Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 27 2023 08:30:53 FOUS30 KWBC 270830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Sat May 27 2023 Day 1 Valid 12Z Sat May 27 2023 - 12Z Sun May 28 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE COASTAL PLAIN OF NORTHEAST SOUTH CAROLINA THROUGH THE CRYSTAL COAST OF NORTH CAROLINA... ....Carolinas... Consolidating area of low pressure off the SC coast will slowly strengthen and lift onshore the SC coast later Saturday evening and overnight as it gets pulled northwestward by a closed mid-level low over the Southeast. To the north of this feature, an impressive surface high pressure will expand southward from New England the Great Lakes, producing a tightening pressure gradient across the area. This will result in intensifying easterly winds through a deep layer of the atmosphere, leading to strong moist advection and widespread heavy rainfall. NAEFS ensemble tables indicate the negative U-wind or easterly component will become as strong as 4 to 5 standard deviations above normal across much of the eastern Carolinas on Saturday, surging integrated water vapor transport or IVT anomalies between +3 and +4 and PW anomalies to +1.5 to +2. Aiding in this robust moisture transport will be 850mb winds reaching 35-50 kts, with the resultant WAA driving warm cloud depths to around 13,000-14,000 ft, sufficient for optimal warm rain processes. This evolution will release 500-1000 J/Kg of MUCAPE across along the coast from northern SC into NC. Meanwhile, with 850mb winds peaking around 2x the mean of the 0-6km winds will allow convective bands to back-build across the Gulf Stream, leading to cell training. Together, these parameters produce an environment extremely supportive for heavy rain, supported by the uptick in HREF exceedance probabilities. The 00Z HREF in fact shows several hours of elevated probabilities (40-60+ percent) of >1"/hr rainfall rates migrating slowly northeast across the Moderate Risk area during much of the day 1 period (through ~10Z Sun). Meanwhile, probabilities of >2"/hr rates peak between 25-35 percent across the southern NC Coast (around Cape Fear) between 13-18Z Sat. Probabilities of the 24hr QPF exceeding 3" peaks above 50 percent over the entire Moderate Risk area, while probs of >5"/24hr peak between 60-70% over coastal southern NC along and north of Cape Fear. Individual cell motions are expected to be relatively quick at 20-25 kts to the west/southwest. However, Corfidi vectors anti-parallel to the mean wind indicate a likelihood for backbuilding into the greater instability just offshore, which will yield training of echoes from east to west. Where these heavy rates can train, rainfall will likely quickly eclipse 2-4" through Saturday across the Coastal Plain, with locally higher amounts of more than 8 inches possible per the latest CAM output, highest near Cape Fear where the aforementioned HREF probabilities are greatest and frictional convergence onshore could enhance ascent. The Moderate Risk area was largely unchanged from yesterday afternoon's Day 2 outlook, though we did extend it slightly southward (just south as Winyah Bay, SC). Despite the sandy soils inherent to this area, this rainfall will likely produce instances of flash flooding, with urban areas being most sensitive to this rainfall. Farther to the west, a secondary maxima of rainfall is likely into the Blue Ridge mountains of NC as the impressive and moist easterly wind upslopes efficiently into the terrain, driving robust confluence of the moisture transport vectors. This is reflected both by deterministic guidance as well as a relative peak in probabilistic rainfall data noted by 20-25% probs for 5 inches per the 00Z HREF. ....High Plains through the Northern Rockies... Another day of widespread convection is likely downstream of a longwave trough ejecting slowly across the Intermountain West. Significant mid-level ridging elongated across the Great Lakes will gradually shift east allowing for at least some eastward translation of this trough, but at the same time it will be slowed by a reinforcing shortwave deepening and dropping across the Pacific Northwest. Downstream of the longwave trough and pinched between it and the high pressure, low-level meridional flow will intensify noted by 850mb winds surging to 20-30 kts across the Southern Plains, and 40+ kts farther north into the Dakotas and Montana. This air will originate from the Gulf of Mexico as return flow develops around the high to the east, helping to drive PW anomalies of +1 to +2 standard deviations northward as far as the Canadian border. This helps to lengthen the moisture transport vectors northward, coincident with increasing MUCAPE during the aftn/eve which is progged to reach 2000-3000 J/kg along a ribbon draped across much of the High Plains. While this setup should support scattered convection across a large area once again, there may be two areas of focus for a greater chance of flash flooding. The first is across Montana and northwest ND where rainfall over the past 24hrs along with the 7-day rainfall anomalies of 200-300% of normal per AHPS produce more favorable antecedent conditions for flash flooding. Additionally, low-level flow will back to the east around an area of low