Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 17 2023 20:12:41 FOUS30 KWBC 172012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Mon Jul 17 2023 Day 1 Valid 16Z Mon Jul 17 2023 - 12Z Tue Jul 18 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE EASTERN GREAT LAKES, THROUGH THE OHIO VALLEY AND PORTIONS OF THE LOWER TO MIDDLE MISSOURI VALLEY, AS WELL AS THE EAST CENTRAL TO SOUTHEAST FLORIDA COAST, SOUTHERN LOUISIANA, AND ARIZONA... 1600Z Update... Mainly cosmetic changes were made to the early morning D1 ERO. The 06Z and 12Z HREF guidance suggests a bit of a stronger/heavier QPF signal to areas of southeast FL later today with some relatively high probabilities of seeing 3+ inch rainfall totals from convection in the 18Z to 00Z time frame near the urban corridor. Thus have expanded the Marginal Risk farther south to include Miami-Dade County. Otherwise very minor adjustments/tweaks were made to the Marginal Risk areas across the central Gulf Coast and also the OH Valley, Middle MS Valley and central Plains. Finally, with some monsoonal moisture making a somewhat stronger presence in the Southwest, it was decided to pull the Marginal Risk a little farther north to include portions of the Mogollon Rim and this will account for some terrain-induced and slow-moving convective cells this afternoon and evening. Orrison Previous discussion... ....Eastern Lakes, Ohio Valley into southern Missouri, southeast Kansas and far northeast Oklahoma... A broad marginal risk area was maintained for the upcoming day 1 period along and ahead of the next frontal boundary pushing through the eastern Great Lakes and toward the Ohio Valley. The latest simulated hi res radars depict potential for scattered convection across these regions where PW values are near seasonal norms and instability will range from 1000-200 j/kg through the Ohio Valley and values 3000+ j/kg from eastern Kansas/far northeast Oklahoma into southern Missouri. The previous marginal risk was extended westward across southern Missouri into southeast Kansas/far northeast Oklahoma to capture model heavy qpf potential in the max instability axis. The marginal risk area coincides well with the higher HREF neighborhood probabilities for 1"+ amounts, peaking at 50-80% across the marginal risk area. At the moment, convection should be fairly progressive limiting the upside potential for heavier totals. This is reflected in much low HREF probabilities for 2"+ amounts. ....Middle Missouri Valley... The front dropping south from the Northern Plains into the Central Plains/Mid Missouri Valley will become nearly stationary Monday afternoon from the Central Plains, east southeast into the Mid Mississippi Valley. Strengthening southeasterly to southerly inflow into this front will support potential for organized convection to press east southeast along and north of the front late Monday afternoon into the early hours of Tuesday. There is general model agreement on heavy rainfall potential as this convection presses southeastward, but not very good agreement on placement of max amounts. This is reflected in very low EAS probabilities for 1"+ amounts, maxing out at 5% over southern SD into far north central NE, while HREF neighborhood probabilities for 1"+ amounts are higher 40-50%+ across this area. Risk level kept at marginal given this spread. ....East central Florida coastal area... An axis of above average PW values expected to persist day 1 across much of central FL, with anomalies 2 to 3 standard deviations above the mean. With the low level flow expected to remain westerly, convection that does fire will likely progress eastward and maximize somewhere along the east central Florida coast Monday afternoon. HREF probabilities reflect this with the highest 2"+ probabilities, 50-80%, along the east central FL coast from approximately Melbourne to Miami. This is also where the Ensemble Agreement Scale (EAS) shows the highest probabilities for 1 and 2"+ amounts. For these reasons, the previous broader marginal risk area across FL was decreased in size and concentrated along the east central FL coast where the aforementioned higher probabilities coincide with the more urbanized regions. ....Southern Louisiana... Only some slight changes to the previous marginal risk outlook with the northern edge trimmed south by approximately 50 miles to fit the latest model qpf output. At the moment, the model max axis of additional precip is across far southeast LA just to the south of the urban areas from New Orleans to Baton Rouge and Lafayette, but can not rule out convection still affecting the urban areas where there will be the greatest runoff risk day 1 from isolated rainfall amounts of 1-2 inches Monday afternoon. ....Southeast Arizona... The latest model consensus is for any monsoonal showers to remain primarily across southeast Arizona and southward into northwestern Mexico. This is reflected in the latest HREF probabilities that show any probabilities greater than 25% for 1"+ amounts centered close to the border over south central Arizona. Activity over the past 24 hours was mostly south of the southern AZ border with this trend again likely for day 1. While there may be some scattered convection north into the Mogollon Rim area, moisture is limited here and should preclude any runoff issue threat. The previous marginal risk area was trimmed on the north and northeast side from southwest New Mexico into central AZ. Oravec Day 2 Valid 12Z Tue Jul 18 2023 - 12Z Wed Jul 19 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID MISSISSIPPI VALLEY AND LOWER OHIO VALLEY... 