Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 30 2023 07:56:04 FOUS30 KWBC 300756 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 AM EDT Tue May 30 2023 Day 1 Valid 12Z Tue May 30 2023 - 12Z Wed May 31 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE WEST, MIDWEST, AND PLAINS... ....Northern CA northeast across ID and southwestern MT/northwestern WY... Mid-Upper low off the Central CA Coast to start Day 1 (12Z Tue) will slowly wobble through SoCal during the period. Favorable upper-level diffluence north of the compact mid-upper low, south of the digging northern stream upper trough/height falls across the Pacific NW, will generate favorable broad-scale forcing across the outlook area Tue-Tue night. PWATs meanwhile are expected to climb between 0.75-1.00", which is ~1.5 standard deviations above normal for the end of May (per the GEFS) and plenty for heavy rain concerns at elevation. Modest uptick in deep layer forcing and moisture, along with at least some diurnal instability (mixed layer CAPEs peaking between 500-1000 J/Kg) will allow for scattered to more numerous showers/storms within the outlook area -- a few will be capable of hourly or sub-hourly rainfall rates of 0.50-1.00" which is around FFG values. 12Z HREF neighborhood probs max out around 35% for exceeding these FFG values, though perhaps overdone at nearby lower elevations. Nevertheless, this will pose a localized flash flood risk for yet another day, especially over any recent burn scars into northeast CA and southern OR and/or over areas that have received recent appreciable rainfall. ....Northern and Central Plains into Midwest... A pair of mid/upper level shortwave troughs are expected to pivot across eastern portions of the Northern Plains and across the far northern MS Valley today. Models do show some low-level frontogenetical response with these features, along with pockets of upper level divergence which are likely convectively enhanced.=20 PWATs look to peak around 1.3" (near the 75th percentile, per TOP sounding climatology), with SB CAPE rising to as high as 3000 J/Kg into the outlook area. Low-level inflow and effective bulk shear appear sufficient to organize convection that forms. While HREF neighborhood exceedance probabilities continue to suggest the potential for locally extreme amounts (3-5"+), it is too difficult to pinpoint a specific area within this broad region (as indicated by HREF EAS neighborhood probabilities for 1" exceedance, which only peak at 15% for southwest KS/OK Panhandle and the IA/NE/MO border region, respectively). Should the 12z HREF come into better agreement, a targeted Slight Risk upgrade (or two) are possible. ....Southeast OK into Northeast TX and Ark-La-Tex... Precipitable water values of 1.5"+ (near the 90th percentile, per SHV sounding climatology) with SB CAPE expected to increase to 1000-3000 J/kg. Low-level inflow appears weak, only on the order of 10-15 kts, but the mean 850-300 hPa winds are virtually null (which suggests minimal if not chaotic storm motion and better rainfall efficiency than average). Effective bulk shear may be as high as 25 kts on the western edge of the MRGL risk area, which may help to organize storms (and would potentially organize a complex, should upscale growth be sufficient). Even so, FFG values are quite high in this area so any excessive rainfall concerns should remain isolated. Churchill/Fracasso/Roth Day 2 Valid 12Z Wed May 31 2023 - 12Z Thu Jun 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE WEST INTO THE NORTHERN ROCKIES AND PLAINS, AS WELL AS ACROSS THE CENTRAL AND SOUTHERN HIGH PLAINS... ....West into the Northern Rockies and Plains... The localized risk for excessive rainfall looks to continue across portions of northern CA into northwest NV into Day 2 (beginning 12Z Weds), also extending farther northeastward into more of MT and into ND as well. This is primarily due to the aforementioned mid-upper low over SoCal beginning to lift northeastward through the Four Corners region (with another shortwave trough in the northern stream digging on its approach into the PacNW). While these two features may have larger implications going into Day 3, it's possible that there will already be some more organized flash flood potential taking shape late on Day 2. Will continue to hold off on any introduction of a Slight Risk area until the full breadth of CAM guidance is in later today, but the ingredients (PWATs up