Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 16 2023 20:18:11 FOUS30 KWBC 162018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 317 PM EST Mon Jan 16 2023 Day 1 Valid 16Z Mon Jan 16 2023 - 12Z Tue Jan 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA AND CENTRAL ARIZONA... ....16Z update... ....Southern and Central California... The Marginal Risks were maintained with this update for both central and southern California, mainly due to antecedent conditions. While the axis of higher IVT has pushed south into Mexico, 2-4 inches of rain has been reported across southern and central portions of the state over the past 24 hours and localized heavy rain may still occur through the rest of the day today. Showers are expected to continue for most of the period for coastal southern California into the Peninsular Ranges with shallow instability developing beneath a relative warm nose in the 700-500 mb layer which could continue to support hourly totals in the 0.25 to 0.5 inch range, mainly focused into the terrain. Farther north, a relatively deeper layer of instability, with values locally above 500 J/kg, is expected to develop beneath the low tropopause (~400 mb) despite precipitable water values of only 0.5 to 0.8 inches given cold mid-level temperatures as the upper low moves overhead. For both locations of California, additional rainfall totals of 1 to 2 inches, locally 3+ inches, will be possible through 12Z Tuesday, which may support a continued threat for runoff. ....Central Arizona... 12Z HREF supports hourly totals locally above 0.5 inches late this afternoon into early tonight across portions of central Arizona along the leading edge of higher low level moisture transport into the region with 25 to 35 kt of 850-700 mb flow from the southwest. Rainfall totals through 12Z Tuesday are forecast to be in the 1-3 inch range with generally weak instability below 500 J/kg. Wetter than average antecedent conditions supports maintaining the Marginal Risk. Otto ....previous discussion follows... ....Southern and Central California... Additional height falls will be pushing inland day 1 into central to southern California continuing localized heavy rainfall potential through at least the first half of the upcoming forecast period. No changes made to the previous two marginal risk areas over California, one from the Peninsular Range west to the coast, including the eastern portions of the Transverse range and the second across central California in the upslope of the central to southern Sierra. For the southern marginal risk area, the threat appears to be for the first few hours of the day 1 period when ..25"+ hourly rates are possible. This may add up to an axis of 1"+ rains from the eastern portions of the Transverse Range, south through the Peninsular Range, with HREF neighborhood probabilities for 1"+ amounts high, 60-90% across these areas. By late morning, the anomalous pw axis will have shifted farther south into northwest Mexico with any additional showers likely to have hourly rates maxing out around .10". Given elevated stream flows from recent heavy rains, additional runoff issues are possible early day 1. For the northern precip area in the upslope of the Central to Southern Sierra, precip may be more prolonged, supporting .25-.50" hourly precip amounts into late morning, followed by a second period of showers Monday afternoon as a second area of height falls push off the Pacific into Central California. Hourly rates not expected to be as great with the second period of rains,=20 mostly .10"+. Similar to the areas across Southern California, additional runoff issues possible across areas where stream flows are high and soils saturated from recent heavy rains. ....Central Arizona... An axis of above average PW values, 2 to 3+ standard deviations above the mean, will persist across central to southern Arizona day 1. Favorable upslope flow of the anomalous PW values into the Mogollon Rim will support persistent showers for most of the upcoming day period. While hourly rates are expected be generally below .25", the prolonged showers conditions will support a concentrated axis of 1"+ totals day 1 across south central Arizona in the upslope of the Mogollon Rim. With stream flows mostly near normal across this area, the threat level was maintained at marginal. Oravec Day 2 Valid 12Z Tue Jan 17 2023 - 12Z Wed Jan 18 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 3 Valid 12Z Wed Jan 18 2023 - 12Z Thu Jan 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI AND LOWER TO MIDDLE OHIO RIVER VALLEYS... A closed mid-level low is forecast to track from western KS Wednesday morning to IA through Thursday morning while an occluded/cold front follows suit at the surface. While the front and preceding axis of low level moisture transport are expected to be fairly progressive toward the east, precipitable water values of 1.5 up to 2 inches are forecast by the 12Z model consensus to track across the middle/lower Mississippi Valley/central Gulf Coast while values of 1.2 to 1.4 inches are forecast for the Ohio Valley. These precipitable water values equate to standardized anomalies of 2 to 3+ for the entire region. Prefrontal convection is likely to expand across portions of eastern Texas into the lower Mississippi Valley late Wednesday morning with 30-40 kt of 850 mb flow from the south. Unidirectional flow from the southwest will support areas of training from heavy rain echoes that align from southwest to northeast, mostly with any activity that forms ahead of the main frontal band of thunderstorms (given the progressive nature of the front). CAPE will be a limiting factor for heavy rain north of the warm front (warm front to lift north from the lower Mississippi Valley into the upper Ohio Valley through the period) but FFG is lower with northward extent (FFG of 2 inches or less in 3 hours for many locations from northeastern Arkansas/western Tennessee into southwestern Ohio). There will also be better jet support for lift (divergence/diffluence) across the Ohio Valley given the track of the upper trough which should help compensate for reduced instability. South of the warm front, CAPE is forecast by the 12Z NAM/ECMWF (GFS is lower) to be 500 to 1000+ J/kg which could easily support rainfall rates of 2+ in/hr with training given the available moisture. There was generally good agreement with the mass fields from the 12Z models with only minor timing differences and uncharacteristically weaker moisture return/low level jet in the 12Z NAM Wednesday morning across the South. The outlook remains largely the same from the previous issuance with generally 1-3 inches (locally higher possible) expected within the Slight Risk area. The Marginal Risk is broader to the south and east of the Slight, covering portions of Alabama, central/southern Mississippi to the west-central Gulf Coast where the probability of heavy rain is lower, but the potential for locally higher totals is present given forecasts of moisture/instability. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!916LBSg88b1seR1ocrWKFNOVHsIkZjJwhEK0hbMoobrZ= 5MOoeNEfiqvo4M0XTGJiZ-V1OVV2FNuLnp9my1vHH7XH9dI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!916LBSg88b1seR1ocrWKFNOVHsIkZjJwhEK0hbMoobrZ= 5MOoeNEfiqvo4M0XTGJiZ-V1OVV2FNuLnp9my1vH3_PPVyE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!916LBSg88b1seR1ocrWKFNOVHsIkZjJwhEK0hbMoobrZ= 5MOoeNEfiqvo4M0XTGJiZ-V1OVV2FNuLnp9my1vHqWyI4sc$=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) .