Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 16 2023 20:08:29 FOUS30 KWBC 162008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM 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 EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHEAST TO THE CENTRAL GULF COAST AND ALONG THE MOGOLLON RIM OF ARIZONA... ....Mogollon Rim of Arizona... Much of the same environment and weather pattern will remain in place during the day 2 period when compared to Tuesday. A weak, but still notable upper level trough will track southeast across northwestern Mexico on Wednesday. PW values are to remain quite anomalous when compared to climatology, approaching 1" (+4 sigma), as southerly flow off the Pacific and up the Gulf of California advect into the region. These PWs would be much more common between July and September. As the trough moves east, favorable upper divergence will also remain over much of the Southwest. The Marginal Risk was expanded somewhat to include the broader threat for isolated convection capable of intense rainfall within flood-prone regions. Convection is likely to initiate near the terrain of the Mogollon Rim of Arizona and nearby ranges, but the potential exists for storms to migrate slowly into the lower terrain before rapidly weakening given the dry air infiltration. The flash flooding threat is expected to be on the low-end and highly isolated, thus the Marginal designation. ....Southeast and Central Gulf Coast... The slow-moving front will settle across the Gulf states and portions of the Southeast by Wednesday, with additional rounds of unorganized, impulse convection the most likely storm mode. Deep moisture drawn northward (PWs around 1.5") over this stalled boundary will keep the threat for intense rainfall rates, though the flash flooding threat will be low due to very high FFG and sandy 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 northward expansion was made to the Marginal Risk into more of MS and AL due to the potential for morning convection along a sheared out upper-trough. Snell/Campbell Day 3 Valid 12Z Thu May 18 2023 - 12Z Fri May 19 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL/SOUTHERN PLAINS... ....Central/Southern Rockies to the Plains... A cold front surging southward on Thursday between a closed upper-level low over the Upper Midwest and separate closed upper-level low over northern Baja California will focus a risk of heavy rain from the High Plains of southeast CO/northeast NM to north-central OK and south-central KS. The tandem upper-level lows will situate this region within both a right-entrance region and left-exit region of upper jet streaks, promoting ample upper divergence. PWs are expected to be modest and between 1.25-1.5", around the 75th-90th climatological percentile, which combined with sfc-based CAPE up to 2000 J/kg is likely to aid in scattered convection growing upscale into a potential MCS further east across the Central Plains by the evening. Different antecedent soil conditions exist across parts of the highlighted risk area. Widespread severe to exceptional drought is found across western and south-central KS, as well as the western and north-central OK. This will limit any long-duration flooding and contain the risk to any immediate flash flooding due to overlapping thunderstorms with rainfall rates up to 2"/hr. Elsewhere, soils are much more prone to flooding across southeast Colorado and within any urban areas. The widespread drought conditions remain the limiting factor within much of the Slight Risk area over KS and OK, which will be monitored for any future downgrades should guidance depict lower rainfall rates or more progressive convection. ....Southeast... Persistent showers and thunderstorms expected across the Southeast near the stalled frontal boundary and near a newly forming weak upper-level low near the Southeast coastline. Convection should remain widely scattered and unorganized, while also occurring over parts of the country where terrain can handle brief periods of heavy rain. Urban areas and typically prone low-lying regions will be most at risk to the isolated flash flood threat. There has also been a trend for the aforementioned upper-level low and associated surface low to remain closer to the GA and SC coasts, which has increased the potential for slow-moving locally heavy rain across this region. A Marginal Risk remains in effect and was expanded to include the South Carolina Lowcountry. Snell/Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Uy4L2OPH3qc5SmTc531yZNQpn_LIbzTZ1hkdmIM6C-g= pSLw66f4JBlFlGol5Sk4aI9U9rX0ajCRIfO3Xu1OWL2uCg0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Uy4L2OPH3qc5SmTc531yZNQpn_LIbzTZ1hkdmIM6C-g= pSLw66f4JBlFlGol5Sk4aI9U9rX0ajCRIfO3Xu1O2y_vC8s$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Uy4L2OPH3qc5SmTc531yZNQpn_LIbzTZ1hkdmIM6C-g= pSLw66f4JBlFlGol5Sk4aI9U9rX0ajCRIfO3Xu1Oc1WvFMk$=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. 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