Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue May 16 2023 15:25:00 FOUS30 KWBC 161600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue May 16 2023 Day 1 Valid 16Z Tue May 16 2023 - 12Z Wed May 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY TO THE CENTRAL AND SOUTHERN APPALACHIANS... ....Eastern Ohio/Tennessee Valleys and the Central/Southern Appalachians... A frontal boundary draped from the Missouri Bootheel and east towards southwest VA along with a mid-level shortwave near southern IL is providing a focus for showers and thunderstorm development organized in a mostly west-east orientation this morning. These storms tracking east into the central/southern Appalachians may result in locally intense rainfall rates as the mountains act as additional forcing within largely southwest flow. Estimated rainfall rates per MRMS have remained under 1"/hr thus far outside of highly localized areas, but coverage and intensity is expected to increase this afternoon and evening as >1500 J/kg MUCAPE flows northeastward up the Tennessee Valley ahead of the approaching shortwave within an atmosphere that contains 1.5-1.75" PWs. In fact, the 12z sounding out of RNK contained a PW of 1.35", which is just under the daily record per SPC's sounding climatology. 12z HREF guidance has rates increasing across central KY after 19z and continuing on an eastward track towards southern VA tonight. Storms may weaken and dissipate some in the immediate lee of the mountains of southern Virginia, but are likely to reintensify as they move across the Virginia Tidewater. The 16z ERO update incorporated an expansion of the Slight Risk across south-central VA to account for the overlapping potential that spans back to the central Appalachians. This expansion also overlaps with the highest probabilities (40-50%) from the 12z HREF for 6-hour QPF to exceed 2" before 03z this evening. Much of the Ohio/Tennessee Valleys and the Appalachians have 3-hr FFG in the 2-2.5" range with embedded locations as low as 1 inch. There will be plenty of moisture, forcing and instability available over this region to yield rainfall totals as high as 3". Confidence is greatest across extreme southern WV, southwest VA, and eastern KY for rainfall amounts to potentially exceed FFG, where flashy basins are plentiful and can be more prone to flooding. While similar amounts are possible across southern VA, urban areas would be of most concern should west-east oriented bands set up and contain 1-1.5" hr rainfall rates. ....Mogollon Rim of Arizona... Abnormally high amounts of atmospheric moisture remain over the region, holding steady around 0.75+ inch which is +3.5 sigma for central Arizona this time of the year. Although the forcing will be on the weaker side associated with a meandering upper-level low near northern Baja California, it should be adequate to trigger convective activity that could produce heavy rain and isolated flash flooding. Flooding concerns will be elevated specially over any old burn scars, slot canyons, and dry washes. The Marginal Risk area was expanded slightly for this 16z update to the Mexico border based on 12z hires guidance, as well as into southwest UT to incorporate prone slot canyons that fall within the risk of scattered convection. ....Northern Great Basin and Northern Rockies... Showers and thunderstorms are expected across the region as a strong shortwave trough skims well northeast of the area. Antecedent moisture and orographically enhanced forcing for ascent may lead to locally heavy downpours and isolated flash flooding. Snowmelt and elevated rivers may exacerbate flooding concerns where heavier rates occur. The Marginal Risk was expanded somewhat to capture the more widely scattered convection highlighted by most 12z hires guidance. Snell/Campbell Day 2 Valid 12Z Wed May 17 2023 - 12Z Thu May 18 2023 ....THERE IS A MARGINAL RISK OF FLASH FLOODING FOR PORTIONS OF THE COASTAL SOUTHEAST AND ALONG THE MOGOLLON RIM OF ARIZONA... ....Mogollon Rim of Arizona... Much of the same environment and weather pattern will remain in place during this period. A weak, but still notable upper level trough will track southeast across northwestern Mexico on Wednesday. PW values to nudge even higher, approaching 1 inch (+4 sigma), as southerly flow off the Pacific and up the Gulf of California advect into the region. As the trough moves east, the signal for potential heavy rain shift as well. The Marginal Risk already in effect remains in good order as it highlights the areas with an elevated threat for flash flooding and excessive rainfall. ....Coastal Southeast... The slow-moving front will settle across the Gulf states and portions of the Southeast by Wednesday, with will continue to provide a focus convection. Deep moisture drawn northward over this stalled boundary will keep the threat for thunderstorms high, though the flash flooding threat will be low, as FFGs are very high over this region despite somewhat wetter than normal soils. The main flash flooding threat will be in the urban areas of the Marginal Risk, including New Orleans, Gulfport, Mobile, Pensacola, Tallahassee, Valdosta, and Savannah. A small westward expansion was made in South Carolina. Campbell Day 3 Valid 12Z Thu May 18 2023 - 12Z Fri May 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/SOUTHERN HIGH PLAINS... ....Central/Southern Rockies to the Plains... The risk for excessive rainfall and areas of flash flooding will be elevated during this period as a front hangs up in the south-central Rockies on Thursday and Friday, with upslope flow on its backside in an environment with 90+ percentile moisture anomalies. Much of this part of the country has had nearly saturated soils with high streamflows. Models suggest areal averages of 1 to 3 inches to fall in the High Plains and points east. These amounts could exacerbate the flash flooding risk. The segment of the front over the Plains in a west-east orientation and its possibly slow movement late Thursday could lead to training of storms with possibly high rain rates that form on the cusp of the instability/moisture near the boundary. A Slight Risk spans from southeast Colorado to south-central Kansas. ....Southeast... Persistent showers and thunderstorms expected across the Southeast near the stalled frontal boundary. Soils across the region will likely have increase saturation after multiple days of rainfall, thus lowering local FFG and increasing the threat for flash flooding concerns. A Marginal Risk remains in effect from southeast Alabama to the Florida/Georgia stretch of the coastline. Campbell $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .