Area Forecast Discussion (AFD)
Issued by Huntsville, AL (HUN)
019 FXUS64 KHUN 250203 AFDHUN Area Forecast Discussion National Weather Service Huntsville AL 903 PM CDT Wed May 24 2017 .NEAR TERM...(Tonight) Issued at 903 PM CDT Wed May 24 2017 A rather highly amplified upper level pattern continued over and near the greater North American domain. A feature influencing local weather was an upper low approaching the Tennessee Valley from the Mid South. Both GOES 13 and 16 imagery nicely show this mid/upper low. 25/0000Z 500mb heights around this low were near 551 decameters, which is pretty low for this time of year. This has resulted in very steep mid level lapse rates, which has lead to short lived, isolated thunderstorms within shower bands near and southeast of this upper low. Local and regional radar views indicated scattered to at times numerous showers moving in a west to east manner across the forecast area. The back edge of these showers were over Mississippi River between NE Arkansas and around Greenville MS. A few lightning strikes (most of them intercloud) have been accompanying this activity, seen from local LMA and MRMS. For the rest of the overnight, the above mentioned upper low will continue moving a bit south of east across the forecast area. Output from the HRRR indicated 1000/500 thickness values below 550 decameters as this system swings by, especially between midnight and 3 to 4 AM. Shower activity with this system per the HRRR and RAP should end across our eastern areas before daybreak Thu, with the new NAM a bit slower with the drying trend. Update wise, stayed close to previous forecast trends/timing. Did lower rain chances over our eastern areas late tonight more in line with the drier high resolution guidance. Cooler surface temperatures (lows falling into the lower 50s and the already cold air aloft should lessen lightning development after 10 PM, as lapse rates aloft moderate somewhat. .SHORT TERM...(Thursday through Friday night) Issued at 304 PM CDT Wed May 24 2017 Thursday will be another relatively cool day for late May, with consensus highs only reaching the low-mid 70s (normals are 83). There is still an outside chance of a shower in northeast Alabama, since a last-gasp shortwave will round the southwest flank of the departing low. However, with precipitable water values less than an inch over that region, the forecast will be left dry. One more below-normal night is expected Thursday night with highs falling to the mid 50s. The cool spell will come to an abrupt end as abundant sunshine, strong SW flow and subtle mid-level ridging all kick in Friday. Highs should easily reach the mid-upper 80s, with a nudge just above blended guidance. A cold front will begin a slow southward push late Friday into Friday night, but it should not get close enough to the Tennessee Valley to warrant including any PoPs in the forecast through Friday night. .LONG TERM...(Saturday through Tuesday) Issued at 304 PM CDT Wed May 24 2017 The latest med range model output continues to show deep SW flow prevailing from the Mexican Plateau to the Southeast and Ohio River Valley on Saturday. Meanwhile, a surface warm front will be lifting north on Saturday afternoon in response to an approaching mid-level shortwave trough moving within the subtropical jet. This will occur in conjunction with a polar jet trough rotating around a parent upper low moving southeast over Corn belt and Great Lakes region. That particular trough will become important for later in the evening/overnight Saturday night. For Saturday, scattered storms may occur as the warm front moves north during the late morning into early afternoon. Plan view of instability parameters are showing a moderate to high amount of instability during the afternoon, but forecast soundings indicate most of this is elevated due to an EML present likely from the fetch off the Mexican Plateau. Because of this limiting factor, will keep scattered storms in the forecast, but this activity could have even less coverage than forecast. As the warm front moves north, the aforementioned shortwave trough will be pivoting southeast towards the MS River Valley with a trailing cold front. The ensuing confluence of the flow aloft will act to increase geostrophic flow across the MS/TN/OH river valleys for Saturday night. Upstream (NW of here) deep convective initiation is expected to increase in coverage and likely form into a MCS rapidly moving southeast towards the TN Valley on Saturday night. Deep layer shear is around 40-50 kts especially after 7PM. Most unstable CAPE (2000-3000 J/kg) is mostly in the elevated portions of the profile (within the hail growth zone) which would heighten the potential for large hail along with heavy rainfall once the profile becomes saturated. Though some uncertainty exists on the trajectory of the MCS, it should spread a cold pool across the TN Valley which may limit convective activity during the day on Sunday. However, additional showers and storms should develop along the trailing surface cold front that should move over the region sometime late Sunday night into early Monday morning. The latest med range models runs do show a slower trend with the cold front passage which is plausible with the bulk of the upper level forcing with the trough associated with the parent upper low removed further to the north. So, the precipitation may not completely clear before sunrise on Monday morning and may affect Memorial Day outdoor activities. The temperatures should be cooler behind the front for both Monday and Tuesday. Dry conditions should prevail on Tuesday as well. && .AVIATION...(For the 00Z TAFS through 00Z Thursday evening) Issued at 630 PM CDT Wed May 24 2017 Mainly VFR conditions should continue through TAF. That being said, an area of showers and a few embedded thunderstorms will continue moving in a west to east manner across the area this evening. CIG and/or VIS values may briefly be reduced to MVFR, but chances of it are too low to include in the TAF. Stayed with a timing of the shower activity ending around 07Z at KMSL and 08Z at KHSV. VFR altitude lower clouds should persist for the rest of the night and during Thu AM. As the coldest air aloft moves east of the region, expect a broken cloud deck to scatter in the early afternoon. && .HUN WATCHES/WARNINGS/ADVISORIES... AL...NONE. TN...NONE. && $$ NEAR TERM...RSB SHORT TERM...BCC LONG TERM...SL.77 AVIATION...RSB For more information please visit our website at weather.gov/huntsville.