Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 30 2022 20:12:41 FOUS30 KWBC 302012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 311 PM EST Wed Nov 30 2022 Day 1 Valid 16Z Wed Nov 30 2022 - 12Z Thu Dec 01 2022 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND ALONG THE COAST OF OREGON AND CALIFORNIA... ....16z Update... The threat for flash flooding is gradually coming to an end across the Southeast, as the parent upper-level trough responsible for storms overnight lifts to the northeast with meager instability (MU CAPE generally 500 J/kg or less) ahead of the diminishing squall line that is moving into the FL Panhandle. Farther south into South Florida, instability is more plentiful (SB CAPE already 2000-3000 J/kg) with a fair amount of deep layer shear (~20 kts) from the southern periphery of the sub-tropical jet. This is expected to result in some semi-organized convective activity this afternoon with a non-zero chance for localized, urban flooding issues (but below Marginal Risk criteria). However, mid-level lapse rates remain quite poor (5.5-6.0 degC/km) with ridging aloft likely significantly limiting the overall intensity of convection. Looking out to the West Coast, the event has yet to meaningfully begin with no changes to our thinking (or Marginal Risk outlook area itself) since the prior discussion. Churchill ....Previous Discussion... ....Southeast... A broad area of thunderstorms with an embedded squall line have maintained strength and momentum during the overnight hours and continue to trek across the Gulf states and into portions of Southeast and Tennessee Valley this morning. The whole area of convection is expected to shift eastward as the warm front lifts up the eastern Tennessee Valley/Central Appalachians and the cold front sweeps through the Southeast. These storms should be fairly fast moving by this point, using up the last of the instability from Tuesday. Recent rains including the previous day's amount have left parts of the Southeast with nearly saturated soils. Additional rains during this period will keep an elevated threat for excessive rainfall and flash flooding. The forcing and instability diminish through the day, so the Marginal Risk is largely for remaining thunderstorms Wednesday morning. ....West Coast... An approaching low pressure system is expected to usher in 1 to 3 inches, with isolated higher amounts probable, along the west coast of Oregon and northern California and inland to the Cascades. The latest guidance continues to show the highest QPF focusing over southwest Oregon and northwest California as the low sinks southward near the coast. Additionally, the CAMs are suggesting very isolated maxes of 5 to 6.5 inches possible in the highest terrain. Due to the speed of the front and cold air punching in from the north, snow levels will plummet over Oregon. This will reduce the degree of possible flooding concerns. However, the primary area of rainfall will stay ahead of any falling snow levels but hourly rainfall rates may occasionally approach (or even exceed) 0.5-0.75"/hr.; which is enough to be problematic in sensitive prone areas. The Marginal Risk that was already in effect had minor adjustments made to reflect the latest guidance trends and WPC WPF. Campbell Day 2 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... ....2030z Update... No changes to the inherited outlook, as model agreement on the overall QPF footprint remains solid through the day 2 period. With the inclusion of the full HREF suite, there is general agreement that localized rainfall rates will approach (and perhaps occasionally exceed) 0.5"/hr along much of the California coast. The HRRR is notably the most intense of the CAMs, depicting localized rainfall rates occasionally exceeding 0.75"/hr. This is quite an outlier amongst the CAMs, particularly along upslope portions of the Santa Lucia Range and San Rafael Mountains where output indicates localized totals as high as 2-4" (compared to the HREF mean of 1-2"). This will need to be monitored for a potential targeted Slight Risk upgrade(s), but given the outlier status of the HRRR (and so far underperformance of the ongoing event across coastal OR/CA) the Marginal Risk will be maintained for the time being. Churchill ....Previous Discussion... During this period the trough/surface low will continue to sink southward and move inland while spreading rain and mountain snow to western Oregon and California. The plume of moisture directed onshore with this system will be increasing through the day reaching +1.5 to +2 sigma. The nearly perpendicular flow to the terrain will result in a fair amount of orographically enhanced rainfall and hourly rainfall rates may exceed 0.50 inch/hour. Recent rains will have already raised soil saturation and increase sensitivity prior to this period. 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 to only cover portions of central and southern California. Campbell 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!7O0vphZehZEP4-ty0o_sf1r5oeASh031AYCiyncLBWiT= O64p1-uW7nNwj_m5i97W8sahJCI3mw6v_4BmZ2AY8F7Xk2k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7O0vphZehZEP4-ty0o_sf1r5oeASh031AYCiyncLBWiT= O64p1-uW7nNwj_m5i97W8sahJCI3mw6v_4BmZ2AYrDaNPzE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7O0vphZehZEP4-ty0o_sf1r5oeASh031AYCiyncLBWiT= O64p1-uW7nNwj_m5i97W8sahJCI3mw6v_4BmZ2AY7yG6y5M$=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) .