Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 04 2023 20:04:52 FOUS30 KWBC 042004 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Sat Nov 04 2023 Day 1 Valid 16Z Sat Nov 04 2023 - 12Z Sun Nov 05 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE BURN SCARS OF FAR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA... ....16Z Update... The previous MRGL across the Olympic Peninsula was removed with the event coming to an end shortly as the drier air advects in before the cold front. The MRGL risk over FL was also removed as the frontal boundary will stay far enough offshore with the instability gradient far enough off the coast and steering pattern expected to keep primary convection offshore. A few cells may wander onshore within both Miami-Dade and Broward counties, but prospects for heavy rainfall necessary for flash flooding have dropped off precipitously. ....Pacific Northwest... Cold front associated with our low pressure and atmospheric river event is currently positioned just of the coast of WA and northwest OR with low-level WV satellite indicating a tongue of drier air beginning to protrude the immediate coast. This will subsequently diminish the heavy rain potential across WA state where the heaviest rain has fallen overnight through this morning. Further south, seeing the indications of the AR shifting focus into southwest OR with heavier echoes now positioned to come ashore within the next hour. Guidance is very consistent on a period of heavy rainfall between 16-22z across the immediate coast and areas within the Klamath Mtns. 12z HREF blended mean QPF has wavered very little from run-to-run over the past 24 hrs with a general 1.5-2.5" expected over the southwest corner of OR into the very northwest edge of CA over the rest of the morning and afternoon. Smith River complex burn scar just inland of the coastal plain will remain the primary focus for flash flooding as low-topped convection is possible within the area during the peak of the event. With the lower threshold necessary for flash flood risks within the burn scar, there was plenty of merit to continue on with the MRGL risk from prior forecasts. Kleebauer Day 2 Valid 12Z Sun Nov 05 2023 - 12Z Mon Nov 06 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST OREGON AND NORTHWEST CALIFORNIA... A general break between AR events is anticipated during the first portion of the period, but a second AR of lower magnitude is forecast to press into the PAC Northwest with locally moderate to heavy rainfall focused over the coastal areas of Northwest California and Oregon. Forecast intergrated vapor transport (IVT) is a full magnitude lower with indices around 350-450 kg/ms leading to a lower scale of AR event, but compounded on top of the previous events over the past 5 days. The primary focus in the initial surge of precip will be over the southwestern OR and northwest CA coast(s) with some protrusion inland across the Klamath Mtns where a few burn scars are located and have the highest sensivity to heavier bouts of precipitation. 12z HREF blended mean guidance, as well as NBM/Ensemble bias-corrected model guidance indicates a general maximum located over the above corridor leading to a limited ability for any appreciable drying from the previous event occurring today. 12z HREF probabilities for rates of 1"/hr or better are not as robust as the current event unfolding, but a solid area of 15-20% across the Oregon coast and adjacent mountains are being depicted which matches well with the HREF EAS probability of where at least 2" of rainfall over the period could occur (30-40%). Further to the north, the rainfall rates will be much lighter and despite the multiple rounds of precipitation the past few days, flash flooding concerns will be much lower, although some nuisance flooding is possible within any urbanized areas. More rain is also expected over the Olympic Peninsula, but exceeding anything greater than 0.3"/hr will be tough to come by given the best ascent aligned further south as the nose of the jet streak off the Pacific is focused over the southern half of OR and northern California. Have removed the MRGL risk from previous forecast issuance across the northern half of the Oregon coast up through northwestern Washington, but maintained the MRGL outlook over southwest OR and northwest CA where the heaviest rain is forecast and within the confines of the remnant burn scars within the terrain.=20 Kleebauer Day 3 Valid 12Z Mon Nov 06 2023 - 12Z Tue Nov 07 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 AND PORTIONS OF THE SOUTHERN WASHINGTON AND NORTHERN OREGON CASCADES... ....20Z Update... Very little change necessary to the forecasted D3 Marginal risk across the Pacific Northwest. Increased ascent within the left exit of region of a 130kt upper jet will provide an enhancement of the precip field over the northern half of Oregon into Washington state by Monday morning. Low pressure over the northeast Pacific will begin occluding off of British Columbia with the 500mb and 700mb lows becoming stacked by mid-period. The focus of the moisture field will shift north compared to its positioning on D2 with the best IVT axis running across northwest OR with protrusion into the Cascades. Latest ensemble bias-corrected guidance relays a split maximum with one focused over the immediate coast of northern OR with a secondary max positioned over the Columbia Gorge into the southern WA Cascades. This aligns with the projected forecast axis of the best vapor transport, and correlates to the better ascent within the left exit region of the 250mb jet streak. ECENS and GEFS probabilities for at least 1" of rainfall are highest within those two zones as well leading to a positive correlation of the current MRGL risk. The southern and northern periphery of the risk area was maintained due to those areas having the longevity of heavy rainfall the previous 24-48 hrs. The compounding rainfall concerns provide enough merit to warrant a continuation within those two areas, especially the Olympic Peninsula where the highest multiday totals are currently positioned. As a result, did not want to diverge too much from the original forecast at lead and will adjust if necessary with newer guidance, including the suite of hi-res deterministic and accompanying ensemble. Kleebauer ....Previous Discussion... A low starting out the day Monday centered off the coast of the Olympic Peninsula will weaken as it drifts northwestward into Vancouver Island and dissipate by Monday night. The cold front south of the low will move into the coast early in the day, spreading into Idaho by Monday night. While the front will keep the flow westerly/onshore at the coast and into the Cascades through the Day 3/Monday time period, the colder air moving in will result in falling snow levels all across the Pacific Northwest. Thus, much of the moisture moving into the Cascades, particularly the northern Washington Cascades, will likely fall as snow into the higher elevations. The inherited Marginal risk was trimmed a little bit, especially out of the northern Cascades on the assumption that most of the precipitation will be snow. The cold air taking longer to get into southern WA or northern OR should allow enough of the expected precipitation to fall as rain in these areas to justify the Marginal Risk, especially given the many previous days of rain in this area. Overall rainfall amounts broadly should continue decreasing on Monday as compared to previous days, but given the sensitivity of this entire area with almost all of the rainfall likely translating into runoff, the Marginal Risk is justified and largely remains in place. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6mh03YlewZQoxHBIwNyC6h5mIzEmBKwHX2Rl3p7LnTDo= nYehXu-N35tHKkcO7dtw_Wp-gNp7FJLfI1_WM6stittNAX8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6mh03YlewZQoxHBIwNyC6h5mIzEmBKwHX2Rl3p7LnTDo= nYehXu-N35tHKkcO7dtw_Wp-gNp7FJLfI1_WM6stnZ6J7VY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6mh03YlewZQoxHBIwNyC6h5mIzEmBKwHX2Rl3p7LnTDo= nYehXu-N35tHKkcO7dtw_Wp-gNp7FJLfI1_WM6st6Jj0e-I$=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) .