Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 30 2023 19:51:47 FOUS30 KWBC 301951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Tue May 30 2023 Day 1 Valid 16Z Tue May 30 2023 - 12Z Wed May 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE WEST, MIDWEST, AND PLAINS... ....Northern CA northeast across ID and southwestern MT/northwestern WY... A Mid-upper low off the Central CA Coast this morning will slowly push inland and wobble through SoCal this evening. Favorable upper-level diffluence 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 this afternoon. PWATs are expected to climb to between 0.75-1.00", which is ~1.5 standard deviations above normal for the end of May (and above the 90th percentile), which is sufficient for heavy rain concerns at elevation. Combination of upper forcing, moisture, weak flow aloft, along with increased daytime instability (mixed layer CAPEs peaking between 500-1000 J/Kg) will allow for scattered to more numerous showers/storms within the outlook area -- a few of which will be capable of hourly or sub-hourly rainfall rates of 0.50-1.00" which is around FFG values. 12Z HREF neighborhood probs for exceeding 1"/hr rates and/or 1/3-hr FFG values max out around 40%, supporting at least a Marginal Risk outline. Lack of focus for convection (aside from terrain) and dependencies on subsequent convection-induced mesoscale boundaries precludes a focused Slight Risk contour. Generally, a localized flash flood risk, especially over urban/poor drainage areas and any recent burn scars into northeast CA and southern OR, have a locally higher risk. ....Northern and Central Plains into Midwest... A pair of mid/upper level shortwave troughs will pivot across eastern portions of the Northern Plains and across the far northern MS Valley today. Weak/modest upper divergence combined with surface lift near and in advance of a frontal boundary will support a heavy rainfall threat with afternoon and early evening convection. This largely covers a similar area to the SPC area as well. PWATs look to peak around 1.3" (near the 75th percentile, per TOP sounding climatology), with SB CAPE rising to as high as 3000 J/Kg into the outlook area. Low-level inflow and effective bulk shear appear sufficient to organize convection that forms, especially initially along the CO/KS border and moving east. Southern extent has potential for a more appreciable excessive rainfall threat (OK/TX Panhandles, where FFG values are a bit lower) depending on the forward speed and development of the southern end of convection. 12Z HREF neighborhood exceedance probabilities continue to suggest the potential for locally extreme amounts (3-5"+), but it is too difficult to pinpoint a specific area within this broad region (as indicated by a lack of consistent HREF EAS neighborhood probabilities for 1" exceedance). Did adjust the area slightly over southwestern MO and the TX Panhandle to account for some 12Z CAM guidance that showed potential for convection farther south later this evening/overnight. ....Southeast OK into Northeast TX and Ark-La-Tex... Precipitable water values of 1.5"+ and SB CAPE expected to increase to 1000-3000 J/kg support the Marginal Risk area near the Ark-La-Tex this afternoon. Low-level inflow appears weak, only on the order of 10-15 kts, but the mean 850-300 hPa winds are virtually null (which suggests minimal if not chaotic storm motion and better rainfall efficiency than average from storms that do form and maintain). Effective bulk shear may be as high as 25 kts on the western edge of the MRGL risk area, which may help to organize storms (and would potentially organize a complex, should upscale growth be sufficient). Even so, FFG values are quite high in this area so any excessive rainfall concerns should remain isolated. Fracasso/Churchill/Roth Day 2 Valid 12Z Wed May 31 2023 - 12Z Thu Jun 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN HIGH PLAINS... ....West into the Northern Rockies and Plains... The localized risk for excessive rainfall looks to continue across portions of northern CA into northwest NV into Day 2 (beginning 12Z Weds), also extending farther northeastward across ID/MT and into ND as well. This is primarily due to the aforementioned mid-upper low over SoCal beginning to lift northeastward through the Four Corners region (with another shortwave trough in the northern stream digging on its approach into the PacNW). Axis of sufficient instability, above normal PWs (as high as ~0.9" and around the 90th percentile per the NAEFS) and generally weak flow aloft will combine with afternoon heating to support another day of isolated to scattered showers and storms. 12Z CAM guidance shows the focus a bit farther northeast from CA/NV where they have seen some locally higher QPE, where a targeted Slight Risk may be warranted (contingent upon rainfall D1). Lower FFG values over ID could support such an upgrade as well, but scattered/random nature of post-initiated convection has been a hindrance to outlining a higher threat. ....Central and Southern High Plains... In coordination with ABQ, upgraded an area of northeastern NM into the western TX/OK Panhandles to a Slight Risk for Wednesday. Upper low moving through the Desert Southwest and a ~100kt subtropical jet across northern Mexico will aide in providing lift to areas just east of the Rockies. Initiation and focus of convection will likely be a bit southwest of the D1 area with a moist south-southeasterly flow promoting PWs near and over 1.25" (about the 95th percentile) with CAPE to 1000-2500 J/kg. Organized thunderstorms will slip eastward across northeastern NM into the TX/OK Panhandle during the afternoon per the 12Z model consensus, and although it is not universal, the multi-cycle trends support the upgrade over this area as they have been relatively wet as well in the past week (QPE 500-600%+ of normal). This generally aligns with the SPC Slight Risk for severe (southern half) and the CSU machine-learning first guess fields. Farther north, FFG values are lower over northeastern CO into NE/KS, but sufficient QPF may be focused farther south closer to the better inflow/moisture flux and forcing. Fracasso/Churchill Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-dO26be9B12PqRJq_5jicxwaRYL20SeTF8LK5ni9qQaD= pIYal07-bO4fJGbQD6JH6v05e2_lGNkUOud1_1ErrHqZPSs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-dO26be9B12PqRJq_5jicxwaRYL20SeTF8LK5ni9qQaD= pIYal07-bO4fJGbQD6JH6v05e2_lGNkUOud1_1ErEO4BTb0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-dO26be9B12PqRJq_5jicxwaRYL20SeTF8LK5ni9qQaD= pIYal07-bO4fJGbQD6JH6v05e2_lGNkUOud1_1Er6mRNn-M$=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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