Forecast Discussion
000 FXUS64 KMEG 190451 AFDMEG Area Forecast Discussion National Weather Service Memphis TN 1151 PM CDT Thu Apr 18 2024 ...New AVIATION... .UPDATE... Issued at 952 PM CDT Thu Apr 18 2024 The first batch of showers and thunderstorms has moved off to the east but round 2 is approaching the Mid-South along a cold front stretching from central IL into northern AR. A fair amount of instability is still present across the region with MUCAPE still on the order of 1000-2000 J/kg. Surface-based instability appears to be largely capped at the moment. Deep-layer shear has increased over the past few hours with effective bulk wind shear now in the 40-45 kt range along and west of the MS River. The increasing low-level jet has also carved out a looping hodograph which has enhanced storm-relative helicity across the area. The NQA VAD hodograph indicates 0-1 km SRH of 440 m2/s2 which is significantly more than the RAP13 has been alluding to. This could lead to some embedded supercell structures within the approaching line. Fortunately, the 0-3 km bulk wind shear is limited to 25-30 kts from the west-northwest. This would provide only marginally sufficient wind shear to support mesovortex formation along the line which should limit the potential for tornadoes. Damaging wind and large hail look to the be the primary threat should this convection remain rooted in the boundary layer. MJ && .MESOSCALE UPDATE FOR SEVERE WEATHER... Issued at 533 PM CDT Thu Apr 18 2024 We`re finally beginning to see more robust storm development north of Interstate 40. This is where the greatest instability is located. Farther south, the cap is stronger and showers/storms are really struggling to maintain/gain intensity despite an approaching shortwave trough near the Arklamiss. The latest analysis indicates MLCAPE (instability) on the order of 2000 J/kg from near Memphis into southeast Missouri (with 0-3 km MLCAPE near or above 100 J/kg). Mid-level lapse rates are fairly steep across portions of AR/MO/TN by Mid-South standards (near 7 C/km) which will support stronger mid-level updrafts and hail growth. One of the negatives is the lack of stronger deep-layer shear. Bulk shear isn`t all that bad at 35 kts or so, but that is more marginal for supercell development, especially with 0-3 km SRH at or below 100 m2/s2. This suggests more in the way of multicells with weak supercell characteristics in general. Large hail and localized damaging wind are the primary concerns. Weak low-level wind shear will keep the tornado threat low for now. Storms upstream are expected to grow upscale into a linear mode, moving into the Mid-South later this evening. The greatest uncertainty will be whether or not the air mass preceding this line of storms is able to remain surface-based instability. If so, damaging winds will be a larger concern. If not, storms may remain elevated which will limit the severe weather potential. MJ && .SYNOPSIS... Issued at 333 PM CDT Thu Apr 18 2024 Scattered showers and thunderstorms will continue over the Midsouth tonight, in advance of an upper level trough lifting through the mid-Mississippi River Valley. Severe thunderstorms will be possible, especially north of a Jonesboro Arkansas to Jackson Tennessee line. Damaging winds and large hail will be the primary threats. A few short-lived tornadoes will also be possible through the early overnight hours, mainly to north and west of the Memphis metro area. Additional showers are expected across the Midsouth through early Sunday, with isolated thunderstorms over north Mississippi. Severe thunderstorm chances will be low. A reinforcing cold front will push through the Midsouth Saturday night and early Sunday, bringing low humidity and cool conditions for Sunday night through early Tuesday. && .DISCUSSION... (This evening through next Wednesday) Issued at 333 PM CDT Thu Apr 18 2024 Primary forecast concern remains severe thunderstorm chances through tonight. SPC/RAP mesoanalysis showed mixed-layer CAPE of 1500-2000 J/kg along and west of the MS River. Mixed layer CINH was estimated at zero across the Midsouth, but early afternoon convective trends suggest consequential CINH remains. 12Z soundings from LZK, SGF and JAN showed a stout elevated mixed layer (EML) and underlying inversion. 17Z aircraft sounding from LIT showed this EML remained strong, and based at 750mb. GOES visible imagery showed multiple standing wave clouds over northwest AR, suggestive of an overlying capping inversion. Immediately west, frontal convergence was evident over far northeast OK and southwest MO, but deep convection was limited to areas further north along the front, over central MO. To our south, thunderstorms persisted through most of the morning and early afternoon. This convection initiated on an elevated boundary, near the base of a relatively low (900mb) EML. This convection was not initialized on early morning runs of the HRRR, despite a pronounced upstream shortwave evident on GOES water vapor imagery. The Arklamiss/central MS storms persisted into central MS at midafternoon and have spread considerable high cloudiness over north MS. This has limited surface heating and ability to sustain deep convection - at least to this point. Should a storm or two manage to break through the low level capping inversion, robust CAPE and steep midlevel lapse rates will support large hail over any portion of the Midsouth. Any of three following features may aid in the breaking the convective cap: height falls associated with aforementioned shortwave exiting the Arklatex, strong surface heating, and general height falls associated with the longwave trough passage over the mid-MS River Valley. None of these are a sure bet, hence the lower typical Day 1 confidence level in severe storms. Should severe storms fail to materialize this afternoon, it doesn`t mean we`re out of the woods. NAM and HRRR depict a pronounced strengthening of 925mb SSW winds late afternoon and evening. This is likely a contributing factor the looping model- derived hodographs over eastern AR late this evening. If storms manage to become surface-based, a mid to late evening QLCS tornado threat may be realized over eastern AR. After midnight, severe threat should diminish with strengthening low level inversion, weakening elevated instability and weakening/veering of the low level jet. Comparatively tranquil weather will prevail Friday and Saturday, as a surface cold front stalls over north MS, parallel to the upper level flow. An open southern branch trough will lift into TX on Saturday, with downstream moisture bringing showers mainly south of I-40. A few thunderstorms will be possible over north MS, but similar to yesterday, NBM thunder probabilities appear too high/too far north Friday night through Sunday morning. A reinforcing cold frontal passage Sunday night will bring dry and cool conditions, with temperatures Sunday and Monday more typical of early March. PWB && .AVIATION... (06Z TAFS) Issued at 1151 PM CDT Thu Apr 18 2024 A line showers and thunderstorms is currently moving across northeast AR and the MO Bootheel along a surface cold front. This convection is expected to gradually decay as it moves into a more hostile near-storm environment. Confidence is generally low for TS on station at forecast terminals outside of JBR, but VCTS was included at MEM (VCSH elsewhere). Winds will shift from the north as this front passes with low ceilings overspread the area during the early morning hours. Low clouds will persist through much of the day before slowly lifting during the late afternoon and evening hours. IFR probabilities are in the 40-50% range or higher, especially at MKL/TUP for Friday morning. Winds will likely be a bit gusty tomorrow, mainly before 18z. MJ && .MEG WATCHES/WARNINGS/ADVISORIES... AR...None. MO...None. MS...None. TN...None. && $$ PUBLIC FORECAST...PWB AVIATION...MJ