2030Z Update... Compared to the previous forecast, the biggest adjustment was to upgrade portions of the Middle MS and Lower OH Valley region to a Moderate Risk with expectations of seeing strong MCS activity in the 00Z to 12Z/Wed timeframe across areas of especially eastern MO and southern IL with notable cell-training concerns down to the southeast into western KY. The 06Z to 12Z time period in particular is when the 12Z HREF supports the strongest and most organized convective threat and with signals appear for high-end rainfall rates and storm totals. The 12Z HREF did show some low-end probabilities of seeing the 100-year ARI for QPF exceeded in this 6-hour timeframe. Synoptically, with a mid-level shortwave traversing the region and interacting with the nose of a moist/unstable and gradually veering 40 to 50 kt low-level, and quasi-stationary front with a wave of low pressure advancing generally east along it, this has the look of a potential frontal/synoptic Maddox-type heavy rainfall/flash flood event. Regardless, the potential for locally several inches of rain will be in place, potentially greater than 5 inches, where the corridor of heaviest rainfall rates which should be capable of reaching 2 to 3 inches/hour, and cell-training occurs. Additional tweaks spatially and temporally of course may be needed over the next couple of model cycles. Elsewhere, only very minor adjustments to the Slight Risk were made over the Northeast with the arrival of the next strong upper trough and associated cold front. Antecedent conditions remain very sensitive across many areas of New England in particular given saturated soil conditions and high streamflows. The Marginal Risk area over the Southwest was expanded northward a bit to account for some increasing monsoonal moisture presence and expectations of sufficient levels of instability and orographic ascent/forcing for scattered areas of slow-moving convection. Locally there is a fair amount of dry air in the low-level of the column, but increasing mid-level moisture is favoring PW anomalies over parts of NV/UT that are over 2 standard deviations above normal, and the low-levels farther south over AZ have been gradually trending upward with a more favorable location of the deeper layer ridge axis near the Four Corners region. Can't rule out at least some isolated flash flooding concerns with these convective cells and especially if they get into any local burn scar areas or slot canyons. The normally dry washes also will need to be monitored at least locally for impacts. Orrison Previous discussion... ....Lower Missouri Valley, Middle Mississippi Valley into the Lower Ohio Valley... The stationary frontal boundary expected to stretch west northwest to east southeast from the Central Plains into Lower Missouri Valley, Mid Mississippi Valley and Lower Ohio Valley region should be the focus for another round of organized convection late Tuesday afternoon into the early hours of Wednesday. There will be potential for an MCS/MCC to progress east southeastward along and north of this front. The initial southerly low level flow into this front will attain more westerly component after 0000 UTC Wed across northern to central MO into southern Illinois. This will support potential for backbuilding and training of cells in a west northwest to east southeasterly direction. There is a model signal for a narrow axis of very heavy rainfall amounts from northern Missouri into southern Illinois and western Kentucky.=20 The day 2 Excessive Rainfall Outlook continues to depict a slight risk across these areas, although the threat axis was narrowed from the previous outlook, trimmed on the northern side, to reflect the current qpf axis range. Given the training potential with this system, an upgrade to moderate risk is possible with later issuances, especially as we get into the day 1 time period where hi res guidance may offer better agreement with a max axis.=20 In the areas of training, rainfall totals in excess of 5 inches are possible. ....Northeast... The cold front moving through the eastern Lakes day 1 will press east from eastern NY through New England day 2. A narrow axis of above average PW values in the 1.5-1.75"+ range expected along and ahead of this front. These values will be lower than PW values with the heavy rainfall event on Sunday as that anomalous PW axis will have moved well off the northeast coast during day 1.=20 Additional convection likely to stretch along this front on Tuesday, supporting locally heavy rainfall totals from in the vicinity of northern NJ/eastern NY State, northeast through western and northwestern New England. The previous marginal and slight risk areas were trimmed on the western portions across eastern NY State to reflect model qpf consensus. The new slight risk area fits well with the 12 hour HREF neighborhood probabilities ending 0000 UTC Wed. While precip will continue after 0000 UTC Wed, the last 12 hours of the current HREF forecast period captures most of the heavy precip potential with this event. There will be some overlap with heavy rains over the past 24 hours and the day 2 expected heavy axis from southeast NY State into northwest CT and western MA and east central NH. Much of the remainder of the heavy rains over the past 24 hours farther to the southeast over southeast New England will remain to the southeast of the day 2 expected qpf axis. ....Southeast AZ... Scattered convection again possible around the peripheries of the persistent upper high stretching across the Southwest. A small marginal risk area was maintained over southeast Arizona where isolated heavy totals may produce local runoff issues.=20 Oravec Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4wd3z57P13N1xfaiahxQHdGQ31PMm1G2FxktE8yP5NEO= PKRgcW7O617p2UbiO9vyhiZUfTmvnjNBN6RB1a-wyAoKlCM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4wd3z57P13N1xfaiahxQHdGQ31PMm1G2FxktE8yP5NEO= PKRgcW7O617p2UbiO9vyhiZUfTmvnjNBN6RB1a-w-14RoE0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4wd3z57P13N1xfaiahxQHdGQ31PMm1G2FxktE8yP5NEO= PKRgcW7O617p2UbiO9vyhiZUfTmvnjNBN6RB1a-wlu8TOUE$=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) .