Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon Jun 12 2023 17:06:00 FOUS30 KWBC 121928 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 PM EDT Mon Jun 12 2023 Day 1 Valid 16Z Mon Jun 12 2023 - 12Z Tue Jun 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE CENTRAL HIGH PLAINS AND FRONT RANGE & AROUND THE ARKLATEX... ....16Z Update... No significant changes were made to either of the Slight Risk areas today. The Slight in much of eastern CO and the surrounding area is on track, with several areas of convection having already caused flash flooding, with continued upscale growth of the convection likely through the afternoon. This Slight is considered on the higher end of the category. Meanwhile In OK, the ongoing MCS is being handled very poorly in the guidance, overwhelmingly towards there being less signal in the guidance than what is currently occurring. There is significantly more uncertainty with this area, because despite the ongoing MCS the embedded storms are moving very quickly, and that is expected to continue with a possible second round of storms in that same area late tonight. For the guidance that does agree there will be a second round, its behavior is expected to be similar to the current MCS, moving very quickly with limited opportunity for flash flooding since the heavy rainfall rates don't stay in the same area for very long. While the Slight Risk area remains per coordination with SHV/Shreveport, LA; OUN/Norman, OK; and TSA/Tulsa, OK forecast offices, it is much more on the lower side of the guidance. ....Northeast... The inherited Marginal risk area remains unchanged with this update. A slow moving cold front is approaching the area, but as expected it is mostly embedded stratiform rain, and rates have been quite low. Instability will increase a little in the Marginal Risk area later today, though MUCAPE values will remain at or under 500 J/kg. Thus, given how dry that area has been it's likely most of the rain for this system will be beneficial for most and only an isolated flash flooding threat is expected, with the highest threat in the included urban and poor drainage areas. Wegman ....Previous Discussion... Central High Plains into the CO and southern WY Rockies... An upper low drifts northeast from the Desert Southwest today adding some upper level lift to increase coverage in what has been a rather wet period for the region. A plume of west Gulf moisture continues to push up the southern Plains through the CO Front range with PWs of 0.75-1" in these higher elevations (2 sigma above normal). Moderate instability will return this afternoon with 1000-1500 J/kg of MUCAPE. While there is modest low-level upslope flow, the flow aloft is westerly with effective bulk shear promoting organized convection capable of heavy rainfall, up to 2" an hour. Storm motion should be eastward per mean layer flow with southwesterly downwind propagating convective motion vectors suggesting a northward tendency. Clusters of 2-4" QPF are in the 00Z HREF consensus over the CO High Plains into western KS which is where storm mergers and short periods of training should be most prevalent. The Slight Risk area is maintained - merely expanded a bit farther west through the Continental Divide. Oklahoma through ArkLaTex to the Central Gulf Coast... Southerly flow abundant with western Gulf moisture converges over the front already stationary over northeast TX through the lower MS Valley (this front persists through midweek in this blocked pattern) maintaining a pool of 1.5-2" PWs with ample instability, 2000-4000 J/kg of CAPE, just upwind. Heavy rainfall is expected at times along this frontal zone with more discrete activity in the warm sector south. Hourly rain totals to 2.5" with local amounts to 4" are possible. A Slight Risk remains for the area, just with a southward shift to along the Red River into southern AR per the 00Z HREF consensus. Ongoing activity over northeast NM and North Texas tonight will need to be monitored for affecting the local frontal position. Northwesterly mean flow becomes westerly this evening with a warranted expansion of the Marginal Risk over North Texas to the Central Gulf Coast. Intermountain West... Enough moisture and instability will be available to aid diurnal convective development under and ahead of the upper low. Hourly rain totals of 1.5" are possible should cells merge and/or train, but coverage is not expected to be dense enough for more than isolated to widely scattered heavy rain-related issues. The Marginal Risk area was shifted just a bit south. Northern Mid-Atlantic States... Frontogenesis in the low to mid-levels ahead of a re-strengthening mid/upper low will pivot over central/eastern PA/Upstate NY through this evening. Some instability will be present, producing locally heavy rainfall that repeats over the areas with a risk for hourly rainfall over 1" and potential for 3" to 4" in 6 hours where cells repeatedly merge/train and/or where mesocyclones can form. This would overcome dry preconditions and also be a risk in urban areas (including Philadelphia on the southern end of the pivot), so the Marginal Risk is maintained with a bit of a northward expansion around Lake Ontario. Jackson Day 2 Valid 12Z Tue Jun 13 2023 - 12Z Wed Jun 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLAMISS INTO FAR WESTERN GEORGIA... ....1930Z Update... ....Southeast... The most notable change made to the Day 2/Tuesday ERO was to significantly shrink the Slight Risk area across the Southeast, and focus both it and the surrounding Marginal Risk area on the rainfall footprint points south, noting the significant but not unusual southward trend in the guidance as to where the heaviest rainfall totals will be seen from the storms. There were 3 reasons noted for the diminishing confidence in flash flooding for this area, as per the recent 12Z CAMs guidance. 1) Antecedent conditions have been very dry, and as such FFGs are very high across this area. Given some of the favorable atmospheric conditions noted in the previous discussion below, including high PWATs and plentiful instability, there may be some chance a few cells may overcome the high bar set by these dry conditions, but confidence on that is low. 2) The time in between these rounds of storms will be a lot. The rainfall in this area is likely to come from 2 separate MCS's, the first ongoing at the start of the period and the second starting up in the very early morning hours of Wednesday. In between there is good consensus among the CAMs guidance that almost the entire area will be dry. This over 12 hour period of dry weather in between rounds of storms should allow for effective draining of the small creeks and streams before the second round hits, so the 24-hour rainfall totals are not reflective of the overall threat. 3) While the storms are moving across the region, they will be RACING. Some of the guidance indicates they could be moving at 40 kts. Thus, even with 2 or 3 consecutive rounds of storms over a given area due to training or as part of an MCS, the time any one area will see heavy rain will be very short. Given these reasons, the Slight Risk area was shrunken to be largely tailored to the HREF neighborhood probabilities of 3 inches of rain or more, with some buffer added to the south, again given the southward trends in the guidance. Further shrinking of the region or elimination of the area altogether may be needed with future updates. ....Intermountain West to the Southern High Plains... No significant changes were made to the aforementioned area. The forcing should lesser on Tuesday as compared with today, so the Marginal remains for much of the area, though given the very wet antecedent conditions, any organized convection may cause local flash flooding impacts to be greater. Wegman ....Previous Discussion... ....ArkLaTex/Mid-South/Southeast... An upper low currently over the Desert Southwest opens into a shortwave trough and crosses the southern Plains Tuesday afternoon/night. This will track over a stationary frontal zone that is already set up over north Texas east over the interior Deep South. Precipitable water values of 1.5-2" will be maintained along the frontal zone with ample instability (CAPE of 3000-4000 J/kg) will persist just south in the warm sector. There should be enough low-level inflow and effective bulk shear to organize convection with cell training in WNW-ESE motion and occasional mesocyclone formation leading to hourly rain totals up to 2.5". 00Z guidance maintains a consistent signal for local 3-5" totals. As this will be the second day of the upcoming three where heavy rain is expected across the region, flash flood guidance should be compromised to some degree from rainfall through tonight. The Slight Risk was shifted south a bit per global guidance consensus (further investigation into CAMs are necessary - as the 00Z NAMNest is farther north) and expanded zonally along this stationary frontal zone with repeating rounds of heavy convection expected. ....Great Basin/Northern Intermountain West to the southern High Plains... A sharp upper level trough will dig southeast from BC, enhancing aloft divergence over the northern Great Basin into the northern Rockies. Pockets of 0.75-1.25" PWs combined with diurnal max of 1000+ J/kg of MLCAPE will allow for convection to develop in the afternoon and evening hours. Hourly rain totals to 2" are possible within such an air mass where cells merge and/or train. 00Z guidance QPF totals continues to indicate a risk for up to 2" in spots, so the rainfall in any one place could occur within an hour and pulse out/send out outflow boundaries to initiate new convection. As any heavy rain-related issues are expected to remain isolated to widely scattered, have maintained the Marginal Risk for an area now from the Sierra Nevada through the northern Intermountain West, over the CO Rockies through western KS. Jackson $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .