Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 31 2024 20:09:40 FOUS30 KWBC 312009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 309 PM EST Wed Jan 31 2024 Day 1 Valid 16Z Wed Jan 31 2024 - 12Z Thu Feb 01 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS FAR SOUTHWEST OREGON AND PARTS OF NORTHWEST AND WESTERN CALIFORNIA, INCLUDING THE COASTAL RANGES, MUCH OF THE SACRAMENTO VALLEY, BAY AREA, AND CENTRAL CALIFORNIA COAST... 16Z Update: Minimal changes for this update were needed with the main plume of atmospheric moisture moving onshore northern California and far southwest Oregon this morning. The new 12Z HREF (and supported by 12Z hi-res CAMs) still show high (>70%) probabilities for hourly totals exceeding 0.5" through the period, initially over southwest Oregon and northwest California, shifting southward through tonight as the plume of higher moisture advances southward and becomes increasingly displaced from the main cutoff low. There is a very slight (~10%) probability of hourly totals exceeding 1" across extreme coastal areas of northern California into this afternoon. Sufficient forcing/lift and a marginal overlap with minimal instability (~100 J/kg) will support some of those higher rain rates through tonight into central and parts of southern California. The greatest coverage/magnitude of impacts (urban and small stream flooding, rock/landslides) still appear to be over northern CA where 3-5" (locally higher) rainfall is expected, but some impacts are likely further down the coast as well within the favored terrain and where any higher rates materialize. Taylor ---previous discussion--- The Day 1 Slight and Marginal areas were largely unchanged from yesterday's Day 2 outlook. Deep/occluded cyclone currently centered well off the OR coast will meander slowly eastward over the next couple of days, though remain offshore through Friday. Meanwhile, a robust NW-SE oriented upper level jet streak (130-150kt at 250 mb) will help drag the longwave trough farther south into southern CA by Thursday. Strong deep-layer synoptic scale support will move into the outlook areas within an expanding area of increased difluence aloft, as both the upper level divergence and south-to-north transport of low-mid layer subtropical moisture become enhanced within the left-exit region of the aforementioned upper jet streak. Robust 850 mb S-SW flow today will be around 5 standard deviations above normal for late Jan per the 00Z GEFS and SREF, peaking between 50-60kts long the coast. Meanwhile, 850-700 mb moisture flux anomalies are expected to reach +4 to +5 standard deviations this morning as the 1.00-1.25" PW plume skirts the CA coast. During the afternoon and overnight, low-mid layer moisture flux anomalies are expected to scale back a bit (closer to +3-3.5 standard deviations above normal) as the AR drops south into the Central CA Coast. The strength and duration of the IVT anomaly equates to a moderate AR based on the CW3E AR Scale per both the GEFS and ECMWF ensemble means, with peak IVT magnitudes of 600-800+ kg/m/s. However the relatively swift duration of the AR should limit the coverage of short-term runoff impacts across the outlook area, along with the weakening magnitude of the AR with time (especially late Wed-Wed night) as it becomes more dislodged from the cutoff low to the north. The latest CSU GEFS-based first-guess fields, particularly the UFVS-verified version, continue to support the continuation of the Slight Risk that was noted in yesterday's Day 2 ERO. The forward progression of the system and a lack of instability along/ahead of the front both continue to be limiting factors with regards to the magnitude of the flood threat. Nonetheless, strong convergence along the southward shifting cold front combined with favorable upper jet dynamics will likely support some embedded convective elements and locally intense rainfall rates within the rain shield . The 00Z HREF neighborhood probabilities of hourly rainfall exceeding 0.75" through 12Z Thursday are 30%+ over a good portion of the Slight Risk area. Thus it seems likely that some heavier rates will transpire, and so even with a progressive system do expect to see some flooding impacts. The greatest coverage/magnitude of impacts (urban and small stream flooding, rock/landslides) still appear to be over northern CA where 3-5" (locally higher) rainfall is expected, but some impacts are likely further down the coast as well within the favored terrain and where any higher rates materialize. Hurley/Chenard Day 2 Valid 12Z Thu Feb 01 2024 - 12Z Fri Feb 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHERN CALIFORNIA, INCLUDING SOUTHERN PORTIONS OF THE CENTRAL CALIFORNIA COAST ALONG WITH THE LOS ANGELES AND SAN DIEGO METRO AREAS... 