will evolve across eastern MT throughout the day, forcing a warm front to spread northeastward across the TN Valley late Sunday night/early Monday morning. Deep lift generated by the warm front and approaching shortwave trough will produce a fairly widespread coverage of rain and storms that may begin across our western zones as early as 00Z Monday before spreading further northeastward. Although mid-level winds will back to SW and increase to 15-25 knots preceding the arrival of the trough, shear does not appear strong enough at this point to warrant concern for organized storm structures. Most NWP guidance suggests that the effective warm front will lie across the OH Valley by 12Z Monday, with remnants of nocturnal precipitation expected to spread northeastward and out of the region during the late morning. A 500-mb shortwave ridge to the east of the upper low (discussed in the paragraph above) will translate across the region, yielding a lower but non-zero coverage of showers and storms Monday afternoon and night. Low- level SW flow will strengthen in the wake of the warm front, with dewpoints expected to rise into the u60s-l70s. && .LONG TERM... (Tuesday through Friday) Issued at 418 AM CDT Sat May 4 2024 Latest long range guidance from the global models suggests that the presence of a mid-level ridge will continue to limit the coverage of warm sector showers and thunderstorms across the TN Valley on Tuesday, with a breezy, warm and humid day expected as highs rise into the m-u 80s. However, by Tuesday night, we will need to keep a close eye on thunderstorm activity to our northwest that should develop along a Pacific cold front/dryline that will surge eastward ahead of an occluding cyclone across eastern MT/western ND. There are some indications that frontal convection across southern portions of MO/IL may evolve into a broken MCS Tuesday evening that could potentially track southeastward into the local area early Wednesday morning and may feature a risk for strong-severe storms as mid-level westerly flow will likely increase to 35-40 knots prior to its arrival in our region. However, a more reasonable scenario is that the initial MCS will not reach our region due to the development of another weak area of low pressure across the southern High Plains that should eject northeastward into MO by Wednesday evening. In this case, redevelopment of thunderstorms would occur along the path of the surface low (across MO) early Wednesday afternoon that could potentially grow upscale into a larger and more significant MCS that would reach our region late Wednesday night/early Thursday morning. This convective system would be entering an environment of even stronger deep-layer shear (with WSW flow aloft of 45-50 knots) and moderate-high levels of instability driven by surface dewpoints in the lower 70s beneath a plume of steep lapse rates aloft. Thus, a notable risk of severe thunderstorms producing large hail and damaging winds may materialize during this timeframe. A third and final round of storms may occur on Thursday night/Friday morning prior to the eventual passage of a cold front that should bring a drier/cooler airmass into the region by next weekend. && .AVIATION... (06Z TAFS) Issued at 1239 AM CDT Sat May 4 2024 Scattered to numerous showers and thunderstorms continue across the area, with a general motion to the NE around 15kt. Additional development is possible over the next few hours, as well. Outside of convection, VFR conditions are expected. Before daybreak Sat, there is a risk of patchy fog forming in/near areas that recently received heavy rainfall. Otherwise daytime heating and resultant instability could allow more showers and storms to form Sat afternoon. Given uncertainty on development and where the activity will impact, have a VCTS for the latter portion of the TAF at both terminals. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...None. TN...None. && $$ NEAR TERM...70/DD SHORT TERM...70/DD LONG TERM...70/DD AVIATION...RSB