Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 06 2023 19:37:08 FOUS30 KWBC 061937 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 236 PM EST Mon Nov 06 2023 Day 1 Valid 16Z Mon Nov 06 2023 - 12Z Tue Nov 07 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHWESTERN WASHINGTON INTO NORTHERN CALIFORNIA... ....16Z Update... Maintained much of the previous forecasted MRGLs with no adjustment necessary for the risk area outlined across northwest OR and southern WA state. Across the Sierra's, lowering heights occurring through the period will allow for snow levels to fall enough to transition the southern extension of the previous MRGL across the northern Sierra's to snow or a rain/snow mix which will greatly cut the opportunity for flash flooding concerns. Elsewhere, the primary concern over the course of the morning and early afternoon will be the scattered bouts of convection with isolated storms as progressive shortwave energy will continue to traverse over the area off the southern periphery of the mean trough. 12z hi-res deterministic was consistent in lower coverage, but potential for 0.5"/hr rates within any convective cell that propagates over northern CA and southern OR may cause problems, in a local sense. Typically these rates would not be considered an issue, but the combination of water logged top soils from all the recent rainfall and multiple burn scars within the Klamath Mtn chain maintains a risk for localized flooding and mudslide concerns. Totals over the area are generally between 0.5-1" over the past 12 hrs, but some of the embedded convective areas have boosted local amounts up to 1.5-2.5", mainly within the terrain. WV satellite indicates a few more shortwaves off the OR/CA coast that are forecast to pivot onshore by late-morning through the afternoon before conditions really subside after 23z. Further north across northern OR and southern WA, a plethora of shortwaves are forecasted to move over the area through the course of the day with scattered to widespread showers and isolated thunderstorms to impact the coastal zones and Cascades to the east. This is the area where rainfall rates could be the highest over either of the risk areas with probabilities of 0.5"/hr settling around 40-50% over the two defined areas within the MRGL risk to the north. There is also a fairly robust signature for neighborhood probability of at least 2" through the period within the Cascades south of Mount Rainier down into the Columbia Gorge (70-80%). Precip will continue through the overnight hrs across the Cascades with a wanning signature for the coastal areas after 03z. Overall, the setup is still on the lower end of the MRGL risk threshold, but the compounding effect of convective rainfall on top of all the rain received the past 5-7 days could generate localized flooding concerns within the aforementioned areas. Kleebauer ....Previous Discussion... The Marginal risk was left mainly unchanged across portions of northwest OR into southwest WA. Both IVT and PW magnitudes are lower today compared to some of the previous days, so not really looking at an AR type of event today across this area. Instead rainfall will be more showery in nature, driven by the approaching surface low, mid level trough and embedded shortwaves. The increased synoptic ascent associated with these features and the steepening lapse rates will drive the showery rainfall threat. With the height falls and aforementioned steep lapse rates some instability will be present today...thus expecting we will see scattered heavier convective elements move across the area, locally enhancing rainfall rates. Convective cells will be quick moving, but 00z HREF guidance supports some localized 0.5" an hour rainfall within any heavier cells. Additional rainfall after 12z of 1-3" is expected across this area, with a few of the more orographically favored areas exceeding 3". This is on top of the 5"+ amounts that have fallen over the past few days, which has helped saturate the top levels of soil. Given this increased saturation most of this additional rainfall will become runoff..and thus localized areas of excess runoff within the aforementioned convective cells could result in some isolated areas of flooding. We did opt to add in a Marginal risk across portions of northwest CA and southwest OR as well. Generally the same story here, with steepening lapse rates resulting in embedded convective cells moving across the area today. The core of heaviest rainfall here should be exiting by 12z but can not rule out some heavier cells lingering just beyond 12z. Also, some additional convective cells appear likely to move ashore through the day as well...although the activity after 12z should generally be less organized. Nonetheless sub hourly rainfall of over 0.25" appears possible through the day...which combined with the antecedent rainfall and some sensitive burn scars in the area...suggests some localized flood risk could linger. A Marginal risk was also extended into portions of the western slopes of the northern Sierra as well. This area has not seen as much antecedent rainfall, and thus is generally not as sensitive to localized flooding. However this is the corridor where the highest rainfall rates appear likely today, which may offset the drier soils and still produce a localized threat. Areas of enhanced convergence as a cold front drops south and interacts with the terrain should result in a few periodic linear backbuilding convective segments, which may be capable of dropping localized 0.5"-1" an hour rainfall.=20 Chenard Day 2 Valid 12Z Tue Nov 07 2023 - 12Z Wed Nov 08 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard/Kleebauer 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!8o-H4T9UgUv3LGC_PIBUzwuSnAI6n27_PRQAAM0JTjQJ= UjFOnFsC8gY4N3E688YtkBddUcWU6UkRUQasuqjbA4BhR94$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8o-H4T9UgUv3LGC_PIBUzwuSnAI6n27_PRQAAM0JTjQJ= UjFOnFsC8gY4N3E688YtkBddUcWU6UkRUQasuqjbs3RgqCk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8o-H4T9UgUv3LGC_PIBUzwuSnAI6n27_PRQAAM0JTjQJ= UjFOnFsC8gY4N3E688YtkBddUcWU6UkRUQasuqjbLdyFNgY$=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) .