Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 04 2023 00:16:59 FOUS30 KWBC 040016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 815 PM EDT Fri Nov 03 2023 Day 1 Valid 01Z Sat Nov 04 2023 - 12Z Sat Nov 04 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE OLYMPIC PENINSULA OF WASHINGTON STATE... Few changes were made to the Marginal risk area over the Pacific Northwest as core of the next atmospheric river arrives and brings increasing rainfall rates thsis evening. The 12Z HREF still depicts that...in general...rainfall rates will be between 0.25-0.5" with max around 0.75" during the peak of the event.=20=20 The best chances of the heaviest rainfall should include the immediate coastal ranges and the Olympic Range. 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 particular interest will be between about 03Z and 10Z during which period the HREF neighborhood probabilities for 3 inch amounts in 12 hour peaks at or above 80 percent on the Olympic Peninsula and where the probabilities for 5 inch amounts in 12 hours get as high as 35 percent. This still coincides well within the forecast MRGL domain, so will keep continuity from previous forecast issuance. Bann 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 Valid 12Z Sun Nov 05 2023 - 12Z Mon Nov 06 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE ENTIRE PACIFIC NORTHWEST COAST FROM NORTHWEST CALIFORNIA THROUGH THE OLYMPIC PENINSULA OF WASHINGTON STATE... ....20Z Update... A weaker atmospheric river is forecast to impact the Pacific Northwest through the end of the weekend. GFS and ECMWF integrated vapor transport (IVT) continues to signal values within the lower grade AR scale which would theoretically limit the max potential of flooding concerns. However, given the antecedent conditions expected after back to back moderate ARs, the coastal plain of Washington and Oregon will reside within a traditionally lower FFG forecast creating a non-zero threat for localized flooding concerns, especially within urbanized areas from the Olympic Peninsula into the central Oregon coast. The southern edge of the MRGL risk is a continuation of coverage within the Smith River complex burn scar that will have a higher capability for flood impacts, including mudslides. Precip totals on the order of an additional 1-2" will be possible with moderate rainfall rates as the peak intensity expected. There could very well be some adjustment within the extent of the MRGL risk area pending the result of the next AR that occurs this evening through Saturday, so we'll be monitoring for further adjustments pending future precip analysis and short term trends. Kleebauer ....Previous Discussion... On Sunday, a series of upper level shortwaves will continue barreling into the coast as the strong jet streak sets up perpendicular to the coast along the OR/CA border. This will result in multiple rounds of heavy rain that will quickly move inland, only to be replaced by the next. Due to there being multiple days of this kind of rain by Sunday, the Marginal Risk was expanded to include all of the coastal ranges from northwest California north. The area may need to be adjusted with future updates depending on how much rain falls and is forecast for the Friday and Saturday time periods, with less rain resulting in a smaller Marginal Risk area. The area is about the same as was inherited, with only minor tweaks to include more burn scar areas along the OR/WA border. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9SA_ZyFTAeGKDOyf_WXbwFQsIVIf8KCwDY-OxMIjt_sF= lKWoV61fLHoiQ67en6VBhgqEenJyDMluSvGyDLYc6F6ZVxw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9SA_ZyFTAeGKDOyf_WXbwFQsIVIf8KCwDY-OxMIjt_sF= lKWoV61fLHoiQ67en6VBhgqEenJyDMluSvGyDLYca30arTY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9SA_ZyFTAeGKDOyf_WXbwFQsIVIf8KCwDY-OxMIjt_sF= lKWoV61fLHoiQ67en6VBhgqEenJyDMluSvGyDLYcbWq_HzY$=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) .