to an inch, ML CAPE rising to 500-1000 J/kg, and lift/shear from the upper features) look to be there for at least localized flash flooding. ....Central and Southern High Plains... After what may be a bit of a lull in activity on Tuesday across the southern High Plains, the return of convection farther south looks increasingly likely by Wednesday. This will largely be due to the same aforementioned shortwave moving into the Four Corners region, which should direct the favorable left-exit region of a ~100 kt jet streak over the confluence of the Central/Southern High Plains. PWATs pushing 1.25" and CAPE of 1000-2500 J/kg should easily support convection capable of heavy rainfall, and the upper-level support (and resulting shear) may help to eventually organize convection into a complex as well. While there is some better agreement between model guidance with regard to the most likely placement of elevated QPF (from northeastern NM into the TX/OK Panhandles), would prefer to hold off on the introduction of a Slight Risk until the full HREF model suite comes into range (given that there is still some offset between the FV3 and NAM-nest, which tend to run a little too hot as well). Farther north into the central High Plains, will also maintain a Marginal Risk (where a subsequent upgrade to Slight Risk is still possible, but less likely than farther south). Churchill Day 3 Valid 12Z Thu Jun 01 2023 - 12Z Fri Jun 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF EAST-CENTRAL MONTANA AND SOME NORTHERN PORTIONS OF WYOMING... ....Northern/Central Rockies and Plains... Things start to get interesting going into Thursday across the Northern Rockies and High Plains, as a remnant mid-upper low and associated vorticity maxima eject northward from the Four Corners region. This should provide broad ascent via DPVA, along with modest diffluence aloft. While we are not quite in the range of CAM guidance at this juncture, the GEFS does indicate non-zero probabilities (5-10%) for 2" and 3" exceedance over central MT. The GEFS also indicates PWATs increasing to as high as 1.25" (+3.0 SD, or approaching the 95th percentile climatologically). The machine learning GEFS-driven first guess fields from CSU suggest a Slight Risk for much of MT, as a result. Have maintained the inherited Slight Risk for about the same area, as a result, and this area will continue to be fine-tuned with subsequent updates as more CAM guidance comes into range. Farther south into the Central Rockies, any flash flooding is expected to be localized with much less organized convection. ....Central and Southern High Plains... Farther south into the central and southern High Plains, another active day of convection looks likely, with some additional lift being provided by the northward moving aforementioned mid-upper low and associated vort max. While convection overall should be less organized than on Day 2, there is still some potential for the introduction of a Slight Risk area (given the potential for introduction on Day 2, as antecedent conditions could be supportive for additional flash flooding). However, at this juncture have maintained an inherited Marginal Risk area due to the overall lack of agreement between models with regard to the spatial placement of 2-3" amounts (varying across the Marginal Risk area, from west-central KS to southern OK/North TX). ....Central and Southern Florida... Also maintained an inherited Marginal Risk for much of central/southern FL going into Thursday, as a very slow moving upper-trough over the Gulf finally makes its way directly overhead the Florida Peninsula. This should act to enhance typical diurnal convective activity across the region, with strong mid-level lapse rates allowing for more intense updrafts with tropical-like rainfall efficiency (given PWATs ~2.0" and wet bulb zero heights of ~15k feet). Any localized flooding impacts are most likely across urbanized metro areas with poor drainage. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_EbajIr-JstSWVuiiqp2Sp7iKaU7pQrKeO3iAW4XOWfe= L8szQYuYHvZatz5d-Lg_j3duG99iQs-k4shsvcJkFFzygMk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_EbajIr-JstSWVuiiqp2Sp7iKaU7pQrKeO3iAW4XOWfe= L8szQYuYHvZatz5d-Lg_j3duG99iQs-k4shsvcJk0-5mx6k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_EbajIr-JstSWVuiiqp2Sp7iKaU7pQrKeO3iAW4XOWfe= L8szQYuYHvZatz5d-Lg_j3duG99iQs-k4shsvcJkDIWB-Vs$=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) .