Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 08 2023 20:24:57 FOUS30 KWBC 082024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Thu Jun 08 2023 Day 1 Valid 16Z Thu Jun 08 2023 - 12Z Fri Jun 09 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF IDAHO AND MONTANA...=20 ....16Z Update... ....Northern Rockies and High Plains... With this morning's update, portions of west central MT through central ID have been upgraded to a Moderate Risk. 06Z HREF Neighborhood probabilities have increased to over 80% of exceeding FFGs into north-central ID through extreme western MT, with values over 50% into central MT. CAMs guidance shows storms lifting north and west with time, but with potential for heavy rainfall rates up to 2 inches per hour with the strongest storms. There is somewhat better agreement that for western portions of the Moderate Risk area, the storms may merge into an MCS, resulting in a longer period of time with heavier rainfall rates. Thus, while antecedent conditions are a bit drier over ID, the longer period of heavier rains should overcome that, resulting in scattered instances of flash flooding. These updates were coordinated with the OTX/Spokane, WA; BOI/Boise, ID; MSO/Missoula, MT forecast offices. Meanwhile further east into central MT, antecedent conditions are already quite favorable, with a few areas of central MT over 90% saturated based on NASA Grace imagery. Expected convection in this area may be a bit more scattered as compared with further west, but the higher river levels will make for less rainfall needed to result in flash flooding. In coordination with TFX/Great Falls, MT forecast office, the Moderate includes all the way to the Canadian border due to the favorable antecedent conditions, and along the eastern slopes of the Rockies near Glacier NP. The expanded Moderate risk was also coordinated with the GGW/Glasgow, MT and BYZ/Billings, MT forecast offices. The surrounding Slight Risk area was expanded a bit in all directions, but most notably to the east to near the MT/ND border in northeast MT. Similarly wet antecedent conditions are noted here, but once again the convection should be a bit more isolated given the distance from most of the forcing south and west, so the Slight should cover the more isolated expected flash flooding concerns towards the ND border. ....Central US and South Florida... No changes were made to the Marginal Risks in these areas and the previous discussion redrawn below remains in effect. Wegman ....Previous Discussion... =20 =20 ....Northern/Central Rockies into the Pacific Northwest...=20 Closed low over the Great Basin will weaken while lifting north and northeast. This puts the Northern Rockies in the path, with divergence aloft over an area of plentiful moisture and instability to lead to widespread convection on Thursday, with rates likely eclipsing 1"/hr in many areas. Effective bulk shear in the area reached 25-35 kts which should lead to convective organization. One week rainfall has been as much as 600% of normal (highest in MT). This all supports the existing Slight Risk. Even though the ID Stovepipe has had below average rainfall this past week and streamflow is below average, the ingredients in place support hourly rain totals to 2" where cells merge and/or train, which would be problematic even there. Changes made to the risk areas was minimal this cycle.=20 =20 ....Northern Plains...=20 A weakening stationary front will dissipate during Thursday ahead of a secondary cold front approaching from Canada late in the period. This forcing moving across a region of elevated PWs of +1 to +2 standard deviations according to NAEFS collocated with a ribbon of MUCAPE reaching as high as 2000 J/kg will support hourly rain totals which at times may reach 2"/hr based upon the ingredients available. Although Corfidi vectors reach 20 kts, they are aligned to the mean wind suggesting some training potential, while bulk shear of nearly 30 kts indicate the potential for convective organization. These together could result in pockets of heavier rainfall as repeated rounds of storms occur, with locally 3" possible. Overall the flash flood risk looks isolated, with the Marginal Risk shifted to where the guidance indicates the best potential for heavy rainfall. ....Portions of CA & NV...=20 As the closed low opens and drifts northward, sufficient moisture with PWs of 0.75-1 inch and MUCAPE of 500-1000 J/kg supports a Marginal Risk of excessive rainfall/flash flooding, with the potential of hourly rain totals to 1.5" where cells merge and/or train. Convective coverage will be the main reason for the low risk in this region, as any heavy rain-related issues are expected to be isolated. Additional rainfall could reach 1-3" in some areas. If it occurs over the same areas impacted earlier this week, rapid runoff and isolated flash flooding could ensue.=20=20=20 =20 =20 ....Southern Florida...=20 A shortwave aloft lifts through northern FL on Thursday, providing another day of favorable ascent for scattered showers and thunderstorms across the southern FL peninsula. Sufficient moisture, with PWs above 1.75 inches, collocated with a ribbon of 2000 J/kg MUCAPE will fuel 2.5"/hr rain totals within the convective activity. Storms that do develop are progged to move progressively to the northeast on 20-25 kts of mean 0-6km winds, but aligned Corfidi vectors and 25 kts of bulk shear indicate that training/convective organization is likely, which could enhance both the rain rates and rain duration. The highest risk for heavy rainfall occurs within a region that has been more than 200% of normal rainfall during this past week. While the greatest risk for any isolated flash flooding appears to be along the urban Gold Coast where sea breeze pinning is more likely to occur, any place across South Florida that experiences training could see instances of flash flooding.