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HMT forecast discussion for Tuesday 8 January 2008

Echoes are beginning to increase in the ARB area and will continue to do so as an initial broad sw-ne band of echoes moves into the area through the next couple of hours. At KBLU very light snow was first observed at 14z, then again in the 16z obs. 18z KBLU obs has 0.25 miles in moderate snow with south wind gusting to 27 kts, and the temp dropping now to 27F. Only 0.02" melted so far as of 18z. A heavier band of rain is now hitting the Northern California coast, extending north into coastal Washington. The very latest radar at 20z shows a strong band just to the sw of the ARB with reflectivities of 50 dBZ drifting east (some could be melting band but does not look like this is the only part of this band). No cloud to ground lightning has been noted so far. The 500 mb shortwave trough axis is just off the Northern California coast, and is still forecast to pass across the ARB around 01-02z/Wed. The Sloughhouse profiler has shown increasing southerly flow through the lowest km+ over the last couple of hours as the trough approaches. No ESRL BLU mesonet data being displayed yet. Visible satellite imagery overlaid with surface reports indicates the surface front is still just off the Northern California coast.

Some model forecasts are up a bit this morning on forecast precip for this event so this is a good sign. Latest 12z/NAM is fairly robust, but already a bit too much so, with 0.18" at KBLU by 18z (when there is likely to be only a few hundreths at most), with about a third of an inch just to the west. It has the heaviest precip coming after 18z, with 0.5 at KBLU 18-21z, then another 0.58" 21-00z. Still another 0.2" by 03z but precip rapidly winding down. Storm totals then are near 2" at KBLU, and ~2.3" just to the west. Our local 3 km models are also a little fast with the precip by 18z. These models also show a good period of precip from 18z through 02z, with the main frontal band passing KBLU near 22z. Max amounts end up in the 2-3" range in the ARB, likely too much, but overall agrees with other indications that this should be a decent event. Lower resolution models like the GFS also have good precip, with 1.4" at KBLU, with similar timing. A close call at KBLU with the freezing level...our local models shift it above KBLU (to ~800 mb) this afternoon before trof passage, but the official forecast keeps it near 4000 feet with ~10" of snow forecast. The morning OAK sounding had a very high freezing level, at 9500 ft, just below 700 mb, but is much lower as one heads east into the higher terrain. In the 18z special OAK sounding the freezing level lowered to near 7400 ft, with a deep layer below this not a lot above freezing (at about 2C) all the way down to 900 mb (~3200 ft). This is the layer that cools heading into the higher terrain leading to the freezing level below KBLU. BUFR soundings from the NAM indicate the freezing level does get close to going near or above KBLU during the afternoon, with the closest by 22z, when sw winds increase to 50 kts near 800 mb, then wsw at 55 kts higher up thru 700 mb. Cooling in the BUFR soundings above 750 mb after 22z, but no real wind shift aloft until around 03z/Wed. This timing agrees with other models. Per the conference call WFO/SAC reports snow down to near 3000 feet, and the Alta mesonet is right near freezing at 3560 ft.

The next event for Wed night and Thu still looks marginal for an IOP. In the 12z NAM light precip begins ~06z/Thu/10 Jan, with a third of an inch at KBLU by 12z/Thu. Then precip at the rate of ~0.15"/3h through 21z then diminishing. Storm totals just west of KBLU still approach an inch over an 18 h period ending 00z/Fri, but a good portion of this would be in cold advection following a 500 mb trof passage near 15z/Thu, with the main trough farther north where more precip occurs. The GFS and ECMWF (00z run) are similar with a weaker system, but with less precip than the NAM. The 12z GFS only has about 0.15" total at KBLU, falling from ~06z/Thu thru 18z/Thu, but much more (about an inch) closer to the coast in Northern California. The 09z SREF run looks to be in general agreement on a lower end precip event. On the positive side, there is still a plume around to be tapped by the wave, and it looks to be a warmer event with the precip likely to be mainly rain at KBLU, if they were to get a decent amount of precip. HPC forecasts keep most of the precip farther north in WA and OR for this event.

Longer range still looking dry, with ridge building on the West Coast and persisting. One dramatic change was that the wave forecast by most models recently on Saturday (12 Jan) to pass north of the ARB suddenly dips south in the 00z/ECMWF run to produce a quick inch of precip in the ARB. However, the 12z ECMWF just in keeps this wave farther north, in agreement with other model runs. Otherwise, there remains general agreement in the ensembles of a dry period through the end of the runs on 23 Jan. However, there is more spread in regards to positioning of the overall CONUS mean trough, and this is even reflected in the forecasts for next week from the deterministic runs of the 12z ECMWF and GFS. This uncertainty may be a result of the positioning of a broad upper level low in the east-central Pacific next week (developing as a wave breaks off from the mean flow), something that would likely have considerable variation in different model runs. It is positioned farther east (but still far enough from California to keep precip away) in the GFS, and subsequently the CONUS trough is also farther east, while in the ECMWF both are shifted west. In some ensemble members this upper low drifts close enough to perhaps be a threat for precipitation sometime in the 15-22 Jan range, but this would be a minority forecast at this time.

ed szoke NOAA/CIRA/GSD