Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 03 2024 00:32:43 FOUS30 KWBC 030032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 731 PM EST Fri Feb 02 2024 Day 1 Valid 01Z Sat Feb 03 2024 - 12Z Sat Feb 03 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN PORTIONS OF THE SOUTHERN PLAINS TO THE MIDDLE AND UPPER TEXAS COAST... 01Z Update... Trimmed a bit of the Marginal Risk area...mainly the portion over Kansas and far northern Oklahoma where stability should limit rainfall rates and areal average rainfall totals should remain under 1.5 inches. Farther south...early evening radar imagery showed convection growing up scale in a region of better instability. Changes made to the western periphery of the Marginal Risk area was based on that trend in radar imagery. Few changes needed elsewhere and did not reflect a change in the forecast reasoning from earlier outlooks. Bann 16Z Update... The 12Z HREF guidance and recent runs of the HRRR remain generally in good agreement with the overnight guidance concerning the expectation for at least some broken bands of convection to develop across portions of the southern Plains as shortwave energy/height falls advance east out of the Southwest. Some localized swaths of 1 to 2+ inch rainfall totals seem likely given the level of dynamical forcing that will be in place along with at least some modest elevated instability. The strongest instability will most likely be focused down across area of south-central to southeast TX and this is where generally the heaviest rainfall rates should be focused. Based on all of this, the previous Marginal Risk area was maintained with just some subtle tweaks made to account for the new HREF guidance. Orrison Previous discussion... The latest model guidance remains consistent with the increasing shower/thunderstorm activity across the Southern Plains Friday afternoon/evening as strong height falls push through the Southwest and into the Southern High Plains. PW values expected to continue to increase day 1 ahead of these height falls as low level southerly flow strengthens off the western Gulf and spreads northward into the Southern Plains. The well defined upper difluence ahead of these height falls and the axis of increasing PW values will support potential for areas of heavy rains. There continues to be a lot of spread with where the heaviest totals will occur. This is illustrated well with the HREF neighborhood probabilities showing a large area of high probabilities for 1"+ totals across central to eastern Texas, northward into portions of southeast to western Oklahoma and south central Kansas. However, the EAS probabilities for 1"+ amounts day 1 are much lower indicating poor overlap in the latest hi res models. The previous marginal risk area still fits the geographical spread in the latest guidance, leading to no changes at the moment in the marginal risk area. No changes also to the risk level given the current spread in qpf details and consensus that any precip will be generally to the northwest of where the heaviest amounts have occurred over the past few weeks. Oravec Day 2 Valid 12Z Sat Feb 03 2024 - 12Z Sun Feb 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL CALIFORNIA COASTAL RANGES AND FROM SOUTHEAST TEXAS INTO PORTIONS OF THE LOWER MISSISSIPPI VALLEY/CENTRAL GULF COAST... 2030Z Update... The 12Z HREF guidance and the global model consensus overall suggest a bit of a quicker arrival of stronger IVT magnitudes/forcing into the coastal ranges of central CA by 12Z/Sunday. There should be a warm front arriving along the coast by that time period, and the 12Z NAM/GFS solutions show an impressive 50 to 60+ kt south-southwest low-level nosing toward the Bay Area. Enhanced warm air advection/isentropic ascent coupled with strong orthographic ascent over the coastal terrain should promote a marked increase in rainfall rates late Saturday night and especially by 12Z/Sunday. Locally as much as 2 to 3 inches of rain over the coastal ranges will be possible by then. WPC has introduced a Slight Risk as a result of the expectation of heavier rainfall rates arriving earlier with this next atmospheric river. The Marginal Risk area has also been locally expanded as well to account for this. Elsewhere, only modest tweaks were made to the Marginal and Slight Risk areas across the eastern portions of the southern Plains and also the Lower MS Valley where there will continue to be a good setup for strong warm advection/forcing and moisture transport ahead of the ejecting southern stream shortwave energy. The area of heaviest rainfall is expected to be over southeast LA where the guidance shows a robust southerly low-level jet of 50+ kts off the Gulf of Mexico ahead of a deepening low center. Thus there will be rather strong poleward transport of subtropical moisture, and at least an elevated pool of instability for enhanced rainfall rates. There is support for some training convection to set up over southeast LA and this could greatly enhance the rainfall potential. Locally several inches of rain cannot be ruled out which is supported by multiple members of the 12Z HREF guidance. This will likely drive an urban flash flood threat across the greater New Orleans metropolitan area. Orrison Previous discussion... ....Central Gulf Coast into the Lower Mississippi Valley... The expanding convection across eastern Texas at the end of day 1, will continue to push eastward day 2 across the Central Gulf coast and toward the Lower Mississippi Valley. Overall favorable conditions continue for heavy rains day 2 with well defined upper difluence spreading east in an axis of strong southerly onshore flow off the Gulf and PW values of 1 to 1.5". There is better model agreement, however, that the heaviest qpf axis will be closer to the Gulf coast, where instability will be better.=20 Subsequently, we have trimmed the northern end of the previous slight risk area by about 150-175 miles. In the area where the slight risk was removed, rainfall has been less over the past week compared to previous weeks. This is evident in many of the USGS sites that show river gauge heights lowering over the past week.=20=20 Closer to the Gulf coast where the heaviest qpf is expected, the convection will still be fairly progressive. There still is the potential for the convection to be offshore of the central Gulf coast, but with still enough models showing heavy precip across southern LA into far southern Mississippi, did not want to remove the slight risk entirely. The marginal risk area extending northwestward into Arkansas, eastern Oklahoma, eastern Kansas and far southwest Missouri, was narrowed from the previous issuance to better match the narrow axis of moderate to locally heavy precip totals. ....Central California Coast... The previous marginal risk area was expanded to the north by approximately 80 miles to cover the spread of qpf that falls by the end of day 2 associated with the next atmospheric river event. All models are in agreement that the heaviest rainfall amounts will be during the day 3 period. However, with recent heavy rains across this area and potential for additional heavy rains to start by the end of the day 2 period, a marginal risk of runoff issues is warranted. Oravec Day 3 Valid 12Z Sun Feb 04 2024 - 12Z Mon Feb 05 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR COASTAL SECTIONS OF CENTRAL AND SOUTHERN CALIFORNIA... 2030Z Update... Changes to continuity for this update include an expansion of the Slight Risk area into large areas of the Sacramento Valley and northern parts of the San Joaquin Valley, and also the adjacent foothills of the Sierra Nevada given the general northward shift of heavier rainfall across the state as strong low pressure lifts northward offshore of CA this period. This will likely drive at least relatively enhanced IVT/moisture flux even into the Central Valley and the adjacent Sierra Nevada. Additionally, some northward expansion of the Moderate Risk through the central CA coastal ranges (south of the Bay Area) was accommodated given the heavier QPF trends across these locations. Enhanced IVT values across the coastal ranges of central and southern CA coupled with an orthogonal orientation of this IVT axis relative to the terrain will support enhanced rainfall totals, and this will be further facilitated by the pooling of at least modest instability along and just ahead of an approaching cold front. Rainfall rates into the coastal terrain may approach or locally exceed 1"/hour, and for the Transverse Ranges in particular, there will be a growing concern for some repeating/training convective elements which will foster notably heavy rainfall totals. Given the wet antecedent conditions and these heavier totals, there will be concerns for considerable runoff and flash flooding. The cold front associated with this atmospheric river will be slowing its advance down the coast by later in the period extending through Sunday night and Monday morning, and this will foster additional excessive rainfall totals beyond this period which will include the Peninsular Range of southern CA. A High Risk of excessive rainfall for portions of the Transverse Range was considered for this update, but given some of the lingering model spread and QPF trends, it was decided to hold off for the time being. Regardless, very heavy rainfall totals are expected for the coastal ranges with 5 to 8+ inch rainfall totals possible for the Transverse Range. Elsewhere, the Marginal Risk area for the Southeast was trimmed a bit, with a removal of it for this update over the central and southern portions of the FL Peninsula. Somewhat lower QPF trends in the 12Z model guidance and a rather progressive eastward advance of a cold front should mitigate the excessive rainfall potential for these areas, but there will still be locally heavy and concentrated rains farther north and especially over areas of southeast GA. Stronger moisture convergence/forcing across this area should drive as much as 2 to 3+ inches of rain. A Slight Risk upgrade may be needed for this area if the models were to trend any wetter. Orrison Previous discussion... ....Central to Southern California... While there are still some detail differences with deepening low pressure off the Central California coast Saturday night into Sunday, all models agree the next Atmospheric River event will commence as strengthening southerly low level flow impacts the Central to Southern California coastal region during day 3. IVT values are expected to be stronger than the first AR event Wednesday-Thursday, reaching 600-800 kg/m/s along the Central to Southern CA coastal region. Widespread 2-4" totals likely along the Central to Southern CA coast, with the heaviest amounts in the western portions of the Transverse range where the strong southerly low level flow will have the strongest upslope component, with totals of 5-8"+ possible. Overall, not a lot of changes made to the previous outlook. The moderate risk area was extended farther to the north to cover the northwest portion of WFO LOX and the far southwest portion of WFO MTR. ....Southeast, eastern Gulf Coast and Florida... The broad closed upper low over the Southern Plains at the end of the day 2 period will be pushing east southeastward into the north central to northeastern Gulf day 3. Strong southerly to southeasterly low level flow will press across the eastern Gulf coast/FL and the Southeast. This will support a broad region of moderate to heavy precip totals across the Southeast, eastern Gulf Coast and Florida. Heaviest totals likely where this precip area pivots and low level east southeast to easterly flow persists over the same region the longest. Model consensus is for this to be from the Lower coast of South Carolina, across south Georgia , North Florida into southern Alabama. Observed precip amounts have been below average over the past few weeks across this region, warranting just a marginal risk at the moment. The best area of instability and potential for organized convection is across Southwest to South Florida late Sunday afternoon/evening. Any associated precip moving off the eastern Gulf and across Southwest to South Florida likely will be very progressive, which will keep precip totals from being very heavy. This potential convection, however, will be moving over the more urbanized regions of Southwest to Southeast Florida, which will keep some threat of urban runoff issues, although precip totals are expected to be less than areas farther to the north. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5ooR3VTsISCN2EODct8uF_OdDLHv1zIDBO1GH38Hd81b= Uub5lZ0rt9fWq9w8My7ohvt-6tHJ5FmS5sA9daFwqfuDx0k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ooR3VTsISCN2EODct8uF_OdDLHv1zIDBO1GH38Hd81b= Uub5lZ0rt9fWq9w8My7ohvt-6tHJ5FmS5sA9daFwxw4OAoI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ooR3VTsISCN2EODct8uF_OdDLHv1zIDBO1GH38Hd81b= Uub5lZ0rt9fWq9w8My7ohvt-6tHJ5FmS5sA9daFwFi1ihq0$=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) .