Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 26 2022 23:57:03 FOUS30 KWBC 262356 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 656 PM EST Mon Dec 26 2022 Day 1 Valid 01Z Tue Dec 27 2022 - 12Z Tue Dec 27 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL PORTIONS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... The strong atmospheric river is still expected to move onshore later this evening through Tuesday morning and overall very little changes were made to the Day 1 Excessive Rainfall Outlook with this update. The most recent hi-res guidance including the 18Z HREF continues to show the peak of the AR (which has +3 to near +4 PW anomalies) arriving after about 03-04Z, initially oriented across southern Oregon then sagging southward across northern California through 12Z. There is a strong signal for rain rates to peak at 0.5-1.0"/hr, with the most likely locations for 1"/hr rates across the favored the west to southwest sloping terrain along the coast. With snow levels initially quite high, this will support a mostly all rain scenario at least initially (and within the Slight Risk area). The HREF mean looks reasonable for 2-4" widespread totals through 12Z with the 18Z HREF probabilities showing potential for localized 3-5" for the favored west facing slopes. The 18Z HREF has a slight signal for amounts greater than 5" over a 6-hr period ending 09Z to give an idea of the expected intensity. Debris flows may develop for the recent burn scar areas and could lead to flooding. Outside of burn scars, precipitation departures are around normal, so some localized/minor flooding may develop through Tuesday morning. For more details, see WPC MPD 1171 that is valid through 09Z for mesoscale details on this atmospheric river. Taylor Day 2 Valid 12Z Tue Dec 27 2022 - 12Z Wed Dec 28 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL PORTIONS OF CALIFORNIA AND SOUTHERN OREGON AND FOR THE WESTERN FOOTHILLS OF THE SIERRA NEVADA OF CENTRAL CALIFORNIA... ....2030Z Update... In coordination with the STO/Sacramento, CA and HNX/Hanford, CA forecast offices, the previous Marginal Risk area for the western foothills of the Sierra Nevada Mountains of central CA has been upgraded to a Slight Risk. The primary point of uncertainty has been snow levels in this area...but there is good agreement with the latest guidance that snow levels could start off this event on Tuesday above 10,000 feet, when most of the precipitation is expected to fall. Snow levels will fall back to between 4,000 and 5,000 feet by early Wednesday morning, ending the flooding threat. This cooler air moving in will also coincide with most of the heaviest precipitation shifting south out of the area and weakening. The California Nevada River Forecast Center stated that despite the very high snow levels, much of this rainfall should be absorbed by the existing snowpack. The other Slight Risk area for the entire Pacific coast from southwestern OR to Oxnard, CA remains unchanged with this update. The greatest rainfall totals are expected in the mountains between the Bay Area and Oxnard. While folks north from the Bay Area will get less precipitation...the flooding threat will be ongoing at the start of Day 2 from the excessive rain tonight. Wegman ....Previous Discussion... Anomalous moisture associated with a fast moving atmospheric river will glide down the California coast and Sierra on Tuesday. Moderate to heavy rainfall (rates of 0.1-0.25in/hr) will spread down the state of California where a slight risk of excessive rainfall is in effect. Orographic enhancement and forcing from a cold front will allow for higher amounts over the foothills of the Sierra and heavy snowfall at higher elevations, but a marginal risk will suffice in these areas for now. PWATs near 1.5 inches (4-5 standard deviations from normal) will lead to instances of flash flooding from the Bay area early Tuesday morning to Los Angeles County later in the afternoon. Remnant moisture will spread across the Pacific Northwest, generating heavy snow in the Cascades and modest rainfall over the coastal ranges and Cascades foothills Tuesday morning, where a marginal risk is in effect. Kebede Day 3 Valid 12Z Wed Dec 28 2022 - 12Z Thu Dec 29 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARIZONA... ....2030Z Update... No major changes made with this update. In a collaboration call with the PSR/Phoenix, AZ, FGZ/Flagstaff, AZ, and TWC/Tucson, AZ forecast offices held this afternoon, it was agreed that the Marginal should remain as is...with the expectation of some snow at the highest elevations and lack of hourly rainfall rates needed to induce flash flooding. Wegman ....Previous Discussion... The flash flooding threat on Wednesday will remain temporally confined to the morning hours over portions of central and southern Arizona, as remnant moisture from the coastal atmospheric river event pushes through the region along a strong cold front. The latest 00z Euro has come into better agreement with the slower 00z GFS with respect to the arrival of another east Pacific shortwave trough. Thus, heavier rainfall totals are currently forecast to remain off of the northern California/southern Oregon coast on Wednesday. PWATs may hover around an inch with anomalies between 2-3 standard deviations in southern/central Arizona Wednesday morning before quickly shifting southward and diminishing. The marginal risk area is cutoff on the northeastern edge along the Mogollon Rim and White Mountains, where moderate snowfall is expected. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8o8TD75o2_EgzxqxYHwKaUjkMnKnKBXDT2jV8RQz1JVz= p6fm_Hn6yxOtdDFU74V-glNKWMy_txlX_PAnwMACZGDm3t4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8o8TD75o2_EgzxqxYHwKaUjkMnKnKBXDT2jV8RQz1JVz= p6fm_Hn6yxOtdDFU74V-glNKWMy_txlX_PAnwMACPO35rFM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8o8TD75o2_EgzxqxYHwKaUjkMnKnKBXDT2jV8RQz1JVz= p6fm_Hn6yxOtdDFU74V-glNKWMy_txlX_PAnwMACtZ845Lw$=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) .