Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Jun 07 2023 17:11:00 FOUS30 KWBC 072020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Wed Jun 07 2023 Day 1 Valid 16Z Wed Jun 07 2023 - 12Z Thu Jun 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN HIGH PLAINS NORTHWEST INTO THE LEE OF THE CENTRAL ROCKIES AND ACROSS NORTHWEST WYOMING INTO SOUTH CENTRAL MONTANA... ....Southern Plains through the Northern Rockies... Another day of scattered to widespread diurnal convection is likely across much of the High Plains and Intermountain West as the blocking synoptic pattern remains entrenched across the area. An expansive ridge centered over the Northern Plains and an anomalous closed low over the Great Basin will sandwich this area, driving persistent moist advection from the Gulf of Mexico to push PW anomalies to +2 to +3 standard deviations according to the NAEFS ensemble tables. The corridor of highest PWs will be collocated with MUCAPE of 500-1000 J/kg, and lay beneath a persistent upper deformation axis, to favorably produce aftn thunderstorms today. Weak impulses embedded within the flow will likely yield areas of greater coverage, but with minimal overall bulk shear, storms should generally remain disorganized today, which is echoed by the simulated reflectivity among the various high-res members. This suggests that the broad MRGL risk inherited from overnight remains warranted as any of these storms could have rain rates above 1"/hr atop soils that are saturated from 7-day rainfall of 300-600% resulting in anomalous streamflows. The only significant adjustment to the MRGL risk area was to expand it east into the western Hill Country and RGV of TX where guidance indicates an MCS may develop tonight with heavy rain rates falling atop locally more sensitive soils. The inherited SLGT risks were adjusted only cosmetically as they still highlight the two areas of greatest concern. The first, from the Front Range of the Rockies south into the TX Panhandle is driven primarily by the most saturated antecedent soils from repeated days of heavy rainfall. This has lowered FFG to 1-1.5"/3hrs in many areas, which has a 20-30% of being exceeded according to the HREF. While coverage/organization of convection may not be overly impressive here, the high risk of runoff leading to flash flooding persists due to the more compromised soils. Additionally, for the smaller SLGT risk across WY/MT, this area is more firmly embedded within the greatest PW anomalies, and FFG is reduced even more to as low as 0.5"/3hrs due to recent heavy rains. Any storms that move across this area could quickly produce runoff to renew flash flooding. ....Northern Sierra/Northern California into portions of the Great Basin... The anomalous closed upper low over CA this morning will begin to drift northward while slowly filling today. The total latitude and height gain will be pretty minimal today however, so significant forcing for showers and isolated thunderstorms is still expected across northern CA and into the Great Basin. The most significant adjustment today should be a reduction in coverage over central CA which did experience heavy rain on Tuesday, so the MRGL risk has been trimmed just slightly from the south. However, the overlap of impressive PWs noted by NAEFS standardized anomalies reaching above +2 sigma and MUCAPE as high as 1000 J/kg will still support efficient rain rates reaching 1"/hr in the scattered slow moving convection. Across the Northern Sierra and northern coastal ranges, 24-hr rainfall has been more than 2 inches in some areas according to MRMS, compounding already wet soils noted by high USGS streamflow anomalies and elevated CPC soil moisture. The HREF has a more aggressive signal today for more than 3 inches across the Klamath Mountain as well, which, while still only 10-20%, suggests a higher-end MRGL risk for runoff and isolated flash flooding than the past few days. ....South Florida... Increasingly confluent mid-level flow across Florida will push a modest shortwave northeast today, combining with the tail of a 70-90kt upper jet streak to produce deep layer ascent. This lift will act upon an environment with PWs rising above 1.75 inches and MUCAPE above 2000 J/kg to support scattered thunderstorms with rainfall rates exceeding 2"/hr. The important environmental difference today compared to earlier in the week is forecast 0-6km bulk shear reaching 30-35 kts, sufficient for storm organization. This combined with Corfidi vectors aligned to the mean wind to imply training indicates that rainfall duration may be lengthened in some areas today, and the HREF neighborhood probabilities suggests the potential for 3-5 inches of rain in some areas. Although FFG is high, 7-day rainfall has been 150-300% of normal, suggesting the soils have at least a subtly higher threat for runoff resulting in flash flooding. This will be most likely if any training or organized cells can move across urban areas, especially on the Gold Coast where the pinned sea breeze may enhance rates and duration today. ....Northern Plains/Upper Midwest... Stationary front positioned north-to-south across the Dakotas will begin to weak today, but continue to provide an impetus for ascent and scattered thunderstorms this aftn. The environment around the front will be thermodynamically favorable for heavy rain noted by PW anomalies around +2 sigma according to NAEFS ensemble tables and a ribbon of MUCAPE