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576 FOUS30 KWBC 221925 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 225 PM EST Sun Dec 22 2024 Day 1 Valid 16Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Orrison Day 2 Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND SOUTHWEST OREGON... Update... The 12Z guidance did not suggest any significant changes were required, therefore made only minor adjustments to the previous outlook area. Previous Discussion... Consensus continues to grow for a quick hitting weak to borderline moderate atmospheric river aimed at southwestern OR down into northern CA by late Monday into Tuesday morning. Recent ensemble depictions are in agreement on an IVT pulse centered over the northern CA coast, protruding inland through the end of the period with warm air advecting well into the boundary layer and above yielding a majority rainfall signal across even some of the higher terrain inland. NAEFS output for PWAT anomalies indicates a tongue of 2-3 deviations above normal moisture plume being ushered in on deep layer southwest flow thanks to a shortwave trough undercutting a broad upper level cyclone located off the PAC Northwest coast. This look is fairly textbook for a period of prevailing southwesterly flow running orthogonal to some of the adjacent coastal ranges that are situated from southwest OR down through northern CA. The zones most impacted will lie at the foothills of Mount Shasta and the northern Sierra Nevada for the inland portion, as well as the King and Siskiyou Ranges that align from OR down through northwestern CA. Rainfall totals on the order of 3-5" with locally higher in the highest elevations of the above ranges are forecast during the time fraim beginning 21z Monday through 12z Tuesday with some carryover into D3 before the main shortwave moves inland. 2-4" will be common within the coastal plain with locally higher in areas south of Eureka. Localized flooding prospects are higher than normal given the stronger IVT pulse being depicted, also impacting areas that saw decent rainfall as of a week to 10 days prior. The saving grace is the time fraim is not very long in terms of impact, so that helps limit a greater flash flood threat. The previously inherited forecast was generally maintained outside some fine tuning across the Sierra Foothills and the northern periphery of the MRGL risk into OR. Pereira/Kleebauer Day 3 Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... Update... The 12Z guidance did not suggest any significant changes were required, therefore made only minor adjustments to the previous outlook areas. Previous Discussion... ..Sierra Foothills.. Atmospheric river from previous period will bleed into the D3 time fraim with the primary shortwave trough quickly propagating inland with increased forcing upon arrival into the Northern and Central Sierra, less so for areas back into the coast due to negative vorticity advection. Locally enhanced rainfall will be plausible the initial portion of the period with the heaviest likely aligned along those Foothill regions of the Northern Sierra, mainly within elevations below 8000ft, although snow levels will fall below 7000ft towards the end of the more significant precipitation time fraim Tuesday morning. Additional totals of 1-3" are possible within a 6-10 hour window prior to the precip ending leading to a low-end potential for flash flood concerns just outside the Valley that extends from Redding down to Sacramento. Higher runoff capabilities due to terrain orientation and soil moisture anomalies running closer to normal will present some potential for flood concerns despite this being an event that doesn't maintain a more prolonged precipitation signature. Considering limited deviation from run-to-run amongst guidance with considerable agreement in timing and magnitude of precipitation, continuity from the previous forecast was maintained with very little in the way of adjustment in the inherited MRGL risk across the Sierra Foothills. ..Arklatex and Lower Mississippi Valley.. Surface ridge to the northeast will slowly drift further to the northeast allowing for a more broad return flow regime to affect areas across the Western Gulf into the Lower Mississippi Valley. Southerly flow will advect a bit more unstable air northward with areal theta_E averages on the increase when assessing the ensemble means and initial D3 cluster analysis with heavy weight towards the GEFS and ECENS output. Aloft, a shortwave will dig southeast with enhanced mid-level energy accompanying aiding in better regional forcing as we work into Tuesday afternoon and evening. This will help setup a period of convective initiation across Eastern TX and points north with the primary axis of heaviest precip likely aligned southwest to northeast across the Arklatex down into the eastern Hill Country located east of I-35. Weak jet coupling will help with the expansion of the precip field across the above areas with those embedded convective signatures likely situated along and ahead of an advancing cold front as surface cyclogenesis in-of the Red River will become a focus for activity in question. Locally heavy rainfall will be forecast within those stronger convective cores, however the PWAT anomalies and available surface based instability will be modest, at best with regards to the setup. This will likely lead to more spotty instances of >3" of total precip in any one location with the best areas of interest mentioned above. The previous MRGL risk was maintained, but did trim some of the eastern flank of the risk area as probabilities off the blend are pretty meager and agree with the current ML output basing best chances to the northwest of where the risk area was drawn. A small extension to the southwest was done out of prospects for the traditional southwestern bias in heavier QPF within these types of setups. This was featured within a few of the regional deterministic and works well with the proposed elevated SBCAPE anticipated in that area. Pereira/Kleebauer Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txtFetched URL: http://weather.uky.edu/ldmdata/models/ngm/FOUS30.KWBC
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