Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 18 2023 00:52:42 FOUS30 KWBC 180052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Mon Jul 17 2023 Day 1 Valid 01Z Tue Jul 18 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, AND ACROSS ARIZONA... ....PA into the Northeast... Convection moving across central/eastern PA this evening may pose a localized flash flood risk, but some weakening of the activity is expected as it moves into a slightly more stable airmass and we lose daytime heating. Some mid level shortwave and upper jet support should be enough to maintain isolated to scattered activity across NY and into VT tonight, but with limited instability it seems like around or a bit over 1" will be the maximum potential. This should not pose more than a very isolated flash flood risk...but will carry a Marginal risk given the increased flood sensitivity driven by wet antecedent conditions. ....KS/OK into the MS and OH valley... Isolated to scattered convection will persist tonight within the west/northwest mid/upper level flow interacting with a slow moving surface front. An isolated flash flood risk can not be ruled out anywhere along/near this front, but at the moment not seeing much of a signal for a more concentrated threat, thus a broad Marginal should suffice. ....WY/SD/NE... Convection forming over northeast WY should track east southeast across southern SD and into northern NE tonight. This activity should track along a low level boundary/instability gradient, with decent upper support in the right entrance of a jet as well. Some of the recent HRRR runs seem to be undergoing the potential, and thus preferring something closer to the 18z 3km NAM and/or 12z ARW2. The quick forward motion of convection will be a limiting factor for flash flooding, but high short term rainfall rates are likely. 18z HREF data supports a swath of 2" of rain, but the probability of exceeding 3" really drops off. This seems like a reasonable forecast...with rainfall mostly peaking around 2" or so expected. ....AZ... An isolated flash flood risk likely continues for a few more hours across AZ, with convective intensity decreasing thereafter. Chenard 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 Valid 12Z Wed Jul 19 2023 - 12Z Thu Jul 20 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS THE MID TO UPPER OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE CENTRAL/SOUTHERN APPALACHIANS... 2030Z Update... Changes to the previous forecast for this period include adjusting the Slight Risk area a bit farther southwest into larger areas of the TN Valley and the west-facing slopes of the southern Appalachians to account for what should be a convectively very active morning on Wednesday across areas of KY and TN in particular. The 12Z HREF guidance definitely supports locally heavy rainfall potential from training convection in the 12Z to 18Z period as shortwave energy/forcing drops southeastward along and near a frontal boundary across the region. Locally a few inches of rain will be possible along with notable concerns for flash flooding. Elsewhere, the Marginal Risk area was expanded over the Southwest farther to the north across areas of NV/UT and east through the central Rockies and adjacent High Plains to account for the relatively anomalous pooling of moisture across the region which coupled with orographics and low-amplitude shortwave energy should favor scattered areas of locally heavy showers and thunderstorms Wednesday afternoon through Wednesday evening. Isolated instances of flash flooding will be possible across this region. The Marginal Risk of excessive rainfall is maintained for areas of the Upper Midwest where areas of locally heavy showers and thunderstorms are expected with the arrival of a rather strong upper trough for this time of the year from southern Canada. Low pressure and a cold front will help to locally concentrate the activity and there may be locally a couple inches of rain in association with this convection later Wednesday through early Thursday. Orrison Previous discussion... ....Mid to Upper OH Valley, Upper Tennessee Valley into the Central Appalachians... PW values expected to remain above average along and north of the front forecast to stretch from the Mid Mississippi Valley into the Upper Tennessee Valley region. Shortwave energy pushing eastward along the southern edge of the westerlies and the above mentioned frontal boundary will support potential for convection along and to the north of this west to east oriented frontal boundary.=20 There is a typical amounts of qpf detail spread for a day 3 forecast, but general consensus for the potential for heavy rainfall amounts in the vicinity of this front. A slight risk was maintained from the previous forecast issuance, expanded slightly to the northeast into central to southern West Virginia where several models show potentially heavy rains moving over the lower ffg values across this area.=20 ....Central Plains, Lower Missouri Valley, Mid Mississippi Valley... The marginal risk area was expanded westward from the previous issuance, from the Lower OH Valley, into the Mid MS Valley, Lower MO Valley and across the Central Plains of Kansas into southeast CO and the TX/OK Panhandle region. This was to capture heavy rain potential in the vicinity of the west to east oriented front across these areas which also will coincide with the southern edge of the westerlies where enhanced lift is possible from transitory shortwaves.=20 ....Upper Mississippi Valley... The marginal risk from the previous issuance was also expanded south from the Upper Lakes/northern MN area, through the Upper MS Valley. This was for convective potential ahead of fairly strong northern stream height falls forecast to push from the Northern Plains into the Upper MS Valley during day 3. A narrow axis of above average PW values and instability along and ahead of the associated cold front will support potential for scattered convection Wednesday into early Thursday. At the moment, it appears convection ahead of this front should be fairly progressive, keeping the risk level at marginal.=20 ....Southeast Arizona... With no significant changes to the overall pattern across the Southwest, maintained a small marginal risk area over southeastern Arizona similar to the days 1 and 2 time periods. It is possible the most organized activity remains across northwest Mexico to the south of the AZ border, with the northern edge of the precip affecting far southeastern AZ. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7mxiAkUCoHZbJQEBT8MEmG1Jr0tzDIQEhek0AzzcWWU4= bAKHr781sOBPGaNpW0-T8Y67_T9DapHYaGM7qIGf5R3vSVQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7mxiAkUCoHZbJQEBT8MEmG1Jr0tzDIQEhek0AzzcWWU4= bAKHr781sOBPGaNpW0-T8Y67_T9DapHYaGM7qIGfHssgZw0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7mxiAkUCoHZbJQEBT8MEmG1Jr0tzDIQEhek0AzzcWWU4= bAKHr781sOBPGaNpW0-T8Y67_T9DapHYaGM7qIGfvbTnSsw$=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) .