pressure, leading to some enhanced isentropic ascent atop a stalled front, while also increasing the upslope ascent into higher terrain. This moist ascent combined with reduced storm motions in an area of greater storm coverage could result in locally 2-3" of rainfall as forecast by the HREF probabilities reaching 10-20% for more than 3 inches. The inherited Slight Rish was expanded a bit to the east to include more of eastern and northeast MD and a small portion of northwest ND, accounting for recent heavy rainfall (ongoing through the pre-dawn hours Saturday) along with trends int the newer guidance. r Farther to the south across the TX Panhandle south into the Cap Rock, Rolling Plains, and northern TX Hill Country, a secondary area of greater flash flood risk may exist as reflected by 00Z HREF probabilities for more than 3 inches of rain reaching 50%. This area is also pre-saturated with the heavy rainfall over the past 48hrs, as noted by higher USGS streamflow anomalies. Based largely on the trends in soil saturation and streamflows from the recent heavy rains, along with more widespread coverage of additional convection per the 00Z CAMs during the day 1 period (12Z Sat-12Z Sun), we felt more confident in including a more elevated (i.e. Slight) risk of excessive rainfall over these areas in TX. ....In and near Northern California, southern Oregon and northwest Nevada... The inherited small MRGL risk was adjusted slightly, but overall minimally. A shortwave dropping south out of the Pacific Northwest will result in some subtly enhanced ascent and moisture centered near the Shasta/Trinity region of northern CA. PW anomalies exceeding +1 standard deviation are likely, and 0-6km mean winds are weak suggesting slow storm motions. However, bulk shear is quite low to prevent much organization, HREF probabilities for even 0.5"/hr are modest, and antecedent conditions are quite dry. Still, a non-zero threat exists for isolated flash flooding, especially where any storms can tie to the terrain in the 500+ J/kg of SBCAPE, which would reduce motion even further. For this reason the MRGL risk was maintained. Hurley/Weiss Day 2 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 VIRGINIA.. ....Central Appalachians into parts of the Mid Atlantic... The surface low moves onshore Saturday evening and then weakens, almost in place, Sunday, as it becomes vertically stacked across NC. The entire system is likely to decay by the end of D3. However, at least during the first half of D3, Sunday and Sunday evening, periods of heavy rain producing showers are likely, especially in NC and VA. While the onshore flow will weaken, noted by 850mb flow falling to 15-25 kts, NAEFS ensemble tables still suggest v-wind anomalies of -2 to -3 standard deviations driving PWs of around 1.25" inland coincident with up to 500 J/kg of MUCAPE. This should still support rain rates of 1"/hr or more streaming westward and training at times. 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. The exception to this is across the Blue Ridge of NC and into southern VA where a more concentrated area of 1.5-3" of additional rainfall is likely per the latest guidance, due in large part to the additional upslope enhancement. This heavy rain falling atop soils saturated from Day 1 and into the more sensitive terrain could result in more scattered instances of flash flooding, and as such have maintained the Slight Risk area inherited from yesterday afternoon's Day 2 ERO. Otherwise, based on the latest guidance trends (including the extended ranges of the high-res CAM windows), have also expanded the Marginal Risk area northwest to include far eastern portions of KY, southeast OH, and central WV.=20 ....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. Once again, the guidance is struggling to point out any specific area that has a greater chance for convection, resulting in another broad MRGL risk for excessive rainfall. Return flow out of the Gulf of Mexico will continue to surge above normal PWs within this narrow north-to-south corridor, supporting heavy rainfall in any convection that develops in response to a ribbon of elevated SBCAPE reaching 2000 J/kg. The flash flood risk will be somewhat dependent on where rainfall is most intense on Day 1 as well, limiting confidence despite some signals for 2-3+ inches of rain per the latest guidance (including CAMs). One notable change was to extend the eastern portion of the Marginal Risk a little farther to the east across the Plains, based largely on the guidance trends. Hurley/Weiss Day 3 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... ....Parts of southern Oklahoma southward to the Rio Grand 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.=20 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_moc6mZaFCAX3gAUw0MdWugkXDyggEgVkZxJjPxiTgtk= YETtQdb58CG8ZHag5DCqWrJRSZdxjon8QGdWc-f-JwuvttU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_moc6mZaFCAX3gAUw0MdWugkXDyggEgVkZxJjPxiTgtk= YETtQdb58CG8ZHag5DCqWrJRSZdxjon8QGdWc-f-Wb_H4F0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_moc6mZaFCAX3gAUw0MdWugkXDyggEgVkZxJjPxiTgtk= YETtQdb58CG8ZHag5DCqWrJRSZdxjon8QGdWc-f-7mkDQiE$=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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