21Z Update: The main update/change to the Day 2 ERO this cycle was to include portions of southern Nevada and western to southern Arizona in the Marginal Risk area. The axis of higher moisture associated with the atmospheric river will nose toward the region Thursday into Thursday evening, with a notable increase in precipitable water values. While the best forcing/lift and moisture will tied to the southern California coast (enhanced by the terrain), some greater moisture does spread eastward toward southern NV and AZ. The 12Z HREF and hi-res CAMs support localized moderate/heavy rainfall with the hourly neighborhood probabilities of 0.5" reaching above 50-60% there. Some hi-res CAMs support totals in excess of 1" and overall felt there was enough of a signal to support a low-end risk of minor/nuisance flooding around the Las Vegas metro through western/southern Arizona. No significant changes were made to the Slight Risk area for S. California for the potential of flash flooding including minor debris flows. Here, probabilities of hourly totals of 0.5"+ are high (above 70%) during the period with even a slight/non-zero threat of hourly totals greater than 1" (10-20%) during the late morning/afternoon. Taylor ---previous discussion--- Main changes to the inherited outlook was to expand the Marginal Risk area over the San Joaquin Valley as well as eastern and southern portions of the Diablo Range, given the high-res model signal of enhanced hourly rainfall rates coinciding with the marginal post-frontal instability. Otherwise, no changes were made to the Slight Risk. As Noted in the Day 2 ERD, the IVT is on a weakening trend as the plume moves into southern CA, however from an anomaly perspective it is maintaining around the climatological 99th percentile. While still minimal, guidance does suggest at least some weak instability getting into southern CA by this time, probably aided by the approaching mid-level trough axis and embedded shortwave energy. Given this, it seems likely that some locally heavier embedded convection will persist into Thursday as the front pushes across southern CA. 00Z HREF in fact indicate 70-90%+ neighborhood probabilities of >0.50"/hr rainfall rates within the northern portions of the Slight Risk area (Transverse Ranges south to the coast), with 60-80% probabilities over the southern portions of the Slight Risk area. Meanwhile, neighborhood probabilities of 0.75"+/hr rates peak between 40-60% through 18Z, then closer to 30% thereafter. Therefore given the likelihood of at least isolated areas with 0.5-0.75"/hr rainfall rates, do expect some flood risk to exist even despite the progressive front. Hurley/Chenard Day 3 Valid 12Z Fri Feb 02 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... Elongated, robust upper southern stream jet (150+ kts at 250 mb) will gradually build across the international border of northwest Mexico and the Southwest Friday and Friday night, with a deepening area of low pressure over the southern High Plains east of the Four Corners. Strengthening exit region upper-level forcing (difluence/divergence aloft and a rapid uptick in southerly low-level flow, moisture transport, and instability will lead to widespread development of showers and thunderstorms ahead of the approaching surface cold front. The latest deterministic and ensemble guidance is beginning to cluster on northern to east-central Texas where localized 2-3"+ totals are expected, although the GEFS probabilities for 2" are higher than the ECENS but overall do support the Marginal Risk area for localized/minor flooding. Hurley/Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5ygP3HABPNvMvh44pUr84jTwjokvMqcQw_891G0BIkFG= Nc7eFvmzVz12RDTGkzZ2sATw-03zh5bkDJw8tw_a1KDsOpQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ygP3HABPNvMvh44pUr84jTwjokvMqcQw_891G0BIkFG= Nc7eFvmzVz12RDTGkzZ2sATw-03zh5bkDJw8tw_a3hYVtUo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ygP3HABPNvMvh44pUr84jTwjokvMqcQw_891G0BIkFG= Nc7eFvmzVz12RDTGkzZ2sATw-03zh5bkDJw8tw_a-dJy9yQ$=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) .