Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue May 23 2023 15:33:00 FOUS30 KWBC 231958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Tue May 23 2023 Day 1 Valid 16Z Tue May 23 2023 - 12Z Wed May 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST, SOUTHERN PLAINS, NORTHERN PLAINS, AND INTERMOUNTAIN WEST... 16z Update: Not much change to the Marginal risks with this update. Considered introducing a Slight risk across portions of north central TX, but opted to stick with the Marginal. There is a good signal for organized convection across this area tonight, and much of this area saw rainfall yesterday which has slightly lowered FFG. However, while there is some chance we get some cell merging over this area tonight, there is at least an equal chance we end up with a pretty quick moving convective line. Overall would like to see a bit higher 3" neighborhood or 2" EAS probabilities in the HREF to have greater confidence in a more organized flash flood risk. Thus at the moment thinking the flash flood risk will most likely stay localized in nature warranting the Marginal risk. Chenard ....Previous Discussion... ....Southeast... Heavy rainfall will be likely again across portions of the Southeast Tuesday into Tuesday night as the upper flow remains broadly difluent, while at the surface a boundary remains draped quasi-stationary across the region. Moisture will continue to be above normal with the favorable easterly flow bringing in PWs 1.75"+ around the stationary boundary. While the deep-layer forcing will remain rather diffuse, 0-6km bulk shear values averaging 25-30 kts per the guidance will favor more widespread, multi-cellular cluster convection during and after peak daytime heating, with the more intense rainfall rates lasting a bit longer than the more typical pulse-type convection. As a result, upwards of 2-3+" within an hour will be likely underneath the strongest cells, as slow storm motions and colliding boundary interactions may also play a factor into the rain totals, which could top 3-5+" in places for the period. For the time being, what is preventing a higher risk (i.e. Slight) is the absence of more focused deep-layer forcing, which is leading to quite a bit of spread among the CAMs with respect to the QPF maxima given how much influence the more chaotic mesoscale processes/convective evolution have. Meanwhile, some overlap is possible with respect to heavy rain areas yesterday and again today, which may lead to a more enhanced risk (i.e. Slight) with subsequent updates to the Day 1 ERO. Otherwise, anticipate more isolated instances of flash flooding, particularly for the more susceptible locations and urban areas. ....Southern Plains... Another round of dry-line induced showers and thunderstorms is expected Tuesday into Tuesday night, a bit displaced to the east compared to today's expected convection. The latest guidance shows a better/strong low level jet upwards of 30-35 kts at 850 mb along with a large pool of higher moisture across the region. The 00Z guidance remains on track for showing potential of 1-2" areal average totals in the risk area with some signal for higher amounts possible and the Marginal Risk was only minimally changed for this update. ....Intermountain West into the Northern Plains... A very slow moving longwave trough over the West Coast will direct a large stream of moisture northeastward in advance of the trough. Embedded within the trough is an upper level low that will be over the BC/AT border north of WA/ID. A shortwave disturbance pinwheeling south of the low will approach the intermountain west Tuesday. This will locally increase the forcing (ribbon of upper divergence) over the Marginal Risk area across northeast NV and northern UT into eastern ID and the northern Rockies. Anomalous moisture will still be in place, with PWATs up to 0.75 inches are over 3 sigma above normal. This will support training storms tracking northeast from NV through UT and into ID. The storms that are forced by the local mountains may stay more stationary, which will locally increase the flash flooding potential. Meanwhile, scattered to numerous showers and thunderstorms are likely to develop across portions of North Dakota into northern Minnesota late this afternoon into the evening as a cold front approaches from Canada and intercepts a low level southeasterly jet of 25-30 knots that will be advecting deeper moisture into the region. Localized 1-2" totals are expected based on the latest forecast guidance and although the areal coverage of heavy rainfall isn't expected to be high, the storms that do form will be capable of producing intense rain rates that could be great enough to cause isolated instances of flash flooding. Hurley/Taylor Day 2 Valid 12Z Wed May 24 2023 - 12Z Thu May 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF GREAT BASIN INTO THE NORTHERN ROCKIES AND HIGH PLAINS, CENTRAL ROCKIES AND HIGH PLAINS INTO THE SOUTHERN PLAINS AND MIDDLE-UPPER TEXAS COAST, NORTHERN NEW ENGLAND, AND THE FLORIDA PENINSULA... 