Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 29 2023 00:52:12 FOUS30 KWBC 290051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Tue Mar 28 2023 Day 1 Valid 01Z Wed Mar 29 2023 - 12Z Wed Mar 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE CENTRAL CALIFORNIA COAST... California... Late afternoon satellite and radar imagery showed the heaviest rainfall pushing southward across the central California coast and eastward towards the Sierra foothills. So far...rainfall rates have been fairly modest and rates approaching a half-inch per hour continue to be reported occasionally...but the area has been particularly wet and any additional rainfall is likely to run off even with those kinds of rainfall rates. As a result...confined the Slight Risk area to areas south of the best moisture transport vectors as the southward shift should continue into the overnight hours. Also trimmed the northern portion of the Marginal and Slight Risk areas in the northern portion of California where dry air has been entrained into the circulation ahead of the mid- and upper-low. Given cold temperatures aloft...scattered rain showers at lower elevations and snow at higher elevations are still expect but the same degree of organization and precipitation rates are not expected there during the remainder of the Day 1 period.=20 Southeast U.S.... Removed the Slight and Marginal Risk areas from the Southeast. One area of rain was about ready to exit the outlook areas and move over portions of the northern Florida peninsula. Some consideration was given to placing a Marginal Risk area there as a shortwave trough over the northeastern Gulf of Mexico approaches.=20 Thinking was that an outflow boundary associated with the first area of rainfall would limit rainfall rates by stabilizing low-level lapse rates to prevent more than isolated problems with ponding...and short-term radar trends Bann Day 2 Valid 12Z Wed Mar 29 2023 - 12Z Thu Mar 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL AND SOUTHERN CALIFORNIA... ....2030Z Outlook Update... The ongoing forecast is on track, with an isolated flash flood risk persisting across central and southern California through 12Z Thursday. Onshore flow and IVT will continue on the southeastern periphery of a low west of the California coast. This low will slide southeastward toward the Los Angeles area through the forecast period, with occasional moderate to heavy rainfall occurring due to 400-500 kg/m*s IVT values affecting coastal ranges. Areas of 0.25-0.5"/hr rain rates remain probable in this scenario, with localized flash flooding occurring in hydrologically sensitive/urbanized areas. See the previous discussion for more information. Cook ....Previous discussion... The atmospheric river impacting California will slide southward into Southern California early on Wednesday and orient the axis of highest atmospheric moisture content (400-500 kg/m*s) orthogonal to the mountain ranges of SoCal. Snow levels are expected to be up to 6000 ft across SoCal as well, until crashing Wednesday night and remaining below 4000 ft for much of the state during the Day 2 period. Rainfall rates may briefly peak at 0.25-0.5"/hr while relatively quick moving across the southern part of the Marginal Risk, but saturated soils could support a localized flooding/rapid runoff threat. The second phase of the Marginal Risk (mainly across central CA, but eventually sliding into the Transverse Ranges) involves the vertically stacked strong polar upper-level low sliding across the region. A secondary convergence axis wrapped around this low along with steeping lapse rates and a subtle layer of instability may contain scattered convective showers capable of containing localized heavy rainfall rates. Per the global guidance and GEM-regional, this convergence axis and areas of scattered showers starts near the central CA coast and Sacramento Valley midday until sliding down the central CA coast and into the Transverse Ranges after about 00z Thurs. Total QPF amounts during this entire time period are not that impressive alone, but when combined with the potential 2-3" amounts just prior and the already overly saturated ground, these convective showers and periods of modest rainfall could produce isolated flooding concerns. Hurley/Snell Day 3 Valid 12Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE UPPER MIDWEST... ....21Z Outlook Update... The previous forecast generally remains on track. Very strong low-level flow (50-60 kts) will materialize across the Great Plains overnight Thursday into early Friday. On the nose of that strong flow, enough instability and convergence will materialize to support deep convection that may become surface-based across northeastern Nebraska and vicinity before migrating east-northeastward into a slightly more stable airmass. The orientation of convection could allow for spotty areas of convective training, with FFG thresholds generally around 1 inch/hr supporting isolated potential for excessive runoff. The main change to the ongoing outlook is to expand the Marginal east-northeastward into Wisconsin based on trends from most-recent model guidance that expands the heavier rainfall footprint into that area. The northern fringes of the risk area may experience near- or sub-freezing surface temperatures that could hinder runoff, and the area was adjusted to attempt to account for this. Additional details are located in the prior discussion below. Cook ....Previous discussion... Vigorous deep-layer trough traversing the Great Basin and Southwest on Thursday will lift across the central-southern Rockies Thursday night. Increasingly difluent upper levels across the Upper Midwest during day 3 will make for a more favorable environment for deep-layer ascent, particularly Thu night as the the global models hint at enhanced right-entrance region forcing from the departing northern stream upper jet streak. A rapid increase in low-mid layer moisture transport from the western GOMEX is evidenced by both the GEFS and SREF, as 850-700 mb moisture flux anomalies climb quickly between 3 and 4 standard deviations above normal Thursday night on the heels of robust SSW 850 mb flow of 50-60 kts. At this point, the guidance is fairly transient with the areas of favorable dynamic and thermodynamic parameters, indicative of a fairly progressive warm front, transition into the warm sector (at least across southern portions of IA, where the 00Z ECMWF shows surface-based CAPEs climbing above 1000 J/Kg), and the cold frontal passage west to east on Friday. Because of this, at least for now, will maintain the Marginal Risk inherited from yesterday's Day 4 ERO. However, the ingredients are certainly present for a more enhanced risk, perhaps farther south, depending on the evolution of the system and upper-level pattern, and whether or not any further uptick and/or added duration of low-level frontogenesis can retard the frontal progression, thus prolonging favorable deep-layer forcing and potentially leading to more cell training. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_igO9BQpY73GayN5uAWf4zWqGBM6kOKcjDuRnOcW2TqC= 5dyS3vDQy7UY7Mg4WXL8Qm92Ih6gEqBw56sATiv6qUuCTDA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_igO9BQpY73GayN5uAWf4zWqGBM6kOKcjDuRnOcW2TqC= 5dyS3vDQy7UY7Mg4WXL8Qm92Ih6gEqBw56sATiv60TMNA1o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_igO9BQpY73GayN5uAWf4zWqGBM6kOKcjDuRnOcW2TqC= 5dyS3vDQy7UY7Mg4WXL8Qm92Ih6gEqBw56sATiv6-WNb960$=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) .