=20 ....South-Central to Southeast TX... A system moves across portions of central and eastern TX today, and is expected to interact with a moisture gradient across central TX which could spur development farther to the east and southeast with an uncapped air mass. PWs are ~1.5" and CAPEs should rise to 2000+ J/kg. Low-level inflow is weak but cyclonic. Effective bulk shear appears to be sufficient for some level of convective organization, so any bigger issues could end up being closer to the origination point of convection midday local time before cold pools form and forward propagation commences. Hourly rain totals to 1.75", with local amounts of 2-4", are expected within the new Marginal Risk area. Flash flood guidance values are lower to the west and higher to the east. However, urban areas around Houston and the Golden Triangle of Southeast TX could have localized flooding with these sort of amounts. Coordination with EWX/the New Braunfels TX forecast office led to this new Marginal Risk area. ....Kansas into Arkansas... A southward propagating piece of the polar front at 850 hPa is expected to instigate convection today within an uncapped air mass. PWs are 1.5-1.75", while CAPEs rise to 2000+ J/kg or so.=20 There are some pieces of guidance indicating local 2-4" amounts in this area. Since hourly rain totals could rise to 2", this would be a problem in any urban areas or within any problematic local topography, despite relatively high flash flood guidance values.=20 As issues are expected to be isolated, the new threat area is considered Marginal Risk. Roth/Weiss Day 2 Valid 12Z Fri Jun 09 2023 - 12Z Sat Jun 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MONTANA HIGH PLAINS... ....2030Z Update... ....Portions of the MT High Plains... A Moderate Risk upgrade was included for this afternoon's ERO update in coordination with the TFX/Great Falls, MT and BYZ/Billings, MT forecast offices. HREF probabilities continue to increase with the 12Z guidance, highlighting the Moderate Risk region and points south and east for a heightened risk of rainfall exceeding Flash Flood Guidance (FFG). The merger of 2 fronts over this region will provide the forcing, which in addition to the over 1 inch PWATs, which are more than 3 sigma above normal, MUCAPE values between 1,000 and 1,500 J/kg with locally higher amounts, and saturated soil conditions expected, especially after this afternoon and evening's convection will be all the ingredients needed for scattered instances of flash flooding, with locally considerable impacts. One of the fronts is a cold front diving southward down the Plains through ND, with the other front the stalled out warm frontal boundary helping cause some convection over this region this evening into tonight. These two opposing boundaries will collide with one another, helping lift the anomalous moisture already in place, resulting in widespread convection. Cell mergers will be especially important for causing flash flooding, as the merged cells will have nowhere to go after merging. The convection on the whole will diminish through the evening with the loss of solar heating, but some progress of the cold front westward across SE MT Friday night may result in additional storms there. Drier antecedent conditions in SE MT are the primary factor preventing the upgrade from extending there, as the area is expected to be able to handle more rainfall before flooding, resulting in less widespread flash flooding. This would be the area most likely to see an upgrade with future updates, depending on how much convection forms over this area this afternoon through tonight. Wegman ....Previous Discussion... =20 ....Sierra through the Pacific Northwest and Northern Rockies...=20 A weakening but still anomalously deep mid-level trough will continue to lift north as its eastward progression gets halted by a strong ridge over the Northern High Plains. The incoming system has an extremely moist environment to work with as PWs remaining as high as +4 standard deviations near the Canadian border, while a plume of 1000+ MUCAPE advects westward from the High Plains. This setup should result in scattered to widespread heavy rainfall, which will occur on top of soils that will be highly vulnerable to runoff due to a very wet week/month/two months. The inherited Slight Risk area was extended slightly southeast to account to a consistent signal for heavy rainfall near the MT/SD border. There's a great deal of spread with amounts and progression across the Northern and Central Plains, so kept that area at a Marginal Risk. =20 Farther to the south, another day of scattered slow moving showers with isolated thunderstorms is forecast from the Sierra northward into portions of NV and OR. The organization and duration of convection is expected to be less than points northeast, but given the available ingredients, hourly rain totals to 1.5" are possible, particularly at elevation. This could result in at least isolated instances of runoff or flash flooding on Friday.=20 =20 =20 ....Southern to Central High Plains...