Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Jun 04 2023 07:58:00 FOUS30 KWBC 040806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 AM EDT Sun Jun 04 2023 Day 1 Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern to Central Plains into the Central to Northern Rockies... No significant changes anticipate to the blocking pattern across much of the nation during the day 1 period. A closed upper high will remain centered across the Northern Plains with an elongated area of troffing extending northwest to southeast on the western side of this ridge from the Northern Rockies into the Central to Southern High Plains. PW values in this elongated trof axis will remain above average, especially from northeast Colorado into Montana where PW anomalies will run 2+ standard deviations above the mean. Weak vorts in the high PW axis will again support another day of widespread scattered convection stretching from the Southern to Central Plains into the Central to Northern Rockies. This entire region has seen much above average precip over the past few weeks, with many areas having observed values 300 to 600% of normal. This will continue the risk of isolated flash flooding where any additional day 1 rains fall across areas that have received recent heavy amounts. With confidence low with respect to where any heavy day 1 totals will occur, much of this region was kept in marginal risk. The one exception continues over the Southern High Plains from far northeast New Mexico, across the Texas/Oklahoma Panhandle region and southwest Oklahoma, where a slight risk area was maintained. This region has seen heavy rains over the past two days in the general same region and has a larger region of high 1"+ HREF probabilities for the upcoming day 1 period. ....Florida... The remnants of Arlene expected to pass to the south of South Florida early Sunday with any associated heavy rains on the north side likely having already pushed to the southeast of the southern portion of the peninsula by the beginning of the day 1 period. In its wake, another day of scattered to widespread afternoon thunderstorms is likely across the southern half of the Florida Peninsula as the upper trof that the remnants of Arlene have been rotating through pushes across the Florida peninsula. HREF probabilities are high across the southern half of the Florida peninsula for 1 and 2"+ amounts. This may lead to isolated flash flooding, especially in urbanized areas where hourly rate of 1 to 2" are possible. Oravec Day 2 Valid 12Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS... ....Southern Plains through the Northern Rockies... With little in the way any significant changes to the overall large scale flow across the central portion of the nation day 2, the day 2 Excessive Rainfall Outlook is very similar to the day 1 outlook. PW values are forecast to remain above average from the Southern to Central Plains through the Central to Northern Rockies. Similar to the day 1 period, shortwave energy embedded in this high PW axis will support another day of widespread scattered convection across these areas. A slight risk area was continued from northeast New Mexico into the Texas/Oklahoma Panhandle region. There continues to be somewhat better agreement that this area will see a better chance of organized heavy precip than other areas in the high PW axis. As stated in the day 1 discussion, isolated flash flooding is possible across the entire area given recent rainfall amounts that have been as much as 400-600% of normal. ....Sierra Nevada... One of the biggest changes to the large scale flow across the U.S. during the day 2 period will be the closed mid to upper level low approaching the central to southern California coastal region. Increasing upper difluence Monday afternoon to the north and northeast of this closed low will support scattered convection through the Sierra, northern California and portions of the Northern Great Basin. With PW anomalies rising to 1 to 2+ standard deviations above the mean in this upper difluent region, locally heavy rainfall amounts of up to an inch possible. With stream flows remaining much above average through the Sierra, isolated flash flooding is possible Monday afternoon into Monday evening. Oravec Day 3 Valid 12Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS NORTHEASTERN NEW MEXICO... The story remains the same for the upcoming day 3 period with little changes to the overall stagnant blocked pattern across the nation. Another day of widespread scattered convection likely from the Southern to Central High Plains, northward through the Central to Northern Rockies into the Northern High Plains. There is model consensus for a westward shift in the axis of heaviest rains over the Southern High Plains day 3. This will remove portions of central Texas from the marginal risk area. A slight risk area was maintained over northeast New Mexico where model consensus is for a max precip area. This is a region where upper difluence strengthens day 3 well to the east of the upstream coastal California mid to upper level low. This will bring three days of potential heavy rains over this region, warranting the continuation of a slight risk. Similar to the day 1 and 2 periods, given recent heavy rains, 400-600% of normal through portions of this area, isolated flash flooding will again be possible from the additional day 3 heavy rain potential across the remainder of the broad marginal risk area extending into the Central to Northern Rockies and Northern High Plains. There is also a model signal for a precip max along the western Gulf coast from the middle to upper Texas coast into southwest Louisiana. With recent rains not as anomalous across this area as other areas in the Southern Plains, the threat was kept at marginal for the time being. ....Northern Sierra/Northern California into portions of the Great Basin... The closed low that approached the central to southern California coast day 2 is not expected to move much day 3. This will keep a favorable upper difluent pattern in place to the north and northeast of this system and support another day of scattered convection. Consensus is for a broader region of upper difluence compared to previous days, resulting in a broader area of potentially locally heavy rains in what will again be an axis of above average PW values stretching across Northern California into the Great Basin. This is reflected in a larger marginal risk area than on day 2 extending into northern California, southern Oregon, northern Nevada and far southwest Idaho. Stream flows are above average across much of the marginal risk region with isolated flash flooding possible. Oravec $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .