Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 03 2023 19:40:24 FOUS30 KWBC 031940 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 PM EDT Fri Nov 03 2023 Day 1 Valid 16Z Fri Nov 03 2023 - 12Z Sat Nov 04 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE OLYMPIC PENINSULA OF WASHINGTON STATE... ....16Z Update... There are no changes to the overall synoptic pattern involved with the expected atmospheric river across the PAC Northwest. The forecast remains on track with the heaviest precip footprint lying from a latitude of Eugene, OR up through the Olympic Peninsula. CIPS analog guidance from last night indicated a textbook AR signature for the aforementioned areas with the avg precip for the top 5 analogs producing between 1.5-2.5" across northwest OR and 2.5-4" over the Olympic Peninsula with the highest impact area over the terrain within the Olympic range. Overall rates for the system will run between 0.25-0.5" with max around 0.75" during the peak of the event with best chances of the heaviest rainfall within the immediate coast and into the Olympic mountains. The previous MRGL remains relatively unchanged with the primary focus over the burn scar areas across northwest WA and along the Skokomish river basin where the last event primed the river valley for a slightly better flood potential during the coming event. Time frame of interest will reside between 00-12z within the period with 3"/12-hr neighborhood probabilities running over 80% within the Olympic Peninsula and 5"/12-hr probabilities around 30-35% within the same area. This coincides well within the forecast MRGL domain, so will keep continuity from previous forecast issuance. Kleebauer ....Previous Discussion... A strong shortwave trough at the leading edge of a 150 kt jet streak stretching across the north Pacific will make its way into the coast of the Pacific Northwest today. A plume of moisture with PWATs to 1.75 inches will move into the coast with an occluded front of a strong low, which will be supported by the lift in the left exit region of the jet streak. Peak IVT values will range between 800 and 900 kg/ms, which is on the stronger side of most atmospheric rivers. The result will be a plume of heavy rain that moves into the coast of Washington and Oregon, and is subsequently uplifted by the coastal ranges primarily, with a secondary maximum of heavy rain into the Cascades. Flooding concerns up to this point have primarily focused in the Olympic Mountains, with far fewer impacts noted along the coast further south into Oregon. Thus, the Marginal risk remains largely unchanged except for a few minor tweaks to the area of concern. By the end of the period...early Saturday morning...the rain will have largely pressed inland, resulting in a break in the rainfall...so for most areas this will be a 9 to 12 hour rainfall. Also, the atmospheric river will be rapidly waning in intensity, with maximum PWATs only to 1.25 inches and shifting south by Saturday morning. Expect a widespread 1 to 2 inches of rain along the Pacific Northwest coast, with locally higher totals in the Olympics and portions of the Cascades. While this is the second in the series of atmospheric rivers/fronts, the area has generally had time to recover from the previous rainfall, so flooding concerns remain isolated even on the Olympic Peninsula. As far as timing goes, the rain will start late this afternoon along the coast and quickly spread inland, with heavy rain ongoing through much of the overnight. Wegman Day 2 Valid 12Z Sat Nov 04 2023 - 12Z Sun Nov 05 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE OLYMPIC PENINSULA OF WASHINGTON STATE, THE BURN SCARS OF FAR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA, AND PORTIONS OF SOUTH FLORIDA... ....20Z Update... The Marginal risks across the Pacific Northwest were maintained with little deviation from previous forecast as the heaviest rainfall impacts the northwest corner of WA state with a secondary max located over southwest OR and northwest CA where residual burn scars leave a heightened risk for flash flooding potential, albeit on the lower magnitude of potential impacts. Rainfall rates maxing out at 0.5"/hr could still cause problems within both respective areas with the Olympic Peninsula dealing with a compounding heavy rain concern with multiday totals from both ARs running between 5-8" with locally as high as 10" over the span of 4 days. The area across southwest OR near the Smith River complex will see the secondary max within the period with consistent signatures of 1.5-2.5" possible over the course of Saturday morning. This maintains continuity from the previous forecast lending credence to a continuation of the MRGL risk from the prior package. The biggest change in the D2 is the addition of a MRGL risk across southeast FL towards the Miami/Fort Lauderdale corridor. More on this setup below... ....South Florida... A stalled boundary analyzed over the Florida Straits, northeast into the Bahamas will lift back northward overnight with a bisection of the south Florida Peninsula by tomorrow morning. PWAT analysis south of the front indicates a rich tropical moisture field that will advect back north within the confines of the front. PWAT indices approaching 2-2.25" will be common across the southern tip of the Mainland, correlating with an increasing thermodynamic environment as areal MLCAPE surges into the range of 1000-1500 J/kg by sunrise. A vorticity maxima currently located over the Lower Mississippi Valley will advance eastward fairly rapidly within the progressive mid-level flow over the Gulf. A strengthening area of convergence along the southeast FL coastline will create an environment favorable for heavy rainfall within a convective mode near and within the confines of the urban centers of Miami up into Fort Lauderdale. 12z HREF probabilities indicate a high likelihood of rates upwards of at least 1"/hr (80-90%) with 2"/hr potential into the range of 25-30% which is characteristic of the lower end MRGL risk threat for flash flooding concerns within the above area. The biggest indicator for flooding concerns is the 5"/12-hr neighborhood probability topping out between 35-40%. With the bulk of the convective potential occurring within a 6 hr window between 12-18z with a potential to last 2-3 hrs longer given some hi-res deterministic output, this pins a heightened risk of urbanized flooding within the Miami-Dade to Broward county population centers. A Marginal risk was issued to include the aforementioned area with focus along the coastal areas where flash flooding risks are highest. Kleebauer ....Pacific Northwest... Residual heavy rainfall from the AR event of the Day 1/Friday period will continue through the day on Saturday, as the main atmospheric river plume sinks south towards the OR/CA border. Additional forcing will result in a renewed round of heavy rainfall as the plume moves over the coastal ranges. Recent burn scars in the area will enhance the flash flooding threat. Thus, a small Marginal risk covering several of the burn scars from recent years along the OR/CA border was introduced with this update. For the most part the area is remote enough that impacts are likely to be nuisance level, though with debris flows from the burn scars possible, it could still pose a hazard to anyone in the wrong place at the wrong time. Meanwhile further north into the Olympic Peninsula, for most of the day the rainfall will likely be scattered and light, though with convective elements involved in any showers. Localized flash flooding can't be ruled out with very little time between the showers and the previous AR from Friday night, but the threat is certainly on the lower side. Wegman 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!-4cMqengGATBGw760Tvxun921O1OLEwPTNChNIkANC_j= uJBIXSjpCHW8hJamyz1DvCLL74GpAeY3RUTM5bW6dnHN0Ak$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-4cMqengGATBGw760Tvxun921O1OLEwPTNChNIkANC_j= uJBIXSjpCHW8hJamyz1DvCLL74GpAeY3RUTM5bW6FTDBuBw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-4cMqengGATBGw760Tvxun921O1OLEwPTNChNIkANC_j= uJBIXSjpCHW8hJamyz1DvCLL74GpAeY3RUTM5bW6yfbmTWk$=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) .