922 FXUS64 KJAN 230248 AFDJAN Area Forecast Discussion National Weather Service Jackson MS 948 PM CDT Thu Apr 22 2021 .UPDATE... Updated for evening discussion. && .DISCUSSION... Mid evening satellite imagery showed plenty of mid and high level clouds to our west that will continue to thicken as they move into our CWA tonight. The JAN sounding was rather dry and although there are a few light returns on regional radar, measurable rainfall will likely hold off in our western zones until after sunrise. Temperatures were on track and no significant changes were made to the current forecast. /22/ Prior discussion below: This afternoon through Friday: Surface high pressure remains entrenched over the ArkLaMiss region this afternoon while increasing westerly flow aloft is bringing in abundant subtropical moisture and increasing mid/high level cloudiness. There will still be enough clear skies this evening to allow temperatures to dip well into the 40s over eastern portions of the forecast area. As a strong upper level shortwave trough moves from the southwest CONUS into the Southern Plains Friday, quickly strengthening moisture transport will help to initiate showers over western portions of the area, especially by afternoon. /EC/ Tomorrow night through Saturday: The forecast period will start off with an active round of weather tomorrow night into Saturday. Guidance is in good agreement that a shortwave trough ejecting out of the Desert Southwest will swing through the Southern Plains tomorrow and through the Lower Mississippi Valley tomorrow night. An associated surface low is forecast to slide east along the Red River and through northern Louisiana into northern Mississippi. This will bring a round of thunderstorms, some of which could be severe, along with locally heavy rainfall to portions of the area. Severe weather threat: A warm front will already be lifting north across southern Mississippi at the start of the period tomorrow night. However, return flow trajectories will be out of the southeast owing to a sprawling surface high just off the Carolina Coast. Initially, this will shunt rich boundary layer moisture and surface-based instability further west into coastal Texas and southwest Louisiana. As such, initial showers and thunderstorms along the advancing warm front will be elevated above a surface stable layer. How far north the warm front progresses remains uncertain, but the general consensus is that it will reside in the vicinity of the I-20 corridor. Convective evolution has become more clear as 12z hi-res CAM guidance is in fairly good agreement on warm advection showers and storms breaking out during the evening hours followed by an MCS sweeping through the area during the overnight. Guidance continues to indicate a disjointed parameter space which remains the primary limiting factor in higher severe weather probabilities at this time. A stout 50-60kt low-level jet will support 500-600 m2/s2 of 0-1km SRH and 40-50kts of 0-1km bulk shear, however, the aforementioned southeasterly return flow will delay the arrival of surface-based instability until winds can veer to out of the south/southwest. As such, the severe weather threat will be conditional on the arrival of surface-based instability prior to low-level winds becoming too veered. Once flow veers, attention will shift to more of a flooding threat as convection becomes outflow dominate and begins to train within a shear profile characterized by nearly unidirectional west- east flow oriented parallel to the warm frontal boundary. With that being said, severe weather cannot be completely ruled out within the highly sheared, yet very weakly unstable environment. The HRRR and ARW solutions depict an MCS track north of I-20 with the NSSL WRF along and south of I-20. In these events, the southerly track tends to verify more often owing to the MCS propagating along the instability corridor/gradient as well as cold pools from lead warm advection storms helping to anchor the warm front further south. If this scenario is realized, the slight risk along and south of I-20 remains well placed with a threat for damaging winds and a few tornadoes, as even a few hundred J/kg of SBCAPE will be sufficient given the magnitude of shear present. If a further north MCS track pans out and/or if surface-based instability is slow to return there could still be a threat for severe weather, but it will be more isolated and dependent on how deep the surface stable layer is and whether stronger wind gusts can penetrate said layer. Ultimately, it will come down to mesoscale trends tomorrow evening as to how the event will play out, so will keep the ongoing marginal and slight risk areas in the HWO as is. While the vast majority of storms will shift east of the area Saturday morning, there will be a secondary threat for storms during the afternoon hours along the cold front/wind shift. Forcing will be nebulous and deeper moisture will have already shifted out of the area, but if a few isolated to scattered storms are able to initiate the environment will remain favorable for severe weather. This will mainly be in the form of large hail and damaging winds as moderate instability and steep lapse rates will be in place with sufficient deep-layer vertical wind shear for supercells. The NAM and GFS are most aggressive with afternoon redevelopment and the timeframe is currently outside of the window of the CAM guidance. Therefore, will hold off on introducing another severe weather graphic for now, but a marginal risk area may be needed if CAM guidance comes into agreement on a few afternoon storms being able to develop. Flash flooding threat: PWATs are progged to surge into the 1.75" range with a stout low- level jet aiding moisture transport into the aforementioned MCS. Increasing ML/MU CAPE on the southwest flank of the departing MCS along with veered boundary parallel flow will support a threat for flash flooding from training convection. This will most likely occur in a focused swath in the vicinity of the warm front where deep moisture convergence is maximized, however, trying to pin down the exact location remains elusive right now. The southern solution is being favored, and while still somewhat broad, the elevated area in the HWO/graphics has been focused mainly along and just south of the I-20 corridor. Further refinement to the limited/elevated areas in the HWO/graphics will likely be needed as placement of the heaviest rainfall becomes clear and a flash flood watch may be necessary as well. Sunday through Tuesday: Surface high pressure will quickly build in on Sunday with dry conditions persisting as we head into the beginning of next work week. A strong Pacific jet diving through California will help carve out a stout trough with the synoptic pattern quickly amplifying. This will allow for downstream ridging over the Lower Mississippi Valley with rising heights and warming temperatures. Afternoon high temperatures will quickly rebound into the low to mid 80s by Monday and Tuesday beneath partly cloudy skies. Wednesday through Friday: The pattern will become increasingly amplified by mid to late week as a strong Pacific jet diving down the West Coast helps carve out a stout trough over the Desert Southwest. Guidance is in decent agreement that this trough will eject into the Southern Plains on Tuesday, but predictability starts to decrease by mid to late week as it encounters upper ridging over the Lower Mississippi Valley. The GFS solution is the most progressive and swings the system through Wednesday night into Thursday. The ECWMF, however, quickly slows the system as it encounters the aforementioned ridging and shears out the wave with any active weather later Thursday into Friday. Given the differences in guidance will keep the forecast in line with the national model blend for now, but another round of thunderstorms and heavy rainfall cannot be ruled out during the mid to late week period. /TW/ && .AVIATION... 00Z TAF discussion: VFR conditions and a light wind will prevail through Friday morning. Coverage of -SHRA wl increase from the west Friday afternoon and may result in brief vsby restrictions but VFR conditions are expected to continue to prevail through the end of the TAF period. /22/ && .PRELIMINARY POINT TEMPS/POPS... Jackson 48 72 60 81 / 5 23 96 37 Meridian 45 72 58 80 / 3 10 98 56 Vicksburg 52 72 61 81 / 7 41 97 23 Hattiesburg 47 74 63 81 / 5 12 94 58 Natchez 52 74 62 82 / 6 47 96 22 Greenville 52 69 58 76 / 9 39 98 28 Greenwood 51 70 57 77 / 3 19 98 44 && .JAN WATCHES/WARNINGS/ADVISORIES... MS...None. LA...None. AR...None. && $$ TW/22/EC