Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 01 2022 08:47:22 FOUS30 KWBC 010847 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 346 AM EST Thu Dec 01 2022 Day 1 Valid 12Z Thu Dec 01 2022 - 12Z Fri Dec 02 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS ALONG MUCH OF COASTAL CALIFORNIA... The upper-level trough will move onshore and pass through the Intermountain West while onshore flow with this system overspreads the Coastal Ranges inland to portions of the Sierra Nevadas and southern Cascades. The latest guidance has continued the slight southward trend with the rains that move onshore and the nearly perpendicular flow to the terrain will result in a fair amount of orographically enhanced rainfall. There is a general consensus for localized rainfall rates reaching 0.50 inch/hour (possibly exceeding) along the coastline. The HRRR also is maintaining the lead on some of the highest amounts with rates occasionally exceeding 0.75 inch/hour; but the HRRR and HREF have areal averages near or greater than 2 inches in some of the favored upslope areas. There are several recent burn scars in this part of the state. Rainfall with hourly rates great than 0.50 inch/hour will increase the threat for rapid runoff which may lead to debris flows in the sensitive burn scar locations and other known vulnerable locations. With colder air filtering into the Pacific Northwest and northern California the snow levels will be falling thus reducing the threat for excessive rainfall. The northern portion of the inherited Marginal Risk area was trimmed down once again due to the southward shift over central and southern coastal California. Campbell Day 2 Valid 12Z Fri Dec 02 2022 - 12Z Sat Dec 03 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA, NORTHWEST CALIFORNIA AND WESTERN OREGON... During this period another fast approaching Pacific system will usher in another plume of deep moisture that global guidance depicts areal averages of 1 to 4 inches over recently saturated soils. The latest runs have shown a slow up with the timing and how far inland the higher QPF reaches. The guidance is starting to diverge on keeping the focus of the highest amounts over southwest Oregon and favoring further up the coast to northwest Oregon. Both areas are still within the Marginal Risk area that remain in effect and only minor adjustments were made to reflect the latest trends and WPC QPF. This stretch of the coastline and the adjacent terrain will continue to be sensitive to runoff, debris flow and localized flash flooding. There continues to be appreciable model differences with regard to the timing and intensity of incoming Pacific system and the trailing front across the Great Basin and southern California, so confidence in the outlooks are lower than usual. This means the spatial placement has the potential to change significantly in subsequent outlooks, with outright removal of the Marginal Risk area over Southern California possible (with most support for it now coming solely from the ECMWF/ECENS). The ECWMF probability of exceeding 0.50 over southern California is less than 10 percent. In contrast, the HRRR, NAM CONest and HREF all show isolated maxes upwards of 2 to 4 inches. For now, the Marginal Risk was maintained but the confidence of occurrence is below average. Campbell Day 3 Valid 12Z Sat Dec 03 2022 - 12Z Sun Dec 04 2022 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND THE SOUTHWEST... ....California... A deepening upper low is approaching the West Coast while aiming a long swath of moisture toward the coast and points inland. This will result in an additional 1 to 3 inches of rain overspread coastal Oregon and far northwest California during this period. Given that this stretch of coast has be multiple days with measurable rainfall it will continue to be primed for areas of rapid runoff and increased vulnerability to mudslides and debris flows near sensitive areas such as burn scars. A Marginal Risk is in effect for most of western Oregon and far northwest California. ....Arizona, New Mexico and far western Texas... The frontal boundary is expected to stall over northern/central portions of the Southwest before sinking south again as a cold front during the later half of this period. A deep plume of moisture will stream into the Southwest bringing PW values of 0.50-0.75 inches (~ + 1 sigma) for parts of central Arizona and New Mexico. As the upper-level low nears the California Coast, southwest flow over the Great Basin and Southwest increases the broad areas of forcing for ascent. Convection is expected to fire up across portions of southern and central Arizona and New Mexico. Some of the guidance show areal average near or greater than 1 inch, especially across central Arizona. Rainfall in this quantity for reach or exceed local FFG. A Marginal Risk is in effect for southern parts of Arizona and New Mexico. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5J3sRBzZ50v75vdc6tRZk4ID7vo2K4cQs8zCENhylxLp= h7EVCBgKRWJZrfXZc80hbGCBzKYnWC7Fv0CLvbJZlvNQxz8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5J3sRBzZ50v75vdc6tRZk4ID7vo2K4cQs8zCENhylxLp= h7EVCBgKRWJZrfXZc80hbGCBzKYnWC7Fv0CLvbJZ_Lb8cqY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5J3sRBzZ50v75vdc6tRZk4ID7vo2K4cQs8zCENhylxLp= h7EVCBgKRWJZrfXZc80hbGCBzKYnWC7Fv0CLvbJZAXFtpYM$=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) .