20z Update: Only minor adjustments to the inherited Marginal risk areas. The threat is probably approaching lower end Slight risk probabilities over portions of eastern CO into the OK and TX Panhandles. Convection moving east off the terrain may tend to exhibit some training/backbuilding as they encounter low level southeasterly inflow, with anomalous PWs and sufficient CAPE for heavy rainfall rates. Main question at this point is the organization and coverage of convection and higher rainfall totals. On one hand the ingredients and the CSU ML ERO supports lower end Slight risk probabilities, but HREF QPF probabilities and overall model QPFs are not too impressive, and more in line with a typical Marginal risk. Given the uncertainty, opted to maintain continuity of a Marginal and continue to monitor, with isolated flash flooding the current expectation. Also note a pretty good overlap of ingredients over portions of central and eastern MT...with PWs getting up over the climatological 90th percentile, instability around or over 1000 j/kg and a frontal boundary in the vicinity. Seems like isolated flash flooding is probable with this set up, but again the organization and coverage seems below Slight risk levels at this time. Chenard ....Previous Discussion... ....Great Basin into the Northern Rockies and High Plains... Aforementioned longwave trough will remain quite amplified during day 2 (Wed-Wed night), resulting in a focused area of upper difluence and SSW-NNE ribbon of upper divergence across the outlook area. Continued favorable forcing along with anomalous moisture profiles and some deep-layer instability (MUCAPEs at least 500-1000 J/Kg) will support more numerous showers/storms and an enhanced risk of cell training given the deep-layer wind profiles. There's still a bit of guidance spread to support anything more than a Marginal Risk at this time. ....Central Rockies and High Plains into the Southern Plains and Middle-Upper Texas Coast... The upper flow across these areas will become increasingly difluent as the upper ridge axis pushes into the eastern (lower) Plains. Meanwhile, low-level south to southeast flow ahead of the approaching trough will bring higher deep-layer moisture northward into eastern Colorado and western Kansas. Convection is likely to initiate off the higher terrain then move eastward into the Plains, interacting with any residual surface boundaries. Meanwhile, the guidance (especially CAMs) continue to indicate locally higher maxima (3-5+ inches) farther S-SE across OK-TX, though with considerable spread. This area would otherwise be capped, however given the likelihood of a mesoscale-enhanced vort lobe that will likely drop south of the Red River, agree with the CAMs that the CAP will likely break across at least a portion of the area. While the heaviest rainfall rates would be quite prolific given the degree of moisture and deep-layer instability, the anticipated progressive storm motions (downwind propagation) enhanced by strong outflows will likely inhibit anything more than a localized flash flood threat. ....Maine... A deepening trough crossing the Hudson Bay region is expected to become negatively tilted and close over eastern Ontario by late Wednesday into early Thursday. A sharp cold front will slip southeastward into the region with an area of low pressure developing along the front. The strong frontogenesis and mid/upper level diffluence is likely to lead to an axis of heavier precipitation from the northern portions of New Hampshire into northern Maine along the Canadian border. Moisture return isn't overly impressive and there's likely to be limited instability (perhaps up to 500 J/kg), nonetheless guidance shows fairly strong signals for 1-2" totals over the period which could be in a narrow axis over a relatively short period of time leading to isolated instances of flash flooding. ....Florida... The excessive rainfall outlook area is a bit more confined compared to Day 1 (i.e. over much of the FL peninsula), however the broadly-difluent upper flow pattern and continued favorable thermodynamical profiles will maintain an isolated (marginal) risk of flash flooding. Hurley Day 3 Valid 12Z Thu May 25 2023 - 12Z Fri May 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS A LARGE PORTION OF THE HIGH PLAINS, NORTHERN ROCKIES, AND INTERMOUNTAIN WEST... 20z Update: The broad Marginal risk was maintained, with isolated flash flooding possible anywhere within the large risk area. One focused area could again be portions of central/eastern MT, where ingredients (front, instability, PWs) support a heavy rainfall threat similar to day 2 (Wed). Although again coverage seems below Slight risk levels at the moment. A second area to watch is portions of the Plains from eastern CO, into portions of OK/KS and north TX. Could be a messy setup by this time with MCVs moving eastward from earlier days convection and thus some activity ongoing to start the day. So while a more focused corridor of higher flood probabilities could exist, tough to pin down any area where a more organized threat could exist...so will just maintain the Marginal for now. Chenard ....Previous Discussion... ....Great Basin-Northern Rockies-High Plains... Upper level pattern shifts very little by Day 3 (Thu-Thu night), though compared to Day 2 there is more support for expanding the Marginal Risk area westward toward the upper low to include more of the intermountain region (southern ID into southern OR, northern UT, northern NV, and northeast CA). Deterministic and probabilistic QPF from the models again support a localized or isolated flash flood threat, thus the rather large Marginal Risk area on Day 3. Hurley $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .