Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 04 2022 01:05:46 FOUS30 KWBC 040105 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 805 PM EST Sat Dec 03 2022 Day 1 Valid 01Z Sun Dec 04 2022 - 12Z Sun Dec 04 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL CALIFORNIA COAST... ....01z Update... Little to no changes required to the inherited outlook this evening, as much of the expected rainfall has yet to occur across coastal and inland portions of central CA. The main anticipated shortwave impulse has yet to approach the shore near the Santa Lucia Range (see MPD #1142 and subsequent MPDs for more info), so the Slight Risk upgrade for portions of the central CA coast remains on track. Additional embedded convection is also expected over some of the same areas of southeastern AZ, with only minor adjustments (trimming) needed for this Marginal Risk area. Churchill ....16Z Update... The main change to the Day 1 ERO is an upgrade to a Slight Risk in northern San Luis and southern Monterey counties in California. While the overall lack of instability will keep hourly rainfall rates generally below 0.50"/hr most of the day, after 21Z, southeasterly 850-700mb layer winds will increase and aid in a surge in 850-700mb moisture flux over the terrain. The strengthening wind field oriented orthogonally to the terrain will also support stronger ascent via enhanced upslope flow. Recent runs of the HRRR/3kmNAM have shown the potential for elevated MUCAPE by 00Z that could be ~100-200 J/kg just off the coast, which would be more supportive of hourly rainfall rates >0.5"/hr later this afternoon and evening. Coming into Saturday morning, soil moisture percentiles in the 0-40cm layer were already up to 80% in some portions of these counties according to NASA SPoRT-LIS, and will only continue to moisten up as rain continues to fall over the area this morning. The latest 12Z HREF depicted 3-hr FFG exceedance probabilities of 15-20% between 21-00Z this afternoon and 00-03Z this evening, which aligns closely with when the heaviest rainfall rates are likely to transpire. Localized rainfall totals between 16Z today and 12Z Sunday morning could surpass 5", with the latest 12Z HREF's 24hr QPF >5" probabilities as high as 50% within the newly added Slight Risk area. In collaboration with WFOs San Francisco/Bay Area and Los Angeles, a Slight Risk was added along the higher terrain of northern San Luis and southern Monterey counties where the combination of moistening soils and sufficiently heavy rainfall rates could lead to rapid runoff along complex terrain and localized water rises in poor drainage areas. For the other two Marginal Risk areas, made minor tweaks to account for minor adjustments in QPF trends. The forecast reasoning from overnight still appears on track as of this issuance. Mullinax ....Previous Discussion... ....California... An upper level cutoff low centered off the coast of OR will drift southeastward towards the northern CA coast today. This forcing will team up with a subtropical disturbance and associated Pacific moisture to drive it into central CA today. The front moving into the Point Conception area will continue drifting northward through the morning, with some intensification in the rainfall rates as the upper levels increasingly contribute to the heavier rainfall rates. That said, instability is very low in this area, so the mountains will be the primary driver of heavy rain. This area of rain will stall between Monterey Bay and Point Conception today, which should allow for several (2-4) inches of rain to fall into the coastal mountains. The only real change from inherited was to nudge the southern end of the Marginal a bit north towards Point Conception, since there is a well defined back edge to the rain moving north and is not expected to move back south once that back edge moves through. Hourly rainfall rates have been one quarter inch or less so far, which is well below anything that would cause flooding. Further inland, the unidirectional southwest flow will run up nearly perpendicular to the Sierras of eastern California today. These much higher mountains will then wring out even more precipitation than the coastal mountains. While the highest peaks can expect snow, the high snow levels should allow much of the precipitation to fall as rain. The inherited Marginal Risk area remains unchanged. Expect 3 to locally 5 inches of rain near or over the mountains that will runoff quickly, causing isolated flash flooding in the foothills.=20=20 ....Southeastern Arizona... The inherited Marginal Risk area was trimmed a bit to align with the forecast axis of heaviest precipitation, in coordination with the EPZ/El Paso, TX, TWC/Tucson, AZ, and ABQ/Albuquerque, NM forecast offices. The heaviest rainfall, which could exceed 2 inches in 24 hours in isolated spots, extends from the Mexican border northeast through metro Tucson and then northeast to the=20 New Mexico border. The Marginal Risk was removed from New Mexico as the guidance becomes better aligned further north and west. This separate plume of moisture from the one impacting the California Coast will have some upper level support in the form of a right entrance region of an upper level jet along with highly anomalous moisture featuring PWATs of 1.25 inches, which is over 5 standard deviations above normal for this time of year. Thus, the moisture has very tropical origins. With that said, the lack of instability will be the primary factor reducing rainfall rates over this area. Meanwhile soil moisture in this area is around average for this time of year. Wegman Day 2 Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO... Most of Arizona and New Mexico will remain beneath the right-entrance region of a ~110 knot 250mb jet streak while, simultaneously, an upper low off the Pacific Northwest coast continues to funnel rich subtropical moisture overhead. NAEFS climatological percentiles remain eceptionally high over southeast Arizona and much of New Mexico Sunday morning and afternoon with values above the 99th climatological percentiles in some cases. The IVT also remains quite robust as the upepr low off the Pacific Northwest coast and the ridge over the Gulf of Mexico work in tandem to generate IVT values of 400-500 kg/m/s, which is also above the 99th climatological percentile. Similar to Saturday, however, instability will continue to be hard to come by, keeping a lid on rainfall rates from becoming too heavy. That said, the region will have picked up heavy rainfall from the previous day and soil moisture will be more elevated than it was entering the weekend. Chose to introduce a Marginal Risk for a similar area that was under a Marginal Risk on Day 1, with the expectation that soils will be more sensitive following Saturday's heavy rainfall. Communities and roads that reside along complex/rugged terrain, near dry washes, and in poor drainage areas are most prone to localized flooding. Mullinax Day 3 Valid 12Z Mon Dec 05 2022 - 12Z Tue Dec 06 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN MISSISSIPPI & ALABAMA, NORTHERN GEORGIA, AND FAR SOUTHERN TENNESSEE... A strong cold front pushing through the Midwest on Monday will occur at the same time as a warm front lifts north into the Mid-South. High pressure over the East Coast will work in tandem with lower pressure in lee of the southern Rockies to generate a southerly 850mb moisture moisture fetch out of the western Gulf of Mexico. This slug of moisture will coincidily join the subtropical jet emerging from the Southwest and result in a robust IVT across the Southern Plains and into the Mid-South. 00Z NAEFS suggests IVT values of 400-750 kg/m/s are possible across the South Central U.S. on Monday, which is >97.5% climatological percentile in many cases. As the upper low off the California coast and deep upper low in south-central Canada continue to help generate a strong jet streak oriented WSW to ENE across the U.S., divergent flow will be positioned over the South, setting the stage for a heavy precitipation event along the warm front in the Mid-South on Monday. There do remain some differneces on global and ensemble guidance regarding positioning of the heaviest QPF axis. Latest Day 3 QPF resmblems more of a Euro/GFS/CMC blend, weighted a bit more towards the Euro and its ensembles. This places the heaviest swath of rainfall over northern Alabama, northern Mississippi, northern Georgia, and southern Tennessee. Soil moisture percentiles are right about average for the time of year, but the last 7-days have featured some anomalous rainfall where some locations witnessed 300-400% of normal rainfall (northern Alabama and northern Georgia in particular). Some ensemble guidance does show MLCAPE by Monday evening could approach ~500 J/kg in the Lower Mississippi Valley as the low level jet picks up in strength Monday night. Latest Day 3 QPF shows rainfall totals of 2.5-5" in northern Alabama, southern Tennessee, and northern Georgia, which is where the bulk of the Slight Risk lies. The Slight Risk also extends as far west as northern Mississippi, which is currently forecats to lie closest to the best available instability late Monday. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!531hvebFUDYAyPN1sWS2q6-heC84pQZgpBLNl-u9yKOP= DewzVDw4tk0PwdD5IKs0IgmhLysQpkb6o9mVlIoalrr8SKA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!531hvebFUDYAyPN1sWS2q6-heC84pQZgpBLNl-u9yKOP= DewzVDw4tk0PwdD5IKs0IgmhLysQpkb6o9mVlIoamzVKc9k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!531hvebFUDYAyPN1sWS2q6-heC84pQZgpBLNl-u9yKOP= DewzVDw4tk0PwdD5IKs0IgmhLysQpkb6o9mVlIoaUSghREw$=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) .