Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 09 2023 20:10:21 FOUS30 KWBC 092010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Sun Apr 09 2023 Day 1 Valid 16Z Sun Apr 09 2023 - 12Z Mon Apr 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST AND SOUTH FLORIDA... Pacific Northwest... 16Z update... Very minor adjustments made to the Marginal Risk area across northwest Washington where there were additional upticks in QPF. The latest guidance is showing 1 to 2 inches of rainfall to occur over west/southwest Oregon which is outside of the inherited Marginal Risk area. Therefore the southern bounds of the area was expanded to include Curry County. Campbell The latest guidance trended QPF upward once again over central portions of the Cascade Range with the onshore flow, with consensus showing 1 to 3 inches reaching farther inland and extending farther northward than before. Overall, the big picture remains the same...with yet another round of rain should be ramping up later today as on-shore flow of deep-layered moisture develops ahead of a low pressure system over the Gulf of Alaska/eastern North Pacific ocean. The magnitude of the moisture transport results in rainfall amounts of 1.75 to 3.0 inches or so in the coastal ranges and in the west aspect of the northern Cascades. Given the fairly high snow levels expected...pulled the Marginal Risk a northward. Overall, though, the forecast reasoning remained similar to previous issuances. ....Southern Florida Peninsula... 16Z update... The latest guidance suggests higher QPF moving onshore the eastern coast, especially during the overnight/early morning hours. Hourly rainfall rates of 1 to 2 inches/hour will skirt the coast and move inland for points near/south of Jacksonville to Miami during this period, with some convection continuing after 12Z (the start of the day 2 period). Much of the re-res guidance depict 3 to 4+ inches of rain along the coast. While most of these areas have been relatively dry the past week there are a few pockets where precipitation has been 100 to 300% of normal. The most likely areas to have isolated flooding concerns will be urban areas and those with poor drainage. The Marginal Risk area that was already in effect was expanded further south in Miami-Dade County and then northward to St John County. Campbell Introduced a Marginal Risk area mainly for concerns about locally heavy rainfall occurring in the urbanized corridor that leads to isolated flooding or run-off problems. A cold front will continue to make its way southward across the Florida peninsula during the day...with low level winds becoming east to southeast in the wake of the frontal passage. Given precipitable water values that approach 2 inches...the localized QPF maxima from the NAM and GFS later in the afternoon do not seem unreasonable (although exactly where remains open for debate). Given the amount of uncertainty...felt no more than a Marginal was warranted. Bann Day 2 Valid 12Z Mon Apr 10 2023 - 12Z Tue Apr 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST AND FLORIDA... Pacific Northwest... 21Z update... Incrementally, QPF has continued with its an increased trend further inland across the Pacific Northwest and into the northern fringes of the Intermountain West. With snow levels remaining higher in some locations, and factoring in multiple days of recent/active precipitation, the risk for excessive rainfall and associated flooding concerns the Marginal Risk was upheld along with a few adjustments. The area along the West Coast was expanded further east across western Oregon and southwest Washington. The Marginal Risk area from northeast Washington to northwest Montana was broadened in an east-west fashion to account for the new QPF and additional snowmelt. Campbell Little change in the large scale picture...although persistent on-shore flow of moisture has had a chance to extend farther inland. Maintained the low-end risk of excessive rainfall along the coastal ranges and Cascades where an additional 1 to 2 inches of liquid precipitation are possible as the moisture plume makes its way southward during the period. Introduced a second Marginal Risk from northeastern Washington into northern Idaho where precipitation...mainly in the form of rain due to fairly high snow levels...occurs on Monday. An added concern is that snow water equivalent is expected to drop more than an inch during the same 24 hour period...although the greatest snow water equivalent is forecast in northeast Washington and not in the northern Idaho panhandle where the QPF is maximized. Felt a Marginal was sufficient for the time being. ....Florida Peninsula... 21Z update... With a slowed frontal boundary near South Florida, northeasterly flow of 20 to 30 knots will transport PW values of 1 to 1.5 inches onshore the eastern side of the Florida peninsula. This will support showers and thunderstorms capable of producing periods of higher intensity rainfall, especially some of the same areas that have moderate to heavy rainfall on D1 which will keep an elevated risk for isolated areas of flash flooding/flash ponding. The Marginal Risk area over southeast Florida was expanded a bit north/south, as well as, westward in Miami-Dade County. Additionally, convection is expected to persist from day 1 and continue to focus higher amounts over portions of the Nassau, Duval, Clay, St John, Putnam, Flagler and Volusia Counties. A Small Marginal Risk area was raised for this part of the coast where additional 1 to 3 inches will be possible. Campbell Continued the Marginal Risk area mainly for concerns about locally heavy rainfall occurring in the urbanized corridor as northeasterly low level flow continues across the same portion of the Florida peninsula and keeps maintain a risk of the showers and thunderstorms. Given the amount of uncertainty...felt no more than a Marginal was warranted. Bann 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!69e0YoCquoN6RYf-8YtTIzsnXvFaJn5cRtDGtINBZUup= s71H7wtHvlanoS9cVBMjZB8_lwyu8v8uae1tX5BoCccZwCM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!69e0YoCquoN6RYf-8YtTIzsnXvFaJn5cRtDGtINBZUup= s71H7wtHvlanoS9cVBMjZB8_lwyu8v8uae1tX5BoH3sRn-A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!69e0YoCquoN6RYf-8YtTIzsnXvFaJn5cRtDGtINBZUup= s71H7wtHvlanoS9cVBMjZB8_lwyu8v8uae1tX5BoBJI7Ykg$=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) .