Forecast Discussion
000 FXUS64 KMEG 081757 AAA AFDMEG Area Forecast Discussion...UPDATED National Weather Service Memphis TN 1257 PM CDT Wed Apr 8 2020 .UPDATE... Updated for 18Z Aviation Forecast Discussion below. && .PREV DISCUSSION... /issued 1111 AM CDT Wed Apr 8 2020/ DISCUSSION... Currently, a strong cold front located over Illinois is expected to drop into the area later this evening. As far as near storm environment, mesoanalysis as well as RAP forecast soundings indicate modest instability (SBCAPE 2500 J/kg) over most of the area, with steep mid-level lapse rates. However, parts of eastern Arkansas, and west Tennessee are moderately capped. With that being said, storms could form over northern Mississippi this afternoon. Some of these storms could be severe but our main threat will be later tonight as the front approaches the area. As for tonight, short range models are inconsistent on storm evolution. However, they are consistent about timing. Both show discrete cells forming over Illinois around 6 PM, then moving southeastward toward the Missouri Bootheel and eastern Arkansas by 8 PM. They begin to diverge as the system progresses. The HRRR shows cells forming into a line and moving into west Tennessee and not affecting northern Mississippi. The NAMNest shows the line diving southeast towards northern Mississippi. So far, the NAMNest seems like the best solution. Large hail and and damaging winds will be the main threats as the line moves into eastern Arkansas and the Missouri Bootheel, then the hail threat seems to diminish as the line moves towards northern Mississippi. SGW PREV DISCUSSION... /issued 345 AM CDT Wed Apr 8 2020/ DISCUSSION... A northern branch trough and accompanying surface low will continue to track towards the southeast from south-central Canada today. At the surface, a strong cold front will contribute to an increase in showers and thunderstorms across the Mid-South this evening into early Thursday. Temperatures this afternoon will climb into the low to mid 80s with dewpoints in the 60s. These warm temperatures and humid air mass will contribute to strong surface instability and a decent warm sector across the Mid-South. While forecast soundings indicate a capped environment to start the day off a few showers and storms could develop beneath this cap mainly across portions of North Mississippi. The big forecast question today is how quickly will this cap break and when this does occur how will mesoscale and forcing mechanisms contribute to storm mode. Hi-res guidance is still somewhat inconsistent regarding convective initiation and evolution. However, the majority of guidance depicts some discrete development ahead of the front and eventually taking on more of a linear structure. Given the highly unstable pre- storm environment, sufficient deep-layer shear, and steep mid-level lapse rates, any storms that do develop tonight have the potential to be strong to severe. The primary threats include damaging winds and large hail. A few tornadoes are possible as well given decent low- level curvature on forecast hodographs and veering wind profile. This particularly threat could increase as storms evolve into more of a linear structure (QLCS or squall line). As such, portions of northeast Arkansas, the Missouri Bootheel, and West Tennessee have been upgraded to an Enhanced Risk for severe weather tonight through early Thursday morning. The remainder of the area remains in a Slight Risk during the same timeframe. Following the passage of the cold front Thursday morning, dry conditions and cooler temperatures will prevail until the weekend. Friday will be the coolest day with highs in the 50s to low 60s and overnight temperatures in the 40s. An upper trough will swing across the Desert Southwest on Saturday eventually taking on a neutral to negative tilt. Similarly, an accompanying surface low will eject northeast through the Southern Plains. Therefore, rain chances will return and be present for much of the weekend. At this time the best chance for thunderstorms will be Saturday night through much of Sunday. The Euro is a little slower with the eastward progression of this system whereas other guidance is a little quicker. This leads to the question of how much of the Mid- South will be encompassed within the warm sector of this next system. This will help determine whether or not severe weather can be expected during the holiday weekend. Definitely a system to keep eyes on. By late weekend long-term guidance solutions diverge. For now, have only included low chance PoPs for early next week. 17 && .AVIATION... 18Z TAFs Primary concern TSRA coverage south of TUP this afternoon, and evening/early overnight TSRA likely affecting all Midsouth terminals. Expect a semi-discrete cluster of storms to develop into a broken to solid line, racing southeast through JBR, MEM- MKL, TUP - in that order from 03Z to 07Z. Convection-allowing models (CAM) consensus on timing was relatively tight, leading to increased forecast confidence. That said, the HRRR model remains an outlier in failing to develop a cold outflow pool over northern AR early evening and subsequent TSRA at MEM a few hours later. This expected outflow pool will be responsible for the storms evolving into a broken to solid line, oriented southwest to northeast, and moving southeast. This broken-solid line is the most likely scenario. Large hail potential will be greatest around JBR, gradually decreasing as storms move southeast to MEM and MKL and eventually TUP. As large hail threat decreases, strong wind threat will increase. Once storms exit, TSRA will rapidly decrease to near zero, with only strong gradient winds remaining. By MEM outbound push, FL020 winds will prevail around 36045KT, with a few gusts near 25KT making it to the surface. PWB && .MEG WATCHES/WARNINGS/ADVISORIES... AR...None. MO...None. MS...None. TN...None. && $$