Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 29 2023 22:38:24 AWUS01 KWNH 292238 FFGMPD NVZ000-CAZ000-300415- Mesoscale Precipitation Discussion 0382 NWS Weather Prediction Center College Park MD 637 PM EDT Mon May 29 2023 Areas affected...Northern California...Northwest Nevada... Concerning...Heavy rainfall...Flash flooding possible Valid 292235Z - 300415Z SUMMARY...Well above average moisture, favorable upper-level pattern and slow cell motions across complex terrain poses scattered incidents of flash flooding through evening. DISCUSSION...GOES-W WV loop shows deep compact closed low off the Central West Coast of California with a highly diffluent pattern across the central and northern Sierra Nevada Range, particularly over the eastern slopes into North Central Nevada. Well above normal (2-2.5 Std Dev) deep layer moisture remains pooled along the eastern slopes of the Range with total PWat values over .7" though 1" values exist in the northern Sacramento Valley as through the northerly return flow regime out of W OR across far NW coastal CA. Strong solar heating across the range sparked an above normal upslope mountain flow and with moist low levels, CAPE up to 1000 J/kg and the convergence has sparked scattered thunderstorm development in the best diffluence region near Lake Tahoe, increasing with time toward the north into Lassen and northern Washoe counties. Moisture flux convergence is solid and should support rates of 1-1.25"/hr. Inital motions have been very slow with the first updrafts allowing for increased rainfall to fall with some highly focused spots of 1-1.5" likely adding to increased runoff due to snow melt. As such, flash flooding in/near these down drafts are possible to induce flash flooding and/or rock slides (especially on the drier eastern slopes, where erosion is less common). Given weak steering, rain may fall in on the initial updrafts or cold pool generation should spur outflow.=20 New strong development is likely to spout on outflow boundaries, and cell motions should likely keep overall totals in the .5-1" range given forward motions. Further Northwest across the Trinity/Siskiyou Ranges, a bit more heating is necessary as upper-level support is a bit less favorable. However, cells across Lake county will spring up first as there is modest divergence in the entrance to the 65kt 3H jet north and northwest of the closed low. This outflow aloft should support stronger updrafts that should spread northward with time. Slow cell motions and slightly increased moisture flux (due to availability) should support similar 1-1.25"/hr rates and spots of 1-1.5" totals are possible. Given the rain-forest nature of the region, the rates will be key here for inducing flash flooding, as so will be limited to the cores of the downdrafts, particularly if they can stay anchored to slopes due to combined easterly and westerly upslope flow. Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!9nZWCKHnTB9pDzunj4S5aHwUZZ1YjJ_kdvde6aNoPr4_G4LhFG4GI9ZvAwmBFAIRlGo4= ZonGPva61Qy7GZG9u2IZ-9U$ for graphic product... ATTN...WFO...EKA...LKN...MFR...REV...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 41892181 41881942 41881743 41681678 40841665=20 40541734 40311840 39311881 38281861 38001880=20 38201965 38702027 39722098 40822216 40332265=20 39352268 39602332 40792381 41712356=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) .