Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 16 2023 23:36:42 FOUS30 KWBC 162336 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 635 PM EST Mon Jan 16 2023 Day 1 Valid 01Z Tue Jan 17 2023 - 12Z Tue Jan 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA AND ARIZONA... ....Southern and Central California... Upper level troughing pushing inland into central to southern California continues localized heavy rainfall potential, with periods of showers and thunderstorms for the remainder of Monday afternoon and evening. As MU CAPE values have risen above 100 J/kg, hourly rain totals have occasionally reached 0.5" in the central Valley. The 18z HREF shows a brief uptick is possible again in southernmost CA, so left the previous Marginal Risk area down there intact. 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.=20 Favorable upslope flow of the anomalous PW values into the Mogollon Rim will support persistent showers and thunderstorms into tonight. A broad area with MU CAPE of 100-500 J/kg has been building across portions of AZ and the CO River Valley during the past several hours -- should instability build further, organized convection can not be ruled out as effective bulk shear in the region in 30-50 kts. The 18z HREF shows hourly rates briefly eclipsing 0.5" over the next several hours, which could be a concern in slot canyons and locations with burn scars.=20 Adjustments to the previous Marginal Risk areas were small, generally based on the 18z HREF probabilities of 0.5"+ in an hour. Roth 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!6Tpu4cjXr7rKaH52EBQTUsSOITgZXtejT9R5OiRNBG4Q= Tj34NZc00PLRxses-nKHFms5sLrrPpmRz0_wOC1ucaUzgfA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Tpu4cjXr7rKaH52EBQTUsSOITgZXtejT9R5OiRNBG4Q= Tj34NZc00PLRxses-nKHFms5sLrrPpmRz0_wOC1u4w4y_PA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Tpu4cjXr7rKaH52EBQTUsSOITgZXtejT9R5OiRNBG4Q= Tj34NZc00PLRxses-nKHFms5sLrrPpmRz0_wOC1uIgnXcnU$=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) .