Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 06 2023 15:55:44 FOUS30 KWBC 061555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM 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... No significant changes again expected to the large scale flow pattern across the mid section of the nation. Blocking features remain in place characterized by a slow moving closed low inching north from south central CA into the southern Great Basin and an upper high across the Northern Plains. Similar to the day 1 period, there will be a broad region of favorable upper difluence stretching from across the High Plains and Rockies between these two blocking features. With PW values again forecast to remain well above average, 1-2+ standard deviations above the mean, there will be a widespread region of convection and potential for locally heavy totals. No changes made to the primary slight risk area from central CO, southeastward into northeast NM and northwest TX where additional runoff issues are likely given the recent heavy rains and resultant high soil moisture and anomalous stream flows. There were no big changes to the CSU first guess fields for the day 2 period compared to the day 3 period across this region, with CSU risk capped at marginal.=20 ....Northern Sierra/Northern California into portions of the Great Basin... While the strong closed low across south central CA day 1 may weaken slightly during day 2 as it creeps northeast into the Southern Great Basin, the overall large scale set up will remain favorable for additional widespread scattered convection across the Sierra, into northern CA and portions of the Great Basin.=20 Well defined upper difluence to the north and northeast of the slow moving closed low in an axis of persistent anomalous PW values will support another day of potential locally heavy rains and isolated flash flooding over regions of high stream flows or recent burn scars. ....Coastal Southeast Florida... Weak mean mid to upper level troffing over the central Gulf of Mexico will keep west southwest mid to upper level flow from the eastern Gulf and across South FL day 2. While not showing great agreement, there continues to be some model signal for heavy southeast coast of FL heavy rains day 2 in the 1.75-2.00 PW axis forecast across South FL. Concerns continue for isolated urban flash flooding from Miami north to approximately West Palm Beach. Oravec Day 3 Valid 12Z Thu Jun 08 2023 - 12Z Fri Jun 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN ROCKIES INTO THE NORTHERN HIGH PLAINS... ....Northern Rockies into the Northern High Plains... The strong closed low creeping northward from south central CA into the Great Basin on day 2 will begin to accelerate to the north northeast day 3. This will be increasing the heavy rainfall threat for the Northern Rockies into the Northern High Plains where strengthening easterly low level flow increases PW values to 2 to 3 standard deviations above the mean. An overall very favorable pattern for heavy rains will exist day 3 with strong upper difluence and anomalous low level easterly moisture flux impacts this region. There is fairly good model consensus for heavy rainfall potential from the MT/ID border region, eastward into central MT, with many of these areas receiving much above normal precip values over the past few weeks, 200-400% of normal. Subsequently, we maintained a slight risk area across this region, expanding it slightly from the old day 4 outlook on the western end across the ID/MT border region. ...Sierra into the Great Basin... PW values will remain much above average day 3, 1.5-2+ standard deviations above the mean, ahead of the accelerating and weakening Great Basin closed low. Precip will begin to focus farther to then north than on day 2, with widespread activity likely from the central to northern Sierra, northern California and into the northern Great Basin. No changes to the previous broad marginal risk area across this region where locally heavy rainfall across area of high stream flow or burn scars from recent years may produce isolated flash flooding. ....Southern High Plains... There is a lessening excessive rainfall threat day 3 into the Southern High Plains. After days of persistently anomalous PW values, veering low level winds to a west southwesterly direction will bring in much lower PW values. Model consensus is for little precip potential day 3 over the Southern High Plains. The southern end of the marginal risk area was trimmed over the Southern to Central High Plains given this from the old day 4 outlook. There will be continued scattered convective potential for north central to northeast CO into WY where a marginal risk is denoted. ....Coastal Southeast FL... With the mid to upper level trof forecast to persist across the central Gulf of Mexico day 3, with west southwest mid to upper level flow moving across South Florida, a small urban centered marginal risk area was added from Miami north to approximately West Palm Beach. Similar to day 2, scattered convection in the tropical PW axis expected across South FL, may result in isolated urban runoff issues.=20=20 Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7Jwssfs39zs-NSs2i_AgUlNUcWETP0hKOPoCAsaiEKoN= oanXlOgqFCnzdScUMgORS7aAig3kMBLqEgf_Y8Heds3ynfI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7Jwssfs39zs-NSs2i_AgUlNUcWETP0hKOPoCAsaiEKoN= oanXlOgqFCnzdScUMgORS7aAig3kMBLqEgf_Y8He1XWt9BA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7Jwssfs39zs-NSs2i_AgUlNUcWETP0hKOPoCAsaiEKoN= oanXlOgqFCnzdScUMgORS7aAig3kMBLqEgf_Y8He7rBk8-E$=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. 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