Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 28 2023 15:44:51 FOUS30 KWBC 281544 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1143 AM EDT Sun May 28 2023 Day 1 Valid 12Z Sun May 28 2023 - 12Z Mon May 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF VIRGINIA, NORTH CAROLINA, & TEXAS.... ....Central Appalachians into parts of the Mid Atlantic... The surface low will migrate onshore near the SC/NC border shortly as it drifts slowly northward and becomes vertically stacked. Even though the surface cyclone and 850 mb inflow will weaken substantially, periods of heavy rain producing showers are likely, especially on the western edge of the mid-level circulation from the higher elevations of western NC to nearby southwest VA, and in the very short term, in and near the Albemarle-Pamlico Peninsula where a heavy rain band is shifting into the region from the south. Other isolated instances of localized heavy rain are possible across central NC and southwest VA into far eastern KY-TN associated with low level frontogenesis and ~500 J/kg of MUCAPE, supporting a Marginal Risk. More organized convection falling atop sensitive terrain is possible across the Blue Ridge of NC and into southwest VA, where local 3-5" of additional rainfall is likely per the 12z HREF probabilities of such, due in large part to the additional upslope enhancement. Some of the terrain is inherently flashy, particularly near the VA/WV border, which could result in more scattered instances of flash flooding. Adjustments to the areas were mainly based on the new 12z HREF output. ....Rio Grande Valley northward through the Northern High Plains... A persistent and slow moving pattern driven by a trough to the west and ridge to the east will promote another day of scattered to widespread showers and thunderstorms from Texas to the Dakotas. Localized rainfall totals of 3" are possible throughout much of the Marginal Risk area, though a more elevated risk for flash flooding will be from southwest OK southward through northwest TX, the TX Big/Hill Countries, then south to the Rio Grande River including the Edwards Plateau and South TX Brush Country. This is due to the potential for additional heavy rain (more organized convection), along with the antecedent wet soils given the recent heavy rainfall. Across TX, PWATs are expected to peak between 1.75-2" along the Rio Grande River Valley while MUCAPE also increases to 2000-2500 J/kg. Shortwave approaching out of northern Mexico and potential lingering surface boundaries from previous day's MCS activity could provide focus for development and brief training of higher rain rates in this area. FFGs across the Edwards Plateau and surrounding regions are very low (1-2"/hr and 1.5-2.5"/3-hr). The Slight Risk area generally covers the spotty 20-50% probabilities of 3" seen in the 12z HREF probabilities for such. The Marginal Risk area saw some slight reconfiguration based on the 12z HREF probabilities as well. ....Southern FL... A convergence zone/baroclinic trough shifts northward back into southern FL today, luring precipitable water values back to 1.5"=20 Effective bulk shear remains 25-30 kts, so any thunderstorm activity has the potential for being more organized than usual.=20 ML CAPE is already 500-2000 J/kg, and daytime heating to raise that another 1000+ J/kg. Probabilities of 3"+ and 5"+ in and near Southeast FL are non-zero. Considering recent rains, thought it prudent to add a Marginal Risk area for eastern portions of the southern peninsula. Roth Day 2 Valid 12Z Mon May 29 2023 - 12Z Tue May 30 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN PLAINS AND MID ATLANTIC REGION... ....Parts of south-central Oklahoma southward to the Rio Grande Valley and the Texas Gulf Coast... Weak, broad mid-upper level trough with embedded convectively-enhanced shortwave energy will make for a favorable dynamical and thermodynamical environment for more numerous thunderstorms with embedded localized areas of more intense rainfall. There is a somewhat more enhanced signal in the guidance for more organized heavy rain farther south (Rio Grande Valley and Lower-Mid Texas Coast), however at this point given the weak kinematic forcing, not enough to support a Slight Risk. Prior MCS activity across south-central TX will also likely influence where additional rounds of thunderstorms set up by the Day 3 period, but those details remain uncertain at the moment. The inherited Marginal Risk from yesterday's Day 3 ERO was not changed across this region, and continues to align with the latest guidance (especially CAMs) in terms of localized deterministic QPFs between 2-4+ inches within an environment characterized by 1.5-2" PWATs and MUCAPEs of 1000-2000 J/Kg. ....Mid Atlantic Region, especially much of Virginia and southern MD-DE... Vertically-stacked low near Upstate SC at the start of Day 2 (12Z Mon) will migrate slowly into eastern NC by the start of Day 3 (12Z Tue). Owing largely to the steepening mid level lapse rates (7-7.5 C/KM between 700-500 mb), mixed-layer CAPEs are likely to peak between 1000-1500 J/Kg during the peak heating hours Memorial Day afternoon into early evening. This along with PWs between 1.25-1.5" will support isolated hourly rainfall rates of 1.5+ inches, which could pose a localized flash flood threat, especially across areas that receive appreciable rainfall on Sunday. Snell/Hurley Day 3 Valid 12Z Tue May 30 2023 - 12Z Wed May 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE UPPER MIDWEST AND EASTERN MISSOURI RIVER VALLEY, ALONG WITH THE NORTHERN SIERRA AND NORTHEAST CALIFORNIA INTO PORTIONS OF THE NORTHERN GREAT BASIN... ....Northern Sierra and northeast CA into southeast OR, northern NV, and southwest-central ID... Mid-Upper low off the Central CA Coast to start Day 3 (12Z Tue) will slowly wobble through SoCal during the period. Favorable upper level difluence north of the compact mid-upper low, south of the digging northern stream upper trough/height falls across the Pacific NW, will generate favorable broad-scale forcing across the outlook area Tue-Tue night. PWATs meanwhile are expected to climb between 0.75-1.00", which is ~1.5 standard deviations above normal for the end of May per the GEFS. Modest uptick in deep layer forcing and moisture, along with at least some diurnal instability (mixed layer CAPEs peaking between 500-1000 J/Kg) will allow for scattered to more numerous showers/tstms within the outlook area, a few capable of hourly or sub-hourly rainfall rates of 0.50-1.00". This will pose a localized flash flood risk, especially over any recent burn scars into northeast CA and southern OR. ....Eastern SD-Northeast NE into northwest IA and southern-central MN... The models show a mid/upper level shortwave trough pivoting across eastern portions of the northern Plains and across the far northern MS Valley on Day 3. Both the GFS and ECMWF do show some low-level frontogenetical response with this feature, along with pockets of upper level divergence which appear to be convectively enhanced. At this point, the QPF signal from the guidance is rather pedestrian, especially ahead of a fortified cold front that will pool PWATs of 1.25-1.5" and mixed-layer CAPEs of 1000-2000 J/Kg into the outlook area. Deep-layer kinematics appear suspect, with the current GFS and ECMWF indicating peak 850 mb S-SW flow of 30-35 kts. However, the pattern appears supportive of at least a Marginal Risk in the Day 3 ERO across this region, which is supported by the CSU GEFS-UFVS first-guess fields. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8U-CEKMPfRCX-uGciPBFGAT1dcc2U_oXYQW27d8vOqoY= bD-tS_NDr5tXE17iuZ8PlSaN4UXabPM2GldjsiNlQWx_H0Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8U-CEKMPfRCX-uGciPBFGAT1dcc2U_oXYQW27d8vOqoY= bD-tS_NDr5tXE17iuZ8PlSaN4UXabPM2GldjsiNlLbkE5OY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8U-CEKMPfRCX-uGciPBFGAT1dcc2U_oXYQW27d8vOqoY= bD-tS_NDr5tXE17iuZ8PlSaN4UXabPM2GldjsiNl-P8w2fk$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .