Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 29 2024 15:37:35 FOUS30 KWBC 291537 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1036 AM EST Thu Feb 29 2024 Day 1 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... 16z update... Lower Mississippi River Valley... Fast moving weakly amplified shortwave is starting to emerge into the Southern Plains currently resulting in low level cyclogenesis across the Lower Mississippi Valley and modest moisture return across Eastern Texas and Louisiana. Guidance remains uncertain on timing/placement of the moisture/instability within the confluent flow out of the western Gulf of Mexico and return flow from the Caribbean/southeastern Gulf merging between Galveston bay to the mouth of the Mississippi today into early tomorrow (Friday) morning. Faster, earlier arriving moisture suggest elevated convection across portions of E TX, central LA into south-central MS, cells may have a tendency to train, but are likely to be more limited in moisture availability and updraft strength limiting overall totals to between 1.5-2.5". This is supported by Hi-res CAMs of the 12z ARW and lesser so the 12z HRRR solution, with other guidance trending away from the higher rainfall values. In addition, very dry soils per NASA SPoRT LIS 0-40cm are values less than 50% and are in the 5-10th percentile for the time of year, any flash flooding risk is contingent on the short-term rates which do not appear to near the required values to pose a risk at this time. However, slower arriving moisture with greater/stronger flux convergence upstream well offshore will have much greater deeper layer moisture (1.5-1.75" total PWATs) and higher unstable Theta-E air resulting in nearer surface based convection initiating much later in the forecast time period. However, deep layer steering will support transport of stronger cells toward the central and southeastern LA coast, perhaps with similar training profiles to allow for enhanced rainfall totals. Most 12z Hi-Res guidance is trending toward this solution lead by the FV3CAM and older ECMWF solutions and given the deeper vertical depth of the cells, rates in the 2"/hr range suggest a few spots of 2-4" are possible mainly over the swamps of southern LA. Strong surface/boundary layer flux with greater unstable air to the south suggests a favorable back-building environment for prolonged training may be overcome by southward propagation of convective line that may form over night on Friday morning. 12z HREF probabilities remain bearish with less than 25% of 2"/3hrs by 12z or 3" totals remaining offshore. As such, confidence in placement/totals crossing areas prone to this level of rainfall may still be limited for risk of flash flooding (minus urban center of New Orleans); and as such will remain below categorized risk for Excessive Rainfall at this time. Given timing after 00z, will continue to watch guidance trend closely for any potential upgrade at the 01z ERO issuance. =20 Southwest Oregon/Northern California... Regional RADAR and GOES-W trends show main core of Atmospheric River continues to press southward through the OR/CA coast as the height-falls associated with the broad/large 2.5-3 standard deviation closed low reaches the Pacific Northwest. Cold air cumulus showers and shallow convective activity will remain north across OR/WA but given fast moving pace and limited overall totals with any shower, the Marginal Risk has been trimmed southward to account mainly for the AR axis. While moisture is not very anomalous, the strength of the flux into the terrain remains supportive of 300-500 kg/m/s IVT values supporting an hour or two of .33 to .5"/hr rates along the supportive terrain. Freezing levels are relatively high (3000-4500Kft) in the warm advective scheme of the AR, but rapidly crash to 1500-3000Kft thereafter.=20 As such, the Siskiyou, Trinity Ranges will quickly turn to snow and the risk areas will be limited to the Coastal Range rainforests. So will maintain a Marginal Risk, expanding a bit further south into northern Central California coast, and similar latitudes along the lower foothills of the Northern Sierra Nevada Range.=20=20 Gallina ---Prior Discussion--- Current WV satellite imagery shows a large, closed upper-low off the Pacific Northwest with a funnel of moisture aimed into coastal WA and OR with a persistent southwest to northeast orientation. Rainfall totals of 2-4" with locally up to 5" have been seen across the Olympic Range in northwest WA down into the northwest corner of OR where the atmospheric plume has since settled over the past several hrs. We'll continue to see a steady southward progression of the moisture plume with flow generally perpendicular to the coastal plain of southwest OR into northern CA with the highest precip totals shifting to those areas through the forecast cycle. A modest surge is anticipated in the initial stages of the precip this morning with 1-3" likely falling across coastal OR and the adjacent ranges just inland over a 6-8 hr period. This will advance into northern CA by the late-morning and afternoon time frame with the main target now focused in-of the Klamath Range out to Mount Shasta and the Northern Sierra's. Snow levels will take a tumble through the period as the first cold front advances inland and changes the higher elevations quickly over to snow with the lower hills and coastal plain leftover as the primary rain beneficiaries. Totals of 2-4" with locally up to 5" are forecast within all of southwestern OR and northern CA with the highest totals mainly within the Sierra where snow will be the primary weather focus. Marginal Risk remains in place from previous forecast issuance, but the primary concern will not be about rates, but the prolonged moderate to heavy rainfall regime thanks to the stout moisture flux pattern associated with our closed upper-level circulation. Localized flooding concerns will be highest within the complex terrain situated below 3000ft MSL, especially within the windward confines of the coastal ranges just inland of OR and CA. Kleebauer Day 2 Valid 12Z Fri Mar 01 2024 - 12Z Sat Mar 02 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER COASTAL SOUTHWEST OREGON AND NORTHERN CALIFORNIA, AS WELL AS THE LOWER ELEVATIONS AT THE FOOT OF THE SIERRA NEVADA... A secondary surge within the life cycle of the atmospheric river will occur during the period with an IVT pulse between 300-500 Kg/ms aimed over northern and central California. A continued onslaught of precip with heavy rain confined to elevations below 3000ft MSL will generate flood potential through much of Friday. Daily totals between 2-3" will be common, on top of what fell the previous period bringing local totals to 3-6" within the concerned areas over the 48 hr time frame. Will be monitoring the progression of the snow levels closely as some guidance is becoming more bullish with the drop of the snow levels further than what has been forecast thanks to an anomalous height fall pattern with the mean trough responsible. Any deviation in the snow level will cause a shift in where any expected flooding could occur, so the adjustments made from the previous forecast package were in reference to the ensemble snow level forecast, and where multiday precip totals are reaching upwards of 2.5" or greater. Highest risk of flooding is the coastal plain north of Eureka on the windward side of the Klamath Range, as well as at the foot of the Sierra's within the Sacramento and San Joaquin Valleys.=20 Kleebauer Day 3 Valid 12Z Sat Mar 02 2024 - 12Z Sun Mar 03 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9mTwkUPiu_u6efcZFbQRgFsj8bbL6zNNEcC3q60g6etp= VG1lFt49VDU8ThtshMqrIDD8XoirmxWFnhap6UuotPQ1gOU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9mTwkUPiu_u6efcZFbQRgFsj8bbL6zNNEcC3q60g6etp= VG1lFt49VDU8ThtshMqrIDD8XoirmxWFnhap6UuooAC7Whw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9mTwkUPiu_u6efcZFbQRgFsj8bbL6zNNEcC3q60g6etp= VG1lFt49VDU8ThtshMqrIDD8XoirmxWFnhap6UuogHU1jAQ$=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) .