=20 A shortwave in the southern stream of the Westerlies moves out of the West and through the Southern Plains Friday into early Saturday. This should strengthen the southerly flow across the region, noted by increasing low level moisture transport through the Southern Plains. While there is some uncertainty as to how this will manifest as convection, there is at least some consensus that a stripe of showers and thunderstorms will develop and push eastward beneath this shortwave into the better moisture, particularly from CO east and southeast through portions of KS, the TX Panhandle, and OK. Model spread with location and amounts remain an issue, but there remains a signal for up to 5" of rain within this area. The inherited Marginal Risk was split in order to differentiate this area from activity expected farther north across the Northern Plains.=20 =20 ....Northeast Florida... A few pieces of guidance has local 2-4" values advertised across this region near a segment of the polar front. PWs rise to ~1.75" and instability of ~1000 J/kg is expected to lie across the region while under an upper level trough somewhat east of its axis.=20 Since the flow is generally out of the west, this should concentrate activity towards the eastern portion of the Marginal Risk area. Still, with the front in the vicinity, it could focus activity along its length should CIN build in and any instability remains aloft. Flash flood guidance values are high, but hourly rain totals rising towards 2.5" potentially would be a problem in urban areas within this region. Thought it prudent to introduce a Marginal Risk area in this region. Roth/Weiss Day 3 Valid 12Z Sat Jun 10 2023 - 12Z Sun Jun 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN HIGH PLAINS AND THE MIDWEST... ....2030Z Update... No significant changes were made with this update to the Slight Risk areas in the northern High Plains and the Midwest. A few cosmetic changes were made to the Marginal Risk, including combining the Marginal risk areas in the Arklatex region with the Midwestern one due to the models beginning to merge these areas of rain. Elsewhere little has changed. Wegman ....Previous Discussion... Northern High Plains... Anomalous moisture continues early on before dropping off and southward as an upper level shortwave potentially splits, with some energy moving north into Canada and some undercutting the ridge and dropping southward. Instability lingers early on, so the existing Slight Risk still makes sense, with storms forming initially across MT before dropping southeast across portions of WY and western SD/NE. Midwest... A shortwave drops south around a forming closed low across southeast Ontario, with the overall system on a strengthening trend. A low- to mid-level low drops southeast and lures PW values up to 1.5-1.75" into portions of KS, MO, NE, and IA.=20 Inflow at 850 hPa and effective bulk shear should be increasing, particularly late in the period when the low-level jet tends to ramp up Saturday night into Sunday morning which portends organized convection. CAPE values upstream are forecast as high at 3000 J/kg on the 21z SREF guidance during daytime heating, falling towards 1000 J/kg overnight. The guidance (outside the 00z NAM) has a cohesive signal for heavy rainfall in and around the offset KS/MO/NE/IA border junction where mid-level heights are diffluent, with the guidance having a decent signal for local 3-5" amounts. Flash flood guidance values, particularly outside MO, are modest, but the Kansas City urban area would naturally be susceptible to heavy rainfall. The ingredients available could support hourly rain totals to 2.5", which would be a problem in such an urban area. Raised a Slight Risk area for these region per the above parameters. A broad Marginal Risk area continues to surround the Slight due to some lingering spread seen in the 00z NAM and 00z UKMET solutions. Portions of CA/NV... An anomalously strong closed low drops into CA Saturday into Sunday. Precipitable water values remain elevated, with moisture increasing in the Central Valley as the cold low develops and moves in. Instability shouldn't be a problem as temperatures aloft decrease. Believe a Marginal Risk of excessive rainfall/flash flood should continue Saturday into Sunday. Hourly rain totals up to 1.5" should be possible where cells backbuild, train, and/or merge. ArkLaTex to the Central Gulf Coast... As a southern stream shortwave phases with a deepening positively tilted upper trough, PWs rise towards 2" along a section of the polar front making some progress eastward. Low-level inflow/effective bulk shear appear to be sufficient for convection with some level of organization, which would support hourly rain totals to 2.5" somewhere between eastern TX, LA, and MS. The guidance shows a great deal of spread in regards to QPF amounts and placement, but enough ingredients appear to be there for the introduction of a new Marginal Risk area. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8ezuBqo2pVf0Ma2BkLL2kzDNvic2yjznMyju6nUTVlOf= LO5FEZuoJ0TaecAidPwiRokH4Qkzq4UAdubRyRsiwp_5oFc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8ezuBqo2pVf0Ma2BkLL2kzDNvic2yjznMyju6nUTVlOf= LO5FEZuoJ0TaecAidPwiRokH4Qkzq4UAdubRyRsiLCwzKhI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8ezuBqo2pVf0Ma2BkLL2kzDNvic2yjznMyju6nUTVlOf= LO5FEZuoJ0TaecAidPwiRokH4Qkzq4UAdubRyRsiegdFlFM$=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) .