reaching 2000 J/kg. This will fuel convection with rain rates that could reach 2"/hr according to the HREF probabilities. Storm motions will generally be around 10 kts, but aligned to the front so some training is possible. Despite the expected scattered nature of convection this aftn, training of these rain rates could produce locally more than 3" of rain, which will fall atop soils still primed noted by USGS streamflows that are above the 90th percentile, especially in eastern ND. The MRGL risk was extended into MN as well which, despite having soils that can likely handle more rain, could also experience a flash flood risk tonight. The high-res simulated reflectivity has some consensus in developing an MCS type event late tonight along a potent CAPE gradient and the elevated front to the east. This could result in training 2"/hr rates, and the HREF has some higher EAS probabilities for 2" tonight. Confidence is not extremely high for this event, and it is possible some further adjustments to this ERO area will be needed later today. Weiss Day 2 Valid 12Z Thu Jun 08 2023 - 12Z Fri Jun 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE NORTHERN ROCKIES INTO THE INTERIOR PACIFIC NORTHWEST... ....Northern/Central Rockies into the Pacific Northwest... Closed low over the Great Basin will open and fill Thursday and then move slowly almost due north as its eastward progression gets blocked by an amplified ridge over the Northern Plains. This evolution will drive increasing height falls into the Northern Rockies, with lobes of vorticity swinging through the trough combining with intense mid-level divergence to drive robust ascent. Moisture will increase significantly on Thursday as well as persistent E/SE flow around this trough advects PWs as high as 1.25 inches, above the daily record according to SPC sounding climatology and approaching +4 sigma according to NAEFS. This exceptional PW will overlap with MUCAPE of 500-1000 J/kg to support intense rainfall in widespread convection on Thursday, with rates likely eclipsing 1"/hr in many areas. 0-6km bulk shear reaching 25-35 kts and southeast upslope flow will contribute additionally to storm organization and intensity, and with this rain falling atop soils pre-conditioned from 7-day rainfall as much as 600% of normal (highest in MT), the flash flood risk is increasing. The inherited SLGT risk was expanded as far west as Spokane, WA, and cosmetically adjusted for the new ensemble guidance. Despite eastern WA and the stovepipe of ID having been dry recently as reflected by AHPS rainfall data and below normal USGS streamflow, these intense thermodynamics and resultant rainfall rates should still produce rapid runoff and scattered flash flooding. However, the higher threat, which is reflected by the CSU first guess fields indicating a greater than 25% risk, is across north-central MT where antecedent soils are more saturated and HREF exceedance probabilities eclipse 50%. ....Sierra/Northern CA/Great Basin... As the closed low opens and drifts northward, the overlap of intense synoptic ascent and impressive thermodynamics will persist one more day. Forcing provided by height falls and downstream divergence will continue to impact the environment with PWs of 0.75-1 inch and MUCAPE of 500-1000 J/kg. This will again support scattered diurnal convection, and HREF rain rate probabilities reach as high as 25% for 1"/hr. While the coverage of convection may be a bit less/more scattered than anticipated on D1, it will be occurring across the same areas, suggesting the soils may be more saturated, resulting in a faster onset of runoff. Additional rainfall could reach 1-3" in some areas, which, especially if occurring over the same areas as on Tuesday or Wednesday, could yield rapid runoff and isolated flash flooding noted by 3-hr exceedance probabilities peaking at 30%. ....South Florida... The tail of an upper level jet streak centered over the Atlantic will remain over the Florida Peninsula on Thursday, providing another day of favorable ascent for scattered showers and thunderstorms. Mid-level flow will become increasingly confluent on Thursday as well the anomalous New England trough digs farther south, within which subtle impulses will rotate over the region. This ascent into PWs above 1.75 inches collocated with a ribbon of 2000 J/kg MUCAPE will fuel 2"/hr rain rates 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 or some multi-cell clusters are likely, which could enhance both the rain rates and rain duration. The HREF probabilities for more than 3 inches of rain reach 40%, with the highest risk occurring atop more saturated soils from 7-day rainfall that has been more than 200% of normal according to AHPS. While the greatest risk for any isolated flash flooding appears to be along the urban Gold Coast where the sea breeze pinning may occur, any place across South Florida that experiences training could see instances of flash flooding. ....Northern Plains/Upper Midwest... 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 intense rain rates which at times may reach above 1"/hr according to HREF probabilities. 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 some multi-cell clusters. 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 MRGL risk tailored to the highest antecedent streamflows suggesting a greater risk for rapid runoff but any heavy rain. Weiss $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .