Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 28 2024 22:18:16 FOUS30 KWBC 282218 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 517 PM EST Wed Feb 28 2024 Day 1 Valid 2215Z Wed Feb 28 2024 - 12Z Thu Feb 29 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF NEW YORK AND NEW ENGLAND...THE COASTAL PACIFIC NORTHWEST AND THE SOUTHWEST U.S... 2215Z Update...Given short term trends, felt the risk of excessive rainfall has ended over portions of the Central Appalachians so removed the area. Father north in parts of New York and New England...the combined effects of fairly modest rainfall and snow melt still has the potential to result in run-off problems or ponding in regions of poor drainage. That portion of the Marginal Risk area was largely unchanged. Short term convective trends over Arizona in proximity to an upper level has promoted the addition of a Marginal Risk area from southern Arizona into far southwestern New Mexico. The 28/18Z run of the HREF and recent runs of the HRRR have been suggesting a low end risk of one hour rainfall amounts exceeding 1 inch and a low-end neighborhood probability of flash flood guidance being exceeded in the 1- and 3-hour time frame. Thinking is that the risk of excessive rainfall will taper off later tonight with the loss of daytime heating and diminishing low-/mid-level lapse rates. See WPC Mesoscale Precipitation Discussion 0074 for additional details. No changes made to the Marginal Risk area in the Pacific Northwest as the area of moisture continues to make its way inland from the eastern North Pacific. Bann 16Z update...The latest hi-res and global guidance suggest portions of western New York/Pennsylvania and coastal New England will either have lesser rainfall rates/QPF, more progressive cells or a combination of both that will lower the threat for excessive rainfall and associated flooding concerns. Therefore portions of the Marginal Risk area was reduced for those locations to reflect this trend. The majority of the risk for eastern reaches of the Tennessee and Ohio Valleys will be late morning and early afternoon with the lines of convection ahead of the advancing cold front while the Northeast will continue its risk into the evening and overnight hours. No changes were made for the Marginal Risk area across the Pacific Northwest for the moderate to heavy rain moving onshore. Campbell ....Mid-Atlantic and Northeast... Our potent shortwave trough is currently positioned over the north-central plains, moving eastward and beginning to take on a more neutral tilt which will occur by morning. Large scale ascent downstream of the mean trough has aid in multiple rounds of convection, some severe as they encounter a favorable instability axis thanks to a prominent theta-E advection regime and strong vorticity maxes ripping northeast off the broad southwest flow. This setup will continue in its progression with a sharp cold front pressing east along the tail end of a surface low as it pushes east-northeast through the Great Lakes, eventually into northern Quebec by the latter half of the period. All this will create a fairly dynamic setup over the eastern Ohio Valley up through the Northeast as large scale ascent shifts towards the eastern seaboard and the cold front aids as a focal point for linear convection to develop and propagate eastward with time. Out ahead of the mean trough, PWAT anomalies ranging from +2 over the Mid-Atlantic to as high at +4 over northern New England will enhance rainfall potential within the evolving synoptic pattern. Locally heavy rainfall will occur at any given point over the next 18-24 hrs with a few locations having the best potential for flooding prospects thanks to the combination of heavy rain and residual snow that adds the water loading provided by the rain. The main targets for flooding will be within the terrain of the Central Appalachians with a focus in-of the WV high country where snow-water equivalent readings of 2" or more remain based on the latest NOHRSC data set. Forecast rainfall between 1-2", much of which falling within a 6 hr time frame between 12-18z today will create a higher risk for localized flooding set within a complex topographic environment which which is well depicted within the recent FFG threshold of <1"/hr and <1.5"/6-hrs to generate flooding potential. The flood threat will reach a halt by later in the afternoon as rapid height falls behind a steady moving cold front will flip the rain to snow on the backside of the precip field, eventually lowering the flood threat for the second half of the forecast period. Further north, a more formidable snowpack remains across Upstate NY in-of the Tug Hill Plateau, Adirondack, and northern New England where moderate to heavy rainfall will occur prior to the frontal passage, creating another environment capable of localized flooding depending on rapid snowmelt coupled with any appreciable rainfall. QPF forecast of 0.5-1" with locally higher will be sufficient to tap into a high SWE pack still remaining within the complex terrain of each respective locale. This setup is similar to the WV/western PA high country with a similar premise of rain changing to snow after the passage of the cold front. This will allow for a sharp decline in flood potential and a shift to winter weather impacts. Further east of the mountains, pockets of heavy rain over urbanized areas could spell for low-end flood concerns, especially with any repeating heavy rainfall episodes over those more impactful locations. Overall, the setup is low to medium end of the Marginal Risk spectrum, but is enough to warrant a continuation of the previous forecast risk placement with little to no change from the last forecast package. ....Pacific Northwest... The next atmospheric river is taking shape across the PAC Northwest with a minor to bordering moderate AR forecast expected as IVT values between 500-700 kg/ms are expected to advect into WA/OR by the early morning hours, onward. This event will be characterized by longevity of moderate rainfall and less of short-term magnitude of rainfall as probabilistic fields are all fairly tame for the prospects of any hourly rates reaching the 1" mark. Most of the heaviest rain will settle between 0.5-0.75"/hr with the heaviest rainfall positioned over northwest OR and the higher terrain of the Olympic Peninsula where all guidance now pinpoints the heaviest precip totals within the period. Expect 2-4" to be fairly common within the coastal plain extending from the Olympic Peninsula down through southwest OR as we move into Thursday morning. While this is not a textbook flash flood concern, any poor drainage areas and urbanized locales will be at the highest risk for flooding during the period. The atmospheric river will continue to propagate southward down the Pacific coast with a continued threat heading into the D2 period. The previous MRGL risk was largely unchanged as the synoptic pattern and associated rainfall forecast remains on track. Kleebauer Day 2 Valid 12Z Thu Feb 29 2024 - 12Z Fri Mar 01 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF COASTAL OREGON AND NORTHERN CALIFORNIA, AS WELL AS THE LOWER ELEVATIONS AT THE FOOT OF THE SIERRA NEVADA... 21Z update... Heavy precipitation will persist along the Oregon and California coast while spreading southward during this period. The latest guidance and WPC forecast increased QPF along the central/southern end of the Sierra Nevada Range. While prolific snow is expected, particularly above 5000 feet, the threat for excessive rainfall at the lower elevations will remain for this period. The Marginal Risk area was extended further south on the windward side of the Sierras and along the northern California coastline. Campbell First round of the atmospheric river will push inland with a continued southwest to northeast moisture flux off the Pacific with locally heavy rainfall aimed over southwest OR into northern CA. IVT values will remain elevated within the 500-650 kg/ms range which is still good enough to classify as a minor atmospheric river in terms of magnitude. Similar areas from today will experience significant rainfall with the primary areas being the southwest OR coast down into northwest CA with the heaviest precip falling in-of the Klamath Range. An additional 2-4" of precip will occur within the confines of the coastal plain and adjacent terrain leading to localized flooding concerns given the already primed soils and incessant rainfall pattern. Rates will be generally capped at 0.5"/hr with a few waves breaching up closer to 0.75"/hr as signaled within the NBM probabilistic fields. Latest 00z HREF probabilities of 1"/hr are at 0% with only a speckle of 5-10% showing up over Mount Shasta which will be receiving snow in the current pattern. Marginal risk for flooding will extend further south this go around within the foothills of the Sierra's below 4000ft MSL as upslope flow will create a favorable environment for uninterrupted heavy precip to the east of the Sacramento Valley. The previous Marginal Risk was maintained with only a minor adjustment on the northern periphery of the risk area along coastal OR. Kleebauer Day 3 Valid 12Z Fri Mar 01 2024 - 12Z Sat Mar 02 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF COASTAL OREGON INTO THE LOWER ELEVATIONS OF NORTHERN AND CENTRAL CALIFORNIA AT THE BASE OF THE SIERRA NEVADA... 21Z update...The strong winter storm will still be raging on during this period with prolific snow above 5000 feet and moderate to heavy rainfall in the lower elevations. An additional 2 to 3 inches are expected to fall during this period over some of the same locations as the previous day. Multiday accumulations of 3 to 6+ inches will keep the threat for excessive rainfall and associated flooding concerns elevated for this period. The Marginal Risk that is currently in effect encompasses where this elevated threat will be and only minor adjustments were made to reflect the latest guidance and trends. Campbell Atmospheric river will continue to push southward with heights falling rapidly across the PAC NW as the mean trough pivots towards the coast by the end of the period. A secondary shortwave kicker will help bring the base of the trough into northern California leading to colder air filtering further into the column over the region, and snow levels steadily declining into Saturday morning. A steady stream of moisture will be positioned from southwest to northeast with flow generally banked against the terrain over northern CA where heavy precip will likely continue through much of the period before waning as a cold front pushes inland. Totals of generally 2-3" will be common across the northwest CA coast, inland into the foothills of Mount Shasta down into the northern Sierra. This will lead to storm totals pushing 3-6" for some of the areas that receive several waves of rainfall over the course of the 48 hour period. Further south into the central Sierra's, precip will be fairly prolific with 3-4" likely within this time frame, leading to the lower elevations on the windward side of the mountains seeing a risk of flooding where the precip is expected to be all rain. As of now, the best chance for flooding will be along the coastal plain and areas below 3000ft MSL leading to a MRGL risk to encompass the lower elevations from coastal OR down through the lower elevations within the upslope region of the Sierra to the east of the Sacramento and San Joaquin Valleys. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6UpQ3zj8WhWo8Vf4kdhmA1gxjzaokZcs7y65y_l5TqBA= VTElfH-Tigoh1v-wKT-DQGJoOB2cj2f5BTUllt5uvwCgUHg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6UpQ3zj8WhWo8Vf4kdhmA1gxjzaokZcs7y65y_l5TqBA= VTElfH-Tigoh1v-wKT-DQGJoOB2cj2f5BTUllt5uu7_Oi6I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6UpQ3zj8WhWo8Vf4kdhmA1gxjzaokZcs7y65y_l5TqBA= VTElfH-Tigoh1v-wKT-DQGJoOB2cj2f5BTUllt5uzB8UGDk$=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) .