Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 01 2024 20:12:19 FOUS30 KWBC 012012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 311 PM EST Thu Feb 01 2024 Day 1 Valid 16Z Thu Feb 01 2024 - 12Z Fri Feb 02 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF COASTAL SOUTHERN CALIFORNIA, INCLUDING THE LOS ANGELES AND SAN DIEGO METRO AREAS... 16z Update: Only change was to cut back on the northern extent of the Slight risk, removing areas that are now behind the main core of heavy rainfall. Hourly rainfall as high as 0.5-0.75" is ongoing near Los Angeles, with this activity expected to continue to push south today. 12z HREF data suggests the coverage of these higher rates should generally be on a downward trend as the rain shifts south with weakening IVT values. Nonetheless, localized 0.5"/hr rainfall is still probable. Post frontal instability upwards of 500 j/kg will result in continued showery conditions even behind the main rain shield...although lower PW values by this time and quick cell motions should limit rainfall magnitudes with this activity. Maintained the Marginal over AZ, where some weak instability should support some embedded convective elements as rain moves into this area tonight. Overall the flood threat appears pretty low, but some localized 0.5"/hr rainfall is indicated by the latest HREF. Chenard ....Previous Discussion... No significant changes made to the slight risk area across the coastal Southern California, including the Transverse and Peninsular Ranges. The ongoing atmospheric River event focused into northern to central California early Thursday morning, along and ahead of the associated surface frontal boundary, will continue to press south into coastal sections of Southern California after 1200 UTC Thursday. PW values along and ahead of this front will remain anomalous, 2 to 3+ standard deviations above the mean, along with similar 850-700 mb moisture flux anomalies. This will lead to widespread heavy precip totals across coastal southern California day 1, including the Los Angeles and San Diego metro regions. Model qpf continuity is very good with this event, resulting in little in the way of changes to the previous Excessive Rainfall Outlook. While the PW and 850-700 mb moisture flux values are anomalous, the progressive nature of the front will be a limiting factor for very heavy totals. This is reflected in the HREF neighborhood probabilities for .50"+/hour, which are high across the slight risk area, dropping off significantly for values of 1"+/hour. The current slight risk area fits in well with where HREF neighborhood probabilities are high for 1"+ totals Day 1, 90%+ and 2"+ totals Day 1, 50-90%. Oravec Day 2 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... 20z Update: No changes to the inherited Marginal risk. Convection is likely across portions of the central and southern Plains by Friday evening, pushing eastward through the overnight hours. The inherited risk area matches well with where guidance overlaps some instability and localized areas of heavy rainfall. The most organized convection will probably end up over portions of east central to southeast TX, although higher FFG values here should still keep any flood risk isolated in nature. Chenard ....Previous Discussion... The strong height falls associated with the Southern California atmospheric river event day 1, will continue to be progressive day 2, moving through the Southwest and into Southern High Plains.=20=20 Increasing PW values in a region of enhanced upper difluence across the Southern Plains will lead to increasing shower/thunderstorm activity late Friday afternoon into Friday evening/night from west central to eastern Texas, central to eastern Oklahoma into central Kansas. There is potential for some of these showers to become organized after 0000 UTC Saturday, producing north-south to southwest-northeast oriented heavy precip axes. The only changes to the previous marginal risk outlook was to extend it farther to the north-northwest into central Oklahoma and south central Kansas to cover the spread in the latest model qpf guidance for heavy totals. At the moment, there was no changes made 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. One concern area, however, is along the central to northern Texas Gulf coast where models do show boundary layer moisture flux convergence strengthening in the 0600-1200 UTC Saturday period associated with a potential surface coastal wave.=20 Would not be surprised to see convective coverage greater than the models suggest as they are often slow to show the development at this time. This may result in a greater risk along the immediate central to northern Texas Gulf coast, including the Houston metro area.=20 Oravec Day 3 Valid 12Z Sat Feb 03 2024 - 12Z Sun Feb 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EAST TEXAS INTO PORTIONS OF THE LOWER MISSISSIPPI VALLEY/CENTRAL GULF COAST... 20z Update: We added in a Marginal risk across portions of the central and southern CA coast. The main impacts from this significant atmospheric river event are still expected to be after 12z Sun (see the days 4 and 5 EROs), but there has been a quicker trend noted in the GFS and ECMWF. Ensemble spread through 12z Sunday is still high, so no guarantee heavy rain ends up impacting these areas through that time. But given the trends in the GFS and ECMWF, and the agreement between the deterministic 12z GFS and ECMWF, introducing a Marginal risk seemed warranted. These deterministic runs both indicate 1-3" of rain over portions of this area by 12z Sunday. The Marginal and Slight risk areas over the Plains into the Lower MS Valley were left unchanged. Will say that this appears to be a lower end Slight risk at this time, as only weak instability and a progressive system suggest rainfall totals will generally stay in the 1-2" range. Considered downgrading the risk to a Marginal, however opted to maintain continuity for now. This will allow future shifts to look at additional model runs and this event will also begin to enter into the high res guidance time frame, which should add some insights into the convective nature of the rainfall and the upper bound potential for rainfall magnitudes. At the moment the best chance for localized 3"+ totals appears to be over southeast LA, closer to the developing low pressure which may allow for more vigorous convection and brief training potential. Chenard ....Previous Discussion... The potential developing convection along the mid to upper Texas Gulf coast at the end of day 2, will expand and push eastward day 3, in a region of strongly difluent upper flow on the southeast side of the closed low over the Central to Southern Plains, across east Texas and into the Lower Mississippi Valley/Central Gulf coastal region. Activity that does form along the mid to upper Texas coast should be fairly progressive day 3, but with the overall strong upper dynamics, strong southerly onshore flow in an axis of above average PW values streaming off the western to central Gulf, heavy rains are likely. However questions for day 3 remain if the developing convection stays offshore, just along the Gulf coast or expands farther northward into northeastern Texas, Louisiana and southwest Arkansas. The current model qpf spread covers these solutions, giving low confidence at the moment.=20 Changes to the previous outlook were to expand the slight risk area to to the north across northern Louisiana, northeast Texas and southwest Arkansas to cover the overlap of where the heaviest rains have fallen over the past few weeks and where there is some model support for heavy rains day 3.=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6iRXGrC1_zseowbuiOT-hwn3Tn-Y2ZZo9B7Qw1955tRO= qW-CFdb-V3GQ-I3iwp7ghOz-cFpiqdWXQogwCfNjBYsvqnU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6iRXGrC1_zseowbuiOT-hwn3Tn-Y2ZZo9B7Qw1955tRO= qW-CFdb-V3GQ-I3iwp7ghOz-cFpiqdWXQogwCfNj5Xbu2vM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6iRXGrC1_zseowbuiOT-hwn3Tn-Y2ZZo9B7Qw1955tRO= qW-CFdb-V3GQ-I3iwp7ghOz-cFpiqdWXQogwCfNj8kfPGg0$=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. 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