Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 28 2023 08:50:47 FOUS30 KWBC 280850 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 449 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 ACROSS THE BLUE RIDGE OF NORTH CAROLINA AND FAR SOUTHWEST VIRGINIA, PARTS OF SOUTHWEST NEBRASKA SOUTH OF THE SAND HILLS, AND FROM PORTIONS OF NORTHWEST TEXAS SOUTH TO THE RIO GRANDE RIVER.... ....Central Appalachians into parts of the Mid Atlantic... The surface low will migrate onshore near the northern SC shores this morning, then weaken while drifting slowly northward and becoming vertically stacked across NC. 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 if western NC to nearby southwest VA. For areas farther east, additional organized north-south bands of convection are possible in eastern NC Sunday morning within an environment of 1.5" PWATs. Despite some of this rain falling atop soils that are almost guaranteed to be fully saturated from the previous day's rainfall, the coverage and intensity suggest a MRGL risk is sufficient to cover the excessive rain risk for most of the region. Other isolated instances of localized heavy rain are possible across central NC and southwest VA into far eastern KY-TN associated with low level fgen 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 1.5-3" of additional rainfall is likely per the latest guidance (including the 06Z HRRR), due in large part to the additional upslope enhancement. This heavy rain falling atop soils saturated from Day 1 and into the inherently flashy terrain could result in more scattered instances of flash flooding. 12z HREF neighborhood probabilities depict 20-35% chances for 2" of rain in 6 hours into Saturday evening, which provides enough confidence to maintain the previous risk area. Per collaboration with WFO's RNK, RLX, and LWX, given the guidance trends (limited instability and less intense short-term rainfall rates), have trimmed the northern portion of both the Slight and Marginal Risk areas across central and southern WV-VA. ....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 1-3" are possible throughout much of the Marginal Risk area, though a more elevated risk for flash flooding will be across parts southwest NE as well as parts of the TX Big Country south to the Rio Grande River, including the Edwards Plateau and western portions of the Hill Country and South TX Brush Country. This is due to the potential for additional heavy rain (more organized convection) during Day 1, 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 per the 00Z HREF. 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). Per the 00Z HREF, probabilities between 25-50% of QPF exceeding 3" during Day 1 are scattered throughout the Slight Risk area. Hurley/Snell 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. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7SvRcpKCB1bsnayQGkMxVEptkaEu4KqW5XZ302lBzANH= exenFTBokBVY_DZ_Q-BIVpLLorLxI7W4wB7wwfQ32Q0uxj8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7SvRcpKCB1bsnayQGkMxVEptkaEu4KqW5XZ302lBzANH= exenFTBokBVY_DZ_Q-BIVpLLorLxI7W4wB7wwfQ3J7ffjIM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7SvRcpKCB1bsnayQGkMxVEptkaEu4KqW5XZ302lBzANH= exenFTBokBVY_DZ_Q-BIVpLLorLxI7W4wB7wwfQ3LYDmmeo$=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) .