Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Jun 06 2023 15:54:00 FOUS30 KWBC 061953 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 352 PM EDT Tue Jun 06 2023 Day 1 Valid 16Z Tue Jun 06 2023 - 12Z Wed Jun 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FRONT RANGE OF THE ROCKIES THROUGH THE SOUTHERN HIGH PLAINS AND IN PORTIONS OF MONTANA... ....Southern High Plains through the Northern Rockies... Pinched flow beneath a broad trough will continue to drive warm and moist air across much of the High Plains and Intermountain West today, as the synoptic pattern slowly begins to evolve today. Above this trough, an axis of upper level deformation will continue to be the axis upon which scattered to widespread aftn convection develops as this ascent works across an environment with PWs of +2 to +3 standard deviations according to NAEFS and MUCAPE surging to 1000-2000 J/kg. The high-res simulated reflectivity is again depicting widespread convection blossoming this aftn, but struggling to depict any consensus as to where the most organized convection will occur. It is this setup that drive the nearly ever-present broad MRGL risk from west TX northward through MT as rainfall rates of 1+"/hr within slow-moving storms move across soils saturated from 7-day rainfall that is more than 300% of normal in many places leading to low FFG. Despite the broad signal, there are two inherited SLGT risks that have been adjusted only cosmetically to account for new guidance. The first is in a large stripe from the Sacramento Mountains northward through the Front Range and east onto the Southern High Plains. This area consists of more vulnerable soils noted by many USGS streamflow gauges between the 75th-90th percentile, as well as numerous sensitive burn scars. This is also an area that will likely have a lee trough providing at least a subtle focus for convection, aided by weak impulses moving overhead and at least the distant LFQ of a subtropical jet streak, and modest 0-6km bulk shear of 25 kts to encourage modest storm organization. The HREF probabilities across this area, for both 3-hr FFG exceedance and 3"/24hrs are greater than the surrounding region, indicating this SLGT remains warranted for a higher potential for runoff and flash flooding today. The other SLGT is a targeted smaller area focused near Billings, MT where the guidance features a better consensus for a pocket of heavier rain. The HREF neighborhood and EAS probabilities are both more aggressive in this small region, and there appears to be a significant overlap between the highest probabilities and the 7-day AHPS rainfall exceeding 600% of normal. This is additionally reflected by HREF FFG exceedance probabilities reaching 60-70%, so the SLGT risk is definitely needed and was adjusted slightly for updated guidance. ....Upper Midwest and Northern Plains... A ribbon of MUCAPE above 1000 J/kg and PWs of around 1.25" will provide a favorable environment for thunderstorms today as a stationary boundary wavers across the Upper Midwest. This front will produce low-level convergence, which into the favorable environment should result in scattered to widespread diurnal convection. The coverage of this is still uncertain as guidance has backed off a bit based on the high-res simulated reflectivity. However, 0-6km mean winds of just 5-10 kts parallel to the front will support some short-term training and slow movement of any cells that develop. This could produce more than 3 inches of rain in some areas as reflected by 24-hr HREF neighborhood probabilities as high as 30%, but confidence as to where those higher rain totals will occur is low. Despite generally dry antecedent conditions, USGS streamflow anomalies are still running high, especially in eastern ND and down towards the Buffalo Ridge, so the inherited MRGL was maintained for the isolated flash flood risk today. ....Central California through the Northern Great Basin... A strong closed low with 500mb heights below -3 sigma lifting onto the CA coast this morning will move little while maintaining intensity. This will drive pronounced deep layer ascent through downstream divergence and upper diffluence across northern CA and into the Northern Great Basin region. Low-to-mid level onshore flow south of this system will rotate around it, increasing PWs to 0.75-1" as far north as central OR, while coincidentally MUCAPE climbs towards 1000 J/kg. This overlap of impressive ascent and thermodynamics will force diurnal convection once again, with the HREF probabilities indicating a chance for short-duration rain rates nearing 1"/hr. The high-res appears to have backed off a bit on coverage today, especially for central CA, which may be due to more enhanced cloud cover noted on visible imagery. However, there is still a strong signal for the 2-yr ARI exceedance from the HREF where antecedent soils are more saturated using the 7-day AHPS rainfall and USGS streamflow anomalies as proxy, so only a few cosmetic adjustments were made to the inherited MRGL risk. ....Central Gulf Coast... A generally weakly forced but thermodynamically favorable environment will again support slow moving thunderstorms this aftn/eve. PWs on the morning U/A soundings at KLCH and KLIX were both just around the median of 1.4-1.5 inches according to the SPC sounding climatology, but with high WBZ heights and freezing levels. This indicates that as the column moistens further this aftn, convection should be capable of producing efficient warm rain rates above 2"/hr as show by HREF probabilities. 0-6km mean winds on the morning soundings were around 5 kts, and this is expected to continue today, indicating slow movement of these storms, with some more widespread development possible as a weak shortwave traverses just south of the coast. This area has been dry the past 7 days, but scattered convection did produce locally as much as 3 inches on Monday according to MRMS, increasing at least top-level soil moisture, and another day of these scattered thunderstorms could produce an isolated flash flood instance. Weiss Day 2 Valid 12Z Wed Jun 07 2023 - 12Z Thu Jun 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FRONT RANGE OF THE ROCKIES THROUGH THE SOUTHERN HIGH PLAINS AND FOR PARTS OF MONTANA... ....Southern High Plains through the Northern Rockies... The mid-level blocking pattern will begin to evolve, albeit slightly, on Wednesday as a closed low over CA gradually expands a trough across the Intermountain West while an omega-type ridge extends over the Plains. Between these two features, shortwave impulses will continue to advect NW across the region beneath a seemingly ever-persistent upper level deformation axis. The changes D2 involve primarily height falls and stronger lobes of vorticity around the approaching trough from CA rotating into the region to drive more pronounced ascent. This will occur in tandem with continued moisture surging northward noted by PW anomalies reaching +3 sigma according to NAEFS. This increase in moisture combined with the more intense and focused deep layer ascent will result in more widespread convection, with more convective organization also anticipated as 0-6km bulk shear climbs to 20-30kts. This is reflected by high-res simulated reflectivity showing rapid growth and expansion of showers and thunderstorms from NM through MT/ID, where HREF probabilities indicate widespread 10-30% chance for 1"/hr rain rates. These storms may exhibit short-duration training in any location, which could produce instances of flash flooding as they occur atop soils saturated from 7-day rainfall that is 200-400% of normal in most areas, and results in the large MRGL risk area. However, embedded within the MRGL risk are two smaller corridors where the excessive rain and flash flood risk appears to be greater, and is handled by SLGT risk areas. The first is in a stripe from the high plains of NM and TX Panhandle northward through the Front Range of the Rockies. Here, a third day in a row of slow moving thunderstorms will move atop extremely vulnerable soils with FFG around 1-1.5"/3hrs. The HREF exceedance probabilities in this area show local maxima of 20-30%, with recent sensitive burn scars likely needing even less than that. The inherited SLGT risk here was adjusted only marginally for new guidance as any heavy rain rates could quickly turn to runoff. The other area of higher risk again appears focused around Billings, MT, similar to D1, where recent heavy rain has dropped FFG to just around 0.75"/3hrs, with more rain expected during D1. Although the signal for more organized thunderstorms with heavy rain is a bit less here on D2, any shortwave and enhanced rain falling atop these soils would likely quickly become runoff and produce a flash flood risk, so the SLGT risk was unchanged. ....Northern Sierra/Northern California into portions of the Great Basin... The strong closed low centered over CA will begin to fill slowly Wednesday into Thursday as it drifts into the Great Basin. The total eastward translation and loss of amplitude will be rather minimal on D2 however, so expect another day of scattered slow moving thunderstorms from the Sierra into southern OR and the northern Great Basin. PWs will again be above 0.75", over 1" in some areas, which is above the 90th percentile according to the SPC sounding climatology. Steep lapse rates invof the upper low will help promote MUCAPE of 1000-2000 J/kg, especially in northern CA through southern ID, which will support short term rain rates of 1"/hr at times. This will be the third consecutive day of potential heavy rainfall across this area, but despite more saturated soils and still very high streamflows, the excessive rainfall risk appears to still be in the MRGL category due to mostly less intense rates without strong organization. ....South Florida... The inherited MRGL risk was expanded to cover the entirety of the southern peninsula for Wednesday. A shortwave over the Gulf of Mexico will weaken as it shifts northeast across the peninsula Wednesday, embedded within increasingly confluent mid-level flow south of a large trough centered over New England. This weak PVA/height falls will combine with upper diffluence becoming increasingly favorable within the RRQ of a jet streak to drive impressive deep layer ascent. At the same time, S/SW 850mb flow, although modest in intensity around 10-15 kts, will draw PWs of 1.75-2 inches northward, with moist transport vectors featuring considerable convergence across South Florida. This suggests that convection will become widespread during the aftn/eve, which with bulk shear rising to 30-40 kts, indicates a likelihood for storm to organize into clusters. The mean 0-6km wind will be around 15-20 kts, which combined with the higher shear could allow storms to move quickly off the eastern coast limiting the flash flood risk. However, aligned Corfidi vectors to indicate training potential which could enhance rainfall as rates reach 2+"/hr. The greatest risk for any flash flooding remains across the Atlantic coast urban areas as the sea breeze likely gets pinned over the more impermeable surfaces, but isolated flash flooding is possible anywhere storms training on Wednesday. Weiss $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .